Dennis Briggs

Biology of Yeasts

Brewers have repeatedly experienced the need to identify a yeast in order that they can search the appropriate literature for information on the yeast in question, or be able to recognize the yeast on a subsequent occasion. It may be that they wish to be assured that a particular sample of pitching yeast is the strain which they propagated initially from laboratory stocks. On the other hand, they may be anxious to know if a yeast strain which has contaminated their pure culture of pitching yeast is, according to other brewers’ experience, likely to cause the beer to spoil. Because yeasts are microscopic in size, and change both their shape and size considerably according to the medium in which they grow, simple microscopic examination rarely suffices and identification is normally based on a range of morphological and biochemical criteria.

Metabolism of Wort by Yeast

Yeast will grow fermentatively in simple media which contain fermentable carbohydrates to supply energy and carbon ‘skeletons’ for biosynthesis, adequate nitrogen for protein synthesis, mineral salts and one or more growth factors. Yeasts also require molecular oxygen (see Chapters 16, 18 and pp. 604–608, this chapter).

Chemical and Physical Properties of Beer

Beer is a complex mixture; over 400 different compounds have been characterized in beer which, in addition, contains macromolecules such as proteins, nucleic acids, carbohydrates and lipids. Some of the constituents of beer are derived from the raw materials and survive the brewing process unchanged. Others are the result of chemical and biochemical transformation of the raw materials during malting, mashing, boiling, fermentation and conditioning. Together all these constituents make up the character of beer but, in general, different beers and lagers contain different proportions of the same compounds rather than novel constituents. Nevertheless, accidental or deliberate contamination of beer with micro-organisms other than yeast may well produce new metabolites.

Microbiological Contamination in Breweries

There has always been a measure of disagreement among brewers on the degree of microbiological control needed in breweries. On the one hand, there are those who wish to eliminate all micro-organisms from the brewery except for pure culture yeast and thereby help to achieve a consistently satisfactory quality of extremely uniform beer. There are however those who, while wishing to restrict rampant growth of other micro-organisms, nevertheless emphasize the distinctive, desirable flavours which these organisms can on occasion impart to beer. As with wines and ciders, outstanding flavours in beers are often associated with the metabolic activities of a mixed microbial population. Because there is no detailed knowledge available on achieving a desirable balance of strains, usually involving both bacteria and yeasts, such a balance can be achieved only fortuitously. A change in the proportions of the organisms in the balanced mixture may, and usually does, lead to beer with inferior taste and aroma. It is particularly true of modern beers of comparatively low original gravity (say under 10° Plato and SG 1040), undergoing a very short maturation treatment. This is because in stronger beers the bacterial strains are inhibited to different degrees by (i) low pH values, (ii) high concentrations of alcohol, (iii) high contents of hop resins, and (iv) in some cases the yeast strains used.

Beer Flavour and Beer Quality

The final arbiter of beer quality is the palate of the consumer and this can show wide variation between individuals, between geographical areas, and even from one time to another. Quality is defined as ‘degree of excellence, relative nature, or kind, or character’ and accordingly the brewer refers to the many varieties of ale, beer, stout, and lager which he brews to satisfy the varied demands as different qualities. When the customer has chosen the quality he wishes to drink he demands that his beverage shall have the ‘degree of excellence’ which he expects and that this shall not change from day to day. Much of the brewers’ art is therefore concerned with quality control, with producing a constant product from variable raw materials by a biological process.

Chemistry of Wort Boiling and Hop Extraction

Wort boiling may be regarded as the turning-point in the brewing of beers; it is omitted in distilling and vinegar brewing. At its simplest, in Bavarian practice, the all malt wort is boiled with hops for 1–2 hr, no other additions being permitted by aw. Elsewhere the sweet-wort may be produced from a mixed cereal grist (see Chapters 10 and 11) and additional carbohydrate may be included in the boil either as brewing sugars or wort syrups (Chapter 8). Further, part or all of the hop grist may be replaced by a suitable hop extract. Indeed, it will be shown in the sequel that hops can be utilized more efficiently if the hop principles are extracted and isomerized independently of the boiling process. The pre-isomerized extract is then added to the beer after fermentation.

Methods of Wort Boiling and Hop Extraction

The previous chapter was concerned with the many complex chemical processes and physical changes taking place during wort boiling. Under no set of practical conditions can all of these be fully optimized. It is understandable therefore that various breweries select different conditions of wort boiling and that these differences are reflected to some degree in the character of the beers produced. Illustrating the complexity of the situation, the various unit operations involved in wort boiling, hop separation, and wort cooling are set out below [1].