Jin Xie | 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 Jin Xie. This seminar is open to the general public to attend, either in-person or online:

https://ualberta-ca.zoom.us/j/95352627172

Meeting ID: 953 5262 7172 | Find your local number: https://ualberta-ca.zoom.us/u/abIrYDkGA9

Thesis Topic: Formation of taste-active metabolites in sourdough fermentation and the fermentation of plant cheese analogous

PhD with Dr. Michael Gaenzle.

Seminar Abstract:

Fermentation is an oldest and effective method to improve the flavor and quality of foods. Taste-active compounds generated during food fermentation contribute to food taste. The aim of this PhD dissertation was to investigate the synthetic mechanism of taste-active γ-glutamyl dipeptides during sourdough fermentation and fermentation of plant cheese analogous, and their effect on taste of foods.

Kokumi γ-glutamyl dipeptides accumulate in sourdoughs. To understand the role of γ-glutamyl-cysteine ligases (Gcls) from lactobacilli in the formation of taste-active γ-glutamyl dipeptides in sourdough or sourdough bread, three Gcls from strains of Lm. reuteri, designated as Gcl1, Gcl2 and Gcl3, were characterized. In addition, sourdoughs were fermented with strains of Lm. reuteri or its Gcl-deficient mutant strains, subsequently used for the production of sourdough breads. The results showed that in enzymatic reaction with an individual amino acid, Gcl3 exhibits the broadest substrate specificity among three Gcls and all three Gcls were inactive to Val, Asp and His. All three Gcls of Lm. reuteri with a mixture of amino acids were most active to Cys among 12 amino acids. Gcl1 exhibited a stronger activity with hydrophobic amino acids Ile, Leu, and Phe when compared to Gcl2 and Gcl3. In sourdough fermentation, the accumulation of kokumi γ-glutamyl dipeptides was attributed to the combined activity of cereal enzymes, Gcls of lactobacilli and other microbial enzymes in lactobacilli. Although several γ-glutamyl dipeptides were synthesized during sourdough fermentation, baking processing strongly influenced the concentrations of γ-glutamyl dipeptides in bread. The effect of γ-glutamyl dipeptides in taste of bread remain to be investigated further.

Consumer demand for plant cheeses is increasing but challenges remain to improve the flavor and quality. The study described in thesis investigated microbiological and physicochemical impacts of seed germination and fermentation with Bacillus spp. on proteolysis in plant cheese analogues and the contribution of adjunct cultures on the accumulation of kokumi γ-glutamyl dipeptides in fermentation of plant cheese analogous. Bacilli but not seed germination enhanced proteolysis of plant cheese and release of glutamate during ripening. Lp. plantarum and Lc. lactis served as starter cultures for plant milk acidification; Lv. hammesii, Ff. milii, or Lt. buchneri were assessed adjunct culture for the ripening of plant cheese analogous. During ripening of plant cheese analogous, Lc. lactis and Lp. plantarum was inactivated. Cell counts of Lv. hammesii remained stable over 45 d of ripening while Ff. milii and Lt. buchneri grew slowly. The combined activity of seed germination, Bacillus spp., and adjunct culture lactobacilli contributed to the synthesis of kokumi γ-glutamyl dipeptides in plant cheese analogous.

Overall, this research revealed the synthetic mechanism of taste-active γ-glutamyl dipeptides in sourdoughs fermentation and fermentation of plant cheese analogous, providing insights into the control of formation of taste-active compounds in food fermentations.