The Extent And Mechanism Research Of The Protective Effects Of Carrier Composition On Lactobacillus Rhamnosus GG During Convective Droplet Drying

Active dry probiotics in powder form has high international trading values in food and pharmaceutical industries. Such dry probiotics powder can be produced by spray drying, which is a rapid and cost-effective powder production approach. However, during spray drying viable probiotic cells in the atomized droplets could suffer from multiple environmental stresses, which are unfavorable for the preservation of cell viability. To improve the survival of probiotic cells after spray drying, protective carriers could be adopted. It is relatively difficult to investigate the protective effect and the relevant mechanism of a specific carrier material in a conventional spray dryer due to the dryer design, which makes it difficult to monitor and measure changes in the droplet properties as drying progresses. In this study, a Single Droplet Drying（SDD） device was utilized to study the drying kinetics of an isolated droplet under conditions analogous to spray drying. Changes in the viability of a model strain, Lactobacillus rhamnosus GG（LGG）, during droplet drying was experimentally measured, to compare the difference in the inactivation history and the injury history as affected by different carrier materials. The inactivation history was correlated to the drying kinetics that cells experienced, to explore the protective mechanism of the carrier material. The results would benefit the design of carrier formulation to enhance the survival of probiotics cells after spray drying.Firstly, the protective mechanism of reconstituted skim milk（RSM）, an outstanding protective carrier reported in the literature, was investigated. The inactivation history of LGG in four carrier materials was compared, i.e., RSM, lactose, MRS and reconstituted whole milk（RWM）. At a drying temperature of 90 oC, the viability of LGG was maintained around 109 cfu/m L at the initial drying stage regardless of the carrier material used, followed by a transition to rapid viability loss at the later drying stage. The transition of viability maintenance to viability loss occurred at 135 s in the RSM carrier, compared to at 105 s in all three other carriers tested. Comparing the inactivation history against droplet drying kinetics, it was found that RSM could better maintain LGG viability upon both high droplet temperature and low moisture content than the other three carrier materials. As RSM was composed mainly of lactose, milk proteins and abundant inorganic cations, the prominent protective effect was attributed to milk proteins（whey proteins） and calcium cations, whereas lactose did not show protective effects according to the experimental results. Upon the rapidly increasing droplet temperature at the later drying stage, Ca²⁺ might enhance the heat resistance of the cells, whereas proteins might lead to a mild temperature variation rate which is beneficial to cell survival. In addition, LGG cells dried in the RWM carrier showed the most advanced transition of rapid viability loss; the detrimental effect could be due to the high level of milk fat content.Based on the results above, the experimental investigation was then extended to the protective effect of calcium cation and whey protein isolates（WPI）. WPI could well protect the viability of LGG cells, in agreement with reported studies in the literature. Adding 1 m M Ca²⁺ to lactose carrier protected LGG viability and reduced cellular injuries as drying progressed, which could be possibly due to stabilization of sub-cellular structures upon heat stress, consequently an enhanced stress tolerance of LGG cells. The protective effect of Ca²⁺ was lost upon supplementing a calcium-binding agent. Adding Ca²⁺ to trehalose carrier barely enhanced the protective effect, as sole trehalose could effectively protect the viability, sub-cellular structures and regrowth capability of LGG cells, which might overlap the effect of Ca²⁺. The results will be useful for optimizing the survival of probiotics and designing effective carrier formulation in industrial spray drying operation to produce active dry probiotics.In summary, this study established the inactivation history and injury history of LGG strain during droplet drying in different carrier materials, using the SDD technique. The findings unveil the protective mechanism of RSM by analyzing the effects of droplet drying kinetics on the inactivation history, and further explore the protective effects of Ca²⁺ on the subcellular level.