The calibration curve for spectrophotometer optical density readings of fermented grape juice samples can be constructed with solutions of water and yeast or should be made with grape juice and yeast? Which standard to use?
Note: I have seen some methodology that made the calibration curve with only water and yeast and used during the readings of the fermented grape juice samples used as standard the grape juice before the beginning of the fermentations. However, I think that the standard before the beginning of fermentation can not be considered as standard for the other samples since the content of solids (sugars) of the juice will decrease with the fermentation time.
By performing 1:100 dilution with 1:10 and 1:10 serial dilutions, you can construct the growth curve in ddH20. The initial OD at t=0 could be used your reference point. Depending on absorbance at different times, you can find how much sugar could be used during fermentation. The amount of cell pellets should be measured after well-dried at 140oC for 2 -day, and you can correlate the amount of sugar used with the number of cells.
You can use Formazin as your absolute standard. Thus your results will be constant FTU or 'formazin turbidity units'. It is commonly used in the 'environmental arena'. However, there is no correlation between anything (yeast in a must) growing and FTU.
We used to take our samples, centrifuge, wash with saline buffer, resuspend and read absorbance in saline at 540nm. Gave us good results but was a bit time consuming.
We made our calibration using hydrated yeast (in dH2O) @ 500nm and when measuring samples we performed at least one centrifugation/wash/re-suspension cycle. Two were often necessary depending on extent at which the matrix/substrate interferes with the absorbance. Good luck!
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