By Moritz Kallmeyer
Chief Brewer Drayman’s Microbrewery, 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
- 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
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.
Yeast cropping technique:
Assuming conical fermentation vessels, it is GBP (good brewing practice) to crop yeast as soon as it has flocculated and settled in the cone. Even if cone chilling is an option, yeast should not be left in the cone for more than a few hours because yeast acts as an insulator and start to warm and autolyse. GBP requires early warm cropping rather than late chilled cropping. The heat produced inside the mass of metabolizing yeast will cause the temperature to rise so much that the yeast quality deteriorates and the flavour of the beer in contact with the yeast will be spoiled.
Cropping is very much a trial and error process which requires a lot of attention, fine tuning and brewer’s skill. Changes in yeast consistency and colour need to be observed. Too low (thin slurry) is indicative that cropping is taking place too early or cropping technique is bad – pulling beer holes. Too high (too thick) is not really bad but might indicate that cropping is taking place too late which makes physical removal of the yeast difficult and more importantly, it compromises health of yeast.
The portion of yeast selected for repitching purposes should be collected at an early or active stage of fermentation (at or near the end of primary fermentation) together with the bulk of the yeast to prevent autolyzed yeast flavours affecting the beer. The little that remains either in suspension or in the cone will not cause heat increase, or beer flavour damage.
Yeast collected for re-pitching is therefore collected while it is still in the active stage of fermentation – or at least shortly thereafter. Yeast collected at this stage, if used without much delay, can still ferment maltose and maltotriose when pitched into fresh wort without first having to form the appropriate permeases enzymes. Their cell membranes are relatively robust and the internal food reserves, such as glycogen are plentiful. The yeast can rapidly absorb the O2 provided and is not easily damaged by transfer to wort which is at a very different temperature or with a much higher osmotic pressure. Such transfers can cause shock to a less healthy cell and result in leakage or even lysis.
What happens to yeast while it is autolysing?
The yeast start releasing cell contents such as amino acids, riboflavin,
organic and inorganic phosphorus compounds such as nucleotides into the
- As the yeast further deteriorates in the absence of nutrients,
it releases proteolytic enzymes which degrade beer-foam proteins and
also increase protein and carbohydrate hazes.
- There is a leakage of
large lipids out of the cell membrane, spoiling flavour attributes.
products leakage will result in an increase in the pH of the beer. This
will affect perceived flavour of other compounds such as bitter isohumulones