Research progress on the influence and mechanism of fermented feed on pork quality

　　China is the world's largest producer of pig farming, and pork is also the main meat product consumed by Chinese residents. Pig farming plays a very important role in China. At present, about 90% of the live pigs raised in China are foreign breeds such as "Du Chang Da". The long-term pursuit of high lean meat percentage and high growth performance has led to a serious decline in meat quality indicators such as sensory quality, nutritional value, intramuscular fat (IMF), and flavor substance content. The production of high-quality pork in China is facing huge challenges. Genetic breeding for variety improvement is an effective measure to enhance the quality and flavor of pork, but it has the problems of long cycle and high cost. Therefore, how to improve the quality and flavor of pork through feed nutrition is of great significance for the production of high-quality pork and meeting people's needs for a better life. Fermented feed is an active feed containing probiotics and their metabolites. Recent studies have shown that fermentation can improve the nutritional composition of feed, providing probiotics, fatty acids, antioxidants, and flavor compounds, thereby increasing feed utilization, promoting animal intestinal health, and improving pork quality and flavor. Therefore, using fermented feed may be a convenient measure for producing high-quality pork.

　　1. Current situation of fermented feed

　　At present, China's fermented feed industry is developing rapidly, with over 1000 enterprises engaged in fermented feed. By 2025, the output will reach about 40 million tons, and the output value will exceed 100 billion yuan in the next five years. By searching for "Fermented feed" as the keyword in the PubMed database, the number of relevant research articles has increased from 92 in 2000 to 841 in 2021, indicating a rapid increase in the attention and research and development of fermented feed. Although the production of fermented feed has gone through a period of mixed fermentation strains and raw materials, unclear fermentation conditions, and uneven quality of fermented products, with the introduction of corresponding policies and standards by the country and industry, the development of the industry and market has gradually moved towards orderliness and standardization. On January 1, 2018, China released the first group standard in the field of biological feed - "Classification of Biological Feed Products", followed by the release of 12 group standards related to fermented feed, including "Microbial Enzyme Collaborative Fermentation Feed for Breastfeeding Pigs", "Microbial Enzyme Collaborative Fermentation Feed for Growing and Fattening Pigs", "Feed Raw Materials - Yeast Hydrolysis Products", and "Feed Additives - Clostridium butyricum". In November 2021, the General Office of the Ministry of Agriculture and Rural Affairs issued a notice on the "Guidelines for Identification and Safety Evaluation of Production Strains of Microorganisms and Fermented Products Directly Fed". The establishment of these policies and standards has played an important role in promoting the standardization and healthy development of China's fermented feed industry.

　　At present, fermented feed mainly includes single feed raw material fermentation and mixed feed fermentation. The selection of fermentation raw materials has gradually developed from conventional feed raw materials (corn, soybean meal, rapeseed meal, cottonseed meal, etc.) to unconventional low value raw materials (corn germ meal, rice bran meal, distiller's grains, tea residue, mulberry leaves, etc.). Fermentation can be divided into solid-state fermentation and liquid state fermentation based on the proportion of added water. It can also be divided into aerobic, anaerobic, and facultative anaerobic fermentation based on oxygen requirements. Fermentation can be divided into single bacteria, mixed bacteria, and bacterial enzyme synergistic fermentation based on the compatibility of fermentation agents. Probiotic fermentation can degrade the content of anti nutritional factors in feed, provide beneficial metabolites, improve nutrient utilization, regulate intestinal microbiota balance, enhance antioxidant and immune levels, and promote intestinal health and improve pork quality. The future development trend of fermented feed mainly focuses on the selection of characteristic fermentation strains, in-depth exploration of fermentation mechanisms and processes, establishment of fermented feed databases, product safety evaluation, and construction of feeding systems.

　　The Improvement Effect and Mechanism of Fermentation on Feed Quality

　　High quality fermentation strains are the key to producing fermented feed. The currently widely used fermentation strains include Bacillus, lactic acid bacteria, and fungi. Bacillus and fungi can decompose large molecular substances, alter the microstructure of feed, degrade anti nutritional factors, and thus enhance the nutritional value of feed. The reason is that these fermentation strains can degrade large organic molecules such as proteins, starch, cellulose, and fats into easily absorbable small peptides, amino acids, oligosaccharides, and fatty acids by secreting extracellular enzymes such as protease, amylase, cellulase, and lipase. Lactic acid bacteria, Clostridium butyricum, and yeast can reduce the pH of fermentation substrates, improve feed palatability, inhibit harmful bacteria proliferation, and prolong feed storage time by producing organic acids, short chain fatty acids, and bacteriocins. Some lactic acid bacteria can also produce metabolites such as antioxidant enzymes, gamma aminobutyric acid, and anthocyanins, which enhance the antioxidant and probiotic functions of feed. In addition, yeast cell structure, cell wall, and cell contents have multiple effects on improving animal antioxidant and immune functions [6].

　　Probiotic fermentation can improve the nutritional value of feed by increasing the content of amino acids, fatty acids, flavor compounds, and biological enzyme activity. In addition, fermentation conditions are also crucial for fermentation quality, and appropriate fermentation conditions can avoid bacterial contamination and mold growth. Our team used Bacillus subtilis and lactic acid bacteria to ferment corn, soybean meal, soybean bran, and wheat bran mixed feed, and found that the content of unsaturated fatty acids in the feed significantly increased, while the content of saturated fatty acids significantly decreased. The content of antioxidant substances such as glutathione and folate increased significantly by 60.00% and 106.63%, respectively. The activity of antioxidant enzymes such as superoxide dismutase (SOD), glutathione peroxidase (GSH Px), and catalase (CAT) increased significantly by 45.60%, 8.65%, and 9.44%, respectively. At the same time, the in vitro digestibility of essential amino acids such as crude protein, dry matter, isoleucine, leucine, arginine, phenylalanine, threonine, valine, and non essential amino acids such as alanine, glutamic acid, tyrosine, and glycine were significantly improved. Among them, methionine and phenylalanine The in vitro digestion rate increased by more than 10% [7]. Feng Xin et al. [8] used mixed probiotics to ferment cottonseed meal, increasing the crude protein content and reducing the fiber content. The content of free gossypol was significantly reduced by 75.39%, and the activities of acid protease, neutral protease, and xylanase were significantly increased by 44.25%, 181.08%, and 65.00%, respectively. Our team used Bacillus and Lactobacillus to ferment yellow wine lees, which significantly reduced the cellulose content by 37.85%, increased the small peptide and total amino acid content by 23.08% and 12.20%, and significantly increased the in vitro digestibility of true protein, total energy, and total amino acids by 12.38%, 9.95%, and 21.13%, respectively. At the same time, the fatty acid composition was improved [9]. Fu Xiaoyu et al. [10] found that mixed bacterial fermentation significantly reduced the tannin content in tree leaves by more than 60%. Large molecular proteins were degraded into amino acids and small peptides, while crude fibers were degraded to produce aromatic substances. In addition, Cui Yiyan et al. [11] studied the fermentation of tea residue by Aspergillus niger and found that the crude protein, essential amino acids, flavor amino acids, reducing sugars, and flavonoids content of tea residue significantly increased by 28.52%, 185.64%, 202.82%, 211.03%, and 69.17%, respectively, after fermentation, while the fiber content significantly decreased.

　　During the fermentation process of feed, changes in the microbial community cause changes in the physicochemical properties and metabolic products of the feed, ultimately affecting the quality of the feed. Park et al. [12] found that bacterial fermentation mainly produces sulfur-containing volatile compounds and fatty acid derived volatile compounds (such as alcohols and ketones) when fermenting soybeans with different strains of bacteria; Fungal fermentation produces volatile products derived from phenylalanine, such as aldehydes, esters, and acetates. Our team found that during the synergistic fermentation of rice bran meal by Bacillus subtilis, Lactobacillus plantarum, brewing yeast, and phytase, microorganisms participate in the degradation of carbohydrates in rice bran meal through metabolic pathways such as glycolysis, pentose phosphate, tricarboxylic acid cycle, and fatty acid synthesis, as well as the synthesis of amino acids, unsaturated fatty acids, and nucleotides [13]. In addition, our team also found that during the fermentation of corn, soybean meal, and Huangjiu residue compound feed by Bacillus subtilis and lactic acid bacteria, fiber degradation occurred within 8-12 hours, and protein degradation occurred within 12-24 hours; The secreted biological enzymes mainly include neutral protease, xyloglucidase, and β - glucan endonuclease; Bacillus subtilis rapidly proliferates in the first 12 hours of aerobic fermentation, inhibiting the proliferation of harmful bacteria in feed and reducing the diversity of microbial communities in feed. Anaerobic fermentation increases the number of beneficial bacteria such as lactic acid bacteria, significantly improving the diversity of feed microbial communities. Bacillus subtilis and lactic acid bacteria are dominant bacterial genera in the fermentation process. Bioinformatics analysis found that environmental information processing and cell processing are the main metabolic functions of microorganisms in the fermentation process [14].

　　The impact and regulatory mechanism of fermented feed on pork quality

　　3.1 Pork Quality and Key Factors Influencing Its Formation

　　The quality of pork mainly includes sensory quality, nutritional quality, processing quality, and hygiene quality [15]. Among them, sensory quality mainly refers to the comprehensive perception of meat by consumers' visual, gustatory, olfactory, and tactile senses. Evaluation indicators include meat color, water holding capacity, flavor, muscle pH, marble texture, tenderness, juiciness, etc. Meat color is closely related to the body's antioxidant capacity, pH, and muscle fiber type. Within a certain range, the pH of pork is positively correlated with the meat color score [16]. The tenderness and water holding capacity are related to pH and IMF content. Pork flavor compounds mainly include fatty acids, nucleotides, flavor producing amino acids, and umami peptides [17]. Nutritional quality mainly refers to the nutritional value of meat, including the content and composition of amino acids, lipids (such as fatty acids), minerals, vitamins and other nutrients, which are closely related to people's dietary nutrition and health.

　　IMF is an important factor affecting pork quality. IMF is closely related to the sensory quality of pork, and its distribution and quantity determine the marble texture score; Meanwhile, the increase of IMF can help improve the flavor and juiciness of pork, as well as enhance its tenderness and flavor [18]. The fat distributed between muscle bundles (intermuscular fat) can effectively reduce muscle fiber density, and an appropriate fat content is beneficial for the water holding capacity and juiciness of pork. In terms of meat flavor, the fat in muscles can provide precursor substances for most volatile flavor compounds, directly affecting the type and content of volatile flavor compounds. IMF sedimentation is mainly influenced by factors such as species, environment, and nutrition. Our team found through comparing the IMF of different pig breeds that the IMF content of the longest dorsal muscle and lumbar muscle of 180 day old Jinhua pigs was 3.42% and 3.58%, respectively, significantly higher than that of Changbai pigs (2.28% and 2.59% of fresh weight) [19]. Zhang et al. [20] found that the expression level of proteasome subunit alpha 6 (PSMA6) gene in Jinhua pig liver and dorsal longest muscle tissue was significantly higher than that in Bactrian pigs, and the mRNA expression level of PSMA6 gene in these tissues was significantly positively correlated with IMF content, which can promote the proliferation of adipocyte precursor cells and adipogenesis. Our team found through analyzing the gene expression profiles of the longissimus dorsi muscle in Jinhua and Changbai pigs that the pig differential gene FLJ36031 (pFLJ) may regulate body fat deposition by regulating the expression of fat synthesis related genes such as fatty acid synthase, acetyl CoA carboxylase, triglyceride lipase, and hormone sensitive lipase [21]. Chen et al. [22] found that microRNA-331-3p (miR-331-3p) is highly expressed in the liver, muscle, and back fat of Laiwu pigs. It can act as a regulatory factor for adipocyte proliferation, differentiation, and fatty acid metabolism, inhibiting cell proliferation and promoting preadipocyte differentiation. Wang et al. [23] conducted genome-wide association analysis on pig IMF and found that the axonal guidance factor 1 (NTN1) gene can regulate the proliferation and differentiation of myoblasts, playing an important role in the formation of IMF.

　　Muscle fiber is another key factor determining the quality of pork. The type and composition of muscle fiber affect the color, pH, water holding capacity, and tenderness of pork, as well as its flavor [24]. Muscle fibers can be divided into oxidative muscle fibers (type I, type IIa) and glycolytic muscle fibers (type IIx and type IIb). The myoglobin content of oxidized muscle fibers is significantly higher than that of fermented muscle fibers. Therefore, muscles with a high proportion of oxidized muscle fibers have a bright red color and a higher meat color score, and pork exhibits better sensory quality [25]. Research has shown that the MyHC Ⅰ gene in the longest dorsal muscle of pigs can inhibit muscle oxidative stress, increase the proportion of oxidative muscle fibers, and enhance fatty acid oxidation through the proliferation of peroxisome activated receptor alpha (PPAR alpha), thereby improving meat quality [25]. Our team's research found that compared with Changbai pigs, the longest dorsal muscle of Jinhua pigs contains more type I and type IIa oxidized muscle fibers and lower content of type IIb glycolytic muscle fibers. The expression level of extracellular regulated protein kinase (ERK) in the longest dorsal muscle of Changbai pigs is significantly higher than that of Jinhua pigs; After overexpression of ERK, the content of type I oxidative muscle fibers was significantly reduced and the content of glycolytic muscle fibers was increased, indicating that the ERK signaling pathway plays an important role in regulating the transformation of muscle fiber types [26]. In the myotubes, the small RNA-22-3P (miR-22-3p) and adenylate activated protein kinase (AMPK)/cell silencing regulatory protein 1 (Sirt1)/peroxisome proliferator activated receptor gamma co activator factor-1 alpha (PGC-1 alpha) pathways can promote the transformation of muscle fiber types from fast to slow muscle fibers [27]. Mitochondria and genes related to sugar and lipid metabolism, such as AMPK α 1, Sirt1, PGC-1 α, mitochondrial transcription factor A (TFAM), mitochondrial transcription factor B1 (TFB1M), cytochrome c, ATP synthase lipid binding protein (ATP5G), carnitine palmitoyltransferase-1B (CPT-1B), and peroxisome proliferator activated receptor delta (PPAR delta), can increase the activity of succinate dehydrogenase and malate dehydrogenase in the longest dorsal muscle of pigs, while reducing the activity of lactate dehydrogenase, thereby achieving the transition of muscle fiber type from fast muscle fibers to slow muscle fibers [28].

　　In addition, gut microbiota is closely related to the quality of pork. Chen et al. [29] reported that Prevotella copri in the porcine intestinal tract can upregulate the expression of genes related to fat production and accumulation through Toll like receptor 4 (TLR4) and mammalian rapamycin target protein (mTOR) signaling pathways, while inhibiting the expression of genes related to fat breakdown, lipid transport, and muscle growth. Qi et al. [30] found that the content of Clostridium butyricum in the gut of black pigs was positively correlated with the content of IMF. The content of Butyricicoccus, Eubacteria, Phascolarctobacteria, and Oribacteria in the gut was significantly negatively correlated with the mRNA expression levels of abdominal fat and triglycerides lipase; Among them, Eubacterium belongs to the Clostridium group XIIa, whose main functions include polysaccharide fermentation and bile acid dehydroxylation, and has potential regulatory effects on obesity and related metabolic disorders. Fang et al. [31] used association analysis to identify 119 operational taxonomic units (OTUs) significantly correlated with IMF content in the study of gut microbiota in Erhuamian and Bama Xiang pigs. Among them, Ruminococcus, Prevotella, and Treponema play important roles in polysaccharide and amino acid metabolism, and are associated with IMF generation, abdominal fat, and backfat deposition. The above results indicate that specific microorganisms in the pig gut may regulate fat deposition and pork quality formation by participating in body energy metabolism.

　　3.2 Effects and Mechanisms of Fermented Feed on the Nutritional Quality of Pork

　　Feed nutrition plays a key role in regulating the nutritional quality of pork [7]. Huang Xiaocheng et al. [32] found that feeding fattening pigs with straw micro storage feed fermented with yeast, Aspergillus niger, white rot fungus, and non starch polysaccharide enzymes significantly increased the crude protein content in the longest back muscle by 4.73%. Qiu et al. [33] reported that corn soybean meal feed fermented with brewing yeast, Bacillus subtilis, and lactic acid bacteria may increase the protein content of pork by increasing the crude protein and amino acid content in the feed. Han Qichun [34] found that pigs fed with corn soybean meal feed fermented with 8 types of bacteria showed an increasing trend in protein content in their meat. Lu et al. [35] found that feeding growing and fattening pigs with corn soybean meal feed fermented with 10% lactic acid bacteria, clostridia, and bifidobacteria significantly increased the content of aromatic amino acids such as aspartic acid, glutamic acid, and alanine in the longissimus dorsi muscle by 2.21%, 3.08%, and 0.93%, respectively. Xu et al. [16] found that fermenting wheat alcohol residue with 10% koji significantly increased the content of essential amino acids, non essential amino acids, and total amino acids in pork by 5.57%, 5.45%, and 5.50%, respectively, thereby improving the nutritional value of pork. Our team's research found that corn soybean meal feed fermented with 8% Bacillus subtilis and Streptococcus pentosus can significantly increase the IMF and essential amino acid (valine, leucine, phenylalanine, methionine, lysine) content of pork. The content of linoleic acid, oleic acid, linolenic acid, and arachidonic acid were significantly increased by 14.86%, 3.99%, 34.48%, and 20.69%, respectively [7]. Tang et al. [17] found that feeding fattening pigs with a complete diet fermented by Lactobacillus plantarum, Bacillus subtilis, Candida albicans, and Aspergillus niger significantly increased the content of oleic acid, linoleic acid, arachidonic acid, and total unsaturated fatty acids in the longissimus dorsi muscle by 5.23%, 5.16%, 11.76%, and 5.41%, respectively, while reducing the content of total saturated fatty acids (SFA). Fang et al. [36] also found that feeding apple pomace silage significantly increased the content of polyunsaturated fatty acids with carbon 18 to carbon 20 in the backfat of fattening pigs by 14.62%. The above results indicate that fermented feed plays a significant role in improving the nutritional quality of pork.

　　Fermented feed plays an important role in improving the nutritional quality of pork. Its mechanism may be that fermented feed can increase the nutritional levels of protein, amino acids, and fatty acids in the feed, increase the feed intake of fattening pigs, improve feed digestibility, and enhance the deposition of protein and fat in pork, thereby increasing the content of nutrients such as crude protein, amino acids, crude fat, and fatty acids in pork [32]. In addition, the increase of probiotics and their metabolites such as lactic acid and acetic acid in fermented feed may also be the cause of nutrient deposition and lipid metabolism changes in pork. Lin et al. [37] found that feeding corn cob fermented with 6% Lactobacillus, Bacillus subtilis, and Saccharomyces cerevisiae can significantly increase the feed intake, daily weight gain, and nutrient digestibility of fattening pigs, reduce feed to weight ratio, significantly improve meat color, IMF content, and pork fatty acid composition, significantly increase the content of lactic acid bacteria in the intestinal tract of fattening pigs, and reduce the abundance of Escherichia coli. He et al. [38] found that feeding growing and fattening pigs fermented wheat bran with 5% Bacillus subtilis, Bacillus coagulans, and lactic acid bacteria resulted in increased daily weight gain, decreased feed to weight ratio, improved immune function, significantly reduced abundance of Streptococcus, and significantly increased abundance of Lachnospiraceae and Turicibacter. From this, it can be seen that fermented feed may also improve the growth performance, serum antioxidant and immune function of fattening pigs by regulating gut microbiota, thereby enhancing pork quality. However, the direct regulatory effect of fermented feed on pork through gut microbiota still needs further research.

　　3.3 Effects and Mechanisms of Fermented Feed on Sensory Quality of Pork

　　Fermented feed also has a significant improvement effect on pork sensory indicators such as meat color and marble texture, antioxidant and water holding capacity, and flavor compounds. Our team conducted a meta-analysis of over 30 literature published on fermentation materials (soy protein, apple pomace, oats, corn, wheat, soybean meal, persimmon shell, apple feed, potato slurry, rapeseed meal, wheat alcohol residue, ginkgo), fermentation additives (Chinese herbal medicine, garlic powder, red ginseng powder, microalgae), and 3562 pigs. The results showed that fermented feed can significantly improve the sensory quality of pork, including pork brightness (standard mean deviation 0.4, confidence interval 0.04.0.7), redness (standard mean deviation 0.7, confidence interval 0.1.1.2), marble pattern (standard mean deviation 2.1, confidence interval 1.2.3.0), and flavor. After dose-response analysis, the optimal doses of fermentation materials and fermentation additives were 8% and 8%, respectively. And 0.15% [39]. Further research by our team showed that feeding corn soybean meal feed fermented with 8% Bacillus subtilis and Streptococcus pentosus significantly increased pork eye muscle area, flesh color redness, and marble score by 16.62%, 9.12%, and 50.49%, respectively [7]. In addition, Lu et al. [35] found that feeding corn soybean meal feed fermented with 10% lactic acid bacteria, clostridia, and bifidobacteria significantly increased the redness and yellowness values of the longest back muscle of growing and fattening pigs by 12.24% and 43.99%, respectively, resulting in a brighter and more vibrant color.

　　Lu et al. [35] found that feeding growing and fattening pigs with corn soybean meal type feed fermented with 10% lactic acid bacteria, clostridia, and bifidobacteria can significantly reduce the drip loss and shear force of the longest back muscle, improve the water holding capacity and tenderness of pork. Ren Xianglei et al. [40] fed a corn soybean meal diet fermented with 10% Bacillus subtilis, Enterococcus faecalis, and Lactobacillus acidophilus. The total antioxidant capacity, SOD and GSH Px activities of fattening pig hind leg meat were significantly increased by 51.90%, 32.51%, and 29.63%, respectively, while drip loss was significantly reduced by 29.80%. After feeding fattening pigs with corn soybean meal feed fermented with 8% Staphylococcus aureus and Bacillus subtilis, our team significantly increased the activities of SOD and GSH Px in pork by 22.29% and 21.10%, respectively. The content of tissue lipid peroxides was significantly reduced, and drip loss and shear force were significantly reduced by 20.96% and 10.64%, respectively. This improved the antioxidant capacity of pig serum and muscle, as well as the tenderness and water holding capacity of pork [7]. Research by Zhu Kun et al. [41] found that feeding fattening pigs with fermented full price feed containing Lactobacillus salivarius, Bacillus subtilis, and brewing yeast significantly increased the levels of metabolites such as xanthine, eugenol, acrylamide, and p-coumaric acid in their serum, improved the antioxidant capacity of pork, and significantly reduced shear force by 6.38%, thereby enhancing pork tenderness.

　　Our team found that feeding fattening pigs with corn soybean meal feed fermented with 8% Bacillus subtilis and Streptococcus pentosus can significantly increase the content of unsaturated fatty acids (arachidonic acid, linoleic acid, oleic acid, linolenic acid), umami amino acids (glutamic acid, asparagine, threonine, serine, alanine, glycine), alcohols (isooctanol, nonanol), aldehydes (nonanal, octanal, heptanal, hexanal, benzaldehyde), and ketones (3-hydroxy-2-butanone, acetophenone, methyl heptene) and other flavor compounds in the longest back muscle [7]. Research by Huang Xiaocheng et al. [32] also showed that feeding with yeast, Trichoderma harzianum, white rot fungi, and non starch polysaccharide enzyme fermented straw feed significantly increased the content of flavor amino acids such as glycine, proline, alanine, palmitic acid, and linoleic acid in the longest muscle of fattening pigs by 8.20%, 14.33%, 10.44%, 3.54%, and 54.11%, respectively, improving pork flavor. Song Bo et al. [42] reported that feeding fattening pigs with 10% yeast, lactic acid bacteria, and cellulase fermented mulberry tree feed significantly increased the contents of histidine, arginine, glycine, taurine, and β - aminoisobutyric acid in the longest dorsal muscle of pigs by 45.20%, 30.87%, 43.00%, 20.64%, and 40.52%, respectively. Ren Xianglei et al. [40] found through electronic nose analysis and sensory evaluation that feeding corn soybean meal feed fermented with 10% Bacillus subtilis, Enterococcus faecalis, and Lactobacillus acidophilus can increase the content of fatty compounds in the hind leg meat of fattening pigs, and significantly improve the tenderness, flavor, and juiciness scores by 6.26%, 5.73%, and 15.01%, respectively. The above research results suggest that fermented feed has the function of improving pork flavor.

　　The mechanism by which fermented feed regulates the sensory quality of pork may be that organic acids promote the secretion of gastric juice and enhance the binding of myoglobin and iron, improving meat color, tenderness, and water holding capacity [41]. In addition, the fermentation of feed increases the content of flavor compounds such as umami amino acids, inosinic acid, organic acids, and volatile substances, which is beneficial for improving various flavor compounds in pork and enhancing the flavor of meat products. At the same time, probiotics with antioxidant capacity such as lactic acid bacteria, small peptides, and antioxidant enzymes contained in fermented feed enhance the antioxidant capacity of the body and pork. The regulation of IMF deposition by fermented feed may be achieved by modulating the expression of peroxisome proliferator activated receptor gamma (PPAR gamma) in fat of fattening pigs [34]; The possible mechanism for regulating muscle fiber development and type is to increase the cascade reaction of insulin/protein kinase B (AKT)/mammalian rapamycin target protein 1 (mTORC1) protein synthesis and activate the muscle atrophy factor (MAFbx)/forkhead transcription factor 1 (Foxo1) pathway, as well as regulate ribosomal proteins, muscle contraction, and muscle hypertrophy related proteins [33].

　　4 Summary

　　Fermented feed can provide probiotics, amino acids, flavor compounds, antioxidant enzymes, etc., enhancing the nutritional value of feed and improving the sensory and nutritional quality of pork, playing an important role in the production of high-quality and safe pork. Different fermentation strains, fermentation forms, and conditions may have varying degrees of impact on the improvement of pork quality and flavor, but the mechanism of how fermented feed regulates pork quality and flavor is currently not well studied. At the same time, fermented feed also has problems such as incomplete evaluation system, incomplete product quality testing, and unclear fermentation mechanism. In the future, research on fermented feed will mainly focus on the selection of characteristic fermentation strains, excavation of active metabolites, production of high-quality pork, standardization of production processes, and establishment of safety and product quality evaluation standards.