934 Journal of Food Protection, Vol. 57, No.8, Pages 934-936 Copyrighl@, International Association 01 Milk, Food and Environmental Sanitarians A Research Nole Death Kinetics of Lactobacillus bulgaricus in a Spray Drying Process P. C. TEIXEIRA, M. H. CASTRO, and R. M. KIRBY* Escola Superior de Biotecnologia Rua, Dr. Antonio Bernardino de Almeida, 4200 Porto, Portugal (MS# 94-293: Received 14 December 1994/Accepted 6 March 1995) ABSTRACT Survival of Lactobacillus bulgaricus during spray drying was studied at various outlet air temperatures. During spray drying the logarithmic survival ratio decreased with increased outlet air tem- perature with first-order kinetics; the pseudo-z value for Lactobacil- lus bulgaricus was 17.3°C. Plots of the death-rate constant for Lactobacillus bulgaricus versus reciprocal outlet temperature during spray drying in skim milk show a curve with two different activation energies (EJ The calculatedE values were 33.47 kJ/mol above 70°C and 85.77 kJ/mol . below 70°C. Thermodynamic quantities for spray drying of Lactobacillus bulgaricus are also presented. Results show that the relationship between the entropy of activation and the enthalpy of activation for both spray drying and heating in liquid medium is linear, with all the data for drying falling in the range of a negative entropy. Key words: Spray drying, Lactobacillus bulgaricus, kinetics, activationenergy The development of concentrated cultures for inoculat- ing bulk starters or the production vat directly has eliminated many of the problems customarily involved in preparing and maintaining starter cultures in the dairy plant. Since the culture concentrates can be evaluated and standardized for activity before shipment to the processor, it should be possible to produce consistently high-quality dairy products (8). Dried preparations have the advantages of long-term preservation and convenience in handling, storage, marketing, and con- sumption. Tamine (17) reported that freeze drying is the most commonly used technique for preparation of commercial dried starter cultures. Freeze drying, however, is not an ideal technique. Cul- tures often have extended lag phases and the technique is more expensive than spray drying ( 16). The delay to onset of growth is largely due not to a decrease in total viability but to reversible cellular damage. Injured cells must repair damage before growth commences; this means that injured cells will take longer to start their desirable activities in food fermenta- tions (2). Teixeira et al. (19) reported that no significant differences in survival and acid production were found when L bulgaricus cells were dried by spray drying or freeze drying. The extent of survival or destruction of bacteria during spray drying may depend upon the temperature-time combi- nations used and upon the heat resistance of the organism. Many studies have been devoted to the influence of the air temperature on the survival of microorganisms during spray drying (6, 10, 12, 15). Lactic acid bacteria, which are fre- quently used as starter cultures in the food industry, have however received little attention (1, 9). No agreement was found in the literature about the effects of operating conditions on survival of different microorgan- isms during spray drying. It is possible that survival might be dependent on the microbial strain or even on the type of spray dryer used. Our aim was to study the destruction of Lactobacillus bulgaricus during spray drying in skim milk and to try to relate death with thermodynamic parameters. Generally L bulgaricus is not used alone as starter culture. It was however selected for this study due to its exceptional sensitivity to drying processes (17). MATERIALS AND METHODS Organism and growth conditions Lactobacillus delbrueckii subsp. bulgaricus NCFB 1489 was used. Cultures were maintained as in Teixeira et al. (18). De Man, Rogosa, Sharpe (MRS) broth (LAB M, Bury, UK) was inoculated from MRS agar (LAB M) slopes and incubated for 24 h at 42°C. This broth was then used to inoculate a second MRS broth (1% voVvol). The cultures were incubated at 42°C for 24 h in a shaken water bath. Cells were harvested by centrifugation at 16,266x g for 10min at 4°C and washed with sterile phosphate buffer (0.01 moVl of ~HP04 [Merck, Frankfurt, Germany] and 0.01 moVl of KHl04 [Merck], both dissolved in a solution of 0.15 moVlof NaCI [Merck], adjusted to pH 7.0 ± 0.1 and sterilized). Spray drying Skim milk was inoculated with L. bulgaricus cultures. This suspension was incubated for 30 min at 37°C to allow for cell adaptation, constantly agitated, and then spray dried in a laboratory- scale spray drier (Niro Atomizer, Gladsaxevej, Denmark). Moisture in spray droplets produced by the atomization of the feed liquid into JOURNAL OF FOOD PROTECTION, VOL. 58, AUGUST 1995 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/58/8/934/1665851/0362-028x-58_8_934.pdf by guest on 16 January 2023