(Source: http://c.ask.nate.com/imgs/qrsi.php/11793224/21217261/0/1/A/17c782b771bb9b8a2768bbaebf4269ca.jpg)
Unlike what many foreigners think, Kimchi actually has a sour taste. Kimchis that can be found in grocery stores of America are in many cases something what's called 'Raw-Kimchi', a type of Kimchi that hasn't fully undergone fermentation. Where does this sour taste come from? How is the process of fermentation related to Kimchi's unique sourness loved by many Koreans?
Glycolysis, the first step of cell respiration in which a glucose (C6H12O6) splits into two pyruvates (C3H4O3), is anaerobic, meaning that it is capable of occurring without oxygen. Anaerobic respiration however undergoes a completely different path from that of aerobic respiration which eventually generates about 38 ATPs. Fermentation is one type of anaerobic respiration. Fermentation also generates certain amount of ATPs, 'recharging' the used up NAD+s to once again form NADH. In aerobic respiration, these NADHs can later on give up electrons to oxygen to eventually generate H2O. But in anaerobic respiration, as its name suggests, there exists no oxygen within the system that can accept electrons.
(Source: http://blog.naver.com/dbok1234?Redirect=Log&logNo=110165450698)
Once again take a look at the chemical equation of anaerobic glycolysis:
C6H12O6 + 2NAD+ -> 2C3H4O3 + 2ATP + 2NADH + 2H+
Without O2, the 2NADH and 2H+ undergoes what's called lactic acid fermentation. A total of 4 H+s, two of which dissociated from NADH, attaches to two pyruvic acids (two each) to form lactic acid (C3H6O3). Lactic acid is what gives a sour flavor to fermented foods including Kimchi, cheese and yogurt. Making all this happen are lactic acid bacteria better known as lactobacillus. The presence of lactic acid Bacteria is crucial to fermentation. Without lactobacillus, A food would undergo putrefaction, or simply rot instead of fermentating.
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