Protection Effect Of Alginate Microcapsules On Lactobacillus Plantarum ST-Ⅲ

Lactic acid bacteria(LAB) have been widely used in the food industry due to its beneficial function, such as alleviating the symptoms of lactose intolerance and restoring intestinal homeostasis. However, probiotics faced multiple environmental stresses in the process of production and human digestion. Microencapsulation can effectively maintain the probiotic activity and has been a research focus in food science. In the study, alginate microcapsules containing Lactobacillus plantarum ST-Ⅲ were prepared using the method of emulsification/internal gelation and the influence of acidification parameters in microcapsule preparation on probiotic protection was investigated. KGM(Konjac glucomannan) and GB(Guluronate block) were used to prepare alginate microcapsules with the same particle size but different mechanical strength and the correlation between cell survivals and microcapsule mechanical strength was then systematically studied. Finally, the protection mechanism of alginate microcapsule was studied through the analysis of physical characteristics of microcapsules and physiological properties of encapsulated cells. The main conclusions are as follows:1. With increasing acid/Ca molar ratio and acidification time, the average diameter of microcapsules decreased and their mechanical strength increased, while the shrinking ratio of microcapsules in simulated gastric juice(SGJ) and swelling ratio in bile salts solution(BS) decreased. Encapsulation yield first increased from 22.9% to 87.3% and then decreased to 77.8% with increasing acid/Ca molar ratio, while it decreased from 90.0% to 22.7% as acidification time increased, indicating that acid/Ca molar ratio and acidification time had a significant impact on the encapsulation yield of microcapsules. In SGJ, the survival rate of encapsulated probiotics increased with increasing acid/Ca molar ratio, but increased first and then slightly decreased with prolonged acidification time. In BS, cell survival rate increased with the increasing of both acid/Ca molar ratio and acidification time. Thus, acidification conditions strongly affected physical properties of alginate microcapsules, resulting in different encapsulation yield and cell protection efficiency.2. Using KGM’s high viscosity and GB’s texture-modulating capacity, 9 groups of alginate microcapsules were prepared with particle size of about 400.0 μm and different mechanical strength(varying from 5.4 N to 51.9 N). Encapsulation yield of microcapsules were 77.2%~90.6%, with no significant difference. Cell survival rate in SGJ increased from 23.3% to 63.7%, while that in BS went up from 2.0*10-1% to 6.4*10-1%, which had significant differences in statistics analysis. Linear-regression analysis showed that cell survival rate in SGJ and BS both positively correlated with mechanical strength of microcapsules. Thus, mechanical strength is an important factor for alginate microcapsules in protecting the LAB.3. Five groups of microcapsules with increasing mechanical strengths were selected for further investigation. The microcapsules shrank in SGJ, while swelled in BS, and their swelling or shrinkage ratio decreased with increasing mechanical strength of microcapsules. The diffusional permeability values of three tested chemicals(MB, VB12, FITC-dextran) decreased with increasing mechanical strength of microcapsules. Thus, cell protection mechanism of alginate microcapsules with high mechanical strength is that microcapsule resisted the osmotic effect of digestive juices, delayed the diffusion speed of digestive juices into the microcapsules and finally reduced cell damage. In SGJ, cell membrane integrity of microencapsulated bacteria varied from 33.0% to 60.3%, cell membrane fluidity decreased, and internal cell pH was at 6.73~6.98. With increasing the mechanical strength of microcapsules, these physiological properties of encapsulated cells tended to maintain at normal values. In BS, cell membrane integrity of microencapsulated bacteria was lower than 4.0% and membrane fluidity was significantly enhanced. Thus, in SGJ and BS, the physiological indexes of LAB verified the physical protection of microcapsules with different mechanical strength.