Lesson 2 - Yeast Morphology and Evaluation

How well does wort perform as a growth medium? Not very well is the answer. Organisms require a balanced supply of nutrients to enable optimal growth. In a batch culture, growth stops either when conditions become inhibitory, or when an essential nutrient runs out. Wort seems to be a rich medium, and it does indeed supply all the nutrients necessary for growth with the exception of zinc (which is often added as a supplement). However, it also contains a lot of sugar, with quite low concentrations of everything else. As a result, growth during fermentation is minimal and usually consists of only three to four doublings before something essential runs out. This might be nitrogen, or it could be sterol/unsaturated fatty acid limitation, which are all themselves controlled by the quantity of oxygen supplied at pitching.   

Nowadays, many brewers use very concentrated worts. This might be so that they can brew a very alcoholic beer. More often, it is used in high gravity brewing as a strategy for increasing the fermenter’s productivity. Here, a concentrated wort is fermented and later diluted to sales strength (discussed fully in the Fermentation unit).

Very concentrated worts can cause a number of problems:

• Low water activity: In very concentrated worts, the high number of dissolved molecules reduces the amount of water available (A_w) to the cell. This stops the growth of many microorganisms and explains why, for example, products such as sugar syrups or fruit jams do not easily spoil. Tolerance to such conditions varies widely. Those which can tolerate low water activity are said to be xerotolerant; those which are relatively unaffected by the high concentration of solute molecules are said to be osmotolerant. Both effects are related.

Saccharomyces yeasts are not particularly tolerant in either respect. As a result, most beers use worts with a starting gravity of 10-15°P. To cope with low water activity, yeast produces compatible solutes. These are neutral compounds, which reduce the difference between the osmotic potential of the cell and the environment. One of these is glycerol, which gives beer mouthfeel or body; it is usually found at quite high concentrations (1-3 g/L).

• Alcohol (ethanol) tolerance: Very high ethanol concentrations stop yeast cells from growing. In extreme cases, they cause the death of  yeast cells. It is not yet overly clear how this works, however we know that it probably involves several different mechanisms. An important factor seems to be the ability of ethanol to disrupt the structure and function of the cell membrane. It is difficult to distinguish between the effects due to the alcohol itself, and those resulting from low water activity /osmotic effects.

Various yeast genera and species can tolerate a range of concentrations. This seems to have a genetic basis. For brewing yeasts, when a malt wort is fermented, they will withstand a maximum alcohol concentration of around 10-12% ABV, although there are some exceptions. Wine yeasts are more tolerant, usually withstanding up to 15% ABV. Yeasts used for sake and tokay wines can withstand up to 20% ABV. There is still much ongoing research into this area as many brewers, distillers, and biofuel producers would like to be able to ferment very concentrated media.  

The high concentrations of alcohol formed in beers made from concentrated worts are, of course, also accompanied by the formation of high levels of CO₂. Remember for each molecule of glucose used in fermentation two molecules each of alcohol and CO₂ are formed. There is some evidence that CO₂ itself may also be inhibitory although this is far from conclusive. It is possible that it does exert an inhibitory effect, possibly by intracellular acidification, caused by the formation of carbonic acid due to CO₂ solution. At very high concentrations this could overwhelm the mechanisms used by yeast to regulate intracellular pH. On the other hand, inhibitory effects could simply be a result of the increased in pressure as a result of CO₂ accumulation.