How Does Bacteria Produce Yogurt From Milk?

Bacteria ferment milk to make yogurt.

Yogurt is a fermented milk product that keeps longer than milk itself. Yogurt is thicker than milk, tastes sharper and can be easier to digest. Yogurt makers introduce bacteria to begin the fermentation process.

History

The word yogurt is Turkish in origin. According to food scientist David B. Fankhauser, yogurt as we know it comes from Turkey, where it has been made for thousands of years. The fermentation of sugars in milk occurs naturally in hot, humid climates. Yogurt provided a way to store milk before refrigeration.

Bacteria

Yogurt making uses two specific bacteria: lactobacillus bulgaricus and streptococcus thermophilus. These bacteria partially digest, or ferment, the lactose sugars in milk to produce lactic acid. Lactic acid thickens milk protein and produces the sharp taste. Acid also keeps yogurt fresh, since damaging bacteria cannot survive in an acidic environment.

Making Yogurt

Make yogurt by boiling milk to kill any pre-existing bacteria. Cool it to 122 degrees Fahrenheit. Introduce bacteria by stirring a small amount of yogurt into the milk. Keep it warm for at least three hours. Keep everything sterile and don't forget to save a little yogurt to start your next batch.

References

• U.C. Clermont College: Yogurt Making Illustrated