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February 11, 2010 |
2 responses In the case of Yogurt, fermentation is the conversion of carbohydrates (sugars) to organic acids by the action of bacteria. In yogurt made from milk, the carbohydrate is lactose (milk sugar) and the end result organic acid is lactic acid.
We have spoken before about the bacteria involved which can be one or more strains of the lactobacillus family or the streptococcus family. Now these bacteria just love sugar and they just eat it up – can’t get enough of it, and as they eat it they convert it to lactic acid which is what gives yogurt it’s tangy and slightly acidic taste.
But that is not all that happens. This feast for bacteria also coagulates or curdles the milk. What happens is that the proteins in the milk (casein) tangle together and form solid masses called curds. The remaining milk proteins form the liquid portion which is called whey. In milk roughly 80% of the milk proteins are casein which is why milk makes a lovely thick substance which we call yogurt.
For the fermentation process to occur, the milk needs to be kept at a constant temperature between 42 and 44 degrees Celsius – around 108 and 111 degrees Fahrenheit. If the incubation temperature drops too low or goes to high, then the bacteria die and you can end up with an inferior end result.
I once had the temperature go too high while making a batch and I ended up with about 25% yogurt and 75% whey. Whey is the liquid made from milk protein that will sits on the top. Rather than waste the batch I just mixed them together and ended up with a tangy thick sort of milk and used it to make shakes.
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lactic acid bacteria,
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In your article you said that if the milk gets too hot, it does not produce the right texture for yogurt, so what then is the right temperature and how would you measure it?
The milk needs to be kept at a constant temperature between 42 and 44 degrees Celsius – around 108 and 111 degrees Fahrenheit.
There are actually many ways to make yogurt – in the oven, in a crock pot, in a yogurt maker – and each might need a different type of thermometer to take the temperature. I use a clip on thermometer with a long metal pole underneath which sits in the mixture.