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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.

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Introduction

Brewing Yeast Yeasts are unicellular micro-organisms which can obtain the energy they need in the presence of oxygen (aerobic) by respiration and in the absence of oxygen (anaerobic) by fermentation. 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. Selected strains of this yeast are systematically isolated and grown as pure culture brewers' yeasts. Because yeast does not only produce alcohol during fermentation but a range of other flavour materials, its metabolism has a great influence on the taste and character of the beer. Knowledge of the structures and composition of yeasts, their metabolism and their growth is important in producing high quality beer. Yeast cells contain about 75% water. The composition of yeast dry matter consists predominantly of proteins and carbohydrates: * Proteins 45 - 60% * Carbohydrates 25 - 35% * Fat 4 - 7% * Inorganic 6 - 9% In addition yeast has a high vitamin and enzyme content, the main vitamins in yeast are B1 and B6. 10 grams pressed yeast has a surface area of 10 m2 contact surface, this explains the tremendous activity of yeast. To grow and multiply yeast is adapted to use organic substances, particularly carbohydrates in the form of sugars. Yeast is able to utilise these sugars both in the presence of oxygen and when oxygen is excluded. ...read more.

Middle

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. ...read more.

Conclusion

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. ...read more.

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Response to the question

A good example of an essay containing lots of accurate detail, in this case yeast used in the beer brewing process, showing a great deal of research that expands out of the A level syllabus e.g. specifics of beer fermentation. ...

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Response to the question

A good example of an essay containing lots of accurate detail, in this case yeast used in the beer brewing process, showing a great deal of research that expands out of the A level syllabus e.g. specifics of beer fermentation. Despite this, however, it is not brilliantly structured as ideally there would need to be more linking between the various aspects mentioned in the essay and to other areas of the biology curriculum.

Level of analysis

The research is great but A level essays require less listing and more linking and analysing with regards to a particular question. Always try and include various topics, for instance you could include genetics and try and discuss how the various strains of yeast arose, incorporating your knowledge of mutations etc. Usefully the writer has included some interesting points and highlighted them in red which are ideal if you’re looking for some less obvious facts to include in an essay and show your depth of knowledge. To be better it would need to include an introduction and conclusion, giving a concise description of what the essay will contain and answering a particular question while linking back to any pertinent points made in the main body, respectively. However the candidate has successfully incorporated a graph showing the growth of yeast and accurately explaining it which is perfectly fine to do in an exam if you want to make a concept clearer.

Quality of writing

Spelling, grammar and punctuation are all accurate but more linking and less listing would improve the fluency of the piece.


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Reviewed by tomcat1993 01/04/2012

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