The action of the yeast and the conditions of production determine whether a beer is a top or a bottom fermentation beer. Bottom fermentation is fermentation at low temperature where the yeast settles to the bottom at the end of the process. The strain Saccharomyces carlsbergensis yields alcoholic strengths of approximately 5 percent vol (alcohol is a degradation product of yeast, i.e., in principle, toxic in high concentrations). “Top fermentation” means fermentation at room temperature or higher, the yeast rises to the surface at the end of the brewing process. The strain Saccharomyces cerevisiae can tolerate alcoholic strengths of up to 12 percent vol. There is of course a direct connection with the amounts of ingredients (the so-called starting materials), i.e., malted barley and other cereals, for example, wheat or rice. The starting for top fermentation beers is much higher than for bottom fermentation beers, expressed in values of “original density'' or degrees Plato - that is, the sugar content of the wort (wort is the sweet malt infusion obtained when the cereal starch converts into fermentable sugars). Values can vary by a few degrees (specific density of sugars per volume) around the 25 degrees mark. Higher-density beers have not only higher alcohol contents but also more “bits and pieces” originating from the raw materials and the changes that they undergo in the various stages of the brewing process. Top fermentation beers should therefore have a more favourable effect than bottom fermentation beers for the body. 

Beer & health