Na Liu

In this study, the spoilage potential and molecular mechanisms of a single specific spoilage organism (SSO) Acinetobacter septicus H-MN10 (A. septicus H-MN10) isolated from off-flavoured rice sour soup (RSS) were investigated through the assessment of its metabolic behavior in the fermentation system of RSS. Compared to natural fermentation, the inoculation with this strain significantly altered the physicochemical properties of RSS, including increased viscosity, a darker yellow coloration, elevated pH, decreased levels of amino nitrogen, total acidity, and organic acids. GC-MS analysis identified 168 volatile organic compounds (VOCs). The main off-flavour VOCs included aldehydes (e.g., decanal, nonanal, and 2-octanal), acids (e.g., 2-methylheptanoic acid, acetic acid, and isovaleric acid), and ketones (e.g., 2-octanone, 2-heptanone, and 2-pentadecanone). Transcriptomic analysis further revealed significant changes in gene expression of A. septicus H-MN10 during RSS spoilage and identified the core metabolic pathways and key differentially expressed genes involved in the formation of off-flavour VOCs. This study provided the theoretical basis for understanding food spoilage mechanisms and realizing targeted prevention and control strategies.

Fermented sour soup is a unique traditional sour food of the Miao and Dong ethnic groups in China. However, due to the commonly used traditional fermentation, the unstable fluctuation of microbial community often leads to some unpleasant inherent off-flavours in rice sour soup. In this study, the microbial composition and volatile flavour components of off-flavour and normal rice sour soup were examined by high-throughput sequencing (HTS) and headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS). Then, based on Pearson correlation coefficients, the correlations between fungi and bacteria and those between core microorganisms and key volatile flavour components were investigated. The dominant fungal genera included Candida, Dekkera, Pichia, Rhizopus, and Issatchenkia, whereas the dominant bacterial genera included Lactobacillus, Pectinatus, Bifidobacterium, Pseudomonas, and Acetobacter. Compared to normal rice sour soup, off-flavoured rice sour soup contained the significantly increased relative abundance of Candida and the significantly decreased relative abundance of Dkkera. In addition, 80 volatile flavour compounds detected in off-flavoured rice sour soup, mainly including esters, acids, and alcohols, were different from those detected in normal rice sour soup. Especially, some substances were only detected in off-flavoured rice sour soup, such as 1-hexanol (68-207 μg/kg), butyric acid (20-91 μg/kg) and heptanoic acid (11-44 μg/kg), which might be the main source of off-flavours. These results provide new ideas and strategies of removing or abating odour from fermented rice sour soup.

In this study, the effects of Lentilactobacillus buchneri (L. buchneri: CCTCC M 2023228) and Kazachstania bulderi (K. bulderi: CCTCC M 2023227) on the quality characteristics and volatile flavor substances in fermented red sour soup were explored based on natural fermentation. Compared to natural fermentation (nitrite: 5.5 mg/kg; amino acid nitrogen: 0.17 g/100 g; lycopene: 63.73 µg/mL), three fortified fermentation methods using L. buchneri, K. bulderi, and both strains together significantly reduced the concentrations of nitrite (2.62, 2.49, and 2.37 mg/kg), amino acid nitrogen (0.03 g/100 g, 0.02 g/100 g, and 0.05 g/100 g), and lycopene (26.64, 32.45, and 51.89 µg/mL). Total acid content (11.53 g/kg) and lactic acid bacteria count (285.9 ± 1.65 × 10⁶ CFU/mL) were the elements most significantly increased by fortified fermentation with L. buchneri relative to other fermentation methods. A total of 99 volatile compounds were determined in red sour soup and could be roughly classified into alcohols, aldehydes, ketones, and esters. Fortified fermentation with two strains and fortified fermentation with K. bulderi increased the content of methyl butanoate and 3-hydroxybutan-2-one-acetoin (D). This study confirmed the effects of L. buchneri and K. bulderi on the quality and flavor of fermented red sour soup and provided a theoretical basis for the fortified fermentation of red sour soup.