Re: how does temperature affect yeast activity?

Most microbes, such as bacteria and yeast, have an optimal 
temperature range in which they grow best. If you get either too low or 
too high then they grow much slower and finally cease growing at all. 
However this temperature range can vary with different organisms.

For example, yeast grows optimally around 30-35 degrees Celsius. If 
you get much above 40 it won't grow at all. Below 30 its grow slows 
down dramatically and when you get below about 10 it really doesn't 
grow much at all. So what causes this?

The main reason is the ability of the enzymes that catalyze all the 
biochemical reactions in the yeast cell to function at those 
temperatures. Enzymes have an optimal temperature range. When 
you get below that range there ability to catalyze the intended reaction 
really slows down. Above that temperature and the enzyme begins to 
denature or unfold and becomes inactive. Each enzyme will have a 
different range where it becomes inactive. 

For an organism to stop growing at some temperature, you don't 
need all the enzymes to become inactive, you just need for one 
essential enzyme to stop working and then the organism fails to 
grow. So whichever is the first essential enzyme to get inactivated 
defines the maximal temperature at which that organism can grow. 

At the lower end it gets more complicated. Usually the enzymes are 
not inactivated, but rather just slow down. Eventually you get to a point 
where they produce to little of their product to support growth. So 
growth of the yeast or bacteria stops for all practical purposes. 

Some bacteria for example have become adapted to grow at unusual 
temperatures. There are thermophilic bacteria that grow in hot 
springs and similar places and grow fine at very hot temperatures. 
Some grow at 60-70 degrees Celsius. A few grow above 80 or even 
90 degrees Celsius. The enzymes in these microbes have changed 
so that they can tolerate the high temperature. But generally these 
bugs can not grow at normal temperatures. A microbe that grows 
optimally at 65 deg Celsius may be completely unable to grow at 37. 

Likewise there are some bacteria which are cold adapted and grow 
at very low temperatures but would be unable to grow at our room 
temperature. All for the same reasons as I stated.