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theses

Kombucha, a fermented tea beverage produced using yeast and acetic acid bacteria, tastes like apple-cider vinegar and contains bioactives such as tea polyphenols, acetic acid, and gluconic acid. While traditional probiotic bacteria were not observed in Kombucha, the bioactives could potentially act as postbiotics and correct dysbiosis, an imbalance in gut microbiota. Challenges in Kombucha industry include batch to batch inconsistencies in flavor quality and mislabeling the amounts of alcohol, acid and sugar. Producers frequently change tea types, inoculum types (age and solid or liquid), and fermenter characteristics without a clear understanding of their effect on bioactives and flavor quality. Our objective was to understand the relationship between key process parameters and microbial activity to help design processes that could achieve better quality control. Microbial activity was measured in terms of change in Kombucha sugars, acids and catechins using HPLC methods and correlated with total plate counts. Flavor quality was evaluated by taste and compared to a matrix developed using a 9-point hedonic scale.

Black tea Kombucha tasted harshly acidic (sugar/acid ratio (S/A) of ~10), compared to a pleasant tasting green tea Kombucha (S/A of ~14) that corresponds to an overall taste acceptance rate of 64%. The differences could be explained by the antimicrobial activity of green tea catechins. Black tea when dosed with green tea catechins (EGC and EGCG) reduced the bacteria counts in Kombucha. While there was no difference in yeast counts, metagenomic sequence analysis showed a different microbial composition in Black and Green Tea Kombuchas’.

Compared to a 10-day inoculum, Kombucha made with a 15-day solid and liquid inoculum was overly fermented and unpalatable (lower S/A of 6.6) which corresponds to an overall taste acceptance rate of 27%. These taste characteristics were not necessarily due to higher bacteria and yeast counts in 15-day inoculum but has to do more with different microbial composition, as confirmed by sequencing data. When only liquid inoculum was used, the rate of fermentation reduced in half, resulting in an overly sweet Kombucha with S/A ratio of 28 which corresponds to an overall taste acceptance rate of 16%. There was an increase in total polyphenol content and catechins (EGC+EGCG) in Kombucha made without solid inoculum.

Kombucha made in a vessel with a specific interfacial area 0.16 (SA/V) produced glucose and fructose 5-7 times as much as the one with lowest SA/V 0.09. Specific interfacial area of the fermentation vessel was found to be directly proportional to the rate of fermentation as confirmed by S/A ratios, residual sucrose and the flavor profile.

A comprehensive analysis was performed to provide a better understanding of relationship between few key process parameters, microbial activity and quality parameters. Changing key process parameters resulted in wide range of sugars, acids and alcohol concentration. These findings will add to a list of parameters that producers will need to monitor for better Kombucha quality control and to deliver a balance between bioactive content and flavor profile.

ETD_9251

doi:10.7282/T3GX4G6R

Ph.D.

Includes bibliographical references

by Saikiran Chaluvadi

ETD doctoral

The author owns the copyright to this work.