Yogurt is made when milk is transformed by helpful bacteria through a process called fermentation. This food science process matters because it changes the flavor, texture, acidity, and nutrition of milk. Students can connect yogurt making to biology through microbes, chemistry through changing molecules, and health through nutrients like protein and calcium.
A yogurt cup is a small example of how living organisms can help make safe and useful foods.
Key Facts
- Yogurt is usually made by heating milk, cooling it, adding starter cultures, incubating it, then chilling it.
- The main yogurt bacteria are Lactobacillus bulgaricus and Streptococcus thermophilus.
- Fermentation converts lactose into lactic acid: lactose + bacteria -> lactic acid + energy.
- As lactic acid builds up, pH drops and milk proteins thicken into a gel.
- Casein proteins form a network that traps water, fat, and nutrients, giving yogurt its creamy texture.
- Yogurt provides protein, calcium, phosphorus, and sometimes live active cultures that may support digestive health.
Vocabulary
- Fermentation
- Fermentation is a process in which microorganisms break down sugars to produce new substances such as acids, gases, or alcohol.
- Starter culture
- A starter culture is a measured group of helpful microbes added to food to begin fermentation.
- Lactose
- Lactose is the natural sugar found in milk that yogurt bacteria use as an energy source.
- Lactic acid
- Lactic acid is the acid produced by yogurt bacteria that gives yogurt its tangy flavor and helps it thicken.
- Casein
- Casein is a major milk protein that forms a gel-like network when milk becomes acidic.
Common Mistakes to Avoid
- Adding cultures when the milk is too hot, because high heat can kill the bacteria before they ferment the lactose.
- Skipping the heating step, because heating milk helps unfold proteins so the final yogurt can become smoother and thicker.
- Assuming all bacteria are harmful, because yogurt depends on specific helpful bacteria that are safe when food is handled properly.
- Thinking yogurt thickens because water disappears, because the main change is protein gel formation as acidity increases.
Practice Questions
- 1 A yogurt recipe uses 1000 mL of milk and 50 mL of starter culture. What percent of the mixture is starter culture by volume?
- 2 Milk begins at pH 6.6 and finishes fermenting at pH 4.6. By how many pH units did the pH decrease?
- 3 Explain why yogurt becomes thicker and more sour during incubation, using bacteria, lactose, lactic acid, and milk proteins in your answer.