Yeast and Water

Classification of yeast

Yeast is a simple, single-celled micro-organism, of which there are many hundreds of strains. Only some are appropriate for brewing.

Yeasts are classified according to genus (family), for example, saccharomyces, and then sub-categorised by species - e.g. cerevisae.

There are two basic types of brewers' yeast:

saccharomyces cerevisae (usually simply referred to as ale yeast), which is a 'top-working' yeast, and is used in making English beers.

saccharomyces uvarum (formerly known as s. carlsbergensis) which is a 'bottom-working' yeast, used to produce lagers.

Top-working yeasts are ones which sit on the top of the wort in the fermenting vessel during fermentation. These are used in English ales.

Bottom-working yeasts are ones which fall to the bottom of the fermenting vessel. Bottom-working yeasts ferment at lower temperatures than top-working ones. They are used in producing the Pilsner-style lagers.

The terms 'top' and 'bottom' working are slightly misleading, as these are merely descriptions of the yeasts' behaviours - both types of yeast leave plenty of cells in suspension, where the 'work' is really being done.

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Respiration

Brewers' yeast have two methods of respiration:

aerobic - using oxygen
anaerobic - without oxygen

Brewing yeast will respire aerobically out of preference, and reproduces very quickly - this can be seen by the rapid formation of a yeast head on a fermenting vessel of wort that is rich in dissolved oxygen.

However, once the yeast has used all of the dissolved oxygen, it has to change to anaerobic respiration. Under anaerobic respiration, the yeast processes the sugar molecules (which are made up of carbon, oxygen and hydrogen atoms) to obtain nutrients and oxygen. This process causes the formation of alcohol and carbon dioxide as by-products.

Alcohol can only be produced when the yeast is operating in its anaerobic mode.

Conversion of sugars - alchohol production

In order to convert the sugars present in a wort, the yeast cell uses enzymes to break the sugars down into two basic sugar types that it can use: glucose and fructose.

The yeast will take in glucose and fructose first. It will then release an enzyme, invertase, to break down the sugar sucrose into glucose and fructose.

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Diagram of sugar uptake by yeast cell

Maltose follows, which is brought through the cell wall by a permease enzyme, maltase. Once inside, another enzyme, a-glucosidase, breaks the maltose down into glucose.

The final sugar to be converted by the yeast is maltotriose, which undergoes a similar process to that of maltose.

It is only when yeast goes into the mode of anaerobic respiration, that it releases these enzymes, and so begins the production of alcohol.

Sugars, and alcohols, are compounds of three elements: carbon, oxygen, and hydrogen. The process of breaking down the sugar compounds - and therefore, the molecular structures of these sugars, creates other compounds - alcohol and carbon dioxide.

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