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An investigation to find the lowest temperature that kills all the yeast cells in a suspension of either dried or fresh baker's yeast

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Introduction

An investigation to find the lowest temperature that kills all the yeast cells in a suspension of either dried or fresh baker's yeast. The aim of this investigation is to determine at what exact temperature all the yeast cells in a 10% suspension die with no interference from other factors which may affect the results. As shown in my appendix I am looking for when the decline part of the mixtures metabolic state is absolutely 0. (ref. appendix 1) Bakers yeast is a species of yeast formally known as Saccharomyces Cerevisiae. It is eukaryotic and its key metabolic process under normal conditions used in producing cellular energy required is aerobic respiration. When a yeast cell respires aerobically alike human tissue cells sugar (glucose/sucrose) and oxygen are used and produce along with energy (ATP) the waste products of water and carbon dioxide.(1) C6H12O6 + 6O2 6CO2 + 6H2O A main use for yeast is its ability to produce alcohol (ethanol) from sugar during anaerobic respiration. Anaerobic respiration is commonly known as fermentation and occurs only when there is no oxygen present. When Yeast cells respire anaerobically they produce energy (ATP), alcohol and carbon dioxide.(2) C6H12O6 2C2H5OH + 2CO2 Microbes such as yeast have optimal temperatures within which they work most efficiently, the reason for this is that the enzymes within the yeast cell are the direct reason that yeast has its particular qualities and the biochemical reactions they carry out are most active at the optimal temperature. ...read more.

Middle

minute, I shall therefore set a control sample which I shall heat up to 100oC which is more than overkill to the yeast cells and will definitely kill all of them (and turn the mixture blue) with this sample I shall take a colorimeter density reading and with this and I can then take samples of each temperature samples and compare them to that of the control using the colorimeter. I think that the cells will be completely killed at around 70 - 80oC the reason for this is that as I said earlier yeast cells generally work optimally around 35oC and after optimal level (exponential phase) it reaches a deceleration phase which usually is quite gradual and I predict that it would take around 30 - 40oC above the optimal temperature to reach the decline phase which is a quick drop in number of living cells and also my preliminary work (ref. appendix part 2) shows that this is the region of temperature where all of the yeast cells were killed. Apparatus 10% suspension of yeast 10% solution of glucose 1.0% methylene blue indicator 16 50cm3 beakers 2 25cm3 measuring cylinders Stop clock 6 digital thermometers Water bath Bunsen burner Colorimeter Method 1. Firstly I will preheat the 6 water baths to their corresponding temperatures i.e. 40, 50,60,70,80 and 90oC ready for the main part of the investigation. ...read more.

Conclusion

I shall also handle the boiling tubes and thermometers with care as they are made from glass and if cracked are sharp and dangerous. Results tables Temperature (oC) 100 (control) 40 50 60 70 80 90 Colorimeter density reading I shall repeat my test 3 times for each temperature, these repeats of the test from 40oC up to 90oC show weather my results are anomalies if no anomalies seem present an average can be worked out by adding the first, second and third density reading for the particular temperature together and then dividing by 3.(if it seems that there is an anomaly in my readings I shall repeat another time and use the 3 that correlate most. Presume the control density was 83, there will be 2 temperatures within this table which the first will be less than this value and the later would be the same. Lets say 70oC and 80oC are these temperatures 70oC having a density of 77 and 80oC having 83. The lowest temperature at which all the yeast cells are killed lies between these two points and I shall then produce a graph showing the temperatures between 70oC and 80oC and their densities I shall then do 3 tests at each of these temperatures starting with 75oC (this meaning less time wasted as you can tell if it is above or below the middle value) which will give me the exact temperature at which all the cells in a 10% suspension of bakers yeast are killed correct to 1oC. ...read more.

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