Production of High-Value-Added Biomass by Saccharomyces cerevisiae Using Lignocellulosic Substrate

Abstract

The aim of this study was to increase the availability of high-value-added compounds by applying S. cerevisiae to rice bran substrates (whole and defatted). The substrates were subjected to solid-state fermentation with yeast (3% pp⁻¹) and water (30%) for up to 8 h at 30 °C. The fermentation of brown rice bran resulted in increased ash, protein, and fiber contents, while the fermentation of defatted rice bran led to higher lipid and fiber levels. Additionally, the fermentation process influenced the mineral profile. The phenolic compound content of the fermented brown rice bran increased over fermentation, reaching values of 1165 µg g⁻¹ per sample. Brown rice bran fermented for 6 h yielded the best results in terms of nutrient and bioactive compound availability. Principal component analysis (PCA) revealed correlations between variables, suggesting that modifications could further enhance the availability of various compounds.

1. Introduction

Rice bran, a co-product obtained by the rice milling industry that is a rich source of nutrients like fiber, vitamins, proteins, minerals, lipids, and polyphenols, is gaining commercial importance in the world because of its nutritional and biological effects, showing that it can be incorporated into human diets to improve their health, especially in developing countries. Cellulosic and hemicellulosic materials are considered to be sustainable biomass resources, and several efforts have been made to produce different compounds using yeast and other microorganisms, in addition to bioethanol [1]. Fermentation is used for the conversion of lignocellulosic materials, present in hulks and bran, into compounds that have the potential to provide solutions to nutritional problems, such as the inadequate intake of proteins, minerals, fibers, and lipids. This also reduces waste and environmental pollution and increases co-product quality. Fermented products contain nutrients that can be used in humans’ and animals’ diets and can replace conventional sources. However, it is important to promote the use of microorganisms that are “Generally recognized as safe (GRAS)” in fermentation and cannot cause harm to their consumers. The use of Saccharomyces cerevisiae in solid-state fermentation is widely accepted due to the history of its use in traditional fermentation and nutritional quality [2,3].

Minerals present in products that are rich in lignin, such as rice bran, can become bioavailable and bioaccessible when solid-state fermentation is applied [4]. They are essential in the metabolism of humans and must be incorporated through the diet, improving the functioning of the human body due to their interactions in genetic and physiological processes. The deficiency of minerals could bring structural and physiological problems. In addition to these nutrients being present in rice bran, they also contain high levels of phytochemicals such as vitamins, γ-oryzanol, phytic acid, tocopherols, tocotrienols, carotenoids, γ-aminobutyric acid, octacosanol, squalene, unsaturated fatty acids, phytosterols, and phenolic compounds [5,6]. Phenolic compounds have been extensively investigated due to having a varied range of bioactivities, such as antioxidants, antimicrobials, antivirals, and anti-inflammatory properties that can promote human health. The bioactive properties of these compounds can play an important role in preventing certain chronic diseases such as diabetes, chronic inflammation, cardiovascular diseases, and certain types of cancer [7,8].

In addition to the nutritional importance of these compounds in the diet, they also have an influence on the generation of new compounds during fermentation. Parameters such as acidity and pH combined with the presence of several compounds inherent to the matrix can activate different metabolic pathways and enzyme activation, making the origin of specific compounds favorable. In other words, fermentation can be optimized in order to obtain different compounds and applications, making biotechnology a versatile option for creating nutritious food inputs [9,10,11].

Xie et al. [12] investigated the impact of sprouting and fungal fermentation on the nutritional value and flavor of rice bran, a nutrient-rich but tasteless by-product of rice. They found that sprouting increased the nutrients, while fermentation significantly altered the taste, depending on the type of fungus used. Sprouting reduced phytic acid by 13.3% and increased the total fiber content by 40% and soluble fiber content by 60%. Fungal fermentation increased the protein content by 19%. Su et al. [13] found that the co-fermentation of defatted rice bran with Bacillus subtilis, Saccharomyces cerevisiae, and Lactiplantibacillus plantarum improved its nutritional value, reduced antinutritional factors, and altered its microbial and metabolic composition, reducing the pH from 6.47 to 4.25, increasing the crude protein content by 21.54%, and decreasing phytic acid by 43.3%.

In this context, studies involving the solid-state fermentation (SSF) of rice bran with GRAS microorganisms such as Saccharomyces cerevisiae have shown the potential for enhancing this by-product’s nutritional and functional value. However, most reports focus on fermentation with filamentous fungi or liquid-state fermentation, and limited studies address the impact of S. cerevisiae on both whole and defatted rice bran. Therefore, this work aims to explore the effects of SSF with S. cerevisiae on the nutritional profile, mineral content, and phenolic compound release in rice bran, and to understand the metabolic interactions via multivariate analysis (PCA). The hypothesis is that fermentation improves the bioactive compounds’ availability and modifies the bran’s nutritional composition, with distinct responses occurring between defatted and whole substrates.

2. Materials and Methods

2.1. Material

The brown rice bran (BRB) was obtained from the polishing of the rice grain, and the defatted bran (DRB) from the co-product generated after the pressing process for oil extraction. Brown and defatted rice brans were supplied by the grain processing industries localized in the state of Rio Grande do Sul, Brazil (latitude −31.776, longitude −52.3594, 31°46′34″ South, 52°21′34″ West). Bakery yeast (lyophilized Saccharomyces cerevisiae) was obtained in the local market and used as the source of cells.

2.2. Solid-State Fermentation

Solid-state fermentation was carried out according to Christ-Ribeiro et al. [14], where rice bran (brown and defatted) was distributed in tray bioreactors (29 × 17 × 5.5 cm dimension) in layers with a maximum height of 1 cm and was then sterilized. Afterward, Saccharomyces cerevisiae (3% ww⁻¹), which was previously hydrated, was added on the medium and the moisture content was adjusted to 30% with the addition of sterile water. Negative controls were included: samples of BRB and DRB incubated under the same conditions without S. cerevisiae. The process was carried out in an oven with air circulation at 30 °C for up to 8 h. Biomass aliquots were removed every 2 h of fermentation and frozen at −18 °C for later use.

2.3. Chemical Composition

The chemical composition of the brans was determined and the moisture (No. 935.29), protein (No. 920.87; conversion factor of 5.75), lipids (No. 920.85), and ash (No. 923.03) content were determined according to the AOAC methods [15]. The fiber content was determined by gravimetry using the basic residues and acids obtained in the digestion of samples. Carbohydrates were estimated by difference. The pH measurement was performed using a digital Lutron pH meter (model PH-206, Coopersburg, PA, USA) and acidity was expressed as the percentage of acetic acid (%) in samples according to AOAC [15].

2.4. Minerals

The brown rice bran, defatted rice bran, and the fermented brans obtained were weighed (~300 mg) in triplicate (n = 3) and digestion was performed in a microwave oven model “Speedwave four” (Berghof, Berchtesgaden, Germany).

Table 1. Microwave heating program.

Steps	Time (min)	T (°C)	Length of Stay (min)	Pressure (bar)
1	20	170	10	35
2	5	200	25	35
3	5	50	20	35

shows the program used for quantification. For digestion, 5 mL of concentrated nitric acid (HNO₃) (Merck, Darmstadt, Germany) and 0.5 mL of 40% hydrofluoric acid (HF) (Merck) were used. After digestion, the samples were diluted by 5 aliquots in 5% HNO₃ for further quantification. The determinations were made by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) with model Optima 4300 DV (Shelton, CT, USA). The samples were introduced into the spectrometer using a GemCone nebulizer and a cyclonic chamber nebulizer. The other operating conditions of the equipment are described in

Table 2. ICP-OES operating conditions.

Parameters	Conditions
Potency (W)	1400
Main gas flow (L min−1)	15
Auxiliary gas flow (L min−1)	0.2
Neutralization gas flow (L min−1)	0.7
Wave-length (nm)	Ca—315.887 Fe—238.204 Mg—279.077 Na—589.592 P—214.914 S—181.975 Cr—267.717 Cu—324.759 Mn—259.374 Zn—213.857

. The plasma was formed from argon (White Martins, São Paulo, Brazil) with 99.996% purity. The microwave digestion program used for mineral analysis is presented in Table 1, and the ICP-OES operating conditions are described in Table 2.

2.5. Phenolic Compound Extraction and Quantification

The phenolic compounds were extracted using methanol (Vetec, São Paulo, Brazil) as the extracting solvent at a proportion of 1:8 (w/v). The mixture underwent orbital agitation (Tecnal TE-141, São Paulo, Brazil) at 160 rpm and at room temperature for 2 h. After resting for 15 min, another 10 mL of solvent was added and agitated for 1 h. The methanolic extracts were evaporated in a route-evaporator (Quimis^®, Q-344B2, São Paulo, Brazil) and the phenolic compounds were resuspended with distilled water. The resulting extract was clarified, centrifuged, and filtered to obtain the phenolic compounds. The total content of the phenolic compounds was determined by spectrophotometer (Varian Cary 100/UV-Visible, Palo Alto, CA, USA) at 750 nm with Folin–Ciocalteau reagent and was quantified using a gallic acid (Sigma-Aldrich, Tokyo, Japan) standard curve (2 to 30 μg mL⁻¹) [10].

2.6. Statisctical Analysis

Statistical analysis was performed using the software STATISTICA (version 7.0, StatSoft Inc., Tulsa, OK, USA). All assays were performed in triplicate and the results were reported as means. One-way ANOVA was applied on the data and Tukey’s post hoc test was used to determine the significant differences between the variables with three or more treatments. T-student tests were applied when the means were compared between the BRB and DRB. Differences with a p-value < 0.05 (95% confidence) were considered significant. Principal component analysis (PCA) was performed in Past 4.0.2 software to establish correlations between chemical composition, minerals, and phenolic compounds.

3. Results and Discussion

Table 3. Chemical composition (dry bases) of BRB and DRB submitted to different fermentation times.

	Ash (%)		Protein (%)		Lipids (%)		Fiber (%)		Carbohydrate (%)
	BRB	DRB	BRB	DRB	BRB	DRB	BRB	DRB	BRB	DRB
NF	15.6 cA	14.1 bB	13.9 cB	33.2 cdA	40.3 aA	3.2 aB	6.1 eA	5.3 bA	23.9 aB	44.1 aA
0 h	20.2 bcA	18.1 aB	17.9 bcB	35.9 bcA	28.8 bA	4.1 aB	7.4 dA	8.1 aA	25.4 aB	33.7 bA
2 h	25.7 aA	18.1 aB	18.1 abcB	31.9 dA	31.6 abA	4.5 aB	7.8 dA	8.2 aA	16.6 abB	37.1 abA
4 h	20.0 bcA	17.7 aB	29.5 aB	38.3 bA	28.6 abA	5.6 aB	7.9 cdA	7.6 aA	12.8 bB	31.9 bA
6 h	21.3 abA	17.9 aB	24.3 abcB	42.5 aA	26.6 bA	5.8 aB	9.3 aA	7.8 aB	18.4 abA	25.8 bA
8 h	20.7 abcA	17.7 aB	25.8 bB	38.5 bA	34.5 abA	5.1 aB	8.9 bcA	6.2 bB	9.9 bB	32.3 bA

BRB—brown rice bran; DRB—defatted rice bran. The results are expressed as means and they show a coefficient of variation < 20%. The values in columns with the same lowercase letter do not indicate significant differences in fermentation time by Tukey’s test (p > 0.05). Values in rows with the same uppercase letter do not indicate significant differences in the t-student test (p > 0.05).

shows the results obtained for the chemical composition of the BRB, DRB, non-fermented (NF), and fermented biomasses up to 8 h. According to Table 3, there was an increase in the ash, protein, and fiber contents and a decrease in lipids and carbohydrates when S. cerevisiae was applied to the BRB and DRB after 8 h of fermentation. The 6 h of fermentation showed the highest levels of the analyzed compounds for both brans. Considering the best fermentation time (6 h), the ash, lipid, and fiber contents were higher in the BRB, and the protein and carbohydrate contents showed higher levels in the DRB. This can be explained by the fact that some proteins can bind to the fatty acids present in the samples. Since DRB has a low lipid content due to the oil extraction process, the unbound proteins are available [11].

During alcoholic fermentation, the metabolic pathway of S. cerevisiae is characterized by a complex interaction between the acetylation, production, and degradation of amino acids [16]. In certain lignocellulosic substances, cellulose is tightly bonded with hemicelluloses and lignin, serving as an obstacle to biomass compound production. This bond resists degradation and provides hydrolytic stability and structural strength to the rice bran cell walls. To overcome the resistance of lignocellulose, one alternative is to apply treatments to alter the structure and microscopic chemical composition of the materials. The enzymatic hydrolysis of the carbohydrate fraction into monomeric sugars is a biotechnological treatment that can achieve higher yields of compounds more quickly. The growth of the microorganism on a substrate alters its chemical composition due to the production of exocellular enzymes which can be used to obtain nutrients and to produce other metabolites suitable for the fermenting agent. This metabolization can enrich the substrate depending on the intrinsic components of the fermenting agent, or due to the availability of nutrients present in it that were inaccessibly associated with chemical or enzymatic extractive processes before the hydrolysis [17].

The results presented in Table 3 are in agreement with Dos et al. [17], who obtained an increase in ash and fiber of 13 and 2%, respectively, and a decrease in carbohydrates of 4.8% with 6 h of fermentation by S. cerevisiae. According to Aruna et al. [18], the solid-state fermentation of yam peels using the same yeast for 96 h in the presence of ammonium sulfate showed an increase in crude protein content, real protein, lipid, and ash content; the initial contents were 6.6%, 4.3%, 1.1%, and 4.4%, reaching up to 15.5%, 13.3%, 2.1%, and 8.0%, respectively.

Table 4. pH and acidity of fermented and non-fermented brown rice bran (BRB) and defatted rice bran (DRB).

	BRB
	NF	0 h	2 h	4 h	6 h	8 h
pH	6.57 aA	6.44 bA	6.42 cA	6.38 dA	6.31 eA	6.24 fA
Acidity (%)	0.18 eA	0.29 aA	0.26 bA	0.24 cA	0.22 dA	0.22 dA
	DRB
	NF	0 h	2 h	4 h	6 h	8 h
pH	6.27 aB	6.17 bB	6.10 cB	6.06 dB	6.06 dB	6.02 eB
Acidity (%)	0.17 bB	0.23 aB	0.22 aB	0.24 aA	0.23 aA	0.21 aA

The results are expressed as means and they show a coefficient of variation < 20%. Values in columns with the same lowercase letter do not indicate significant differences in fermentation time by Tukey’s test (p > 0.05). Values in rows with the same uppercase letter do not indicate significant differences in the t-student test (p > 0.05). NF = non-fermented.

shows the pH and acidity during the fermentation of rice bran. The BRB and DRB showed a decrease in pH during fermentation, reaching pH values of 6.24 and 6.02, respectively, during 8 h of fermentation. Non-fermented (NF) DRB had an original pH value of 6.27; in comparison, the BRB fermented for 8 h reached 6.24, while the DRB became acidic after the same length of fermentation time, reaching pH 6.02. This pH value obtained for DRB after 8 h of fermentation can be attributed to the increase in organic acids produced by the microorganism through the degradation of carbohydrates during fermentation, providing the fermented biomass with increased digestibility and availability for use of nutrients and vitamins; this acidic pH reached with fermentation also helps in food preservation [18]. Xu et al. [19] evaluated the fermentation of the feed for 72 h, which caused a notable drop in pH from 7.15 to 5.24, indicating a significant increase in the medium’s acidity, a finding corroborated by the broader context of solid-state fermentation (SSF) described in this study, where decreased pH was a common observation alongside enhanced nutritional value and metabolite production, driven by the dynamic interactions of microorganisms like Saccharomyces cerevisiae. Both brans showed an increase in their acidity levels when the microorganism was added to the substrate (0 h). The acidity content decreased in BRB after 8 h of fermentation; these results can be linked with the number of fatty acids present in this sample. Organic acids present in foods influence taste, odor, color, stability, and quality maintenance [20].

Table 5. Mineral contents present in fermented and non-fermented brown rice bran (BRB) and defatted rice bran (DRB) during fermentation.

Analyte	BRB						DRB
	NF	0 h	2 h	4 h	6 h	8 h	NF	0 h	2 h	4 h	6 h	8 h
Ca	559 aB	545 aB	479 aB	553 aB	575 aB	607 aB	858 aA	746 aC	794 aB	784 aA	1053 aA	991 aA
Fe	134 bA	251 abA	161 abA	271 aA	184 abB	189 abA	309 aA	218 aA	243 aA	398 aA	290 aA	259 aA
Mg	14,086 aA	13,793 abA	13,407 bA	13,231 bA	13,418 bA	13,543 abA	13,261 aB	12,840 aB	13,228 aA	13,450 aA	13,010 aA	13,304 aA
Na	80.5 cB	116 cB	136 bA	128 cA	140 aA	135 bB	116 bA	144 bA	125bA	131bA	145bA	229 aA
P	25,569 aA	25,607 aA	24,456 cA	24,456 cA	25,543 abA	24,455 bcA	23,427 abB	22,846 bB	23,661 abB	23,500 abB	23,020 abB	23,966 aA
S	1889 aA	1777 aB	1893 aA	1973 aA	1923 aA	2018 aA	2142 aA	2151 aA	1966 aA	2227 aA	2047 aA	1938 aA
Cr	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46	<0.46
Cu	7.50 aA	8.00 aA	7.50 aA	7.20 aA	8.00 aA	7.40 aA	8.5 aA	7.60 aA	7.80 aA	7.60 aA	7.60 aA	7.90 aA
Mn	194 aB	191 aB	192 aB	192 aB	192 aB	195 aB	276 aA	258 aA	260 aA	262 aA	260 aA	264 aA
Zn	111 aA	115 aA	84.2 aA	105 aA	84.6 aB	95.3 aB	117 aA	111 aA	106 aA	121 aA	136 abA	116 aA

The results (µg g⁻¹) are expressed as means. Values in rows with the same uppercase letter do not indicate significant differences by Tukey’s test (p > 0.05). Values in columns with the same lowercase letter do not indicate significant differences in fermentation time by Tukey’s test (p > 0.05).

shows the minerals calcium (Ca), iron (Fe), magnesium (Mg), sodium (Na), phosphorus (P), sulfur (S), chromium (Cr), copper (Cu), manganese (Mn), and zinc (Zn) and their variations during fermentation. These elements, particularly Ca, Mg, Na, K, Fe, Zn, Cu, and Mn, are classified as being essential not only for human health, due to their roles in numerous biochemical and physiological processes, but also for microbial metabolism during fermentation. The minerals present in rice bran, in addition to their nutritional relevance, play a fundamental role in microbial metabolism during fermentation. Many of these elements serve as essential enzymatic cofactors, directly participating in biochemical pathways that involve polysaccharide degradation, protein synthesis, electron transport, and intracellular pH regulation. Thus, their availability in the fermented matrix influences both cell growth and the production of secondary metabolites, such as organic acids and phenolic compounds [21].

During solid-state fermentation with S. cerevisiae, variations in the levels of minerals such as calcium (Ca), iron (Fe), magnesium (Mg), sodium (Na), and phosphorus (P) are observed, reflecting their active participation in metabolic processes. The increase in iron of up to 50% in BRB after 4 h of fermentation may be related to the more significant release of this mineral from complexes with phytate, potentially facilitated by the acidification of the medium and by enzymatic activities activated during fermentation. Iron, in turn, is essential for the functioning of respiratory chain enzymes and for the activity of dehydrogenases, which contributes to the regeneration of NAD⁺, which is necessary for the continuation of alcoholic fermentation [22].

Phosphorus and magnesium, which remain in high concentrations, are key cofactors for enzymes such as ATPases and kinases, in addition to participating in the stabilization of structures like phospholipids and nucleic acids. Phosphorus is also involved in cellular energy balance (ATP/ADP) and is essential for the growth and metabolic activity of yeast. Maintaining its levels during this process may have sustained cell viability and favored the synthesis of bioactive compounds. The presence of manganese (Mn), even without significant variation, is also essential, as this element participates every day during fermentative metabolism in the activity of superoxide dismutase (SOD) and other enzymes that protect cells against oxidative stress. Furthermore, Mn and Cu are associated with lignolytic enzymes that participate in the degradation of the lignocellulosic fraction, facilitating the release of fibers and phenolic compounds from the plant matrix [4]. The increase in sodium (Na) content observed, especially in DRB, after 8 h of fermentation, may be related to the secondary metabolism of the yeast and the formation of soluble salts resulting from the degradation of the structural components of the biomass. In fermentative systems, Na can also contribute to the osmotic balance and enzymatic stability, favoring the solubilization of nutrients [22].

Therefore, the dynamics of minerals during fermentation not only influence the nutritional quality of the final product but also play a direct role in the biochemical transformation of the biomass, contributing to the synthesis, modulation, and release of high-added-value compounds. This complex interaction between nutrients and microbial metabolism underscores the potential of fermentation as a versatile biotechnological tool for utilizing agricultural by-products.

Silva et al. [21] found that brown rice contributes significantly to the recommended daily doses of some minerals, including Mn (61%) and Mg (20%), while parboiled rice contributes to Cu (45%) and white rice to Mn (14%). Biological or synthetic materials can be transformed into products of interest by fermentation with biological agents. The variation in the mineral content between non-fermented and fermented biomass occurs due to its use in mycelial synthesis [17].

A similar effect to this study was observed by Oduguwa et al. [23] when the authors cultivated R. oligosporus and S. cerevisiae using BRB as substrate; the ash content, for example, increased by 24.5% after 48 h of fermentation. According to Orlean [24], the cell wall of S. cerevisiae has 1.4% ash, 2% total lipids, and 3.8% insoluble fibers. In addition to minerals, the extraction of phenolic compounds during 8 h of fermentation is evaluated and is shown in Figure 1. BRB shows a higher content (10%) of extractable phenolic compounds than DRB at all fermentation times. This result can be explained by the ways in which phenolic compounds can be formed—examples include by the decomposition of the bonds between lignin, cellulose, and hemicellulose or by the production of a part of rice bran oil [10]. In the case of rice bran fermentation, the increase in the content of phenolic acids is caused mainly by the cleavage of compounds complexed with lignin [17].

According to Schmidt et al. [22], the application of R. oryzae in solid-state fermentation using BRB as a substrate influenced the phenolic acid content, increasing it by more than two times. Abduh et al. [25] demonstrated that solid-state fermentation with Aspergillus niger significantly increased the phenolic extract yield, enhancing the ferulic acid content by up to 58 times and improving the antioxidant activity of the defatted rice bran.

Fungi synthesize lipids to supply their basic functions and form cell membranes, although some species of fungus can accumulate lipids, generating biomass with up to 20% of lipids. In the cultivated substrates, the increase in lipid content can occur through a direct method of cellulolytic enzymes converting the lignocellulosic biomass into sugars, and consequently, lipids can be developed [11,26,27]. To evaluate the effect of the different substrates during fermentation by S. cerevisiae and the relationship with the compounds obtained, the principal component analysis (PCA) obtained is presented in Figure 2.

The principal component analysis (PCA), presented in Figure 2, allows us to understand the correlations between the different parameters evaluated—chemical composition, minerals, and phenolic compounds—and how they are influenced by the characteristics of the substrate (BRB vs. DRB) and the fermentation process. The first principal component (PC1), responsible for 54% of the total variance, clearly separates the groups based on the presence of lipids, which are strongly associated with BRB. This indicates that the lipid fraction plays a determining role in the metabolic configuration of fermentation. Lipids can act as physical barriers to the penetration of water and enzymes into the lignocellulosic matrix, partially limiting the release of other compounds, such as proteins and phenolics. This explains the inverse association observed between lipids and proteins, suggesting that, in the absence of lipids (as in DRB), there is greater protein availability. The second component (PC2), responsible for 20% of the variance, highlights the direct effect of fermentation on biomass transformation. The separation of points along the y-axis represents the changes induced by fermentation time, especially related to the release of phenolic compounds and variations in mineral and fiber content. The presence of minerals such as Cu and Mn close to carbohydrates reinforces their association with the activity of lignolytic and carbohydrolytic enzymes, such as laccases, manganese peroxidases, and glycosidases, which promote the cell wall degradation and the release of bioactive compounds.

The correlation between pH and lipids suggests that biomass with a higher lipid content presents lower acidification, probably due to the lower degradation of fermentable carbohydrates and the consequent production of organic acids. The drop in pH, associated with an increase in proteins and phenolics, indicates that the acidification of the medium is closely related to the metabolic activity of the yeast and the solubilization of compounds previously inaccessible in the plant matrix.

Another relevant point is the association of minerals such as Fe, Na, Ca, and S with protein content, suggesting the possible formation of mineral–protein complexes or their participation as cofactors in reactions related to protein synthesis or release. The presence of sulfur-containing amino acids, such as cysteine, contributes to this correlation. These relationships show that the type of substrate (BRB vs. DRB) and the fermentation conditions do not affect each parameter in isolation, but rather promote an integrated modulation of the biomass, with cascading effects on the final composition. PCA, therefore, not only confirms the trends observed in isolated analyses, but also reveals subtle and relevant metabolic interactions that can guide future process optimizations [28,29,30].

4. Conclusions

This study demonstrated that solid-state fermentation with Saccharomyces cerevisiae was an effective strategy for the nutritional and functional modification of rice bran, promoting the valorization of a widely available agricultural by-product. The application of yeast resulted in significant changes in the chemical composition of the biomass, with an emphasis on the increase in the levels of proteins, fibers, and phenolic compounds, in addition to a modulation of the mineral profile. Multivariate analysis (PCA) revealed the relevant correlations between the biomass constituents, highlighting the central role of minerals as metabolic cofactors and their direct influence on the compounds generated and transformed throughout the fermentation. The difference between the integral and defatted substrates showed that the initial composition of the matrix directly impacted the activated biochemical pathways, allowing for distinct nutritional profiles to be obtained from the same fermentation process.

In addition to contributing to the sustainable use of agro-industrial waste, the results obtained indicate that fermented biomass can be applied as a functional ingredient in foods or supplements, with the potential for human and animal use. Understanding the interactions between the matrix, microorganism, and metabolism opens up paths for the development of optimized formulations, customized according to the desired nutritional or technological objective.