Brewing. During beer production the sugar in the wort is fermented by the yeast into alcohol. For this purpose yeast fungi of the species Saccharomyces cerevisiae are used.

by yeast. An important parameter for distinguishing

individual yeast species is their ability to respire or

ferment various sugars. Whether a sugar is used

aerobically or anaerobically depends principally on the

oxygen availability. In the presence of oxygen, yeasts

obtain their energy by respiration. When the oxygen is

removed metabolism changes to fermentation.

Yeast is the only living organism which can change from respiration to fermentation. The production of alcoholic

beverages, which has taken place for thousands of years, depends on this fact.

During the initial stage of brewery fermentations, the

yeast requires a lot of energy, a great deal of oxygen

must be supplied to the yeast to initiate this process

quickly. In the subsequent phases of fermentation and

maturation the processes are performed anaerobically

and oxygen would be detrimental to the flavour of the

beer.

The preferred carbohydrate sources for brewing yeast

are low molecular weight sugars. Yeast is able to use

many mono-, di- and trisaccharides. Polysaccharides,

such as starch and cellulose, are not used by yeast.

For beer production it is important which sugars are

fermented by the yeast. The fermentable

carbohydrates include:

• Monosaccharides - glucose, fructose, manose, galactose.
• Disaccharides - maltose, sucrose.
• Trisaccharides - raffinose, maltotriose (by some yeasts)

Yeast metabolism depends on an adequate supply of

inorganic substances. The following are needed to

maintain good yeast viability:

• Potassium
• Sodium
• Calcium
• Magnesium
• Copper
• Iron
• Manganese
• Zinc
• Sulphate
• Phosphate
• Nitrate

Normal wort contains sufficient amounts of the above

salts or ions.

The fermentation performance of a yeast cell

is enormous, under optimal conditions it is

able to split its own mass of glucose, that is about

200 million molecules into alcohol and carbon

dioxide in 1 second.

Yeast normally reproduces by budding. During

budding a small bubble like protuberance from the

mother cell is formed into which part of the cytoplasm

as well as a daughter nucleus, formed by division,

passes. In some yeast strains the mother and daughter

cells separate from one another completely, in other

strains the cells remain connected to one another and

form chains. When the yeast is pitched into fresh wort

in the brewery they begin to grow. This growth does

not occur at a constant rate but is divided into six

phases.

1. Lag Phase - The yeast cells metabolism becomes active, the length of this phase depends on the type of yeast, its age and the conditions within the wort. The lag phase ends with the first cell division.

2. Acceleration phase - The rate of cell division continuously increases.
   
3. Exponential phase - The growth rate is constant and at a maximum. The cell number doubles every 90 to 120 minutes.

4. Deceleration phase - Because of various factors such as the reduction in the amount of nutrient and the increase in the amount of growth inhibiting products, the lag phase occurs for only a limited time. Then the growth rate gradually decreases.
   
5. Stationary phase - There is a balance between the number of newly formed cells and the cells which die.
   
6. Declining phase - In this last phase the rate of cell death exceeds the rate of new cell formation.

In brewing yeasts are divided into two major groups -

top and bottom fermenting yeasts. There are a number of

differences between the way these yeasts ferment.

Bottom Fermenting Brewer’s Yeast

Top and bottom yeasts can be differentiated under the

microscope by means of their budding behaviour.

Bottom fermenting yeasts occur mainly as single cells

or pairs of cells, whereas top fermenting yeasts form

chains of budding cells. In the case of top fermenting

yeasts the mother and daughter cells remain attached

to one another for a longer time and as a result

branched chains of cells are formed. In the case of

bottom fermenting yeasts the mother and daughter

cells separate from one another when the division is

complete.

The most important physiological difference between

top and bottom fermenting yeasts relate to the

fermentation of the trisaccharide raffinose. Bottom

fermenting yeasts can ferment raffinose completely

whereas top fermenting yeasts can ferment only a third

of the trisacchride.

Top Fermenting Brewer’s Yeast

Another characteristic of top fermenting yeasts relates

to their different flocculation behaviours. Top

fermenting yeasts are therefore divided into powdery

and flocculent yeasts. In the case of powdery yeasts

the cells remain very finely divided in the fermentation

medium and sink slowly to the bottom only at the end

of fermentation. The cells of flocculent yeast, clump

together after a short time to form large flocs and then

settle rapidly. Bottom fermenting yeasts do not form

flocs.

The flocculation ability of yeast is of great practical

importance. Flocculent yeasts produce a clear but less

fully fermented beer, whereas powdery yeasts and

bottom fermenting yeasts produce a turbid beer with a

high degree of attenuation.

Top and bottom fermenting yeasts differ with regard to

fermentation temperature. Fermentation's with

bottom fermenting yeasts are performed between 4 oC

and 15 oC. In the case of top fermenting yeasts 16 oC to

25 oC is used.

---

Yeast Propagation

There are three stages in pure yeast propagation:

1. Isolation of suitable yeast cells.

2. Multiplication of the yeast in the laboratory till sufficient vigorously fermenting yeast is obtained.

3. Yeast multiplication in the brewery till a sufficient amount is obtained to pitch a complete brew.

The yeast should be isolated from the active phase of

fermentation. Drops containing single cells can be

isolated under the microscope and several grown in

wort at the normal fermentation temperature. The

most vigorously fermenting yeast colony is absorbed

onto sterile filter paper and added to 5ml of sterile

wort. If the yeast cells are not going to be used

immediately the yeast cells are grown on a solid

medium, usually wort agar. The sealed sample is kept

in a fridge at 0 oC to 5 oC.

The increase in the number of yeast cells is achieved by

transferring the vigorously fermenting contents of one

vessel into another vessel containing 10 times as much

sterile wort. Sterile air or oxygen is bubbled into the

wort to maintain the yeast in growth phase rather than

fermentation phase. Volumes of wort up to 25 litres

can be grown this way, which is sufficient to pitch a

1000 litre brew.

The main points of importance with yeast propagation

are:

• The operation must be performed under sterile conditions right through to pitching the yeast in the brewhouse wort.
  
• Intensive sterile aeration or oxygenation of the yeast is necessary for rapid yeast growth.
  
• Brewing wort should be used for yeast propagation as the hop bittering compounds exert an inhibiting effect on the growth of bacteria.

For propagation of a pure culture yeast, cells which

have performed well in practice are used.

---

1. Sterile air filter - allows excess pressure to escape.
   
2. Sample tap - allows sterile air or oxygen to be bubbled through the wort.
   
3. Inoculation connection - for adding sterile yeast to wort.

Note - The cut away section in the main body of the

vessel is to allow a view of the inside and does not

exist in the real vessel.