Probiotics in dry state can maintain viability for several months to several years and retrieve metabolic activity upon rehydration, making them a promising ingredient as dietary supplements.  However, when dehydrating the cells, dehydration is often considered as a stressful factor causing cell inactivation, particularly in spray drying where dehydration simultaneously occurs with heating.  To date, little is known about how probiotic cells respond to the multiple stresses during the rapid droplet drying.  In this study, the influence of the interplaying stresses on probiotic cells was dissected for the first time with the use of the Single Droplet Drying (SDD) technique.  The results showed that dehydration could alleviate the detrimental effect of thermal stress to a certain extent, evidenced by that probiotic cells withstood higher temperature in the SDD process compared to sole heat treatment.  Changes in the viability of a model strain, <i>Lactobacillus rhamnosus</i> GG (LGG), along with the increase of the protectant temperature were experimentally monitored during the two processes at 90 °C.  The inactivation of LGG cells was initially observed at 61-65 °C in the SDD, approximately 10 °C higher than in the heat treatment.  The cytoplasmic membrane of LGG was also more stable at elevated temperatures of 60-65 °C during drying, which might be related to the high viscosity of semi-dried particles mitigating the leakage of intracellular substances.  Analyzing the changes of the kinetics of the protectant showed that the favorable temperature history in the SDD might favor cell survival.  At the middle stage of drying, the low water activity of semi-dried particles, as estimated with the Reaction Engineering Approach (REA) model, may be another reason for the enhanced heat stability.  The effect of the protectants, trehalose and lactose, was different between the two processes, indicating that the interaction between probiotic cells and protectant matrix is crucial for maximizing cell survival in dried particles.

Enhanced thermotolerance of probiotic cells during dynamic droplet drying compared to heat treatment

255th National Meeting (2018)

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