What Temperature Kills YeastBy: Bob's Red Mill | February 21 2018
Making bread is an art. Or perhaps a science. In any case, with breadmaking, there are two kinds of leaveners typically used in the baking process. One is baking soda or powder, and the other is yeast.
Yeast is a live fungal organism made of a single cell. Yeast has over 160 different species that live in us and all around us. The type of yeast that is used when making bread is usually the kind that comes in little paper packets. It looks like beige colored granules that essentially lie dormant until they come into contact with warm water at just the right temperatures.
When the warm water hits the yeast, it reactivates it and “wakes it up.” Then it begins to eat and multiply. The yeast organism feeds on the simple sugars found in flour. As they feed, they release chemicals and gases like carbon dioxide and ethanol, along with energy and flavor molecules.
This is part of the process used to give bread its rise, and it is sometimes referred to as the fermentation process. As the carbon dioxide gas expands, the bread dough rises. This process of rising happens a lot slower though with yeast than it does with baking powder or baking soda used as the leavening agent. Yeast is also what imbues the bread with all of its yummy flavors and smells.
Some professional bakers believe that carbon dioxide is the sole rising agent, while ethanol is the sole flavoring agent, but it's not entirely so black and white. Ethanol is formed in equal parts to the carbon dioxide, so ethanol also contributes to the fermentation process every bit as much as carbon dioxide does.
Not only does the yeast help produce carbon dioxide and ethanol, it also assists in the development of gluten. Gluten is the substance that traps gas bubbles and gives the dough its structure. With no-knead recipes, this process is even more important, because as these gas bubbles move around inside the dough, it helps to push and rearrange the proteins into the necessary structure without any kneading required.
The short story is that without yeast, your bread won't rise properly, and you won't get the same look or flavor that you would when yeast is used.
Proofing the Yeast
How do you prepare the yeast to be mixed into your next batch of dough? This process is sometimes referred to as proofing the yeast. It is when you add yeast to water, then feed it sugar and stir it together. As the yeast sits in the water, it begins to dissolve and the yeast is activated. Once the yeast has been activated or “awakened,” it will begin to feed on the sugar in the water.
The next step when proofing yeast is to let the yeast mixture sit for several minutes. A good benchmark is to allow 2 to 3 minutes for it to completely dissolve, and then an additional 2 or 3 minutes for the yeast to start growing and show signs of life.
Signs of lively yeast include little surface bubbles on the top of the water. Depending on the variety of yeast, sometimes the mixture may expand even more than you expect!
If you do all of these steps and find that nothing is happening and you are sure you kept your water at an appropriate temperature, then it could be a sign that you need a new batch of yeast, as the batch you’re trying to use may be too old.
Yeast that is older and doesn’t respond to the proofing process is sometimes referred to as “tired” yeast. The reasoning behind the whole method of proofing your yeast is so that you can prove the yeast is viable and ready to do its job before you mix it into your bread dough.
Once your yeast has been proved, the next step is to begin stirring in your flour and salt. Be careful that you stir in the flour first as a bit of a buffer, because yeast organisms don’t like salt. If you pour the salt in first, then your yeast organisms will not be happy campers!
The Magic Temperature for Yeast Growth
At what temperature can you see the best results when proofing your yeast? Good question. Yeast is a finicky little single-celled organism.
With dry yeast, if your water is too cold, the yeast will not activate. Or, if they do wake up, they might release a substance that hinders the formation of gluten. Then again, if your water is too hot, you will kill the little buggers and they will be useless.
Typically, hot water somewhere in the range of 105° and 115°F is ideal for proofing dry yeast. 95°F is often recommended for live yeast, but it may not be hot enough at 95°F for activating the dry yeast.
At this temperature, once you pour it into the bowl and dissolve the sugar, it will cool a little bit and be the perfect temperature range for dissolving and activating your bread risers.
Not sure if your water is the right temp? One way to test this is to do the wrist test. Drizzle a few drops of your water onto the inside of your wrist. If it is warm and comfy for you, then it will no doubt be warm and comfy for your yeast too. However, if it is not warm and instead feels hot, it most likely will be too hot for your yeast to survive. By the same token, if it is too cold, then your yeast will simply remain dormant.
Fresh, Live Yeast
If you’re using fresh yeast, then you can shoot for temperatures that range between 95° and 100°F for the proofing process. This is because fresh yeast (sometimes called cake yeast), doesn't need to be dissolved in the water. It simply needs to be combined with water, and when it is combined, it will start feeding and growing right away.
Too Hot to Survive
Regardless of the type of yeast you use, if your water reaches temperatures of 120°F or more, the yeast will begin to die off. Once water temps reach 140°F or higher, that is the point where the yeast will be completely killed off. If you’re doing the wrist test, 120°F feels pretty hot, whereas 140°F feels extremely hot. If you don't trust the wrist test, you can always use a candy thermometer to test the temperatures and get a more accurate reading that way.
The High Heat Caveat
Is there ever a time you can use higher water temperatures? Yes, but only when you are using instant yeast.
Instant yeast, sometimes referred to as rapid rise yeast, doesn’t require proofing with warm water before using it. This type of yeast is mixed with flour first, instead of water right away, so the temperatures that are suggested are much higher and can range from 120° to 130°F.
Keep in mind that even though this type of yeast doesn't require proofing, you can proof it if you suspect it might not be lively. You would simply proof it the same way you would proof the active dry yeast. Also, since flour is usually around room temperature, this could be the reason higher temperatures are tolerated.
The bottom line is that yeast thrives in warm water, sleeps in cold water and dies in hot water. So, like Goldilocks and the Three Bears, it’s important to get the temperatures “just right” for your yeast to thrive and your bread to obtain the best rise and flavors possible.
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