- 1.1 Introduction to the Biorefinery Concept
- 1.1.1 Integration with Existing Industrial Value Chains or Development of New Value Chains
- 1.1.2 Biorefinery Scale
- 1.1.3 Biomass Supply: Harbour (Import of Biomass or Intermediates) or Rural (Locally Produced Biomass)
- 1.1.4 Biorefinery Concepts in 2030
- 1.2 Extractable High-Value Chemicals in Food and Byproducts
- 1.2.1 Cereal Crops
- 1.2.2 Oat Extracts
- 1.2.3 Oilseed Extracts
- 1.2.4 Root Crops
- 1.2.5 Wood and Bark
- 1.2.6 Herbaceous
- 1.2.7 Summary
- 1.3 Transformation of Glycerol into High-Quality Products through Green Chemistry and Biotechnology
- 1.3.1 Aqueous Phase Reforming
- 1.3.2 Selective Reductions
- 1.3.3 Halogenations
- 1.3.4 Dehydrations
- 1.3.5 Etherifications
- 1.3.6 Esterifications
- 1.3.7 Selective Oxidations
- 1.3.8 Pyrolysis
- 1.3.9 Biotransformations
CHAPTER 1: Green Chemistry and the Biorefinery
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Published:09 Oct 2013
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Special Collection: 2013 ebook collection , ECCC Environmental eBooks 1968-2022 , 2011-2015 food science subject collectionSeries: Green Chemistry
A. Kazmi, in The Economic Utilisation of Food Co-Products, ed. A. Kazmi and P. Shuttleworth, The Royal Society of Chemistry, 2013, pp. 1-24.
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The concept of Green Chemistry is now mainstream with industry and society taking the lead. Recently, on UK national radio the issue of waste cooking oil from domestic households and restaurants that ends up blocking drains was being discussed. The waste congeals and forms a “Fatberg”, and unsurprisingly it costs Thames Water, £1 million per month to remove the 100,000 blockages that arise each year. Instead of just accepting that this materials destiny is landfill, a commercial energy company has collaborated with the water agency to burn this fat and generate renewable energy. The plant will generate enough energy to support nearly 40,000 homes, although the company plans to use the electricity to run internal processes and the surplus amount will be distributed in the national energy grid. This example shows how the concept of “waste” is disappearing and being replaced by the concept of “resource”. We know that waste cooking oil is being used to manufacture biodiesel by leading retail outlets such as McDonald’s to fuel the companies transport fleet. These types of closed-loop approaches not only make sense from an environmental perspective but also commercially. Let us not forget that when the price of crude oil spiked to $150 it caused global chaos and according to some economists, it was the trigger of the deepest recession we have seen for nearly a century. Alternative supplies of fuel and energy may provide a buffer during high crude oil prices, and the same applies to alternative chemicals and materials. Society needs to implement a biorefinery approach where renewable carbon is used to provide the necessities of life; food, chemicals, materials and energy.
1.1 Introduction to the Biorefinery Concept
The core component of all biorefinery definitions is the conversion of biomass into several products (materials, chemicals, energy, food and feed) and the integration of various technologies and processes in the most sustainable way. The definition developed by the International Energy Agency (IEA) Bioenergy Task 42 Biorefineries has been widely accepted due to its general and broad character:
“Biorefining is the sustainable processing of biomass into a spectrum of marketable products and energy”
This definition includes the following key words:
biorefinery: concepts, facilities, processes, cluster of industries;
sustainable: maximising economics, minimising environmental aspects, fossil-fuel replacement, socioeconomic aspects taken into account;
processing: upstream processing, transformation, fractionation, thermo-chemical and/or biochemical conversion, extraction, separation, downstream processing;
biomass: crops, organic residues, agroresidues, forest residues, wood, aquatic biomass;
spectrum: more than one;
marketable: a market (acceptable volumes and prices) already exists or is expected to be developed in the near future;
products: both intermediates and final products, i.e. food, feed, materials, and chemicals;
energy: fuels, power, heat.
Using biomass as a sustainable renewable resource is the only way to replace carbon from fossil sources for the production of the carbon-based products such as chemicals, materials and liquid fuels.
In order to be competitive with crude-oil-based products, an integrated biorefinery strategy has been developed to optimise the added value from biomass. This strategy is mainly based on the transfer of petroleum refineries logic to biomass (raw material fractionation, integration of mass and energy fluxes; integration of processes) in order to be able to produce a spectrum of products and therefore maximising the added value. The approach requires the valorisation of the whole biomass. In other words, a biorefinery concept is based on a zero-waste concept.
Moreover, the biorefinery concept goes beyond the petroleum refineries logic, as it includes the management of sustainability based on a cycle concept. This is obvious for renewable carbon at global scale. The cycle also concerns water and mineral nutrients at the local scale, especially nitrogen, phosphorus and potassium (NPK). Contrary to carbon, these elements have to be left on or reincorporated into the soil to avoid depletion, and thus the use of fossil-based fertilisers to compensate for that soil depletion. More generally, the biorefinery concept includes the management of all sustainability issues, including environmental, economic and societal factors.
According to the project Biorefinery Euroview,1 “Biorefineries could be described as integrated biobased industries using a variety of technologies to make products such as chemicals, biofuels, food and feed ingredients, biomaterials, fibres and heat and power, aiming at maximising the added value along the three pillars of sustainability (Environment, Economy and Society)”.
All biorefineries are biomass-based industries, whereas not all biomass processing plants are biorefineries. It is important to clarify the respective differences in the next section, in order to understand the focus of this biorefinery vision document.
In conventional biomass processing plants, biomass is directly transformed (1st conversion) into a single main product (usually already marketable). In a biorefinery, however, raw products are firstly converted into intermediate products (1st conversion), which are partly or entirely preproducts. These are further processed (2nd conversion) to several end-products or semifinished goods by additional conversion and conditioning steps, predominantly at the same location.
The additional conversion and conditioning steps are carried out to achieve a better valorisation of biomass by transforming the raw product(s) as completely as possible into various value-added end-products.
For a better differentiation of biorefineries, the following listing provides examples of biomass processing plants that are not considered to be biorefineries;2
Plants for biomass conversion that convert the feedstock into one quantitatively dominating, marketable product directly after the primary refining step. Examples are biodiesel plants (main product: biodiesel) or agricultural biogas plants (main product: bioenergy, namely power and heat).
Plants for biomass conversion that have no combined primary and secondary refining step at the same location. Examples are paper mills without connected pulp mills, separate fermentation plants or starch mills without connected conditioning processes.
Plants for biomass conversion, where the biomass compounds are not separated, but unmodified or only slightly modified biomass is used or processed. Examples are wood-processing saw mills, or plants producing natural fibre insulation.
A 2030 vision for biorefineries was developed during the FP7 Star-Colibri project that involved European Technology Platforms, Industry Leaders and world-leading academic centres such as the Green Chemistry Centre of Excellence (University of York). Although the work was done in great detail, summaries can be found of this and the 2020 research road map on the project website (www.star-colibri.eu/).
1.1.1 Integration with Existing Industrial Value Chains or Development of New Value Chains
In 2030 many biorefineries will operate at a large-scale commercial level. Most of these biorefineries will be developed based on the integration with existing industrial value chains (top-down approach).
Different biorefineries will be developed based on industrial specificities (sector types) or on geographical specificities (biomass type, quality and availability, infrastructure, presence of a certain industry, etc.). The choice of the technological options (processes, feedstocks, location and scale) within the biorefinery will be made by the industrial actors, based on their competitive advantages (available industrial equipment, technological and industrial know-how, access to biomass). Biorefinery development will be driven by the industrial leaders from sectors such as agroindustry, forest-based industry, energy sector (power and heat), (bio)fuels industry and chemicals.
However, another interesting development path for biorefineries is envisaged on the development of new industrial value chains (bottom-up approach). This refers to newly developed, highly integrated, zero waste sites to produce a broad variety of products for different markets from different, pretreated and preseparated (lignocellulosic) biomass fractions. Usually, the whole biomass crop is used (e.g. woody lignocellulose, grains and straw from cereals, green grass). In 2011 this approach is often still only operating at pilot or demonstration stage (e.g. a lignocellulosic biorefinery in Leuna, Germany). However, a lot of research and development will lead to implementation on a commercial scale in 2030. Preferably these biorefinery plants for new industrial value chains should be integrated in an already existing industrial park to profit from the infrastructure.
In any case, the sustainability and the competitiveness of the different value chains will always rely on a close collaboration within industry sectors and also on a high level of integration between the different production processes.
1.1.2 Biorefinery Scale
The choice of the optimal biorefinery scale has to accommodate the constraints that arise from logistics, production costs and processes. The chosen scale will have a major impact on the emergence of industrial biorefineries and their distribution;
Large-scale integrated biorefineries, mainly based on thermochemical process, are likely to emerge in Northern Europe and/or in industrial harbours.
Small/medium-scale integrated biorefineries, mainly based on biotech processes, are likely to emerge in rural areas in “mid” Europe (western, central and eastern Europe).
Decentralised biorefineries will also emerge in both regions, based on the development of a network of pretreatment units.
As a consequence, the scale has a major impact on the technology choice and on the industrial strategies as it could limit the size of the production facilities (limited biomass quantity per industrial unit). Basically, three possibilities are offered:
small/medium-sized production facility;
medium-sized/large production facility linked to a network of decentralised biorefineries (biomass fractioning and/or concentrating units);
very large production facility, located on industrial harbours with importation of biomass.
1.1.3 Biomass Supply: Harbour (Import of Biomass or Intermediates) or Rural (Locally Produced Biomass)
As a consequence of the biomass supply form, there will be not one but several biorefinery types in Europe, with a predominance of certain types according to the geographical biomass location.
The biorefineries based on wood (locally produced biomass) are likely to be developed in Northern Europe or in dense forested area in “mid-Europe”.
The biorefineries based on classical agricultural crops (cereal, sugarbeets, oilseed crops) are likely to be developed in “mid-Europe”.
Biorefineries based on imported biomass will be established mainly in or very near to large harbours (like Rotterdam).
The development of biorefineries in South Europe is more difficult to predict. It could be either connected to the area of industrial harbours or to (new) regional crops.
The BIOPOL (2009) project gave some predictions about the most likely regions for biorefinery development in connection to the biomass availability. The main conclusions were: “Western Europe has the best prospects for biorefinery development. It has: high agricultural yields, vast amounts of lignocellulosic agricultural side streams, considerable forestry and good possibilities to sell biorefinery side products. The countries in the East of Europe have good opportunities to improve agricultural yields. Thus, they could become interesting countries for biorefinery establishment. Northern Europe is currently a natural market leader of lignocellulosic biorefinery due to the presence of large forests.”
1.1.4 Biorefinery Concepts in 2030
Some more traditional biorefinery concepts were already established on an industrial scale in 2011. They are based on an extension and/or on upgrading processes of existing industrial plants in the respective sectors. However, there will emerge other, newly developed biorefinery concepts that will be well established in 2030. In 2011, these biorefinery concepts were still only in the research, development or demonstration stage. For some of the following future biorefinery concepts the first pilot plants are being built in Europe in 2011.
1.1.4.1 Starch and Sugar Biorefineries
Starch and sugar agroindustries have a long experience in starch fractioning and/or fermentation and distillation. They are therefore a perfect candidate to integrate biotech processes for first- and second-generation bioethanol and, in a second step, other fermentation products. Starting from production based on starch and sugar crops, the industrial units will progressively use lignocellulosic feedstocks and integrate fractionation processes. The first steps will be the integration into the supply chain of cereal straw and, in a second step, of dedicated lignocellulosic (mainly herbaceous) crops.
The integration scenario does not concern only biomass diversification but also the valorisation of side products of the lignocellulose deconstruction: lignin, C5-sugars from hemicellulose and C6-sugars mainly from cellulose. The utilisation of lignin as an energy source by cogeneration will be progressively replaced by the development of new chemistry based on lignin. The ethanol production from C5-sugars, on account of the poor conversion yield, will be replaced by the development of a new C5-chemistry (by biotech and/or chemical processes) to produce higher-value chemicals (2020–2025 horizon). The ethanol production from C6-sugars will be progressively replaced by new fermentation processes and the production of higher-value chemicals. This development is likely to occur several years after the C5 switch (2025–2030) as the bioethanol European market will still be growing until this period.
The required biomass quantities per biorefinery are in the range of 200 to 400 kt/year of dry biomass, which enables reliance on a local production area, and the integration of the management of sustainability parameters in the production chain (carbon sequestration, nitrogen and other mineral nutrients cycles).
This will lead to the development of small/medium-scale rural biorefineries close to the agricultural production areas producing the required biomass. These rural ‘starch and sugar’ biorefineries will be implanted in the most efficient production and supply areas. Ideal localisation will be “mid-Europe” (from West to East Europe).
1.1.4.2 Oilseed Biorefineries
The oilseed agroindustry will develop different strategies. Today, it focuses on first-generation biodiesel and the development of the oilseed-based biorefineries could involve the integration of the glycerol valorisation and the development of a glycerol-based chemistry. However, the main evolution of these biorefineries will be based on the development of a new oleochemistry, based on long-chain fatty acids from European oilseeds (mainly rapeseed and sunflower) and the progressive integration of oleochemical processes into the biodiesel production chain. Moreover, this shift will be supported by the evolution of biofuels production in Europe and the relative decrease of first-generation biodiesel on account of its low energy efficiency per surface unit.
1.1.4.3 Forest-Based (Pulp and Paper) Biorefineries
The forest-based (pulp and paper) industry is located close to the main forest areas in Europe (mainly in Northern Europe). The industry has a long experience of woody and lignocellulosic biomass logistic. Wood and pulp byproducts (such as bark) are relatively dry biomass and are therefore well suited for new thermochemical conversion processes such as gasification. The industry is therefore a good candidate to integrate advanced second generation biofuels production and/or chemicals production from syngas (e.g. DME). The industry has also access to a huge amount of lignin, which is currently mainly used for combustion to produce bioenergy. Higher added-value chemicals will be obtained by integrating a chemical valorisation within forest-based biorefineries, particularly focusing on black liquor.
1.1.4.4 Biofuel-Driven Biorefineries
In 2011, there is no industry in Europe yet that has biorefineries using gasification in combination with the Fischer–Tropsch process to produce liquid second-generation biofuels. Since huge investments are needed to set up a large-scale industrial unit, the best candidates will be either energy companies or traditional oil companies, because of the infrastructure availability and the economy of scale. In 2030 oil companies will have installed large-scale biorefineries based on thermochemical process into their existing oil refineries located in the main European harbours. The required biomass (wood, forest residues, dedicated lignocellulosic crops or urban wastes) will be imported and also collected locally. Another interest of the traditional oil companies will be the utilisation of hydrogen from syngas as a source for hydrogenation of heavy crude oil. The gasification units will also be used to produce higher-value chemicals by catalysis processes.
1.1.4.5 Green Biorefinery
A Green Biorefinery processes wet biomass, such as grass, clover, lucerne and alfalfa (BIOPOL, 2009). The wet biomass is pressed to obtain two separate products: fibre-rich press juice and nutrient-rich press cake. The press cake fibres can be utilised as green feed pellets or as a raw material for chemicals. The press juice contains valuable compounds, such as proteins, free amino acids, organic acids, minerals, hormones and enzymes. Lactic acid and its derivatives as well as ethanol, proteins and amino acids are the most favourable end-products from press juice.3 The bio-organic residues in press juice are mainly used to produce biogas with subsequent generation of heat and electricity.
An example in 2011 of a pilot of this future biorefinery concept is the production and demonstration plant of the Biowert GmbH in Brensbach, Germany, where insulating material, reinforced composites for production of plastics and biogas for heat and power are generated from grass in an integrative process. In 2030 many of these smaller-scale Green Biorefineries will be established in regions that traditionally produce high quantities of wet biomass (like grassland areas).
1.1.4.6 Future Lignocellulosic Biorefineries
The lignocellulosic biorefinery concept based on dry biomass is not only applicable for previously described pulping process. Two different approaches can be distinguished in 2030 for the lignocellulosic biorefinery: thermochemical and biochemical.
The thermochemical approach is based on gasification of lignocellulosic feedstocks, and further processing the syngas to transportation fuels and chemicals. Many different biomass types are taken into consideration as raw material for this type of lignocellulosic biorefinery concept: dry agricultural residues (e.g. straw, peelings, husks), wood, woody biomass, and biogenic residues (e.g. waste paper, lignin).
Another option is the biochemical approach, that is based on the biochemical fractionation of the lignocellulosic raw material (cellulosic agricultural residues, forest residues or dry biogenic waste materials) into three separate product precursors: cellulose, hemicellulose and lignin. This is done during the primary refining step. These chemical fractions are then treated separately, and converted into value-added products during the secondary refining step. Cellulose can be hydrolysed into sugars and then used as fermentation substrate to produce alcohols (e.g. ethanol), organic acids and solvents. The second-fraction hemicellulose can be converted to xylose, gelling agents, barriers, furfural and further to nylon. Finally, lignin can be applied as a binder and adhesive or can be used for the production of fuels, carbon fibres for materials or syngas, which can be used for energetic or industrial purposes.
The thermochemical (gasification) approach will be mainly based on woody biomass (forest biomass and residues or dry biogenic waste materials), while agricultural residues (e.g. cereal straw) and dedicated lignocellulosic herbaceous crops will be more likely used in the biochemical fractionation and hydrolysis processes. We assume that the technical problems in the biochemical approach to overcome the “biomass recalcitrance” will be solved in the period 2011–2020, and that both approaches will lead to commercially viable lignocellulosic biorefineries in 2030.
1.1.4.7 Aquatic (Marine) Biorefinery
Aquatic (marine) biomass (microalgae and seaweeds) is an interesting new feedstock for aquatic biorefinery, characterised by high productivity per area unit, and a high content of valuable components for the biobased economy, including: oils, proteins, polysaccharides and specific biomolecules. Aquatic biomass cultivation and processing are regionally or locally integrated. Due to the varied composition, microalgae and seaweed biomass are highly suited for biorefinery with end-products ranging from fuels and bulk chemicals to specialty chemicals and food and feed ingredients.
Microalgae, like some other micro-organisms and plants, produce storage lipids in the form of triacyglycerols (TAGs) that can be used to synthesise fatty acid methyl esters (a substitute for fossil-derived diesel fuel). Microalgae represent a very attractive alternative compared to terrestrial oleaginous species, because their productivity is much higher, and they do not compete for land suitable for agricultural purposes, providing therefore food security. Attractive features of microalgae include: high areal productivity as compared to terrestrial crops, location of cultivation systems on marginal lands or other low-grade surfaces, the unique biomass composition including the ability to accumulate large amounts of oils, and the great variety in species and products. Furthermore, algal cultures lend themselves for combination with wastewater treatment, CO2 removal from flue gas, and useful applications for low-temperature waste heat.
Typical products from microalgae include:
oils for food applications and nutraceuticals such as omega fatty acids;
oils and derived fractions for nonfood applications, i.e. transport fuels, chemicals;
proteins and derived products (amino acids, N-chemicals);
other bulk chemicals (e.g. ingredients for coatings);
a range of high-value specialties.
In 2030 several commercial biorefinery concepts for integral microalgae-based production chains will exist for:
biofuels and coproducts;
feed for aquaculture, including recycling of nutrients;
production of food or food ingredients and coproducts.
The cultivation of seaweed can be an enormous source of biomass in the future. Use of land for cultivation of biomass for the production of fuels, products and fibres is subject to debate due to the competition with food production. Expansion of biomass cultivation to the sea would increase the potential amount of biomass being available for nonfood and nonfeed purposes, and thereby increasing the potential share of biomass in renewable energy supply in the future.
For aquatic biomass several biorefinery processes seem to fit. Depending on the type of feedstock (the seaweed species), and the possibilities of process integration, a processing route will be selected. Hydrothermal upgrading (HTU), anaerobic digestion to methane, and ethanol fermentation and distillation, are possible processes.
1.2 Extractable High-Value Chemicals in Food and Byproducts
Secondary metabolites are extracted from numerous plants and flowers and are used as pigments, health products, food ingredients and cosmetic applications. This section focuses on mainly European crops. The biomass has been divided into seven distinct groups that are cereal, oilseed, root, wood, herbaceous and marine biomass.
The bulk of secondary metabolites are used in the food, cosmetics and health industries and good sources of information are available on the internet. For example, hundreds of secondary metabolites are used in the cosmetics industry and in the food and health industry.
1.2.1 Cereal Crops
Human society has depended on cereal crops for millennia and today there are thousands of products based on wheat, barley, oats and rice. Although much of the focus is on the grain for food purposes, there are very important metabolites that are not used in the growth of the plant, however when extracted, they have many uses in various applications. Applications exist in the cosmetics industry where extracts are used in products such as hair conditioner, shampoo, bleacher, styling gel/lotion, antiageing cream, facial moisturiser, hair spray, body wash, foundation and mascara.
Secondary metabolites mainly exist in the straws of the crops in the form of waxes. However these are only 1–3% by weight of the straw, therefore any components that are to be extracted must be of very high value for the process to be economical. For example, the composition of wheat straw wax contains several commodity chemicals, however, the likelihood of using this as a feedstock are low. A list of wheat straw extracts is shown in Table 1.1.
Product . | Market . | Volume . | Current Feedstock . |
---|---|---|---|
Cetearyl Alcohol | Personal Care | Large | Crude Oil and coconut oil |
Benzoic acid | Food preservatives, Chemical precursor | 139,000 tonnes + | Crude Oil |
Succinic acid | Chemical intermediate, Numerous applications | Large | Biomass via fermentation |
Fumaric acid | Medicine, Food, Chemicals | Large | Nonbiomass |
rn-Toluic acid | Insect repellent, PVC Stabiliser | Low | Nonbiomass |
Salicylic acid | Medicine, Cosmetics | High | Biomass |
Maleic acid | Cosmetics | Low | Nonbiomass |
p-Hydroxybenzoic acid | Cosmetics, Chemical intermediate | – | Nonbiomass |
Gentisic acid | Mass Spectrometry, Medicine | Low | Nonbiomass |
Vanillic acid | Flavouring agent, Chemical intermediate | – | Biomass |
p-Resorcylic acid | Cosmetics and fine chemicals | – | – |
Protocatechuic acid | Antioxidant | – | – |
Azelaic acid | Medicine | – | Biomass |
1,2,3,5-Tetrabromobenzene | – | – | – |
Dihydroferulic acid | – | – | – |
trans-p-Coumaric acid | Antioxidant | – | – |
Syringic acid | – | – | – |
cis-Ferulic acid | – | – | – |
trans-Ferulic acid | – | – | – |
1-Naphthoic acid | – | – | – |
Product . | Market . | Volume . | Current Feedstock . |
---|---|---|---|
Cetearyl Alcohol | Personal Care | Large | Crude Oil and coconut oil |
Benzoic acid | Food preservatives, Chemical precursor | 139,000 tonnes + | Crude Oil |
Succinic acid | Chemical intermediate, Numerous applications | Large | Biomass via fermentation |
Fumaric acid | Medicine, Food, Chemicals | Large | Nonbiomass |
rn-Toluic acid | Insect repellent, PVC Stabiliser | Low | Nonbiomass |
Salicylic acid | Medicine, Cosmetics | High | Biomass |
Maleic acid | Cosmetics | Low | Nonbiomass |
p-Hydroxybenzoic acid | Cosmetics, Chemical intermediate | – | Nonbiomass |
Gentisic acid | Mass Spectrometry, Medicine | Low | Nonbiomass |
Vanillic acid | Flavouring agent, Chemical intermediate | – | Biomass |
p-Resorcylic acid | Cosmetics and fine chemicals | – | – |
Protocatechuic acid | Antioxidant | – | – |
Azelaic acid | Medicine | – | Biomass |
1,2,3,5-Tetrabromobenzene | – | – | – |
Dihydroferulic acid | – | – | – |
trans-p-Coumaric acid | Antioxidant | – | – |
Syringic acid | – | – | – |
cis-Ferulic acid | – | – | – |
trans-Ferulic acid | – | – | – |
1-Naphthoic acid | – | – | – |
When a low-cost method of extracting such waxes is developed then the components could have significant commercial value. For example, the waxes can be used in the lucrative cosmetics industry for applications such as lipstick.4 The waxes also contain a series of chemicals that act as insect repellents and therefore could have commercial value in this field. However, the most valuable components of the waxes are likely to be the sterols and polycosanols that have been shown to act as cholesterol-reducing agents. Polycosanols are widely used in the world in the form of pills and are commercially extracted from sugar-cane waxes. Furthermore the wheat straw contains a relatively high amount polycosanols5 (164 mg/kg) which makes it an attractive component to extract. Sterols also exhibit similar health benefits and are currently extracted from vegetable oil and pinewood, which do not meet the ever increasing demand.6 Although polycosanols and sterols employ different mechanisms to reduce cholesterol levels, mixtures of both extracts could offer enhanced effectiveness and this would mean more of the wheat straw wax is used.
The use of sterols in food products has also been approved by the EU Novel Food program in 2004, where cholesterol reducing ingredients can be used in a wide variety of products such as margarine, milk, yoghurt, cheese, soymilk, dressings and rye bread.7 The sterols are now being used in one-shot drinks and special milks with the market still growing. Large companies such as Unilever have taken the lead, however, smaller companies are also entering this lucrative market. Although in Europe there are some restrictions for use in some foods, in the US there are no restrictions and sterols are generally regarded as safe. A report from Frost & Sullivan valued the European plant sterols market at $185 million and this is estimated to rise to $395 million in 2012, see Figure 1.1.
1.2.2 Oat Extracts
Sterols also exist in oats, however, oats also have a considerable amount of antioxidants such as tocols (vitamin E), phytic acid, phenolics and avenanthramides.8 These important components are concentrated on the outer layers of the kernel and although they are consumed as part of the grain, they can be extracted using solvents such as methanol or choloroform/propanol mixtures to be used in food supplements or blended with other food products.
The tocols found in oats are mainly in the form of α-tocotrienol and concentrations vary depending on genotype and location as shown in Table 1.2.
Region . | Concentration of tocol (mg/kg) . |
---|---|
USA | 19–30 |
Hungary | 15–48 |
UK | 19 |
Region . | Concentration of tocol (mg/kg) . |
---|---|
USA | 19–30 |
Hungary | 15–48 |
UK | 19 |
On average, barley contains twice as much tocols as oats therefore it is also a good source for such molecules. Phenolic acids are found in oats in the form of ferulic, vanillic, sinapic, ρ-coumaric and ρ-hydroxybenzoic acid. Clearly the concentrations of these acids vary depending on the species and the section of the plant that they are extracted from. Table 1.3 shows that high concentrations of ferulic acid that can be obtained from the groats and hulls, whilst high concentrations of vanillin, hydroxybenzoic and ρ-coumaric acid can also be found in hulls.
Compound . | Groats, free acid fraction . | Groats, soluble esters . | Groats, insoluble bound . | Hulls, free acid . | ||||
---|---|---|---|---|---|---|---|---|
Sosulski et al.19 . | Xing & White20 . | Dimberg et al.21 . | Emmons & Peterson22 . | Sosulski et al.19 . | Sosulski et al.19 . | Xing & White20 . | Emmons & Peterson22 . | |
Caffeic acid | 1.0 | 16.8 | 2.2 | 2.4 | 1.6 | 0.9 | ||
Catechol | tr | 0.1 | ||||||
Coniferyl alcohol | 0.8 | |||||||
o-Coumaric acid | 6.9 | |||||||
p-Coumaric acid | 0.7 | 44.9 | 1.6 | 0.9 | 0.5 | 0.8 | 59.7 | 9.7 |
Gallic acid | 1.3 | 0.6 | ||||||
Ferulic acid | 2.4 | 147.2 | 2.3 | 1.2 | 8.6 | 55.3 | 142.3 | 1.7 |
p-Hydroxybenzoic acid | 0.7 | 3.5 | 0.7 | tr | 50.0 | |||
p-Hydroxybenzaldehyde | 0.9 | 0.3 | 7.7 | |||||
p-Hydroxyphenylacetic acid | 0.4 | 0.6 | tr | 4.6 | ||||
Protocatechuic acid | 0.5 | 0.7 | tr | tr | 2.1 | |||
Salicylic acid | 3.1 | |||||||
Sinapic acid | tr | 0.5 | 4.3 | tr | 5.6 | 0.6 | ||
Syringic acid | 2.3 | 3.0 | tr | |||||
Vanillic acid | 0.7 | 16.1 | 1.2 | 1.6 | 3.5 | 24.3 | 4.0 | |
Vanillin | 3.4 | 2.3 | 1.0 | 54.2 | 6.3 |
Compound . | Groats, free acid fraction . | Groats, soluble esters . | Groats, insoluble bound . | Hulls, free acid . | ||||
---|---|---|---|---|---|---|---|---|
Sosulski et al.19 . | Xing & White20 . | Dimberg et al.21 . | Emmons & Peterson22 . | Sosulski et al.19 . | Sosulski et al.19 . | Xing & White20 . | Emmons & Peterson22 . | |
Caffeic acid | 1.0 | 16.8 | 2.2 | 2.4 | 1.6 | 0.9 | ||
Catechol | tr | 0.1 | ||||||
Coniferyl alcohol | 0.8 | |||||||
o-Coumaric acid | 6.9 | |||||||
p-Coumaric acid | 0.7 | 44.9 | 1.6 | 0.9 | 0.5 | 0.8 | 59.7 | 9.7 |
Gallic acid | 1.3 | 0.6 | ||||||
Ferulic acid | 2.4 | 147.2 | 2.3 | 1.2 | 8.6 | 55.3 | 142.3 | 1.7 |
p-Hydroxybenzoic acid | 0.7 | 3.5 | 0.7 | tr | 50.0 | |||
p-Hydroxybenzaldehyde | 0.9 | 0.3 | 7.7 | |||||
p-Hydroxyphenylacetic acid | 0.4 | 0.6 | tr | 4.6 | ||||
Protocatechuic acid | 0.5 | 0.7 | tr | tr | 2.1 | |||
Salicylic acid | 3.1 | |||||||
Sinapic acid | tr | 0.5 | 4.3 | tr | 5.6 | 0.6 | ||
Syringic acid | 2.3 | 3.0 | tr | |||||
Vanillic acid | 0.7 | 16.1 | 1.2 | 1.6 | 3.5 | 24.3 | 4.0 | |
Vanillin | 3.4 | 2.3 | 1.0 | 54.2 | 6.3 |
Cholesterol-reducing medication is also available such as LIPITOR, which had sales of $12 billion in 2008. Synthetic antioxidants such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) are dominant in this market, however, these synthetic molecules have some key disadvantages as they are suspected to have toxic properties, are highly volatile and restricted by certain countries.
1.2.3 Oilseed Extracts
Oilseeds residues contain significant amounts of phenolics, falvanoids and sinapine that can be used as natural antioxidants. Furthermore, they can add value to residues that normally would be used as animal feed. Using conventional solvents such as methanol, acetone, water, and ethyl acetate/water these components can be extracted with yields of 3 to 19% of all extracted material.9 As shown in Table 1.4, significant amounts of these valuable antioxidants are found in various oil seed crops.
. | EC (mg/g) . | PC (mg/g) . | PC/EC (%) . | flavanoids (mg/g) . | sinapine (mg/g) . |
---|---|---|---|---|---|
70% methanol extract | |||||
B. carinata | 223.0 | 13.6 | 6.1 | 7.49 | 99.9 |
rapeseed | 187.3 | 11.8 | 6.3 | 7.96 | 94.6 |
L. campestre | 311.0 | 35.6 | 11.5 | 128.80 | 0.5 |
B. Vema | 159.0 | 13.1 | 8.2 | 11.47 | 100 |
crambe | 214.0 | 8.1 | 3.8 | 9.36 | 9.6 |
sunflower | 102.0 | 16.1 | 15.8 | 12.03 | 0.0 |
C. sativa | 166.7 | 11.1 | 6.6 | 142.79 | 56.5 |
mustard | 245.0 | 17.6 | 7.2 | 0.97 | 122.1 |
70% acetone extract | |||||
B. carinata | 192.7 | 9.4 | 4.9 | 9.32 | 129.7 |
rapeseed | 143.3 | 6.6 | 4.6 | 5.20 | 85.6 |
L. campestre | 150.7 | 7.4 | 4.9 | 72.49 | 1.8 |
B. Vema | 156.7 | 8.5 | 5.4 | 18.61 | 100 |
crambe | 224.0 | 9.6 | 4.3 | 5.69 | 11.5 |
sunflower | 110.0 | 4.7 | 4.3 | 11.23 | 0.0 |
C. sativa | 309.3 | 11.1 | 3.6 | 36.15 | 47.5 |
mustard | 228.0 | 17.6 | 7.7 | 1.21 | 85.5 |
water extract | |||||
B. carinata | 466.7 | 36.0 | 7.7 | 2.04 | 17.5 |
rapeseed | 326.0 | 21.3 | 6.5 | 0.74 | 11.7 |
L. campestre | 229.3 | 44.7 | 19.5 | 3.54 | 0.7 |
B. Vema | 188.7 | 25.2 | 13.4 | 5.26 | 57.0 |
crambe | 332.7 | 16.6 | 5.0 | 0.64 | 2.6 |
sunflower | 267.3 | 38.8 | 14.5 | 1.45 | 0.0 |
C. sativa | 275.3 | 21.8 | 7.9 | 11.77 | 9.6 |
mustard | 247.3 | 36.5 | 14.8 | 0.07 | 32.5 |
ethyl acetate extract | |||||
B. carinata | 47.3 | 3.6 | 7.7 | 8.25 | 37.9 |
rapeseed | 72.0 | 4.0 | 5.5 | 4.19 | 39.4 |
L. campestre | 59.3 | 8.3 | 13.9 | 47.92 | 0.0 |
B. Vema | 60.0 | 5.4 | 9.0 | 19.83 | 0.0 |
crambe | 74.0 | 2.6 | 3.6 | 9.30 | 13.7 |
sunflower | 80.7 | 2.7 | 3.3 | 4.49 | 0.0 |
C. sativa | 58.0 | 3.2 | 5.5 | 59.75 | 0.0 |
mustard | 60.7 | 9.2 | 15.2 | 2.18 | 60.9 |
. | EC (mg/g) . | PC (mg/g) . | PC/EC (%) . | flavanoids (mg/g) . | sinapine (mg/g) . |
---|---|---|---|---|---|
70% methanol extract | |||||
B. carinata | 223.0 | 13.6 | 6.1 | 7.49 | 99.9 |
rapeseed | 187.3 | 11.8 | 6.3 | 7.96 | 94.6 |
L. campestre | 311.0 | 35.6 | 11.5 | 128.80 | 0.5 |
B. Vema | 159.0 | 13.1 | 8.2 | 11.47 | 100 |
crambe | 214.0 | 8.1 | 3.8 | 9.36 | 9.6 |
sunflower | 102.0 | 16.1 | 15.8 | 12.03 | 0.0 |
C. sativa | 166.7 | 11.1 | 6.6 | 142.79 | 56.5 |
mustard | 245.0 | 17.6 | 7.2 | 0.97 | 122.1 |
70% acetone extract | |||||
B. carinata | 192.7 | 9.4 | 4.9 | 9.32 | 129.7 |
rapeseed | 143.3 | 6.6 | 4.6 | 5.20 | 85.6 |
L. campestre | 150.7 | 7.4 | 4.9 | 72.49 | 1.8 |
B. Vema | 156.7 | 8.5 | 5.4 | 18.61 | 100 |
crambe | 224.0 | 9.6 | 4.3 | 5.69 | 11.5 |
sunflower | 110.0 | 4.7 | 4.3 | 11.23 | 0.0 |
C. sativa | 309.3 | 11.1 | 3.6 | 36.15 | 47.5 |
mustard | 228.0 | 17.6 | 7.7 | 1.21 | 85.5 |
water extract | |||||
B. carinata | 466.7 | 36.0 | 7.7 | 2.04 | 17.5 |
rapeseed | 326.0 | 21.3 | 6.5 | 0.74 | 11.7 |
L. campestre | 229.3 | 44.7 | 19.5 | 3.54 | 0.7 |
B. Vema | 188.7 | 25.2 | 13.4 | 5.26 | 57.0 |
crambe | 332.7 | 16.6 | 5.0 | 0.64 | 2.6 |
sunflower | 267.3 | 38.8 | 14.5 | 1.45 | 0.0 |
C. sativa | 275.3 | 21.8 | 7.9 | 11.77 | 9.6 |
mustard | 247.3 | 36.5 | 14.8 | 0.07 | 32.5 |
ethyl acetate extract | |||||
B. carinata | 47.3 | 3.6 | 7.7 | 8.25 | 37.9 |
rapeseed | 72.0 | 4.0 | 5.5 | 4.19 | 39.4 |
L. campestre | 59.3 | 8.3 | 13.9 | 47.92 | 0.0 |
B. Vema | 60.0 | 5.4 | 9.0 | 19.83 | 0.0 |
crambe | 74.0 | 2.6 | 3.6 | 9.30 | 13.7 |
sunflower | 80.7 | 2.7 | 3.3 | 4.49 | 0.0 |
C. sativa | 58.0 | 3.2 | 5.5 | 59.75 | 0.0 |
mustard | 60.7 | 9.2 | 15.2 | 2.18 | 60.9 |
The oilseed cake is a material that is left after the oil has been extracted from the seeds and contains significant levels of protein and chemicals that have numerous applications. Currently, there exists a stable, but low-value animal feed market for oilseed cake. However after extraction of nutrients and proteins, the cake will no longer be viable for animal feed and the only application would be as fuel for energy production, also a low-value market. The Sustoil project, funded by the EU under the 7th framework programme, has identified a number of valuable extracts that can be obtained from oil seed rape cake.10
For example, glucosinolates are an important group of chemicals found in oil seed rape, turnip, broccoli and mustard that can be broken down by enzymes to produce isothiocyanates that have pesticidal properties. There is great potential to use such pesticides in organic farming. An even greater market exists in the food sector as it has been shown that glucosinolates offer anti-inflammatory, antimicrobial and chemopreventive effects. Oil seed rape also contains significant amounts of phenols in the form of sinapine, which is present at around 1% by weight in the cake.
1.2.4 Root Crops
Sugarbeet pulp contains a variety of valuable components, most notably pectin, a complex polysaccharide consisting of D-galacturonic acid and a series of neutral sugars such as L-rhamnose, L-arabinose, and D-galactose.11 Pectins are used widely in the food industry as gelling agents in jams and are mainly sourced from apple pomace and citrus peels. Sugarbeet is also a potential source as it contains a very high amount of pectin, reported to be 15–30% of the dry weight, and its pectin also offers superior emulsifying properties, however, poor gelling is observed. Pectin can also be extracted from sunflower residues at 15–25% dry weight.
The global pectin market stands at 30,000 tonnes and the price ranges from $11–$13/kg, see Table 1.5.12 The San-Ei Gen F.F.I. Inc company has recently developed a process that modifies sugarbeet pectin to offer enhanced emulsifying properties and is close to commercialisation.13
Application . | Growth Rate . |
---|---|
Fruit preparations | 15–20% |
Fruit spreads (low-sugar jams) | 10%+ |
Yoghurt and dairy | 15–20% |
Acidified milk drinks, high-sugar jams | 2–5% |
Fruit juices and high-calcium drinks | 10–15% |
Baked goods, confectionary | 5% |
Fat replacers | 20%+ |
Application . | Growth Rate . |
---|---|
Fruit preparations | 15–20% |
Fruit spreads (low-sugar jams) | 10%+ |
Yoghurt and dairy | 15–20% |
Acidified milk drinks, high-sugar jams | 2–5% |
Fruit juices and high-calcium drinks | 10–15% |
Baked goods, confectionary | 5% |
Fat replacers | 20%+ |
1.2.5 Wood and Bark
Tannins, or wood polyphenols, are important extracts from wood and bark and have been used in various products such as leather. Other applications for tannins include use in particleboard adhesives and anticorrosive primer. A lucrative market for tannins is developing in the field of medicine where they have shown to have anti-inflammatory, antiviral, antimicrobial and antiparasitic effects.
Another important component called oleoresin is found in pine and other softwood. The oleoresin, isolated by tapping of living trees, is fractionated into gum rosin and gum turpentine. Approximately 75% of the global oleoresin production (over 1,000,000 tons) and processing takes place in China. Rosin is used in numerous applications ranging from industrial inks to soaps. Rosin can be used as a glazing agent and is used in medicine and chewing gum. A small percentage of rosin is also in flux, required for soldering. Other valuable components of wood that have great potential include phytosterols, flovonoids (e.g. Pycnogenol), lignans and other bioactive substances, and other specialty chemicals.
In addition, various hemicelluloses can be isolated from wood for the applications in their polymeric or monomweric forms, after hydrolysis. For example, some arabinogalactan isolated from larch wood by hot-water extraction has been on the markets since the 1970s.
1.2.6 Herbaceous
Grasses are an abundant source of biomass and can be grown in many areas throughout the world. When grasses are pressed a green juice is produced that contains a cocktail of valuable substances. The juice can contain hundreds of individual components that fit into the following groups; proteins, lipids, glycoproteins, lectins, sugars, free amino acids, dyes, hormones, enzymes, minerals and others.14
Sugars such as glucose, fructose, fructan, erythrose, rhamnose, xylose, galactose, mannose, mannitol and maltose are contained within the juice and have considerable value. Sugars can be converted to high-value chemicals and can therefore penetrate several markets from fuels to cosmetics. Beta-carotene and xanthophyll are possible anticarcinogens and used for applications in cosmetics, food, textiles and toys. The juice also contains other vitamins such as B1, B2 and E that all have considerable market value. The juice also contains high-value fatty acids such as palmitic acid, linoleic acid and linolenic acid that not only provide health benefits but can also be used in the cosmetics industry. The proteins within the juice are of high value as they can be used in medical diets to promote recovery from brain damage and can be consumed by people with kidney problems.15
1.2.7 Summary
Secondary metabolites exist in almost all types of biomass, as shown above, and are mainly used in speciality markets such as food ingredients, cosmetics and health products. There are certain biomass types that have not been covered in this study but are promising alternatives. In order for a secondary metabolite to be used in any product its production cost must be low. In exceptional cases where the extract is of very high value then costly processing may be utilised but in most cases a low-cost production method is required. For example, wheat straw is already used as a low-value product in animal feed and for energy production, however, extracting valuable components should not increase the cost to such an extent that the straw is overpriced. Alternatively, there are many biomass sources such as certain fruits and vegetables that are already being used for consumption purposes and also contain high-value secondary metabolites. In Table 1.6 a list of important metabolites found in various types of fruit and vegetables that have potential health benefits are shown.
Name . | Source . | Uses . |
---|---|---|
Terpenoids /Isoprenoids | ||
Carotenoid Terpenoids | ||
Lycopene | Tomatoes | Cholesterol reducing, suppresses tumour growth, antioxidant |
Beta-Carotene | Carrots, Pumpkin | Cornea protection |
Alpha-Carotene | Carrots, Pumpkin | Anticarcinogenic |
Lutein | Kale, spinach, watercress and parsley | Eye protection |
Zeaxanthin | Kale, spinach, watercress and parsley | Eye protection |
Astaxanthin | Fish | Powerful antioxidant |
Noncarotenoid Terpenoids | ||
Perillyl Alcohol | Cherries, Mint | Anticarcinogenic |
Saponins | Chickpeas, Soybeans | Removes cholesterol, anticolon cancer |
Terpeneol | Carrots | Anticarcinogenic |
Terpene Limonoids | Orange peel | Anticarcinogenic |
Polyphenolics | ||
Flavonoid Polyphenolics | ||
Anthocyanins | Blueberries, blackberries, black raspberries | Endothelial cell protection |
Catechins | Tea leaf, Chocolate | Anticarcinogenic |
Isoflavones | Soy beans | Anticarcinogenic, Cholesterol reducing |
Hesperetin | Citrus fruits | Antioxidant, anticarcinogenic |
Naringin | Grapefruit | Cholesterol reducing |
Rutin | Asparagus, buckwheat, citrus fruits | Anti-inflammatory, Strengthens capillaries |
Quercetin | Apple skins, red onion | Antihistaminic, antioxidant, blood thinner |
Silymarin | Artichokes, milk thistle | Antiatherosclerotic, antioxidant, anticarcinogenic |
Tangeretin | Tangerines | Strong anticarcinogenic |
Punicalagin | Pomegranate | Antioxidant, anti-inflammatory |
Phenolic Acids | ||
Ellagic Acid | Raspberries, strawberries | Reduces esophagal and colon cancer |
Chlorogenic Acid | Blueberries, tomatoes and bell peppers | Antioxidant |
P-Coumaric Acid | Red/Green bell peppers | Antioxidant |
Phytic Acid | Legumes, whole grain | Reduces cancer growth, reduces blood-sugar levels |
Ferulic Acid | Brown rice, whole wheat, oats | Antioxidant, anticarcinogenic |
Vanillin | Vanilla bean | Antioxidant, anti-inflammatory |
Cinnamic Acid | Balsam tree resin, wood, inner bark | Antibacterial, pigment |
Other Nonflavonoid Polyphenolics | ||
Curcumin | Tumeric | Anti-inflammatory, antioxidant, asprin alternative |
Resveratrol | Grape skin | Anti-inflammatory |
Lignans | Flaxseed, wood | Cytotoxic agent |
Glucosinolates | ||
Isothiocyanates | ||
Phenethyl Isothiocyanate | Watercress | Anticarcinogenic |
Benzyl Isothiocyanate | Cruciferous plants | Anticarcinogenic |
Sulforaphane | Broccoli | Anticarcinogenic |
Indoles | ||
Indole-3-Carbinol | Broccoli | Anticarcinogenic |
Thiosulfonates | Garlic, onions | Reduces blood pressure |
Phytosterols | ||
Beta-Sitosterol | Black cumin seed, cashew fruit, rice bran | Reduces cholesterol, anticarcinogenic |
Anthraquinones | ||
Senna | Leaves of luguminous herbs | Laxative |
Barbaloin | Aloe vera | Laxative |
Name . | Source . | Uses . |
---|---|---|
Terpenoids /Isoprenoids | ||
Carotenoid Terpenoids | ||
Lycopene | Tomatoes | Cholesterol reducing, suppresses tumour growth, antioxidant |
Beta-Carotene | Carrots, Pumpkin | Cornea protection |
Alpha-Carotene | Carrots, Pumpkin | Anticarcinogenic |
Lutein | Kale, spinach, watercress and parsley | Eye protection |
Zeaxanthin | Kale, spinach, watercress and parsley | Eye protection |
Astaxanthin | Fish | Powerful antioxidant |
Noncarotenoid Terpenoids | ||
Perillyl Alcohol | Cherries, Mint | Anticarcinogenic |
Saponins | Chickpeas, Soybeans | Removes cholesterol, anticolon cancer |
Terpeneol | Carrots | Anticarcinogenic |
Terpene Limonoids | Orange peel | Anticarcinogenic |
Polyphenolics | ||
Flavonoid Polyphenolics | ||
Anthocyanins | Blueberries, blackberries, black raspberries | Endothelial cell protection |
Catechins | Tea leaf, Chocolate | Anticarcinogenic |
Isoflavones | Soy beans | Anticarcinogenic, Cholesterol reducing |
Hesperetin | Citrus fruits | Antioxidant, anticarcinogenic |
Naringin | Grapefruit | Cholesterol reducing |
Rutin | Asparagus, buckwheat, citrus fruits | Anti-inflammatory, Strengthens capillaries |
Quercetin | Apple skins, red onion | Antihistaminic, antioxidant, blood thinner |
Silymarin | Artichokes, milk thistle | Antiatherosclerotic, antioxidant, anticarcinogenic |
Tangeretin | Tangerines | Strong anticarcinogenic |
Punicalagin | Pomegranate | Antioxidant, anti-inflammatory |
Phenolic Acids | ||
Ellagic Acid | Raspberries, strawberries | Reduces esophagal and colon cancer |
Chlorogenic Acid | Blueberries, tomatoes and bell peppers | Antioxidant |
P-Coumaric Acid | Red/Green bell peppers | Antioxidant |
Phytic Acid | Legumes, whole grain | Reduces cancer growth, reduces blood-sugar levels |
Ferulic Acid | Brown rice, whole wheat, oats | Antioxidant, anticarcinogenic |
Vanillin | Vanilla bean | Antioxidant, anti-inflammatory |
Cinnamic Acid | Balsam tree resin, wood, inner bark | Antibacterial, pigment |
Other Nonflavonoid Polyphenolics | ||
Curcumin | Tumeric | Anti-inflammatory, antioxidant, asprin alternative |
Resveratrol | Grape skin | Anti-inflammatory |
Lignans | Flaxseed, wood | Cytotoxic agent |
Glucosinolates | ||
Isothiocyanates | ||
Phenethyl Isothiocyanate | Watercress | Anticarcinogenic |
Benzyl Isothiocyanate | Cruciferous plants | Anticarcinogenic |
Sulforaphane | Broccoli | Anticarcinogenic |
Indoles | ||
Indole-3-Carbinol | Broccoli | Anticarcinogenic |
Thiosulfonates | Garlic, onions | Reduces blood pressure |
Phytosterols | ||
Beta-Sitosterol | Black cumin seed, cashew fruit, rice bran | Reduces cholesterol, anticarcinogenic |
Anthraquinones | ||
Senna | Leaves of luguminous herbs | Laxative |
Barbaloin | Aloe vera | Laxative |
1.3 Transformation of Glycerol into High-Quality Products through Green Chemistry and Biotechnology
Glycerol, or propan-1, 2, 3-triol, is an important byproduct of biodiesel production generated from the transesterification reaction of triglycerides from virgin vegetable oils or fats as well as waste oils, with alcohols including methanol and ethanol, in the presence of a homogeneous base catalyst such as NaOH or KOH, and acid catalyst. In general, the production of 10 kg of biodiesel yields approximately 1 kg of crude glycerol (10% (w/w)), and currently the world’s capacity for biodiesel production is dramatically increasing. Further increases in biodiesel production rates will significantly raise the quantity and surplus of crude glycerol and partially purified glycerol in the environment. In contrast to the surplus of impure glycerol, high-purity glycerol is an important industrial feedstock that finds applications in the food, cosmetic and pharmaceutical industries, as well as other more minor uses. However, its refining is generally costly, especially for medium- and small-sized plants.
Glycerol can be used as a building block for many chemicals such as 1, 3-propanediol, lactate and succinate. In fact many companies have initiated commercial plans to manufacture high-value chemicals such as epichlorohydrin (Solvay SA) and proplylene diol (Ashland/Cargill) from glycerol feedstocks. The market volatility in the price of glycerol has caused concern for these projects, however, the long-term fundamentals remain strong.
Glycerol has been known since 2800 BC mainly as a byproduct of soap production.17 Currently, glycerol has numerous applications in personal care, food, tobacco, detergents, cellophane, explosives and pharmaceuticals.18 Leffingwell and Lesser identified 1582 applications for glycerol in 1945.19 However, in recent times, many glycerol production plants are closing and new plants utilising glycerol as a raw material are starting.20 Global glycerol production has increased from 60,000 tons in 2001 to 800,000 tons in 2005, partly due to biodiesel production. The amount of glycerol being used in technical applications is around 160,000 tons, and this is expected to grow at a rate of 2.8% per year.21
Glycerol is a raw material for the production of flexible foams and rigid polyurethane foams. It is known to provide properties including flexibility, pliability and toughness in surface coatings and paint regenerated cellulose films, meat casings and special quality papers.22 Glycerol has the ability to absorb moisture from the atmosphere and is therefore used in many adhesives and glues to prevent early drying. In food applications, nontoxic glycerol is used as solvent, sweetener and preservative. Many polyols such as sorbitol, manitol and maltitol are used as sugar-free sweeteners; however, they are facing fierce competition from glycerol. Glycerol has similar sweetness to sucrose and has the same energy as sugar. Furthermore, it does not raise blood sugar levels and does not feed plaque bacteria. Glycerol is also employed as an emollient, humectant and lubricant in many products in the personal care industry including toothpaste, mouthwashes, shaving cream and soaps.23
A detailed revision on glycerol transforming processes and applications can be found in ‘The Future of Glycerol- New usages for a raw material’ authored by Mario Pagliaro and Michele Rossi.23 The book was published in 2008 and focuses on key chemical and biochemical transformations with detailed processing conditions. In this book, relevant information on the sustainability and economics of glycerol and biofuels production is discussed. The detailed synthetic chemistry involved in the transforming processes has also been reviewed by Behr et al. in his paper entitled “Improved utilisation of renewable resources: New important derivatives of glycerol”.24
A more detailed revision on chemicals that can be derived from glycerol was conducted in 2008 by Zheng, Chen, and Shen in “Commodity Chemicals Derived from Glycerol, an Important Biorefinery Feedstock”.25 Many important chemicals have been identified that can be produced from glycerol-derived platform chemicals and their respective industrial applications are discussed. Furthermore, this review maps the reaction pathways of a glycerol-derived platform chemical that can form many other commodity chemicals that are not easily identifiable. Some of the important commodity chemicals identified include acrolein, dichloropropanol, epichlorohydrin, dihydroxyacetone, 1,3-propanediol, 1,2-propanediol, glycerol carbonate, diacylglycerol (DAG), monoglyceride (MG), oxygenate fuels, glyceric acid, tartronic acid, and mesoxalic acid.
Biochemical methods can likewise be employed to transform glycerol into commodity chemicals as highlighted by Yazdani et al.26 For some transformations a detailed description of the processes involved are shown including the overall production costs as shown in Figure 1.2.
Some of the important commodity chemicals produced using anaerobic fermentation include succinic acid, 1,3 propanediol, propionic acid, formic acid, butanol and ethanol. A recent paper by Silva et al. reviews glycerol as a source for industrial microbiology.27 This report identifies various microbial reaction pathways to produce many chemicals from glycerol-derived platform chemicals.
1.3.1 Aqueous Phase Reforming
One of the major achievements in glycerol chemistry is the development of aqueous phase reforming processes (APR) that involve the conversion of glycerol to hydrogen and carbon monoxide (synthesis gas). The reported process conditions require 250 °C and the utilisation of a Pt–Re catalyst in a single reactor.28 This process can also theoretically produce high yields of hydrogen from glycerol at low CO concentrations due to favourable water-gas shift (WGS) thermodynamics. This requires significantly lower energy consumption than traditional methane reforming.
The synthesis gas can be used as a building block for chemicals and fuels via Fischer–Tropsch synthesis (FTS). Through FTS, syngas can be converted to a range of useful liquid hydrocarbons (mainly linear alkanes, although alkenes and alcohols can also be produced under certain conditions) using iron and cobalt catalysts. The temperatures used in the process typically range from 150 to 300 °C and pressures of one to a few atmospheres are common. High temperatures lead to gasolines and linear low molecular mass hydrocarbons, whereas lower temperatures and high pressures favour the formation of longer-chain hydrocarbons (e.g. waxes).
1.3.2 Selective Reductions
The main processes utilised to reduce glycerol to glycols are hydrogenolysis, dehydroxylation and biotechnology via bacteria.
Propylene glycol is commercially produced via hydrogenolysis using a copper chromite catalyst at 200 °C at pressures below 10 bar. Wang and coworkers showed that it was possible to produce 1,3 propanediol via selective dehydroxylation.29 The central hydroxyl group of glycerol is selectively converted to a tosyloxy-group that is removed using hydrogenolysis. The biological reduction to 1,3 propanediol involves the use of bacterial strains from groups such as Citrobacter, Enterobacter, Ilyobacter, Klebsiella, Lactobacillus, Pelobacter and Clostridium. Freund showed in 1881 that PDO could be produced using Clostridium, a widely available micro-organism found in nature.30 The process involves a two-step enzyme-catalysed reaction sequence in which a dehydratase catalyses the conversion of glycerol to 3-hydroxypropionaldehyde, which is subsequently reduced to PDO by a NAD+-linked oxidoreductase.
1.3.3 Halogenations
The chlorination of glycerol via a 1,3-dichloro-2-propanol intermediate yields epichlorohydrin, an important and valuable chemical. 1,3-dichloro-2-propanol can be produced directly from glycerol using HCl as catalyst. Its subsequent dehydrochlorination using NaOH generates epichlorohydrin and NaCl.
1.3.4 Dehydrations
Glycerol dehydration can also produce relevant chemicals including acrolein, 3-hydroxypropionaldehyde and acrylic acid. Protonated glycerol is more susceptible to dehydration due to reduction in the energy barrier of the intermediate state. Therefore, acrolein can only be produced in acidic conditions. The reaction can be conducted in both liquid or gas phase at high temperatures and/or vacuum that are normally used to drive the dehydration. In the presence of molecular oxygen, acrylic acid can be produced via a one-step oxy-dehydration step.
1.3.5 Etherifications
Glycerol alkyl ethers can be synthesised by etherification of alkenes including isobutylene in the presence of an acid catalyst at temperatures from 50–150 °C. The typical molar ratios used in the reaction are 1∶2 (glycerol∶isobutylene) and the yield can be improved by optimising the reaction conditions. Glycerol can be etherified to form polyglycerol via anionic polymerisation of glycidol through a cation exchange equilibrium initiated by partially deprotonated 1,1,1-tris(hydroxymethyl) propane. The resulting polymer usually has a polydispersity of below 1.5 and a molecular weight ranging from 1000 to 3000 gmol−1.
1.3.6 Esterifications
Glycerol can be esterified with carboxylic acids or via carboxylation and nitration.31 Reaction with carboxylic acids results in the formation of monoacylglycerols and diacylglycerol. Monoacylglycerols are produced on a commercial scale by either continuous chemical glycerolysis of fats and oils (250 °C, alkaline, N2 atmosphere) or by direct esterification with fatty acids.32 The reaction of glycerol with dimethyl carbonate can also produce a high yield of glycerol carbonate in the presence of a biocatalyst (e.g. lipases). Glycerol can be converted to glycidyl nitrate by nitration that can be subsequently polymerised to form a valuable polymer.
1.3.7 Selective Oxidations
The oxidation of glycerol can be catalysed using highly active aerobic catalysts such as platinum and palladium. Supported gold catalysts are well known for catalytic stability, resistance to oxygen and tolerance against inhibition by aliphatic and aromatic amines. Organocatalysts such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) can be used for the selective oxidation of glycerol to mesoxalic acid. TEMPO has also been used in electrochemical oxidation where glycerol is converted to 1,3 dihydroxyacetone (DHA). The reaction proceeds by applying a small electric potential to a solution containing glycerol, water and 15 mol% TEMPO using a glassy carbon anode. DHA can also be produced using biological oxidation via micro-organisms or enzymes. Other oxidation products include glyceraldehydes, glyceric acid, glycolic acid, hydroxypyruvic acid, oxalic acid and tartronic acid.
1.3.8 Pyrolysis
Glycerol was identified as a feedstock for pyrolysis in 1985, well before the growth in the biodiesel market. Recent research by Valliyapan and coworkers has focused on optimising conditions for hydrogen or syngas production. Pyrolysis carried out in a continuous down-flow fixed-bed microreactor can take place with flow rates of nitrogen from 30–70 mL/min, temperatures of 650–800 °C and at atmospheric pressure. It was shown that the type and size of packing material in the tubular reactor can affect the conversion of glycerol and subsequent product distribution. Typical products include carbon monoxide, hydrogen, carbon dioxide, methane and ethane. At lower temperatures under steam or supercritical water conditions, longer molecules such as acrolein, formaldehyde and acetaldehyde are observed.
1.3.9 Biotransformations
Glycerol can be converted to a very large number of chemicals using micro-organisms and enzymes. The aerobic conversion of glycerol to 3-hydroxypropionaldehyde (3-HPA) was reported in 1985 by Slininger and Bothast.36 The cells of klebsiella pneumonia can be grown on a rich glycerol medium and production of 3-HPA starts when these micro-organisms are added to a buffer containing semicarbazide and glycerol. It was shown that a yield of up to 84% could be obtained, however this yield is sensitive to cell age and cultivation medium. The optimal processing conditions for this experiment were 32 °C, pH 7–8 and glycerol concentrations of 20–50 g/L.