Yogurt experiment: explaining the science

Making yogurt is a protected process.  This means that sterilization is not required.  By using standard sanitizing practices such as dipping utensils into boiling water, we can reduce the number of microbes to a sufficient level so that these microbes will not interfere with our yogurt-making process.

What is a protected process?
This phrase is used to describe certain microbial fermentations whereby microbes will produce a product as a result of their natural metabolic reactions. This product will accumulate and alter the overall environment so that pathogenic microbes are unable to proliferate.  In other words, the good bacteria will out-compete the bad bacteria.
Why is yogurt a protected process? 
By seeding milk with a yogurt culture, you are introducing millions of Lactobacilli microbes.  These microbes start growing at a rapid rate, consuming the lactose present in milk and converting the sugar to lactic acid.  The production of lactic acid lowers the pH to a level that makes the environment inhospitable for the growth of pathogenic bacteria.

Why do we bring the milk to almost a boil? 
This is necessary to reduce the number of possible pathogenic microbes already present in the milk.  Although the milk has been pasteurized, once opened, bacteria can contaminate the milk.  Normally this isn’t a problem because the milk is consumed within a short period of time and it is kept at a cold temperature, both of which help keep these bad bacteria at such low levels that they do not cause problems.  However, when making yogurt, we are going to incubate our milk at a warm temperature for several hours, so we will want to make sure that those bad bacteria aren’t present in sufficient quantity to cause problems.

Why does the milk become creamy and thick after incubation?  
This is due to the production of lactic acid by the yogurt microbes.  Lactic acid denatures milk proteins which causes the milk to “curdle”.

Why do we have to wait until our milk has cooled to at least 115 degrees Fahrenheit?  By maintaining the temperature at a warm 115F, we are providing the optimum growth temperature for our yogurt bacteria.  Too high a temperature, we will kill our bacteria.  Too low, we will not achieve adequate growth.

Any other questions?  Post on the blog or at our forums, we will be happy to try to answer them!

This entry was posted in Experiments. Bookmark the permalink.

Leave a Reply