Correlations between microbiota succession and flavor formation during fermentation of Chinese low-salt fermented common carp (Cyprinus carpio L.) inoculated with mixed starter cultures
Introduction
Suanyu is a traditional low-salt fermented freshwater fish product prepared using pieces or the whole fish. It is mainly consumed in Southern China. These fermented fish products retain almost all the desirable components of fish while also having a long shelf-life and characteristic flavors (Zeng et al., 2013). However, it is usually manufactured using spontaneous fermentation in small-scale processing units using traditional techniques, resulting in a high level of product variability.
Inoculation with starter cultures has become increasingly necessary to improve and stabilize the quality of fermented fish (Nie et al., 2014). Compared with spontaneous fermentation, products inoculated with starter cultures not only had the typical characteristics of Suanyu, but had better flavor and product quality (Zeng et al., 2017). Flavor is a particularly important indicator of consumer acceptance of Suanyu. However, fermentation is actually a complex microbial process. The metabolism of microbiota including carbohydrate hydrolysis, proteolysis and lipolysis determines the types and intensities of the flavor. Meanwhile, the free amino acids (FAA) and the free fatty acids (FFA) are important flavor compounds or precursors of flavor compounds (Flores and Toldra, 2011). It has previously been reported that both microbial and endogenous enzymes contributed to the liberation of FFA and FAA in fermented fish, while the microflora are dominant in the chemical reactions generating flavor (Xu et al., 2018). However, the mechanism of flavor formation during fermentation has not been fully documented and the association between microbiota and flavor and flavor precursors requires more study.
High-throughput sequencing (HTS) is a powerful technique involving sequencing of whole microbial community DNA to explore the microbial communities without the need to culture microbes (Reuter et al., 2015). In addition, bidirectional orthogonal partial least squares (O2PLS) is an effective statistical method that can simultaneously analyze two data sets to better understand the potential relationship between different levels of biological function (El Bouhaddani et al., 2016). Recently, this approach had been used to investigate the microbiome and metabolites in fermented vegetables (Xiao et al., 2018). In addition, dynamics and diversity of microbial community succession during fermentation of Suanyu has been successfully determined by HTS in our previous study (2018b). The aim of this paper was further to explore the role of microorganisms in the formation of flavor metabolites and establish the relationship between flavor and flavor precursors based on O2PLS.
Section snippets
Preparation of fermented suanyu and sampling
Strains of Lactobacillus plantarum 120 (L. plantarum120), Staphylococcus xylosus 135 (S. xylosus 135) and Saccharomyces cerevisiae 31(S. cerevisiae 31) previously were isolated from traditional Suanyu and shown to be an appropriate starter culture formulation (Zang et al., 2018a, Zang et al., 2018b). They were cultivated using the method of Zang et al. (2018a). Briefly, the L. plantarum120 strain was cultured twice in De Man Rogosa and Sharpe (MRS) broth at 30 °C for 2 days; S. xylosus 135 was
Change of volatile components at different fermentation stages using GC–MS
As shown in Table 1, 81 aroma compounds were found during Suanyu fermentation, including 15 esters, 9 alcohols, 21 aldehydes, 6 ketones, 4 acids, 8 alkanes and 18 other compounds. The aroma compounds changed during fermentation. Some compounds decreased, while others first increased and then either stayed stable or declined. For example, the content of esters fluctuated slowly in the first two weeks of fermentation, but increased significantly (p < 0.05) in the later fermentation period,
Conclusion
The correlations between microbiota and characteristic of flavor compounds were obtained. Esters and aldehydes played important flavor roles in different fermentation stages. Polyunsaturated fatty acids might be the main precursors of aldehydes. Furthermore, it was possible to infer which microorganisms were involved in the formation of certain flavor compounds. In particular, among starter cultures, only Lactobacillus were able to dominate during fermentation and were involved in the most
Declaration of competing interest
The authors attested that there were no conflicts of interest with respect to this paper.
Acknowledgement
This research was financially supported by the National Natural Science Foundation of China (NSF31671885), the Earmarked Fund of the China Agriculture Research System (CARS-45-27), the National First-class Discipline Program of Food Science and Technology (JUFSTR20180201), and the program of the “Collaborative Innovation Center of Food Safety and Quality Control in Jiangsu Province”.
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