First, what is yeast? For a scientist yeast is any single celled fungus. This makes it a very large and diverse grouping of organisms in the Fungi Kingdom. Brewers really focus on just a few of the many yeast that are in nature. All the varied strains that brewers use fall into the Class Saccharomycetes (Sugar Fungus) Order Saccharomycetales (containing 13 families).
The most common two brewery yeast strains are are Saccharomyces cerevisiae and Saccharomyces pastorianus, these are commonly referred to as Ale and Lager yeast. All of these strains get there name from the latin words Sacch – meaning sugar, and myces – meaning fungus. Appropriately named these strains all eat sugar and turn it into a variety of metabolites, predominantly alcohol and carbon dioxide.
The main difference between ale and lager yeast, according to Graham Stewart at the International Center for Brewing and Distilling, Heriot-Watt, Riccarton, Edinburgh, Scotland.
- Lager yeast produce greater amounts of sulfur dioxide
- Diacetyl reduction is less efficient in lager yeast
- Ale yeast has a higher upper temperature limit
- Ale strains sporulate easier
- Lager yeast have a higher stress tolerance
- Lager yeast strains have greater maltotriose uptake
- Ale yeast are more tolerant to drying
The main sugars that brewers strains (cerevisiae and pastorianus) are able to consumer are glucose, maltose, maltotriose, fructose, and sucrose (table sugar). Sugars with greater than 4 saccharides are generally referred to as “dextrins” and the typical brewers strains are unable to consume this. Brewers use this to their advantage as dextrins provide body and sweetness to the beer.