Isolation And Performance Evaluation Of Saccharomyces Cerevisiae From On Palm Wine Elaels Guinneensis At Different Temperature Of Proofing During Bread Making

INTRODUCTION
1.1 PALM WINE
Palm wine is a milky alcoholic beverage produced from the inflorescence of palm tree it is the most widely used and cherished natural traditional alcoholic beverage especial in the southern part of Nigeria, and is the juice of the oil paolm ( Elacis guinneensis) and raffia palm ( Rapia hooker) ( Ihekoronye and Ngoddy 1985).
A milky juice containing initially well over 13% sucrose is collected in the calabash which is hung at the base of the incision of the inflorescence of palm tree soon after leaving the tree, yeast spares, especially those of Saccharomyces cerevisias infect the juice and soon start to ferment the fermentable sugar.
Palm wine can be consumed as an alcoholic beverage. It could be allowed to ferment and subsequently distilled into gin it could also be used for the leavening of dough for bread making ( Somiari and Udoh 1993).
The use of palm wine and as a leavening agent for dough is attributed to the presence of a yeast strain contained in the palm sap. This yeast strain is saccharomyces cerevisiaem or baking yeast is called in the bakery industry.
Palm wine when fresh, tastes like ginger bear and can be used as like yeast ( Irvine 1961). Initially, the sap is sweet, dirty brown in colour. The fermentation process results in the sap becoming milky white in appearance. This is due to the presence of large number of fermenting bacteria and yeast.
Fermentation occurs between 36 – 38 hr period during which PH of sap falls from 7.0 –7.2 to < 4. (Jay 1986).

1.2 COMPOSITION OF PALM WINE
Palm wine has the average alcoholic content of 2.00 percent to 4. 69 percent. The sugary syrup, which is dirty brown in colour contains about 10 – 12 % sugar mainly sucrose.
Studies made by faparusi et al1986 found the following genera of bacteria to be the most predominant in finished produced Lactobacillus, Micrococcus, Leuconostoc, Streptococcus and Acetobacter. The predominate yeast found are Saccharomyces and Candida spp with the former being the more common.

1.3 YEAST (saccharomyces cerevisiae)
Yeast is a unicellular micro-organism and fungus type. It makes possible many of the products made by bakers. This is because various types of bread and certain other bakery precuts are leavened (raised) by yeast. Many of them produce ethanol and carbon dioxide as waste products of their metabolism. They are therefore useful in the food industry for fermentation and aeration.
Yeast usually used in the temperate region for baking are carefully selected strains of Saccharomyces cerevisiae. This is a simple chlorophyll – free plant which feeds on sugar in the batter or dough to produce carbon dioxide (Kotshever 1980). By enzyme action, it converts fermentable sugars and some of the starch present in the dough into carbon-dioxide gas and alcohol and provides desirable controlled fermentation (Sultan 1982).
The discovery of the use of yeast to leaven bread centuries ago lead to the growth of bakery industry. Bakers used brewers yeast till about 80 –1000 years ago due to its performance in the bakery which was low and variable. The isolation of a special yeast strain which possessed the desired characteristic needed, brought about revolutionary changes in the bakery industry. This strain is known as saccharomyces cerevisiae or otherwise “ bakers” yeast in the form of cakes of compressed fresh yeast cell, with moisture content of about 70%.

NUTRITION AND GENERATION
Since the yeast cell is a living organism, it has numerous nutritional needs and it is only if these are met that it will grow vigorously and produce a large quantity of carbon dioxide. Food and moisture are needed for this growth.
Saccharomyces cerevisiae differs from other yeast species. It has more aerobic growth habit, maximum yielding capability stability during storage. It is usually derived from special selection of fast growing (short generation time) naturally occurring yeast strains ( Oyawoye and Bassey 1997) .
The plant grow best at the temkperature between 80 – 900 cooler temperatures retards their growth and a temperature as high as 1110 F kills the plant within an hour 1400 F destroy them within 5 minutes.

IMPORTANCE OF BAKERS YEAST
This principle involved in dough leavening is based on the utilization of the carbohydrate by the ‘bakers’ yeas to give out carbon dioxide entrapped in the dough. As the yeast multiply in the dough, fermenting at room temperature. More and more carbon-dioxide is produced and the dough expanse because of the pressure of gas. This yeast activity is destroyed during baking at the temperature of 1400 F.
Bakers yeast is useful chiefly in three different ways.
1. TESTURE FORMATION: They produce carbon dioxides gas, which leavens or raises the dough, giving the bread the desired loose porous texture.
2. INCREASE OF DOUGH VOLUME: The use of bakers yest as a leavening agent help to achieve great great increase in volume and make a wide variety of baked products. This means that it is no longer a necessity to rely on egg white foam to encompass enough air
1 FLAVOUR PRODUCTION: Bakers yeast is reported to contribute to the flavour of bread and other yeast leavened products. During dough fermentation, many secondary metabolites such as ketones, higher alcohols, organic acids, aldehydes, and esters are produced by the yeast. Some of these alcohols escape during baking. Others react with one another and with other compounds found in the dough to form a new and more complex flavour compounds. The resultant flavour diffuses into the crumb of baked bread ( Graw – hill 1971).

1.4 BREAD
Bread is food produced by mixing flour with water and yeast and baking in an oven ( Hornby 1995). In some cases, other ingredients such as eggs, butter, milk and sugar are added basically to augment its nutritional value. Like all other foods produced from cereals, bread is eaten primarily as a cheap source of energy. It contains valuable amount of protein, iron and vitamins flour is the basic ingredient in the production of bakery goods. This is because of its gluten. The unique properties of wheat protein, glademin and can prdice bread dough of the strength and elasticity required to produce low density of bread of desired texture and flavour (Ihekoronye and Ngoddy 1985). The elasticity of gluten retains the gas and supports the structure of the loaf.
The dough is made by mixing together the flour, water, yeast, salt and other additions ingredients. Dough raising is the act of producing carbon dioxide in the dough. This is a results of yeast cell saccharomyces cerevisiae incorporated into the dough, and this helps in the puffing up of the dough during the process of bread making.
Bread is baked at the temperature of 2500 – 2600 for a period of 30 –50 minutes. During baking, the mixtures (dough) are expanded by air, steam or carbon dioxide. The proteins present (gluten) coagulates and the starch takes on water and sets (gelatinizes). The coagulated proteins and gelatinized starches give baked product their structure and colour ( Kotschevar and Lunderg 1970).
Bread production is of vital importance as its is a product which is not liable to seasonal fluctuation and can be made from varieties of flour.
In this work, palm wine was selected as a medium for the isolation of saccharomyces cerevisiae for the production of bakers yeast. This is due to ability of the yeast cells present in the palm sap to produce alcohol ( ethanol) and carbon dioxide form the fermentable sugar in the palm juice.
Past work showed that other yeast strains are present in which wine apart from saccharomyces cerevisiae which have the desired characteristics of the bakers yeast. These other strains are generally referred to as “ wild yeast” ( Frazier and Hoff 1988).
The presence of this Wild yeast “ as well as mold and bacterial flora limits the direct use of palm wine for bread production. Those loaves produced with 90% palm wine dreg developed sour – taste and pronounced palm wine flavour and were unacceptable after 3days thereby limiting leavening , for this reason, Saccharomyces cerevisiae, which occur naturally in palm wine and have the desirable characteristics to the bakers yeast is isolated, propagated and used as pure culture for dough fermentation.
Somiari, and Udoh (1993) worked on the isolation of yeast from palm wine using malt extract agar ( MEA DIFCO) for the leavening of the dough.

1.4.1 AIMS AND OBJECCTIVES
1. To isolate the yeast saccharomyces cerevisiae from palm wine for the production of bakers yeast whose performance can compete favourably with commercial bakers yeast in bread production.
2. To embark on bread production with locally sourced yeast rather than commercial bakers’ yeast.
3. To determine the optimum proofing temperature for the isolated yeast during bread production