Fermentation is life without oxygen
Louis Pasteur


The role of yeast in fermentation

Yeasts are micro-organisms which have been associated with man since prehistoric times. The ability of certain yeasts to quickly and efficiently convert sugars into alcohol and carbon dioxide has given mankind many foods, including beer, wine and leavened breads. The use of ferments in bread making and brewing has been depicted in wall carvings of the ancient Babylonians and in the hieroglyphic writings of ancient Egypt dating back over 2,000 years. The yeast which todays bakers use to produce leavened breads is considered by some as being one of the oldest plants cultivated by man.

The early use of yeasts for bread making was dependent upon wild yeast cells from the surrounding environment falling into a batch of dough. Such fermentation was highly variable due to the unknown quantity, type of organisms present and the conditions to which the dough was subjected. Eventually it was seen that when a piece of this dough was saved for the next batch, the subsequent dough was more consistent and fermented fasted. Hence, the birth of the starter dough. To leaven bread became an "art" and remained as such for thousands of years. Even today some sours are handled as in generations past.

In 1859, Louis Pasteur established the fact that fermentation was the metabolic by-product of yeast feeding on sugar. This discovery changed the past assumption that fermentation was only a spontaneous or inorganic reaction like iron oxidized into rust. Pasteur's work demonstrated that the sugar in grape juice was converted into two primary compounds: 1) carbon dioxide and 2) ethyl alcohol. He found that by heating the wine he killed the organisms; thereby halting fermentation. Thus the pasteurisation process began.

Around 1868, Charles Fleischmann brought to the United States two test tubes of pure yeast culture from Europe. This yeast species, Saccharomyces cerevisiae, has endured over the years with modifications for improved bakery performance. New strains are being looked at today for their specific qualities to impart improved characteristics to the dough and the finished product.

Yeasts are generally unicellular organisms which are non-mobile and have no chlorophyll. They are looked at today as neither animal nor plant, but fall under the new Kingdom of Fungi. The yeast can be either fresh or dry. Fresh yeast can be either compressed or liquid (the so called cream yeast). Also dry yeast can be obtained in different forms: active dry yeast or as instant dry yeast. Fresh yeast must be kept under refrigeration during storage. Dry yeast can be kept at room temperature.

The yeasts produced for bakers mainly come from one species called Saccharomyces cerevisiae. Most strains selected from this yeast have special qualities desired by the baking industry. Some of these qualities are its fast rate of gas production and its ability to utilize maltose sugars. Not all yeasts have these qualities, as will be seen later. These yeast cells measure approximately four by seven microns in size. One gram of compressed yeast contains about 25 billion cells.

The following table summarises the main characteristics of both types of yeast.

 
compressed yeast
cream yeast
storage temperature
2C - 7C
2C - 4C
storage life
3 to 4 weeks
10 to 14 days
% water
67 - 72 %
80 - 84 %
conversion factor
-
1,5 - 1,8

With the conversion factor I mean that if you use 1 kg of compressed yeast, you will need 1,5 to 1,8 kg of cream yeast.

The compressed yeast-cake form comes prescaled in blocks of 500 g or of 1 kg, individually wrapped in wax paper or plastic. This yeast has approximately 70 percent moisture content and is highly perishable outside of refrigerated storage conditions. Expected shelf life is from three to four weeks with proper handling. Compressed yeast can be added directly at the mixer with the other ingredients or possibly dispersed in water ahead of time to allow for quicker dispersion. It is not recommended that fresh yeast be allowed any contact time with dry ingredients, other than flour, (e.g., such as prescaling dough ingredients ahead of time and placing yeast on sugar, salt, milk, etc.). This may cause slack, sticky doughs with longer proof times.

The compressed yeast in crumbled form is packaged in 20 kg multi-walled poly-lined bags. This yeast is similar to cake yeast in all respects except for the particle size, which is smaller. It is primarily used in the larger bakeries because of its added convenience due to less packaging material and lower handling costs. Only one hour of yeast requirements should be brought onto the shop floor at a time before mixing. Any unused portions should be handled by evacuating the air from the bag and closing it tightly. This is done to prevent the fresh yeast from being unnecessarily exposed to higher temperature, moisture and oxygen which will cause autolysis. The temperature of the yeast will gradually rise even if kept in the fridge. It is best, once the bag of yeast is opened, to use it completely until it is empty.

Autolysis is the process by which the yeast destroys itself through its own enzymes breaking down the cellular structure of the yeast. This, in turn, allows the release of glutathione from the yeast cells. Glutathione acts as a reducing agent on gluten proteins and weakens the dough structure. The baker sees this as dough that mixes faster than normal; a dough that is stickier and slacker, requiring more dusting flour for proper machining. It may also cause longer proofing times and greater pan flow along with weaker sidewalls.

Cream (liquid) yeast is manufactured like compressed yeast but with the last two processing steps of dewatering and packaging eliminated. The dewatering step means the difference between a pumpable liquid form of yeast and a cake or crumpled form.

As baking plants have become larger and more automated, the desire to handle yeast in bulk has increased; some bakeries have already been using yeast slurry systems to improve the quality of yeast handling through better temperature control and reduced labour costs by automatic batching of ingredients to the mixer.

The advantages of cream yeast include:

  1. More consistent product; activity can be standardized for one bulk tank.
  2. Easier handling; it is a pumpable ingredient.
  3. Better sanitation; there is no packaging or spillage.
  4. Savings in raw material costs; no product remains in bags.

The main disadvantage is the need for specialized handling equipment at the bakery. Generally, this consists of two jacketed stainless steel storage tanks maintaining storage temperatures in the range of 1C to 4C. The reason for two tanks is flexibility and ease of use. While one tank is being used, the other can be cleaned, sanitized and made available for another delivery. The storage life of this yeast is reported to be from 10 to 14 days. During production, the cream yeast circulates between the holding tanks and the mixer in well insulated pipes. This prevents the yeast from settling out and provides for faster scaling and very consistent temperature control. Slow sweep agitators (which equal 20 revolutions per minute) in the holding tanks keep the yeast suspended. The specific gravity of cream yeast is 1.04 to 1.06. About 1.5 litres of cream yeast is equal to one kilogram of compressed yeast. The solids content of cream yeast generally ranges from 18 to 20 percent. It must also be remembered that ingredient water must be adjusted to compensate for the increased water from the cream yeast as compared to compressed yeast.

Active dry yeast, first produced in the 1940's, needs no refrigeration and today has from 2 to 12 months storage life, depending on packaging. Active dry yeast must be rehydrated with water 40 to 43C for about 10 to 15 minutes before use. Rehydrating at temperatures above 43C will damage the yeast and longer proof times will result. Rehydrating at too low a temperature will result in leaching glutathione from the yeast cells, resulting in quicker mix times, weakened gluten structure, and again, slower gas production. When converting from compressed yeast to active dry yeast, a conversion factor of 0,45 to 0,5 is generally used. One kg of compressed yeast equals 0,45 to 0,5 kg of active dry yeast.

 
active dry yeast
instant dry yeast
storage temperature
room temperature
room temperature
storage life
2 - 12 months
1 year and more
% water
6 - 8 %
4 - 6 %
conversion factor
0,4 - 0,5
0,33 - 0,4

The shelf life of the active dry yeast depends on the packaging. The product can be packed under protective atmosphere or vacuum packed.

Instant dry yeast came on the market in the early 1970's. This type of yeast is prepared from special yeast strains that have the ability to retain a high degree of activity through the special drying process. Like active dry yeast, no refrigeration is required and storage life is one year or more due to packaging in inert gases or under vacuum. Once the package is opened, it is recommended that it be used within three day's time. The short shelf life is due to the porous nature of the yeast particles which allow available oxygen and moisture to easily penetrate.

Unlike active dry yeast, instant dry yeast does not have to be rehydrated. It can be added directly with the other dry ingredients and blended, or delayed addition to the dough until no loose water is visible. The yeast can be shocked if water that is too cold is allowed to make contact. If a liquid ferment system is being used, it will then become necessary to rehydrate the yeast. The instant yeast supplier should be contacted to find out the best water temperature for rehydration to obtain 100 percent activity. Differences do exist between instant yeast products. Slower proofing times have been associated with the instant dry yeasts, but improvements have been seen in the last few years. When converting from compressed yeast to instant dry yeast a conversion factor of 0,33 to 0,4 is generally used; one kg of compressed yeast equals 0,33 to 0,4 kg of instant dry yeast.

The basic function of yeast in bakery products is leavening. The production of carbon dioxide (CO2) causes the dough to expand and rise. The yeast has the capability of taking sugar and breaking it down through its natural metabolism. The process of this metabolism we refer to is fermentation. It was thought that yeast multiplies in number during normal bread fermentation, but this is not the case because few cells are produced in a dough's anaerobic environment. In order to be able to multiply yeast needs oxygen and a dough is basically a non-aerobic system. Sugars are then fermented by yeast into CO2, ethanol and other organic materials.

C6H12O6–> 2CO2 + 2C2H5OH + 113 kJ

From 1 gram of sugar the yeast will produce 0,464 g of carbondioxide (CO2), 0,486 g of alcohol (C2H5OH) and 0,05 g of aromatic compounds. The reaction also generates heat.

Wheat flour contains monosaccharides (glucose, fructose, and galactose), disaccharides (sucrose and maltose), trisaccharides (glucofructose and raffinose), and oligosaccharides (glucofructans).

The ability of bakers yeast to ferment dough is related to the amount of LMW sugars in the flour. The low molecular weight (LMW) sugars of greatest importance are sucrose and maltose followed by glucose, fructose, and glucofructans. Because of the potent invertase of yeast, sucrose is converted almost immediately to glucose and fructose. Yeast ferments glucose at a slightly faster rate than it does fructose. If maltose is the only sugar, the rate of gas production drops appreciably after indigenous fructose and glucofructans are exhausted until yeast enzymes adapt to maltose. The fermentation of yeast in bread dough leads to the expansion of the bubbles, which were occluded to the dough during mixing. This creates the open crumb structure of baked products. Therefore, the measurement of yeast activity and content of LMW sugars can be an important factor in the manufacture of consistent products.

The pathway of the alcohol fermentation by yeast can be represented as follows:

Through the production of these fermentation products, we look at three major changes taking place in doughs. The first is the production of CO2 which causes a dough to expand, making it lighter, more airy and giving a product with improved palatability It should be stated that it is wheat gluten that allows gas to be retained in dough for such expansion.

The second change is the development of fermentation flavours in the product. The organic materials produced, such as glycerol, acids, aldehydes, alcohols, fusel oils and other flavour precursors, all contribute to the development of bread flavour. These are developed in the oven by the high temperatures and found mainly in the crust. As fermentation is extended or made more vigorous, the fermentation flavours will become stronger.

The third contribution of yeast fermentation to doughs is that of "maturing" or "developing" the dough. This process is complex and not fully understood. Some of the factors which influence the dough are the lowering the pH of the dough, the effect of ethanol on the gluten protein and the stretching of the dough during expansion. Each of these has an influence on dough handling and development.

Generally, there are four factors which control the rate of yeast activity.

The food supply for yeast must be sufficient to fully leaven the baked product. Sugars such as dextrose and fructose are easily fermentable by yeast. If sucrose is used it is split into its component parts of dextrose and fructose by an enzyme called invertase. This is quickly accomplished by the yeast. Maltose is another food source for yeast but it also must be broken down into its component parts by the maltase. Baker's yeast has the capability to produce maltase but does so only when the fructose and dextrose have been utilised.

Flour supplies about one percent naturally occurring sugars which the yeast can use directly. But this is generally not enough to produce a fully leavened product. More food for the yeast can be supplied by two methods:

  1. We can add sugar directly to the formulation.
  2. We can supply sufficient amylase enzymes which will break down flour starches into maltose sugars

Flour contains some amylases but usually not at a high enough level. Yeast obtains maximum activity if 4 to 6 % of sugar is used, based on the weight of the flour. Higher sugar levels increase the osmotic pressure on the yeast, decreasing its metabolism. For bread containing more then 6 % of sugar, more yeast is generally added to the dough in order to speed up proofing times

trial n
1
2
3
4
5
6
% of sugar
0,0 %
1,0 %
2,0 %
4,0 %
8,0 %
16,0 %
fermentation rate
100
104
108
94
74
41

A dough which is made stiff (low water content) will take longer to ferment. Likewise, a dough which is made slack (higher ingredient water content) will ferment much faster. This is due to the concentration of soluble solids in the free water of the dough. For example, as the ingredient water increases, causing the dough to become softer, the soluble solids are diluted causing a decrease in osmotic pressure on the yeast and thereby increasing its activity.

trial n
1
2
3
4
5
% of water
40 %
44 %
48 %
52 %
56 %
fermentation rate
77
86
94
100
103

Dough temperature is the basic means by which the baker controls fermentation in the bakery. It has been estimated that a 1C rise in dough temperature will accelerate yeast activity by about 10 % i.e. 10 % more gas is produced in the same amount of time. Correspondingly a decrease will cause a similar slowdown of yeast activity. As a dough ferments heat is generated, raising the dough temperature and accelerating the yeast activity This temperature increase can be monitored and indicates the progress of fermentation, At the end of a standard fermentation time higher temperatures than expected may indicate an excess of fermentation has been accomplished.

The optimum temperature range for yeast activity is between 32 to 40C. Temperatures higher or lower than this range will slow yeast metabolism. The general practice in bakeries, depending on the dough system used, is to ferment doughs between 23 to 30C so as to maintain proper dough handling through production. One must also bear in mind that the quantity and the type of aromatic substances will vary with the temperature.

The initial pH of a dough should range between 4.0 to 6.0. However, if the pH is higher or lower than this range a slower yeast activity is seen. As a dough is fermenting the pH will begin to drop because of acid production. A freshly made dough will have a pH slightly above 6, but the pH will drop during fermentation (due to the formation of lactic and acetic acid).

Salt is one ingredient that should be mentioned because of the strong retarding effect it has on yeast fermentation. As salt in a formulation increases by as little as 0,2 percent, the yeast activity will slow. Normal salt ranges for most breads are 1,75 to 2,25 percent, based on flour weight.

trial n
1
2
3
4
5
6
% of salt
0,0 %
1,8 %
2,0 %
2,2 %
3,0 %
4,0 %
fermentation rate
125
104
100
94
66
34

A dough with no salt will rise 1,25 times faster then a dough with 2 % salt. The crust of the bread will also be pale. This can be explained by the fact that the yeast will have consumed "all" the available sugars so there are less left to take take part in the Maillard reaction during baking.

Obviously also the fermentation time will influence the volume of the bread. The table below refers to a bread of 880 g dough baked in a baking tin.

trial n
1
2
3
4
5
fermentation time
0'
30'
60'
90'
120'
volume of dough
1,1 dm3
2,0 dm3
2,9 dm3
3,8 dm3
4,7 dm3
volume of bread
2,2 dm3
3,1 dm3
4,0 dm3
4,6 dm3
4,9 dm3
oven spring
1,1 dm3
1,1 dm3
1,1 dm3
0,8 dm3
0,2 dm3

Mold inhibitors also show retarding effects on yeast activity and are generally noted with slightly longer proofing times. Increases in yeast level will compensate for this effect.

We now have some of the basics of yeast. What it is, how it is used and why it is used. But man continually strives for something better. There are certain products and production techniques which put differing demands on yeast today. Some of these demands can be seen as subjecting yeast to high sugar levels or freezing temperatures for prolonged frozen dough storage.

Doughs which contain sugars in the 10 to 13 percent range pose a problem for baker's yeast. High sugar creates high osmotic pressure on the yeast cell and decreases its rate of activity. Generally higher yeast levels (five to 10 percent) are used to produce acceptable proof times for these products. But if a new yeast was found to gas faster at these higher osmotic pressures, yeast usage could be decreased and product cost reduced. There have been some notable attempts crossing Saccharomyces cerevisiae and Saccharomyces rouxii species to produce a hybrid yeast, one of which was shown to produce CO2 more rapidly in high sugar doughs. None of these yeasts, though, are commercially available.

Yeast manufacturers can modify their production process to improve any yeast strain's tolerance to osmotic pressure. This is done by creating an environment of higher osmotic pressure while yeast is being grown in manufacturing. The resultant yeast will have improved osmotic tolerance and greater gassing capability in higher sugar doughs. There are some instant dry yeasts available which claim improved gassing for sweet dough- production.

Lean or doughs with little or no sugar pose a slightly different problem for yeast. The yeast must acquire all of its food from the flour. This is done by first utilising the naturally occurring sugars in the flour. Next, the yeast produces maltase enzymes to split the maltose sugars which have been formed by the amylases breaking down the damaged starches of the flour.

This whole system relies on:

  1. The level of amylases present in the flour (they can be supplemented by the baker or the mill).
  2. The available level of damaged starches on which the amylases work.
  3. The temperature at which the fermentation is being carried out (higher temperatures increase rate of amylase activity).
  4. The ability of the yeast to produce maltase enzymes.

Baker's yeast has this ability but there are other strains with better gassing power for non-sugared doughs. These yeasts are widely available.

For frozen (bread) dough one needs a special yeast depending on the shelf life needed. The problem with baker's yeast is about a 50 percent loss of activity due to freezing and prolonged storage time. Most yeast-leavened doughs can be frozen and stored from 12 to 18 weeks with proper formulation adjustments and dough handling techniques. When a dough has past this storage period, exceedingly long proofing will occur and a porous open grain will result. This is believed due to yeast damage and leaching of glutathione, weakening the dough structure.

Yeasts can be used mainly for their flavour contribution to the finished product. Two such yeasts have been mentioned in a patent to be used in conjunction with baker's yeast, Candida lusitaniae and Saccharomyces delbrueckii. These two yeasts have claimed to increase the bread-like aroma and taste over that of baker's yeast. Also, these yeasts are somewhat slower gassing and do not convert sucrose as effectively as does Saccharomyces cerevisiae.

In the early 1970's, a yeast strain, Saccharomyces exiquus, was discovered to coexist with a lactobacillus in sours used to produce San Francisco sour dough bread. It was found that this yeast was the main leavening organism. This yeast does not have the ability to utilise maltose and so it is not competitive with the bacterium strain.

Brewer's yeast, as well as baker's and other yeast strains, can be dried after being suspended in water to produce inactive dried yeast. The resulting yeast flakes are then milled into a powder. This powdered dead yeast is used as a carrier and stabilizer of flavours for such foods as meats, crackers and snacks. Dried brewer's yeast contains approximately 48 percent protein and nine percent ash, thus becoming useful as a nutritional supplement. These dried inactive yeasts, autolysates and extracts:

Dried inactive yeasts have also been used as reducing agents for shortening mixing time and improving machinability of pizza doughs. Enhancing the crust browning, improving flavour retention and, in some cases, like Torula yeast, acting as a food binder have also been cited.

Autolysates and extracts of yeast are produced by autolysing the yeast cells. This process, as we have discussed, is the self-destruction of the yeast by its own enzymes with the aide of salts or solvents, but under controlled time and temperature conditions. The yeast proteins are hydrolyzed into their component amino acids and smaller peptide chains. This, in turn, releases the other soluble components of the cells. Particular flavours, such as cheesy, yeasty, meaty or savoury, have been created by this technique. Autolysates generally consist of the whole yeast cell that has been broken down, whereas extracts are characterized by the solubilised components that are separated from the cell walls by filtration. These products have been used with chemically leavened products when a yeast flavour is more desirable.



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