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The secret behind kimchi’s sour taste

It might not be love at first taste for everyone when it comes to kimchi. To the unaccustomed, the Korean pickled cabbage can be mouth-puckeringly sour – and once the chilli smeared on it hits your sinuses, eye-wateringly spicy. But put one bite after another, and sooner or later you may find yourself looking forward to that fermented tang. It certainly inspires devotion in many people: Flourishes of kimchi now adorn fancy hot dogs in New York City, kimchi stews take pride of place on foodie sites and recipes for making it at home are everywhere.


But though kimchi is quite clearly, um, alive – if you keep a jar of it in the fridge for a while, you may notice it bubbling – its natural history is still a bit mysterious. Early studies into the microbes that transform the cabbage and other ingredients into their final state involved taking swabs from the fermenting fluid, culturing them on dishes, and identifying the bacteria that grew, which we now know doesn't give you anything close to the full picture. Many bacteria do not deign to grow in the lab.



Researchers have been trying out other approaches, though – the same approaches used to classify the riot of bacteria in the belly button or in the vagina. This involves extracting DNA from a sample and looking for a barcode sequence that varies between bacterial species to get a sense of who's there.


Studies of that kind have given a clearer picture of what's creating that funky kimchi flavour. In one study, the researchers had 30 vacuum-sealed bags of kimchi made at a factory in Korea, kept them all at the factory's usual kimchi-making temperature of 4 C (39 F), and every few days opened three bags. They drained the fluid off, spun it in a centrifuge so the bacteria collected at the bottom and cracked the cells open to get at the DNA. They also collected data on the molecules the bacteria were releasing.


The picture they paint is that the microbial population changes fairly radically over time. At first, the cabbage, which was soaked in brine, drained, and mixed with a variety of seasonings, was populated mostly by unidentified bacteria and ones from the Deferribacterales group, whose representatives have cropped up in oilfields and in the guts of deep-sea shrimp. These bacteria may have been on the cabbage to begin with, rather than having anything to do with the fermentation, because as the days passed, and the oxygen ran out, others began to take over.


By the seventh day, DNA from the Leuconostoc group, which converts sugars into lactic acid and are behind the fermentation of kefir, a fermented milk drink, and sourdough bread, was on the rise. By day 13, members of the Lactobacillus and Weisella groups had joined in. Like Leuconostoc, Lactobacillus and Weisella produce lactic acid from sugar, and they're also part of the team behind cheese, kefir, pickles, and other fermented products. Together the three groups dominated the kimchi for the rest of the experiment.


Viruses upon bacteria


As the microbial ecology of the kimchi changed, so too did its chemistry. The pH gradually became more acidic over the first 15 days, then dropped precipitously, coming to a stop at around pH4.5. That's about same as pureed tomatoes and it's considered optimum for kimchi. As the lactic acid bacteria took over, the levels of glucose and fructose dropped and acetate and lactic acid levels rose. Amounts of mannitol, a sweet-tasting sugar alcohol produced in the conversion process, rose as well, fitting in with the general picture of fermentation. This chemical transformation happens at different speeds and abundances of the microbial players at different temperatures, as numerous other papers note. But the end result is still a nice mix of funky-tasting acids and alcohols.

当泡菜的微生物生态环境发生变化时,它的化学成分也会随之改变。最初的15天,pH值的酸性逐渐增强,然后突然下降,最终停留在pH 4.5左右。这与西红柿浓缩汤的酸度相同,也是泡菜最适宜的酸碱度水平。随着乳酸菌占据主导,葡萄糖和果糖含量下降,醋酸盐和乳酸含量上升。甘露醇(在转化过程中生成的一种甜味糖醇)含量也会上升,符合常见的发酵流程。很多论文都指出,在不同的温度下,这种化学变化的速度会有所改变,参与其中的微生物数量也会有所不同。但最终的结果仍然将酸度和醇类的口味完美地融合到一起。

Oddly, this particular team also noticed a ton of viral DNA as well from viruses that just infect bacteria. They propose that the level of infection might influence the fermentation process somehow.


Most homemade kimchi recipes suggest you start eating the stuff within a week – as time passes, the funk grows and the cabbage starts to disintegrate. In the end, there's just too much living going on for kimchi to be truly shelf-stable. When you are next enjoying its spicy-sour kick, think about the bacteria that gave it to you – and ponder all the things we are still figuring out about life in our food.

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