Yeast

By Sylvie Van Zandycke, PhD, Lallemand Brewing

History and characteristics of brewer’s yeast

The yeast Saccharomyces cerevisiae was used for thousands of years in the fermentation of alcoholic beverages before anyone realized it! The Dutch scientist, Anton Van Leeuwenhoek observed the mighty cells for the first time under the microscope in 1680.

But it wasn’t until 1857 that the French scientist, Louis Pasteur, discovered and informed the world that yeast was live and in fact responsible for converting sugar to alcohol. In the late 1800′s a Danish scientist, Emil Christian Hansen, was the first to isolate pure cultures of yeast. Afterwards, brewer’s yeasts were commonly divided into two categories, ales and lagers. Ale strains are top fermenting, ferment at higher temperature, and produce fruity beers. Lager strains are bottom fermenting, favor lower temperatures, and produce a crisper, cleaner, less fruity beer.

The terms top or bottom fermenting came about because it was observed that at a point in the fermentation process, ale yeast tends to clump and float to the surface, whereas lager yeast tends to settle to the bottom (see section on fermentation).

 
Ale
Lager
TaxonomySaccharomyces cerevisiaeSaccharomyces cerevisiae
Growth TCan grow at 99F (differentiation test) Optimum temperature range 68-104FCan grow at 99F (differentiation test) Optimum temperature range 68-104F
Melibiose utilization<No (differentiation test)Yes (differentiation test)
Fermentation Temperature55-70F40-54F
FlocculationTopBottom

Yeast Q&A

Any question you may have on fermentation, yeast and nutrients can be asked on this forum and a Lallemand representative will address it as soon as possible.

Propagation/liquid yeast/dry yeast

The brewer’s choice in pitching yeast is influenced by numerous factors: company policy, production size, equipment availability, technical expertise, type of beer to be produced, etc…the following describes common scenarios in small and large facilities. Microbreweries are often limited in terms of equipment and hygienic safeguarding required for yeast propagation. Typically they purchase ‘ready-to-pitch’ yeast from manufacturers like Lallemand or the Siebel Institute. Ready to pitch yeast is available in dry or liquid form in variable volume and strain depending on the brewer’s need. Dry yeast offers a practical alternative to liquid yeast for microbrewers in that the yeast can be stored for over a year and is ‘always ready’ to be used whenever the brewer is.

Liquid yeast, on the other hand, does not maintain viability over a long period of time and needs to be used within a few days impacting on flexibility of the brewing schedule. However, a greater selection of strains is available in liquid form. Both liquid and dry yeast can be recycled. (More information on yeast recycling below)

Large Breweries will often have a yeast propagation unit and a proprietary yeast strain which has been used in the company since its creation. This does not prevent them from using dry yeast from time to time for a seasonal brew or very high gravity brew for which the yeast cannot be re-used. Propagation encourages yeast growth rather than production of alcohol. Hygiene at this step is critical and viability/vitality must be very tightly controlled (see section on propagation).

Yeast maintenance

The quality of the yeast will have a significant impact on the outcome of the final product and as such should be treated with care and looked after. Any changes in the integrity or health of the yeast and this may impact on the characteristics of the beer. Breweries which practice yeast propagation can either keep their yeast stock on-site or order them regularly from an outside banking company. As an alternative they can also use yeast strains from manufacturers and order them in regularly. Even if using on-site stocks it is recommended to keep a backup in an outside banking company in case of problems. If kept on-site, both backup and working stocks should be produced. Two back up stocks should be kept unopened until the working stock is running out. Using the working stock, two new stocks must be produced: a new working stock and a new back up stock. That way there is always 2 back up stocks (unopened) and one working stock. These stocks must be ideally kept at -80°C either in 20% glycerol or coated on beads (recommended, see http://www.pro-lab.com/products-microbank.php). At this temperature the yeast will be protected from genetic drifts which may alter the characteristics of the yeast. It is good practice to have yeast stocks checked up for genetic drift once a year using DNA fingerprinting methods. [Read more]

Propagation

The criteria of a good propagation are: adequate cell counts, good viability (>95%), hygiene, good vitality or metabolic activity with high glycogen and sterols levels. To ensure this, the propagation should take place in sterile wort in presence of oxygen, nutrition can be added, particular attention should be paid to Zn levels. The temperature should be between 68-77°F to avoid a stressful environment, especially for lager strains. Most importantly, propagation should occur in several increments. The Siebel Institute of Technology recommends 8-fold increment but the first step is often between 25 to 100 fold. As an example, starting with some yeast colonies from a plate, 10ml of wort is inoculated:

10ml    250ml    2L    16L

It is important to transfer the yeast while it is in exponential phase, to keep the yeast active and avoid potential contamination which could take place if the yeast is resting.

Pitching rates

The pitching rate for brewer’s yeast will depend on the original gravity of the wort to be inoculated. As a rule of thumb, 1 million viable cells per ml of propagated or recycled yeast are inoculated per Plato degree. Liquid and dry yeast manufacturers may have their own recommendation according to their particular product.

Example: For a beer at 10°P or 1.040 gravity (to convert gravity in Plato degrees, divide the last 2 decimals by 4 (i.e. 40/4 = 10). Cell concentration to be inoculated: 10x1x106 cells ml-1 = 1×107 cells ml-1

Fermentation

The main purpose of the fermentation is to allow the yeast to convert the sugars from the wort into alcohol. Other yeast byproducts are also produced as a result of normal cell functions of survival and growth. These include fusel alcohols, esters and aldehydes which will determine the character of the beer. [Read more]

Yeast storage and recycling

Cropped or skimmed yeast can be discarded if it is contaminated (especially with wild yeast – see section below) or if it has been through very high gravity brew. If intended for recycling, the yeast should be re-used as soon as possible to preserve vitality and viability. If the yeast cannot be re-used immediately, storage time needs to be kept to a minimum in a closed vessel (but should allow for gaseous exchange) ideally equipped with a cooling system; temperature should be 39°F with intermittent agitation to avoid hot spots which can cause the yeast to autolyze. Depending on the strain and company policy, the yeast can be re-used 5 to 15 times. Some breweries re-used their yeast over 100 times or until fermentation performance declines. The recycled yeast can be acid washed between brew in order to reduce bacterial contamination. The recommended procedure for acid washing to avoid damaging the yeast is to use food grade phosphoric acid to achieve a pH of 2.2-2.5 and wash for 2 hours maximum at a temperature below 39°F
Acid washing instructions (pdf)

Bottle conditioning

The aim of bottle conditioning, depending on the product is to increase CO2, increase alcohol levels and bring stability to the beer by incorporating yeast into each bottle. Liquid or dry yeast can be used for that purpose. Cropped yeast is mostly used at a concentration of 1×106 cells per ml. Dry brewer’s yeast can also be used at a dosage of 100 mg per liter and has the advantages of being always in the same ideal physiological condition as opposed to cropped yeast for which conditions vary greatly. Dry yeasts used for Champagne production have also demonstrated some excellent results, producing fine bubbles in the beer bottle. Try it!

Contamination

Contamination affects the quality of the beer. Most contaminants will produce off-flavors, acids and non-desirable aromas. They can also produce hazy beers and films. They may compete with the production strain for essential nutrients; they can also induce stuck fermentation or … [Read more]

Yeast Tracking Chart

Download this pdf to track each strain in your brewery.

Monitoring of yeast condition

The best means of monitoring yeast health is to check its microscopic appearance. By using the brewery standard dyes methylene blue or methylene violet, viable and non-viable cells can be distinguished in a given yeast population. Viable cells can reduce the dye to a colorless form due to the presence of active enzymes in the cells. In contrast, dead cells remain stained. A high percentage of dead cells will lead to sluggish fermentations as less viable cells are being pitched. This will increase the opportunity for infection to take place and also increase the risk of off-flavor formation due to the yeast autolyzing and releasing byproducts into the wort. By using a special microscope slide, known as a haemocytometer, one can count the number of cells in a given suspension. This can be useful in determining the concentration of pitching yeast for primary fermentation and bottle conditioning. Microscopic examination of your yeast and beer can also enable you to detect any substantial contamination which may not otherwise be evident (see section on detection of contamination). A microscope is therefore is an invaluable tool for good quality control and should be considered by all brewers. It is also obtainable at modest cost.

Terminology and Definitions

A glossary from adjunct to wild yeast.