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Yogurt does indeed go bad -- witness the expiration dates on commercial yogurt containers. But when you're talking about spoiling, otherwise known as fermentation, the schedules are flexible. Most retailers suggest that refrigerated "live" yogurt is perfectly edible several days after the expiration date. Yogurt is simply milk that has been supplemented with a couple of friendly bacteria (more specifically, actobacillus bulgaricus and Streptococcus thermophilus), then been left alone to ferment. The bacteria convert the milk's sugar, or lactose, into lactic acid. The lactic acid accounts for yogurt's tangy taste, as well as its texture. So
when you buy a container of yogurt, you are effectively purchasing an ongoing chemical process. Allowing this process to continue unchecked (i.e., leaving the yogurt alone for three weeks) will result in a lot of dead bacteria and lactic acid. The yogurt will taste sour. Feel free to try. Some yogurts are pasteurized, meaning the bacteria have been killed through heat. Since this halts the fermentation process, the yogurt should last longer and may not have to be refrigerated. If your power goes out for a couple of hours, you probably don't have to throw out your yogurt. But check the expiration dates just to be safe.
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