Introduction

Beverages fermented by a consortium of acetic acid bacteria, lactic acid bacteria and yeasts have been gaining a lot of attention recently, due to their potential to promote several health benefits (Leonarski et al., 2021). The fermentation process involves using bacteria and yeast in the presence of sucrose to change a substrate through both aerobic and static fermentation (Laureys et al., 2020).

These fermented drinks contain probiotic bacteria that can survive the digestive process and settle in the intestines while staying active. Once there, these active bacteria interact with the existing gut bacteria, leading to changes in both the composition and function of the gut microbiota. This interaction helps in developing a well-balanced and diverse gut microbiota, which is linked to various health benefits. These benefits include improved digestion, a stronger immune system, and reduced inflammation.

Additionally, the organic acids produced during kombucha fermentation, particularly acetic acid and gluconic acid, can create an environment in the gut that supports the growth of beneficial bacteria while inhibiting harmful bacteria. This can contribute to a healthier gut microbiome (Costa et al., 2021; Jung et al., 2019; Permatasari et al., 2022).

The key metabolic functions of the bacteria and yeast culture involved in the fermentation process are illustrated below as an overview of the process.

Process dynamics#

The process dynamics during kombucha fermentation involve the transformation of various bioactive compounds by the symbiotic culture of bacteria and yeast (SCOBY).

The bioactive compounds in fermented drinks come from the substrate such as phenolic compounds, polysaccharides, vitamins, minerals, and amino acids as well as from the metabolic activity of microorganisms in the fermentation process. During fermentation, certain chemical factors can disrupt the microbial communities, producing bioactive peptides and biogenic amines.

The changes in major bioactive compounds during kombucha fermentation has been mentioned in several studies:

• Organic acids: The total acidity increased from 0.11% (0th day) to 0.37% on the tenth day of black tea fermentation, with the most significant increase between the 3rd and 7th day (de Noronha et al., 2022). In black tea fermentation at 20 or 30 °C, gluconic acid concentration increased more than glucuronic acids (Aung & Eun, 2021; Filippis et al., 2018). A 3-fold rise in acetic acid was observed in a Laver kombucha sample from the start of fermentation; α-ketoglutaric acid levels were highest on day 14 (2.25 g/L). Acetic, gluconic, glucuronic, citric, L-lactic, oxalic, tartaric, pyruvic, and malic acid seemed to be produced during coffee kombucha fermentation (Bueno et al., 2021). Acetic and gluconic acid concentrations increased with prolonged Zijuan kombucha fermentation time, reaching a maximum of 24 g/L and 2.3 g/L, respectively (Zou et al., 2021). Total acidity increased during 12 days of red grape kombucha fermentation from 25.9 to 104.2 meq/L (Ayed et al., 2017).

• Phenolic & Flavonoid content: The initial carbon source for kombucha is sucrose, which is hydrolyzed into glucose and fructose during fermentation. Day-by-day fermentation sugar kinetics are rarely documented (Aung & Eun, 2021; Ivanišová et al., 2020; Jayabalan et al., 2007; Valiyan et al., 2021; Zou et al., 2021). Phenolic compounds increased gradually with fermentation time, and flavonoids varied positively from unfermented to fermented kombucha beverages over time (Ahmed et al., 2020; Bortolomedi et al., 2022; Gaggìa et al., 2018; Júnior et al., 2022; Kayisoglu & Coskun, 2020; Lobo et al., 2017; Lopes et al., 2021; Rahmani et al., 2019; Ulusoy & Tamer, 2019; Zubaidah et al., 2018).

• Vitamins & Minerals: According to reports, water-soluble vitamins (B complex and C) increase with fermentation time. However, reports on the dynamics of minerals are scarce (Buzia et al., 2018; Ivanišová et al., 2020; Tamer et al., 2021; Jakubczyk et al., 2020).

In the following section we will analyse each bioactive compound involved in kombucha fermentation in more detail.

Section references#

Leonarski, E., Cesca, K., Zanella, E., Stambuk, B. U., de Oliveira, D., & Poletto, P. (2021). Production of kombucha-like beverage and bacterial cellulose by acerola byproduct as raw material. LWT, 135, 1–8. https://doi.org/10.1016/j.lwt.2020.110075. August 2020.

Laureys, D., Britton, S. J., & De Clippeleer, J. (2020). Kombucha tea fermentation: a review. Journal of the American Society of Brewing Chemists, 78(3), 165–174. https:// doi.org/10.1080/03610470.2020.1734150

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