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Preparation of freeze dried and vacuum dried yeast starter cultures: evaluation of relevant viability detection analyses - YEAST-SPECIAL
S. Khler, M. Schmacht, S. Malchow, L. Wolff and M. Senz

Dried yeast starter cultures have gained considerable importance and popularity in food and beverage industry for the production of different goods. The viability of the preparations used is of utmost importance as this parameter largely influences the manufacturing process and the quality of the later product. However, there is a plethora of possible methods to be applied for the evaluation and making the right choice is often subjective. Therefore, the progress of viability of differently dried yeast preparations was evaluated by different techniques, i. e. colony forming units in relation to total cell concentration, flow cytometry, Oculyze system and NucleoCounter. The processes of freeze drying and vacuum drying were carried out for the production of dry yeast and the obtained preparations were stored at refrigerated and challenging conditions in order to achieve a large range of survival rates. It was found that Oculyze and NucleoCounter measurements clustered at rather high viability levels over the full investigated time, whereas flow cytometry and classical microbiology gave much lower values for the respective samples. The detailed track of the processing course showed that this gap rose with extended processing and storage. The differences can partly be explained by the diverse mechanisms to which the different methods refer. Yet, it can generally be concluded that classical microbiology and flow cytometry were better suited to show a broad range of viabilities. The study data helps users of viability detection methods to make an evidence based decision that represents a good compromise between cost-benefit considerations and the needed level of information.

Descriptors: yeast viability, freeze drying, vacuum drying, viability measurement, yeast preparation, performance test

BrewingScience, 73 (March/April 2020), pp. 41-50