Yeast

YEAST  

DEFINITION- 

Any of the certain economically important single celled fungi, most of which are class Aascomycetes, only a few being basidiomycetes.The  Ascomycitan yeast include the common bread ,beer and wine producing varitiesof Saccharomyces Cereviseae. 

---Cookery Year              

Yeast is the single celled plant from the fungi family.  

---Perfect Bakery and Confectionery 

HISTORY 

We have been eating raised bread for 6000 years, but it was not until the investigations of Pasteur a little over a century ago that the process of leavening began to be understood. Today we know it as the gas-producing metabolism of a particular kind of Fungus, the yeast. 

 The word yeast is as old as time itself and originally meant the froth or sediment of a fermenting liquid that could be used to leaven bread. 

 The word "Ferment" is derived from the Latin word "Fervere" to boil or to seethe. 

In Egypt beer froth was eventually used, but the first leaven was simply a piece of left over dough in which yeast was already growing.The first raised doughs must have originated spontaneously, since yeast spores are ubiquitous and will readily infect a hospitable environment. 

In Rome the production of yeast was quite regularized, though beer froth was not used. It is said that bread made in Barbarian, beer drinking Spain and France ‘is lighter than that made elsewhere" because of the peculiarly bubbly leaven used. 

During the 17 century the preferred leaven in England was beer froth which could be preserved for later use by drying it slowly; when needed, it was mixed with water, flour and sugar and allowed to ferment for a day. It is said that this method made the finest bread. Sourness in bread had become a standard complaint by this time. By the beginning of the 17 century the term "artificial yeast" was applied to what was probably the original way of leavening: Flour was mixed with water and allowed to ferment spontaneously by mid century English bakers were producing yeast by fermenting malt and hops mixture themselves rather than going to the brewery .they omitted flour from the formula on the grounds that it imparted sourness to the yeast. 

Historically it is has been reported that yeast cells were first seen in a droplet of beer mounted on a crude microscope by van Leeuwenhoek in 1680. 

Then in 1857 Pasteur proved the biological and later in 1876 Pasteur demonstrated that yeast can shift it’s metabolism from a fermentative to an oxidative pathway when subjected to aeration.    

METHOD OF PRODUCTION 

Production of baker's yeast- Saccharomyces cerevisiae is produced on a large scale commencing with a single healthy cell of a vigorous growing strain. This cell is placed in a nutrient growing culture containing molasses and nutrient salts ammonium chloride and calcium sulphate,then it multiplies itself by budding into many daughter cells. 

The growing yeast is transferred to large stainless steel culture tanks of molasses and nutrient salts. It multiplies for 12 hours producing several tons of yeast. Which is separated by filtration, washed with pure water or dried under a vacuum. During this process of growth pure air is bubbled through it to provide necessary oxygen for aerobic respiration. 

Yeast can also be produced by feeding them on alkali hydrocarbons instead of molasses mainly for animal feed and T.V.P. textured vegetable protein production. 

The cell walls of bakers yeast contains 30-35 %glucan (yeast cellulose), 30%Mannan (yeast gum) which is bound to protein (7%), 1-2 % Chitin 8-% lipid material and in organic material largely phosphates. 

Gross chemical composition of baker` s yeast is opproximately70% moisture, and in the dry matter , and in the dry matter 55%, 6% ash, ,1.5 % fat and the remainder mostly polysaccharides including about 15 % glycogen and 8 % trehalose.      

CHOOSING QUALITY YEAST 

Fermentation capacity: Yeast should have a particularly good capacity for breaking down maltose and sucrose; this is especially important for the early stages of fermentation. 

Raising power: The yeast should have a uniform and excellent raising power. As little raising power as possible should be lost under normal storage conditions. 

Reproduction power: The yeast should be tested for its reproduction power, it should have a good reproductive power. This is especially important in long bread making processes. 

Taste and smell: Yeast should have a pleasant, fresh, characteristic smell and taste. The smell should be yeasty, but not sour. 

Consistency: it should be moist and homogenous, but not pasty, and should have an “oyster shell” structure on breaking. 

Solubility: when mixed with water it disperses quickly and easily.  

Forms of yeast 

Compressed Yeast: Is made up of moist living cells pressed into cake form with a small amount of starch as binder. IT is perishable and must be refrigerated. A fresh sample is creamy white in colour, is moist but not slimy crumbles easily and has a distinctive odour. When stale the yeast might become slimy and brown in colour and develop a strong  “ off” odour. 

Dry Yeast :  It is made from a strain of yeast, dried in fine granular form, packed in metal foil and sealed in atmosphere of nitrogen to exclude air. Air , moisture and warm temperature hastens it’s loss of viability. It keeps well for several months. It can also be stored at room temperature but under refrigeration it lasts for as long as six months. When the yeast is required for baking it is mixed with a little warm water and becomes active and ready for the fermentation process. Dry yeast is convenient for domestic baking . 

Dried Fake Yeast : This is an inactive form not used in leavening. It is intended as food for man either as such or in the form of yeast extract. 

Other Leavening Agents:  

Baking soda: Baking soda exploits the reaction between certain acidic and alkaline compounds, which results in evolution of carbondioxide, the same gas as yeast produces. It is also called Sodium Bicarbonate Baking soda can be the sole added leavening if the dough or batter it acidic enough to react with and evolve carbondioxide 

Yogurt \ Sour Milk : Yogurt and sour milk contain lactic acid and are often used instead of water  or ordinary milk in the baking process to make cakes, muffins e.c.t. Sour milk can also be added along with the baking powder as a “neutral” component of the leavening. Two general rules of thumb  : Half tea spoon of baking  soda is neutralized by one cup of sour milk: and baking soda to be neutralized by sour milk provides the leavening action of four it’s the of baking powder. Sweet milk can also be soured for use with baking soda by adding a tea spoon of lemon juice or vinegar, or ¼ spoon of cream of tartar, for every cup of milk.  

Baking Powder : Baking powder contains baking soda and an acid in the form of salt crystals that dissolve in water. Ground dry starch is also added to prevent premature reaction in humid air by absorbing moisture, and to dilute the powder. Most baking powders are “double reacting” : that is : they produce an initial set of gas bubbles upon mixing the powder in the batter and a second set during the baking process.  

  

.Cream Of Tartar : A fine white powder used as a stabilizer.,cream of tartar is a by-product of wine fermentation process. Crystals containing potassium bitartrate form on the inner surface of the barrels in which wines are fermented. The tartaric acid, produced when grapes ferment, is first isolated in the form of sodium salt. The crystals are then ground , purified, dehydrated, and reground to the powder known as cream of tartar.Since 1835 cream of tartar has been combined with baking soda to be used as dry yeast. 

Non Fermentation Methods of Leavening :  

  

Air :  Air may be incorporated by beating eggs ,by folding batters and rolling dough such as puff pastry.                                                                

 Gas Produced by Bacteria : Bacteria also produce leavening gas in flour fixtures, they produce hydrogen and carbondioxide gases biscuits , by creaming flour and sugar together , by sifting flour or by beating batters. 

Steam : Flour mixtures can also be leavened by steam. Foods entirely leavened by steam are porus. One volume of water increases to more than 1600 volumes when converted to vapour. Thus steam has tremendous leavening power. Egg whites bring about 2 to 3 times the expansion in baking as they contain enough water to complete the expansion.   

   

A Starter : This is some of the sponge from previous baking saved for future use (to replace yeast). It has sugar added as food for yeast and must be used frequently to keep yeast alive and prevent souring. it is a more uncertain source of yeast cells than compressed and dry yeast. It is only used for convenience and economy. 

Liquid Yeast : Can be made from potato water, sugar and yeast. It is also an uncertain source of yeast cells and is used only for economic purposes. 

Gas Formation From Chemical Action :  Due to delay in time for gas formation from fermentation ,experiments were conducted for producing gas by combining sodium bicarbonate with various acids. Some of the substances  which will combine with soda to form gases are : 

  

Sour Milk (Containing lactic Acid) 

Molasses (containing mixtures of organic acids) 

Brown Sugar 

Honey 

Corn Syrup 

Chocolate 

Out of these only sour milk and molasses are gas forming. For one  cup flour use half teaspoon soda. Molasses also varies in acidity since the pronounced flavour of soda masks an undesirable flavour resulting from excess of soda hence 1 teaspoon of soda is often required to be used with 1 cup of molasses. In some cases salt of various other acids are left as residue in the mixture leavened by chemical leavening agents. 

Phosphate Powders : phosphate powders have either mono sodium phosphate or mono calcium as the acid. The baking trade uses sodium pyrophosphate   

All types of baking powders to law must contain at least 12 % of co2 

Ammonia Gas : Ammonium bicarbonate gives off gas when heated . Ammonia gas is formed and is evident in the mixing room. If the ammonia bicarbonate is not completely decomposed then the flavour of the product is impaired. This is used in crackers and cookies. 

  

USES 

Fresh or dried yeast can be used in bread making dried yeast is more concentrated than fresh yeast: 1/2ounce of dry yeast is the equivalent of 1 ounce of fresh yeast. 

Methods of using 

Fresh yeast is added in flour in 3 ways: it is rubbed in, blended with liquid or added as a batter. Rubbing in is suitable for soft doughs, quick-breads, and sweet dough. Blending in liquid is the basic way and is suitable for all bread recipes the batter method is best suited for rich yeast dough, and work equally well with fresh and dried yeast. It is not advisable to cream fresh yeast with sugars this results in breakdown of some of the living yeast cells. 

Rubbing in method 

  Crumble in the yeast in the in the sifter flour and the salt with the fingers. Add specific amount of liquid to the flour and yeast mixture to make a soft dough. Work the dough with the finger tips to distribute the yeast evenly  

Blending with liquid 

Blend the yeast with part of the measured liquid ;add this mixture to the flour and salt , together with the remaining liquid. 

Batter method 

Mix about 1/3of the measured flour with the yeast, blend with all the liquid and sugar . Leave in a warm place until frothy, about 20 minutes then add the rest of the flour, the salt and any other ingredients specified.  

Methods Of Adding Baking Powder And Soda : Dry leavening agents are best sifted in with the flour and should not be wet until one is ready to have the gas firming reaction taking place .Brown spots in the baking powder mixture are due to excess of soda in the baking powder. 

Plasticity Of Fats: Plasticity is the capacity to be moulded or shaped . 

It is important in terms of shortening value. Fats indirectly acts as a  

 leavening agents. 

Butter and Fats: these are also used as shortening or the leavening agents. Fats and oils produce their shortening effect by coating the particle of flour and forming layers with separate parts of the dough structure and prevent them from coming together like in puff pastry with every fold that is given the fat coats the flour particles and forms a separate layer altogether giving the pastry its characteristic “melt in the mouth” quality. 

To shorten well a fat must have the following : 

• The capacity to coat or spread well 
• To be rolled and pressed into thin layers. 
• To adhere well to flour particles so as not to be pressed out of the mixture. 

Temperature of fats affects plasticity. eg at 18 degrees Celcius  butter is less plastic than at 22 degree Celcius . 

Plasticity can be increased by manipulation. 

Degree of saturation of fat as related to shortening power: 

Fats and oils contain a higher percentage of  unsaturated fats have more shortening power than those made up of saturated fats, because they cover a larger surface area per molecule of fat. 

Liquids: Help in the leavening action as they hydrate the starch and gluten. 

They dissolve constituents like baking powder and it is only when baking powder is wet that the evolution of carbon dioxide gas is saturated. 

Eggs; They help in incorporating air & enriching the product because egg protein coagulates with greater rigidity of cell walls occur in mixture containing eggs. It adds to the volume of the finished product.  

Sugar: it gives food to yeast which helps to leaven the product .it increases tenderness & has a solvent action on gluten. 

Yeast reproduction: In a dough the yeast will reproduce and breathe simultaneously .The yeast requires sufficient oxygen to breathe during reproduction. Primary dough(sponges ) which are added to a dough to improve reproduction ,should be kept cooler than the main dough (knockback once or twice ) 

Fermentation: the process develops quickly in the main dough, because the yeast is able to ferment much quicker than it can reproduce .Within a short time a lot more carbondioxide is produced . 

Alcohol is also produced during fermentation which improves flavour in bread . 

Storage Of Yeast 

Yeast is stored as follows  

• Storage temperature :1 -5 degrees centigrade  
• Fresh yeast should be stored no longer than 3 weeks. 
• Yeast being a living organism has to be able to breathe during storage ,therefore yeast packs should be stored with spaces in-between, to allow air circulation. 
• Yeast should not be stored at high temperatures as increasing the temperature would cause the yeast to breathe more and in doing so breaks down the protein content in its cells. 
• Dehydrated yeast should be stored at cool temperatures in a dark cabinet away from sunlight 

• Freezing Yeast:  
• Only fresh yeast should be stored at -20 degrees centigrade and below. 
• Freeze the pack separately and not in one box 
• Avoid temperature fluctuation. 
• Only defrost what is required for immediate use  
• Never add frozen yeast to the dough 

Hint : In order to ensure production of good quality bread, it is advisable to use half the quantity of fresh compressed yeast and one fourth dry yeast. 

Cooking  Possibilities:  

• Production of all breads and cakes batters, pastries. 
• Production of beer and wine 
• Production of cheese.