Study of Chemoreaction Drying with Calcium Oxide and Its Impact on the Stress and Damage of Saccharomyces cerevisiae Culture

Abstract

The objective of the research was to study the parameters of the drying process and their effects on the viability of dried Saccharomyces cerevisiae culture. These parameters include the pattern of water isotherm sorption, the critical water contents, stress and mortality on a variety of bound water conditions, as well as morphological and micro-structural changes of damaged and leaked stressed cells. Layer thickness of the sample and high ratio used of calcium oxide speed up the drying process. Using a calcium oxide to sample ratio of 10:1 (w/w) for drying, 1.3 mm thickness of sample with 78-80% water content required 24 hours to dry to a final 4.42 % water content. The viability and cell content of the final product were 72% and 109 cells per g dried sample respectively. A sigmoid curve sorption isotherm pattern consist of 3 bound water regions of the dried yeast culture was similar to that of common dried foods. In the primary area most of the dried cells were dormant, in the secondary area the majority of the cells were under stress condition, while in the tertiary area a lot of them died. Likewise, the viability curve up to aw 0.2 was very low as due to high number of dormant cells, in between aw 0.2 - 0.6 sharply increased due to decreasing number of dormant cells, in between aw 0.6-0.75 sharply decreased due to increasing number of dying cells, and above 0.75 the cells was dying off to totally died. As shown by SEM observation, normal cells were oval-shaped with a smooth surface. In the primary region the dried cells showed and oval to round shape and small number of deformed cells, in the secondary region wrinkled surface and some number of deformed cells, while in the tertiary region the cells were sticky with wrinkled surface and most cells were damage. Meanwhile, TEM observation showed that the cells in the primary region had a well-defined, thick and solid cell wall with smooth surface with visible nuclei. In the secondary region the cells showed a wrinkled surface with less visible nuclei, while the wall was thin and cracked in some places. In the tertiary region the most number of cells were damaged, the remaining defined cells were flat, empty, irregular shape and no nuclei were visible. The increasing cell damaged was mainly due to the leaking out of cell or cytoplasm and the nuclear content through the damage cell wall. This was shown by increased electrical conductivity as well as the nucleic acid, nucleotide, protein and amino acids content of the cell-free supernatant from the culture. The Ca++ and K+ content of the cell mass decreased due to the leakage out of cell content. Sharp increase of K+ in secondary region of bound water was due to cell wall leakage, while high level of K+ in the tertiary region of bound water was due to high lysis of the cells with increased cell content damage.