4. Discussion
Generally, once yeast adaptation is complete, the population of
viable yeast cells rapidly increases until conditions become unfavorable.
According to Gutierrez et al., (2012), successful adaptation
leads to the growth or exponential phase in which the culture increases
from 106 cfu/ml (inoculated population) to approximately
108 cfu/ml. However, in this study, the number of viable yeast cells
in blueberry musts began to decrease gradually in all samples, and
the population reached 103~104 cfu/ml at the end of fermentation
(38 days). Namely, the log phase was not detected, and thestationary and decline phase were atypically long during the
fermentation of blueberry musts. It is worth nothing that from the
12 days of fermentation, the addition of yeast nutrients maintained
the long stationary phase following the decline phase, thus indicating
that nitrogen shortages affected the ability of the yeast to
multiply during fermentation.
In a previous study, we observed that the sugar consumption
rates of blueberry must decreased in proportion to the amount of
water added prior to fermentation (Seo et al., 2014). These results
suggested that a nutrient deficiency may be the cause of the sluggish
fermentation of blueberry wine rather than toxic substances
inhibiting the growth of yeast. In particular, nitrogen is the most
important growth-limiting substrate during wine fermentation
(Varela, Pizarro, & Agosin, 2004), and its deficiency represents one
of the main causes of stuck or sluggish fermentations (Bisson,
1999). Agenbach (1977) reported that the amount of total yeast
assimilable nitrogen (YAN) sufficient for full completion of
fermentation in grape wine was shown to be at a level of 140 mg/l.
4. DiscussionGenerally, once yeast adaptation is complete, the population ofviable yeast cells rapidly increases until conditions become unfavorable.According to Gutierrez et al., (2012), successful adaptationleads to the growth or exponential phase in which the culture increasesfrom 106 cfu/ml (inoculated population) to approximately108 cfu/ml. However, in this study, the number of viable yeast cellsin blueberry musts began to decrease gradually in all samples, andthe population reached 103~104 cfu/ml at the end of fermentation(38 days). Namely, the log phase was not detected, and thestationary and decline phase were atypically long during thefermentation of blueberry musts. It is worth nothing that from the12 days of fermentation, the addition of yeast nutrients maintainedthe long stationary phase following the decline phase, thus indicatingthat nitrogen shortages affected the ability of the yeast tomultiply during fermentation.In a previous study, we observed that the sugar consumptionrates of blueberry must decreased in proportion to the amount ofwater added prior to fermentation (Seo et al., 2014). These resultssuggested that a nutrient deficiency may be the cause of the sluggishfermentation of blueberry wine rather than toxic substancesinhibiting the growth of yeast. In particular, nitrogen is the mostimportant growth-limiting substrate during wine fermentation(Varela, Pizarro, & Agosin, 2004), and its deficiency represents oneof the main causes of stuck or sluggish fermentations (Bisson,1999). Agenbach (1977) reported that the amount of total yeastassimilable nitrogen (YAN) sufficient for full completion offermentation in grape wine was shown to be at a level of 140 mg/l.
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