The Effect of Dryer Inlet and Outlet Air Temperatures and Protectant Solids on the Survival of Lactococcus lactis during Spray Drying

Abstract

The influence of spray-drying conditions, inlet air temperature (130°C to 200°C), outlet air temperature (38°C to 65°C), drying medium (air and nitrogen) and milk-derived protectants (10%, 15%, and 25% lactose; 5% and 10% sodium caseinate; 10%, 25%, and 35% lactose:sodium caseinate (Lac:NaCas, 3:1)) on the survival of Lactococcus lactis ssp. cremoris was studied using a laboratory-scale spray dryer. An inlet air temperature of 130°C and 65°C as the outlet air temperature maintained high survival of the bacteria without sacrificing low moisture content. Inlet air temperature, previously considered to have no significant effect, was shown to play an important role in the survival of bacteria during spray drying. A mixture of Lac:NaCas (3:1) showed a better protective effect on the survival of bacteria than lactose and sodium caseinate individually, and this effect increased with increasing amount of protectant. The results were generalized by substituting whey protein isolate for sodium caseinate. Finally, the positive effect of elimination of oxygen was demonstrated both by replacing air with nitrogen and adding ascorbic acid as an oxygen scavenger to improve survival of the bacteria. Adding an oxygen scavenger would be a better candidate for industrial application considering the potential high cost of manufacturing if nitrogen was used as the atomization and/or drying medium.

Keywords:

• Drying/atomization medium
• Inlet air temperature
• Lactococcus
• Outlet air temperature
• Oxygen injury
• Spray drying
• Survival

ACKNOWLEDGMENTS

The authors wish to acknowledge Dairy Innovation Australia for supporting this project through its competitive grant (Project 08210C). We are especially grateful to Dr. Martin Palmer and Dr. Malcolm Broome for fruitful discussions during project meetings. The authors also express their appreciation to Bruce Armstrong (University of Ballarat), Dr. Huihua (Amy) Liu and Dr. Mithila Jayasundera (University of Ballarat) for their help in some of the experiments. The first author acknowledges the scholarship provided by IDP Education Australia to undertake this study.