Effect of shear rate and oxygen stresses on the survival of Lactococcus lactis during the atomization and drying stages of spray drying: A laboratory and pilot scale study

• Published in: Journal of food engineering 2012-11, Vol.113 (2), p.194-200
• Main Authors: Ghandi, Amir, Powell, Ian B., Howes, Tony, Chen, Xiao Dong, Adhikari, Benu
• Format: Article
• Language: English
• Subjects: Ascorbic acid; Atomization; Atomizing; Bacteria; Biological and medical sciences; Death; Drying; Food engineering; Food industries; Food microbiology; Fundamental and applied biological sciences. Psychology; General aspects; Lactococcus; Lactococcus lactis; Oxygen injury; Shear rate; Spray drying; Survival
• ISSN: 0260-8774; 1873-5770
• DOI: 10.1016/j.jfoodeng.2012.06.005

► Integral effect of shear and oxygen on survival of lactic acid bacteria during spray drying was investigated. ► Much higher percentage of bacterial death was found to occur in atomization rather than in drying stage. ► Shear-induced bacterial death at atomization stage was much higher in pilot than in laboratory spray dryers. ► Use of oxygen scavenger in the feed substantially lowered the bacterial death during atomization and drying. ► The use of N2 gas during atomization substantially reduced the bacterial death during atomization and drying. The effect of shear rate and oxygen injury during atomization and the combination of these factors on the survival of Lactococcus lactis subsp. cremoris in spray drying was studied using laboratory and pilot scale spray dryers. The atomization was carried out using a two-fluid nozzle in the laboratory study and a two-fluid nozzle or rotary atomizer in the pilot scale study. The extent of oxygen-induced death was determined using ascorbic acid in the feed and atomizing the feed with gaseous nitrogen. The lowest levels of bacterial death were observed at lowest characteristic shear rate and in the presence of nitrogen and ascorbic acid. Quantitative analysis showed that lower shear rate, creating an oxygen-limiting environment during atomization and drying, and using oxygen scavengers in the feed were successful in enhancing bacterial survival in spray drying. We also report for the first time that, at least for L. lactis, the extent of death during the atomization stage far outweighs death during the drying stage, and that the majority of bacterial death (up to 93%) occurs during the atomization stage. The death of bacteria was found to be less when using a rotary atomizer or when using a two-fluid nozzle atomizer at lower flow rate. This work shows that bacterial death during spray drying can be minimized by using oxygen scavengers such as ascorbic acid and/or an anaerobic atomizing medium (such as nitrogen), and by altering the spraying conditions.