- 1.1 Introduction
- 1.2 Structure and Chemistry in Determining Food Quality
- 1.3 Spectroscopic Methods for Determining Food Quality
- 1.3.1 UV–Visible Spectroscopy
- 1.3.2 Fluorescence Spectroscopy
- 1.3.3 Infrared Spectroscopy
- 1.3.4 Mid-infrared Spectroscopy
- 1.3.5 Near-infrared Spectroscopy
- 1.3.6 Far-infrared Spectroscopy
- 1.3.7 Raman Spectroscopy
- 1.3.8 NMR Spectroscopy
- 1.3.9 Atomic Emission Spectroscopy
- 1.4 Conclusion
- References
CHAPTER 1: Spectroscopic Techniques for the Analysis of Food Quality, Chemistry, and Function
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Published:20 May 2022
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Special Collection: 2022 ebook collection
M. Mishra, in Advanced Spectroscopic Techniques for Food Quality, ed. A. K. Shukla, The Royal Society of Chemistry, 2022, pp. 1-22.
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Food is a broad term covering a basic necessity of life. Food regulates our physiological and metabolic activity and hence our health. In the modern era, many transformations have been made to improve the properties of foods, and food colouring and processing have also changed the quality of many foods. These modified foods can also regulate the commencement of many diseases. As a consequence, it is essential to know the chemical and structural composition of foods. Many techniques have been applied to determine the quality of food, and spectroscopy has played a prominent role. This chapter summarizes various spectroscopic techniques that have been used to determine the quality of foodstuffs.
1.1 Introduction
The term ‘food’ is a compound word that is used to describe a material that provides nutritional support to alleviate the hunger of every organism.1 Materials covered by this broad term can be obtained from diverse resources such as plants, animals, and fungi. The food is ingested by different means to meet the physiological needs of a particular animal2 and, according to these needs, the types of food vary from animal to animal. Based on their food habits, animals can be classified as herbivorous, carnivorous, and omnivorous.3 With the continuing advancement of science and technology, the quality of food consumed by humans has improved over time, and consequently surpluses and deficiencies in the amounts or types of food have occurred within the body, causing several metabolic disorders including diabetes. This suggests that food has a determinant role in regulating the physiological and metabolic activity of the organism.
The primary composition of foods includes carbohydrates, proteins, lipids, vitamins, and minerals in different combinations. Phenols, flavones, and colourants are often found as secondary components of food.4 These secondary components help to enhance the aroma and taste of a food by stimulating the sensory organs.5,6 Similarly, the presence of bacteria can alter the metabolic, physiological, and behavioural conditions of humans. Recently, an oroneural connection was established in humans.7–9 Different types of food are also known to improve the psychological condition.10 As a consequence, the determination of food composition, authentication, assessment, prediction, and microbial composition are essential for determining the quality of foods11 (Figure 1.1).
A mechanistic representation of how the structure of food stimulates the sensory organs.
A mechanistic representation of how the structure of food stimulates the sensory organs.
1.2 Structure and Chemistry in Determining Food Quality
The structure of a food gives an impression of its quality at a first look. The nanostructures present in a food contribute to its colour and shape, which stimulate the sensory organs (Figure 1.2). For example, the actin–myosin complex in meat, starch granules of plant foods, and micelles of milk are nanostructures present within the food. The amorphous and helical region of starch determines the starch quality. Each food has a shelf life, which changes as a function of time. During processing of a food, consideration of the shelf life is important since it has a role in obesity and diabetes. Hence great importance is attached to understanding the structure of foods. Self-assembly of β-lactoglobulin and α-lactalbumin occurs in whey protein. A correlation between the particle size of a food and its effect on the tongue and palate has been described.12
In addition to nanostructure, other parameters are also used to detect the quality of food.13 One such parameter is flavonoids, which are low molecular weight polyphenolic substances having a heterocyclic ring (ring C). Flavonoids are resistant to heat, oxygen, humidity, and light. A hydroxyl group on C3 of ring C (flavanol) contributes to the photostability.14 The distribution of flavonoids is species specific for plants. Tea, red wine, fruits, and vegetables are enriched with flavonoids. Although quercetin is the main flavonoid, myricetin, kaempferol, apigenin, and luteolin are also found in foods. Humans need a minimum of 1 g per day of flavonoids for the body.15 The amount of flavanol can be determined from plasma and urine analyses, hence it is often used as a biomarker. External factors such as the season, ripeness, and food preparation and processing can change the amount of flavanol present. A minimum of 10 nM of flavanol is needed in our daily intake of food.16 The amount of quercetin depends on the types of bacteria present in the gut. Flavonoids are conjugated in the liver and are eliminated from the kidney. Flavonoids can be determined using many spectroscopic techniques.17 The matrix of the food is analysed in several industries such as the milk, meat, coffee, and wine trades.18 Although the quality of food can be roughly determined using the sensory organs, many sophisticated types of equipment are also used for this purpose.19
1.3 Spectroscopic Methods for Determining Food Quality
Among numerous techniques, spectroscopy is indispensable for determining the quality of foods.20–24 It is based on the combined action of absorption, transmission, and emission of electromagnetic light radiation with additional materials based on the wavelength of radiation.25 It is an interaction between light and the molecules following the effective collision theory. Light is electromagnetic radiation and the spectra vary according to the wavelength, frequency, and energy.26 It is used as an analytical tool in several branches of science. It can detect the composition of foods, microbes, pest pathogens, and adulteration that occur within the food27–29 (Figure 1.2). Spectroscopic techniques such as UV–visible, fluorescence, infrared, mid-infrared, near-infrared, Raman, and nuclear magnetic resonance spectroscopy are used to determine food quality.
1.3.1 UV–Visible Spectroscopy
UV–visible spectroscopy is used in several food industries. It is based on the principle of the Beer–Lambert law30 (Figure 1.3):
where I0 is the intensity of the incident light, I is the intensity of the transmitted light, c is the concentration of the sample, l is the pathlength through which the light travels in the sample, and ε is the molar extinction coefficient at a particular wavelength.
UV–visible spectroscopy uses electromagnetic radiation in the range 100–750 nm. The UV range covers 100–380 nm and the visible range covers 380–750 nm.31 It detects two different aspects: (1) colour and (2) fat oxidation. In oils, the greater the carotenoid (a pigment or chromophore such as chlorophyll, which most plants contain) content, the better is the antioxidant activity.32–35 The presence of chlorophyll makes olive oil bitter.36,37 To determine the fat oxidation, the p-anisidine value (AV) is measured.38 This is the quantity of aldehyde produced during fat oxidation as a function of temperature, oxygen, and light.39 In oils, an AV of >8 is not permissible.
1.3.1.1 In the Oil Industry
UV–visible spectroscopy is widely used to determine the quality of oil by measuring anisidine, which is generated during the oxidation of food. The AV also determines the total oxidation value (Totox), which indicates the fat deterioration.40 The Totox value is represented by two times the peroxide value (PV) plus AV (2PV + AV). It is measured at 350 nm. The deterioration of fat is measured by the PV. When lipids are exposed to heat, daylight, and oxygen, they undergo decomposition and form peroxides and hydroperoxides. The PV value can be measured easily using a UV–visible spectrometer at an absorbance maximum of 240 nm.41 The quality of oil is also determined by the presence of chlorophyll and carotenoids. The greater the amount of carotenoids, the better antioxidant properties the oil has. Carotenoids can be determined at 440 nm42 and chlorophyll at 666 nm. The absorbance peak also varies with the solvent that is used in the analysis. The green colouration of olive oil is due to the presence41 of chlorophyll and pheophytin.43,44 Since pheophytin-α has oxidative properties, the lower its content the better is the oil quality.44
1.3.2 Fluorescence Spectroscopy
The application of fluorescence spectroscopy is a rapid means to analyse a sample in a non-destructive way. It detects the fluorescence based on a naturally present fluorophore within the sample.45 Many microbes and their colonies possess fluorescence, hence it is easy to detect the presence of any microbes based on their fluorescence spectra.46,47 Many foods possess fluorophores. There is also a chance that food may become contaminated due to the presence of microbes. Since foods contain fluorophores, many industries use the fluorescence technique to determine food quality. The method is based on the emission of light after absorption of ultraviolet or visible light by a fluorophore.48,49 The fluorophores present in foods include rhodamine B, quinine, Acridine Orange, fluorescein, and pyridine. This technique is often used in combination with other techniques such as high-performance liquid chromatography (HPLC). It is a very sensitive method that can determine food quality with high specificity in several industries.
1.3.2.1 In the Dairy and Honey Industries
Milk possesses many proteins, amino acids, and vitamins that have fluorescence properties. Among the amino acids, tryptophan, tyrosine, and phenylalanine are commonly found, and among the vitamins, vitamins A and B2 are found in milk. The numbers of vitamins, proteins, and amino acids within milk vary with how it has been processed. The fluorescence spectra of vitamin A present in cheese was found to vary among eight different types of cheese.50 Yogurt possesses three different types of fluorophore, tryptophan, riboflavin, and lumichrome, and the presence of these three fluorophores allows the determination of the quality of yogurt using fluorescence spectroscopy.51 Honey is a product made by bees from the nectar of flowers,52 and its properties change during packing and transportation. Honey possesses phenolic compounds that are derivatives of phenolic acid present within the flower. The amount of phenolic compounds can be determined by fluorescence spectroscopy.53
1.3.2.2 Meat and Seafood
Meat and seafood lose their quality owing to oxidation, autolysis of enzymes, and growth of microbes.54,55 All these phenomena can be detected by monitoring the fluorophores present within the sample. Meat possesses fluorophores such as tryptophan, nicotinamide adenine dinucleotide (NADH), porphyrins, riboflavin, and vitamin A.56,57 Collagen is present in the adipose or connective tissue58 and provides texture or tenderness to the tissue. The greater the amount of collagen, the better is the quality of the meat. Beef tenderness is detected using fluorescence properties.59 Fat/lean meat is also analysed by monitoring the fluorescence of tryptophan.60–62 In spoiled meat, oxidation of lipids and proteins occurs. Thiobarbituric acid reactive substance (TBARS) is considered a parameter of lipid peroxidation.63 The amount of TBARS can be measured from the fluorescence spectra. The extent of oxidation varies within the meat product with different storage and processing conditions. Seafood and fish possess the naturally present fluorophore vitamin A, amino acids, NADH, riboflavin, and oxidation products.64 Oxidation causes spoiling of food. The quality of fish such as mackerel, salmon, and cod can be determined using fluorescence spectroscopy.65 The fluorescence spectra vary with the storage time in cold conditions.65
1.3.2.3 Detection of Bacteria
Microbes can degrade the quality of food and in extreme cases it can make it toxic. Hence it is essential to keep foods free from microbes. Fluorescence spectroscopy is widely used in poultry to distinguish different bacteria. Lactic acid bacteria are often detected in sausages and can be identified using fluorescence spectroscopy.66 The toxins of these bacteria are identified using fluorescence spectroscopy.67–69 Salicylates can also be determined by using this technique. This method can also detect structural changes in proteins and carbohydrates. By altering the excitation and emission wavelengths, the food quality of several compounds can be determined. Most mycotoxins have blue fluorescence;70 the exception is aflatoxin G1 and G2, which have yellow–green fluorescence.71 Accordingly, using fluorescence spectroscopy, the toxin can be detected. Ochratoxin A, a fluorescent compound, may be present in roasted coffee, corn, and sorghum,72 and fluorescence spectroscopy can help to determine the food value of these materials. Cereals such as rice and maize have also been investigated using fluorescence techniques.73 In a corn sample, fluorescein was labeled to detect the fumonsin B1.74 For a rapid test for the toxin deoxynivalenol in wheat, a fluorescence polarization immunoassay was developed.75 Fluorescence spectroscopic properties can distinguish between rice and maize flour.76 For wheat kernels, red and white emission spectra were recorded.77 The morphological variation between two different varieties of wheat can be distinguished by using the fluorescence technique.78,79 The refinement and milling of wheat flour can be monitored by measuring the emission spectra of ferulic acid and riboflavin.80,81 The quality of olive oil was determined by using fluorescence spectroscopy.82 Oils such as sunflower, cotton, soybean, and corn oils have a fluorescence band at 439–450 nm.83 Only olive oil has a band at 440 and 455 nm, a medium band at 681 nm, and a strong band at 525 nm.84 The 681 nm band represents chlorophyll85 and the 525 nm band represents vitamin E.84 Fluorescence spectroscopy can be used to determine the anisidine and iodine values of oligomers and monomers.84
1.3.3 Infrared Spectroscopy
Infrared (IR) radiation was discovered in 1800 by William Herschel.86 Its range varies between 78 nm and 1 mm. This technique uses the vibration of atoms and molecules. Different vibrations were observed in the IR region. IR spectra provide evidence of molecular structure from the frequency of the normal mode of vibration. In the case of the normal modes, the sample executes harmonic oscillations. There are six normal modes. The vibrations of functional groups such as OH, NH2, CH3, and C=O are responsible for bands near the IR spectrum.87 Any compound containing a C=O group shows strong bands at 1899 and 1650 cm−1. The NH2 group gives an IR band between 3400 and 3300 cm−1. A compound with a C6H5 group gives peaks at 1600 and 1500 cm−1. Thus the IR spectrum is considered as the fingerprint of the molecule. Based on the range, it is divided into three forms: (1) near-infrared (780 nm–5 µm), (2) mid-infrared (5–30 µm), and (3) far-infrared (30–1000 µm).88
1.3.4 Mid-infrared Spectroscopy
Mid-infrared (MIR) spectroscopy can detect functional groups and carbon, nitrogen, and lignin.89 It is used to study soil and food. MIR spectroscopy is used with attenuated total reflectance (ATR). It is also used in combination with the Fourier transform process. It uses the diffuse reflectance infrared Fourier transform (DRIFT) process. Soil properties, organic matter, and the presence of fungus in the food can be detected using this technique.
This technique helps to establish the structure–function relationships between foods, hence it is valuable in the food industry. Fourier transform infrared (FTIR) spectroscopy is widely used. The MIR region stretches from 4000 to 400 cm−1. The X–H range is 4000–2500 cm−1, triple bond 2500–2000 cm−1, and double bond 2000–1500 cm−1. The X–H stretch is due to the presence of O–H, C–H, and N–H stretching. The vibrations of C≡C, C≡N, C=C, C=O, and C=N can be determined using this technique. MIR spectroscopy is used to investigate the chemistry of fats and oils. The presence of a 996 cm−1 band corresponds to a trans double bond.90 Excess cis–trans conversion occurs during hydrogenation, the conversion of oil to fat, to enhance the oxidative stability of polyunsaturated oils.91 Spoiling of oils, fats, and lipids occurs due to lipid autoxidation. The peroxide value determines the oxidative status and firmness of refined and olive oil. This technique detects a peak at 3444 cm−1, which corresponds to hydroperoxide (–OOH). Corn, peanut, sunflower, cottonseed, and soybean oils are also analysed using this technique.
1.3.4.1 Soil Study
MIR spectroscopy detects the composition, properties, and organic matter present within a soil. The DRIFT technique can detect the composition of soil and humus chemically. ATR also detects soil organic matter.92 Using different MIR wavelengths, feldspars, quartz, silicates, clay, and minerals such as carbonates and calcites can be detected in soil.93 MIR spectroscopy can also detect the organic matter present within the soil, such as lignins, cellulose, carbohydrates, fats, and proteins.94
1.3.4.2 Analysis of Humus
Natural compounds originating from plants and animals are present in soil, rivers, and coal. The composition of humus can be determined by using the DRIFT method.95
1.3.4.3 Quality Control
MIR spectroscopy is employed to detect fungal disease and mycotoxins in cereals during processing and storage.96,97
1.3.5 Near-infrared Spectroscopy
Near-infrared (NIR) spectroscopy is a primary technique for the rapid detection of moisture, carbohydrates, proteins, and fat within a food. However, the composition of an unknown sample needs to be calibrated with a known sample, and recalibration is essential for each sample. Hence the method lacks sensitivity for a sample having only a minor amount of the component of interest. The vibration of C–H, O–H, N–H, S–H, and C=O bonds occurs with absorption in the NIR region.98 Wheat and wheat products are easily analysed using this technique.99 In Australia, the optimization of fertilizer is achieved by determining the total nitrogen and carbohydrate in plant tissue.100 Spectroscopy coupled with chromatography can be used to determine the quality of wheat.101 Seventy-five relative reflectance intensities were extracted from the scanned images of bulk wheat samples and used for the differentiation of wheat classes using a statistical classifier and an artificial neural network (ANN) classifier.102,103 It was found that kernels damaged by insects contained less starch than healthy kernels. NIR spectroscopy has also been used to check the adulteration of milk powder, orange juice, sugar, vegetables, and coffee, and also to measure the amount of dry matter as a parameter to determine the maturity of fruit and vegetables.104 The amount of dry matter indicates the right time to harvest the food product so that it can be transported and stored safely for a longer time.105 Other applications of NIR spectroscopy include maturity determination, processing, pest detection, toxin measurement, drought management, fertilizer application, and post-harvest quality control:
Processing: NIR spectroscopy is used to determine the dry matter and water content during the processing of food samples. It can be used to check the freshness of mushrooms, cereals, fruits, and vegetables.106
Pest, disease, and toxin detection: NIR spectroscopy can detect the amount of mycotoxin in cereals and grains.107
Drought management: The correct water level is essential for the proper growth of plants. The amount of water present can be determined so that appropriate irrigation can be applied to maintain the water supply.108
Application of fertilizer: The growth of desired plant leaves can be monitored so that the required nutrients can be added to improve the growth of the plant.109
Post-harvest control: After harvesting and during transportation and storage, damage may occur, which can be assessed by using NIR spectroscopy. The structure and ripeness of apples can be measured and the effect of the ethylene storage atmosphere can be determined.110
Several modern types of advanced optical equipment based on the NIR principle have been developed for fine-tuned analysis. An example is the Felix Instruments leaf spectrometer, used with a quality meter.111 The CI-710 Miniature Leaf Spectrometer was used to detect the leaf quality and the effects of pests, drought, and pathogens.112 The quality meter can detect the acidity, colour, and total soluble solids of fruit. The instrument is marketed as the F-750 product quality meter for mango113 and the F-751 quality meter for avocado testing.114
1.3.6 Far-infrared Spectroscopy
The far-infrared (FIR) region lies between 400 and 10 cm−1. The region below 200 cm−1 is difficult to interpret. Hydrogen atoms, organometallic compounds, and inorganic compounds absorb in the FIR region, and hydrogen bond stretching can also be detected in this region.
1.3.7 Raman Spectroscopy
When photons interact with molecules in matter the Raman effect or Raman scattering is produced. During this process, the photon loses vibrational energy in the Stokes process and gains vibrational energy in the anti-Stokes process. Such communication is possible for the interactions of atoms, which moderate the polarizability of the molecule. Such communication is possible for the cross-talk of incident photons with atoms. Intense Raman bands are detected from non-polar groups, predominantly from aromatic rings, and the vibrations produce an inflection of polarizability. The Raman spectrum is obtained as wavenumbers (cm−1) and the variation between excited and emitted energies is detected as vibrational spectra. This has a weak effect since the possibility of energy exchange is very small. For non-polar groups, particularly aromatic rings, the vibrations produce considerable modulation of the polarizability. Raman spectroscopy is a vibrational spectroscopic technique that uses the Raman effect, and is used to identify molecules via the measurement of the vibration of atoms. Each molecule has its own identification features associated with the groups present within it, and the positions, widths, and intensities of bands can be measured. Raman spectroscopy has several advantages, e.g. (1) it uses the vibrations of atoms, (2) samples can be analysed in the normal environment with no interference from the solvent, (3) it shows a relative enhancement of intensity, and (4) it can analyse samples at the picosecond level.115 Raman spectroscopy is a complementary technique to IR spectroscopy and can detect carbohydrates, proteins, and fats present within a food.116 Interaction of water with food proteins causes a decreased intensity of H–H stretching at 3250 cm−1 and the C–H band at 2938–2942 cm−1,117 due to the interaction between water and food proteins. The –CO–NH– amide or peptide bond can be determined by many sensitive methods.118 Amide III bands have been used to detect the secondary structure of proteins.119 The storage process can modify the structure of carbohydrates, especially the presence of water, which can be detected by using Raman spectroscopy. The –CO–NH amide or peptide bond is used for secondary structure determination. Raman spectroscopy can characterize and quantify the lipid content of foods.120 It detects the degree of unsaturation of cis and trans isomers and alterations of foods, such as isomerization and autoxidation. The quality of olive oil can be determined by low-resolution Raman spectroscopy. The peroxide value can be determined directly using this technique. Soybean, corn, and olive residue quality can also be measured by Raman spectroscopy.121
More recently, Raman spectroscopy has been combined with microscopy to detect the chemical composition of heterogeneous foods.122 This combined technique uses quality and quantitative methods to detect food values. Several organic compounds can be identified from their unique absorption pattern. Microscopic samples can be analysed without changing their properties. Confocal Raman microscopy can determine the chemical composition of wheat.123,124 This technique can also determine the protein content during milling. Alterations of the secondary structure of proteins, conformational changes of puroindolines, and lipid-binding proteins can also be detected using Raman spectroscopy.125
1.3.8 NMR Spectroscopy
NMR spectroscopy uses the cross-talk between the magnetic properties of atoms and compounds and the applied magnetic field. It is used for measurements for soil testing, plant tissues, and food products.126 It can detect the genotype of grapes used to make wine. It also analyses the soil in which the vines are grown. It can monitor the ripening, drying, and adulteration of food materials. NMR spectroscopy provides information on mixtures of metabolites127,128 .1H NMR spectroscopy provides information on the metabolic composition of a sample without further knowledge of its composition.129,130 For liquid samples, preparation steps such as derivatization and centrifugation are not needed. NMR metal profiling has been carried out on grape samples.131 1H NMR spectroscopy is combined with multivariate analysis to determine the biotic and abiotic stress in plants132 and variations associated with genes.133,134 Metabolites in fruit juice, olive oil, wine, tomatoes, and beer were detected using this technique.135–138 Flavonoids are often detected using the NMR technique.17 Soluble sugars, organic acids, and amino acids can be distinguished by their 1H NMR spectra. 1H NMR spectroscopy cannot determine phenolic compounds in a precise manner. Phenols, flavanols, and anthocyanins can be determined in a more precise manner by using 2D NMR spectroscopy.139 1H NMR spectroscopy has also been used to determine the classical p-anisidine value (AV) of oil, and 79 edible oils were checked for their AV values.140 Portable NMR instrumentation is available for food analysis.141,142 The metabolomics of foods can be determined from NMR spectra. Since the spectra contain a considerable amount of information, they are analysed by using Student's t-test and analysis of variance. Overlapping of the spectra sometimes prevents the identification of compounds.
1.3.9 Atomic Emission Spectroscopy
Atomic emission spectroscopy (AES) is used to determine the chemicals and also elements present within foods. Light from a plasma or flame excites the atoms present within the sample, generating photons or light of a particular wavelength. AES can detect trace elements. Most foods include many major and minor elements, and also some trace elements such as arsenic, nickel, silicon, boron, and cobalt, which must be within permitted levels. Cadmium, iron, and sodium are mandatory elements and can be detected by AES. The medicinal properties of plants and the composition of wine have also been investigated using AES.
1.4 Conclusion
Spectroscopic techniques are indispensable methods for determining the quality and composition of foods, both quantitatively and qualitatively. They provide structure–function relationships between proteins, and can be used alone or in combination with other analytical techniques. For some spectroscopic techniques, sample preparation is not needed, which saves time. Spectroscopic techniques can measure the AV value, PV value, and colour to determine the quality of the food. They can rapidly differentiate the variations between different types of wheat and oils and bacterial infection. The lipid, carbohydrate, fat, and water content of foods can be determined easily using spectroscopic techniques.
The author's laboratory is supported by financial assistance from the Department of Biotechnology, Ministry of Science and Technology [SERB/EMR/2017/003054, BT/PR21857/NNT/28/1238/2017, and Odisha DBT 3325/ST (BIO)-02/2017].