Fresh baked bread possesses the powerful hand of magic. As your nose fills with the yeasty aromas inherent in the process, images of comfort saunter into your unsuspecting brain. Realtors use it to sell houses, and grocers capitalize on specialty cheeses, pickles and sauces, positioned in close proximity to the earthly scents associated with a touch of heaven.
Those who have delved into the art of breadmaking understand the source of this erotic scent. The brewer understands the key to this secret as well. It’s the yeast!
Yeast has the ability to impart specific flavors to beer, just as surely as sourdough starter imparts a distinctive flavor to a loaf of French bread. These single celled organisms reproduce by budding, and convert fermentable sugars into alcohol and other by-products. The most common of these by-products is carbon dioxide, but it may convert into other chemicals, such as esters, fusel alcohols, diacetyl and sulphur compounds.
In the early nineteenth century, yeasts were thought to be chemical substances, much like hydrogen or iron. Critical thinking in Europe had been more clearly articulated during the Enlightenment of the late eighteenth century, and a thirst for expanded knowledge reigned supreme as time advanced into the next century. In 1836, Cagniard de Latour showed that yeasts were living organisms. By 1860, Louis Pasteur, in his Etudes sur la Biere (Studies concerning Beer), proved that fermentation was caused by living organisms. Pasteur identified the conditions in which yeast used oxygen – while multiplying – or worked without oxygen – causing fermentation.
In 1883, Emil Christian Hansen, a brewing scientist at Carlsberg Brewery, Copenhagen, Denmark, isolated a single cell culture of brewing yeast. This was a bottom fermenting yeast, and became known as Saccharomyces carlsbergensis, also known as Saccharomyces uvarum. These bottom eaters are generally used to produce pilsner, dortmunder, märzen, bock and American malt liquors.
Yeast strains that rise to the surface during fermentation were classified as Saccharomyces cerevisiae and are used to produce top-fermenting beer such as ale, porter, stout, altbier, kölsch and wheat beer.
Due to reclassification of the Saccharomyces species, both ale and lager yeasts are currently considered to be Saccharomyces cerevisiae. “There will probably be another reclassification in the future,” says Christopher E. White, PhD, of White Labs in San Diego, CA, USA. “Lager yeasts are physiologically different from ale yeasts, but they were put together because you can get them to mate but that doesn’t have anything to do with how they behave in beer.”
Despite this reclassification, brewers continue to use the previous nomenclature to distinguish the differences in the way the yeast strains perform their work. Top fermenting yeasts work most effectively at higher temperatures between 65 and 72 degrees F. They create beer that is high in complex esters and aromas with a thick, yeasty head.
Bottom fermenting yeasts are most effective at 42 to 52 degrees F. They grow less rapidly, lack esters, create clean flavors with less surface foam, and are able to ferment raffinose.
California common yeast is a bottom fermenting yeast that works at the higher temperatures associated with top fermenting yeast strains. Anchor Brewing produces Anchor Steam Beer using this method of fermentation.
The life cycle of yeast moves from dormancy, when it is added (pitched) to the wort, through activation in four phases:
Lag period – The yeast begin to reproduce, causing a drop in pH due to the utilization of phosphate and a reduction in oxygen. Intracellular carbohydrate reserves of glycogen become the energy source, and are broken down into glucose.
Growth phase – During this respiration phase, the wort becomes covered with foam, and acid compounds are oxidized. The pH drops significantly in this phase.
Fermentation phase – As the oxygen supply depletes, any that remains is “scrubbed,” or stripped out of solution by the carbon dioxide produced by the yeast. The production of CO2, ethanol and beer flavor can be observed by a reduction in wort gravity.
Flocculation phase – In this sedimentation process, the yeast begins to prepare itself for dormancy by flocculating, or gathering together, and settling to the bottom. It produces glycogen as a self-sustaining energy source for use in the next life cycle.
Usually, 75-85% of the fermentable sugars are assimilated by the yeast, leaving polysaccharides and oligosaccharides behind as non-fermentable sugars. Yeasts consume the simple sugars of sucrose, glucose and fructose within the first 24 to 49 hours. Maltose continues to assimilate within 70-72 hours of the onset of fermentation. Maltotriose may be consumed by some yeast strains after 72 hours, but most strains leave the maltotriose and dextrins behind.
Yeast growth affects beer flavor, and optimum temperatures and sanitation are necessary to create an environment that is conducive to making flavorful, well-balanced beer.