http://orcid.org/0000-0002-3942-5095
ABSTRACT
Cereals are important grains that are largely used for food and source of bioactive compounds such as starch, dietary fibre, amino acids, minerals, vitamins, and phenolic compounds. However, the use of cereals in the food industry subjects these cereals to different processes that can alter the phenolic content and cause degradation of their antioxidant properties. Understanding the influence of processing operations such as germination, fermentation, and thermal processing on the phytochemicals of cereals grains is important in retaining the health benefiting properties of these antioxidative compounds in processed food products. Most of the previous investigations in the literature review on cereal grains have shown that food processing operations reduced the antioxidants of the processed foods. However, in the last decade, some articles in literature reported improvement in antioxidants properties of processed cereals grains. In this review, more than 60 articles from indexed scientific journals were reviewed in order to understudy the effect of diverse pre- and post-processing methods on the phenolic and antioxidant status of cereals grains.
Introduction
Cereals and pseudo-cereals constitute a major source of food and nutrients in the world, especially among the populace of developing countries. The nutritional and health benefits of this plant food are subjects of extensive research and epidemiological studies.[Citation1] Regular intake of whole-grain cereals has been inversely linked to the risk of developing chronic diseases such as cancers, cardiovascular disease, type 2 diabetes, and obesity.[Citation2–Citation5] These diseases are results of oxidation of living cells and tissues by radical and radical oxygen species (ROS).[Citation6–Citation8] Health benefits derived from cereals are not attributed solely to any single compound but the combined effects of its dietary fibre, phenolic compounds, and other bioactive components.[Citation2] Extracts of cereals have also been linked to the prevention of chronic diseases associated with oxidative stress such as type 2 diabetes, cardiovascular diseases, parkinson disease, cancer, Alzheimer’s disease, monogolism, and dementia.[Citation9–Citation11]
Flavonoids, tannins, and phenolic acids are major compounds present in cereal grains which have been correlated to their antioxidative properties and invariably health benefits. Phenolic acids have been shown to modulate cellular oxidative status and prevent oxidative damage to biological molecules such as DNA, proteins, and membrane lipids.[Citation12–Citation14] They also act as reducing agents, hydrogen donors, and singlet oxygen quenchers[Citation15,Citation16] and are excellent antioxidants due to a 30–40 dihydroxy group in their B ring and the galloyl ester in the C ring of flavonoids which is an important structure in ion-metal chelation.[Citation3,Citation17–Citation20] In addition to direct antioxidative functions, they also exert effects in the regulation of cell signalling pathways, thus modifying gene regulation and cell redox status.[Citation13,Citation21] The health effects of antioxidants and other phytochemicals from consumption of whole-grain cereals were reviewed by Liu.[Citation22] shows some suggested protection of polyphenol ingestion.
Figure 1. Suggested protection of polyphenol ingestion.
Food-processing operations are primarily focused to improve the palatability of cereals products. Other function of processing includes the inactivation of microorganisms and enzymes and reducing the moisture content so as to improve the shelf-life properties of the product. shows the in vitro antioxidant activity of processed cereals grains. Generally, food-processing procedures are known as one of the major factors on the destruction or changes of natural phytochemicals, which may affect the antioxidant capacity in foods.[Citation23] Recent research has now established that food processing operations have positive effects that improve the quality and health benefits of cereals. This review provides concise information on the effect of thermal, fermentation, and germination food-processing operations on the phenolic antioxidants status of processed grains.
Table 1. In vitro antioxidant activity of processed cereals grains.
Chemistry and occurrence of plant polyphenols
Phenolic compounds are important contributors to the antioxidant properties of cereal grains and are known to play essential roles in the prevention and control of degenerative diseases. [68] Antioxidant activity correlates with the occurrence of phytochemicals including phenolics, flavonoids, and anthocyanins in foods.[Citation24] They are poly-hydroxyl phytochemicals synthesised by plants to provide defence against oxidative stress and invasive pathogens, such as bacteria, viruses, and fungi.[Citation12,Citation25] They are secondary metabolites with a common structure, esterified with glucose and other glycosides or existence as free aglycones. Their preventive effects of oxidative induced disease such as cardiovascular, neurodegenerative diseases and cancer have been deduced from epidemiologic data as well as in vitro and in vivo studies.[Citation16,Citation26–Citation29] Polyphenols are abundant in certain cereal grains and subdivided into three main subclasses: flavonoids, phenolic acids, and stilbenoids (). Polyhydroxylated stilbenes belong to the smaller classes of stilbenoids with resveratrol been the main representative. The most isolated and study of the polyphenols belongs to the subclass of the flavonoids. Flavonoids are compounds consisting of two or more aromatic rings each having one or more phenolic hydroxyl groups connected by a carbon bridge.[Citation20,Citation27,Citation30] Most flavonoids bear the phenyl-benzopyrene structure.[Citation62] Phenolic acids are usually divided into two main groups which include those derived from benzoic acids, containing seven carbon atoms or those comprising nine carbon atoms (cinnamic acid).[Citation31] Gallic acid, chlorogenic acid, caffeic acid, ferulic acid, p-coumaric acid, p-hydroxybenzoic acid, and salicylic acid are flavonoids which have been successfully isolated from plants. shows the structures of some important polyphenol compounds.
Figure 2. Structures of polyphenol compounds. Source: Dai and Mumper.[Citation61]
![Figure 2. Structures of polyphenol compounds. Source: Dai and Mumper.[Citation61]](/cms/asset/deab6e59-c49f-4f39-a235-9e530155c115/ljfp_a_1315130_f0002_b.gif)
Effect of processing on polyphenols
Fermentation
Fermentation has been widely applied in the food, chemical, and pharmaceutical industries in the production, extraction, and modification of bioactive compounds.[Citation32,Citation33] It has enhanced the shelf-life and nutritional and organoleptic properties of cereals.[Citation34] It has also been linked to increase antioxidant activity in legume plants.[Citation33,Citation35] Fermentation is able to improve the bio-conversion of conjugated forms of phenolic compounds into their free forms which improves their health-linked functionality.[Citation35] Examples of food products with improved antioxidant properties after fermentation process are vinegars and wine products. In a study by Seki et al.,[Citation36] vinegar produce from rice vinegar was reported to contain tyrosol and ferulic acid with both exhibiting Lipid peroxyl radicals. Antioxidant properties of cereals are affected by food-grade bacteria fermentation. Likewise, fermentation increases the phenolic, flavonoid content and enhances the antioxidant activities of the following cereals plants; adlay, chestnut, lotus seed, and walnut in a study conducted by Wang et al.[Citation37] The microorganisms used for this fermentation process were Bacillus subtilis and Lactobacillus Plantarum. In another study, the combined effect of yeast fermentation and hydrolytic enzyme treatment (mainly xylanase, β-glucanase, α-amylase, cellulase, and ferulic acid esterase) on wheat bran improves the bio-accessibility of free phenolic acids such as ferulic acid, p-coumaric acid, and sinapic acid at a rate higher than samples subjected to fermentation process alone.[Citation38] Hydrolytic enzymes have also been observed to improve the structural breakdown of bran cell walls which invariably results in improvement of bio-accessibility and bio-availability of phenolic compounds. Baker’s yeast is a typical microbe which has been reported to increase solvent-extractable phenolic compounds in a fermentation process on the wholegrain of rye. In their explanation of the inter-relationship between fermentation and antioxidant properties, Ðordevic´et al.[Citation8] noted that the modification of bioactive compounds by the activities of microbes is a reason for this observation. The same investigators reported that enzymes such as amylases, xylanases, and proteases produced from cereals by the action of microbes during fermentation process result in the modification of grain composition and release of bound phenol prior to extraction thereby resulting in the structural breakdown of cereal cell walls/synthesis of bioactive compounds. Lactobacillus rhamnosus-fermented cereals were likewise observed to result in the production of cereal product with stronger antioxidant capabilities than Saccharomyces cerevisiae-fermented cereals though no explanation was given for this occurrence.
Lee et al.[Citation35] reported the effect of fermentation on the total phenolics, anthocyanin contents, and antioxidative activity of fermented black beans. The same investigator also reported the enhancement of bioactive contents after the fermentation process which was attributed to the liberation of lipophilic aglycones from iso-flavone glucosides such as genistin by the catalytic action of β-glucosidase. Fungi have also been reported to produce different types of enzymes during the fermentation of cereals. Examples of such enzyme synthesis during this process include glycoside hydrolase, cellulose or xylan degrading enzymes, and esterase. These enzymes soften kernel structure, break down cereal cell walls, and release it stored esterified insoluble-bound.[Citation39] These enzymes are able to hydrolyse the β-glucosidic bonds of several phenolic compounds that occur mainly as conjugates with one or more sugar residues linked to hydroxyl groups thereby increasing the concentration of free polyphenols.[Citation40] A presumable mechanism for fermentation-induced enhancement of phenolic and antioxidant activities in cereal grains is a factor of degrading enzymes present in secretes of fermenting microbes, which results in structural breakdown of cell wall matrix and increased accessibility of bound/conjugated phenolic compounds to enzymatic attack.[Citation8] Likewise, the synthesis and enzymatic transformation of bioactive compounds occurring during fermentation processes[Citation41,Citation42] are further explanation proposed for this observation. In summary, the effects of fermentation on phenolic compounds are a factor of types of grains,[Citation8] microorganism species,[Citation8,Citation43,Citation44] and fermentation conditions, particularly temperature, pH, and time.[Citation6,Citation44] Ðordevic´ et al.[Citation8] however observed that additional non-phenolic organic substances produced during fermentation process react with Folin–Ciocalteu reagent organic in the test for polyphenols which give rise to elevated apparent phenolic concentrations thereby compromising the true antioxidant capacities of fermented cereal product.[Citation45]
Thermal processing
Thermal processing methods such as cooking, boiling, steaming, microwave heating, and autoclaving have been extensively applied in the improvement of the taste, palatability, colour, stability, and shelf-life of cereals grain. This process has been reported to have mixed effects on the phenolic antioxidants of processed cereals. While some processing techniques previously stated have been reported to improve the extractability of phenolic compounds from cereal grains, other processes have been reported to have a degrading effect on phenolic compounds. For example, Dona[Citation46] reported that autoclaving of wheat bran was observed to increase total phenolic content in water-soluble extracts by 50% compared to untreated counterpart. The investigators attribute this effect to the release of bound phenolic compounds as a result of the breakdown of cell wall structure and the partial hydrolysis of fibre polysaccharides resulting from high temperature, pressure, and rapid depressurisation. This report is in agreement on an earlier study by Bryngelsson et al.[Citation47] on the effects of commercial processing on the levels of antioxidants in oats. These investigators observe increased p-coumaric and ferulic acids which was attributed to breakage in the linkage bond between hydroxycinnamic acids and fibre components. Degradation of conjugated polyphenolics to simple phenolics is also attributed to post-harvest treatment and heat stress.[Citation48] In contrary findings, no obvious change in total phenolic content was observed in either dark or white buckwheat flour when roasting was done at 200°C for 10 min in a study by Sensoy et al.[Citation49] In similar findings, thermal processing of Tartary buckwheat flour results in significant decline in total phenolics, total flavonoids, and antioxidant activity.[Citation50] Studies on the effect of hydrothermal processing by Chandrasekara et al.[Citation8] reveal that phenolic compounds in five species of millet grains (Kodo, Foxtail, Prosoo, Little, Pearl) were stable against the treatment. In contrast to the report by Min et al.[Citation49], it was observed that hydrothermal processing significantly decreased free phenolic contents in rice cultivars by 16%–91%. These authors attribute this observation to the types of the grains, the nature and location of phenolic compounds in these grains, severity and duration of heat treatment. Likewise, Ti et al.[Citation52] observed that cooking decreases free phenolic, bound phenolic, flavonoid contents, and antioxidant activities of brown and polished rice. This was attributed to the breakdown of phenolic molecules by thermal treatment which promotes the decarboxylation and polymerisation of free phenolic acids. In another study by Siah et al.,[Citation53] it was observed that though the thermal processing of Faba beans reduces its phenolic contents significantly, considerable amount of phenolic compounds with potent antioxidant activities was contained in their broth. In explaining the phenomenon behind the thermal-induced reduction in antioxidant activities, Massaretto et al.[Citation54] stated that (i) depolymerisation of higher oligomeric and polymeric structures into dimers and trimers, (ii) concomitant polymerisation reactions, and (iii) formation of strong complexes of soluble phenolic with macromolecules of the food matrix, which might have reduced their polyphenol solubility,[Citation54] might be responsible for this occurrence. To achieve a more accurate estimation of phenols and antioxidant capacities of cereal products, Massaretto et al.[Citation54] suggested that free and bound phenolic components be taken into consideration during its analysis.
Germination
Several studies indicate that germination could improve the phenolic and antioxidant properties of post-processed cereals. In a study by Chen et al.,[Citation55] it was reported that free, bound, and total phenolic contents of canary seeds significantly (p < 0.05) increased after germination of the seeds compared to un-germinated seeds. Bound ferulic acid was observed to also show the highest improvement in phenolic content which was attributed to phenolic biosynthesis and hydrolysis of polyphenolic compounds bound to cell walls. In another study, rye grains germinated at 5, 10, and 25°C for 6 days all revealed an increase in methanol-extractable phenolic compounds,[Citation56] and germination at 25°C resulted in the highest increase. It was suggested that germination induced the synthesis or activation of a range of hydrolytic enzymes in the germinated grain, which results in structural modification and synthesis of new compounds with high bioactivity or nutritional value. Germination has been suggested to have the potential of increasing solvent-extractable phenolic compounds.[Citation57,Citation58]
Germination has also been reported to increase the phenolic content as well as ferric reducing antioxidant activity of rice flours.[Citation30] The investigator attributes this effect to the biosynthesis of phenolic compounds caused by enzyme hydrolysis during the germination process. In a study on germinated barnyard millet seeds, it was suggested that cell wall-degrading enzymes active during germination process result in the modification of cell wall of the seeds thereby releasing bound phenols and improving its antioxidant properties in the process. Sharma et al.[Citation59] likewise observed increase in the concentration of phenolic compounds in germinated barnyard millet seeds and suggested that cell wall-degrading enzymes active during the germination process result in the modification of cell wall structure. Contrary to these findings, Abderrahim et al.[Citation60] noted that total antioxidant capacity of canihua germinated at 96 h of germination does not correlate with extractable flavonoids, and it was suggested that other antioxidant compounds, among them phenolic compounds, could have been induced during the germinating processes. Therefore, it is imperative in understudying the mechanisms required for the improvement of the antioxidant properties of germinated cereals to better understand this process .
Conclusion
The major sources of antioxidant in cereal grains are phenolic and flavonoid compounds. They have remained a stable component of the diet of the populace of both the developing and the developed countries for centuries. Certain studies have however shown that cereal processing methods like thermal, fermentation, and germination improve the antioxidant properties of cereals. Of the processing techniques discussed in this study, germination was reported to be a simple and an inexpensive processing technique which improves the availability of nutrients present in the bran of cereals grains while also reducing various anti-nutritional factors. From various reports, it cannot be stated categorically that processed cereals have better phenolic and antioxidant properties when compared to their unprocessed counterpart. Further study was however suggested by a few investigators on the need of understanding the biochemical mechanism by which this processing method brings about its effect on the antioxidant properties of cereal grains.
Related Research Data
References
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