Yeast terminology and definitions

Adjunct: unmalted grains (corn, rice, oats etc…) used in brewing to supplement the main mash ingredients (malted barley). Can be used to create different types of beer or to cut cost on using less barley malt.

Autolysis: also called self-lysis, refers to the rupturing of the yeast cell and subsequent release of yeast intracellular components into the beer (mostly off-flavors). Autolysis mostly occur when yeast is subjected to stress or temperature shock.

Biofilm: complex aggregation of microorganisms marked by the excretion of a protective and adhesive matrix. Cropping: refers to the removal of the yeast which has settled to the bottom of the fermenter at the end of the fermentation. Cropped yeast can either be discarded, stored or re-used straight away.

Diacetyl: vicinal diketone also known as 2,3 butanedione. Diacetyl is a by-product of the yeast biosynthesis of the amino-acid valine. During the synthesis, the yeast produces ?-acetolactate which is spontaneously decarboxylated into diacetyl which has a very low flavor threshold of 0.1 mg/l. The yeast will then re-absorb diacetyl and reduce it to acetoin and 2,3 butanediol which are relatively flavorless. Diacetyl is not a desirable flavor in beer, it is usually described as butterscotch. Diacetyl is also produced by bacterial and yeast contaminant which can contribute in spoiling the beer.

reversible aggregation of cells due to the binding of protein lectin on one cell and sugar receptor on another. Calcium is necessary for the binding to occur.

Genetic drift: refers to changes in DNA integrity of a particular cell or cell population. A cell which undergoes genetic drift is commonly named a ‘variant’ or a ‘mutant’. Genetic drifts can cause changes in fermentation performance and flavor profile. They can also cause changes in the aspect of the colony morphology. Genetic drifts can be detected using fingerprinting techniques.

Gram stain: procedure aiming at differentiating the two groups of bacteria: gram-positive and gram-negative. The two groups differ by the nature of their cell wall. Gram-positive bacteria can retain a purple dye whereas gram-negative cannot and have to be counterstained in pink.

refers to the density of the wort and expressed as a ratio to the density of water. The density of the wort depends on the amount of sugars dissolved in the wort. As an example: gravity is expressed as either 1040 or 1.040.

Haemocytometer: thick microscope slide containing a grid of 25 large squares, each of them containing 16 small squares. The volume of liquid between the grid and the coverslip placed above it is 1mm high x 1mm width x 0.1mm depth, which equal to 0.1mm3. By counting the number of cells in the grid and multiply it by 104, one can obtain the cell concentration in number of cells per ml (cm3).

Melibiose utilization: lager strains possess an enzyme called melibiase (?-galactosidase) which can break up melibiose into the fermentable sugars galactose and glucose. However, ale strains do not possess this enzyme and therefore cannot utilize melibiose. Melibiose utilization is a common test to differentiate between ale and lager strains and is perform in test tubes using liquid media.

Pitching: refers to the inoculation of yeast into the wort to start the process of fermentation. Pitching yeast can either be propagated yeast, recycled yeast or ready-to-pitch dry or liquid yeast.

Plato degree: unit used to measure the density of beer wort in terms of percentage of extract by weight. Plato = g of sugars in wort/100g wort.

Skimming: refers to the removal of excess top-fermenting yeast which rises to the top of open-air fermenters during the fermentation process.

Superattenuation: refers to the gravity of the beer going further down than theoretically calculated (this being dependent on the concentration of fermentable sugars in the wort). The reason for superattenuated beers is often the presence of wild yeast or bacteria which have the ability to ferment dextrins and starch.

Taxonomy: practice and science of classification.

Yeast viability: refers to the percentage of viable cell in a population. The percentage of dead cells can be determined using either brightfield or fluorescent dyes. The ability of a cell to divide is also sometimes used as an assessment of viability. Yeast vitality: refers to the activity or metabolism of the cell. Yeast vitality has been linked to fermentation performance. Vitality can be assessed using a variety of methods; examples include vital dyes, measure of intracellular pH, production of CO2, consumption of sugars and oxygen.

Yeast propagation: process by which the yeast uses its energy to create more biomass and not ethanol. Propagation consists of a series of steps to increase the volume of yeast until the volume needed is reached (depending on brewery need). Yeast fermentation: biochemical process of energy production under anaerobic conditions. Yeast can also ferment in presence of oxygen as long as the sugars in the wort are over 0.1%. Typically in a brewing fermentation, sugars are transformed preferably into ethanol and carbon dioxide.

Wild Yeast: any yeast not deliberately introduced during the brewing process.

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