Yeast is an organism consisting of living cells, more precisely a fungus. Since yeast is sensitive to environmental influences, it is very important to treat it properly so that it can function optimally. It plays an important role in the production of bread and various bakery products.
In short: Yes, yeast is killed by high temperatures in the oven, during baking. Yeast activity slows down above 40°C/104°F and ceases completely at 60°C/140°F.
However, if we don’t treat the yeast with care, it may die too soon.
What factors can destroy yeast?
1. Overproofing the dough
The yeast will die if the dough is over-proofed. In this case, the dough emits an alcoholic smell similar to beer. The proofing process takes place faster in a warm environment, but we can slow it down by cooling the dough.
If the dough is cooled to 4°C / 40°F, the activity of the yeast will slow down, but it will not be damaged by the cooling. When stored in the refrigerator, the proofing time of the dough is extended, which helps the development of good flavors. This process is called dough retarding.
You can keep preferments and bread dough in the fridge for up to three days before the yeast cells start to die, their activity weakens and they develop a bad taste.
2. Direct contact with the salt or sugar
Salt is necessary for yeast dough, as it flavors the bread and helps brown the crust. The salt slightly inhibits the work of the yeast, which is good because it prevents the formation of uneven and excessively large air bubbles.
Since salt absorbs water, it inhibits hydration and also slows down the fermentation process. Therefore, we usually do not use salt when making sourdough starters and preferments.
When assembling the dough, you should make sure that the yeast and salt do not come into direct contact with each other. The best way to add salt to bread dough is to mix it with flour. This way, you can prevent the salt from coming into contact with the yeast and it will be evenly distributed in the dough.
3. Adding too much sugar
Once hydrated, the live yeast begins to bubble and convert the sugars into carbon dioxide gas. The sugar that feeds the yeast can come from flour or from added sugar.
A small amount of added sugar, therefore, helps the yeast work, but if you add more than necessary, you can destroy the yeast. One fresh yeast cake does not require more than a teaspoon of sugar.
Sugar is hygroscopic, just like salt. This means that it absorbs water and draws it away from the yeast and inhibits fermentation.
4. The liquid is too hot
Yeast is activated when it comes into contact with liquid, and when flour is added, the fermentation process begins. The liquid in contact with the yeast should not be above 40°C/104°F.
At higher temperatures yeast fermentation stops, which will have a negative effect on the quality of dough and bread. The optimum temperature for proofing yeast dough is between 35-40°C/95-104°F.
While on the subject of liquid bread ingredients, you may also wish to find out what happens if you don’t add enough water to bread dough.
5. The yeast is too old
Fresh compressed yeast keeps its quality for 15 to 30 days. The fresher you use it, the better. If you see that the yeast is discolored or moldy, throw it away. Yeast is also spoiled if it becomes slimy and smells bad. In this state, it is no longer safe to use. Fresh yeast must be stored in the refrigerator, and carefully wrapped in foil. Fresh yeast dies if it dries out.
Due to its low moisture content, instant dried yeast can be stored much longer than fresh yeast. The shelf life of dry yeast can be up to 24 months. Dry instant yeast also contains live cells, but they are in a dormant state.
Yeast is a living organism that ideally dies during baking at temperatures above 60°C/140°F after it has completed its activity in the dough. However, if we don't handle it carefully, we can cause the yeast to die prematurely, so it won't do its job as it should.
You may also be interested in my related article on what happens if you bake under-proofed bread dough.