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Our last Science Digested piece on kimchi looked at the effect of red pepper powder on the progress of fermentation, and its impact on which bacteria dominate at the end. This paper backs up a step and asks: where do these lactic acid bacteria come from in the first place? Which ingredients bring microbial life to kimchi?
To answer this question, the team made five kimchis, using the same recipe, but with sets of ingredients purchased from completely different markets. The total bacterial count per gram was taken for each of the key ingredients (cabbage, leek, ginger, garlic, and red pepper powder) from each source. Then DNA sequencing was used to investigate the communities of bacteria present in the raw materials and in ‘kimchi soup’ samples taken at intervals from the five different kimchis as they fermented. Thus, the scientists were able to show what was present in each raw material, and what came to dominate each experimental kimchi over the course of its fermentation.
Summary of Findings
With the exception of the red pepper powder, in which no microbes were found, the team found enormous variability in the bacterial (and lactic acid baterial) loads of their raw materials. In general, the garlic samples had the highest total bacterial counts, while the cabbage had the lowest. Interestingly, despite being the most plentiful ingredient in kimchi—and even when the volume was corrected for—the cabbage was not the predominant source of lactic acid bacteria (LAB). A few ingredient samples (in particular one sample of garlic) were loaded with LAB, while in most cases, they were dominated by other types of bacteria not considered desirable in food, such as the Proteobacteria (which includes in its ranks E. coli, Salmonella, and the spoilage organism Pseudomonas, among many other more innocuous members).
As in the previous paper, as the fermentation process progressed, LAB (particularly Leuconostoc, Lactobacillus, and Weissella) quickly came to dominate the community. The total number of bacteria present increased by more than a thousandfold and the acidity increased from slightly alkaline to strongly acidic (<pH 4.3). However, different species of these lactic acid bacteria dominated depending on the set of ingredients used; after 30 days, some kimchis were dominated by Weissella, some by Leuconostoc, and some had significant proportions of multiple different LAB. The team concludes that different strains of LAB may be more or less competitive, and that in order to get a truly consistent fermention, and a standardized kimchi “with good taste and high quality…starter culture may be surely necessitated.”
Once again, the results of this paper demonstrate the power of natural fermentation to select for beneficial and useful microbes from amongst the motley crew of bacterial species present in variable raw materials. The low final pH of kimchi is very strongly selective for LAB, which probably accounts for the fact that no commercial starter cultures have been required or developed for this extremely popular food, even up to now.
The paper drew the conclusion that starter cultures are necessary for a standardized kimchi, without assessing the taste and quality of the five kimchis. Was there a significant difference between kimchis depending on which organisms (Lactobacillus, Weissella, Leuconostoc) dominated at the end? They also do not take into account the effect that a given environment, such as a kimchi plant where a native population of lactic acid bacteria is already likely to be present, might have on inoculating new batches in an indirect way. How does the speed of the fermentation affect the complexity of the flavour? Could adding a pure starter culture of a few strains to create a fast and ‘clean’ fermentation strip the final product of its richness and character? These are questions that other fermented food industries, from bread to wine to cheese to beer, are grappling with. Given that kimchi is so robust (unlike, for example, some types of cheese, which due to their higher pH are capable of maintaining any pathogens that might be present as well as beneficial microorganisms), it would be interesting to see more concrete evidence from home or professional kimchi makers of why standardized starters are necessary or useful.
Post written by Bronwen Percival
Source Tracking and Succession of Kimchi Lactic Acid Bacteria during Fermentation
Lee et al., Journal of Food Science. 2015. Link to abstract here.