Friday, June 15, 2012

How long does yogurt keep unrefrigerated?

I've been making my own yogurt for years now and am interested in knowing how long it will keep unrefrigerated. Yogurt probably originated as a way to keep fresh milk for longer in warm weather and I'd like to know just how long it can go. The Internet was remarkably unhelpful in answering this question, and it will be good to know how it works given my specific conditions anyway, so I'm experimenting.

I made a batch of yogurt on Friday, June 8th. (To make yogurt, heat the milk to 180F to kill bacteria, cool it to 115-120, add yogurt saved from the last batch, and keep it warm in an oven or cooler for several to many hours to allow the cultures to grow.) In this case I used Meadowbrook Farms pasteurized, homogenized whole milk (some fresh and some more than a week old and slightly sour) and kept it in a warm cooler for about 10 hours before moving two 8-ounce jars into my basement root cellar.

My root cellar is not yet designed to stay as cold or dry as it should, but it is significantly cooler than the rest of the house in June. The temperature was around 67. I pulled out the first jar on the morning of June 12th, four days after I put it down there. There was no mold or evidence of other nastiness, though there was a small amount of whey on top which rarely happens when it's refrigerated. It smelled stronger than usual and tasted a bit more sour, which I was expecting because it would have continued to ferment for longer than typical. (Refrigerator temperatures stop fermentation quickly but I'm not sure how much/ how quickly it continues at temperatures that are warmer than that but below 90F.) It was good though and didn't make me sick.

Jar number two came out on the afternoon of June 15th, seven days post-incubation. It had separated slightly so its texture was curdier- it still was more like yogurt than cottage cheese, but it wasn't as smooth as yogurt usually is. Its taste and aroma were similar to that of Day Four, but I'll admit that taking the first bite required steeling myself rather more firmly. My body has registered no distress however, which means that the next batch of yogurt I make will include samples to be tested up to two weeks out.

Variables other than temperature and length of storage may affect how yogurt keeps. Cooling after incubation, incubation time, fat content, age and type of culture, and age of milk come to mind. I'll start playing with them once I figure out how long it takes for this yogurt to go bad on me.