SURVIVAL OF ALGINATE-ENTRAPPED CELLS OF AZOSPIRILLUM-LIPOFERUM DURINGDEHYDRATION AND STORAGE IN RELATION TO WATER PROPERTIES

Survival of alginate-entrapped cells of Azospirillum lipoferum was stu died during dehydration using a dry air stream and during prolonged st orage at various constant water activity values (a(w)). During the dry ing operation, the viability loss remained almost constant from the in itial water content to 0.35 g water/g dry weight (DW) corresponding to a 98.5% water removal, strongly increased until a water content of 0. 25 g/g DW and then stopped until the end of the drying operational (fi nal a(w) 0.18). A water content of 0.25 g/g DW (a(w)=0.55) corresponde d to the critical point of the moisture sorption isotherm curve from w hich water became restricted to the dry material. A high drying rate ( 5 g/g DW per hour) was shown to be more detrimental for cell viability than a low drying rate (1.18 g/g DW per hour). When the product was s tored in a closed chamber with a regulated a(w) (0.23), the number of living cells decreased during a short period (less than 15 days) corre sponding to the product a(w) stabilization, and then remained constant for more than 150 days. In addition, cell survival during storage was not affected by a(w) values in the range 0-0.55. Above a(w)=0.55, the higher the a(w) and the storage duration, the lower the residual surv ival percentage. The influence of the drying and storage conditions on the cell death rate are discussed with regard to both the mechanisms generally involved in viability loss and the hydration properties of w ater.