Saturday, October 22, 2011

Eating bacteria -- on purpose

Last Tuesday night, I found myself wondering what my students would say when I told them that yogurt is made by bacteria multiplying in milk. Let's face it, that sounds like a dubious way to make a food product, doesn't it?

I picked yogurt for two reasons:

1) We are studying the U.S. and are going region by region. We've started with New England and making yogurt is a way to celebrate our home state's dairying tradition.

2) This week we had limited time for any cooking project because of the last round of standardized testing and a teacher inservice day on Friday. Yogurt is a quick no-brainer recipe.

My husband and I have been making yogurt weekly for over twelve years. When we first got married we used a yogurt maker, which is a glorified incubator for the warm, cultured milk. It stopped working a year or two and a friend told me how to incubate the milk the old fashioned way, which is what I planned to do with my class.

I started off by asking the kids what they knew about bacteria. I got the responses you'd expect: it can make you sick, they're really small, etc. I explained that not all bacteria are harmful, and that some are even beneficial.

They looked at me with skepticism.

Then I explained how yogurt is made: milk is warmed to just below the boiling point to kill off bacteria that while not harmful, prevent the bacteria we would add to the milk from multiplying. I had done this step as they were coming in about 45 minutes earlier. Then the milk is cooled to about 110 degrees, a temperature that is just right for the bacteria we were going to add. We talked about how if the bacteria doubles, then doubles again, and so on, there would quickly be a lot of bacteria in the milk, and this is what would turn it into yogurt. The last step is to keep the pre-yogurt warm enough for the bacteria to do its job. We checked the milk and it was the right temperature, so we moved from the meeting area to the horseshoe table near my desk.

Usually a demonstration like this is tricky. Can everyone see? Is everyone focused or do some people take the opportunity to stand next to a friend and chat. This time, I knew there was a high degree of interest. Everyone crowded around respectfully and was listening and asking good questions as we proceeded.

I often use my own yogurt as starter, but after several generations we usually buy a new tub of commercial yogurt to get a strong culture. The day before, I stopped at the local general store and the only choice was Chobani. A bit of a risk, since I hadn't tried using it as starter before, but knowing Chobani yogurts are very thick, I decided to give it a try.

We added several tablespoons of starter to a cup ful of warm milk, mixed it in thoroughly, and then poured it back into the pot of milk. As I stirred I reminded everyone that I was spreading the bacteria through all the milk so it could do its job. Then I ladled milk into 8 oz Ball jars, sealed them up tight and put them in a large pot. We poured hot water in up to lid height, covered the pot and then wrapped it in some foam and a towel to keep the heat in.

Five hours later, we peeked to see what was going on. (At home I usually leave it overnight to culture.) Done! The yogurt was solid in the jars. I refrigerated it overnight and at snack the next day I served it with a drizzle of maple syrup. Sixty four ounces of yogurt were devoured in minutes.

A couple things struck me about this activity:

- Yogurt making is a science. And we will explore it in this vein over the winter. Several kids naturally asked, “What if you did...” some step a slightly different way. Yogurt making is totally ripe for changing one variable and applying the scientific method.

- Some basic recipes I have been using for years are completely novel even to adults who work in my room. One of whom has said a couple of times that she plans to try making yogurt at home herself. This recipe is so simple once you see it done, it doesn't even require written directions!

- We didn't really cook. I did a demonstration and the class ate the product. I am not sure how I feel about this, but am making note of it.

For most regions, we will only prepare one dish, but because I want to do something representative of the entire New England region, if that's possible, we are going to make chowder next week. Stay tuned!

1 comment:

  1. My mother used to make yogurt the old fashioned way too. She didn 't even use a thermometer.... When she could put her little finger in the hot milk and count to 10, it was cool enough! Then she wrapped the big bowl with towels and left it on the counter. Long long ago they made yogurt (at least in the Middle East, where weather was really warm) as a way of preserving milk for a longer time when they had no refrigeration,

    Hope you all had a good long weekend!