Microbial succession and metabolite changes during fermentation of dongchimi, traditional Korean watery kimchi
Introduction
Kimchi is an emblematic traditional Korean food that is produced through the fermentation of salted vegetables with various seasonings at low temperatures to ensure ripening and preservation (Chang and Chang, 2010). Kimchi is classified into hundreds of varieties, mainly based on the vegetables used, but it also can be classified into two general types, ordinary and watery (mul) kimchi, according to the preparation method (Cheigh and Park, 1994). Dongchimi is one of the most typical forms of watery kimchi and is usually made by the fermentation of radish with a seasoning mixture including Korean leek, garlic, ginger, and other seasonings in plenty of solar salt water (2–4% (w/v)). Red pepper or sliced apples are sometimes added to impart spicy or sweet taste.
Spontaneous fermentation without sterilization of the raw materials in kimchi preparations leads to the growth of diverse lactic acid bacteria (LAB), which in turn influence the sensory qualities of kimchi (Kim and Choi, 2001, Park et al., 2002). Kimchi metabolites, such as free sugars (glucose and fructose), organic acids (lactic and acetic acids), and other flavoring compounds (such as mannitol and amino acids) are important determinants of kimchi tastes and flavors, and represent more direct and collective phenotypic outcomes resulting from microbial activities in kimchi fermentation (Ha et al., 1989, Jung et al., 2011b, Jung et al., 2012). Studies of not only microbial communities but also of metabolite changes are needed in order to understand the relationships between microbial populations and metabolites in kimchi fermentation since a rational approach to the control of the microbial community is currently almost impossible. The few previous studies on this subject have shown that members of the bacterial genera Leuconostoc and Lactobacillus and yeasts probably play key roles in dongchimi fermentation based on culture-dependent approaches or denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes (Song and Park, 1992, Kim and Kim, 1999, Kim and Choi, 2001, Lee et al., 2005, Park et al., 2008). However, comprehensive studies of microbial succession during the entire dongchimi fermentation period have not yet been performed. Flavoring compounds in dongchimi fermentation were investigated at a particular time of dongchimi fermentation (Kim and Sohn, 2001), but metabolite changes during the entire dongchimi fermentation period have never been explored. Therefore, little is known about the microbial dynamics and metabolites present during dongchimi fermentation. In particular, to the best of our knowledge, no study has investigated the relationships between the microbial community and metabolites during the entire dongchimi fermentation period.
Culture-dependent approaches and DGGE of 16S rRNA genes have been applied to investigate the microbial communities of fermented foods (Lee et al., 2005), but these methods have many limitations for monitoring microbial communities during the entire fermentation period because they are time-consuming and laborious and produce limited amounts of information. However, many recent studies have reported that multiplex barcoded pyrosequencing strategies allow for the investigation of complex microbial succession in fermented foods without the limitations associated with other methods (Humblot and Guyot, 2009, Ercolini et al., 2011, Kim et al., 2011, Sakamoto et al., 2011, Nam et al., 2012). In addition, proton nuclear magnetic resonance (1H NMR) is one of the most comprehensive, easy, and powerful techniques for the simultaneous monitoring of multiple metabolites present in a given sample (Figueiredo et al., 2006, Jung et al., 2012, Jeong et al., 2013). Therefore, in this study, we applied a combination of massively parallel pyrosequencing and 1H NMR in addition to a culture-based approach to investigate microbial succession and metabolites as well as the relationships between the microbial community and metabolites during the entire dongchimi fermentation period.
Section snippets
Dongchimi preparation and sampling
Three samples of dongchimi were prepared using radish (Raphanus sativus var. sativus L.) at the World Institute of Kimchi, Korea following a traditional manufacturing method. Briefly, radishes of approximately 150–200 g each were cut into four pieces and then dispensed into three plastic bags so that each bag held a total of 3 kg of radishes. One hundred-fifty grams of a seasoning mixture containing Korean leek, garlic, and ginger (6:3:1, w/w/w) and 4.5 l of 4.0% (w/v) solar salt (Shinan, Korea)
The profiles of pH and microbial cells during dongchimi fermentation
The initial pH of the dongchimi liquids was about 6.2, which quickly decreased to reach approximately 3.6 after 18 days of fermentation (Fig. 1), yielding a pH profile that is similar to the one obtained in a previous dongchimi fermentation study (Park et al., 2008). After 18 days of fermentation, the pH values became relatively stable until the end of dongchimi fermentation (100 days).
Colonies of bacteria, Saccharomyces, and Candida were easily differentiated on the basis of their colony
Discussion
Kimchi is classified into ordinary and watery kimchi in Korea, according to preparation methods (Cheigh and Park, 1994). In the preparation of ordinary kimchi, vegetables are salted for a substantial period of time (5–10 h) using solar salts or a solar salt solution (approximately 10%) for the preparation of kimchi supplemented with seasonings (Jung et al., 2011b, Jung et al., 2012, Jeong et al., 2013). On the other hand, watery kimchi is prepared by simply putting vegetables into enough
Acknowledgments
This work was supported by the Technology Development Program for Agriculture and Forestry (TDPAF) of the Ministry for Agriculture, Republic of Korea.
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Sang Hyeon Jeong and Ji Young Jung contributed equally to this article.