This Fall I took the online edX course from Harvard called “Science and Cooking”; these courses are free and are quite intensive. Through the use of class videos, lectures, homework assignment, labs, videos of famous chefs making their specialities, and a final lab project, this was very much a college level investigation into food and cooking. I learned, among other things, about how heat affects proteins in cooking, how gluten protein is involved in forming baked goods and how acid and fermentation work to create some of our traditional foods. I chose to study yogurt making for my final project, and I’ve just completed it. Since the making of yogurt is related to several units in the course and because I’m interested in making fermented foods at home, studying how using different cultures may result in slightly different yogurt products seemed like a useful and interesting project. I purchased two commercial dry yogurt cultures (NE Cheesemaking Supply and Yogourmet) and purchased two fresh plain yogurts advertised to have active cultures from the supermarket (Dannon and Stoneyfield Farms). I incubated milk with the four different cultures and followed the formation of solid yogurt as the bacteria grew, converted lactose to lactic acid and resulted in solid strands of casein protein (just as happened when we added acid to milk to form ricotta cheese in a lab early in the course). The Dannon culture formed yogurt the fastest and gelled with a syrupy consistency. Stoneyfield was second fastest and formed a rougher curd. NE Cheese culture was third fastest, but like Dannon solidified quickly after forming a syrupy texture. Yogourmet was the slowest to form. Dannon and Stoneyfield produced the tartest yogurt while the other two yogurts were milder in taste. Both supermarket yogurt and purchased cultures can be successfully used to make yogurt in the home. I monitored the growth of the bacteria in the incubating yogurt by putting some of the culture under the microscope and observing every hour and by monitoring the dropping of the pH as the bacteria converted the lactose in the milk to lactic acid. The acid helps to coagulate the casein proteins in the milk resulting in solid yogurt.
This was a fun and interesting project. I think I may be back to culturing my own yogurt rather than just purchasing it at the store. Below are some photos of the equipment and the progress of the experiment.
The final products ready for refrigeration and tasting!