Yeast Maintenance and the Control of Infection

by Tony Parkes

Introduction

As the means by which wort is converted into beer, yeast is clearly of fundamental importance to brewing. To those commencing brewing the first aspect of good yeast management is the selection of a good yeast.
One requires a yeast which:
1) Produces beer of desirable flavour.
2) Gives good attenuation.
3) Is sufficiently flocculent to enable it to be easily separated from the beer at the end of the fermentation
Having chosen your yeast you need to look after it properly – it is a valuable resource! In order to brew good beer it is essential that your yeast is healthy and free from infection. Yeast is usually “recycled” from brew to brew, the crop from one brew forming the inoculum for the next.

Yeast Health

The factors which affect yeast health are:
A) Its’ nutrition.
B) The growth temperature.
C) The time of harvesting (skimming).
D) Storage conditions.

A) Yeast Nutrition

A good quality wort will contain all (but one) of the essential nutrients for yeast growth; sugars, nitrogen source, vitamins etc. The missing nutrient and in many ways the most important one is Oxygen. One cannot over-emphasise the importance of oxygen. When one looks at the growth curve for yeast during a typical fermentation one sees that there is an initial lag phase, whilst the yeast adjusts to its new environment, followed by a period of rapid growth. This is important to produce the mass of cells necessary to rapidly convert wort sugars to alcohol. This growth is affected by the availability of nutrients of which oxygen is crucial. Oxygen is essential to produce certain fatty substances used in making the yeast cell membrane.

Lack of oxygen will have a number of consequences:
1) Poor growth.
2) Poor attenuation.
3) Poor flocculation, leading to cloudy beer and a poor yeast crop.
4) The yeast crop will not be “healthy” and will die more readily on storage.

B) Growth Temperature

This will also affect the rate of growth of the yeast. If the temperature is too high, yeast growth will be too vigorous, producing an excessive demand on nutrients and resulting in beer depleted in these nutrients. This can have an effect on subsequent conditioning. In addition to this, and probably more importantly a higher growth temperature will change the yeasts metabolism producing a different range of by products which can have a major effect on flavour. If the temperature is too low, the fermentation will be sluggish, resulting in an increased opportunity for infection.

C) Time of Harvesting (Skimming)

The aim when skimming is to remove the yeast head at a point in the fermentation where there is still sufficient activity to keep the head on the surface and to leave just sufficient yeast in suspension to complete the fermentation and form a thin but stable cover over the surface of the beer for protection. If the yeast is allowed to remain on the surface of the beer until the end of fermentation a number of things may occur.
1) It is possible that autolysis may occur due to overheating from ambient temperatures. This will result in a decrease in yeast cell viability, which could cause, slow, problematic subsequent fermentations.
2) In addition if the yeast head is allowed to remain too long on the beer it may result in yeast off flavours, due to the products of autolysis.
3) The longer the yeast head is kept at ambient temperatures, exposed to air, the greater the possibility of aerial infection.

D) Storage of Yeast

The way in which yeast is stored after skimming is of the upmost importance if it is to remain in top condition. Yeast will rapidly deteriorate if kept at too high a temperature. It is therefore important to decrease the temperature of the yeast, which will be skimmed at about 21°C to 2° – 4°C as soon as possible. The simplest way to store yeast is to mix it with chilled sterile water and store it in 1 – 2 gallon containers with lids in a refrigerator. The lids should not be tightly sealed so as to allow gaseous exchange. The depth of yeast should not be too great otherwise a temperature difference between the surface and the centre of the yeast mass can build up resulting in temperatures of 8° – 10°C higher at the centre. Under these conditions the yeast at the centre will be more active but in the starvation conditions prevailing will be forced to undergo autofermentation i.e. break down their own constituents. As a result they will autolyse and die causing a gross reduction in the overall viability. Another possible consequence of storing yeast in poor conditions is that they will become altered with respect to their flocculent character. This may cause problems in subsequent fermentations. One other factor which can affect yeast health is the level of infection. This is because the yeast must compete for essential nutrients with contaminating organisms.

Monitoring the Condition of Your Yeast

If you take account of all the factors I have mentioned and follow the best practice, you should have no problems. However, even in the best establishments things can go wrong. It is important to have some system of quality control in place, bearing in mind that such things as a decrease in your yeast cell viability or a low level of infection may not be immediately evident. If action is not taken to remedy these problems at an early stage one could have bigger headaches later, as such effects become exaggerated in subsequent brews. The best means of monitoring the health of your yeast is to check its microscopic appearance. By using the dye “Methylene Blue” one can distinguish between viable cells (i.e. cells capable of growing) and non-viable (dead) cells in your yeast population. The cell membrane of viable cells is impermeable to this dye so cells appear colourless. Dead cells in contrast have damaged membranes and therefore take up the stain and appear blue. Too high a percentage of dead cells will lead to sluggish fermentations, as you will in effect be pitching less yeast than you think. This of course will increase the opportunity for infection to take hold. 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 number of cells in suspension in your beer at rack. Microscopic examination of your yeast and beer can also enable you to detect any substantial contamination which may not otherwise be evident. (Note however, that if contaminating organisms are easily visible under the microscope, then they are present in appreciable numbers. For more sensitive methods of detection of contaminants it is necessary to set up laboratory cultures.) 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.

Remedial Actions:

If the yeast cell viability is low, you should replace your yeast stock and review the causes, as indicated in the previous sections. Similarly, if the yeast becomes infected then it can be replaced. Alternatively, if the infection is bacterial, it may be possible to eliminate it by acid washing. This involves treating the yeast with acid for a period immediately prior to pitching. The principal of this procedure is that the low pH produced (pH 2·0 – 2·2) is sufficient to kill the majority of contaminating bacteria, whilst the yeast (which is more tolerant of low pH) is left unscathed. In order to carry out this procedure it is essential that the correct conditions are obtained or the yeast may also be damaged.

Correct Conditions for Acid Washing Yeast
pH 2·0 – 2·2
Temperature less than 5°C
Contact time approx. 1 hour.

The yeast slurry should be stirred to ensure an even temperature and distribution of the acid.
Many acids are suitable for this procedure but we recommend the use of food grade ortho-phosphoric acid. Yeast should be pitched immediately after acid washing otherwise it will deteriorate. If an infected yeast has a low viability it is preferable to replace it rather than attempt to acid wash it, as the yeast cells will be under stress and therefore more likely to be adversely affected. It is important to time acid washing correctly, to start an hour or so prior to pitching. The acid washed yeast is then pitched directly into the brew – do not attempt to store acid washed yeast.

Maintenance of Yeast Stock

At first sight, it seems reasonable that one should maintain ones yeast stock indefinitely, as each brew yields more than enough yeast to pitch subsequent brews. Some breweries have in fact managed to sustain this system for many years. It is however a risky policy for a number of reasons. You should always have a source of new yeast of consistent quality available in case the need should arise to replace your yeast stock.

Possible causes of loss of yeast stock

1) Infection.
An infection with wild yeast cannot be eradicated other than by replacing your yeast stock (possibly several times). Also, if your yeast is unhealthy, it may not be wise to acid wash to remove a bacterial infection.

2) Mutation.
Mutation is always occurring in a population of yeast cells. Most of the mutants do not survive but over time there is an increasing possibility that one will arise which may have undesirable properties. The most common adverse mutations involve a change in flocculent character or the loss of the ability to ferment maltotriose.

Sources of New Yeast

1) A nearby brewery.
Are you confident of the quality of yeast derived from this source?. You could be inheriting problems. Is continuity of supply assured?.

2) A laboratory propagated culture.
Samples of your yeast can be stored under laboratory conditions at very low temperatures at which no changes in yeast character are possible. On request these can be re-cultured to provide a starter culture of assured quality.

3) Use of Dried Yeast.
Products such as Nottingham Ale Yeast are finding increasing favour in the world of Brewing. There are a number of advantages of using fresh innocula of dried yeast:

a. Guaranteed consistency of product
b. Freedom from infection
c. No problems related to yeast storage during periods of low brewing activity
d. Excellent flavour, attenuation and flocculation characteristics
e. Guaranteed source of supply

Infection

As previously indicated, infection can have an effect on yeast health because pitching yeast must compete with the infecting organisms for essential nutrients. More importantly however infection will have a major effect on the quality of your beers.

The principal effects of contamination are:
1) Flavour/odour effects.
Sour taste due to the production of acetic or lactic acid and/or unusual aromas, often vegetable, sulphury, phenolic or stale.
2) Hazes.
3) Poor attenuation or over attenuation.

There are two major groups of microorganisms which can cause infections:
1) Wild Yeast
2) Bacteria

Wild Yeast

Gilliland (1967) defined “Wild Yeast” as “any yeast which is not deliberately used and under full control”. Standard brewing yeast, Saccharomyces cerevisiae is only one of 400 different species of the genus Saccharomyces. There are also many hundreds of different strains of the species Saccharomyces cerevisiae each with different brewing characteristics. A “wild yeast” could therefore be a different strain of Saccharomyces cerevisiae but is more commonly a different species of Saccharomyces or one of several other genera of yeasts e.g. Candida, Brettanonmyces, Hansenula, Kloeckera, and Pichia.
All yeasts have certain properties in common which make them potentially serious contaminants of brewing yeast and/or beer; They are relatively tolerant of low pH and alcohol concentration.

Bacteria

Bacteria can grow phenomenally rapidly in wort and beer if allowed the opportunity and any fresh wort must be pitched as soon as possible with active yeast to prevent bacterial growth causing deterioration. As well as flavour effects concern has been raised over the ability of bacteria to convert nitrites in beer into carcinogenic nitrosamines. This should be considered in situations where an infection is not obvious because no other effects (i.e. on flavour or clarity) are evident. Bacteria form a very diverse group. When viewed under the microscope they appear very small in comparison with brewing yeast. They have two basic shapes; round (cocci) and rod shaped (bacilli). They can further be divided into two broad groups using a staining procedure known as “Grams Stain”. Bacteria will usually be either Gram positive or negative depending on how they react to this stain. This property is related to the structure of their cell walls. Many bacteria are sensitive to the adverse conditions prevailing in wort and beer, namely low pH, alcohol content, hop extracts and low oxygen concentration. However some groups are able to thrive under these conditions and are therefore particularly dangerous. Listed below are some of the main groups involved in brewing infections.

Obesumbacterium proteus (Hafnia protea)

Otherwise known as common brewers bacteria. They thrive in wort at pH 5·0 – 5·5 but are inhibited in beer at pH 3·8 – 4·2. They are not likely to grow in beer unless its pH is unusually high i.e. above 4·5. They cannot however be ignored as beer spoilage organisms because, if they are present at sufficiently high levels in the pitching yeast, they will impart off flavours in the beer described as fruity, parsnip-like. When the wort pH becomes inhibitory during fermentation, these bacteria tend to associate themselves with pitching yeast cells, as they flocculate. If the fermentation gets off to a slow start due to decreased yeast cell viability or a lack of oxygen, then the bacteria have a prolonged period in which to multiply before the pH becomes inhibitory. It is therefore important to get a vigorous start to fermentation to prevent this happening.

Lactic acid bacteria

These are potentially the most dangerous spoilage organisms due to their ability to grow in low oxygen conditions and their tolerance of low pH, high alcohol and hop antiseptics. They can either be rod shaped (Lactobacilli) or cocci (Pediococci).
Acetic acid bacteria
These are Gram negative short rods. As the name implies, they produce vinegar (acetic acid) from ethanol. Some members of the group will produce more extensive gassing of an infected beer, often leading to strong head retention. They are obligate aerobes (i.e. they require oxygen for growth) so prevention of access of air to the beer at all stages of processing after fermentation offers the readiest means of control.

Zymomonas

This is another Gram negative short rod. It is also a potent spoilage organism since it thrives in low oxygen conditions where the pH is low and the alcohol concentration relatively high. Fortunately it is not often isolated in breweries.

Sources of Contamination

Contamination can arise from a number of sources.
1) The starting materials i.e. the wort, the yeast and any other additions.
2) The brewing plant i.e. Vessels, pipework etc.
3) The brewing environment.

1) The Starting Materials

When looking at contamination one tends to think primarily of its effect on beer quality and the subsequent losses which might result. However, as each brew is often the source of yeast for the next one, we should consider the possibility of carry-over of contamination from one brew to the next in the pitching yeast. It is important that your yeast management should ensure that the pitching yeast be substantially free from contamination. Wort leaving the copper, having been boiled, should be virtually sterile. However, if the wort cooling system is not sufficiently clean, there is scope for the wort to pick up contamination before it enters the fermenting vessel. Any other additions made at this stage e.g. yeast foods should also be free from contamination. Remember that any water added will contain bacteria and wild yeast unless it is first sterilised.

2) Brewing Plant

All surfaces which come into contact with wort, beer or yeast must be thoroughly cleaned and sterilized. This includes vessels, pipework and implements. Soiled surfaces can form a focus for microbial growth which is later transferred to beer.

3) The Brewing Environment

Microorganisms are present in the air, particularly associated with dust particles or small droplets of moisture. They can also be carried by insects (e.g. fruit flies) and other pests. Every effort should therefore be made to keep the brewing environment as clean as possible and to minimize the ingress of dust and soil from outside. Wherever possible all vessels should be covered to reduce the risk of aerial contamination.