Yeast plays a very vital role in fermentation. This key ingredient has various strains, which produce unique aromas and flavor profiles characteristic to that region. Today microbiology and packaged yeast have made it possible to transport these wonderful biomes to our brewing vats globally. Yet, yeast is one of the least understood aspects of fermentation and in some aspects continues to remain an enigma. Dutch scientist Anton van Leeuwenhoek first spotted yeast cells in the 17th century through his microscope but their role in fermentation was largely ignored. The common perception, at that time, was that ‘all fermentation required a divine intervention’. There were Gods and Goddesses for brewing who had to be appeased for a successful batch. Fermentation has its roots from the Latin word fervere, which means to boil. The yeast is derived from a similar term gist, which means to boil.
But Microscope was not enough. It was only in 1859 that Germ Theory (by Louis Pasteur) was proposed and scientists started studying them. Today yeast is being actively researched and genetically modified to help brewers hone their craft. The first year of any microbiology lab is spent researching yeast, preparing its slants, and culturing them.
Let us cover some major types of yeasts used in fermentation:
- The most common yeast species is Saccharomyces cerevisiae. It is the most omnipresent form of life in the world. Although unicelled, these organisms are highly specialized and yield varied results based on the conditions of the medium. During baking, one would have come across it under the trade name Active Dry Yeast. This dry yeast is produced in tons and is the most commonly used yeast. Once hydrated, they will start foaming in 10-15 minutes and they can leaven dough in a matter of hours. Since they are chosen for aerobic fermentation and leavening, they specialize in producing a lot of CO2 in a very short period. Wines and beers need an ability to have higher alcohol tolerance and not result in stressed yeast flavors when deprived of oxygen for extended periods.
- The second type is wine yeast and Brewer’s yeast, which is another strain of Saccharomyces cerevisiae. Unlike baker’s yeast, they are designed to flourish in adverse environments (low pH, high alcohol, high sugar, and without oxygen) and yet ferment the fruit or sugar wash (chashani).
- The third strain is the Ale beer yeast. They have an additional capability of being able to metabolize maltodextrins (complex soluble sugars produced during mashing). Baking yeast, may not be able to metabolize them and will result in a heavy-bodied sweet brew. Unlike the typical wine yeasts, beer yeasts have low alcohol tolerance.
- The fourth yeast, which is talked about is the Lager yeast- Saccharomyces pastorianus. This is a bottom-fermenting yeast that can operate at low temperatures to produce the crisp clear lagers a.k.a. bottled beers. In nature, this yeast is not found. It was probably synthesized accidentally when the tree yeast from Argentina traveled on a Spanish boat to Europe and ended up mutating with the ale yeast in an ice-brewing cave in Belgium. Its cell walls are hydrophilic, while the cerevisiae’s walls are hydrophobic. As a result, it does not attach to CO2 bubbles and float at the top. The interesting aspect is that it is able to ferment at 5oC where most of the other microbes go dormant, making the much-coveted winter beers.
- The fifth kind that people come across is the Acetobacter. Technically, it is not a yeast (fungus) but a bacterium. It specializes in oxidizing the ethanol produced by yeast into vinegar (acetic acid) and hence the name. It is found on all fruit surfaces and in the air. It plays an important role in vinegar production, pickling, or pro-biotic preparation.
- Sixth variety is Koji. Technically, it is not yeast but a fungal mold (Aspergillus orzyae). It is heavily used in most Asian recipes from Sake to Soy sauce to Miso. Koji has the ability to produce enzymes that can breakdown starch and proteins. Hence, it is used to ferment rice, legumes, and soybeans.
- The seventh variety is Lactobacillus. As the name suggests, it metabolizes milk sugar (lactose). It is widely found in curd, pickling, and most pro-biotic wild fermentation. This friendly bacteria dominate our skin pores, digestive tracts, urinary system, and genitals. They are responsible to maintain acidic levels and keep the disease-causing microbes away. Most wild fermentations relies heavily on lactobacillus to ferment sugars into lactic acids.
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Although there are several trade books and literature praising the various strains, brands, and logos of the microbial culture, one should not underestimate the significance of wild fermentation. Until about 50 years ago, laboratory isolated strains were not available commercially. Food scientists were relying on the local biome (the symbiotic wild microbes co-existing in a particular farm or region) to ferment their food. Today with awareness towards Probiotics & Eco-immunology, there is a revival of using wild strains to ferment. We will be discussing about this in detail towards the second half of the book.
To get the best from any yeast, below are the six golden rules:
- Maintain proper temperature: Either most home brewers do not have a temperature controller or they get impatient. As a result, they end up with improper temperatures or give thermal shocks (steep day and night temperature differences). Usually, the lower end of the temperature scale is preferred for better (clean and crisp) sensory notes. However, at the higher range, the fermentation will complete at twice the pace and produce 5X more esters. Buying a thermostat is one of the must-have for a brewer. The seasonal variation in beers is attributed to this temperature difference.
- Improper yeast strain: Each yeast has its characteristic profile, which when not matched will lead to a confusing beverage. Just looking at temperature range, flocculation and attenuation are not sufficient. Review the yeast’s esters, bio-chemical chemistry, phenols and another characteristic to match it with the intended product. Trying to ferment an extra-strong brew by stressing the yeast to its maximum sugar tolerance is a bad idea.
- Yeast Nutrients: Lack of yeast nutrients is a notable problem with fermenting honey (meads), certain fruits, and sugar wash. Inadequate nutrients yield a sugar only diet for the yeast, which is detrimental for their health. The yeast starts autolysis (metabolizing dead yeasts) and competing for nutrients leading to a variety of off-flavors.
- Insufficient yeast count: Yeast require oxygen to multiply and do not multiply in anaerobic conditions. As a result, they start dying or mutating towards the latter half of the fermentation. Not using adequate healthy yeast would yield an altered flavor profile.
- Improper wort aeration: This problem is more profound in industrial-scale lagers than in small-batch beer makers. Essentially, during boiling and mashing, the dissolved oxygen is stripped away from the wort. Without this oxygen, the yeast is not able to synthesize the required precursor’s fatty acids and sterols to multiply themselves for optimal performance. Winemakers also keep their fermenters open for the first couple of days to allow some level of oxygen to seep in.
- Poor yeast health & infection: Using old or expired yeast without hydrating them could result in stressed yeast traits & incomplete fermentation. Wild fermentation is gaining popularity, which poses a bit of a challenge for amateurs.
Yeast Spec Sheet
Most commercial strains have a detailed spec sheet which can be very technical and confusing. Few of the most important parameters in them are:
- Aroma wheel: Depicted either through a spider diagram or through a yeast profile paragraph. It talks about the different kinds of esters, phenols, and flavors produced by the yeast. It is a good idea to match this with the kind of fermentation being used.
- Optimal temperature: This is the most important parameter for home brewers. Fermenting ales at lager temperatures (or vice versa) can result in miserable results. Also, the same yeast produces different quantities of esters, phenols, and higher alcohol at different temperatures. High-temperature brewing can produce fusel alcohols and bio-chemicals that can induce a hangover.
- Attenuation: It is a measure of malt fermentability for the respective yeast strains. This is important for beer (grain) yeasts. Most yeasts are able to completely ferment out glucose, fructose, and sucrose but their ability to ferment maltose is limited. Low attenuation (~70%) yeasts result in a good body and high attenuation (~85%) results in a thinner lager-like mouthfeel.
- Flocculation: It measures the rate at which the yeast clumps together and settles to the bottom once all the fermentable sugar/ maltose is consumed. Most American style beers and IPA need high flocculation and fast sedimentation. However, some unfiltered German-style beers need a medium to low flocculation. In ciders and wines, inadequate flocculation yield bready or yeasty notes. Hence, choose the flocculation as per the recipe style.
- Alcohol tolerance: A lot of amateurs have an obsession with strongest wine or brew. Personally, I rarely look at this parameter. If the yeast can produce 10% (for wine) and 5-7% (for beer), it is OK. Some special styles like barley wines and sparkling wines need very high alcohol tolerance.
Yeasts are akin to the workers in our fermentation and will create off-flavors if not treated well. We have a chapter on handling off-flavors that specifically helps to diagnose signs of improper fermentation and yeast activity. I hope that it will help you rectify some of the issues and refine your craft.