|MadSci Network: Microbiology|
As bacteria reporoduce they require energy and building blocks from substrates. Basically, the substrate is enzymatically converted into the bacterial biomass. In yogurt, portions of milk are converted into bacterial biomass as the milk is fermented and byproducts of bacterial metabolism including lactic acid. The lactic acid interacts with the milk proteins to form the curd-like consitency of yogurt. Another byproduct that can be produced by bacteria in this process is carbon dioxide - a gas. The gas that escapes actually leads to a loss in mass in the yogurt.
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