Chris White and Jamil Zainasheff’s Yeast: The Practical Guide to Beer Fermentation is a thorough review of the subject of yeast, with the practical (home)brewer in mind. It is mostly a treatment of commercial brewer’s yeast but there are some interesting observations about wild yeast, too. The authors define wild yeast as yeast “that is not in the brewer’s control.” For example, commercial Brettanomcyes is not wild yeast but native strains of Saccharomyces that (unintentionally) are introduced during cooling of the wort or barrel aging would be. Of course, today’s commercial strains of Brettanomyces may still have a lot in common with yeasts that are found in the wild, but one could imagine a scenario where the use of Brettanomyces becomes so popular that commercial yeast sellers increasingly select these strains for certain properties. As a consequence, wild yeast is not characterized by its aroma and flavor properties (such as tartness or funkiness) but by its involvement in (ambient) spontaneous fermentation.
There are a number of distinct traits that have been retained in wild yeast. Wild yeasts are usually diploid, form spores, and are still capable of mating. Commercial yeast, in contrast, has lost this ability because mainstream brewers desire consistent characteristics from their yeast. Wild yeast usually has low flocculation, which can produce higher attenuation because the yeasts will not quickly drop or rise in the wort. In commercial yeast, however, such a property is not desirable for many beer styles, where a quick and clean beer is the goal. Unlike wild yeasts, which have evolved to compete against each other, commercial yeast can often co-exist and ferment at similar rates.
The book also includes sections on Brettanomyces and capturing wild yeast. Although the name Dekkera is often used interchangeably with Brettanomyces, it is only Brettanomyces that is of the non-spore forming type. One of the intriguing things about Brettanomyces, much to the chagrin of wine makers, is that it produces the enzyme Beta-glucosidase, which can convert the wood sugar cellobiose into glucose, a phenomenon that is more prevalent in new barrels that have higher concentrations of cellobiose. Brettanomyces is quite sensitive to oxygen, with moderate concentrations most favorable to its growth, and lower and higher concentrations, unfavorable. Increased oxygen produces more acetic acid as a fermentation product.
Instead of inoculating wort with commercial Brett, some (home)brewers aim to capture real wild yeast for fermentation. There is no shortage of methods for doing this, including ambient exposure of the wort, fermentation in “infected” barrels, the use of wild fruit and herbs to start fermentation, or using dregs from the bottles of traditional lambic brewers. Of course, such methods usually introduce souring bacteria as well, and the art is to discover and perfect a method that leads to consistent, favorable outcomes. Because many brewers prefer not to waste multiple batches of wort on spontaneous fermentation experiments, and the yeast captured in the wild may not be sufficient to start a healthy fermentation, one approach is to create ambient spontaneous starters (there is a lot of information about creating conventional starters in the book). At this stage, such efforts are still largely the work of some adventurous (home)brewers, and documentation of such efforts is still in its early stages (the Mad Fermentationist blog is an excellent resource). In the case of spontaneous starters it is important to avoid sampling at an early stage, where aerobic conditions, higher pH, and low alcohol still permit the presence of dangerous pathogens.
Because the book is mostly written for brewers who have control over their yeast and fermentation, a lot of information is not completely applicable to brewers who use spontaneous fermentation or incorporate spontaneous fermentation. But there is some information that is interesting for “wild” brewers as well. For example, proper wort aeration is important for healthy yeast growth but brewers who use barrels for (primary) fermentation may have problems in getting enough dissolved oxygen at the start of fermentation. The authors report on a New Belgium method where olive oil was added to the wort to supply the sterols that yeast cell membranes require for proper structure and function. One also wonders how the use of coolships (with their large surface to volume ratio) influences initial wort aeration. Temperature is another topic that affects conventional brewers as well as those using wild yeast. As far as I am aware, traditional lambic brewing does not necessarily exclude temperature control, but I think it is safe to assume that most fermenting lambic wort is subject to substantial seasonal and overnight temperature changes that would be contra-indicated for conventional brewers (Cantillon’s Jean-Pierre Van Roy once looked horrified when I asked him about active temperature control). It would be quite helpful to quantify and characterize the effect of ambient temperature fluctuations on wild yeast and bacterial growth, fermentation, and flavor.
Much of the information on yeast growth, handling, storage, and labs is not applicable to spontaneous fermentation but some of the techniques (such as wild yeast tests and forced fermentation) can be used by adventurous brewers to study wild yeast and the conditions that influence spontaneous fermentation. Ultimately, there is an increasing need for an extensive book treatment on (home)brewing with non-conventional and wild yeast. Modifying or ignoring (!) procedures for brewing with domesticated yeast will only take you so far, and the homebrew recipes that can be found in some classic lambic and wild beer books give little guidance about expected fermentation behavior and troubleshooting. Of course, no matter how much our knowledge about spontaneous fermentation grows, beer that is produced in this way will always have more variability than beer that is produced with domesticated yeast under highly controlled conditions. But this is also one of its strengths, and like authentic wine, can lead to surprising results. Many readers of this blog will agree that the best beer in this world remains a product of spontaneous fermentation. If you brew conventional beer in addition to wild beer, Yeast is an invaluable resource.