## 3 Results and Discussion
### Determination of Residual Glucose and Sucrose, pH and Total Titratable Acidity  
As shown in Figure 1A,B, the concentrations of residual glucose and sucrose were determined at regular time intervals during fermentation. A significant increase in glucose concentration (_p_ < 0.05) was observed after 24 h for all experimental sets (Figure 1A). This could be due to the metabolism of sucrose by yeast, yielding glucose and fructose.  
In the absence of sucrose addition, glucose utilisation is maximized. Set 7 (12 g/L of glucose) and Set 4 (6 g/L of glucose) demonstrated high glucose utilisation values of 70% and 74%, respectively. Set 2 (supplemented with 6 g/L of sucrose) and Set 8 (supplemented with 12 g/L of glucose and 6 g/L of sucrose) showed high sucrose utilisation values of 79% and 81%, respectively. In general, sucrose was utilised more than glucose. The utilisation of sugars is reflected by an increase in the TVC of LAB, yeast, and AAB (Table S1, Table S2 and Table S3, respectively). The utilisation of sucrose by water kefir microorganisms has been reported by [PERSON] and [PERSON] [40]. Sucrose can be hydrolysed to D-glucose and D-fructose by dextran-sucrace (invertase) secreted by microorganisms, such as LAB and yeast. D-fructose is utilized as an energy and carbon source, whereas D-glucose is used for the biosynthesis of dextran via polymerization [42; 43].  
During fermentation, glucose and sucrose were metabolised to produce acids, which was supported by a drop in pH (Figure 1C and Table S6), and an increase in the overall acidity of the medium. This total titratable acidity (TTA) (Figure 1D and Table S7) increased significantly over time (_p_ < 0.05). Set 3 (supplemented with 12 g/L of sucrose) showed the highest amount of TTA (13.65 \(\pm\) 0.04, \(p\) < 0.05) at 96 h compared to the other sets. A significant increase in TTA (up to 1% at the end of fermentation) was previously reported for milk kefir grains by [PERSON] et al. [44]. This was accompanied by a decrease in pH and an increase in lactic acid and other organic acids. The transformation of sugars to lactic acid and other organic acids by LAB is an important step during fermentation that influences the pH and acidity of fermented products, such as kimchi [45; 46]. Studies have also been carried out on milk kefir with or without supplementing soy [47]. pH in this study was found to steadily decrease during fermentation for up to 21 days. In a separate study on milk kefir by [PERSON] et al. [44], fresh milk was provided daily for 28 cycles and the pH was observed to gradually decrease to 3.4 during fermentation.  
In the present study using CWK grains, sucrose was utilized more efficiently compared to glucose during fermentation. To date, no studies have compared the efficiency of sucrose and glucose utilisation during CWK fermentation. [PERSON] and [PERSON] [48] reported that the efficiency of sucrose uptake by LAB and yeast depends on the genetic architecture of microorganisms. The changes in pH values in this study are similar to fermentation studies carried out on milk, where a steady drop in pH was reported after 96 h without any replenishment of the medium during fermentation. According to [PERSON] [49], the pH of milk kefir made using cow's milk with 5% kefir grains (fermented for 24 h between 18-24 \({}^{\circ}\)C) varied between 4.2 and 4.6. This was attributed to the production of acid and metabolites by the kefir microorganisms, which supports the results obtained in this study. Lower pH values were also observed by [PERSON] et al. [50], who attributed this to the metabolic activity of kefir grains.