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i know you can delete posts but i have left my other to help people because i have managed to answer it myself....
Anyone stuck on the science behind the yeast shin dig i found this site quite helpful.......
http://www.madsci.org/posts/archives/ja ... .Gb.r.html
............ knock yourselves out
I am stuck with the method.
I also dont understand why we need the sucrose solution in this experiment. i understand that the yeast breaks down the sucrose into alchol but why do we need it to find out the lowest temperature that kills all the yeast cells in a solution?
where does it come into use in the experiment? do we add it in with the yeast and methylene blue?
what temperature do you think should be the minimum temperature to test the effect of yeast?
This experiment seems to be generating a lot of confusion for some reason. Maybe the confusion is mine, I don’t know, but it seems straightforward to me—never having actually done the experiment, mind you.
First point of confusion seems to be the use of methylene blue. It’s only an indicator—as one or two have said. Its purpose is to detect the presence or absence of metabolic activity after yeast cells have been heated to some temperature for a given amount of time. As a redox indictor, methylene blue is blue in its oxidized state, and colorless when reduced. You shouldn’t need to count cells to determine if there is metabolism going. The entire solution should clearly change (or not). Dye exclusion as a test of viability won’t work using methylene blue (maybe some are thinking of trypan blue, but that’s something else); methylene blue will diffuse into cells whether they are viable or not.
Second point of confusion is what to do with the sucrose solution. Its primary function is to act as the carbon source for the yeast cells. If, after treatment, there are cells that can take up and metabolize the sucrose/glucose, then there will be a level of metabolic activity which should reduce (decolorize) methylene blue either due to the scavenging of O2 for use in oxidative phosphorylation, or by direct uncoupling of electron transport by methylene blue (I don’t know the emf potential for methylene blue, so I don’t know if that is real possibility or not). Sugar and glucose are reasonably stable compounds and should withstand brief exposure even to boiling water. Extended exposure to high temperature can carmelize sugars, so limit the incubations to no more than 10-15 min—at 100ºC that should be plenty of time to kill bakers yeast without doing anything untoward to sugars.
The biggest potential error is reading the color change, it seems to me. I would suggest always having a "positive" control (room temperature would be my choice, but you can imagine others) where you’re pretty sure that all of the yeast can survive, and a "negative" control (boiling water would be my choice—again, you can imagine others) where you’re pretty sure none of the yeast have survived. The amount of color/colorlessness of the test solutions should fall somewhere between the two control tubes. After treatment at the various temperatures, swirl in 1-3 drops of the dye solution (whatever cooling effect there might be is not likely to influence the experiment, I think), wait a set period of time (5 minutes?—I wouldn’t wait too long, either, as the dye can be re-oxidized by atmospheric oxygen re-dissolving into the solution—try vigorously shaking the positive control after you’ve finished; I bet you see a little blue color return), and interpret the color changes. As far as accuracy goes, that really depends on a) how many temperatures you’re willing to look at, and b) how accurate your heating device is.
Other things to consider might be how much of the yeast suspension to use for each test. And another way to “get around” any possible temperature effects might be to heat the yeast as 10% suspensions, but then dilute the treated samples into sugar/indicator solutions held at room temperature to make the reading. You can get even more elaborate, if you want to. Then, to those who don’t seem to know what temperatures to try, consider that most yeast do well at 27-37 ºC (like your own cells and bacteria), so you’re probably going to go higher that 40 ºC, and 100ºC is almost certainly lethal, but most likely overkill—so the lowest lethal temperature is likely to lie within the range of 40-100 ºC.
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