By Moritz Kallmeyer
Chief Brewer Drayman’s Craftbrewery, Silverton Pretoria, March 2005
The more I know about yeast, the more I realize I need to know more about yeast!
Any excess over need results in the formation of food reserves in the form of carbohydrates or fat. When given access to a plentiful supply of nutrition, plants like barley make barley starch in the seeds to provide the energy for germination. Yeast is no exception and energy reserves are found particularly in the form of the non-reducing disaccharide trehalose and the polysaccharide glycogen. These carbohydrates will have normally accumulated in cells at the end of brewery fermentation (when famine sets in). Both in storage and early in fermentation, yeast uses them as reserves to provide glucose for glycolysis. Brewers are thus cautioned to handle yeast correctly or the yeast may autolyze.
Yeast autolysis or self-lysis is the breaking open or rupturing of the yeast cell and the transfer (leaking out) of undesirable substances and off-flavours to the beer. The flavour is described as yeast-bite, broth-like, meaty, sulphury and dirty diaper.
Reasons for autolysis:
- The cell membrane of any unhealthy cell can become more prone to lysis during fermentation if exposed to any stressor.
- Older yeast cells become weaker and less active with age and eventually their cell membranes rupture.
- Sudden exposure to shock caused by too rapid cooling or warming can cause some cells to lyse which would otherwise remain intact.
- If beer which contains yeast is stored for a long time, or when beer which still contains yeast when it leaves the brewery is kept for a long time before it is drunk.
- Poor storage conditions of cropped yeast. 6. Intentional acceleration of autolysis by heating the yeast to make autolysates for the food industry, such as Marmite.
Yeast and nutrition:
The composition of the environment influences the metabolic processes the yeast uses. Yeast has the ability to use a range of different sugars and even ethanol for growth. Fermentation with the goal to achieve beer can not be separated from yeast growth – and for growth to occur the brewer needs to supply the correct nutritional environment. If there is no growth there is no fermentation. Growing yeast never flocculate and flocculated yeast never grow.
Brewers yeast needs:
- Readily usable (assimilable) sources of carbon and nitrogen (FAN – free amino nitrogen) which they get from malt.
- B-vitamins from malt.
- Trace elements from malt and brewing water, namely ions of calcium, magnesium, zinc, phosphate and sulfate.
- Fermentable sugars, mainly the disaccharide maltose from malt which is transported into the cell and hydrolyzed to glucose. Lesser amounts of the monosaccharides, sucrose, glucose, fructose and the trisaccharide maltotriose .
- Molecular O2 in small amounts (1ppm O2 for every degree plato) supplied by the brewer at pitching referred to as wort DO (dissolved oxygen). If oxygen is supplied, yeast can manufacture (synthesize) unsaturated fatty acids and sterols on their own. These two compounds are irreplaceable constituents of cell membranes. If they are not present, yeast can not grow because they are unable to biosynthesize cell membranes.
Causes of stress in yeast:
- Extended period of cold storage before re-pitching.
- Inadequate temperature during cold storage.
- Poor temperature control during fermentation.
- Nitrogen starvation.
- Excessive re-pitching without cleaning.
- Vitamin and mineral deficiencies.
- O2 deficiencies.
- Low oxygen related lipid levels.
- Glucose / maltose ratio out of balance.
- CO2 toxicity.
- Ethanol toxicity.
- Contaminants accumulation.
- High gravity wort.
- Osmotic pressure shock.
- Low pH.
If yeast is stressed for whatever reason, it will stop growing.The flocculation mechanism is initiated as survival method. The cell surface change, they clump together and drop to the floor in a coma awaiting more energy. Autolysis always follows “hot on the heels” of flocculation.
The yeast cell membrane:
The cell membrane consists of a double layer of lipid molecules and proteins. Only small organic nitrogen compounds (amino acids and di- or tri-peptides) can be absorbed by yeast. Several flavour active compounds can pass out into the beer.
Structures in a yeast cell
*All have lipid membranes.