Warm milk, tangy yogurt
Milk sugar is lactose. Yogurt cultures, mainly Lactobacillus and Streptococcus thermophilus, feed on it and make lactic acid. The acid lowers the pH, and the casein in milk clumps into a soft gel. The acid gives the tang, and the gel gives the thickness.
Keep the milk at 40 to 45°C
Yogurt cultures work best around 40 to 45°C (105 to 113°F). Cooler, they stall. Hotter, they die or the curd turns grainy. Many recipes first heat milk to about 82°C (180°F) and cool it, which unfolds the whey proteins for a thicker gel.
Test yourself
Questions from the interactive lesson, with the answer and the why.
What makes milk thicken and turn tangy as it becomes yogurt?
Cultures turn lactose into acid that sets the casein. Lactic acid lowers the pH, and casein clumps into a gel.
True or false: You can stir yogurt starter into milk straight off the boil.
False. Hot milk kills the cultures. Cool it to about 43°C (110°F) before adding starter.
What went wrong? You stir starter into just-simmered milk and leave it overnight. It's plain milk in the morning.
The milk was too hot and killed the cultures. Very hot milk kills them. Cool to about 43°C (110°F) first.
Fill the blank: The tang of yogurt comes from ___ acid.
The tang of yogurt comes from lactic acid. Cultures turn lactose into lactic acid, which gives the tang.
About how warm should milk be when you add the starter?
About 43 C. Yogurt cultures grow best when milk is about 43°C (110°F).
Your yogurt is thin and watery. What should you change next time?
Heat milk to 82°C (180°F) first, then cool it. The heated whey proteins help the gel hold water, so it sets thicker.