## 1 Introduction  
Milk kefir and water kefir are traditionally made using different gelatinous particles that contain probiotics. These particles are called \"milk kefir grains\" and \"water kefir grains\", respectively [1]. Both types of kefirs can influence the composition and activity of gut microbiota [2]. However, people who are vegan, severely lactose intolerant, or allergic to dairy products cannot consume milk kefir. Hence, water kefir is fast becoming a cheaper and more popular alternative to developing milk kefir beverages. Water kefir can be made with fruit juices, coconut water, organic sugar water, or filtered water. Some of the widely predominant microorganisms in kefir are _Lactobacillus kefir, L. kefiranoaciens, L. paracasei, L. acidophilus, L. delbrueckii, and L. plantarum_; Acetobacter is also an integral component of kefir grains [3; 4; 5; 6; 7]. A detailed list of all of the species identified in kefir and beverages fermented with kefir has been reported by [8]. [PERSON] et al. [9] provided a comprehensive review of non-dairy kefir products that described their characteristics and potential health benefits. Kefir grains contain a complex mixture of lactic acid bacteria (LAB), acetic acid bacteria (AAB), and various yeasts embedded in a polysaccharide matrix [10; 11; 12]. The main polysaccharide speific to milk kefir grain is known as \"kefran\", while water kefir grains contain a dextran matrix [1]. Milk kefir grains and water kefir grains have different microbial composition. It was reported that water kefir grains do not grow in milk, mainly because _L. hilgardii_ does not assimilate or produce polysaccharides from lactose [13]. Milk kefir grains, on the other hand, can grow in sugar solutions and juices [14]. Kefir grains can grow on different substrates, such as sucrose, molasses, honey, fruit and vegetable juice, and water-soluble plant-based extracts, acting as a starter culture [15]. Kefir made from coconut water has shown a promising sensory profile in green coconut water kefir and sourdough bread [16]. Coconut water has grown rapidly in the functional beverages category as it is low in calories, rich in potassium, fat and cholesterol free, and has a good hydration quality [17]. Recently, a fermented beverage was developed using water kefir in a water-soluble coconut extract (WSCE) with inulin [18]. The WSCE obtained from the pulp (endosperm) of the coconut was an interesting option as it has a white colour like milk. The authors reported that the growth of the kefir grains was greater when the concentrations of inulin were close to 2.8 and 3.0% (_w/v_) and up to 0.26% (_w/v_) xanthan gum. Monitoring the chemical changes during the kefir fermentation process is important to improve the characteristics of kefir products. [PERSON] and [PERSON] [19] evaluated a mixture of pomegranate juice and whey as potential substrates to produce a novel probiotic beverage using kefir grains. They found that fermentation temperature and kefir grains inoculum influenced pH, acidity, lactose consumption, and organic acid formation over 32 h of fermentation. The growth of microorganisms in kefir results in enzymatic reactions involving invertase that can increase the content of reducing sugars, such as glucose and fructose [20]. Glycolytic enzymes can further act on these sugars to produce metabolites, such as ethanol, CO\({}_{2}\), and organic acids. Lactic, citric, malic, and succinic acids are also found in kefir beverages [20; 21]. The production of these acids depends on both the type of microorganisms in kefir and substrate used in the fermentation. For example, the use of glucose as a substrate, compared to lactose and sucrose, was found to increase LAB counts and, consequently, the production of lactic acid in soymik [22]. In general, the decrease in pH and resulting increase in acidity coincides with the fermentation progress; while, the microbial growth increases and soluble solids decrease [20].