The pH is one of the most important environmental parameters affecting the fermentation of Kombucha, because some of the acids formed as acetic and gluconic, could be responsible of the biological activities of the resulting beverages. It is also closely related to the microbial growth and the structural changes of the phytochemical compounds which may influence the antioxidant activity (Hur et al., 2014). However, the lowest acceptable pH value should not decrease below 3, which is the one of digestive tract (Lončar et al., 2006). Also, in accordance with Šaponjac and Vulić (2014), to obtain a pleasant sour beverage, the fermentation should be ended when the total acidity reaches the optimum value of 4 to 5 g/L. However, the period of time to obtain this value may differ depending on the origin of the culture medium and fermentation conditions.

Oxygen transfer rate and scaling‐up process

Most fermentation processes are aerobic and, therefore, require the provision of oxygen. If the stoichiometry of respiration is considered, then the oxidation of glucose may be represented as:where 192 g of oxygen are required for the complete oxidation of 180 g of glucose. However, both components must be in solution before they are available to a microorganism and oxygen is approximately 6000 times less soluble in water than is glucose, thus, it is not possible to provide a microbial culture with the necessary amount of oxygen for completing the oxidation of the glucose or any other carbon source, in one addition.

At the beginning of the process, significant amounts of ethanol and monosaccharides required for AAB are provided by Kombucha yeasts. The oxidation of ethanol into acetic acid requires one mole of oxygen (32 g) to completely oxidize 1 mole of ethanol (46 g), therefore, the activity of AAB as strict aerobic organisms depends on the transfer of oxygen from the air into the fermentation broth. For that reason, a microbial culture must be supplied with oxygen during growth at a sufficient rate to satisfy the organisms demand (Stanbury et al., 2013). As being a beverage in constant study and evolution, Kombucha has mainly been studied at lab‐scale, from 200 mL to 2 L. However, few researchers have studied its fermentation in higher volumes. Malbaša et al. (2006) applied a regression analysis method to a batch process of 8 L and concluded that the pH is a variable that can allow scale‐up estimation. Later, Cvetković, Markov, Djurić, Savić, and Velićanski (2008) studied the impact of the specific interfacial area as a variable that could control the production of Kombucha tea, using reactors of 90 L and concluded that reactors having the same interfacial area, although being different in size could provide similar mass transfer conditions. And recently, Coton et al. (2017) worked with volumes of 1,000 L and studied the microbial ecology of the produced tea by meta‐barcoding and culture‐based methods. They observed that the microbial population seemed not to be affected by the industrial‐scale microbial stress factors, which could lead to the standardization of Kombucha tea for industrial production. Besides the volume, there are several parameters for bioprocess development to be taken into account, where the most important include the vessels geometry and the agitation type (Junker, 2004). In Kombucha fermentation, according to some authors, the agitation processing affects the structure of the biofilm due to the reported loss of mechanical strength (Chawla et al., 2009). In static cultures, substrates have to be entirely transported by diffusion and the oxygen availability might become the limiting factor for cell metabolism, which could have a negative effect on the production and quality of cellulose. The kinetic factor that expresses the relationship between the dissolved oxygen and the surface/volume of the medium is the specific interfacial area, which is directly related to other factors, such as the reactor cross‐section and the mass transfer coefficient (Cvetković et al., 2008). This means that the rate of Kombucha batch fermentation without agitation and without introducing gas depends on the specific interfacial area. Cvetković et al. (2008) developed a mathematical model to scale Kombucha tea fermentation based on several specific interface areas. The verification of the model was made in reactors of large volume (90 L) and very small vessels of 0.33 L. The model standardizes the optimal conditions as: 70 g/L of initial substrate (sucrose), interfacial area of 0.0231 to 0.0642 cm−1, and 14 days of fermentation. They concluded that regardless of the vessel size or volume, if the value of the interfacial area is constant they could ensure the production of Kombucha tea with similar properties. In the specific case of batch fermentation of Kombucha tea, several biological factors should be taken into account. Especially in the absence of agitation, where a microbial disintegration may occur between the aerobic acetic bacteria which will tend to occupy the surface layer and the yeast which may precipitate to the bottom of the vessel (Lončar et al., 2006), and this might have negative effects in the fermentation process. Besides the fact that microbial cellulose has been already well studied by some authors (Campano et al., 2016; Czaja et al., 2006), the available information defining the optimal reactor conditions for its development such as surface/volume or surface/height are limited. In order to investigate the influence of the volume in the processing, Lončar et al. (2006) worked with several conditions and found that the best geometric conditions for scaling‐up the fermentation were obtained with a reactor of 4 L and a diameter of 17 cm. Goh and others (2012) investigated the relationship between the yield, the properties of the biofilm produced from Kombucha fermentation, and the surface area, and found that the production of the biofilm was increased with an intensification of surface area, and was decreased with a broader depth. This can be explained because the metabolic process is completely aerobic and it is constantly generating carbon dioxide, which might be trapped in the pellicle and end up being accumulated in greater quantities especially in the deeper mediums. However, Caicedo, Da França, and Lopez (2001) found that even though the surface area is determinant, the height is not unimportant, since it was observed that a minimal height is needed for the development of the pellicle, this taking into account the production of several layers of cellulose throughout the fermentation which will occupy part of the initial volume.

Beside all the previously mentioned parameters, Kombucha's elaboration process may also affect its final properties. The process still remains quite artisanal and the exact proportion of components may vary depending on the expected product. However, it generally follows the next order: Tea leaves or plant extracts are added to boiling water and allowed to infuse for about 10 to 15 min after which the leaves are removed. Sucrose is next dissolved in the hot tea and after the infusion is left to cool. The sweetened beverage is subsequently poured into a container and inoculated with about 3% w/v of already prepared Kombucha biofilm, afterwards the container is covered with a clean cloth and incubated at room temperature.

Nevertheless, in order to optimize the industrial production of Kombucha tea, as being a functional beverage, a complete study including high‐volume production, microbiological identification, and biological assays should be performed.