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Yeasts in the National Collection of Yeast Cultures showed substantial variation in biochemical and morphological properties following maintenance by conventional subculturing for prolonged periods. Variation was greatly diminished in yeasts maintained by freeze-drying. Although the initial viability of cultures immediately after freeze-drying had not always been high, only a very small subsequent decrease in viability has been detected after storage for periods of between 10 and 20 years. Detailed study of the properties of brewing yeasts following prolonged storage in the freeze-dried state showed that brewing performance remained unchanged.
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B. H. Kirsop (1974) studied this question.
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