## 1 Introduction  
Fruit juices commonly contain high levels of monosaccharides (mainly glucose and fructose), which can contribute significantly to a high caloric intake for consumers. An effective and cost-efficient strategy to reduce this sugar content involves fermenting the juices using either single cultures of probioticic strains [1, 2, 3, 4, 5, 6] or kefir grains [7, 8, 9]. This process not only reduces the sugar content but also yields a potentially probiotic fermented beverage with a pleasant aromatic profile. Additionally, this approach may support the valorization of fruits with a low commercial value (such as undersized fruit) or those resulting from surplus production. In recent years, the development of fruit-based kefir beverages [7; 8; 9; 10; 11; 12; 13] has been explored as an alternative to traditional milk kefir, which contains lactose and may not be suitable for lactose-intolerant individuals. These beverages have been shown to contain high levels of lactic acid bacteria (LAB) and yeasts, often exceeding \(10^{6}\) CFU/mL, the minimum threshold required to confer probiotic benefits to the consumer. Furthermore, fruit-based kefir beverages are rich in volatile compounds, derived both from the original fruit juice and from microbial activity during fermentation by organisms present in kefir grains [9; 10; 11; 12; 13] or kefir-based inocula [7; 8]. Several studies have shown that both the shaking rate [11; 14; 15] and the amount of kefir grains [11; 14; 16; 17; 18] can influence the nutrient consumption, kefir grain growth, viable cell production, and synthesis of fermentation metabolites during the fermentation of various dairy and non-dairy substrates. However, the results have been controversial, as the optimal values for the shaking rate and proportions of milk kefir grains varied for different types of beverages. For example, the growth rate of kefir grains was maximal under static conditions in milk [14] or at 125 rpm in skim milk [15]. Within an inoculum range of 1% to 5% (\(w/v\)), the highest kefir grain growth rate in milk was observed at an inoculation level of 1.86% [14]. In contrast, in whey-based media inoculated with 2.5%, 5.0%, and 7.5% (\(w/v\)) kefir grain concentrations, the greatest kefir grain biomass production was achieved at 5% (\(w/v\)) [17]. Furthermore, in an apple juice and whey-based fermented beverage, increasing the kefir grain concentration from 2% to 8% (\(w/v\)) led to higher acidity, and kefiran and lactic acid production, as well as elevated counts of lactobacilli and yeasts [16]. Conversely, in a whey-date beverage, the highest viability of LAB and yeasts was observed at the lowest inoculum level, within a kefir grain concentration range of 2% to 5% (\(w/v\)) [18]. Additionally, in kiwi-based kefir-like beverages, nutrient consumption, microbial growth, and the synthesis of fermentation metabolites were strongly influenced by both the shaking rate (25 to 147 rpm) and kefir grain concentration (1.32% to 4.68% \(w/v\)) [11]. Based on these observations, it is reasonable to assume that both the shaking rate and inoculum (kefir grains) concentration could also influence the aroma characteristics of fermented drinks. However, studies investigating this impact are limited. The effects of the shaking rates and inoculum proportions on the chemical and microbiological compositions of fermented kiwifruit-based beverages were previously investigated by our research team [11]. Building on that work, the present study aimed to analyze the volatile composition of these beverages in order to provide a more comprehensive characterization. Based on the findings from both the previous and the current study, kiwifruit-based beverages exhibiting a broad aromatic profile, high probiotic cell counts, and low concentrations of sugars and ethanol were identified and selected as promising candidates for probiotic kiwi-based kefir-like products.