Graduate Thesis Or Dissertation
 

Prevalence of Volatile Phenols in Kombucha and Evaluation of Factors that Influence their Formation

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tx31qs129

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  • Kombucha is a traditional fermented beverage that has become increasingly popular in the United States due to its perceived health benefits and unique flavor profiles. The drivers of kombucha flavor have not been fully characterized, at least in part because it is produced using a complex microbial community, leading to challenges for kombucha producers in achieving product consistency. The management of fermentations to achieve desired flavors has been well established in systems like beer and wine, where inoculation with single yeast starter cultures is practiced and preservatives are utilized to control spoilage organisms. Kombucha is typically made using organic ingredients and is intended to be consumed “live”, therefore preservatives are not utilized and currently there is a lack of knowledge regarding how to manage kombucha fermentations to achieve desirable outcomes. The fermentation of kombucha is initiated by the transfer from batch to batch of a SCOBY (Symbiotic Culture of Bacteria and Yeast), which is a solid-phase cellulosic biofilm that can harbor a wide array of yeast and bacterial species. The most prevalent and abundant yeasts found in North American kombucha SCOBY were recently shown to belong to the notorious wine-spoilage genera, Brettanomyces. The same yeasts are known to contribute positively with regards to quality, in certain styles of beer. Their divisive contributions to flavor revolve around production of volatile phenols (VPs), such as 4-ethylphenol, from odorless hydroxycinnamic acid precursors (HCAs), which are present in steeped tea. They are also found in common kombucha flavorants like juices, herbs, and spices, which may be added pre- or post-fermentation. With Brettanomyces as a common production organism and the potential for HCAs to be present in the tea matrix, this research initially sought to determine if VPs are a feature of North American kombucha. To determine the prevalence of these compounds in kombucha, a random sampling of 57 products available in retail outlets in the Pacific Northwest were obtained. Using gas-chromatography mass-spectrometry stable-isotope dilution assays (GC-MS-SIDA), the concentrations of four VPs (4-vinylphenol, 4-vinylguaiacol, 4-ethylphenol, and 4-ethyl guaiacol) were determined. While median values of each VP were below known perception thresholds (established in beer and wine), some individual products contained relatively high concentrations. Furthermore, repeat analysis of some products highlighted bottle-bottle variation, suggestive of either variation occurring during fermentation, or VP production post-packaging. Machine-learning analysis of product ingredient lists inferred that ABV (alcohol by volume), addition of Lactobacillus probiotic supplements, and addition of certain flavorants (ginger and tangerine) corresponded with variation in VP concentrations in packaged kombucha. Ginger and tangerine have been shown to be rich in HCAs, and some Lactobacillus species display capacity to produce VPs, therefore it is possible that products containing high VP concentrations had undergone post-packaging conversion of HCAs into VPs. To evaluate whether variation in precursor content and microbial community structure could impact volatile phenol formation during fermentation, commonly reported kombucha yeast and acetic acid bacterial species were screened for capacity to degraded hydroxycinnamic acids. Yeasts displaying high (Brettanomyces bruxellensis), moderate (Meyerozyma guilliermondii), and low (Zygosaccharomyces bisporus) capacity were incorporated into design of 10 different co-cultures and de-novo SCOBYs, which were used to ferment sweetened tea with and without added hydroxycinnamic acids. Brettanomyces dominated the kombucha microbial communities into which it had been inoculated and produced high amounts of volatile phenols in the presence of added HCAs in our lab-scale kombucha fermentations. Treatments with both Brettanomyces, and Zygosaccharomyces also had the ability to produce ethylphenols without the addition of HCAs. The presence of Brettanomyces in the treatments also significantly increased the level of acidification and decreased the final weight of the SCOBY compared to the other treatments. Prior to this study, there was no published data on prevalence of volatile phenols in kombucha, despite the prevalence of microorganisms capable of their synthesis and the potential abundance of the necessary precursors: hydroxycinnamic acids. This work confirmed that volatile phenols are a feature of the kombucha flavor profile and provides guidance on potential drivers of volatile phenol production during fermentation.
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  • Pending Publication
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  • 2021-09-21 to 2022-10-22

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