Population growth has increased dramatically since the mid-20th century, tripling from 1950 to the present (from 2.5 billion to 7.9 billion).

According to United Nations’ research, the world’s population could grow to 11 billion by 2100. Figure 1.1 shows the size of the world’s population and its growth over the years.¹ 

As a result of widespread improvements in nutrition, personal hygiene and public health, among other factors, the world’s population has grown since 1950.¹  However, such rapid population growth has meant a rapid increase in the demand for food required to meet the needs of the population. Indeed, according to the Food and Agriculture Organization of the United Nations (FAO), it is estimated that by 2050, a 70% increase in food production will be needed to satisfy the food demand. Population growth is directly proportional to the increase in food demand and, consequently, to the increased amount of food production that is accompanied by the large-scale generation of food waste (edible and non-edible parts).²  According to the FAO, one-third of all food produced globally is lost or wasted along the supply chain. The main stages at which this waste is produced are consumption and manufacturing, with 56% and 38% (approx.), respectively.²  The total quantities of food waste generated at the European level are shown in Figure 1.2.

Of the total food waste generated globally, the beverage industry accounts for 26%, the dairy industry for 21%, the fruit industry for 15% and the cereal industry for 13% (see Figure 1.3).³ 

Food waste has economic (estimated losses of $1000 billion per year), environmental (water and biodiversity loss, land degradation and air and water pollution)³  and social impacts.⁴  These environmental, social and economic impacts, or problems, have arisen over time in both developed and developing countries. These problems have contributed to the deterioration of the environment and human well-being, as little effort has been made to mitigate them. Nevertheless, the most prominent effort made by the United Nations (UN) began in 2015, with the creation of the 2030 Agenda for Sustainable Development, which sets out a plan for improvement to be achieved in 15 years.^3,5  Sustainable Development Goals (SDGs) are guidelines for improving the global social, economic and environmental situations. In total, 17 SDGs have been proposed to end poverty, protect the planet and improve the quality of life of everyone everywhere.⁵ 

The SDGs framework is a standardised way of organising the various strategies focused on specific issues, be they poverty, quality of life, environment and society. In addition, this framework leads to greater coherence in government policies in most UN member countries. Therefore, SDGs integrate concepts, principles and criteria, providing UN member countries with a structured analysis to enable the 2030 Agenda to be met.³ 

SDGs can be divided into three groups:⁶ 

• Natural resource use: the goals included in this group are considered the basis for the organisation of the SDGs, as they focus on the protection, conservation, restoration and sustainable use of significant natural resources.

• Poverty and human well-being: SDGs related mainly to human well-being can be seen as an outcome of the fulfilment of other SDGs, because they are related to a more sustainable society.

• Sustainable production and consumption: these SDGs can be achieved by changing the current economic model from a linear to a circular one, for example, by shifting from fossil fuels to renewable energy sources.

As the SDGs focus on sustainable production and consumption, two new concepts are introduced: bioeconomy and biorefinery.

The bioeconomy concept is based on the need to overcome the damage caused to non-renewable resources. This concept can also be related to accomplishing the SDGs, since the transition towards a bioeconomy decreases the use of non-renewable resources. However, the concept of bioeconomy has been interpreted in different ways by different countries, organisations and/or institutions. For example, according to the FAO, bioeconomy is based on the generation of renewable biological resources and their conversion into high added-value products, such as bioproducts or bioenergy. Another concept, given by the Organisation for Economic Co-operation and Development, conceives of bioeconomy as “a world in which biotechnology contributes a significant share of economic input”.⁶  To really develop the concept of bioeconomy in any area, this concept and that of sustainability must be integrated to understand what is achieved when many bio-based products are obtained from biomass instead of non-renewable resources.

The biorefinery concept is an area of bioeconomy that is gaining much interest. Biorefineries use biomass as feedstock to produce one or several products.⁷  The use of waste as a feedstock in biorefineries is seen as a potential strategy that contributes to the achievement of the SDGs and, simultaneously, to reduce the use of both fossil and bio-based resources, supporting the transition towards a circular bioeconomy, with a lower intensity of use of virgin materials.⁸  The purposes of biorefineries are:

• To valorise biomass resources.

• To produce competitive and marketable products.

• To mitigate the environmental damage caused by the excessive use of fossil fuels.

• To encourage a sustainable future through the implementation of non-oil-based processes.

• To serve as the starting point to boost a bio-based economy in developing and developed countries.

Biorefineries can produce multiple high-value products such as proteins for food and feed,^9,10  biofuels,¹¹  biochemicals^12,13  and bioplastics.¹⁴  In addition, the biorefinery concept can be fully integrated with established biotechnologies such as anaerobic digestion, which can produce bioenergy and biofertilisers.^9,15 

The implementation of biorefineries can directly or indirectly address the SDGs, as biomass conversion can increase the economic growth without sacrificing the environmental performance. Most of the SDGs address cleaner production, sustainable development, sustainable agriculture, affordable energy, business alternatives and responsible production and consumption. Biorefineries thus play a key role in driving the achievement of the SDGs, as renewable resources (i.e., biomass) can be established as the new pillar of human development. Table 1.1 presents a summary of the main SDGs that can be achieved through biorefineries.

Due to the incessant growth of the world’s population and industrialisation, the amount of waste generated around the world is rising every year causing problems. The elimination and disposal of this untreated waste into the environment can lead to the deposition of pollutants in the ecosystem that will eventually affect humans and other living organisms.¹⁶  Therefore, because of the impact that the lack of biowaste management generates at the social, economic and environmental levels,¹⁷  it is necessary to propose a change of paradigm where residues are no longer waste and, instead, considered raw materials capable of providing high added-value products.

Solid waste management is a universal problem that affects everyone, making it necessary to look for more environmentally sustainable solutions from a circular economy standpoint. According to the data reported by Kaza et al. (2018),¹⁸  2 billion tonnes of municipal solid waste were already produced annually in 2016; however, the projection is that with the increase in population and social development, by 2030 it will exceed 2.5 billion tonnes of municipal solid waste, reaching 3.4 billion tonnes of waste by 2050. Figure 1.4 depicts the overall average composition of global municipal solid waste. This composition varies greatly depending on the development level of the country, with the amount of organic waste being higher in lower-income countries. However, higher-income countries are found to have a wider range of waste, as they include more materials such as paper, wood and plastic.¹⁸ 

Biowaste accounts for more than 60% of the waste generated annually (see Figure 1.4). Biowaste refers to biodegradable residues, such as fruit, vegetable, animal or forestry waste, as well as some household or industrial waste, among others.¹⁹  Throughout this chapter, emphasis will be placed on the importance of the valorisation of vegetable waste, from fruits and vegetables to trees and their residues.

Agriculture, by definition, includes the set of techniques and procedures for cultivating land with the aim of producing food of plant origin, such as fruits, vegetables and cereals, among others.²⁰  The agriculture sector not only generates lignocellulosic waste such as dried crop stalks, pruning residues, molasses, husks (corn, wheat and rice) and shells (walnuts, almonds, peanuts, etc.),¹⁹  but also produces a minor amount of food waste, such as that of fruits or vegetables due to their discarding when they do not fulfil product quality criteria. Food loss is regarded as the decrease in the quantity and quality of food that is produced due to the decisions and actions of the suppliers in the food chain (producers and the food industry).²¹  However, food waste is the decrease in the quantity and quality of food due to the decisions and actions of retailers, food service providers and consumers.²¹ 

Food loss and waste occur during the five main stages of the food supply chain: production, handling and storage, processing, distribution and marketing, and consumption.²²  The sum of both, food loss and waste, corresponds to one-third of the total food produced (around 1.3 billion tonnes per year).¹⁷  According to the Foodwaste Index Report, ∼931 million tonnes of food waste were generated in 2019 (61% household, 26% food service and 13% retail waste).²³  The main food groups contributing to nutrient and food waste or loss are cereals and pulses, fruits and vegetables (22% loss), meat and animal products, and roots/tubers/oil crops (26% loss).²¹ 

An analysis of the distribution of food waste in Europe shows that consumption generates 52% of waste, followed by production (23%), handing and storage (11%), processing (5%) and distribution and sale (9%), resulting in a waste generation rate of 180 kg per inhabitant per year.²²  Some of the most common wastes generated in the food industry are seeds, leaves, roots, tubers, fruits including their peel/skin and pomace, among others.²⁴  Food waste generally consists of carbohydrates (between 30 and 60%), proteins (between 5 and 20%), lipids (between 15 and 40%) and nutraceuticals.^17,22  Vegetable waste is richer in carbohydrates, while proteins and lipids are mainly obtained from meat and egg by-products.²⁵  This chapter focuses on the valorisation of the different vegetable wastes produced along the whole food chain, so animal waste and waste produced by animals will not be analysed.

The forestry sector involves forest resources and the production, trade and use of forest products and services.²⁶  This includes forestry and the different related industries, such as wood and furniture, paper and pulp, and bioenergy production. The timber industry, in addition to the uses derived from wood, also produces other non-wood forest products, such as cork and resin. The waste generated along the entire chain in this sector is mostly lignocellulosic waste, which consists of a three-dimensional polymeric network synthesised by plants through photosynthesis. Lignocellulosic biomass consists mainly of hemicellulose, cellulose and lignin (structural compounds), in combination with other minor compounds (pectins, inorganic compounds, proteins and extractives).²⁷ 

Forestry residues (chips and dust from tree discards, such as branches, bark, foliage and stumps) can reach 50–68%.²⁸  Moreover, Europe harvests around 426 million m³ of logs per year, which results in the generation of an annual large amount of waste that needs to be managed.²⁹  Pulp and paper production generates around 50% residues of wood pulp, since this process implies the removal of lignin, in addition to hemicelluloses and other extractives, carried out during the pulping and bleaching processes.³⁰  Timber production, on the other hand, represents up to 60% of waste from saw-logs due to the discard of different parts from the sawmill, such as branches, bark or other trimmings, in addition to the production of sawdust.³¹ 

The diversity in the constituents of the studied biowaste allows a wide range of valorisation processes, leading to the production of a countless number of compounds. There are many high added-value compounds that can be recovered from each of the different residues (see Figure 1.5), and their applications can be versatile, ranging from the production of biomaterials to the pharmaceutical, cosmetics, food, textile and chemical industries, among others.³²  This fact consolidates the idea of biorefineries as the sustainable future pathway for moving towards a circular economy.

Lignocellulosic residues, with their high cellulose, hemicellulose and lignin contents, represent a natural source of these high added-value compounds with an unlimited number of applications, from the production of biomaterials to the production of biofuels or food supplements.^33–35  This type of waste is also formed by other compounds, which means that polyphenolic compounds (like in bark), pigments (mainly in the leaves), as well as other compounds in smaller quantities can also be obtained. Oil crops produce a wide variety of residues (olive leaves, olive pomace, empty palm bunch, rapeseed meal, etc.), which in turn will facilitate the acquisition of various high added-value products such as polyphenolic compounds, monosaccharides and polysaccharides, dietary fibres, pectin and organic acids, among others.³² 

Vegetable and cereal manufacturing industries produce a lot of lignocellulosic by-products during the agricultural production stage; however, a large number of potential by-products are also generated in the vegetable and cereal preparation chain. Vegetable processing results in waste such as broccoli or asparagus leaves and stalks, and cabbage waste, among many others. By using different fractionation technologies, diverse compounds with high added-value such as phenolic compounds, pectin, glucosilates, carotenoids, saponins and sterols could be produced from these raw materials. The cereal processing industry could also be able to obtain sugar monomers, dietary fibres, phenolic compounds, xylo-oligosaccharides, organic acids, enzymes and proteins, among other possible high added-value products, derived from wheat bran, corncob, rice husks, brewers’ spent grain, etc. ³² 

Considering the wide range of products that can be obtained from this type of by-product, it is worthwhile to exploit this opportunity to achieve the goal of a circular economy promoting sustainable development. To this end, it is essential to carefully preselect the technologies to be employed for the best fractionation of each different biomass. The most critical stages in a biorefinery are commonly the separation processes. These stages are usually the most resource-intensive, and consequently, are the most expensive stages³⁶  with the highest environmental impact. Thus, there is a need to choose the right technologies to be used in biorefineries.

The aim of this chapter is to analyse and evaluate the different available techniques for the valorisation of biomass and its manufacturing industries, from the point of view of environmental sustainability. For this purpose, together with introducing the different available techniques, both conventional and alternative, an environmental impact assessment is presented considering the life cycle assessment (LCA) approach by assessing the environmental burdens associated with the processes.

Conventional techniques have been widely applied to isolate solid matrix compounds, both in the laboratory and in industry. These methods are based on solid/liquid extraction of compounds according to their polarity differences using suitable solvents (e.g., methanol, ethanol or water).³⁷  The most influential operating factors in these techniques are temperature, extraction times and the sample/solvent ratio.³⁸  The advantages and disadvantages of each of the methods explained below are listed in Table 1.2.

It is a solvent-free method that has been traditionally used to extract edible oil. The two main stages involved in this method are the sample preparation and the extraction stage.³⁹  Sample preparation consists of applying various pre-treatments (e.g., crushing, grinding or boiling) to the sample to destroy its cellular structure, thus reducing the moisture content in order to ultimately achieve higher extraction yields from the next stage.⁴⁰  Although it is a solvent-free method, solvents are sometimes used to continue the extraction process due to the amount of oil (20–30%) remaining in the press cake.³⁹  The pressing stage can produce between 70% and 80% oil.⁴¹  Yields are affected by some factors, such as the type of seed sample, the extraction temperature, the pressure exerted during pressing and the operating time.⁴⁰  Extraction by pressing is an inefficient process, but it is still the only extraction method in some rural areas. Furthermore, this type of extraction is only carried out in small industries due to limitations in scaling up, which has a negative effect on production costs.⁴² 

In these small industries, there are two types of extraction by pressing: cold pressing and hot pressing. As the name suggests, the main difference between the two methods is that cold pressing does not involve any heating during the pressing process, apart from the heat generated by friction.⁴¹  Cold pressing is considered the first choice in the industries because this technique can produce higher quality oil products due to its thermal sensitivity. This means that, as oil is a heat-sensitive product, the heat applied in the hot-pressing process can promote the oxidation process of the oil, reducing its quality.⁴¹  However, cold pressing has a lower extraction yield than the hot-pressing method.³⁹  Because of its good quality, cold-pressed oil usually commands a higher price on the market. However, given the wide use of this technique in the industry, the low extraction yield in cold pressing needs to be addressed. Therefore, various types of pressing equipment, such as hydraulic presses, have been applied to optimise the extraction process.⁴¹ 

It is considered the simplest extraction process used for the extraction of thermolabile compounds. The process to extract bioactive components consists of pre-treatment of milling, increasing the contact surface of the feed to improve the extraction efficiency, and then immersing the sample in a solvent for a long period of time. Finally, the liquid is filtered and the remaining solid is pressed to recover as much of the dispersed extracts as possible.⁴³  Among the most used solvents are hexane, benzene and toluene. The range of maceration times is very wide, from one hour to 15 days.^44,45  In addition to the long extraction times, one of the main disadvantages of maceration is the low extraction yields. However, adding agitation to the process results in homogenisation of the system, which provides an increase in the extraction yields. It is also well known that one of the other parameters that has great influence on the extraction yield is the sample/solvent ratio.⁴⁶  In general, the extraction yield increases with time until the extract concentration equilibrium between the raw material and the solvent is reached.

Therefore, the extraction process can be optimised by collecting at set times to determine the time to equilibrium.⁴⁶  Ćujić et al. (2016) achieved high yields of total phenols and total anthocyanins from chokeberry fruits under optimised conditions (50% ethanol, a solid–solvent ratio of 1 : 20, and 0.75 mm particle size), which suggested that maceration was a simple and effective method for the extraction of phenolic compounds from chokeberry fruits.⁴⁷  Albuquerque et al. (2017) conducted a study on the extraction of catechin from Arbutus unedo L. fruits using maceration, microwave-assisted extraction and ultrasound techniques.⁴⁸  Their results showed that microwave-assisted extraction was the most effective. However, if maceration was performed at low temperatures, similar extraction yields were obtained, which could translate into economic benefits.

Jovanović et al. (2017) evaluated the extraction efficiency of polyphenols from Serpylli herba using different extraction techniques (maceration, heat-assisted extraction and ultrasonic-assisted extraction).⁴⁹  Based on the content of total polyphenols, ultrasound-assisted extraction resulted in the highest total yield of flavonoids, with no statistical difference between maceration and heat-assisted extraction. In all cases, the method with the lowest extraction results was the maceration method.

It is widely used in a variety of industry sectors, such as waste treatment, food engineering and pharmaceuticals. Solvent extraction is a separation process that uses solvents (either polar or non-polar) to separate liquids from solid–liquid samples. The most commonly employed solvents are hexane, ethanol, methanol, chloroform, diethyl ether, petroleum ether and acetone.³⁹  This extraction method is based on refluxing and siphoning by combining the processes of percolation and refluxing, using heat. In Soxhlet extraction, the solvent is evaporated by a heat source and then condensed in a reflux condenser, dripping onto the sample compartment. When the solvent charged with the extracted product reaches the top of the sample chamber, the solvent is discharged back to the bottom through a siphon.⁵⁰ 

Solvent extraction efficiency could be affected by diverse independent factors, such as the residence time, moisture content of the sample, extraction temperature, sample size and selected solvent type. The sample should be crushed to a smaller size during the sample preparation stage to enhance its surface area. This helps to increase the contact area between the solvent and the sample, increasing the extraction yield. Some of the advantages of Soxhlet extraction are (1) conservation of the sample in contact with a fresh solvent, (2) the relatively high extraction temperature being maintained due to the heat of the distillation flask, (3) no requirement of filtration after lixiviation and (4) the simple and cheap operation of the system.

Nevertheless, the main disadvantages of Soxhlet extraction are the long extraction time, the use of a large amount of solvent, the inability to speed up the process by agitation, the need for a concentration step by evaporation, and the possible thermal decomposition of the target compounds.⁵¹ 

In these methods, the applied heat is the main cause of vegetable cellular disruption, allowing the release of essential oils. Therefore, the heating temperature must be enough to break down the vegetable cellular structure and to release the desired compounds.⁵²  In steam distillation, water is boiled in a chamber, and the resulting steam flows through plant matter in another compartment. Alternatively, steam can be introduced directly into the sample for extraction. On the other hand, in hydrodistillation, water and the raw material are placed in the same vessel and boiled together. Both techniques obtain the extracts dissolved in the steam for extraction. Then, the steam is condensed by indirect contact with cold water, and the mixture is separated into two fractions: extracted compounds and water. This separation is usually performed by decantation based on the difference in density between the compounds.⁴³ 

Despite the similarities between these methods, steam distillation could improve the yield of essential oils and the capture of volatile compounds. The small amount of volatile compounds recovered in hydrodistillation can be due to the hydrolysis reactions caused by the combined action of water and high temperature, which can lead to the degradation of some compounds. Nonetheless, steam distillation can avoid this degradation and allow the production of better-quality essential oils because the vegetal material is not in direct contact with water. Another important factor affecting the extraction yield is the extraction time, which varies on the scale of hours. While Santos et al. (2019)⁵³  needed only 6 h, Meullemiestre et al. (2017)⁵⁴  needed 8 h for the extraction. Both techniques, hydrodistillation and steam distillation, allow the recovery of compounds with different properties, mainly depending on the employed raw material.

This method is a biological process in which organic matter is metabolized and converted into biogas through complex reactions in the absence of oxygen.⁵⁵  Anaerobic digestion is common in nature, for example in animal digestive systems and wetlands. Anaerobic digestion is most-commonly performed worldwide in several ways, such as the digestion of primary and secondary sewage sludge, in anaerobic sludge bed reactors and upstream from activated sludge plants. The process consists of four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis, which can occur sequentially or simultaneously in one step.³  Anaerobic digestion of food waste generates products, such as methane (CH₄), volatile fatty acids (VFAs), acetic acid, iso-butyric acid, valeric acid and hydrogen (H₂), among others.

Anaerobic digestion can be performed in single or two-stage operations. For single-stage configurations, all reactions are performed in one reactor, helping to reduce the operating costs and reactor complexity. However, the formation of intermediate products in the reactor could accelerate the inhibition of subsequent processes. Therefore, with this reactor configuration, a lower product yield and conversion efficiency are achieved.

In general, process instability, reactor acidifications and co-production of hydrogen and methane are common problems in single-stage reactor configurations.⁵⁶  The two-stage process in which the acidogenic and methanogenic processes are physically separated appears to be efficient and can overcome the problems associated with primary digestion.⁵⁷ 

Anaerobic digestion of food waste mixtures, poultry litter and sewage sludge increased the biogas production to 640 L kg⁻¹ VS with a mixture ratio of 2 : 1 : 1 (sewage sludge : food waste : poultry litter).⁵⁸  Anaerobic digestion of food waste produced a methane yield of 276.5 mL CH₄ g⁻¹ VS at mesophilic temperature (34 °C), while 307.5 mL CH₄ g⁻¹ VS was obtained at the thermophilic temperature of 55 °C.⁵⁹  Lukitawesa et al. (2020) observed that VFA production can be increased by controlling the pH of the acidogenesis process (at pH 6) during the anaerobic digestion of food waste.⁶⁰  The highest VFA production (0.8 g VFA g⁻¹ VS) was obtained at 1 : 3 ratio (inoculum : substrate).⁶⁰  The combination of dark fermentation (acidogenesis) and methanogenesis of food waste resulted in a 1.22-fold increase in biohethane (H₂ + CH₄) production.⁶¹  The amount of methane produced by anaerobic digestion of one ton of food waste can reach 90.6 m³.⁶² 

The anaerobic digestion reaction configuration is strongly influenced and controlled by the process parameters (pH, acidity, temperature, substrate composition, C/N ratio, reaction time and inoculum) and the desired final product. Therefore, by optimising the process parameters, the yield of the desired final product can be improved.

Conventional methods have several disadvantages such as long reaction times, high volumes of solvent, additional processes for purification and concentration of the compounds,⁶³  and the generation of degradation products due to the reaction conditions,⁶⁴  among others. These drawbacks make the processes expensive and have a high impact on the environment.⁶⁵  Therefore, in order to overcome these problems, other so-called non-conventional methods are currently being studied. The main objective of these technologies is to eliminate the drawbacks of conventional methods in order to achieve competitive and environmentally sustainable processes. These non-conventional methods need to fulfil certain requirements, such as a lower energy and chemical solvent requirement and low target compound degradation.⁶⁵  The advantages and disadvantages of each of the non-conventional methods explained below are listed in Table 1.3.

The pre-treatments necessary for the fractionation of biomass are the most expensive processes in a biorefinery, so it is essential to make a good process selection. The purpose of the pre-treatments is to facilitate the release of the desired compounds in a selective way, avoiding contamination by insoluble macromolecules or micro-organisms.⁶⁶  Thus, it is necessary to determine what end products can be obtained from the different biomass in order to choose the most suitable pre-treatment depending on the type of biomass and the desired product. The selection of the ideal pre-treatment should satisfy the following requirements according to Jesus et al. (2022):⁶⁷  simple and economical operations, low energy, water and chemical consumption, low corrosion, ability to modify the biomass structure, high selectivity, high extraction yield, low production of degradation products and limited waste generation.

The different pre-treatments can be grouped into four large categories defined by the way in which they act on the biomass to modify its structure: physical, chemical, physicochemical, and biological pre-treatments. Physical treatments are those that only modify the form of presentation/structure of the biomass. Some physical pre-treatments have the unique objective of reducing the particle size of the biomass in order to increase the contact surface and decrease the polymerisation degree (e.g., grinding), while others have a direct effect on the biomass structure (e.g., ultrasound, extrusion, etc.).⁶⁸  Particle size reduction pre-treatments are often considered mandatory since they support subsequent fractionation steps. Chemical pre-treatments are those that use catalysts to separate the compounds of interest, such as acids, bases or oxidising agents. Physico-chemical pre-treatment, on the other hand, is a combination of chemical and physical methods applied simultaneously to favour the extraction of the desired compounds. On the other hand, biological pre-treatments are based on the use of biocatalysts (biological agents) to separate the target compounds.⁶⁷ 

Physical treatments include not only mechanical operations, but also different types of radiation, such as ultrasound or microwave radiation.

The extrusion process is based on two unitary operations, mixing and heating. First, the raw material injected under pressure is vigorously agitated using up to two rigid screws, mixing everything perfectly. Subsequently, this mixture is heated during further intensive mixing, thereby allowing the mechanical action to create a change in the structure of the starting biomass when it passes through the grate.⁶³  This technique permits continuous processing of biomass, which saves both space and energy. The most important advantages of this pre-treatment are shorter time, less water consumption and low corrosion, among others.⁶⁹ 

Delvar et al. (2019) reported an extraction process using a twin-screw extruder for the extraction of polyphenolic compounds from passion fruit residues.⁷⁰  Using this technique, the group was able to extract up to 67% of the total polyphenol content of the different studied biomass.⁷⁰  A recent study evaluated the combined effect of the freeze-drying process and extrusion to improve the quality of flour from banana waste.⁷¹  The improvement in flour composition resulted from the retention of phenolic compounds in the flour due to the process combination.

Regarding lignocellulosic waste, the work carried out by Wang et al. (2020) with corn stover confirms that extrusion pre-treatment not only favours the subsequent stages for biomass recovery, but also achieves the extraction of both proteins and cellulose.⁷²  Duque et al. (2020) evaluated the benefits of combining extrusion with an alkaline pre-treatment followed by an enzymatic pre-treatment of barley straw.⁷³  The bioextrusion favours the mixing of the media with the biomass, increasing the efficiency of the separation processes.

Ultrasound-assisted extraction (UAE) pre-treatments are based on the principle of acoustic cavitation generated by ultrasound radiation, which intensifies the transfer and transport phenomena between the solvent and the biomass. This effect occurs due to the propagation of the ultrasound waves in the medium, which generates bubbles that increase in size until they reach a maximum, when these bubbles collapse. The collapse of the bubbles generates different physical effects, such as micro-jets, shock waves and turbulence.⁷⁴  When these phenomena occur close to the biomass fractions, it promotes cell disruption, which favours the transfer of matter between the solvent and the biomass, facilitating the separation of the compounds. The main advantages of this pre-treatment are the improvement in extraction yield, time and energy savings, and lower working temperatures, among others.⁵⁰  This technique is usually used in combination with solvents, which can be conventional solvents, in general, different organic solvents, or more recent solvents, such as ionic liquids (ILs) or deep eutectic solvents (DES).

Banožić et al. (2019) conducted a research study to optimise the extraction of both phenolic compounds and solanesol from different tobacco plant residues using UAE.⁷⁵  Leaves were the residues from which most phenolic compounds and solanesol were extracted using pre-treatment at temperatures below 45 °C and with a duration below 43 min. This confirmed UAE as a potential technique for the extraction of these compounds. Medina-Torres et al. (2019) used UAE to maximise the extraction of phenolic compounds from Persian lemon residues, requiring only 10 minutes.⁷⁶  This technique is also being widely studied on residues generated from the forestry industry. An example of this is the results published by Cetera et al. (2019) on their work related to the extraction of polyphenolic compounds with antioxidant activity from Turkey oak.⁷⁷  Using this pre-treatment, a considerable amount of polyphenolic compound was extracted in just one hour, reducing considerably the extraction time of conventional methods.

Microwave-assisted extraction (MAE) pre-treatments are based on electromagnetic irradiation (frequencies between 0.3 and 300 GHz), usually using 2.45 GHz for these processes to take advantage of the dielectric heating principle leading to a more efficient material heating. This heating principle is based on the dipole moment rotation mechanisms and ionic conduction,⁷⁸  so the heating will depend on the ability of the material and the solvent to absorb microwave energy. Microwaves interact with polar molecules by making them vibrate, so that the dipoles of the molecules are constantly being aligned with the magnetic field produced by the microwaves, thus resulting in the heating of the solvent or mixture.⁷⁹  In addition, they can penetrate into the cell matrix of the biomass, since water and other polar compounds present in the cells are also affected by microwaves causing cell disruption.⁵⁰  In this way, the use of microwaves, in addition to being a more efficient heating mechanism, enhances the penetration of the solvent into the matrix, making it easier to separate the desired compounds. The most outstanding advantages of this technology are the high reaction yield, the reduction of reaction time and the reduction of the required solvent volume.⁸⁰ 

In recent years, MAE has been one of the most studied non-conventional methods, mainly applied to intensify different processes. Jesus et al. (2019), for example, used MAE to extract polyphenolic compounds more efficiently from pruning residues.⁸¹  Recently, Dávila et al. (2021) carried out optimisation to maximise the extraction of oligosaccharides from vine shoots by intensifying the autohydrolysis process with MAE.⁸²  They concluded that the use of MAE required less reaction time, which translates into less energy consumption, making the process more environmentally sustainable. Among the latest work published on the valorisation of pumpkin peel and pulp waste, Sharma and Bhat (2021) investigated the extraction of carotenoids using MAE.⁸³  In this study, after only 30 min, the carotenoid content extracted was almost double compared to conventional methods, in addition to exhibiting higher oxidative stability, thus confirming the potential of this technique.

In recent years, the simultaneous use of UAE and MAE techniques is also being evaluated in order to improve the limitations of these techniques separately. The simultaneous use of microwaves and ultrasound generates a synergistic effect that induces the improvement of both heat (microwaves) and mass (ultrasound) transfer.⁸⁴  The synergistic effect generated by the simultaneous use of both techniques results in higher extraction yields in shorter reaction times.⁸⁵ 

In a recent study conducted by Estrada-Gil et al. (2022), they confirmed that the simultaneous use of microwaves and ultrasound has a better extraction yield of phenolic compounds from Mexican Rambutan peel residues than the separate use of those techniques.⁸⁶  On the other hand, Sillero et al. (2020), by optimising the extraction of pine bark by simultaneous use of microwave and ultrasound, concluded that this pre-treatment not only improves extraction yields, but also reduces reaction times by up to 47 times compared to the conventional method.⁸⁵  Recently, the use of this pre-treatment has also been studied to favour depolymerisation by acid treatment of lignin.⁸⁷  The results of this work show that the hybrid use of these techniques favours the yield of bio-oil, and monomer production, which converts it into a technique with potential application in this field.

This non-thermal pre-treatment is based on the phenomenon of electroporation, which can be defined as the increase of cell membrane permeability due to the application of a high-voltage pulse. It facilitates the recovery of intracellular compounds by diffusion. During the treatment, the biomass to be processed is suspended in a solvent, and then the mixture is placed between two electrodes where the pulses are generated, using an electric current of 0.1–100 kV cm⁻¹ for a short period (microseconds or milliseconds).⁶⁴  The electric potential applied on biomass, whose cell membranes have a dipolar nature, results in the separation of the membrane molecules forming pores and increasing the permeability of the membrane, thus enhancing the movement of intracellular compounds.^24,63  The resulting membrane changes, depending on the adjustment of the reaction parameters, could be reversible or irreversible depending on the objective. The main advantages of this pre-treatment are related to the ability to conduct this process at room temperature and the short reaction times, which reduce energy consumption.⁶⁸ 

Lončarić et al. (2020) reported research on the extraction of polyphenolic compounds from blueberry pomace using PEF.⁸⁸  In this study, the results confirmed that PEF achieved a higher yield of phenolic compounds, compared to those obtained by UAE, with an energy input of only 41.03 kJ kg⁻¹. In the study carried out by Visockis et al. (2021) to obtain betalain from red beet, it was shown that only 3 pulses of 1 Hz and a pulse duration of 100 µs for each resulted in extraction of up to 70% of the total initial betalain.⁸⁹ 

Physico-chemical methods are carried out due to the combination of effects, which tends to make them more efficient in biomass.

Supercritical fluid extraction (SFE) pre-treatment is based on the thermodynamic properties of the fluids, meaning that through the application of pressures and temperatures over the critical point, a fluid acquires the properties of both gas and liquid.⁹⁰  In this way, the fluids maintain the properties of surface tension, high diffusivity and low viscosity characteristic of gases, and the solvating power of liquids.⁹¹  Therefore, the separation and extraction of the target compounds are enhanced. The solvents that can be used include ammonia ethylene, toluene and carbon dioxide, which are used at a pressure of 10–30 MPa and at temperatures of 40–50 °C.⁹² 

This pre-treatment has been extensively studied for the extraction of oil from different agricultural residues. In the study performed by Ferrentino et al. (2020), it was confirmed that the use of SFE with CO₂ achieved similar results to Soxhlet extraction but in less than half time, saving both energy and the required amount of solvent.⁹³  Thus, using SFE, they achieved an oil yield from apple seeds of more than 20% in only 140 min at 40 °C. In the work carried out by Marić et al. (2020) for the treatment of raspberry seeds, they also managed to reduce the reaction time by half compared to Soxhlet extraction, and the reaction temperature was only 50 °C.⁹⁴ 

In a recent work developed by Bukhanko et al. (2020) for the valorisation of different Norway spruce residual fractions (branches, needles and bark), SFE with CO₂ has been proven to slightly improve the extraction yield compared to Soxhlet for similar reaction times, but with the advantage of solvent savings, as well as the use of green solvents.⁹⁵  In addition, this technique proved to be more efficient for the extraction of both resins and sterols. In another recent study, up to 50% of the total extractives have been removed from different forest residues by CO₂ SFE, which favoured the subsequent pyrolysis treatment to obtain charcoal.⁹⁶ 

Subcritical fluid extraction (SubFE) pre-treatment, also considered an environmentally friendly and non-toxic technique, is based on the variation of the working temperature as well as the pressure, which directly influences the dielectric constant of the solvent, but always works below the critical point.⁸⁰  Different polar solvents can be used; however, water is the most common of all due not only to its properties, but also to its green solvent character. Thus, when water is heated to temperatures between 100 and 320 °C, at pressures between 20 and 150 bar, the dielectric constant of the solvent is modified and can be equal to that of methanol or ethanol. Thanks to this decrease in the dielectric constant, other properties such as surface tension, viscosity and polarity are also reduced, which favours the dissolution of the target compounds.⁷⁴  The major advantages of using this pre-treatment are the reduction of the extraction time, the selectivity of the procedure and the use of a green solvent.⁸⁰ 

Subcritical water pre-treatment has been used for the extraction of different compounds, from polysaccharides to peptins and polyphenolic compounds.⁹⁷  In the recent work carried out by Muñoz-Almagro et al. (2019) for the valorisation of cocoa pod husk using subcritical water conditions, they reached an extraction yield of almost 11%, compared to the 8% obtained by conventional extraction.⁹⁸  In addition to improving the yield, the extraction time was reduced by up to 3 times, obtaining a pectin-richer product. In another study carried out for the extraction of pectin from jackfruit peel residue, it was concluded that the use of subcritical water is an efficient method, which reduces the extraction time and has a lower environmental impact.⁹⁹  Gonçalves Rodrigues et al. (2019) have also used this technique for the extraction of polyphenolic compounds from papaya seeds as can be seen in their recent work.¹⁰⁰ 

Pressurised liquid extraction (PLE) pre-treatment, also known as accelerated solvent extraction, as well as the previous two, is based on the relationship between the temperature and pressure variables. In this case, different solvents are used at high temperatures and pressures in order to enhance the extraction processes, thanks to the improvement of the kinetics and distribution coefficient.¹⁰¹  The increase in pressure allows working at temperatures higher than the boiling temperature at atmospheric pressure. The increase in temperature (between 50 and 200 °C), which improves the diffusivity of the solvent, together with the high pressure (10–15 MPa) facilitates both the solubility and the mass transfer rate, thereby promoting the extraction of the target compounds.⁵⁰  The main advantages of this technique are the reduction of reaction times, the reduction in solvent use, and the lack of subsequent filtration steps.^66,101 

It has recently been demonstrated that the use of PLE for the valorisation of chestnut wood leads to an almost doubling of the extraction yield of polyphenolic compounds using only 15 min of extraction, compared to the 7 h required in conventional methods.¹⁰²  The use of this pre-treatment on other agro-industrial wastes also allows an increase in the extraction yield of polyphenolic compounds. One example of this is the work published by Alexandre et al. (2019), in which researchers improved the amount of extracted phenolic compounds from pomegranate peel with a treatment at 300 MPa for only 15 min.¹⁰³  In the work carried out by Pereira et al. (2019), PLE was implemented for the valorisation of grape marc in order to maximise the phenolic compound content.¹⁰⁴  This only required the use of ethanol–water (50/50 w/w) as the solvent at 100 °C, without using any catalyst and with only 20 min of reaction time.

Steam explosion pre-treatment involves exposure to high-pressure saturated water vapour for short periods, followed by immediate depressurisation while the biomass is forced through a small hole. This process applies high thermo-mechanical energy to the biomass, allowing the alteration of the cell wall,⁶⁵  thus enhancing the solubilisation of the target compounds. The ranges of the working parameters in this process are temperatures between 160 and 270 °C and pressures of 0.7–5 MPa.¹⁰⁵  The main advantages of this pre-treatment are the reduction of reaction time and energy consumption and the fact that the process is free of chemicals.⁶⁸  This process is widely used to obtain hemicelluloses from lignocellulosic biomass.

In the work developed by Álvarez et al. (2020) to obtain xylooligosaccharides from barley straw, the good performance of steam explosion pre-treatment was confirmed.¹⁰⁶  Using this technology, high-purity oligosaccharides were obtained using only 130 min of reaction time at 180 °C. The result of this technique for oligosaccharide extraction is so promising that, in a recent study, Swart et al. (2022) performed the scale-up of this pre-treatment in order to process brewery wastes.¹⁰⁷  Under the optimised scale-up conditions, they were able to extract more than 70% of the total oligosaccharides from the waste in only 10 min, working at 180 °C. This result is promising for the industrialisation of this technique, since the low time required will permit the processing of large amounts of waste. Bhatia et al. (2020) also carried out a pilot plant study of steam explosion treatment of herbaceous plants, where they managed to extract more than 50% of the total xylooligosaccharides from the biomass under the following reaction conditions: 10 min, 15 bar, and 200 °C.¹⁰⁸ 

Chemical methods tend to be more aggressive in biomass fractionation, causing a permanent alteration of the biomass structure. Within the different pre-treatments, it is possible to separate, on the one hand, those carried out with acids, bases or organic solvents and, on the other hand, the more recent ones that are carried out with ionic liquids (ILs) or deep eutectic solvents (DES). Moreover, it has been confirmed that the use of co-solvents at low concentrations can promote the extractions. The most commonly used co-solvent is carbon dioxide as it reaches its supercritical state at 7.38 MPa and 31 °C.⁶³  In addition to being easy to operate with, carbon dioxide is inert in nature and is non-toxic and non-inflammable, making it a potential green solvent. Apart from the advantage derived from the use of green solvents, this pre-treatment also reduces the amount of generated waste in the process, as well as decreasing reaction times and energy consumption.^74,105 

Acid pre-treatments are carried out by mixing an acid solution with the biomass to be treated, at a given temperature for a controlled time. This treatment always requires a subsequent filtering stage to separate the solid and liquid fractions. Acid pre-treatments lead to hydrolysis of the biomass allowing the separation of the desired compounds. For this purpose, acid solutions are usually used at low concentrations, between 0.2 and 2.5 w/w%, mixing at temperatures between 130 and 210 °C.⁶⁸  The most used acids are sulphuric acid and nitric acid, although in recent years organic acids are also being used. These types of pre-treatments, although yielding good results for the separation of compounds, have the disadvantage of generating toxic products, as well as corrosion in the equipment.¹⁰⁵  This technique is widely used as a delignification method for lignocellulosic biomass.

In a recent work conducted by Morais et al. (2020), the use of a phosphoric acid solution to perform acid hydrolysis of sugarcane biomass was investigated.¹⁰⁹  The results showed an improvement in the saccharification of the biomass using a 4.95% acid solution at 80 °C for more than 6 h. Although a long period of time was required, the use of the pre-treatment resulted in a hydrolysate with 98% of glucose. Previously, Chiranjeevi et al. (2018) developed an acid pre-treatment of rice straw in order to facilitate the delignification of this lignocellulosic biomass.¹¹⁰  In this work, where they combined the use of sulphuric acid and boric acid, at 150 °C for 20 min, they achieved the total elimination of the hemicellulosic fraction, in addition to removing 44% of the lignin. This process, besides saving time and energy, facilitated the subsequent bioethanol production stage.

Alkaline pre-treatment, as in the case of acid treatment, consists of mixing the biomass with an alkaline solution at a given temperature for a given period. The most commonly used solutions are those prepared with potassium hydroxide, sodium hydroxide, calcium hydroxide, and ammonium salt, among others.⁶⁵  The process causes the biomass to expand, which affects the physical properties of the biomass, including porosity, crystallinity and the surface area.¹⁰⁵  Thus, it facilitates the separation of the biomass components, leading to their extraction. Although this pre-treatment results in good yields, it has the disadvantage of requiring long reaction times and a subsequent filtering and neutralisation stage, which increases both the economic and energetic cost of the process.⁶⁸  This process is being widely used, mainly for the delignification of lignocellulosic biomass.

In a recent study conducted by Morales et al. (2020) to choose the best biorefinery sequence to obtain the highest yield from almond shells, alkaline hydrolysis with NaOH was selected to perform the delignification.¹¹¹  Using this technique, they were able to remove 40% of hemicelluloses and up to 35% of lignin from the biomass that had been previously submitted to an autohydrolysis step. Moreover, this technique made subsequent stages of the proposed biorefinery easier. Cavali et al. (2020) used alkaline hydrolysis (NaOH) for the recovery of sawdust, so that with a 170 °C treatment almost all the lignin in the biomass was removed.¹¹²  This work confirmed that the sequential use of acid and alkaline treatments promotes the total elimination of hemicelluloses and lignin from the lignocellulosic biomass, achieving the removal of 87% of hemicelluloses and 94% of lignin.

In organosolv pre-treatments, different organic solvents (e.g., ethanol, acetone, methanol, etc.) are employed, usually in combination with water, for biomass defragmentation. This procedure sometimes additionally requires the use of different catalysts (e.g., oxalic acid, salicylic acid, etc.) to increase the yield and purity of the separated compounds.¹¹³  Organosolv pre-treatments are usually carried out at temperatures above 150 °C, with a duration of 1–3 h.¹¹⁴  Once the temperature treatment has been completed, a subsequent filtration process is required for phase separation. This pre-treatment is mainly used with lignocellulosic biomass in the delignification process, achieves a high yield and high purity of the separated compounds, and facilitates the subsequent processes.¹¹³  This approach, in addition to high efficiency, has other advantages such as the use of mild-operation conditions, the ease to recycle solvents and the simplicity of the processes.¹⁰⁵ 

The recent work published by Morales et al. (2022) validates organosolv pre-treatment with an ethanol/water mixture as a very good technique for the delignification of walnut shells.¹¹⁵  Here, up to 94% of the lignin was removed by the consecutive application of three organosolv delignifications, with the first one having the highest yield, removing up to 60% of the lignin. In another study performed for the revalorisation of olive pomace, they used an ethanol/water solution (50%) with 1% H₂SO₄ as a catalyst.¹¹⁶  The use of a catalyst led to a pre-treatment at lower temperatures, 140 °C being the best temperature in this case, where a 53% delignification was achieved and high-purity lignin was produced. Rivas et al. (2021) used a 1-butanol/water mixture for the delignification of vine shoot.¹¹⁷  By intensifying the organosolv pre-treatment using microwaves in addition to a catalyst, solubilisation of both part hemicelluloses and lignin (almost 68%) was achieved in a single step. The optimum reaction conditions of the microwave-assisted organosolv pre-treatment were 2% catalyst, 52% 1-butanol and 190 °C. Other investigations have been performed with different alcohols with good results, such as pentanol, propanol, butanol, and glycerol.^118,119 

One of the novelties of the last decades is the use of ionic liquids (ILs) for biomass fractionation. ILs consist of organic cations combined with organic or inorganic anions in a wide range of possible combinations. This new class of non-molecular solvents has unique properties that convert them into potential green solvents for biomass fractionation. Their most important properties include negligible vapour pressure, low combustibility, good thermal and chemical stability and adaptable solubility, among others.^120,121  In general, ionic liquids tend to have a high viscosity, so it has been confirmed that the use of ILs in combination with a not very high percentage of water not only improves viscosity, but also makes the pre-treatment more effective.¹¹³  Pre-treatments with ILs consist of mixing the solvent (IL) with the biomass to be treated at a given temperature for a defined period. In this case, the working temperatures, due to the properties of the ILs, range between 90 and 130 °C at ambient pressure.⁶⁸  The advantages of this pre-treatment are good efficiency, the use of mild operation conditions, easy recovery and recycling of the solvent and energy savings, among others.¹⁰⁵  This chemical pre-treatment has been investigated not only for delignification processes, but also for the separation of other compounds from the biomass, such as polyphenolic compounds, polysaccharides, etc.

Sillero et al. (2021) investigated the use of ionic liquids in combination with water for the extraction of flavonoid compounds from pine bark.¹²²  The use of the ionic liquid (1-butyl-3-methylimidazolium bromide) gave an extraction yield of more than 15% with a total flavonoid content of 779 mg CE g⁻¹ DBE, but this pre-treatment also solubilised part of lignin and hemicelluloses. Li et al. (2020) used 1-dodecyl-3-methylimidazolium chloride IL for protein extraction from tobacco leaves.¹²³  This pre-treatment improved protein extraction by almost 2-fold compared to a conventional technique. In a work where the use of ILs is combined with UAE, it was demonstrated that acetate-based ILs were able to dissolve bamboo biomass in only 40 minutes.¹²⁴  This reduces energy and solvent costs, since the IL was found to be recyclable. Recently, Rauf et al. (2022) studied the use of IL triethylammonium hydrogen sulphate for the pre-treatment of corn straw, achieving removal of 80% of the lignin from the initial biomass at 78 °C only.¹²⁵  Many ILs can be used for delignification, and proof of this is the work done by Neubert et al. (2020), where they examined the use of eight different ILs for the valorisation of sugar cane bagasse.¹²⁶  The delignification yields range from 23% to 86%, when 1-butyl-3-methylimidazolium H₂SO₄ was used. Another promising IL is HTEA H₂SO₄, with an efficiency close to 80%, thereby extending the IL possibilities.

Deep eutectic solvents (DES) are formed by the complexation of a hydrogen acceptor (HBA) and a hydrogen donor (HBD), usually formed by non-ionic species. DES are formed by mixing two or more non-toxic compounds under mild-conditions for a certain period of time (minutes or hours)¹²⁷  forming a eutectic mixture that allows these solvents to be liquid at temperatures below 70 °C. The compounds that form DES can be natural compounds, such as choline derivatives, organic acids and amino acids, among others, forming the so-called natural DES (NADES). Similar to ILs, DES have good chemical stability, adjustable solubility and negligible vapour pressure,⁹¹  as well as being biodegradable and recyclable.¹¹³  Pre-treatments with DES are carried out by mixing them with the biomass to be treated at fairly low temperatures, between 80 and 150 °C, at room pressure.⁹¹  Due to the high viscosity of many DES, the use of co-solvents, especially water, has been confirmed in order to improve the efficiency of the extraction processes.¹²⁸  One of the most important advantages of DES pre-treatment is based on the inherent characteristics of DES (non-toxic and biodegradable products), which makes this technology more environmentally friendly.¹¹⁴ 

A recent study on the valorisation of pine bark using green solvents concluded that the use of choline chloride-based DES (ChCl:1,4-butanediol) in combination with a low percentage of water (25%) promoted the extraction of flavonoid compounds.¹²⁹  Recently, Elgaharbawy et al. (2019) optimised the pectin extraction process from grapefruit peel using NADES in combination with UAE.¹³⁰  The pre-treatment carried out at 80 °C for 60 min of sonication was efficient for pectin extraction, especially by using ChCl:malonic acid and ChCl:glucose–water as solvents, extracting up to 94% of the pectin.

DESs are also being widely investigated as green solvents for use in delignification processes. Kohli et al. (2020) demonstrated that DESs are effective for the delignification of different lignocellulosic biomass, extracting more than 85% of the starting lignin.¹³¹  In this case, the DES providing the best results were ChCl:oxalic acid and ChCl:formic acid. Recently, the delignification process based on DES assisted with phosphotungstic acid catalysts has also been evaluated.¹³²  This pre-treatment resulted in the extraction of 86% of the lignin and 90% of the hemicelluloses at medium temperatures (100–150 °C), for periods of 3 h.

In addition to the pre-treatments mentioned here, other less-studied treatments could also be interesting from a biorefinery point of view. Examples of this are ozonolysis, high-pressure homogenisation, different biological treatments, hydrodynamic cavitation, plasma treatment, and gamma irradiation, among others.⁶⁴ 

The biorefinery approach processes and supply chains are multi-sectoral, starting with raw material procurement, transport, and industrial processes up to the reuse or disposal of the sub-streams generated in each process. Therefore, in order to understand the biorefinery products' sustainability, it is important to consider the impacts of each sector involved in their production.⁷  LCA methodology is a systemic evaluation of the environmental aspects of a product, process or activity during its life cycle stages, analysing its associated environmental impacts.¹³³  With regard to non-conventional techniques, it is interesting to know which ones are the most recommendable from an environmental point of view, as they are called “green” techniques. Nevertheless, the use of this term does not guarantee their sustainability, thus the environmental burdens arising from their operation should be considered.

The LCA methodology is driven by four phases: goals and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA) and interpretation of results.^133,134  An attributional LCA approach was considered since averaged data were used in all analysis scenarios.¹³⁵ 

Regarding the conventional and non-conventional techniques outlined above, it is interesting to know which are the most environmentally friendly. Therefore, this study aims to consider the different extraction sequences of phenolic compounds considering the LCA methodology from an environmental perspective. This methodology takes into account the entire life cycle of the extraction sequence (cradle-to-gate approach) and allows the identification of the main processes or key steps responsible for environmental burdens.

The objective of this study is to assess the environmental performance of various scenarios of valorisation routes to obtain phenolic compounds as a target product. The first step in performing an LCA is to define the functional unit, as it supplies the reference to which the inputs and outputs of the system under study are related.^133,134  The selection of the functional unit in a biorefinery study can be on the basis of the feedstock or the quantity of the target product.¹³⁶  In this case, due to the variety of biomass used as feedstock and that the main objective is the comparative environmental study between conventional and non-conventional extraction techniques, one kilogram of the target product (e.g., phenolic compounds) has been chosen as the functional unit. This functional unit would allow effective comparison of extraction methods in terms of their environmental performance.

In order to ensure that the input and output of the valorisation scenarios are as similar as possible, the impacts associated with the construction and installation of the biorefinery plant during its lifetime stayed out of the study. It has not been considered to include biomass storage or any chemical compounds within the system boundaries. These assumptions have been previously considered in other similar LCA studies.^13,137–139  The system boundaries of this environmental study include the process units of each of the studied scenarios (see Figure 1.6). The general composition of all scenarios is three subsystems: pretreatment, extraction and solvent recovery.

The use of non-conventional extraction processes is widely discussed as the “green” future. However, it is necessary to conduct an environmental impact assessment of the different processes before confirming this. Conventional methods are assumed to have high environmental impacts, mainly due to high energy consumption and the use of high amounts of solvents. Nevertheless, environmental impact studies need to be completed to confirm this and to be able to compare with more modern processes.

Therefore, in this study previously published different extraction processes have been selected to obtain phenolic compounds. In the work developed by Barjoveanu et al. (2020), three extraction methods are mainly described for obtaining phenolic compounds from spruce bark (Picea abies).¹⁴⁰  The authors use three extraction methods, Soxhlet extraction (SE-1), solid–liquid extraction with NaOH (NaOH-SLE) and ultrasound-assisted extraction (UAE). SE-1 was carried out at 78 °C for 6 h and 34 min. The solvent selected in this case was ethanol (70%), and the solid–liquid ratio (SLR) used was 1 : 20. The extraction with NaOH (1%) was conducted at 90 °C for only 1 h, with an SLR of 1 : 10. Finally, the UAE was performed using an ultrasonic bath (35 kHz and 320 W) at 50 °C for 1 h. The solvent used in this case was ethanol (70%), with an SLR of 1 : 10. NaOH extraction was the best in terms of yield, with 21.66 mg GAE g⁻¹ bark. Soxhlet extraction was the poorest in phenolic compounds (12.39 mg GAE g⁻¹ bark), while UAE yielded 19.1 mg GAE g⁻¹ bark.

In the study carried out by Solana et al. (2015), a comparative study of the use of three techniques for the extraction of polyphenolic compounds from asparagus stalks was carried out.¹⁴¹  The first extraction technique used was Soxhlet extraction (SE-2) at 100 °C, for 4 h. The solvent used was ethanol, according to the modification proposed by Santiago et al. (2021).¹³  This method was compared with the optimised pressurised liquid extraction (PLE). The reaction parameters were 65 °C, 10 MPa, 30 min and a 50% ethanol/water mixture, with a flow rate of 2 ml min⁻¹. Finally, supercritical fluid extraction (SFE) was tested, using an ethanol/water mixture (50%), but with 8% CO₂ as an additive. This reaction was conducted for 1 h, at 65 °C and 15 mPa, with a CO₂ flow rate of 0.25 kg h⁻¹. The highest concentration of phenolic compounds was obtained by Soxhlet extraction (4 mg g⁻¹); while looking at the antioxidant capacity, the best result was achieved with SFE (2.4 mg rutin equivalent g⁻¹).

The last non-conventional extraction process studied is that developed by Murugan et al. (2021).¹⁴²  In this work, a green solvent (choline chloride:ethylene glycol) was used for the extraction of phenolic compounds from the seeds of Dacryodes rostrata. To this end, the ground seeds were mixed with the solvent keeping the SLR 1 : 10 and heated at 40 °C, for 100 min. To enhance the extraction and reduce the viscosity of the solvent, 10% water was added to the solvent. The extraction achieved excellent results, 105 mg GAE g⁻¹ sample. The use of this type of solvent allows working at lower temperatures with good final results, which suggests that its environmental impact should be lower than that of conventional methods. However, this needs to be confirmed by calculating the environmental impact of the process.

The life cycle inventory stage involves collecting primary and secondary quantitative input and output data for the system under consideration for the environmental assessment. In some cases, data are obtained from publications, research or official databases. In this study, the inventory data, such as electrical demand for all equipment, chemical consumption, process water and thermal energy, have been obtained from environmental studies previously developed by Barjoveanu et al. (2020),¹⁴⁰  Santiago et al. (2021)¹³  and Murugan et al. (2021).¹⁴²  Table 1.4, which is a summary of the inventory tables of the works described above, is employed to build the life cycle inventory. The input–output structure of the inventory entries, as well as their associated impacts, were sourced from the Ecoinvent® database.¹⁴³  SimaPro v9.3.0.3 software was used for the computational implementation of the life cycle inventory data.¹⁴⁴  Due to the differences between the alternative valorisation scenarios (e.g., extraction technique, raw material, location and consequent energy and chemical production), two assumptions have been considered: (1) all processes are located in Europe, including background processes such as the production of chemicals and energy (electrical and thermal), as Saavedra et al. (2021) considered in their environmental study,¹⁴  and (2) the heat source is obtained from steam from the chemical industry, while the cooling energy consists of the recovery of the cooling utility in a CHP unit. This information is accessible in the Ecoinvent® database.¹⁴³  The main processes of the Ecoinvent® database considered in the comparative environmental study are listed in Table 1.5.

For the environmental analysis, the hierarchical midpoint method of ReCiPe Europe v1.13 (2016) was used to perform the environmental profiling.¹⁴⁵  The research focused only on the global warming (GW) category, without including other categories such as eutrophication or land use, as would be contemplated in a holistic LCA. Thus, the characterisation factors reported by this method were considered to estimate the environmental loads in terms of a single impact category: global warming (GW).

In order to assess the sustainability of different extraction processes employed for the extraction of phenolic compounds from different biomass, in this work the GW of previously published processes has been calculated (see Figure 1.7). This figure provides a straightforward interpretation of the GW burden of each of the processes. The conventional extraction methods that have been studied are shown in yellow. This group includes two Soxhlet extractions (SE-1 and SE-2) and one extraction with NaOH. This last process is the one with the highest impact on the GW category, with a total of 45.5 tonnes of CO₂ equivalent per kg of product. According to the LCA calculations performed by the authors of the research, the impact of this process is entirely due to the energy production demanded in the extraction stage, as temperatures of up to 90 °C must be reached, and with the processes involved in the solvent recovery stage.¹⁴⁰ 

Proceeding with the results of the conventional methods, it could be noted that there is a considerable difference between the impacts associated with the two Soxhlet extractions. Furthermore, compared to the NaOH-SLE scenario, the GW environmental burden of the SE-1 and SE-2 scenarios decreases by 43% and 79% respectively. As in the previous case, energy consumption has the greatest impact on the GW of the Soxhlet extraction processes, with an impact of more than 90%.¹⁴⁰  The extraction process described for the extraction of polyphenolic compounds from spruce bark (in Figure 1.7 presented as SE-1) was found to have a higher impact than the extraction process described for asparagus stems, being 63% higher. Despite the difference in the temperature at which each of the processes is performed, with 78 °C in SE-1 and 100 °C in SE-2, the main reason for this large difference is due to the scale at which the environmental analysis is performed, i.e., SE-1 was studied at the laboratory scale while SE-2 was performed at the pilot scale. When data are provided at the laboratory scale, it results in a higher impact compared to data at larger scales.¹⁴⁶ 

Figure 1.7 illustrates in green the results of the GW impact of the non-conventional extraction processes investigated in this study. It is evident at first view that both UAE and PLE are technologies with lower environmental loads. The UAE conducted at 60 °C for only 1 h, in addition to achieving a good extraction yield, as mentioned before, is also a process with a low environmental impact (7.5 tonnes of CO₂ eq. per kg of product). It was found to have a burden of less than 83% of that generated in the extraction with NaOH of spruce bark and up to 71% less than the Soxhlet extraction (SE-1) of the same residue. For the GW impact, electricity production is once again the protagonist, in particular due to the processes involved in solvent recovery. The ethanol recovery process requires energy, but water recovery is more energy demanding. On the other hand, PLE (4.0 tonnes of CO₂ eq. per kg of product), although it has a lower impact than UAE, also improves the results achieved by conventional methods. This decrease in the environmental load is due to the lower energy required for the process compared to the rest, in addition to the lower solvent demand required in the extraction stage.¹³ 

The SFE scenario has an environmental burden similar to the UAE scenario (7.1 tonnes of CO₂ eq. per kg of product). The main cause of this environmental burden is the production of electrical and thermal energy demand for the process of converting CO₂ into a supercritical fluid.¹³ 

Finally, the DES scenario has been found to have a higher environmental burden than Soxhlet extraction (SE-2), being approximately 48% higher. However, as it is a non-conventional extraction technique, a lower environmental impact was expected in this scenario. Since this extraction process was carried out at a low temperature (40 °C), the environmental load was expected to be lower. However, in this case, apart from energy consumption, chemical reagents also appear as the main problems. Therefore, the GW result rises to 18.2 tonnes kg CO₂ eq. per kg of product. The calculation of the GW in this case was based on existing processes in the Ecoinvent® database, which limited some of the calculations. The chemicals were blamed for the unexpected increase in the environmental load of this process. This may be due to the fact that since the DES production process does not exist in the database, and the data required to create a process were not available, it was decided to assume a process of a conventional organic chemical. However, choline chloride:ethylene glycol is a compound obtained by mixing pure compounds of choline chloride and ethylene glycol. The eutectic mixing of these two compounds, unlike the processes required for the synthesis of many organic compounds, is carried out at temperatures not exceeding 80 °C for 1–2 h.¹²⁹  Therefore, the loads involved in the production process of the compound are assumed to be lower (this needs to be confirmed), and the properties of organic conventional compounds are different from those of DES, which means that the impact will also be different. One of the most important properties of these compounds, as already mentioned, is that they are not volatile, with the benefits that the use of these compounds in diverse processes can have for the reduction of pollutant emissions. In conclusion, it is also necessary to carry out an environmental analysis of the DES preparation processes, so that the results of the extraction processes with this type of compounds fit more adequately with the reality.

According to the FAO statistics, around 33% of the food produced globally is lost or wasted along the supply chain. The world’s population is increasing, so is food consumption as well as waste generation due to its production, so it is necessary to develop techniques to minimise, eliminate and treat this food waste. SDGs were created, among other things, as an opportunity to implement recycling and reuse strategies in the production and supply chains to reduce food loss. Therefore, in addition to the interest in reducing the amount of food waste, the biorefinery concept aims to convert biomass into high added-value products, which can even be commercially exploitable such as biofuels, biofertilisers or proteins, among others. Depending on the desired final product, as well as the quality, quantity and composition of the biomass, several parameters determine the choice of the production process in a biorefinery. So far, maceration, pressing, hydrodistillation or Soxhlet extraction, known as traditional or conventional techniques, have been the most common techniques for obtaining high added-value products. However, to overcome the limitations of conventional methods for the extraction of these high added-value compounds, advanced techniques and strategies are being continuously developed. The advantages of these techniques, also known as “green” techniques, are the production of extracts with high quality and yield, low solvent and energy consumption and short extraction time. Recent research has shown that “green” extraction methods offer excellent alternatives to traditional methods. Many studies are still ongoing in this field, to further improve these new green extraction techniques, to reduce the extraction cost and extraction time and to improve the extract quality, environmental safety and health. However, the wealth of information available on the extraction of high added-value compounds from different types of biomass makes it difficult to draw general conclusions. Fortunately, the understanding of the process and how variables can be used to control is rapidly increasing, leading to innovative approaches to maximise yields and reduce degradation while minimising the environmental impact.

In addition to the study of the different techniques available for the extraction of high added-value compounds, this study has also focused on the comparative environmental analysis of different biomass valorisation scenarios, based on the LCA methodology, to obtain phenolic compounds as the main high added-value product, under a biorefinery approach. The global warming impact caused by the use of different techniques, both traditional and non-conventional, in the production of one kilogram of phenolic compound has been calculated. The overall results show that the PLE (“green” technique) has the lowest environmental burdens, while the extraction using soda (conventional technique) has the highest value, with a difference between them of 91%. When comparing the two scenarios focusing on the Soxhlet extraction technique, a difference of 67% could be seen in the climate change category. The main reason for this is the difference in the scaling of the environmental analysis. When comparing the non-conventional techniques with each other, a similarity between the UAE and SFE scenarios could be seen, reporting an approximate value of 7 tonnes of CO₂ eq. with respect to the GW category. The DES scenario reports the highest value in the GW category, being approximately 60–80% higher depending on the scenario with which it is compared. Furthermore, this scenario reports a higher environmental load value (48%) when compared to the conventional Soxhlet technique, namely the one using asparagus residue as the raw material. This is because the DES production process does not exist in the database used and traditional organic chemistry processes have to be used. The LCA methodology makes it possible to establish a comparative framework that can represent a useful tool for identifying the advantages and disadvantages of each selected technique, as well as the possibility to determine the operating parameters on which to act in the short and medium term. However, it is important to carry out the comparative environmental analysis on the same scale, trying to avoid the lab-scale, as there will be a noticeable difference between the environmental impacts for the different scenarios analysed. For this reason, plant and larger scale studies to ensure the environmental viability of the processes are needed. Considering the environmental results, it could be confirmed for the researched studies that the non-conventional extraction methods have lower environmental burdens, except for the DES scenario, as well as better extraction yields. This confirms the promising future of these methods. Finally, the production of energy demand is the main hotspot in many processes. Therefore, special attention should be paid when using renewable energy sources (instead of taking it directly from the national grid) to satisfy and optimise the energy demand, as well as when reusing some of the internal steams to reduce the environmental impact of all scenarios. In addition, once the environmental analysis of these techniques has been carried out, it is suggested that a cost analysis be carried out as a future project to determine their economic viability.

DES

Deep eutectic solvents

FAO

Food and Agriculture Organization of the United Nations

HBA

Hydrogen acceptor

HBD

Hydrogen donor

IL

Ionic liquids

LCA

Life cycle assessment

LCI

Life cycle inventory

LCIA

Life cycle impact assessment

MAE

Microwave-assisted extraction

PEF

Pulse electrical field

SDG

Sustainable development goal

SFE

Supercritical fluid extraction

SLR

Solid–liquid ratio

SubFE

Subcritical fluid extraction

PLE

Pressurised liquid extraction

UAE

Ultrasound-assisted extraction

UN

United Nations

VFA

Volatile fatty acid

VS

Volatile solid

This research has been partially supported by the SENSE project granted by the FEDER/Spanish Ministry of Science, Innovation and Universities, Spanish National Research Agency (CTQ2016-75136-P) and by the project Enhancing Diversity in Mediterranean Cereal Farming Systems (CerealMed) funded by the PRIMA Programme and FEDER/Ministry of Science and Innovation – Spanish National Research Agency (PCI2020-111978). We also like to thank the Spanish Ministry of Science, Innovation and Universities for the project RED2018-102623-T. B. Santiago would like to thank the Spanish Ministry of Science, Innovation and Universities for the financial support (grant reference BES-2017-081715). Dr Sillero would like to thank the Spanish Ministry of Universities for the Margarita Salas grant for the re-qualification of the Spanish University System funded by the European Union-Next Generation EU. The authors belong to the Galician Competitive Research Group GRC 2013-032 and to CRETUS Strategic Partnership (AGRUP2015/02). All these programmes are co-funded by Xunta de Galicia and FEDER (EU).