Microbial and genes diversity analysis: Relationship between starch conversion and carbohydrate metabolism during Niandoubao fermentation via the glutinous proso millet (GPM) process

Niandoubao is an ancient Chinese northeast fermented food mainly manufactured by glutinous proso millet (GPM). Succession and carbohydrate metabolism of complex microorganisms during GPM soaking (primarily spontaneous fermentation) may affect pH and starch content and then change the texture properties of Niandoubao. This study determined physicochemical parameters during the GPM spontaneous fermentation period, including pH, starch components, and reducing sugar content. Analysis of microbial diversity and the functional pathways related to these physicochemical parameters were also performed by 16S rDNA and metagenomics sequencing. Results showed that the pH value significantly decreased during the GPM spontaneous fermentation process. The total starch content and amylopectin content of fermented samples decreased significantly (p < 0.05), and amylose content increased overall but decreased slightly during fermentation while reducing sugar content increased. In addition, the Lactococcus genus was a predominate genus during the spontaneous fermentation period according to high throughput sequencing results. The metagenomic results further confirmed that Lactococcus lactis species encodes α-amylase (amyA/malS), α-glucosidase (malZ), and l-lactate dehydrogenase (ldh) contributed the most to starch degradation and increased of reducing sugar content and lactic acid production. Our study provides a new research angle to 16S rDNA high throughput sequencing combined with metagenomic sequencing analysis on microorganisms and metabolisms in spontaneous GPM fermentation for starch's acidic and enzymatic degradation. It is also concluded that Lactococcus lactis showed the potential use as a starter for Niandoubao manufacture.