Yeast doesn't need sugar to grow. Yeast readily makes its own food supply by transforming flour's starch into sugar. Yes, sugar jump-starts yeast right at the beginning, but yeast dough without sugar will soon catch up.
Most yeasts require an abundance of oxygen for growth, therefore by controlling the supply of oxygen, their growth can be checked. In addition to oxygen, they require a basic substrate such as sugar. Some yeasts can ferment sugars to alcohol and carbon dioxide in the absence of air but require oxygen for growth.
Purist bakers will sometimes make the case that adding sugar to the proofing liquid for active dry yeast is not necessary. And the truth is, it isn't. Active dry yeast will proof just fine without sugar, albeit a little more slowly.
Yeast is a single-celled organism and only certain strains are used for fermenting grain. Yeast activation and the initiation of fermentation are triggered by hydration, from either water or some other liquid, and the presence of a food source.
Yeasts feed on sugars and starches, which are abundant in bread dough!
Yeast readily makes its own food supply by transforming flour's starch into sugar. Yes, sugar jump-starts yeast right at the beginning, but yeast dough without sugar will soon catch up.
The thing about respiration is that the yeast cells only do it for the sake of producing energy. And they can only produce energy when the have glucose (sugar).
You should have a bubbly mixture that smells very “yeasty”. If the bubbles are non-existent, the yeast is unfortunately dead as a doorknob. IF the yeast is still quite bubbly, kind of frothy, and has a yeasty aroma, then it can still be used for recipes that call for yeast.
Water: For best results, use water that is heated to 110 – 115 degrees Fahrenheit and use a thermometer. Any temperature between 75 degrees and 130 degrees should work, but yeast dies at 138 degrees. Some recipes use milk instead of water to activate yeast, so just follow your recipe.
Dry yeast is the most common type used at home. To activate dry yeast, you'll need to leave it in a bowl of warm water (100–110℉) mixed with a pinch of sugar for 10–15 minutes. You can also use warm milk if you nix the sugar. Yeast is used in baking as a leavening agent, meaning it makes cakes and bread rise.
Yeasts produces enzymes that react with sugar. The yeasts, like most fungi, respires oxygen (aerobic respiration), but in the absence of air they derive energy by fermenting sugars and carbohydrates to produce ethanol and carbon dioxide.
Aspartame (Equal) was the sugar substitute that worked the best when combined with yeast and warm water by producing the most carbon dioxide.
Yeast feeds on sugar so by adding a tablespoon or two provides yeast a readily available food. This increases yeast's activity and speeds up fermentation as well. However, adding a large amount of sugar to your dough will affect yeast's metabolism. Sugar is hygroscopic which means it absorbs moisture from the dough.
Yeast makes alcohol through a biological process. Sugar, dissolved in water, is ingested by the yeast organism. The sugar is metabolized, generating energy for the organism's life processes such as reproduction. The waste product it discards consists of alcohols (primarily ethanol) and carbon dioxide.
If the yeast doesn't get foamy or bubbly, either the water is too hot and killed the yeast or the yeast is no longer active. Toss old yeast that doesn't activate and use new yeast for your recipe. The shelf life for unopened dry active yeast is usually about twelve months.
Most tap water is warm enough to activate yeast without having to be microwaved. Yeast: Instant or active yeast that hasn't expired and looks healthy (not clumped and discolored). Sugar or Honey: Either one will work, and you only need about a teaspoon to feed the yeast.
If the yeast does not foam after proofing, there could be a few reasons why. The most common is that the yeast has either expired or was not stored properly. If you're working with yeast that does not foam, you'll need to discard the yeast and buy a fresh batch before continuing the recipe.
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.
All in all, dead yeast in the water looks like small grainy specks that are difficult to see with the naked eye. Under a microscope, however, they appear as oblong shape cells with no internal workings.
If the water is cold, the yeast still produces carbon dioxide gas, but at a much slower pace. Bakers sometimes let dough rise in the refrigerator for one to three days. Because the yeast works slowly at cold temperatures, it has more time to create lots of special molecules that add flavor to the dough.
If your yeast is “dead” or “inactive” then you will need to get new yeast—there is no way to revive it or liven it up again once it goes bad.
Because the dough is fermenting during both rises, if the process goes on for too long, the finished loaf of bread can have a sour, unpleasant taste. The finished loaf usually also has a dense texture and isn't sufficiently chewy. Over-proofed loaves of bread have a gummy or crumbly texture.
Yeasts can survive in the presence and absence of oxygen (1). In the presence of oxygen, yeast undergo aerobic respiration and convert carbohydrates (sugar source) into carbon dioxide and water. In the absence of oxygen, yeasts undergo fermentation and convert carbohydrates into carbon dioxide and alcohol (Figure 2).
Yeast get their energy from carbohydrates such as sugars. They break down these carbohydrates in a process called fermentation, which produces carbon dioxide gas and alcohol. Without carbon dioxide gas from yeast, bread would not rise.
Another reason both glucose and sucrose seem to be favored by yeast in fermentation could be related to oxygen. Yeast can use oxygen to release the energy from sugar in a process called “respiration.” Thus, the more sugar there is, the more active the yeast will be and the faster its growth.