Objective  Isotherm of dehydration process and kinetics of survival rate of a dried Saccharomyces cerevisiae product were studied for the development of a highly active yeast product with extended shelf life for vinification.

  Method   S. cerevisiae JH301 was used in the fluidized hot air oven dehydration experimentation. Under varied hot air temperatures, the moisture content and survival rate of S. cerevisiae were monitored to construct dehydration isotherms and analyze yeast vitality. Water migration and distribution in the drying yeasts were determined by a nuclear magnetic resonance mothed.

  Results  (1) The dehydration isotherm followed the Henderson exponential equation of M=a×EXP(b×T), where a and b were the constants related to the drying temperature (W) as a=−0.31W+81.36 and b= −0.009 4W+0.27. (2) The yeast survival rate declined gradually at first, and then, rapidly as the moisture content decreased with time in the process. A water threshold appeared at the inflection point of the yeast survival rate curve. The kinetic equations before the threshold point were y₁= a₁x+b₁, and after the point y₂= a₂x+b₂, where a₁=0.014W+0.20, b₁=−0.90W+81.64, a₂=0.36W−14.04, and b₂=−2.77W+159.40. The moisture content at the point where y₁ and y₂ intersected, or the water threshold, positively correlated with the processing hot air temperature, while the yeast survival rate negatively correlated with the rate of bounded water evaporated from the yeast cells. (3) A theoretical minimum drying temperature was determined to be 41.2 ℃. Based on the kinetic model prediction and a follow-up experimental verification, the optimal yeast dehydration was determined to be conducted at 42 ℃ for 20 min. A final moisture content of (5.24±0.12)% with a survival rate of (48.24±0.15)% on the dried yeast product was achieved.

  Conclusion  The vitality of the dried S. cerevisiae could be maximized by controlling the evaporation of bounded yeast cellular water in the hot air dehydration process.