Viability Retention of Probiotics during Spray Drying: Drying Conditions, Carrier Materials and Subcellular Structures

Production of active probiotics in powder form using spray drying could effectively reduce manufacturing cost. The resultant active dry probiotics can be used as food supplements, pharmaceutics, biopreservatives, and starter cultures, having tremendous commercial values. However, it is difficult to monitor the status of probiotics-loaded droplets during spray drying, as the mass and temperature of the droplets are subject to continuous change as drying progresses. The high air temperature and rapid dehydration process could pose a variety of stresses on probiotic cells, such as heat stress, dehydration stress and oxidative stress. Improper spray drying conditions could lead to a reduction of cell viability up to 4-5 orders of magnitude in the dried powders. Hence, understanding the changing history of the viability of probiotic cells during drying would be fundamental for unveiling the mechanisms of viability retention, which is useful for enhancing the survival ratio of probiotic cells after spray drying and further benefits to industrial application of spray drying to produce activity dry probiotics.

In this study, a new type of drying technique, Single Droplet Drying (SDD), was employed to quantitatively study the drying kinetics of probiotics-loaded droplets under droplet drying conditions analogous to spray drying. The corresponding changing history of probiotics viability, i.e., the inactivation history, was experimentally determined for each drying condition. Firstly, the effects of drying conditions on the inactivation history of probiotic cells were investigated. The results showed that the evaporative-cooling effect at the initial stage of drying effectively protected cell viability, and a rapid deactivation of probiotic cells was only observed at the later drying stage when droplet temperature was rapidly increased to a high level. Summarizing the trends at six drying conditions, it was found that the inactivation rate of probiotic cells could be well correlated to droplet temperature, whereas droplet moisture content showed minor effect. Secondly, the effects of carrier materials on the inactivation history of probiotic cells were studied. The inactivation histories of Lactobacillus rhamnosus GG (LGG) in four types of dairy materials, viz., reconstituted skim milk (RSM), lactose, MRS growth medium, and reconstituted whole milk, were compared. Probiotic cells with RSM carrier exhibited better survival ratio at the high temperature and low moisture content range than those with other carriers at the later drying stage. Comparing the different compositions of the four types of carriers suggested that the superior protective effect of RSM could be due to the abundant calcium cation and milk proteins, which enhanced the stress tolerance of bacterial cells and led to mild temperature elevation kinetics. Thirdly, we conducted preliminary experimental analysis on the history of cellular injuries that LGG cells experienced during droplet drying, by which the subcellular damage on probiotic cells at intermediate drying stage was investigated for the first time. Based on the findings, we proposed three possible mechanisms underlying the protection of carrier materials, which could be useful for developing high-performance carrier formulation in industrial spray drying of active dry probiotics. Finally, the injury history of cellular membrane was investigated in both heat treatment and SDD processes. Cells subjected to drying treatment could withstand higher droplet temperature than in heat treatment, possibly indicating a protective effect of dehydration against heat stress. Besides damages on cellular membrane, there could be damages on other cellular structures which caused cell death under elevated temperature.