Organic acids

Organic acids play a big role in how our food and drinks taste, how they’re chemically structured, and how long they stay good. In kombucha, some key organic acids are acetic, lactic, gluconic, and glucuronic acid.

Studies of kombucha made from black or green tea, show that the levels of acetic acid, glucuronic acid, gluconic acid, and ascorbic acid increase during fermentation (Aung et al., 2021; Jakubczyk et al., 2020; Kaewkod et al., 2019). Within 10-14 days the pH to drops from 5 to 3 due to the increased concentration of these organic acids (Zou et al., 2021).

Another study in Taiwan found that the longer you store kombucha made from different teas, the more acetic acid it has, reaching 8 g/L after 60 days (Chen et al, 2000).

The basic parameter that determines the quality of Kombucha is pH. The high content of organic acids in the drink, mainly produced by acetic acid bacteria (AAB), results in a low acidity of the drink. This, prevents the degradation of polyphenols, positively influencing, inter alia, the chemical stability of anthocyanins. The more of these compounds in a drink, the stronger the antioxidant effect (Antolak et al., 2021).

Acetic acid#

Acetic acid is produced in kombucha fermentation through the metabolism of sucrose by yeast cells, which are then converted into fructose and glucose. These sugars are further metabolized by yeast to ethanol, which is subsequently oxidized by AAB to produce acetic acid. The AAB play a crucial role in the production of acetic acid, which contributes to the reduction of the pH and the sour taste of kombucha (Wang et al., 2022).

The acidification phase in kombucha production is characterized by the release of gluconate and gallate from AAB metabolism, and probably from polymeric polyphenols, respectively. This phase has the most impact on molecular diversity, but the type of substrate mainly influences the global composition in polyphenol profile. Black tea polyphenols are more impacted by microbial activity compared to green tea polyphenols (Tran et al., 2022).

The microbial dynamics between yeasts and AAB in kombucha are crucial for the production of acetic acid, which contributes to the reduction of pH and the sour taste of the beverage. The yeast and AAB strains in kombucha have different strategies for the utilization of sucrose. Yeasts and AAB unable to perform efficient sucrose hydrolysis rely on yeasts with high invertase activity to access released monosaccharides. Moreover, the presence of AAB rerouted the metabolism of Saccharomyces cerevisiae towards higher invertase and fermentative activities (Tran et al., 2020).

The microbial dynamics between yeasts and AAB in kombucha fermentation have been studied through the cultivation of the original kombucha consortium and cultures in sugared tea, which were compared to determine the interactive microbial effects during successive phases in open (aerobic) and closed (anaerobic) incubation condition. The results highlight the main impact of yeast metabolism on the product’s chemical composition and the secondary impact of bacterial species on the composition in organic acids (Tran et al., 2020).

Gluconic acid#

Gluconic acid is a major organic acid present in kombucha, and its production is enhanced during the fermentation process. The production of gluconic acid is a result of the metabolism of sucrose by yeast cells, which are then converted into fructose and glucose. These sugars are further metabolized by yeast to ethanol, which is subsequently oxidized by AAB to produce acetic acid.

The production of gluconic acid does not start in the first days of fermentation. A study conducted by Mohd Roby et al. (2020) found that the level of gluconic acid was enhanced to 39 g/L during the 60th day of fermentation, while another study on Zijuan-based kombucha displayed an increment in gluconic acid, reaching a maximal concentration of 2.3 g/L on the 14th day of fermentation (Zou, C., Li, R.Y., Chen, J.X., Wang, F., Gao, Y., Fu, Y.Q., Xu, Y.Q. and Yin, J.F., 2021).

The production of gluconic acid contributes to the overall flavor profile of kombucha, and it is known to enhance the sensory qualities of wine, kinds of vinegar, and honey. Due to the vinegar-like flavor, kombucha’s high acetic acid content has a detrimental effect on its general appeal. Gluconic acid, on the other hand, was observed to be advantageous due to its moderate, gentle, and energizing flavor (Tran et al., 2020).

Glucuronic acid#

Glucuronic acid is produced in kombucha fermentation through the symbiotic relationship between yeasts, AAB, and in some cases, lactic acid bacteria (LAB) (Savary et al., 2019).

Studies have shown that the presence of acetic acid bacteria and yeasts in the kombucha fermentation process is essential for the production of glucuronic acid. The microbial consortium metabolizes natural pomegranate juice, rich in carbohydrates and acids, to produce a fermented beverage with high glucuronic acid content. Factors such as sugar concentration, fermentation temperature, and processing time influence the production of glucuronic acid, with optimized conditions leading to significant glucuronic acid yields (Yavari et al., 2017).

Furthermore, the addition of Lactobacillus casei and variations in fermentation conditions, such as sucrose concentration, temperature, and duration, have been shown to stimulate glucuronic acid production in the kombucha symbiosis model. L. casei acts as a supporter species, enhancing glucuronic acid production by the microbial consortium (Khoi et al., 2015).

The dynamics of glucuronic acid in kombucha fermentation is reported by various researchers (Aung & Eun, 2021, 2022; Jakubczyk et al., 2020a; Jayabalan et al., 2007; Myron et al., 2014; Shahbazi et al., 2018). Nguyen et al. (2015) reported a concentration of 0.03 g/L of glucuronic acid on the 5th day of fermentation with SCOBY. Whereas, Jayabalan et al. (2007) obtained 1.71 g/L of glucuronic acid on the 18th day of fermentation. At the end of the kombucha fermentation, the amount of glucuronic acid reached a range of 0.839-1.158 g/L while it was absent at the beginning of the fermentation (Shahbazi et al., 2018). However, as observed by Yang et al. (2010), the highest glucuronic acid in black tea was 5 g/L. The amount of glucuronic acid rose throughout fermentation at all temperatures, peaking on the 10th day at 25°C (Neffe-Skocińska et al., 2017).

Lactic acid#

Similar to acetic acid, lactic acid also notably lowers the pH of the Kombucha, which increases the capacity of the kombucha drink to fight microorganisms. Lactic acid is produced when glucose is digested by homofermentative LAB using the Embden-Meyerhof-Parnas (EMP) pathway. Lactic acid, Ethanol, and CO2 are the primary by-products when glucose is metabolized by heterofermentative LAB using the pentose phosphate pathway.

During fermentation, the level of lactic acid increases considerably. At the beginning of the fermentation process, its concentration ranged from 0.0221 to 0.0834 g/L, but by the end, it was varied between 0.090.81 and 0.1845 g/L (Shahbazi et al., 2018). Various researchers reported that the lactic acid content significantly increased during kombucha fermentation in various substrates (Aung & Eun, 2021; Jakubczyk et al., 2020; Jayabalan et al., 2007; R. Malbaša et al., 2008).

Other organic acids#

Citric acid is known for its flavor enhancement in beverages. According to reports, it is absent at the beginning of the kombucha fermentation process but subsequently appears to be generated in the range of 0.014 to 0.47 g/L as the fermentation process concludes (Aung & Eun, 2021; Shahbazi et al., 2018).

Minor organic acids, with concentrations lower than 1 g/L, including succinic acid, citric acid, quinic acid, malic acid were detected in all types of kombucha beverages (Zou, C., Li, R.Y., Chen, J.X., Wang, F., Gao, Y., Fu, Y.Q., Xu, Y.Q. and Yin, J.F., 2021).

Section references#

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