Yeast and Human Societies

The English word yeast has cognates in most other languages. In the Romance languages, the current words are levure (French), lievito (Italian), and levadura (Spanish). In German the word is Hefe and in Dutch it is gist. Older German terms are yes (to ferment) and gäscht (ferment or boil). The Greek language has zéein (to boil), zestos (ferment, boil, hot), and zethos (beer). It seems likely that the English word yeast was derived from Scandinavian sources. The medieval English term is zeest or yest, whereas the Danish word was gjœr. Medieval Icelandic was jast as was medieval Swedish. These various terms generally refer to the lifting, frothing, foaming, or bubbly properties that this species confers on the material upon which it is acting. That these cognates exist in most languages is evidence for the long association of this species with man. The genus and species names, S. cerevisiae,also can be traced to earlier origins. Saccharomyces means sugar mold or fungus and cerevisiae has its origin in the Gaelic word kerevigia and the old French word cervoise. Both these ancient words for cerevisiae mean beer (Meyan 1838). Yeast has been an important part of human civilization for a very long time. The earliest evidence of wine making has been dated to Neolithic times ∼7400–7000 years ago. The calcium salt of tartaric acid and the resin of the Pistacia (terebinth) tree have been identified in narrow-necked pottery specimens dated to this period (McGovern et al. 1996, 1998). Tartaric acid occurs in large amounts only in grapes, and it was postulated that thePistacia resin was added as an antibacteria] preservative. This was considered to be strong evidence for the making of wine at these times. It is not known whether the wine made at the Haji Firuz site was made from domesticated or wild grape. The site lies within the ancient and modern distribution zone of wild grapevine, as established by pollen cores from nearby Lake Urmia. The quantity of wine in the “kitchen”, ∼50 liters (14 gallons) if all six jars contained wine and were nearly full, suggests fairly large-scale production and consumption for a household. If the same pattern of usage were established across the whole of the site's Neolithic stratum, only part of which was excavated, it might be concluded that the grapevine had already come into cultivation (McGovern et al. 1997). Chemical evidence for the making of wine and beer at later Sumerian times (4000 to 3000 BC) has also been detailed (Michel et al. 1992; 1993; McGovern et al. 1995). Evidence for the making of beer came through the demonstration of the presence of calcium oxalate and the shape and construction of the vessels. This evidence was found in the village of Godin Tepe in the Zagros mountains in lower Mesopotamia. The cuisine of the Neolithic period included relatively large-scale production of wine, a processed and fermented beverage made from grape juice, as attested by the chemical confirmation of its presence in jars of a Neolithic residence at Hag Fires in northern Saguaros mountains of Iran (McGovern et al. 1997). The making of wine was probably man's first experience with yeast because the process does not require the use of an inoculum of yeast. The concept of yeast as the microorganism that carried out the fermentation, was not developed until ∼7000 years later with the work of Pasteur (1872) and others. These early people found empirically that it was necessary only to crush the grapes or other fruit and leave the juice (called must) to ferment. The yeast cells were already on the fruit and ready to carry out the fermentation (Mortimer and Polsinelli 1999). Recently, we have shown that DNA isolated from a presumptive wine jug at Abydos (3150 BC) had a sequence that closely matched the sequence of modern yeast S. cerevisiae (Cavalieri et al. 2000). The yeast S. cerevisiae is an essential component of many important human activities including baking, brewing, distilling, and wine making. It also is now used as a model eukaryotic organism in research activities in hundreds of laboratories worldwide. It can grow and function both aerobically and anaerobically (fermentative). Its principal use involves fermentation. In all four industries, specialized strains ofS. cerevisiae have been selected and are in use and these strains are not readily interchangeable. Wine-making and baking sometimes involve the use of “natural” strains, and it is apparent that such strains, in contrast to the commercial yeast now used in all these industries, are still evolving. It appears that bread making dates back at least 6000 years, but use of leavening, which required the development of suitable cereal grains with easily removable hulls, gluten, and the introduction of yeast cells, did not appear until around 500 BC (McGee 1984). With the development of agriculture, it was probably found that addition of some of the fermenting wine to dough resulted in a lighter, more pleasant bread. Alternatively, insects may have landed on the dough and inoculated it with yeast. The use of leavening is described in several verses of Exodus in the Bible. It was found empirically that some of the leavened dough could be used as a starter for a new batch of dough. It is likely that, until recently, the yeast used in baking was evolving, with the participation of man, because starters were transferred sequentially from one baking to the next. Following the discoveries of Pasteur and the development of microbiological techniques including the concept of “pure culture,” specialized strains were introduced into these industries and evolution was partially interrupted. In addition to wine, other alcoholic beverages were developed quite early. For example, beer is made from malted barley and other cereals and requires the addition of live yeast cells to promote fermentation. Malting, which was practiced in Egypt and Babylon, dates from at least the third millennium BC. The procedure, which involves sprouting cereal seeds, leads to the conversion of starch to fermentable sugars. Katz and Maytag (1991) have recently made beer using an ancient Sumerian recipe. They state that yeast was inoculated into the original fermentations by addition of grapes and raisins. They also may have been introduced to the malt by the addition of some fermenting wine. An alternative explanation is that insects such as bees, wasps, orDrosophila landed on the malted grain and inoculated it with yeast carried on their bodies. Most distilled beverages are also made from malted grains with the fermentation being carried out by the addition of yeast cells. It seems likely that the yeast cells involved in these processes, until recently, were evolving during sequential transfer. In wine making, the yeast cells are present on the grapes, and later in the fermentation, for only a few weeks in the year. Attempts to find them in the vineyard either a few weeks before or soon after harvest generally have been unsuccessful (Kunkee and Amerine 1970). Recent evidence suggests that the yeast are brought to the vineyard when the grapes are nearly ripe by insects which perhaps transport them from their nests or hives (Stevic 1962; Snowdon and Cliver 1996). We have shown that these insects primarily feed on damaged berries (Mortimer and Polsinelli 1999). These damaged berries along with many intact berries are then crushed and enter the fermentation. Our studies show that the microorganisms on the damaged berries can provide the principal inoculum for natural fermentations and that they also may make significant contributions to inoculated fermentations. Results discussed in a following section show that evolutionary forces may act on the yeast in the hive, in the vineyard or in the fermentation. Another model of yeast evolution proposes that the yeast cells are present on the cellar walls and equipment and from there enter the fermentation. When the fermentation is complete, it is proposed that they return to the cellar walls in a closed cycle and it has been argued that evolutionary forces operate to make better wine yeast (Martini 1993). This situation may apply in some special circumstances (real cellars and even caves) but it seems unlikely that it functions in a modern winery with stainless steel, and often closed, fermentation tanks and other stainless steel equipment. Many modem wineries even have their fermentation tanks outside so there are no cellars or cellar walls. A model proposed by Naumov (1996) states that S. cerevisiae cells are found only in association with human activities. According to this model, the yeast exist in the vineyard and enter the fermentation with the grapes. The model also proposes that the yeast may be transported to the cellar or wine-making equipment by insects and thereby get into the fermentation. Subsequently, the yeast are transported back to the vineyard from the cellar by insects. Evidence in support of this model is that S. cerevisiae has been found only in areas close to human civilization. Attempts to find this species of yeast in regions remote from human activities have been unsuccessful. Closely related species such as S. paradoxus and S. bayanus are, however, found in these remote regions. Today the use of starter cultures in brewing, baking, distilling, and wine making are special cases because, originally, the yeast used in these starter cultures came from a wine or other natural fermentation. Usually these starter cultures were chosen for specific purposes and are often selected from hundreds or thousands of natural isolates. One of the objectives of the companies that sell strains of yeast to wineries and bakeries is to keep the strains without change for many years. Brewers and distillers generally propagate their own yeast but again they attempt to keep the same strains without change from year to year. Such strains would not be in the evolutionary pool. Brewing, baking, and distilling are year-round operations whereas wine-making is seasonal. In the baking and distilling industries, the S. cerevisiae used to carry out these processes is killed by heat. In contrast, brewing is a process that yields a product that still contains live yeast cells. And brewers often use a portion of their fermentations to inoculate a new batch of material for making beer. In these operations, the raw material used does not contain S. cerevisiae. In wine making, in contrast, the raw material (grape must) contains S. cerevisiae which can carry out the fermentations (Mortimer and Polsinelli 1999).