Effects of different temperatures on bacterial diversity and volatile flavor compounds during the fermentation of suancai, a traditional fermented vegetable food from northeastern China
Introduction
Fermented vegetable is a result of complex biotechnological processes, including osmosis by salting, zymosis by microorganisms. Suancai is one of the popular traditional fermented food items in China, especially in northern China. Usually, the vegetables are washed and drained, then immersed in 6%–8% NaCl (w/w) in layers, and fermented at ambient temperature for one month in the winter (Hou, Jiang, & Long, 2013). Enhancing the quality of fermented vegetables may lead to increasing nutritional value and health-benefits to consumers (Liu & Tong, 2017). Besides, optimizing the production process can also facilitate the industrial production of fermented vegetables.
Until now, the researches on suancai fermentation were mainly carried out from two aspects: salinity and temperature. The salt concentration had an effect on the microbial community structure of suancai brine, which could directly or indirectly affect the quality and flavor of suancai (Xiong et al., 2016a). Besides, Salt quality could also impact the fermentation of sauerkraut (Viander, Maki, & Palva, 2003). In addition, the fermentation temperature was also one of the important factors which should be considered in fermented food production. Xiong, Xiao, Li, Peng, and Huang (2016b) investigated the physicochemical properties during suancai fermentation in different temperatures and obtained that the utilization of glucose and fructose during the fermentation was outstanding and the yield of lactic acid was the highest at 37 °C. Previous studies on the effects of different fermentation temperatures on metabolites of Kimchi have been reported (Park, Seo, Kim, Na, & Son, 2018). But there are no studies reported on the effect of fermentation temperature on the bacterial community structures during suancai fermentation.
The quality of fermented food was correlated to the microbial composition during fermentation. The effect of salinity and temperature on fermented food is ultimately determined by its effect on the growth of microorganisms. Therefore, the knowledge of the effect of temperature on the microbial community will contribute to improve the process of suancai. As is known to all, lactic acid bacteria (LAB) are the most prominent microbial group responsible for the fermentation of meat, dairy, grains, and vegetables (Fideler, Johanningsmeier, Ekelof, & Muddiman, 2019). Some researches showed that the mainly strains in fermented vegetable are affiliated with Firmicutes and Proteobacteria, including Lactobacillus, Pediococcus, Weissella, Lactococcus, Bacillus, Pseudomomas, etc. LAB was the dominant bacteria in the whole fermentation (Xiao et al., 2018; Liang et al., 2018a, 2018b). Furthermore, the microorganism in fermented vegetable was related to the flavor formation (Wang, Lu, Shi, & Xu, 2016; Xiao et al., 2018). At present, there are many molecular methods to reveal the microbial structures during the fermentation. High-throughput sequencing has been proved to be an effective technique to analyze the microbiome in fermented food (Liang et al., 2018b; Zang et al., 2018; Zhang et al., 2018a).
Therefore, the aim of this work was to determine the effect of temperature on the volatile flavor compounds (VFCs) and the bacterial community during the fermentation process. This could provide a fundamental basis for the improvement of the suancai fermentation process.
Section snippets
Preparation and collection of suancai
Cabbages (Brassica rapa spp. Pekinensis) were purchased from the local supermarket in Dalian, Liaoning Province, China. The preparation of suancai was performed as follows: fresh cabbages were selected, sliced into quarters, and then submerged into brine solution (6% w/w NaCl) in sterilized jar (5 L). The jar was sealed with water, and then kept at 10 °C, 15 °C, 20 °C and 25 °C for 30 days, respectively. During the fermentation, samples of 1d, 3d, 5d, 7d, 10d, 20d and 30d were collected.
Determination of pH and TTA
During
Physicochemical properties during suancai fermentation
The physicochemical properties during suancai fermentation are shown in Fig. 1. The pH showed a decreasing trend during the fermentation. After fermentation, the pH of suancai at 20 °C was the lowest (Fig. 1A). In contrast, the content of TTA showed an increasing trend during the fermentation and then remained at 2.43 g/L, 3.05 g/L, 3.80 g/L and 3.87 g/L in suancai at different temperatures, respectively (Fig. 1B). The nitrite content of suancai during the fermentation at different temperatures
Discussion
In this study, the bacterial diversity and VFCs in suancai fermented at different temperature were investigated. As a typical lactic fermentation, pH and TTA are basic properties during fermentation as they could affect the growth of the microorganisms and the accumulation of metabolic products, which can change the safety and flavors of suancai (Wu et al., 2015). After fermentation, the pH of all suancai at different temperature was less than 4 (Fig. 1A), which generally indicated that the
Conclusions
In this study, the bacterial diversity and VFCs during suancai fermentation at different temperatures were investigated by using Illumina Hiseq sequencing and GC-MS. The results showed that a total of 5 phyla were detected. Firmicutes and Proteobacteria were observed as the dominant phyla during the fermentation. The abundance of Firmicutes was increased while that of Proteobacteria was decreased. At the genus level, Lactobacillus, Serratia, Leuconostoc, Pseudomonas, Pseudomonas, Pediococcus,
Declaration of competing interest
None.
Acknowledgments
The work was supported by National Key R&D Program of China (Grant No. 2018YFD0400402), and youth project of Department of Education of Liaoning Province (Grant No. 2017J037).
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