Genetic diversity present in cereals is an excellent source of a variety of bioactive chemicals. The polyphenolic molecules in cereal grains include phenolic acids (ferulic acid, ferulic, vanillic, p-coumaric, caffeic, protocatechuic, p-hydroxybenzoic, genistic, chlorogenic, and syringic acids), flavonoids (flavones, flavonols, isoflavones, flavanols, flavanones, anthocyanins), and avenanthramides. Given that some of these compounds are water-soluble and others are insoluble antioxidants, strategies for increasing their bioavailability must be investigated (Jones et al., 2019). Cinnamic acid and its derivatives, especially ferulic acid, are the main phenolic acids found in cereal grains (Zhou et al., 2005). Quercetin glycosides, flavones, and flavonols are the major flavonoids found in cereal grains; however, anthocyanins contribute substantially to the overall flavonoid content of dark-colored grains such as purple maize and purple rice (Francavilla and Joye, 2020). Buckwheat grains have also been shown to contain catechins. Sorghum is one of the few kinds of cereal that contains large amounts of oligomeric and polymeric flavonoids known as condensed tannins (Espitia-Hernández et al., 2020). Purple and red pigmentation in cereals is linked to larger amounts of polyphenols covering a wide array of bioactive compounds such as anthocyanins, proanthocyanidins, flavonoids, phenolic acids, and lignins (Lachman et al., 2018; Mbanjo et al., 2020; Loskutov and Khlestkina, 2021).

The majority of phenolic acids exist in conjugated and bound forms, mostly in the bran (Kim et al., 2006; Chandrasekara and Shahidi, 2010). Lignans, a class of polyphenols with significant health benefits, especially as precursors to mammalian lignans, are plentiful in cereal grains. Whole grain cereals and their products are well-known for containing more protein, minerals, vitamins, dietary fiber, and phytochemicals than their polished counterparts. In cereals, a large proportion of phenolic compounds are concentrated in the bran, where they are typically found as soluble conjugates or covalently linked to sugar moieties or cell wall structural components (Mnich et al., 2020). By cross-linking to lignin, extensins, and glucuronoarabinoxylan components, bound phenolic acids are usually implicated in the structure of the cell wall. In white, red, and black rice grains, ferulic, p-coumaric, syringic, and isoferulic acids are found in bound forms (Shao et al., 2014). Further, they may be physically trapped inside cereal matrices and intact cells (Acosta-Estrada et al., 2014; Wang et al., 2014a; Wang Z. et al., 2020). Due to the fact that these chemicals are often found bonded, organic solvents cannot usually remove them. When coupled with increased dietary fiber content in whole-grain foods, increased levels of bioactive phytochemicals such as phenolic compounds, sterols, tocols, and lignans have been found to protect against non-communicable illnesses (Derrien and Veiga, 2017). Given that the bulk of phenolics are contained in the bran or seed coat of grains, it is more beneficial to consume whole grains and intact seed-based meals. Numerous in vivo and in vitro studies on the health benefits of cereals have clearly shown that diets high in whole grain cereals and cereal-based products contribute to the prevention of a range of chronic diseases with significant public health implications (Chen et al., 2019; Francavilla and Joye, 2020).

Numerous variables may affect the content and possible health benefits of meals along the value chain. The concentration of these bioactives in cereals, food components, and dietary supplements, all of which can have an effect on human health and wellbeing, may be influenced by genetics, growing and storage conditions, post-harvest treatments, food formulation, and processing. Postharvest processing of cereal grains is critical to making them preferentially edible and biofunctional and impart distinctive features for better cooking and organoleptic aspects (Oghbaei and Prakash, 2016). These processes include drying, grinding, and storing, contributing to the product’s shelf life, stability, and palatability. Numerous cooking techniques, particularly thermal processing techniques such as microwave heating, roasting, frying, steaming, autoclaving, boiling, extrusion, and baking, have been extensively used to improve the palatability and taste of cereal grains and their products. Several pre-and post-harvest methods have been devised to liberate bound phenolics from the indigestible matrix and boost the free-soluble phenolics content (Wang et al., 2014b; Călinoiu and Vodnar, 2019). The effects of various pretreatment procedures on specific phenolic compounds are shown in Table 1, while Supplementary Table 1 summarizes the change in the antioxidant capacity of the total phenolic content. Understanding the effect of various processing methods on phenolic compounds in grain is critical for maintaining or enhancing these compounds’ health-promoting qualities in processed cereal products (Kadiri, 2017).

Most of the processing approaches alter the polyphenol composition of cereals and their products and improve the availability and digestibility of phenolics due to the chemical or physical modifications that occur during processing (Oghbaei and Prakash, 2016; Ribas-Agustí et al., 2017). In comparison, several of these post-harvest processing methods have been shown to degrade the natural phenolic components in the end products, resulting in decreased bioavailability during in vivo digestion (Ribas-Agustí et al., 2017). Hence in this review article, we detail the chemical diversity of bioactives identified in cereal crops and comprehensively review the implications of (a) bioprocessing methods such as germination, enzymatic treatment, and fermentation, and (b) novel technological processing methods such as ultrasonication, parboiling, micro- and nano-encapsulation, infrared, and pulse electric field methods for improving the stability of phenolic compounds in functional foods and for specific delivery of cereal phenolic compounds with increased bioavailability (Figure 1). To collect as many relevant citations as feasible, a broad variety of scientific databases were searched. Only data from literature published until 31st August 2021 obtained from the following databases (Google Scholar, PubMed, SciELO, and Scopus) were included in this review. To this end, the following keywords and their combinations were used: cereal, grains, wheat, rice, corn, barley, oat, rye, millet, sorghum, phenolics, phenolic acids, flavonoids, anthocyanin, processing, post-harvest, thermal, extrusion, nixtmalization, microwave, ultrasound, microencapsulation, micronization, microfluidization, enzyme, fermentation, baking, pulsed electric field, product, development, cardiovascular disease, obesity, inflammation, diabetes, or glucose.

Recent years have seen the discovery of novel functions for phenolic chemicals, especially flavonoids. Indigenous farmers have preserved the medicinal characteristics of grain types, landraces, cultivars, and wild forebearers for their therapeutic potential. These traits are now attracting considerable scientific attention. For example, Njavara, a Kerala medicinal rice, is widely utilized in Ayurveda to treat neurological disorders and regeneration. Phytochemical assessments and spectroscopic examinations of the diethyl ether extract of methanolic concentrate of Njavara rice bran resulted in the identification of two rare flavonolignanstricins, namely 4′-O-(erythro-β guaiacylglyceryl) ether and tricin 4-O-(threo-β-guaiacylglyceryl) ether (Rao et al., 2010; Jung et al., 2014). It is now recognized that flavonoids may influence cell signaling pathways at physiological doses much below those needed to impact cellular antioxidant activity (Koch et al., 2014). Through their regulation of cell signaling pathways, flavonoids may help prevent cancer by increasing phase II detoxifying enzyme activity, decreasing proliferation, and triggering apoptosis. When human participants were fed pigmented black rice high in flavonoids, serum polyphenols and flavonoids levels were shown to be enhanced compared to baseline values (Vitalini et al., 2020). Bioactive compounds found in cereals possess antioxidative properties, which in turn counter various diseases such as heart disorders, cancer, aging, and inflammatory diseases (Espitia-Hernández et al., 2020). According to Ghasemzadeh et al. (2018), the free fraction of black rice bran had a higher IC50 value than the free fractions of red and white rice bran. Several phytochemicals found in pigmented rice prevent certain types of cancer (Henderson et al., 2012; Forster et al., 2013; Chatthongpisut et al., 2015). Through activation of effector genes, the p53 protein, and caspase enzymes, the proanthocyanidin-rich fraction isolated from red rice germ and bran extract may suppress cell growth and cause death in HepG2 cells (Wongjaikam et al., 2014; Upanan et al., 2019). This may provide a novel target for cancer treatments.

Shan et al. (2020) demonstrated that the bound fraction of foxtail millet bran regulates the expression of miRNAs to exert anti-multidrug resistance against colorectal cancer. Likewise, the enzymatic extraction of rice bran has anti-cancer potentiality against leukemic cell lines (Revilla et al., 2013). Additionally, blue corn and barley reduced the frequency of colon malignancies caused by 1,2-dimethylhydrazine and mammary carcinogenesis caused by N-methyl-N-nitrosourea, respectively (Reynoso-Camacho et al., 2015; Kubatka et al., 2016). Long-term consumption of an anthocyanin-rich extract from black rice on a continuous basis may aid in the stabilization of plaques in elderly apoE-deficient mice (Xia et al., 2006). Furthermore, black rice extracts act as antioxidants by suppressing cellular reactive oxygen species (ROS) and malondialdehyde (Lee et al., 2014).

Cyanidin-3-glucoside, the main anthocyanin compound of black rice, suppresses the occurrence of high-fat-diet-induced obesity (Kongthitilerd et al., 2020), increases hyperglycemia and insulin sensitivity via AMP-activated protein kinase in type 2 diabetic mice (Takikawa et al., 2010). The black rice pigment fraction significantly decreased plasma levels of soluble vascular cell adhesion molecule-1 (sVCAM-1), soluble CD40 ligand (sCD40L), and high sensitivity C-reactive protein (hs-CRP) in patients with coronary heart disease. In terms of the antidiabetic property, oryzanol has been reported to directly correlate with insulin sensitivity and adiponectin, indicating that it plays a critical role in type 2 diabetes (Ohara et al., 2009). Son et al. (2011) showed that oryzanol could control insulin production to maintain glucose homeostasis, correct liver enzymes’ activity, and decrease the risk of hyperglycemia associated with a high-fat diet. A diet intervention supplemented with 0.160% oryzanol and 0.05% ferulic acid alleviated hyperglycemia developed in a group of rats for 13 weeks utilizing a high-fat, high-fructose diet (HFFD) (Wang et al., 2015). Its nutritional quality and medicinal values have made rice unique among cereals. Therefore, some traditional rice varieties and other sources of cereal grains can now be portrayed as functional foods when we deploy novel processing technologies to retain higher antioxidant and anti-cancer properties in the final food products.

Interest in nutraceuticals and functional foods continues to increase due to growing public interest and consumer demand (AlAli et al., 2021). As a response, food scientists, dietitians, and food industrialists are collaborating to create new functional food products. Pigmented cereal grains such as blue and purple wheat, red and black rice, purple, blue, red, and pink maize, black, blue, and purple barley, and black, purple, red, and lemon-yellow sorghum, all of which contain a variety of functional bioactive components such as polyphenols, anthocyanins, flavonoids contributing to higher antioxidant activity, that can be used to help prevent chronic diseases such as cancer, type 2 diabetes, and hypertension. Colored cereal grains serve as an attractive supplement or whole grain ingredient in the production of functional baked goods such as pan bread, flat bread, buns, rusk, cookies, extruded snacks, breakfast cereals such as pigmented cereal flakes, pigmented popped cereals, snack bars, non-alcoholic beverages, and porridges, as well as in the enhancement and retrofitting of traditional food products. Therefore, the positive health benefits of whole pigmented cereal grains, their bran fraction, phytochemicals, and antioxidant activity as described in this review article may be leveraged by the food industry to create novel nutraceutical cereal-based foods.

Pigmented barley (15.3–132.3 mg Cyn-3-OGlu equiv/kg DM; Suriano et al., 2019), corn (389–7,800 mg Cyn-3-OGlu equiv/kg DM; Suriano et al., 2021), rice (79.5–473.7 mg Cyn-3-OGlu equiv/kg DM; Chen et al., 2012), sorghum (8.62–67.97 mg Cyn-3-OGlu equiv/kg DM; Dykes et al., 2009) and wheat (14.36–27.76 mg Cyn-3-OGlu equiv/kg DM; (Wang X. et al., 2020) contain a relatively higher concentration of anthocyanins and other nutrients than their non-pigmented counterpart. This has resulted in the conversion of colored cereals into a variety of functional foods with increased nutritional value and health advantages. Figure 2 illustrates a variety of food products derived from pigmented cereal grains. Other potential functional foods are shown in Table 2.

Fermented cereal-based meals and beverages have a higher nutritional value owing to an increase in phytochemicals and a reduction in anti-nutrients (Blandino et al., 2003; Das et al., 2011). As shown in Table 3, cereal-based products maintain a high level of phenolics. Recent research has shown the potential for fermented pigmented cereals to increase their nutritional value by boosting phytochemicals such as phenolic compounds and reducing their antinutrient content. Chicha, a traditional Peruvian fermented beverage made from purple maize, has been found to have enhanced phenolics, anthocyanins, and antioxidant capacity while decreasing starch digestibility (Vargas-Yana et al., 2020). A nearly 10-fold rise in gallic acid, catechin, vanillin, and resorcinol was found in pigmented barley (Bangar et al., 2021). The malting process of pigmented rice increases the phenolic acids and GABA (Santos et al., 2020). When several types of fermented rice were examined, black rice had the highest polyphenolic content and antioxidant activity, followed by red and unpigmented rice (Cai et al., 2019). Apart from its nutritional benefits, fermented black rice has good sensory qualities (Jiang et al., 2020), enhancing the product’s use when transformed into fermented snacks and beverages (dela Rosa and Medina, 2021). Pigmented cereals have also been considered in sourdough bread due to the abundance of its phytochemicals and revealed effective anti-inflammatory activities (Luti et al., 2021). Notably, fermentation conditions must be adjusted to get the highest polyphenolic content and antioxidant activity possible. For example, metabolomic analyses of fermented black rice have shown a reduction in phenolic chemicals after 60 h, which may be a result of molecular breakdown and bacterial consumption (Mu et al., 2019). While fermented colored barley, maize, and rice have been widely researched for their nutritional benefits, other pigmented cereals have received less attention. Metabolomics study of fermented pigmented grains has the potential to provide new light on the phenolic diversity and its interaction with microorganisms. Natural pigments such as anthocyanins and proanthocyanidins found in pigmented grains have been used to increase the nutritional content and prolong the shelf life of food items (Chatham et al., 2020). Due to the low price attached to broken-colored cereal, grains may be processed to create food additives (Sapna and Jayadeep, 2020). The encapsulation of phenolic compounds is used as a co-ingredient in bread goods (Papillo et al., 2018; Thiranusornkij et al., 2019), food supplements (Norkaew et al., 2019), beverage hydrogels (Guo et al., 2018), and probiotic products.

Physical mixing of cereal grains rich in phenolic compounds with polymers and food matrix resulted in various product additives and biopolymers (Tiozon et al., 2021). The addition of red sorghum to pasta enhanced the amount of free phenolic acids and resistant starch (Khan et al., 2013). Meanwhile, the inclusion of black rice extracts extends the shelf life of muffins and improves their eating quality (Croitoru et al., 2018). Combining pigmented grain starch has a twofold purpose: starch interacts with other polymers to increase their hydrophilicity, and its phenolic chemicals provide antioxidant qualities. When black rice extracts are combined with a chitosan-starch film, the antioxidant and light barrier characteristics are enhanced (Yong et al., 2019). Additionally, purple corn extracts containing chitosan and silver nanoparticles improved the film’s pH sensitivity and antibacterial activity (Qin et al., 2019). The resultant nanocomposite film based on pigmented grains has been shown to monitor the deterioration of pork (Yong et al., 2019) and seafood items (Ge et al., 2020). The change in pasting, thermal, and rheological characteristics is due to the complexation of phenolic chemicals with starch and other polymers (Zhu, 2015).

Extruded pigmented cereals have gained significant attention due to their nutritive value and ease of consumption. By and large, the overall phenolic content is decreased, but their bioavailability is increased (Hole et al., 2013). For instance, extruded puffed rice increased extractable phenolic acids, namely gallic acid and protocatechuic acid, but significantly decreased cyanidin-3-glucoside (Bagchi et al., 2021). In contrast, extruded blue corn showed cyanidin-3-glucoside stability and retention, which may be related to previous nixtamalization (Escalante-Aburto et al., 2013). Enzymes may be used to enhance the retention of phenolics in extruded goods (Zeng et al., 2018). Additionally, as shown with extruded sorghum, controlling factors such as feed moisture and extrusion zone temperature may avoid additional phenolic loss (Ortiz-Cruz et al., 2020). Other components can be added in cereals like vegetable (Xu et al., 2021) and fruit (Bhat et al., 2019) extracts. Extrusion-based three-dimensional technology has emerged as a new mode to diversify products without sacrificing their nutritional or sensory qualities (Prabha et al., 2021).

In general, promoting wholegrain cereal grain consumption over milled endosperm to prevent non-communicable diseases is widely accepted. However, owing to rancidity, poor shelf-life, and reduced palatability, different processing methods are being used to produce milled goods for broader consumption. As a consequence, we lose the nutritional density found in grains, such as rice bran. The enormous genetic diversity that exists for enriched phenolic chemicals and flavonoids in cereals as purple, variable purple, and red cereal grains has provided new possibilities to market them as future functional food. However, it is critical to evaluate the final concentrations of these free phenolic compounds in pigmented food items after cooking/baking to guarantee bioavailability of these phytochemicals and food safety concerns to remove aflatoxins mycotoxins, and pesticides to translate human health benefits. Identifying the optimal post-harvest processing techniques to maintain greater polyphenolic compounds via traditional cooking and baking processes should be encouraged to preserve the human health advantages. More than 150 million tons of wheat bran and 76 million tons of rice bran nutritional material are produced as a byproduct of milling and are mostly discarded. It can be channeled into technological advancements in processing technologies such as micronization, microfluidization, ultrasound processing, nixtamalization, pulsed electric fields, and micro/nano-encapsulation to extract bioactives and phenolic compounds and produce various functional byproducts with enormous human health benefits.

Furthermore, bioprocessing techniques to produce different functional food products from germination sprouts, fermentation and enzymatic treatments of bran and pigmented grains to enhance the bioaccessibility of phenolic compounds in producing distinct functional byproducts may be used. Implementing holistic strategies to identify donor lines with enriched bioactives using untargeted metabolite profiling to identify novel phenolic compounds with higher antioxidant potential and deploying state-of-the-art processing applications to produce final food products/by-products with higher bioactives will be beneficial in creating novel opportunities to position cereals as staple foods to meet human nutrition needs. Long-term clinical studies for the whole range of functional food items produced from cereals are required to determine long-term benefits. Additional research on the gut-microbiome health advantages of colored grains is needed. We would like to highlight that the consumption of pigmented cereals and their derived functional foods as staples by definition means that improving staples-based diets can deliver maximum benefits, particularly to the poorer sections of society, while also satisfying the urban consumers’ health concerns.

NS conceptualized the review article and edited the article with inputs from DK, RT, KS, HI, PR, AK, and AR. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.