The vegan lifestyle excludes eating any animal products. Extending down into the microscopic realm, some wonder whether vegans can eat yeast. Even those who have been eating meat-free for years may ponder this question and need to come up with an answer when quizzed by omnivores.
Yeast Is a One-Celled Fungal Organism
If your vegan lifestyle includes eating mushrooms, you can feel on safe ethical grounds eating yeast. Yeast are in the fungal kingdom (along with mushrooms) rather than in the plant or animal kingdom. They are a step up the complexity ladder from bacteria in that they have a true nucleus. Like many plants (and unlike many bacteria), they don't have a means of propulsion. Under the microscope they may be seen floating around, but not swimming.
Yeasts consume sugar for their energy source rather than using light for photosynthesis. They are found naturally everywhere, especially on fruits and vegetable matter that has enough sugar for them. If you eat fruits and vegetables, you are eating wild yeasts and other fungi. There is no way to avoid them.
Yeast and Vegan Ethics
Philosopher Jeremey Bentham asked the questions, "Can they reason? Nor, can they talk? But, can they suffer?" Yeasts have no spinal cord, central nervous system, or nerves. This means there's no demonstrated scientific capacity to experience pain, nor to register anything as pain, and that's what makes yeast different from cows. Yeast does not suffer.
If your strict definition of being vegan is not eating anything that has a face, then be assured yeast don't have a face. If you don't eat anything that casts a shadow, you are safe eating yeast. The cells can form colonies so you can see the growth of little "city" of yeast, but each individual yeast organism is far too small to be seen without a microscope.
People for the Ethical Treatment of Animals (PETA) is satisfied that the production of yeast does not involve animal cruelty or exploitation. If you choose not to eat products produced by or containing yeast, you will be in a tiny minority of vegans. Vegans notoriously love their nutritional yeast and have no problem drinking vegan beers and eating vegan bread or other foods containing yeast.
Should Vegans Eat or Drink Fermented Products?
Yeast and its relatives are used in fermentation, whether natural or in food manufacturing. They convert sugar to alcohol and carbon dioxide. In bread, this causes the bread to rise. In making beer, wine, and distilled spirits the alcohol is the desired result. Vegan products such as miso and tempeh are made by fermentation.
You can look into the process of making these products to see if any animal products are used. Some beers and wines are filtered through bone char or fish products, for example. As with checking any other food products, you will need to study the manufacturing process to decide for yourself whether it constitutes cruelty to animals or exploitation of animals.
Level 5 Vegans
The most extreme form of veganism is called level 5 vegan. Some people may be discerning enough that they wonder whether to consider the lives of the millions of yeast cells that are used to produce a bottle of wine or loaf of bread and wonder whether those lives are the equal of the lives of cows, fish, or bees. However, if you decide you can't ethically consume the products of yeast, where does this stop? Yeast are less complex than plants and only a bit more complex than bacteria.
Any way you obtain nutrition must start with plants or bacteria, which also lack a nervous system but are individual organisms. If you eliminate plants, fungi, and bacteria you probably won't be able to eat anything at all. While plants can turn sunlight, air, water, and minerals into nutritional substances, the human body cannot.