Transcriptomic Analysis and Products of Nutrient Transformation in Hongjundouzha, a Traditional Hakka Fermented Okara Food Fermented by Neurospora crassa LY03, during Fermentation

In this study, the nutritional components of Hongjundouzha inoculated with Neurospora crassa at different fermentation times were analyzed. RNA sequencing (RNA-seq), quantitative real-time polymerase chain reaction (real-time PCR) and ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based non-target metabolomics were used to find out the main differentially expressed genes (DEGs), key metabolites and metabolic pathways affecting the fermentation of this strain, and the transcription and expression of cellulase, an important gene, was explored. The results showed that the second day of fermentation was the key period for the transformation of nutrients in Hongjundouzha and the most active period of gene expression, where cellulase expression was the highest. Carbohydrates in okara were metabolized through the starch and sucrose metabolism and carbon metabolism pathways to produce D-arabinose, sorbitol, D-xylose, L-fucose, linoleic acid, gamma-linolenic acid and arachidonic acid. On the third day, amino acid metabolism occurred. Proteins in okara were metabolized into glutamate, alanine, valine and other amino acids through the amino acid biosynthesis pathway. On the fourth day, secondary metabolites were produced. Daidzein, genistein and genistin in okara were transformed into neohesperidin, strychnine, squamolone and other substances with special physiological activities through the secondary metabolite biosynthesis pathway. The mycelia began to age, lysophosphatidyl choline (LPC) 18:4/18:1 and 18:1, lipids indicating food spoilage, appeared. In general, the second day of fermentation was the most active period of nutrient transformation in Hongjundouzha. The product matured on the third day of fermentation, and began to age and rot on the fourth day.