Investigation into the Impact of Food Matrix on Bacterial Survival during Gastric Digestion

Abstract

Over the years, food safety research often focused on the bacterial survival during food processing and storage, whereas physiological studies extensively explored the host-pathogen interaction in gastrointestinal tract. There is a need to understand the intermediate step on pathogen survival during gastric digestion and the potential impact from its food carrier. This study utilized water-in-oil (W-O) and oil-in-water (O-W) emulsion as well as deionized water (DI) as the fundamental model food matrices to study the potential protection by food matrix during simulated gastric digestion. Using Salmonella enterica subsp. enterica serovar Typhimurium as a sample foodborne pathogen, this study investigated the survival kinetics of bacteria using various models of simulated gastric digestion. In a simplified static pH simulated gastric digestion model, inoculated W-O and O-W emulsion matrices were challenged with simulated gastric fluid (SGF) containing HCl and pepsin with mixing using a stomacher for two hours. W-O emulsion showed significant protection of Salmonella survival compared to O-W emulsion and DI water. This protective effect appeared to be matrix dependent regardless of the inoculation location of Salmonella (in dispersed phase vs. in continuous phase). Within the same emulsion type, inoculating Salmonella in water phase or oil phase did not show significant difference in its survivability during simulated gastric digestion. The study was then extended to an improved gastric digestion model where the chyme pH dropped from 4.0 to 1.5 over three hours, and the chyme mixing was achieved by an orbital shaker. In addition, the new SGF was modified to be HCl solution with pepsin, amano lipase A, mucin and NaCl. Under this digestion condition, there was no significant difference in Salmonella survival between W-O emulsion, O-W emulsion, and DI water. Moreover, the dispersed-continuous phase ratio of emulsion composition also showed no impact on Salmonella survival. The simulated gastric digestion model setup was also further analyzed including the role of individual digestive enzyme, the pH impact, and the mechanical mixing approach. In the dynamic pH simulated gastric digestion model, partial activity from lipase accelerated the disruption of emulsion structure for both W-O and O-W emulsion matrices. Mild mixing using an orbital shaker also showed difference in Salmonella survival compared to vigorous mixing using a stomacher. Lastly, this study expanded from using Salmonella as the single bacteria strain into a tailored natural microbiome community. Natural microbiome communities from Golden Delicious (GD) and Empire (EP) apples were manually enriched using bacteria culturing broth at pH 5 and pH 7, respectively. The enriched apple microbiome was then collected and analyzed using 16S rRNA sequencing to study the microbial composition. With a significant decrease in Alpha diversity, the culturable apple microbiome was successfully enriched from less than 3 log CFU/ml to more than 8 log CFU/ml. There was no known foodborne human pathogens detected in the enrichment, and the most abundant genera appeared to be potential plant growth promoting bacteria. The collected apple microbiome was then inoculated in various food matrices to study its survivability during dynamic pH simulated gastric digestion including DI water, apple sauce (AS), chicken puree (CK), sweet potato puree (SP), and W-O emulsion. The enriched apple microbiome showed remarkably high survivability in W-O emulsion throughout the full three-hour digestion treatment. CK also exhibited moderate protective effect compared to SP at the same condition. There was no significant difference between DI and AS on bacterial survivability. In addition, the apple microbiome enriched at two pH levels (5 & 7) showed similar bacteria inactivation kinetics. In conclusion, this study revealed the potential impact from food matrix on bacterial survival during simulated gastric digestion. W-O emulsion offered significant protection of certain bacteria strains or communities in specific simulated gastric digestion models. The parameters in gastric digestion models also affected bacterial survival. Future work should focus on exploring the potential impact from other types of food matrices, expanding the microbial survival study into other bacterial strains as well as a more complex microbiome community, and further comparing the various gastric digestion models.