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Name: Ross
Status: student
Grade: other
Location: Outside U.S.
Country: United Kingdom
Date: Winter 2012-2913


Question:
This is my method for growing yeast and measuring effect of ethanol concentration: 1. Stock yeast suspension was made up of 100ml sterile water and 1.5g dried yeast. The bottle was then incubated at 30c until required. 2. 12 ml of nutruent broth was added to 21 mccartney bottles along with 2 ml of yeast suspension. 3 bottles have 6ml ethanol added and 0 ml sterile water, 3 bottles have 5ml ethanol and 1 ml water, 3 bottles have 4ml ethanol and 2 ml water etc etc so I have 6 different concentrations and a control repeated 3 times. 3. 5 ml of air is left in the Mccartney bottles to allow for erobic respiration. Should i also leave caps loosely screwed? 4. Bottles are left at 30c - do you recommend this temperature for yeast growth? - and measurements are taken on a colourimeter in the morning, lunch and afternoon for 2 days. Does this also sound fine?



Replies:
Hi Ross,

Thanks for the question. I see an issue in your description below. Alcoholic fermentation occurs when there is a lack of oxygen. Thus, if the bottles are left open (and exposed to air), your yeast will not produce alcohol.

How do you plan to use the colorimeter for measurements? Remember that ethanol is colorless and will not be directly measured on the colorimeter. (Hint: There is a way to indirectly measure the alcohol produced, but you will need to do some research on the internet.) I think you will develop your scientific skills by looking up how to measure yeast concentration by a colorimeter.

I hope this helps. Please let me know if you have further questions.

Thanks Jeff


Aerobic Incubation Choose plates without spacers or products with higher filling volumes (i.e. ICR settle plates, LI-plates), if ventilated incubators are used and/or prolonged incubation times are required. The bottles you are using are OK. You'll need a decent sized incubator for 18 bottles plus controls. Sufficient access to air is important lest anaerobic respiration begins. Therefore slightly open screw caps of tubes. Constant revolving or shaking of tubes may be necessary. Incubation temperatures to stimulate growth of bacteria and yeast, culture media are mostly incubated at 36 ± 1°C. Thirty degrees will probably work, but slower growth. You may have to replace some nutrient over time. Two days will probably not be enough. Try to keep volumes constant. Will you be counting colonies? You will have 18 bottles plus controls, yes. The concentration of the EtOH at 5ml/100ml is OK.

It is not clear to me what "measuring the effects of ethanol" means. Are you looking for increased/decreased growth of yeast? How will you measure that? Are you concerned with yeast producing ethanol? Are you looking to see consumption of ethanol? If so, how will you measure the concentration of ethanol formed? It is not easy unless you have access to gas chromatography. I do not know if the "breathalyzers" are good enough

From: Kinetics of ethanol inhibition in alcohol fermentation J. H. T. Luong Article first published online: 18 FEB 2004 DOI: 10.1002/bit.260270311 ================== The inhibitory effect of ethanol on yeast growth and fermentation has been studied for the strain Saccharomyces cerevisiae ATCC No. 4126 under anaerobic batch conditions. The results obtained reveal that there is no striking difference between the response of growth and ethanol fermentation. Two kinetic models are also proposed to describe the kinetic pattern of ethanol inhibition on the specific rates of growth and ethanol fermentation:

The maximum allowable ethanol concentration above which cells do not grow was predicted to be 112 g/L. The ethanol-producing capability of the cells was completely inhibited at 115 g/L ethanol. The proposed models appear to accurately represent the experimental data obtained in this study and the literature data. ====================== Stephen R. Dunn Ass't Professor of Medicine (ret.) Kimmel Cancer Center & Division of Nephrology Thomas Jefferson University



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