Benjamin Bourrie | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Benjamin Bourrie. This seminar is open to the general public to attend.
Thesis Topic: Culture and Characterization of Kefir Organisms to Reproduce the Health Benefits of Traditional Kefir

PhD with Drs. Ben Willing and Paul Cotter.

Seminar Abstract:

Fermented dairy products such as kefir have long been associated with a host of health benefits, however, the role that microbial composition has in these benefits has not been properly examined. This thesis aimed to examine how kefir impacts host metabolic health, as well as how alterations to the microbial populations of the kefir impacts these health benefits and the fermentation dynamics.

In order to examine how kefir impacts metabolic health, four traditional and one commercial kefir we administered to mice using a high fat diet-induced obesity model. Two traditional kefirs decreased weight gain and plasma cholesterol levels with one also lowering liver triglycerides. Conversely, commercial kefir had no beneficial effect. Improvements to liver triglycerides corresponded with decreases in the expression of fatty acid synthase, a gene involved in liver lipogenesis. This study shows that traditional kefir has the potential for improving metabolic dysfunction associated with obesity and indicates that differences in kefir microbial populations may influence the ability of traditional kefir to positively impact host metabolic health.

As the metabolic health benefits associated with kefir were only associated with traditional examples, we set out to create a kefir product that utilized microbes from traditional kefir while maintaining a production method that is viable on an industrial scale. This was accomplished through the isolation of a large collection of organisms from a variety of traditional kefirs representing the most abundant species. Characterization of the isolates was carried out, and individual isolates were selected for use in the pitched kefir based on the characteristics of both the isolate and the kefir from which it was isolated. This proof of concept study showed that it is possible for traditional kefir organisms to ferment milk outside of a kefir grain, and that the resulting product resembles kefir in microbial density and pH.

Following the development of the novel pitched kefir, we investigated the ability of this pitched culture kefir to recapitulate health benefits observed with a specific traditional kefir. This was accomplished using a mouse model of diet induced obesity. Additionally, we examined how differences in the microbial composition of kefir impacted these benefits. Both the traditional kefir and its pitched culture equivalent decreased plasma cholesterol and liver triglyceride levels by similar amounts when compared to commercial kefir. Furthermore, a pitched kefir produced without yeast and pitched kefir produced without lactobacilli did not show cholesterol lowering effects. The traditional and pitched kefir with the full complement of microbes were able to impart corresponding decreases in the expression of multiple cholesterol and lipid metabolism genes in the liver. These results show that traditional kefir organisms can be utilized to create a more health promoting commercial kefir, while also highlighting the importance of microbial interactions during fermentation in the ability of fermented functional foods to beneficially impact host health.

In order to better understand how alterations to the microbial composition of kefir fermentations impact the dynamics of the fermentation, we used GCxGC-TOFMS to examine the metabolite profiles of traditional and pitched culture kefir, included pitched culture kefirs that had the yeast or lactobacilli removed. Interestingly, despite similarities in their ability to improve host metabolic health, the Pitch and traditional kefir differed significantly in the profile of metabolites present, especially in yeast associated metabolites such as ethanol and esters. In addition, the removal of lactobacilli from the Pitch fermentation seemed to have a profound effect on the generation of organic acids during fermentation, while the removal of yeast appeared to result in little alteration in the metabolite profile.

This thesis provides insight into how microbial composition impacts kefir fermentation, and presents a new strategy for the development of fermented functional foods.