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Review

Modulation of Cereal Biochemistry via Solid-State Fermentation: A Fruitful Way for Nutritional Improvement

by
Avneet Kaur
1,* and
Sukhvinder Singh Purewal
2,*
1
Department of Chemistry, University Institute of Sciences, Chandigarh University, Gharuan, Mohali 140413, Punjab, India
2
University Centre for Research & Development (UCRD), Chandigarh University, Gharuan, Mohali 140413, Punjab, India
*
Authors to whom correspondence should be addressed.
Fermentation 2023, 9(9), 817; https://doi.org/10.3390/fermentation9090817
Submission received: 4 June 2023 / Revised: 26 June 2023 / Accepted: 29 June 2023 / Published: 7 September 2023
(This article belongs to the Special Issue Health and Bioactive Compounds of Fermented Foods and By-Products)
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Abstract

:
Cereal grains play a vital role in a dietary chart by providing a required number of macronutrients and micronutrients along with health-benefiting bioactive components. Cereal grains, despite being a good source of bioactive compounds, are not able to provide the full dose of bioactive components to consumers. The biochemistry of cereal grains restricts the release of certain dietary components; therefore, a method like solid-state fermentation could be utilized to modulate the chemistry of bioactive components present in cereals. Once modulated, these components can easily be recovered using an optimized extraction medium and other conditions. Fermented grains are better than unfermented ones as they possess a higher amount of certain dietary and bioactive components along with better quality attributes and shelflife. Fermented-cereal-based products can be promoted because of their health-benefiting nature and hidden industrial potential.

1. Introduction

Cereal grains have played an important role in improving the sustainability of various sectors such as food, pharmacy and biotech industries. A major portion of the world’s population relies on cereal grains to meet their nutritional requirements. Due to their easy cultivation, availability, cost-effective nature and health benefits, people prefer to add them into their routine dietary chart [1,2]. From a nutritional point of view, cereal grains are naturally enriched with health-benefiting macro- as well as micronutrients [3,4,5]. In recent years, different processing methods such as soaking, germination, roasting, drying, steam-cooking and fermentation have been widely used for the purposes of improving or modulating specific nutrients/bioactive metabolites and decreasing anti-nutrients [6,7,8,9,10]. Among these processing methods, fermentation is most often used for these purposes as the process helps to modulate the nutritional profile of substrate and achieve desirable results within a short span of time. Various scientific reports on fungal fermentation demonstrate a positive effect on the bioactive metabolites of cereal grains [11,12,13,14,15].
Bioactive metabolites are mixtures of complex derivatives with a broad spectrum of biological activities ranging from anti-aging, antioxidation and anti-cancerous effects to DNA damage protection [16,17,18]. These metabolites are present in natural resources, such as vegetables, fruits [19], cereal grains and agro-industrial waste material, in complex conjunction with macromolecules. Metabolites assisted with macromolecules need to be present in free forms so as to achieve maximum benefits from them [20]. Therefore, for the conversion of bioactive metabolites from their bound complex forms to free forms, solid-state fermentation (SSF) could be used as a fruitful method of processing, as during the SSF, activity of enzymes such as xylanase, pectinase, β-glucosidase and amylase help in boosting the rate of phenolic enhancement [21,22,23,24]. The increment rate in phenolic compounds depends on both the nutritional composition of the raw material (substrate) used for fermentation purposes and the type of microbial strain being used on the substrate [25,26,27]. Furthermore, the enhancement of bioactive metabolites also depends on the type of extraction solvent and concentration of the extraction medium, the extraction time and the temperature. The efficacy of the extraction medium for the maximum leaching of bioactive compounds from fermented substrates could be optimized using response surface methodology [28,29,30]. This review paper describes in detail the effect of fermentation on the nutritional composition of different grains along with the fermented products.

2. The Effect of Solid-State Fermentation (SSF) on the Nutritional Profile of Cereals

The concentration of phenolic compounds and their derivatives in cereal grains is controlled through a complex mechanism. Research in different sectors has focused keenly on converting phenolic compounds from their bound form to a free form [31]. These bioactive phenolic compounds can be leached out, preferably via food-grade fungus and bacteria, from cereal grains [20]. In the SSF process, different hydrolytic enzymes could be produced directly from a solid substrate and be simultaneously utilized to release the phenolics. Previously, it was reported that fungal β-glucosidase and α-amylase are involved in phenolic mobilization during solid-state growth [7,9,14,15]. Several strategies were adapted to enrich the cereal grains, fruit peels and agro-industrial waste with antioxidant potential under controlled conditions [30,32,33,34,35,36]. The most important factor for the success of the fermentation process is the selection of suitable substrates for microorganisms to be incorporated into as starter culture. A detailed list of starter strains along with suitable substrates is presented in Table 1. The effect of SSF on the TPC value of different substrates is reported in Table 2. Furthermore, the growth of specific microbial consortia on steam-sterilized substrates is solely dependent on the type of nutrients that any particular substrate may possess. This also determines the market value and quality attributes of the final products prepared after the fermentation process. Nowadays, a variety of cereal grains, pseudo-cereals, pulses and legumes are used as substrates during fermentation, and different strains of M.Os (microorganisms) are employed as starter cultures on them [37,38]. Filamentous fungi are the best candidate for the improvement of bioactive compounds from cereal grains and agro-industrial waste [20]. The thermo-stable enzymes produced during the fermentation process help in leaching out the bound phenolic compounds into their free form [15]. A number of optimization techniques are currently being adopted by researchers for better extraction of bioactive compounds from different natural resources. Optimization is the major concern in order to manage the extraction process for the large-scale production of these bioactive compounds [39]. Each microorganism shows the maximum activity at the optimum temperature required to produce enzymes, which plays a significant role in leaching out these bioactive compounds [20]. Statistical software is in greater demand for designing optimization strategies for extraction procedures. Software like RSM (response surface methodology) is commercially used to optimize certain factors like fermentation time, solvent concentration, extraction temperature and extraction time.

2.1. The Effect of SSF on Wheat Grain

Different fungal strains such as Aspergillus niger, Aspergillus sojae, Aspergillus flavus, Rhizopus oryzae, Mucor racemosus and Aspergillus awamori nakazawa have been used as starter culture on wheat as well as wheat bran. Sandhu et al. [64] used Aspergillus awamori nakazawa on six different cultivars of wheat. Changes in the value of phenolic compounds during fermentation were 974–2056 µg GAE/g (gallic acid equivalent/g) (wheat cultivar PBW-343), 1155–2389 µg GAE/g (wheat cultivar PBW-590), 1290–3491 µg GAE/g (wheat cultivar WH-283), 1116–2497 µg GAE/g (wheat cultivar WH-1080), 1069–2289 µg GAE/g (wheat cultivar WH-896) and 1399–3598 µg GAE/g (wheat cultivar WHD-943), respectively. Demir and Tari [66] used Aspergillus sojae on wheat bran for the purpose of determining the effect on polygalacturonase production. The outcome of their investigation was a 3.9-fold enhancement in production and a 7.3-fold increment in the activity of polygalacturonase. Bhanja et al. [15] studied the effect of fermentation assisted by Aspergillus strains (Aspergillus awamori and Aspergillus oryzae) on wheat phenolics. They observed that starter culture leads to changes in the phenolic profile of wheat in different ways. As in the case of Aspergillus oryzae-fermented wheat, the increment in phenolics lasted till the 4th day (7.23–158.9 µmol GAE/g); however, the similar trend of an increase in phenolics lasted till the 5th day of the experiment (wheat fermentation with Aspergillus awamori nakazawa) and increased from 7.23 to 124.2 µmol GAE/g. These changes confirm that every fungal strain actsin a different manner on similar substrates. Dey and Kuhad [67] observed the effect of Aspergillus oryzae-assisted fermentation on the bioactive profile of wheat. They found a positive effect of fermentation on the phenolic compounds of wheat, which increased from 0.81 to 11.61 mg GAE/g. As per their results, there were some changes observed for both the quality and quantity of phenolic acids. There was no gallic acid and 4-hydroxy benzoic acid observed in Aspergillus oryzae-fermented wheat. In a similar manner, there was no protocatechuic acid in Rhizopus oryzae-fermented wheat. However, the concerned phenolic acids were present in the unfermented wheat. On the one hand, a significant improvement in the concentration of phenolic acids was observed, whereas on the other hand, some phenolic acids which were present in the unfermented wheat disappeared after the fermentation. Zhao et al. [68] studied the effect of lactic acid bacteria and yeast-assisted fermentation on the nutritional composition of wheat bran. During their study, they observed significant changes in the bioactive and dietary components of wheat bran. As compared to non-fermented bran, the changes in fermented bran during the fermentation process were 7.7–37.4 (water extractable arabinoxylan), 5.9–8.5 mg/g (alkylresorcinol), 52.36–55.5 g/100g (total dietary fiber), 44–45.5 g/100g (insoluble dietary fiber) and 4.4–8.36 g/100g (soluble dietary fiber), respectively.

2.2. The Effect of SSF on Barley Grain

Barley comes under one of the industrially important cereals which are being widely used by food processing industries for a variety of purposes. Industries process the grains to achieve desirable features in the final product. Processing methods affect the structural, functional and bioactive profile of the substrate (raw material). To meet consumer demands, industrial sectors choose the appropriate processing methods. Food-processing techniques, especially SSF, contribute towards 1/3rd of total consumption of food worldwide. The efficiency of SSF has attracted attention for its use in the processing of food materials.
Sandhu and Punia [65] used Aspergillus awamori Nakazawa-assisted SSF for the purpose of achieving enhancement of the total phenolic content (TPC) of barley. During the study, six different barley cultivars, namely BH 393, BH 932, BH 902, BH 885, DWR 52 and PL 172, were used as substrates for SSF. The rate of change in the TPC value of the barley varies with the nutritional composition of the barley cultivars used in the study. The variation in TPC value was 3.25–4.59 mg GAE/g for BH 393, followed by 3.05–4.27 mg GAE/g for BH 932, 3.76–5.17 mg GAE/g for BH 902, 3.92–5.40 mg GAE/g for BH 885, 2.92–4.08 mg GAE/g for DWR 52 and 2.89–5.15 mg GAE/g for PL 172, respectively. Nelofer et al. [69] used Rhizopus oligosporus on barley for the modulation of its bioactive profile. A significant increase in protein content (10.2–16.8%) and decrease in fat content (2.1–1.6%) were observed. Bartkiene et al. [70] utilized the potential of a Pediococcus acidilactici strain for the improvement in the nutritional composition of barley industry by-products. Due to the effect of fermentation, the protein content changed from 16.7 to 13.1%, 4.2 to 2.5% (crude fat) and 12.3 to 16.85% (crude fiber), respectively. Changes in specific bioactive metabolites during the SSF were as follows: 7.85–9.43 µg/g (vanillic acid), followed by p-coumaric acid (83.1–32.02 µg/g), ferulic acid (547.4–339.1 µg/g), sinapic acid (12.7–7.68 µg/g) and p-hydroxybenzoic acid (4.26–27.03 µg/g), respectively. Lee and Ra [71] studied the effect of SSF on the production of enzymes and β-glucan using hulled barley as a substrate. Optimal pretreatment conditions of hulled barley by SSF were studied in terms of maximum production of fungal biomass, amylase, protease and β-glucan, which were 1.26 mg/g (96 h), 31,310.34 U/g (24 h), 2614.95 U/g (144 h) and 14.6% (w/w) (72 h), respectively, at pretreatment condition. Zhai et al. [72] studied the effect of SSF on water-soluble dietary fibers, protein and β-glucan content. They observed that after SSF, the proteins, water-soluble dietary fibers, and β-glucan contents of both black barley and yellow barley samples were about, respectively, 1.3 times, 4.6 times and 1.2 times higher than those in raw materials.

2.3. The Effect of SSF on Oat Grain

Oats (Avena sativa) are considered a minor cereal crop, which is initially utilized as a fodder crop. Scientific reports have explored the hidden potential of oat grains in terms of health benefits [1]. Storage proteins of oats might have health-benefiting bioactive metabolites which may help to eradicate the symptoms of diseases like cancer, diabetes and other issues related to ageing [73]. Several reports demonstrate that oat grains could be used as a raw material for solid-state fermentation. The nutritional profile of oats demonstrates that the grains have sufficient supporting nutrients for the growth of a fermentation starter culture.
Bei et al. [52] used Monascus anka as a fermentation starter culture on oats with the aim to modulate their bioactive profile. The fermentation was performed for a period of 14 days. SSF significantly modulates the concentration of specific metabolites present in oat grains. The modulation in specific compounds were gallic acid (2.63–8.45 mg/kg), chlorogenic acid (0–186.59 mg/kg), caffeic acid (2.46–42.41 mg/kg), vanillic acid (2.21–97.5 mg/kg), rutin (3.21–103.6 mg/kg), p-coumaric acid (2.21–32 mg/kg), sinapic acid (17–136.3 mg/kg) and ferulic acid (0-49.3 mg/kg), respectively. The choice of starter culture for SSF is crucial, as sometimes a starter culture may modulate the bioactive profile of the substrate in a different way. For instance, Calinoiu et al. [74] observed the effect of yeast-assisted fermentation on oat phenolics. They observed a significant change in oat phenolic profile as di-OH benzoic acid showed an incremental increase in concentration only up to 4th day of fermentation (38.7–52.1µg/g); thereafter, a significant decrease in concentration was observed. Similarly, changes in the concentration of other compounds were observed, such as protocatechuic acid (2.79–3.98 µg/g, 1st day); caffeic acid (3.3–4.14 µg/g, 1st day), vanillic acid (2–2.32 µg/g, 1st day), p-coumaric acid (2.1–3.68 µg/g, 4th day), sinapic acid (4.6–7.35 µg/g, 2nd day) and ferulic acid (7.8–9.68 µg/g, 1st day), respectively.
Chen et al. [44] studied the effect of SSF on the bioactive profile of oats and observed that the bioactive profile of fermented oats improved, with increased contents of compounds such as gallic acid (3.8–50.1 mg/kg), chlorogenic acid (6.9–326.4 mg/kg), p-hydroxybenzoic acid (6.1–13.7 mg/kg), vanillic acid (3.5–65.2 mg/kg), rutin (0–102.7 mg/kg), p-coumaric acid (2.6–41.7 mg/kg), sinapic acid (3.9–123.2 mg/kg) and ferulic acid (1.7–49 mg/kg). Wu et al. [12] demonstrated that Lactobacillus plantarum- and Rhizopus oryzae-mediated fermentation have a synergetic effect on the antioxidant and nutritional profile of oats. During the fermentation period, the proximate composition of oats changed as follows: crude protein 89.1–118.5 mg/g, potassium 2.4–2.8 mg/g, calcium 0.39–0.67 mg/g, magnesium 0.4–1.02 mg/g, leucine 5.10–5.16 g/100g, isoleucine 2.76–2.82 g/100g, lysine 3.44–3.98 g/100g, methionine 1.29–1.24 g/100g, phenylalanine 3–2.93 g/100g, threonine 3.34–3.51 g/100g and valine 4.56–4.34 g/100g, respectively. Xiao et al. [54] observed that Cordyceps militaris-mediated fermentation of oats resulted in significant modulation in specific metabolites such as gallic acid (134.5–165.4 µg/g), vanillin (51.2–109.8 µg/g), ferulic acid (51.2–86.1 µg/g), luteolin (6.4–36.2 µg/g), apigenin (1.5–2.3 µg/g) and total phenolic content (5.8–14.1 mg/g).

2.4. The Effect of SSF on Millet Grain

Millets include foxtail millet, pearl millet, barnyard millet, proso millet, little millet, finger millet, kodo millet, etc. Millets could be an industrially important raw material/substrate, as their grains have various industrially important micro- and macromolecules [5,9,14]. At the industrial level, the substrate might be processed via different processing methods including roasting, soaking, germination and fermentation (especially SSF). Fermentation is usually preferred over other processing types, as leaching of bioactive metabolites occurs at a faster rate during this process. The output of fungal fermentation demonstrates that during the fermentation time, a significant change in the bioactive profile of pearl millet was observed. The bioactive profiles of pearl millet cultivars PUSA 415 and HHB 197 were significantly modulated using Aspergillus awamori, Aspergillus oryzae, Aspergillus sojae and Rhizopus azygosporus, respectively. Salar et al. [29] used Aspergillus awamori as a starter culture during SSF, and they observed significant increases in phenolic compounds within a short span of fermentation time (8 days). Further, Aspergillus oryzae was used as a fermentation starting culture on a pearl millet cultivar (PUSA 415) [27]. During the process of SSF, the enhancement observed in the TPC value was 6.1–18.7 mg GAE/g.
During the fermentation process, starter cultures not only increase or improve nutrients and bioactive compounds, but additionally, they may incorporate some of their own addition metabolites which may have positive effects on health [9,27,75]. Further, the amount and type of metabolites depend on the type of fermenting microorganisms and media composition. The modulation rate of phenolic content in the substrate to be fermented depends on the starter culture and its efficacy for enzyme production. Other factors, such as media composition, incubation temperature and fermentation time, also affect the bioactive profile (both qualitative and quantitative) of a substrate. In another study, Aspergillus sojae was employed to modulate the phenolic profile of pearl millet, where the maximum increase in bioactive metabolites was observed only up to 6 days (of fermentation time). The TPC value increased after SSF from 6.4 to 34.1 mg GAE/g [9]. Rhizopus azygosporus-assisted fermentation of pearl millet modulates the TPC value of pearl millet from 6.58 to 21.78 mg GAE/g within 10 days [14]. Pampangouda et al. [76] reported significant changes after fermentation in different nutrient components of little millet flour. Changes in different components were as follows: 9.8–10.9% (protein), 3.2–2.6% (fat), 39.2–41.4 mg/100g (calcium), 137.6–141 mg/100g (magnesium), 221.4–234.4 mg/100g (phosphorus), 6.6–7.9 mg/100g (iron), 3.8–4.6 mg/100g (zinc) and 188.9–114.7 mg/100g (phytic acid).Omotoke-Azeez et al. [11] promulgated the fermentation effect on in vitro digestibility, antioxidant properties and mineral composition of finger millet flour. Changes in different properties of finger millet flour after fermentation were: protein 9–10.7 g/100g, fat 1.8–1.1 g/100g, crude fiber 3.8–3.4 g/100g, amylose 22.8–20.3 g/100g, resistant starch 7.54–9.1 g/100gand digestible starch 45.1–39.8 g/100g, respectively. Further changes in the mineral profile of finger millet flour were as follows: calcium 124–135 g/100g, iron 181–394 g/100g, magnesium 1095–1120 g/100g, potassium 2120–2165 g/100g, phosphorus 2278–2403 g/100g and zinc 16.1–17.4 g/100g. Fermentation significantly affects the antioxidant properties of finger millet flour, as confirmed by changes in TPC from 122 to 155 mg GAE/100g, followed by an increase in TFC (total flavonoids content) of 119–143 mg RE (rutin equivalent)/100g. Antioxidant properties were modulated from 131 to 154 µmol TE/100g (DPPH: 1, 1-diphenyl-2-picryl-hydrazil radical scavenging assay), 36.8 to 72.1% (ABTS: 2, 2′-Azino-bis-3-ethylbenzthiazoline-6-sulfonic acid assay) and 120 to 147 µmol TE (trolox equivalent)/100g (FRAP: ferric reducing antioxidant power), respectively.

2.5. The Effect of SSF on Rice Grain

Bioprocessing such as bioleaching/solid-state fermentation has proved its potential to be applicable in the food, feed, chemical and pharma industry for the production of nutrient-rich products with the help of microorganisms [76,77,78]. SSF technology is part of the current trend of research and is widely used by researchers to improve the quality of cereal grains [79]. Purwar et al. [77] studied the effect of SSF (for 10–11 days, at 30 °C temperature and 15% moisture) on the nutritional quality of rice using a fungal strain Monascuspurpureus (6 × 105 spores/mL). Their study reported that SSF significantly increases the moisture (13.32%) and protein content (29.62%); however, reductions in the fat, fiber and carbohydrate content by 27%, 0.28% and 11.32% were observed at the end of the fermentation period. A study investigated the effect of SSF at different intervals of time (4, 6 and 10 days) on the nutritional composition of rice bran by using Pleurotus sapidus species [80]. The study reported that the maximum increase in the carbohydrate content of rice bran (36.6–50.2%) was observed on the 4th day of fermentation. However, the protein, ash and moisture content of fermented rice bran samples increased with the increase in fermentation time, as the maximum increment in the protein (7.4–12.8%), ash (7.6–11.5%) and moisture content (20.7–37.9%) of rice bran was observed in the 10-day-fermented samples. As observed in their study, the lipid content of rice bran was decreased (48.5–27.8%) after SSF processing. Previous findings of Ribeiro et al. [81] also state that solid-state fermentation with Rhizopus oryzae significantly increases the protein, fiber, ash and lipid content; however, the process decreases the carbohydrates of rice bran.
The above studies on solid-state fermentation of rice and rice bran suggest that SSF is a promising technique for achieving an increase in nutrients, meaning that it could be used for the production of high-quality food and feed products. Some previous findings [57,62,82] on the solid-state fermentation which uses rice/rice bran as substrate reported that it could also be used to improve the phenolic profile using different fungal strains. Ritthibut and co-workers [57] studied the effect of two fungal strains of Aspergillus (A. brasiliensis, A. sojae) on the phenolic content of rice bran, using solid-state fermentation. They reported that among the studied fungal strains, Aspergillus sojae effectively increased the phenolic content (10.6–36.5 mg GAE/g extract) on the 14th day of SSF. However, the maximum phenolic content in Aspergillus brasiliensis-fermented samples (20 mg GAE/g extract) was observed on the 8th day of fermentation. SSF (for 5 days at 30 °C) with Aspergillus awamori and Aspergillus oryzae also increases the total phenolic content of the black rice bran, as reported by Shin et al. [62]. Results from their study indicate that contents of bioactive compounds such as protocatechuic acid and ferulic acid were increased from 1028.2 to 1660.6 μg/g (1.61-fold increase) and 46.4–566.5 μg/g (12.18-fold increase) in the A. awamori-fermented black rice bran extracts. Different fungal stains have their own effect on the recovery of phenolics. Janarny and Gunathilake [61] reported the effect of SSF with Rhizopus oryzae on the bioactive profile of rice bran. Fermentation of rice bran with Rhizopus oryzae significantly increases the total phenolic content (5.33–8.81 mg GAE/g), total flavonoid content (5.20–14.75 mg RE/g), total antioxidant capacity (0.10–1.65 mg cyanidin-3-glucoside equivalents/g) and total carotenoids (62.49–71.82 mg/g). Fermentation with suitable fungal strains for a controlled period of time could increase the phenolic profile of the selected substrate, which could then further serve as an antioxidant-rich raw material for designing functional foods and nutraceuticals. Other than the improvement in nutritional composition and phenolic profile, SSF could also be used for enzyme production such as xylanase [59], laccase [83] and alpha-glucosidase [84] from rice husk.

2.6. The Effect of SSF on Maize Grain

Maize is one of the most widely cultivated cereal crops (about 40% of total cereal grain production) and it serves as a major part of calorie intake for most of the global population [85,86]. Cereal grains are a cost-effective and easily available staple food which fulfill nutritional requirements of the body and help to protect from diseases. The protective effect of cereals is mainly due to their proteins, fibers, phenolics, lipids and micronutrient content [20,87,88]. However, compared to other cereal grains, the nutritional composition of maize grains is inferior. Populations depending mainly on maize for their nutrition have mostly suffered from protein malnutrition and micronutrient deficiency. SSF is a biological method that can improve the quality of grains and is currently being used in research. Terefe et al. [89] studied the effect of SSF (for 48 h) on protein digestibility, nutritional composition and antinutrients of maize grains. They reported an increase in the protein content of the maize flour from 38 to 55%, 49% and 48% after fermentation with Lactobacillus plantarum, Saccharomyces cerevisiae and their cocultures. Lactobacillus plantarum-fermented samples showed higher in vitro protein digestibility (31–40%) compared to Saccharomyces cerevisiae (31–36%) and their coculture (31–34%). However, the co-culture of the two strains effectively reduce the tannin (75%), phytate (66%) and trypsin inhibitors (64%) from the maize flour after fermentation for a period of 48h. This is an interesting practical aspect of the use of fermentation in improving the nutritional value of maize-based products. The results from the above study indicate that every fungal strain has its own potential to modulate the quality of the substrate. Other than nutritional composition, SSF has also been used by researchers to improve the phenolic profile of maize grain. Salar et al. [20] promulgated the effect of SSF (with 2–6 days of incubation) on the phenolic profile of maize using Thamnidium elegans CCF 1456. Their results reported that SSF with Thamnidium elegans CCF 1456 can increase the total phenolic content from 327 to 409 GAE μmol/g. They observed higher phenolic content in the 5-day-fermented sample. Chen et al. [44] demonstrated that SSF of corn kernels with Monascus anka, Bacillus subtilis and Saccharomyces cerevisiae has the potential to increase the phenolics by 18.08 times compared to unfermented samples.
A mixed culture of more than two fungal strains (Monascus anka, Saccharomyces cerevisiae and Bacillus subtilis) could also be used for SSF, as reported by Luo et al. [90]. They studied the effect of a mixed culture (0, 6, 12 days) on the release of phenolic compounds from corn seeds, and their study demonstrated that SSF with mixed culture results in a 22.56-fold increase in total phenolics compared to unfermented samples. Higher phenolic content was observed in the 12-day-fermented sample.
The effects of SSF on corn waste, such as corn stalk and corn cob, for production of enzymes, pigments and to upgrade the nutritional quality are also reported in the literature [91,92,93,94]. A study by Darwish et al. [91] stated that fermentation of maize stalk with a coculture of Pleurotus ostreatus and Saccharomyces cerevisiae (7-day incubation period, 45 mL inoculums size) significantly improves the protein content (3.60–11.80%) compared to a single fungal strain (Pleurotus ostreatus) which improved the protein content by 3.60–6.30%. In terms of crude fiber, their study reports a decrease in lignin and cellulose content with an increase in fermentation time. Aliyah et al. [93] analyzed the potential of SSF using Aspergillus niger to produce hydrolysis enzymes from corn cob and reported the activity unit as 81.86 U/mL and 95.02 U/mL for α-amylase and β-glucosidase. Grain waste can also be utilized with the help of solid-state fermentation. Corn cob could be used for production of natural pigment using SSF, and these are good alternatives to synthetic dye. SSF of corn cob with Monascus purpureus KACC 42430 yields a 25.42 OD (optical density) U/g pigment production, as reported by Velmurugan et al. [94]. Their results showed the potential of M. purpureus-fermented corn cob for industrial applications.

3. Extraction Medium for Cereal Phenolics

Scientists and researchers are continuously exploring the effect of the solvent system and extraction conditions on cereal phenolics. The phenolic profiles of cereal and other natural resources vary from free to complex bound forms. Based on the availability, cost and simplicity, the extraction medium may be designed and optimized (Figure 1). Response surface methodology is fruitful software, and its applications are being widely utilized in optimization of experimental parameters [95,96,97,98,99,100]. The suitability of an extraction medium helps to raise the level of interaction among phenolics and other bioactive metabolites or complex nutrients (starch, protein, etc.) of cereals. Therefore, an optimized extraction condition is very useful to recover a heap of bioactive metabolites from the samples.

3.1. Conventional Extraction

Solvent-assisted effects on the recovery of bioactive metabolites from different plants parts such as seed, leaves, stem and roots have been explored in different scientific reports. The salient features that determine the extraction medium used at a commercial scale are solvent polarity, safety, temperature and time requirement.Liquid–liquid extraction of secondary metabolites could be achieved using the following ways:(1) Soxhlet extraction, which is an effective and simple method for recovery of secondary metabolites. The time required for the maximum recovery of phenolics may vary with the type of sample and the complexity of bioactive compounds present in them. (2) Maceration is a type of extraction method in which the sample used for extraction is kept in a container in powdered form. Periodic stirring and shaking is required to achieve good results. (3) Hydro-distillation extraction is one of the traditional methods used for recovery of bioactive metabolites from natural resources. The method involves the use of a sample in a container (compartment), followed by the addition of water and boiling.

3.2. Non-Conventional Extraction Methods

3.2.1. Ultrasound-Assisted Extraction

Ultrasound-assisted extraction (UAE) is the key process which could be used to achieve a targeted amount of bioactive metabolites from natural resources. The demand for UAE is significant in the food as well as the chemical industry [101]. With low solvent consumption, product purity and high reproducibility, UAE is dominant over other extraction types. Several different components from botanical resources such as pigments, minerals, aroma, bioactive compounds and other metabolites could be extracted with high efficiency.

3.2.2. Microwave-Assisted Extraction

Microwave-assisted extraction (MAE) is being widely adapted for the extraction of bioactive metabolites from natural resources. During this process, heating of a molecule occurs through ions-mediated conduction and dipole rotation. The water molecules which are present in the inner parts/space of the cells may create pressure on the cellular wall, which may result in cellular breakdown. The success rate of microwave-based extraction methods depends on both the extraction medium and target metabolites. The polarity of the extraction medium plays an important role, as the recovery rate depends on the amount of microwave energy absorbed during the extraction process. A mixture of ethanol and water proved to be an efficient solvent system for the recovery of secondary metabolites from specific materials.

3.2.3. Pressurized Liquid Extraction (PLE)

PLE is a fruitful technique which is performed under pressure and heat. Temperature changes significantly affect the mass transfer rate, solubility and extractability. The temperature fluctuation modulates the efficiency and speed of extraction. The process is convenient from a commercial point of view compared to other processes, as the extraction process could be completed within a shorter span of time. To initiate the PLE process, samples with supporting mediums are kept in a pressurized cell to avoid excess solvent flow and agglomeration.

3.2.4. Supercritical Fluid Extraction

Supercritical fluid extraction is a popular technique used to separate out extractant from a matrix using a suitable extraction medium. To achieve the desired properties in a product or remove an undesired impurity, this process could be employed. Despite a broad spectrum of applications, the process is limited due to the high pressure requirements.

4. Cereal-Based Food Products

4.1. Fermented Drinks and Products

Fermented drinks are widely consumed, have good nutritional properties and lead to many health benefits [102,103]. These drinks and other fermented products are prepared by utilizing the hidden potential of microbial consortia and natural substrates (raw material) [14,27,104]. The most common examples of fermented drinks are shalgam juice, followed by boza, kefir, hardaliye, ayran, etc. [101,105,106]. Boza is one of the traditional beverages (non-alcoholic) which could be prepared using a variety of cereal substrates like millets, barley, oats, rice, corn, etc. [101,107,108]. Microbial strains, especially LAB (lactic acid bacteria), yeast and those which are actively involved in the bacteriocins productions, are being utilized for the preparation of boza. Depending on the substrate and microbial strain type employed for boza production, the quality attributes and stability of the beverage may vary accordingly [109,110,111]. LAB-assisted fermentation significantly alters the pH of boza and alcoholic fermentation, which results in the production of carbon dioxide and results in modulated (enhanced) volume [110]. Earlier documented reports [112,113] claimed that boza could be considered a good source of protein and ACE-inhibitory peptides. Kancabas and Karakaya [112] studied the protein content and ACE-inhibitory activity of boza. They reported the protein content in boza as 1.08 ± 0.079% and ACE-inhibitory activity as 76.7 ± 14.9. Despite being a famous Turkish beverage, studies on boza are limited. Yegin and Uren [111] reported that boza contained 0.50–0.99% protein.

4.2. Tarhana

Tarhana is a famous fermented soup recipe which is prepared domestically as well as commercially for consumers in Turkey. The major ingredients used in the preparation of tarhana are yoghurt, cereal-based flour, baker’s yeast and seasonal vegetables. Tarhana is a good source of minerals and vitamins which could be used by consumers of any age group (children/elders and even patients). Studies demonstrated that tarhana possesses crude protein (12–20%) and amino acids in significant amounts [114,115,116,117,118,119,120]. Further, the soup’s mineral and vitamin profile indicates the presence of calcium (59–191mg/100g), iron (2.1–5.9 mg/100g), sodium (296–1130 mg/100g), potassium (60–182 mg/100g), magnesium (30–134 mg/100g), zinc (0.8–3.2 mg/100g), copper (147–807 mg/100g) and manganese (211–1182 mg/100g).The protein-, vitamin- and mineral-rich nature of tarhana helps sustain metabolic reactions in the body. Further, due to its proteinaceous nature, this fermented food product could be recommended to individuals suffering from protein deficiency.

4.3. Shalgam Juice and Hardaliye

Ulucan et al. [121] promulgated the effect of processing with regard to improving the shelf life of shalgum juice. The major findings from their work indicate that shalgam juice usually only has a shelf life of 3 months, which could be improved up to 2 years using their pasteurization method and suitable preservatives. In addition to this shelf life improvement, processing also mediates dramatic changes in the quality parameters and sensorial attributes. Shalgam is considered an important substrate as it possesses a broad spectrum of appetizing, antioxidant and other health-benefiting properties [122,123]. One of the other fermented beverages, hardaliye (non-alcoholic), is prepared using the traditional method in Turkey (European part). As far as its nutritional composition is concerned, being derived from grapes and processed through fermentation, its components are unique and beneficial to health. A unique blend of nutrients in hardaliye comes from the mixture of cherry leaves, grapes and mustard seeds [122]. Arici and Coskun [124] studied the quality parameters (pH, color, total bacterial count and ethanol content) of different hardaliye samples. The values of different parameters they observed during their study were 3.21–3.97 (pH) and 3.5 × 102–8 × 105 cfu/mL (bacterial count), followed by an ethanol content of 595.5 mg/dL. Aksoy et al. [125] promulgated the effect of hardaliye on malondialdehyde formation during the digestion (in vitro gastrointestinal) of meat products. Among the studied samples, the lowest values were meat doner 1 treated with Shiraz hardaliye (10.27%); burger patties 2 treated with Shiraz hardaliye (22.40%); and meatball 2 treated with Cabernet Sauvignon hardaliye (7.12%). The conclusion of their study supports the fact that hardaliye could be used as a potentially health-benefiting beverage because of its involvement in the reduction in lipid oxidation. Ilıkkan et al. [126] studied hardaliye in terms of its proliferative effect and anticancerous potential in CF-1 and HT-29 cell lines. Two different kinds of dilutions, viz., five- (H5) and ten-fold (H10), were used during the experimental work. Modulated gene expression and apoptosis levels were analyzed using TAA (tali apoptosis assay) and qRT-PCR, respectively. After an interval of 24 and 48h, samples were screened to detect percentage levels of viable, death and apoptotic cells. The observed data from their study suggest that Hardaliye was sufficiently effective to trigger apoptosis and cancer cells via enhanced Bax (6.49 ± 0.1-fold for H10 and 22.45 ± 3.1-fold for H5), as well as CAT and SOD activity.

4.4. Sordough Bread

In a human dietary chart, bread is considered an important staple food which provides both nutrients and energy. The bakery process started with the homemade and artisanal way to produce bread leavened with sourdoughs, prepared from a flour and water mixture. However, with the technical advancements during the last 10 years, the microbiological, morphological, nutritional, technological, textural and sensorial aspects of bread were studied to understand the health-benefiting and hidden potential of bread. The behavior of the starter culture on a specific substrate may vary according to their nutritional type. The quality attributes of bread being prepared using the starter culture depend on the grain types being utilized as a source of experimental material [127,128,129,130,131,132]. The temperature during the sourdough fermentation favors the growth of microbiota and ultimately affects the autochthonous community’s enzymatic activity [133,134]. The fermentation process involved in sourdough bread results in the breakdown of complex proteins and carbohydrates, which ultimately makes it an easily digestible product. With a low glycemic index, sourdough bread may be preferred over normal bread prepared using commercial yeast. The fermentation process slows down the rate of glucose release, and therefore, sourdough bread could be good product for people suffering from diabetes [135]. Nutrients in sourdough bread, such as magnesium, iron, and zinc, remain more readily available for absorption inside the digestive tract. Lactic acid bacteria (LAB) involved during sourdough bread fermentation act as prebiotics in the gut, and thus help to maintain diversity in the gut microbiome which ultimately affects health. LAB are reported to have positive effects on nutritional attributes, flavor, texture and overall acceptability of sourdough bread [135,136].

5. Conclusions

Fermentation of natural resources, especially cereal grains, has emerged as a promising and efficient method for various applications in different sectors such as the food, feed, pharmaceutical and biotechnology industries. A significant improvement in the nutritional profile of fermented products helps to develop specific health-benefiting products. Through SSF, a diverse range of metabolites, enzymes, bioactive compounds and probiotics can be produced using cereal grains. This technology has the potential to enhance the nutritional quality of cereal grains by improving the bioavailability of essential nutrients and enhancing their functional properties. By selecting suitable microorganisms and optimizing substrate composition, moisture content and process parameters, the SSF process can be fine-tuned to maximize product yield and quality. Despite the numerous benefits and applications of SSF of cereal grains, there are still challenges to overcome. These include optimizing process parameters, scaling up production, ensuring consistent product quality and addressing regulatory considerations. Future research should focus on addressing these challenges and exploring new avenues for the application of SSF of cereal grains.

Author Contributions

A.K.: Conceptualization; project administration; original draft preparation; data curation; writing—review and editing; visualization; supervision; project administration. S.S.P.: original draft preparation; data curation; writing—review and editing; visualization; supervision; project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This review paper received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are presented in this manuscript.

Acknowledgments

The authors would like to thank Chandigarh University, Mohali, for providing a peaceful working environment, without which the authors would not be able to prepare the present manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Fermentation 09 00817 g001
Figure 1. Different type of extraction methods.
Figure 1. Different type of extraction methods.
Fermentation 09 00817 g001
Table 1. List of microbial strains used during the solid-state fermentation of cereals and their fractions.
Table 1. List of microbial strains used during the solid-state fermentation of cereals and their fractions.
SubstrateMicrobial Strain Used for FermentationPurposeReferences
OatsRhizopus oryzaeTo study synergetic effects of Lactobacillus plantarum and Rhizopus oryzae on physicochemical, nutritional and antioxidant properties of whole-grain oats (Avena sativa L.) during solid-state fermentation.[12]
CornGanoderma sinenseTo screen edible fungi with laccase activity and determine their effects on the degradation of Aflatoxin B1.[40]
Cereal vinegarKomagataeibacter europaeusTo study the assembly and co-occurrence patterns for
abundant and non-abundant bacterial sub-communities using Zhenjiang aromatic vinegar fermentation as a
model system.		[41]
Wheat branRhizopus oryzaeStudy aimed at evaluating the effect of SSF on flavor and sensory properties of wheat bran containing cake.[42]
Brown riceAspergillus oryzaeTo study the effect of SSF on bioactive compounds of brown rice.[43]
Corn kernelMonascusankaIn this study, regulation of phenolic release and antioxidant activity in corn kernels by co-microbiological fermentation was investigated.[44]
Wheat branAspergillus flavusTo study the optimization of pullulanase production by Aspergillus flavus under solid-state fermentation.[45]
Rice strawPenicillium citrinumSolid-state fermentation of rice straw using Penicillium citrinum for chitosan production and application as nanobiosorbent.[46]
Maize silageCephalotrichum stemonitisFungal pretreatment of non-sterile maize silage and 581 solid digestate with a Cephalotrichum stemonitis strain selected from agricultural biogas plants to enhance anaerobic digestion.[47]
Rice huskTrichoderma harzianumRice husk as a source for fungal biopesticide production by solid-state fermentation using B. bassiana and T. harzianum.[48]
Wheat branAspergillus nigerUsing response surface methodology to improve the L-asparaginase production by Aspergillus niger under solid-state fermentation.[49]
Lime-cooked maizeP. ostreatus Perla and Hericium erinaceusTo study the effects of solid-state fungi fermentation on phenolic content, antioxidant properties and fiber composition of lime-cooked maize by-product (nejayote).[50]
Wheat branAspergillus oryzaeUniform culture in solid-state fermentation with fungi and its efficient enzyme production.[51]
OatsMonascus ankaTo study enzymatic action mechanism of phenolic mobilization in oats (Avena sativa L.) during solid-state fermentation with Monascus anka.[52]
OatsAntrodia cinnamomeaEnhanced antioxidant and antitumor activities of Antrodia cinnamomea cultured with cereal substrates in solid-state fermentation.[53]
OatsCordyceps militarisTo study the effect of solid-state fermentation on antioxidant capacity and DNA damage protective effect of oats (Avena sativa L.).[54]
Wheat strawPhlebia floridensisProduction of lignocellulolytic enzymes and enhancement of in vitro digestibility during solid-state fermentation of wheat straw by Phlebiafloridensis.[55]
Red riceMonascus spp.To detect and reduce citrinin during the fermentation of red fermented rice.[56]
Rice branAspergillus sojaeEnhancement of bioactivity of rice bran by solid-state fermentation.[57]
Rice strawMucor indicusEfficient ethanol production from rice straw through cellulose restructuring and high-solids-loading fermentation by Mucor indicus.[58]
Rice huskAspergillus nigerXylanase production from solid-state fermentation of rice husk.[59]
Broken riceMonascus sanguineusProduction and extraction of red pigment by solid-state fermentation of broken rice.[60]
Rice branRhizopus oryzaeChanges in rice bran bioactives, their bioactivity, bioaccessibility and bioavailability with solid-state fermentation.[61]
Black rice branAspergillus awamori and Aspergillus oryzaeEffects of SSF on phenolic acid composition and antioxidant activity.[62]
SorghumAspergillus fumigatusSorghum xylans as substrates for the production of xylanase enzyme.[63]
Table 2. Effect of SSF on TPC value of different substrates.
Table 2. Effect of SSF on TPC value of different substrates.
SubstrateMicrobial Strain Used for FermentationPurposeModulation in TPC after SSFReferences
Pearl milletAspergillus sojaeTo enhance phenolic content and antioxidant properties.6.4–34.1 mg GAE/g[9]
Finger milletYeastTo enhance phenolic content and antioxidant properties.122–155 mg GAE/100g[11]
Pearl milletRhizopus azygosporusTo enhance phenolic content and antioxidant properties.6.58 to 21.78mg GAE/g[14]
WheatAspergillus oryzaeTo enhance phenolic content and antioxidant properties.7.23–158.9 µmol GAE/g[15]
WheatAspergillus awamoriTo enhance phenolic content and antioxidant properties.7.23–124.2 µmol GAE/g[15]
MaizeThamnidium elegansTo enhance phenolic content and antioxidant properties.327–409 GAE μmol/g[20]
Pearl milletAspergillus oryzaeTo enhance phenolic content and antioxidant properties.6.1–18.7 mg GAE/g[27]
OatsCordyceps militarisTo enhance phenolic content and antioxidant properties.5.8–14.1 mg/g[54]
Rice branAspergillus sojaeTo enhance phenolic content and antioxidant properties.10.6–36.5 mg GAE/g extract[57]
Rice branRhizopus oryzaeTo enhance phenolic content and antioxidant properties.5.33–8.81 mg GAE/g[61]
Black rice branAspergillus oryzaeTo enhance phenolic content and antioxidant properties.1028.2 to 1660.6 μg/g[62]
WheatAspergillus awamoriTo enhance phenolic content and antioxidant properties.974–2056 µg GAE/g[64]
BarleyAspergillus awamoriTo enhance phenolic content and antioxidant properties.3.25–4.59 mg GAE/g[65]
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Kaur, A.; Purewal, S.S. Modulation of Cereal Biochemistry via Solid-State Fermentation: A Fruitful Way for Nutritional Improvement. Fermentation 2023, 9, 817. https://doi.org/10.3390/fermentation9090817

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Kaur A, Purewal SS. Modulation of Cereal Biochemistry via Solid-State Fermentation: A Fruitful Way for Nutritional Improvement. Fermentation. 2023; 9(9):817. https://doi.org/10.3390/fermentation9090817

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Kaur, Avneet, and Sukhvinder Singh Purewal. 2023. "Modulation of Cereal Biochemistry via Solid-State Fermentation: A Fruitful Way for Nutritional Improvement" Fermentation 9, no. 9: 817. https://doi.org/10.3390/fermentation9090817

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