Abstract
Spray drying is a widely used method for producing milk powder. This process is not aimed to cause microbial inactivation, thus sporeforming bacteria may be abundant in the microbiota of milk powder. The first aim of this study was to determine the inactivation kinetics parameters in capillary tubes of three Bacillus cereus strains (436, B63, 540) in three menstrua (whole milk, phosphate buffer, and talc suspension) at 90, 100, and 110°C. D-values for B. cereus in the three menstrua were not significantly different at the highest tested temperature (p > 0.05). Thus, talc was chosen as a carrier agent to allow the recovery of B. cereus from spray dried materials given its low interference on inactivation kinetics. B. cereus spores were also inoculated in whole milk and skim milk following spray drying at 95, 105, and 110°C (outlet temperature). After the spray drying runs, B. cereus spores were counted and the number of decimal reductions (γ) calculated. A correlation between the small diameter of the particles with the survival of spores of three B. cereus strains was found, and B. cereus 436 presented consistently the lowest γ no matter temperature and a carrier agent. The highest γ was found when talc powder was used, which suggest that this carrier agent does not protect B. cereus spores during spray drying. Spray drying of milk can lead to up to 4 γ (strain 540) of B. cereus spores but depending on the strain less than one γ (strain 436) could be observed. This study contributes to the knowledge on the microbiology of low water activity foods by providing novel findings regarding the fate of three B. cereus strains to different spray drying conditions. Acknowledging the variability of inactivation of B. cereus during spray drying is key in the current context of food safety in which the quantification of effects of unit operations must be known for the validation of processes and development of more robust formulations.
Highlights
Milk is an important source of proteins with high biological value, vitamins, and minerals (Pereira, 2014)
The highest D110◦ C values for B. cereus were obtained in talc suspension and phosphate buffered saline (PBS) (p < 0.05)
At 110◦C, the effect of menstruum was only evident for B. cereus 436, for which the highest D-value was obtained in PBS (Table 1) (p < 0.05)
Summary
Milk is an important source of proteins with high biological value, vitamins, and minerals (Pereira, 2014). The complex microbiota present in milk may lead to its spoilage or association with foodborne diseases (Machado et al, 2017). Because of these issues, preservation strategies such as drying have been employed by dairy industry to extend milk’s shelf-life. During the spray drying process, inlet air and outlet temperatures may reach from 200 to 250◦C and about 100◦C, respectively (Anandharamakrishnan and Ishwarya, 2015). When exposed to these conditions for a short time (20–30 s), microorganisms could be injured (Bhandari et al, 2008). Some industries establish a maximum limit of 100 CFU/g of B. cereus group in dried milk intended for industrial applications, such as for formulation of infant formulas (Becker et al, 1994)
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