Alternative Uses of Fermented Wheat Bran: A Mini Review

Abstract

Bran is a by-product primarily derived from the milling of grains, notably wheat and rice. It is rich in dietary fiber, vitamins, minerals, and phytochemicals yet often remains underutilized in its raw form. This raw material is abundant and readily available, offering significant potential for value-added applications. In its unprocessed state, bran boasts a complex chemical composition that includes proteins, lipids, and carbohydrates. However, it also contains antinutritional components such as phytic acid and enzyme inhibitors, which may limit its nutritional efficacy. Through further processing or storage, these components can be transformed to enhance their antioxidant properties and overall nutritional value. Bran is used in both animal feed and human food applications, though its use is often hindered by its high fiber content and antinutritional factors. To maximize its utility, innovative processing techniques are required to improve its digestibility and nutrient availability. Fermentation presents a viable method for enhancing the nutritional profile of bran. This process typically employs microorganisms such as bacteria, yeast, or fungi to break down complex compounds, thereby increasing the bioavailability of nutrients. After fermentation, bran exhibits improved chemical composition and nutritional value. The process reduces antinutritional components while enriching the bran with beneficial compounds like amino acids and probiotics. Utilizing fermented bran in animal feed offers numerous advantages, including enhanced digestive health, improved nutrient absorption, and augmented disease resistance. It serves as a sustainable feed alternative that supports livestock growth while aligning with ecological goals. The processing of bran through fermentation not only maximizes its nutritional potential but also contributes to sustainable agricultural practices by reducing waste. Future research should focus on optimizing fermentation techniques and exploring novel applications in both feed and food industries to fully realize the benefits of this versatile by-product.

1. Introduction

Wheat is recognized as one of the most widely produced cereals globally. An examination of the chemical composition of wheat reveals significant disparities in nutrient and antinutritional factor levels, predominantly influenced by genetic and environmental variables. Furthermore, variations in wheat quality can be quantitatively assessed through physical parameters such as kernel weight and density [1,2].

Wheat can be classified by several criteria, including growth cycle (winter versus spring), hardness (soft versus hard), and color (white versus red). Specifically, winter wheat is planted in the autumn, while spring wheat is sown at the onset of early spring [3]. Additionally, a critical distinction exists between hard and soft wheat types. Generally, soft wheat contains a lower concentration of crude protein (CP) than hard wheat but compensates with a higher starch content [4,5]. Wiseman suggests that soft wheat possesses enhanced digestibility for non-ruminants compared to hard wheat, owing to inherent structural differences in their starch and protein granules [6,7]. In soft wheat, starch and protein granules are integrated within a loose matrix, facilitating enzyme access, whereas in hard wheat, the protein matrix encapsulates the starch granules, restricting the penetration of digestive enzymes.

Classification by color refers specifically to the pigmentation of the aleurone or the outer layer of the wheat kernel [8]. In the United States, a more extensive classification framework is employed, encompassing six distinct categories: hard red winter, hard red spring, soft red winter, durum, soft white, and hard white wheat [9].

Wheat can provide up to 60% of an animal’s total amino acid (AA) requirements [10] and as much as 70% for essential amino acids [11]. The by-products of wheat processing, such as wheat bran (WB) and wheat middlings (WM), are generated during the transformation of wheat into flour for human consumption [12,13]. The economic relevance of wheat milling by-products is substantial, accounting for approximately 25% of the original grain weight [14]. Wheat bran, primarily composed of the outer protective layers of the wheat kernel, constitutes a substantial portion of the grain’s mass. The annual global output of wheat bran exceeds 200 million tons [15,16]. Consequently, it is essential to optimize the utilization of WB resources within the feed industry. While these by-products can significantly mitigate feed costs, their nutritional profiles often exhibit variability, which may restrict their effective incorporation into poultry and swine diets [17,18,19]. Wheat bran, the outer layer of wheat grains, emerges as a by-product during the milling process [10,11,12,13,14]. While commonly used as agricultural feed, its economic value remains relatively modest [20,21]. Despite being replete with non-starch polysaccharides, lipids, lignin, vitamins, and proteins [16], its high fiber content and the presence of antinutritional factors result in poor palatability and diminished protein quality, hindering efficient digestion by livestock and poultry, thus reducing its nutritional efficacy [2]. As a fiber-rich resource, wheat bran not only finds application in animal feed and as a dietary fiber supplement in human food but also demonstrates extensive utility in health supplements, eco-friendly materials, soil conditioners, and research into poultry and livestock nutrition [11,12,13,14,15,16,17,18,19]. This underscores its significant potential in advancing sustainable development and enhancing the efficacy of multiple sectors. Given the abundant availability of wheat bran resources in China [16], effectively harnessing its potential to advance the feed industry has become a pivotal research focus.

Ferulic acid (FA) has garnered attention for its potential as a substrate in vanillin synthesis [22,23]. Extracted from lignocellulosic biomass waste, FA presents a cost-effective feedstock for the production of highly desirable bioactive chemicals. This phenolic compound is embedded within the cell walls of lignocellulosic materials and is covalently linked to a variety of carbohydrates through amides, glycoside conjugates, and ester bonds. Notably, it possesses the intrinsic ability to release ferulic acid upon pretreatment [24,25]. Ferulic acid (FA), with the molecular formula C₁₀H₁₀O₄ and a molecular weight of 194.19, is a derivative of cinnamic acid (3-phenyl-2-propenoic acid) and exhibits weak acidity. It is highly soluble in water. Ferulic acid exists in two isomeric forms: cis and trans. The cis form is a yellow oily substance, while the trans form crystallizes into colorless squares or fibrous structures [26]. This compound contains three functional groups: hydroxyl, methoxy, and carboxyl, which enable it to form covalent bonds with unsaturated carbon, imparting significant antioxidant activity [27,28]. FA is a phenolic compound prevalent in various plants, predominantly existing in an esterified form within the cell walls of plants, where it binds with polysaccharides and lignin. Its chemical structure, characterized by a phenolic hydroxyl group and an extended carbon chain, constitutes the cornerstone of its diverse biological activities [29,30,31,32,33,34]. FA is found extensively in a myriad of plants, with particularly high concentrations in cereals, vegetables, and fruits. Notably, agricultural products such as wheat bran, rice bran, and corn serve as rich sources [35]. FA exhibits a multitude of biological activities, including antioxidant, anti-inflammatory, anti-cancer efficacy, and the potential to enhance cardiovascular health [36,37,38,39,40,41]. Through mechanisms such as lipid metabolism regulation, antioxidant action, anti-inflammatory effects, and inhibition of apoptosis, FA contributes to maintaining animal health. In the body, FA is present in the form of glycosides and methyl esters. In the small intestine, FA binds with sugar molecules as glycosides, facilitating absorption by intestinal cells. The methyl ester structure further enhances its bioavailability [42,43]. Once absorbed, FA and its derivatives are distributed throughout various tissues via the bloodstream, with elimination occurring approximately five hours later in the form of benzoic acid through urine [44,45]. Studies indicate that FA effectively scavenges free radicals, thereby safeguarding cells from oxidative damage and reducing the incidence of chronic diseases [46,47,48,49,50]. Additionally, its role in inhibiting certain enzymes may thwart tumor progression by disrupting substrate binding [30,46,51,52]. Further research underscores FA beneficial influence on angiogenesis, potentially through its regulation of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor (HIF) [39,53,54]. In the food industry, ferulic acid’s intrinsic antioxidant properties lend it to widespread use in preserving food and enhancing nutritional value.

In conclusion, ferulic acid stands out as a pivotal natural antioxidant, promising notable applications in the pharmaceutical domain while also playing an integral role as an additive in the food industry.

Recent research suggests that microbial fermentation is a promising method to amplify the nutritional and functional profile of bran. This process transforms the fiber structure, disrupts chemical bonds, and converts phenolic acids into their free, more active forms, thereby increasing FA content and augmenting antioxidant capacity [55,56,57,58]. Fermentation also diminishes neutral detergent fiber and phytic acid levels, improving the digestible energy value of bran. Furthermore, the fermentation process generates organic acids, bioactive peptides, and amino acids, all of which contribute to enhanced nutritional value and palatability [57,58,59].

Wheat grains consist primarily of bran, germ, and endosperm, with the outer layer being the bran. Wheat bran contains approximately 8–12% moisture, 13–18% protein, 36–57% carbohydrates (typically 40% dietary fiber and 10% starch), 5–6% ash, 4–5.5% fat, and 1% phenolic compounds [60].

Table 1. Regional Variations in the Nutritional Composition of Wheat Bran.

Item	Dry Matter	Crude Protein	Crude Ash	Crude Fat	Crude Fibre	Neutral Detergent Fiber	Acid Detergent Fiber	Ca	P	N-Free Extract	Reference Material
Wheat bran	87.38	15.08	4.16	4.72	7.77	32.28	11.00	0.10	0.99	-	NRC 2012 [61]
Wheat bran	89.00	17.10	6.90	4.40	11.30	-	15.00	0.13	1.38	-	USA FEEDSTUFF [62]
Wheat bran	87.10	14.80	5.00	3.40	9.20	39.60	11.90	-	-	-	INRA [63]
Primary bran	87.00	15.70	4.90	3.90	6.50	37.00	13.00	0.11	0.92	56.00	Chinese feed ingredients and nutritional value Value (Edition 31, 2020) [64]
Secondary bran	87.00	14.30	4.80	4.00	6.80	41.30	11.90	0.10	0.93	57.00
Wheat bran (Korea)	87.14	17.02	5.09	2.88	8.45	34.21	11.24	0.70	3.02	53.70	Park et al. (2019) [65]
Kamut bran	89.89	11.77	3.56	2.38	-	29.07	9.42	-	-	-	Zhu et al. (2018) [66]
Wheat bran	88.29~91.38	13.47~15.97	3.96~5.54	2.33~2.97	-	29.16~46.13	8.27~14.81	-	-	-
Spelt bran	89.57~89.76	14.11~15.4	3.28~7.07	2.83~3.55	-	21.03~36.76	5.46~13.55	-	-	-

presents reference data on the nutritional value of bran from different countries [61,62,63,64,65,66]. Bran is classified as a medium to low-energy feed and exhibits nutritional variations depending on its source. Compared to other medium to low-energy feeds such as rice bran and corn husks, bran has a higher crude fiber content but lower crude protein and crude fat levels, resulting in a significantly lower metabolizable energy value. Additionally, the nutritional composition of bran is influenced by processing techniques; finer wheat milling results in higher residual endosperm content in the bran, thereby increasing its nutritional value [55].

After processing in flour mills, wheat is transformed into flour and bran. Bran comprises the outer layers and aleurone layer, with its quality influenced by processing techniques. Rich in dietary fiber, bran aids digestion and provides energy. Additionally, it contains significant amounts of enzymes, such as β-amylase, which can substitute malt in beer production. Bran is also composed of 13–18% crude protein, offering a variety of essential amino acids. It is abundant in trace elements like phosphorus, calcium, potassium, iron, manganese, and zinc. Wheat bran is abundant in phenolic compounds, including flavonoids, ferulic acid, lignans, and alkylresorcinols, which possess various pharmacological activities [56,57,58,59,67,68,69,70,71,72,73]. Studies indicate that flavonoids in wheat bran can neutralize specific carcinogens and eliminate free radicals in the body [74,75]. These compounds have shown promise in anti-aging, cardiovascular disease prevention [76,77], and cancer prophylaxis [78].

Compared to traditional feed, fermented feed presents notable advantages. According to the research by Wang et al., sensory evaluations indicate that fermented feed exhibits enhanced quality, characterized by a uniform hue, rich aroma, and superior palatability, thereby markedly improving animals’ feed intake. Moreover, fermented feed is cost-effective, adeptly transforming otherwise wasted agricultural crops into high-quality livestock and poultry feed [79]. Numerous studies have underscored the efficacy of solid-state fermentation (SSF) technology as a potent method for enhancing the nutritional quality of by-products, particularly in wheat bran [80,81,82]. Research by Zhao et al. [83] demonstrated that fermentation of wheat bran using Saccharomyces and Lactobacillus species resulted in a three to fourfold increase in the content of water-extractable arabinoxylan. Moreover, this fermentation process elevated both the concentration and bioavailability of soluble dietary fiber and total free phenolics while reducing phytic acid levels by over 20%. Furthermore, the incorporation of 5% dry fermented wheat bran (FWB) into the diets of broiler chickens (Ross 308) yielded significant improvements in growth performance, increased Lactobacillus counts in the ileum, and fostered a favorable intestinal environment [84]. Additional investigations by Zhang et al. [85], Lee et al. [86], and Teng et al. [87] have corroborated the beneficial effects of solid-state fermented wheat bran, noting its positive impact on growth performance, nutrient digestibility, and anti-inflammatory responses in broiler chickens.

The value of feed is primarily assessed through its nutritional composition. Fermented feed is abundant in beneficial microorganisms and proteins, while it reduces antinutritional factors, aiding in the improvement of animal gut microbiota and growth performance. Additionally, fermented wheat bran efficiently utilizes agricultural by-products, mitigating resource wastage and environmental pollution [88,89,90,91,92,93,94].

The implications of these findings are significant: as a renewable resource sourced from agricultural waste, FA fosters sustainable progress within the bioeconomy. Its utility extends into the spice and cosmetics industries, where it is sought after for its unique aroma and skincare benefits. Collectively, the expanded applications and processing advancements of FA not only showcase its substantial economic potential but also align with the principles of green and sustainable development.

The potential of fermented wheat bran in optimizing the utilization of wheat by-products is significant. Figure 1 illustrates fermented bran as a sustainable solution to food waste, emphasizing its nutritional composition, antinutritional factors, fermentation process, benefits post-fermentation, and contributions to sustainable development and ecological objectives. This figure highlights the importance of fermentation in enhancing the nutritional value of bran and demonstrates future research and application directions, such as optimizing fermentation techniques and exploring new applications.

2. Issues in the Application of Wheat Bran

It is understood that approximately 90% of bran is used in the feed processing industry, while around 10% is allocated for human food production and the preparation of other bioactive substances [18]. The limited application of wheat bran in industrial production and food processing can be attributed to the following factors:

Wheat bran is rich in crude protein (13–18%), crude fat (2.33–4.72%), and crude fiber (6.5–11.3%) [61,62,63,64,65,66], all of which exhibit strong water absorption properties. During storage, inadequate ventilation can easily lead to clumping and mold growth [95]. Moreover, wheat bran inherently contains various spoilage bacteria and fungi, making it susceptible to mold and difficult to store, which severely impacts its industrial applications. In grain storage, the presence of mold reflects the quality of the crop. Mold metabolism can produce toxins such as vomitoxin, aflatoxin, and fumonisin, posing obstacles to the use of wheat bran. Additionally, due to its hygroscopic nature, wheat bran can facilitate the rapid proliferation of harmful fungi [96]. Research by Liu Jinrui indicates that the mold content in wheat bran significantly increases with extended storage time, with a notable rise starting from the 14th day. It was also found that wheat bran with a moisture content of 13% is prone to exceeding mold limits when stored at a temperature of 30 °C and humidity of 76% [97].

Wheat bran, as a plant-derived lignocellulosic material, primarily comprises cellulose, hemicellulose, and lignin [98]. Cellulose is the main component of lignocellulose, consisting of linear crystalline polymers formed by β-D-glucosyl units linked through β−1,4-glycosidic bonds, with a degree of polymerization typically ranging from 4000 to 8000 [99], making it difficult for organisms to utilize. Hemicellulose structures are typically composed of various glycosyl units, such as glucosyl, rhamnosyl, mannosyl, and xylosyl, with significant variation depending on the raw material. Additionally, the molecular chain contains varying amounts of acetyl and formyl groups, rendering hemicellulose more complex and amorphous [100]. Lignin is constituted of phenylpropane units, forming aromatic polymers connected by C-C bonds and ether linkages, characterized by a high degree of polymerization and strong viscosity. It often acts as an adhesive, intertwining with cellulose to create a complex lignocellulosic structure, increasing the difficulty of utilizing wheat bran [98]. Furthermore, the dense lignocellulosic chains entrap compounds such as phenolic substances, preventing their release and utilization by organisms. The intricate lignocellulosic structure renders the majority of bran components, primarily insoluble dietary fibers, unusable, leading to resource wastage [101]. The large degree of polymerization and complexity of the structure make it indigestible for animals and humans, thus significantly reducing the bioavailability of wheat bran [100]. In recent years, efforts to modify wheat bran through physical [102], chemical [103], and biological methods [104] have become a focal point for numerous researchers, aiming to substantially enhance industrial production efficiency.

Wheat bran is rich in minerals, characterized by a lower calcium content and a higher phosphorus concentration, primarily in the form of phytic acid [105]. As an antinutrient, phytic acid can bind with minerals, altering their solubility, functionality, digestibility, and absorption, thereby reducing their bioavailability [100]. It also forms insoluble complexes with positively charged metal ions, diminishing the utilization and biological value of mineral elements, which can severely affect normal metabolism and reproductive functions [106]. Additionally, wheat bran contains active compounds such as oxalates and trypsin inhibitors (TI) [107]. Research indicates that the oxalate content in wheat bran can reach up to 457.4 mg/100 g, significantly higher than the 53.3 mg/100 g found in wheat, indicating that oxalates are primarily located in the outer layers of grains, with higher levels in whole grain products compared to refined grains. High oxalate levels are critical in managing recurrent calcium oxalate kidney stones, as excessive amounts can lead to their formation. Trypsin inhibitors significantly impact biological systems, with high levels affecting both organisms and the stability of wheat bran products. Given the crucial role of grains, particularly whole grains, in daily nutrition, it is essential to eliminate or minimize these antinutritional factors in bran to fully leverage its nutritional potential. Improving the antinutritional profile of wheat bran has become a research focal point, with efforts concentrated on employing microbial fermentation technology to expand its application in industrial production.

3. Fermentation Technology and Strain Selection

3.1. Solid-State Fermentation

In the realm of solid-state fermentation (SSF), agricultural by-products are transformed into valuable biological substances under meticulously controlled conditions and minimal free water presence. This intricate process is facilitated by specific microbial metabolism, elevating its biological significance. Typically, the media employed in SSF comprises approximately 50% moisture, often referred to as semi-solid fermentation, and operates within a gaseous phase as the continuous medium.

In the feed industry, SSF manifests in various forms, including breathable bags, barrels, piles, troughs, and drums. The resultant product from this technique is known as solid-state fermented feed [108,109]. When juxtaposed with liquid fermentation, SSF presents several notable advantages: it boasts a lower moisture content and reduced water activity, facilitating easier handling and higher substrate insolubility. Microbial growth is enhanced alongside elevated enzymatic activity. The process is energy-efficient, requiring simplified energy apparatus, and is relatively low-maintenance, obviating the need for stringent sterile environments, thus requiring less capital investment. Furthermore, it minimizes pollution, simplifies post-processing, and is inherently environmentally friendly and easy to execute. The diversity of applicable raw materials is vast, encompassing any carbohydrate-containing agricultural by-products or residues, providing essential nutrients for microbial sustenance.

SSF represents a complex microbial cultivation method whose effective execution is contingent upon the meticulous regulation of various interrelated factors. At the core of this process lies the strategic selection of appropriate substrates, combined with the optimization of their particle size and carbon-to-nitrogen ratio [110,111,112,113,114,115]. These aspects are crucial as they significantly impact the nutrient absorption and growth velocity of the microorganisms. Adjustments in moisture content and pH are pivotal for nurturing an environment that supports the growth of the desired microorganisms while inhibiting the proliferation of unwanted species [115,116,117,118]. The supply of oxygen is equally critical; insufficient oxygenation within solid matrices can severely hinder metabolic functions. Consequently, thoughtful strategies of aeration and agitation are implemented to boost oxygen availability effectively. Furthermore, the control of temperature and humidity requires precise calibration according to the specific physiological requirements of the microbial species involved, safeguarding them against thermal damage. The fermentation period must be dynamically tailored to align with microbial growth dynamics, ensuring peak metabolite accumulation [108,116,117,118,119,120,121,122]. In my earlier research, I conducted an in-depth analysis of these parameters, experimentally confirming their significant influence on both the efficiency of fermentation and the quality of the resultant products. The results highlighted that methodical optimization of these variables can markedly enhance the yield and quality of the target products, thereby emphasizing the indispensable role of precise environmental regulation in SSF. This investigation not only deepens our understanding of the SSF process but also offers valuable insights for its application in industrial contexts [108].

3.2. Liquid State Fermentation

Liquid state fermentation is a method that involves inoculating a fermentation substrate with natural fermentation materials or specific probiotics, then uniformly mixing with water at a ratio of 1.0:1.5 to 1.4 for fermentation [97,108,109]. This process embodies the life activities of microorganisms within a liquid matrix. First advocated by Henderson in 1814, liquid fermentation was initially applied in the Dutch feed industry. It integrates various factors such as temperature, oxygen, and pH to create optimal conditions for microbial growth and reproduction. Compared to SSF, liquid fermentation offers several advantages: thorough exchange of raw materials and fermenting substances, resulting in more uniform fermentation; the possibility of real-time monitoring during the process; production of high-quality, stable feed that can be directly administered after fermentation; and the presence of beneficial microorganisms, such as lactic acid bacteria, bifidobacteria, and yeast, in their growth phase within the final product. These microorganisms maintain high activity and can exert biological functions directly upon entering the animal intestine.

3.3. Influence of Pivotal Strains

Microbial fermentation of feed hinges upon the selection of appropriate microbial strains, a task complicated by the vast diversity and distinct biological characteristics of these organisms. It is imperative that chosen microorganisms are safe, stable, and non-toxic to animals, ensuring their suitability for use [123]. The selection criteria must prevent disruption of the host’s microbial equilibrium and must not lead to the production of harmful substances or toxins. Additionally, the microorganisms intended for feed must satisfy the following requirements [124,125,126,127,128,129]:

• It can reduce antinutritional factors in feed ingredients, thereby enhancing their nutritional quality.
• It promotes the absorption and utilization of nutrients such as proteins, vitamins, and minerals.
• It elevates the levels of bioactive substances in fermentation substrates and generates beneficial metabolic products.
• It regulates gut health in livestock and poultry, boosts immune function, and enhances animal productivity.

The application of feed additives must adhere to the catalog of species stipulated and published by agricultural authorities [130]. According to Announcement No. 2045 from the Ministry of Agriculture in China’s “Feed Additive Variety Catalog 2013”, 33 strains are deemed suitable for feed use, including prominent options such as Bacillus, Lactobacillus, and yeast [131]. Currently, the microorganisms predominantly used in the SSF of wheat bran include beneficial microbes such as lactic acid bacteria, Bacillus species, yeasts, and molds, all known for their exceptional extracellular enzyme activity [132].

(1) Lactic acid bacteria (LAB) are Gram-positive, anaerobic, or facultative anaerobic bacteria capable of fermenting carbohydrates, primarily glucose and sucrose, to produce lactic acid. They are acid-tolerant but heat-sensitive, becoming inactive at 65 °C to 75 °C, except for certain thermophilic strains. LAB can be classified into homofermentative and heterofermentative types, producing lactic acid alone or lactic acid, acetic acid, and CO2, respectively. Studies have shown that LAB metabolites, including enzymes, organic acids, and bacteriocins, inhibit the growth of harmful Gram-positive and Gram-negative bacteria, with effectiveness inversely related to the pH of the viable cell culture [133]. LAB and their metabolites enhance nutrient digestion and absorption in animals and have been reported to bind and degrade aflatoxins through cell wall adhesion and metabolic activity [134].

(2) Bacillus species, Gram-positive bacteria capable of forming endospores, exhibit both aerobic and facultative anaerobic characteristics. Their endospores resist high temperatures, pressure, gastric acid, and bile acids, allowing them to colonize the intestines effectively and consume intestinal oxygen, thereby inhibiting the growth of harmful aerobic bacteria. Bacillus secretes digestive enzymes such as amylases, proteases, lipases, glycosidases, and cellulases, which reduce antinutritional factors, significantly enhance the intestinal environment, and improve nutrient digestion and absorption [135].

(3) Yeast is a facultative anaerobic unicellular fungus that primarily reproduces by budding. It metabolizes carbohydrates to produce carbon dioxide, water, and alcohol under both aerobic and anaerobic conditions. During feed fermentation, yeast generates digestive enzymes, free nucleotides, B vitamins, and amino acids, enhancing the nutritional profile of the feed. It breaks down complex proteins into smaller peptides, improving nutrient absorption and feed palatability. Additionally, yeast produces antibiotics and thiamine, inhibiting the growth of Gram-negative bacteria [136]. In the rumen of ruminants, yeast helps stabilize pH levels and reduces methane emissions [137].

(4) Molds, a general term for filamentous fungi, possess remarkable environmental adaptability and reproductive capabilities, exemplified by commonly used species such as Rhizopus, Aspergillus niger, and Aspergillus oryzae. These molds are prolific producers of digestive enzymes, including proteases, cellulases, amylases, and pectinases, which break down starch and cellulose in feed into simple sugars accessible to yeast, thereby enhancing the protein content and quality of fermented feed [138]. Research indicates that Aspergillus niger can degrade aflatoxin B1 by secreting oxalic acid, inhibiting the toxin production by Aspergillus flavus, and thereby reducing toxin levels in feed [139]. Additionally, studies suggest that Aspergillus niger mitigates aflatoxin synthesis by regulating its biosynthetic pathways [140].

4. Transformation Analysis of Chemical Constituents in Fermented Bran

In comparison to wheat bran, the nutritional characteristics of fermented wheat bran are notably distinguished by several key features, as illustrated in

Table 2. Transformation Analysis of Chemical Constituents in Fermented Bran.

Nutritional Components	Pre-Fermentation (Bran)	Post-Fermentation (Treated with Different Strains)	References
Crude fibre	6.5%–11.3%, Higher content, affects nutrient absorption, possesses antinutritional factors.	Upon fermentation, 20–30% of the crude fiber is degraded, thereby diminishing the obstruction to mineral absorption.	[1,3,4,5,8,9,10,11,12,13,15,16,17,18,19,31,33,56,57,66,80,85,87,105,128,138]
Crude protein	Approximately 11.77% to 18%.	Following fermentation, it is transformed into easily absorbable small peptides and amino acids, with their content increasing, reaching up to 21% at its peak.	[1,3,4,5,8,9,10,11,12,13,15,16,17,18,19,31,33,56,57,66,80,85,87,105,128,138]
Phytic acid	High in content, it reduces the bioavailability of minerals.	Fermentation reduces phytic acid by approximately 50%, thereby enhancing the bioavailability of minerals.	[83,94,141,142,143,144,145,146,147,148]
Phenolic	When bound to cellulose, their bioactivity becomes restricted.	Post-fermentation, 20–30% of these compounds are liberated, thereby enhancing their antioxidant capacity.	[15,16,33,34,49,50,58,149]
Bioactive	Present in low concentrations.	Enhancement of amino acids, small peptides, and related probiotics.	[74,80,82,116,150,151,152]

.

4.1. Enhancing Nutritional Value Through Fermentation

The molecular weight of proteins significantly influences the rate of digestion and absorption in animals. In bran, where the quality of plant proteins is relatively poor, microbial fermentation transforms these into high-quality small peptides and even amino acids, enhancing their bioavailability to animals. This is evidenced by the increased acid-soluble protein content in fermented bran. During metabolism, Bacillus subtilis produces proteases that convert large, hard-to-absorb protein molecules into easily absorbable small peptides and amino acids [153]. Research by Wang et al. [150] demonstrated that yeast fermentation elevated the crude protein content in bran by 119.59% compared to unfermented bran while also enhancing lysine, vitamins, and enzyme content. Additionally, Aspergillus oryzae, commonly employed industrially for its proteolytic capabilities, generates a rich array of proteases to degrade large protein molecules [154]. The activity of neutral proteases in fermented bran can be gauged by assessing the amino acid content post-fermentation with Aspergillus oryzae.

Upon fermentation, the physicochemical properties of bran undergo significant transformation. The crude protein content rises as macromolecular proteins are converted into more readily absorbable peptides and amino acids, evidenced by an increase in acid-soluble proteins. Post-solid-state fermentation and drying, some microbial strains remain viable. The activity of microbial phytase degrades phytic acid in the bran, facilitating the release of phosphorus. Furthermore, crude fiber is broken down, enhancing the dietary fiber content of the bran. Simultaneously, the release of phenolic acids elevates the total phenolic content. Throughout the fermentation process, beneficial microbial metabolites proliferate [149].

4.2. Reduction of Antinutritional Factors

Dietary fiber constitutes the primary structural component of plant cell walls, encompassing cellulose, hemicellulose, lignin, and cutin. Within bran, dietary fiber serves as a principal antinutritional factor, followed closely by phytic acid.

Dietary fiber’s binding to divalent cations and encapsulation of minerals within plant cellulose matrices significantly impede mineral absorption, with studies indicating reductions in zinc, iron, and calcium bioavailability by up to 50%, 50%, and 20%, respectively [155]. Further, dietary fiber accelerates the transit of food through the intestine, thereby diminishing the digestibility of nutrients in feed. Phytic acid comprises an inositol molecule esterified with six phosphate groups. Phytase and citric acid significantly enhanced the solubility of minerals such as calcium, magnesium, and potassium in oat bran, with citric acid increasing the solubility of magnesium from 21% to 70% and manganese from 6% to 54% [156]. Although phytic acid is minimally toxic, its potent negative charge enables it to form insoluble complexes with numerous cations, significantly reducing the bioavailability of these mineral elements. Furthermore, phytic acid can bind with proteins to form phytate-protein complexes, thereby decreasing protein utilization [106]. Phytic acid also hampers the activity of various digestive enzymes, including proteases, amylases, and lipases, acting as a limiting factor for animal growth [157].

5. Functional Study of Fermented Bran

The underutilization of bran remains a significant challenge, as it is often overlooked as a resource despite its potential. Through the process of fermentation, bran can be converted into a high-quality feed ingredient for livestock and poultry, enhancing its nutritional value and digestibility. This transformation not only promotes the efficient and rational use of agricultural by-products but also aligns with the principles of sustainable development. By integrating fermented bran into animal feed, we can make meaningful strides toward resource optimization and ecological responsibility. This review endeavors to thoroughly explore the benefits of fermented bran, providing a theoretical framework for its development and application in advancing sustainable agricultural practices.

5.1. Exhibiting Probiotic Properties

The gut microbiota is predominantly composed of anaerobic bacteria, such as lactobacilli and bifidobacteria. These beneficial microorganisms colonize the intestinal mucosal surface, synthesizing essential vitamins and participating in the metabolism of carbohydrates and proteins while also enhancing mineral absorption. Even when dried, fermented bran retains some of these advantageous bacteria. Upon entering the gut, these beneficial microorganisms or spores constrain the living space of harmful bacteria like Escherichia coli and Salmonella, thereby inhibiting their proliferation. This establishes a microbial barrier in the gut, crucial for maintaining physiological health [158].

Bacillus subtilis-fermented bran contains a significant number of spores, which, upon ingestion, revive and proliferate extensively within the digestive tract. As an aerobic bacterium, Bacillus subtilis consumes oxygen in the gut, creating an anaerobic environment [159]. This condition facilitates the production of lactic acid, volatile fatty acids (VFA), and inhibitory protein-polypeptide substances, indirectly suppressing the growth of aerobic pathogens such as Salmonella and Escherichia coli while fostering the proliferation of beneficial anaerobic bacteria, thereby restoring microbial balance. Furthermore, yeast-fermented bran harbors numerous ascospores that, once reactivated in the gut, also deplete oxygen levels, promoting the growth of anaerobic probiotics such as lactobacilli [160].

5.2. Antioxidants and Immune Modulation

Fermentation of wheat bran can significantly enhance the activity of antioxidant enzymes in animal serum and liver while reducing malondialdehyde (MDA) levels. This process effectively mitigates oxidative damage caused by free radicals to cellular DNA, proteins, and membrane lipids. Consequently, it inhibits oxidative stress and diseases induced by reactive oxygen species [161,162,163].

Administering feruloyl oligosaccharides derived from fermented wheat bran via gavage significantly enhances the total antioxidant capacity (T-AOC) in serum, liver, kidneys, and ileum. It also increases the activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and the levels of glutathione (GSH). Concurrently, it reduces the presence of oxidative stress markers such as malondialdehyde (MDA) and 8-hydroxy-2′-deoxyguanosine (8-OHdG), thereby mitigating oxidative stress-induced damage. This mechanism is likely associated with the microbial fermentation process, which elevates the levels of soluble-bound FA and feruloyl arabinoxylan in wheat bran [151,164,165]. Incorporating fermented wheat bran can enhance the transcriptional activity of the antioxidant pathway nuclear factor E2-related factor 2 (Nrf2). This significantly elevates the expression of heme oxygenase-1 and glutathione S-transferase genes, thereby augmenting the body’s antioxidant capabilities [152]. Methionine and cysteine, precursors of glutathione, see an increase in essential amino acid content through microbial fermentation of wheat bran, promoting the synthesis of glutathione and consequently boosting the body’s antioxidant capacity [166,167].

Bran is replete with an array of functional polysaccharides such as arabinoxylan, β-glucan, and galactan. Through the sophisticated process of microbial fermentation, the concentration of these bioactive components is enhanced. This process bolsters both specific and non-specific immune modulation by activating immune cells, releasing cytokines, and promoting antibody production [127]. Fermented wheat bran has been shown to fortify the mucosal barrier function of the animal intestine, promoting the proliferation and differentiation of T and B lymphocytes. It stimulates the secretion of cytokines by T helper cells 1 (Th1) and T helper cells 2 (Th2), thereby enhancing the activity of natural killer (NK) cells and phagocytes. This process ultimately boosts both humoral and cellular immunity within the organism [168,169].

In conclusion, the fermentation of wheat bran can bolster the body’s antioxidant capacity and activate both innate and adaptive immune responses, thereby enhancing overall immune function.

6. Conclusions

This review explores the potential of fermented bran as a sustainable solution for transforming food waste into high-value animal feed. By employing fermentation processes, the nutritional profile of wheat bran is significantly enhanced, leading to a marked increase in the bioavailability of crucial amino acids, peptides, and probiotics, which are beneficial for animal health and growth. The synthesis of current research highlights the efficacy of fermentation in converting agricultural by-products into nutrient-rich feed components. The article advocates for the broader use of fermentation technologies to meet the growing demand for sustainable livestock nutrition. This approach not only mitigates food waste but also supports agricultural sustainability goals by creating an efficient cycle of waste valorization. Consequently, fermented bran stands out as a promising innovation in the development of eco-friendly and nutritious animal feed solutions.