A strain with high amylase activity, designated as CS3.04, and another strain with high protease activity, designated as CS3.22, were isolated from koji for fermented soybean paste by the Oxford-cup double-layer plate method. Both strains were identified as Aspergillus oryzae by morphological characterization and internal transcribed spacer (ITS) rRNA sequence analysis. The two strains were used to make single-strain and mixed-strain koji for the manufacturing of fermented watermelon-soybean paste. Commercial A. oryzae was served as the control. The results showed that physicochemical indexes of the fermented product exhibited a consistent trend with fermentation time. The contents of reducing sugar and ammonia nitrogen in the mixed-strain fermented product were the highest, which were 107.37 and 6.76 g/kg and increased by 11.02% and 5.56% compared with that fermented by commercial A. oryzae, respectively. The qualitative and quantitative analysis of volatile flavor compounds was performed using liquid-liquid extraction and gas chromatography-mass spectrometry (GC-MS). It was found that the mixed-strain fermentation was superior to the single-strain fermentations in producing aldehydes, alcohols, ketones and short-chain esters, while fermentation with CS3.04 alone was more conducive to the production of acids and phenols. Moreover, the mixed-strain fermentation resulted in higher overall sensory score and better aroma and taste. We concluded that the mixed-strain fermentation improves the quality of fermented watermelon-soybean paste through the complementary effect between the enzymes of strains CS3.04 and CS3.22.

The variation of fungal community structure and dominant fungi during the fermentation of high-salt liquid-state soy sauce were studied by high-throughput sequencing on the Illumina MiSeq platform, and the changes of reducing sugars, amino acid nitrogen, total acids and volatile flavor compounds were analyzed. The results showed that a total of 44 fungal genera were obtained in soy sauce samples. The dominant genus was Aspergillus (62.62% to 93.85%) during the first two months of fermentation, which promoted the production of reducing sugars and amino acid nitrogen. The content of amino acid nitrogen exceeded 10.45 g/L after three months of fermentation. The dominant genus was Kodamaea (85.64% to 99.50%) from the 3rd to the 5th month, and Zygosaccharomyces (78.12%) in the 6th month. The contents of reducing sugars, amino acid nitrogen and total acid were 17.73, 10.72 and 21.68 g/L after 6 months of fermentation, respectively. A total of 61 volatile flavor compounds were detected in all samples by gas chromatography-mass spectrometry (GC-MS), and the total content of volatile flavor compounds was 12404.15 μg/L at the end of fermentation. The contents of alcohols and acids as major volatile compounds were 8826.2 and 2349.97 μg/L, respectively. Correlation analysis between the dominant fungi and volatile flavor compounds indicated that Aspergillus was significantly positively correlated with 1-octen-3-ol (P < 0.05). Kodamaea was not significantly correlated with any changes in flavor compounds. Zygosaccharomyces was significantly positively correlated with phenethyl alcohol, 3-methyl-1-butanol, 2-methyl-1-butanol, benzaldehyde, and ethyl acetate (P < 0.05). This study has proved that the variation of fungal community structure and the dominant fungi are important factors affecting the formation of flavor compounds, and constructing beneficial fungal flora is beneficial to improve the quality of soy sauce.