Vol. XIV · Fermentation Plate 5 of 11yogurt, Streptococcus thermophilus, Lactobacillus bulgaricus, 43 °C, casein, gel, pH 4.6, heating milk, Greek yogurt, 酸奶MT

Yogurt.

Warm milk, a spoon of yogurt, five hours at 43 °C. Two species of bacteria have turned a liquid into a gel.

Yogurt is made by two bacteria that help each other: Streptococcus thermophilus starts fast, using a little oxygen and making formic acid and carbon dioxide that the second needs; Lactobacillus delbrueckii subsp. bulgaricus breaks down milk proteins into amino acids the first uses, and makes the acetaldehyde that gives yogurt its fresh, green-apple tang. Both thrive near 43 °C, where most other microbes do poorly.

Their lactic acid does the setting. Casein, milk's main protein, is held in tiny clusters (micelles) by charge and calcium; as the pH falls towards 4.6, its isoelectric point, the charges cancel, the micelles lose their calcium and stick together into a fine network that traps the whey: a gel. Heating the milk first, to about 85 °C for half an hour or 90 °C for a few minutes, unfolds the whey proteins so they join the network and the yogurt sets thicker and weeps less. The workshop incubates a pot.

Below: the pH of milk inoculated with yogurt cultures against hours at the temperature you choose (microbe bench, teaching culture and a teaching acid-to-pH curve for milk's buffering), with the gel point near pH 4.6.

What you will learn
Workshop
Microbe bench: combined yogurt culture (Tmin 20, Topt 43, Tmax 52 °C) from 6 log per millilitre (a spoon of yogurt per litre); lactic acid 0.95 % at full growth; pH against acid from a teaching curve shaped on milk's buffering (pH 6.6 fresh, about 4.6 at 0.7–0.8 % acid).

Warm, still and patient

In this model a spoon of yogurt per litre of milk sets in about 5.8 hours at 43 °C, 7.3 at 37 °C and 16.5 at 30 °C; above about 48 °C the cultures falter. Longer incubation after setting makes it more sour. Leave the pot undisturbed while it sets: stirring breaks the forming gel into a thin, lumpy yogurt.

Method: heat milk to about 85 °C and hold it there a few minutes (or 90 °C briefly), cool to 43–45 °C, stir in a spoon of plain live yogurt or a starter, cover and keep warm — an oven with the light on, a vacuum flask, a yogurt maker or a rice cooker on warm, checked with a thermometer — until set, then chill. Strain through cloth for thick, Greek-style yogurt (the whey is good for baking or marinating). Keep a spoon from each batch as the next starter, refreshing with bought yogurt now and then.

At the stove

Make two small jars of yogurt from the same milk and starter, one at about 43 °C and one at room temperature. Check them after six hours: the warm one has set; the cool one has barely begun.

Ilya Mechnikov, at the Pasteur Institute around 1905, attributed the long lives of Bulgarian peasants to yogurt, and his popular writing launched yogurt across Europe; the bulgaricus bacterium had been identified by Stamen Grigorov in Geneva shortly before.

Terms

Yogurt cultures
Streptococcus thermophilus and Lactobacillus bulgaricus, working together.
Casein micelle
A cluster of milk proteins held by calcium and charge.
Isoelectric point
The pH (about 4.6 for casein) at which a protein's charges cancel and it clumps.
Whey
The watery liquid that drains from yogurt or cheese.
Two heat-loving bacteria at about 43 °C make lactic acid; at pH 4.6 casein sets into a gel. Heat the milk first for a thicker set; do not disturb it while setting.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.