Microbes Involved:

Lactobacillus bulgaricus and/or acidophilus & Streptococcus thermophilus
What they do
 What is yogurt?
Yogurt is a fermented milk product that can be made from any milk, even soy milk. A mixed culture of Lactobacillus acidophilus (or bulgaricus) and Streptococcus thermophiles (they work better together than they do separately) produce lactic acid which lowers the pH and makes it sour. The partial digestion of the milk (lactose) which occurs during fermentation makes the lactose easily digestible, which is good for lactose-intolerant people.
Yogurt Facts

Factors for successful yogurt making

Food Uses (other than by itself)

How is yogurt different from soft cheese?


  1. Pasteurized milk is fortified with skim milk (adds solids for body)
  2. Mixture is homogenized
  3. Bacteria added; culture allowed to incubate in warm tanks
  4. Cooling
  5. Placed in containers and refrigerated for sale


Yogurt lab/demo

Materials (per class demo):


1. Heat milk to 125oF in double boiler. If using heavy pot, stir frequently to prevent sticking. Keep covered.
2. Immediately remove from heat, place covered pot in pan of clean cool water until stirred milk is very close to 115oF.
3. Stir up yogurt starter with a clean fork, add to 115oF milk, stir thoroughly, (temp should drop a little). Pour still warm mixture into the three bottles, cap immediately.
4. Place filled bottles in styrofoam cooler, add enough 115oF water so that bottles are surrounded, but the lid
rims are not touching water. Do not disturb the yogurt and it will be finished in 3 hrs (temp. needs to stay constant, around 110oF or so) refrigerate until needed.

Lumpy yogurt?

1) You disturbed it during the incubation period. After stirring in yogurt starter, place in the 115oF "cooler" and do not disturb for 3 hours. It should be gelled by then.
2) Many things could cause curdling--too much starter, not properly stirred in at the beginning, too much heat when too much starter is added. Unnatural curdling comes from too much acid too fast, at too high a heat.

Take the Yogurt Quiz and see how you do!

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