Establishment Of Evaluation Method For Yeast Vitality And Its Application For Culturing High Vitality Yeast

During yeast cultivation procedure, cell vitality is often measured to gather the information of physiological situation of the yeast cells. Several methods have been developed over the past decades. However, these methods, such as CO2 evolution, intracellular pH measurement, intracellular magnesium ion release and acidification power measurement, were seldom used as spiked program for yeast vitality due to their low sensitivity and little correlation with fermentation performance of the yeast. We aim to develop a new method to evaluate yeast vitality with high sensitivity could reflect the changes in the fermentation and have well correlation with fermentation performance of the yeast cells. The main research contents are showed blow.First, by comparison with AP test, a new method was developed based on the Methylene Blue dye Reduction Test (MBRT) to evaluate yeast vitality according the rate of decoloration of methylene blue. The results showed that T^1/2 value could accurately and effectively reflect yeast vitality. The lower the value of T^1/2, the higher the yeast vitality is. T^1/2 values of less than 100 s means high active yeast with good physiological state while T^1/2 values of 100 to 250 s indicate a reduced metabolic activity; T^1/2 values above 250 s suggest low metabolic competence of the yeast that could result in sluggish fermentations.After that, the effects of metallic ion and nitrogen sources on Saccharomyces cerevisiae vitality in synthetic medium were studied using this method in this paper. The experimental results showed that: the minimum level of peptone and yeast extract required for yeast is 4⁸ g/L and 2⁴ g/L, respectively; Urea is the better nitrogen source comparing to ammonium sulfate, and reasonable concentration is 2 g/L; The effect of K+ is significant, and its optimum concentration is 15 mmol/L; Mg²⁺ is very important for yeast growth, but a higher concentration beyond 2 mmol/L will impair the growth of the yeast; Low concentration of Na+ and Ca²⁺ slightly promote the yeast growth and their optimal concentration are 2.5 mmol/L and 0.5 mmol/L, respectively.Meanwhile, considerable starch material dregs were not fully utilized and discarded every year in domestic and abroad result in severe environment pollution. In this paper, the quality of yeast seeds gown on starch material dregs was monitored by MBRT. The results showed yeast counts in the exponential mid-term growth phase could reach 1×108 /mL, and the T^1/2 value was about 150 s; yeast counts in the exponential mid-term growth phase could reach 1.5×108 /mL by adding certain nutrients into mash of cassava flour, and the value of T^1/2 was about 100 s; comparatively, yeast cells grew poorly on lycoris radiata dregs. The reason is likely that noxious materials present in lycoris radiata dregs could inhibit yeast growth. The above results further verified T^1/2 value could be used to evaluate yeast growth state.After a preliminary study of the growth situation of yeast seeds on certain starch material, Cassava flour was used to cultivate yeast seeds with higher vitality by adding needed nitrogen source. The results showed yeast counts could reach 4×108 /mL and T^1/2 value was about 100 s when 10 g/L import peptone and 10 g/L import yeast extract were used as nitrogen source. Yeast counts could reach above 3×108 /mL and T^1/2 value of logarithmic period was also about 100 s by adding 2 g/L Urea and 6 g/L ammonium sulfate as nitrogen source. The results indicated that cheap nitrogen source could be used to substitute for the expensive imported products in large-scale yeast cultivation.