Yogurt 101

—by Kathryn Tempas

Yogurt. It’s nutritious, it’s delicious. But how does it work? What do those bacteria do to milk, and why is it so good for you? Follow along with me for a mini microbiology and chemistry lesson.

Yogurt from cow’s milk is made by taking warmed milk and adding two basic bacteria: Lactobacillus bulgaricus and Streptococcus cremoris. These two bacteria, working together, ferment (use for food) the lactose (that’s the sugar in milk) to lactic acid. That lactic acid lowers the pH in the milk, and at a pH of 4.5, this causes the milk protein casein to coagulate, making the milk thicker. The protein unwinds, sort of like a ball of yarn coming unwound.

Temperature is strictly controlled in the fermentation, because bacteria, just like you and me, don’t like it too hot or cold in their work environment.

Other ingredients are often added to yogurt for flavor (flavors galore are available) and to prevent the whey (that’s the liquid) from separating. The latter products include pectin and locust bean gum. Sometimes nonfat dry milk is also added to increase yogurt thickness

Other bacteria can also be added, such as Bifidium species, Lactobacillus casei, Lactobacillus acidophilus and others. These bacteria help colonize your lower intestines with “good” bacteria and prevent “bad” bacteria from taking up residence. These bacteria are sometimes called probiotics. They are pro (positive) and bio (life), so they are definitely good for you.

In addition to the good bacteria, yogurt is an excellent source of calcium and protein.

Yogurt can be made with cow’s milk or goat’s milk, since they both contain lactose and the milk protein casein. Greek yogurt, which is often thicker, has less liquid whey in it. You can approximate it at home by placing plain yogurt in a coffee filter over a sieve for several hours. Whey will gradually drain out of the yogurt resulting in a thicker consistency. Yogurt made from soy milk or other nondairy beverages relies on other methods to form the gel, such as pectin and locust bean gum.

Different subspecies of the basic two bacteria can give different textures to the yogurt, as can whether the yogurt is fermented in the cup, or in a big vat and later transferred to the package.

If you want to make your own yogurt, Hampden Park Co-op carries Yo’gourmet brand freezedried yogurt starter cultures, in the cooler with the yogurt. After getting the culture started, you can often keep it going by saving some of one batch of yogurt to inoculate (start) the next batch. Directions are included in the package for a variety of yogurt makers.

In addition to serving yogurt straight up or with fruit, yogurt can be used in smoothies, salad dressings, marinades, and even baked goods. In some recipes it can be a lower-fat alternative to sour cream. Here’s an example of a family favorite using yogurt.

[Kathryn Tempas works at a local bakery and tries to add culture to family meals.]