Researchers and industry experts acknowledged the nutritional worth and health benefits of fermented dairy products like kefir and yogurt. Probiotics and bioactive substances found naturally in certain foods support immunity, metabolism, and digestion [1]. Antioxidant, antibacterial, and anti-inflammatory lactic acid bacteria (LAB), yeasts, and acetic acid bacteria are all found in kefir [2]. Dairy products can be made even healthier by adding plant-based bioactive ingredients. Fruits rich in polyphenols, for example, can increase nutritional value, strengthen antioxidant stability, and increase consumer demand [3, 4]. These developments reflect increasing scientific interest in combining traditional fermentation processes with plant-derived bioactive compounds to enhance the functional properties of dairy products [5].

The nutritional value and flavor of fermented dairy products are enhanced by plant chemicals called polyphenols. As naturally occurring antioxidants, they aid in oxidation prevention and shield cells from harm brought on by free radicals. Incorporating fruit polyphenols into dairy fermentation might enhance the end product’s flavor and color while also promoting the survival of beneficial bacteria [6]. The tropical fruit Syzygium cumini, popularly known as jamblang or Java plum, has potent antioxidant, antidiabetic, and anti-inflammatory properties, which are associated with its high contents of anthocyanins, flavonoids, and tannins [7]. In addition to providing health benefits, these substances interact with microorganisms and dairy proteins during fermentation to produce more stable and nutrient-dense products [8].

Interest in functional dairy foods is part of a wider trend of mixing plant-based bioactive compounds with animal products. This approach presents scientific and technical challenges, particularly in terms of stability, absorption, and interactions of phenolic compounds in complex foods matrix. Phenolic compounds can undergo changes when pH, temperature, or enzyme activity shifts during fermentation, which may affect their structure and reduce their effectiveness [5]. While polyphenols can support probiotic growth and function, excessive amounts can harm the microbes, so product development must find the right balance [9]. These considerations provide the rationale for evaluating the effects of S. cumini extract on the physicochemical, microbiological, and biochemical properties of goat milk kefir [10, 11].

Goat milk has a superior protein profile, smaller fat globules, and is more easily to digested than cow milk, thus it is frequently selected for fermentation [12, 13]. It has a smoother texture and helps the body absorb nutrients more quickly due to its low αs1-casein and higher quantities of short- and medium-chain fatty acids [14, 15]. LAB convert milk proteins into bioactive peptides that have antihypertensive, immune-stimulating, and antioxidant properties during fermentation [16]. Goat milk kefir is a useful model for researching the interaction between milk and fruit chemicals because these peptides promote gastrointestinal and metabolic health. Additionally, fermentation helps release advantageous peptides and fatty acids and makes micronutrients more accessible [17].

Goat milk kefir gains additional nutritional and sensory benefits when fruit polyphenols from S. cumini are added. The stability and functionality of the product are enhanced when polyphenols are combined with milk proteins. By preventing oxidation, these interactions preserve high antioxidant levels and increase shelf life [18]. Kefir’s texture and appearance are enhanced by anthocyanins that attach to milk proteins, thickening it and preserving its color [5]. These modifications have an impact on the molecular breakdown of proteins and fats, increasing the production of free amino acids and unsaturated fatty acids, which boosts nutritional value [19]. However, information remains limited regarding how different concentrations of S. cumini extract influence the quality characteristics of goat milk kefir.

The lack of official standards and regulations has made research on fruit-enriched fermented dairy products difficult. It is challenging to compare studies or create universal recipes due to variations in fruit concentration, fermenting techniques, and analysis [10, 11]. Instead of integrating data from several fields, most studies focus on specific factors, such as microbial viability or antioxidant activity. However, the effects of fruit polyphenols on kefir remain insufficiently understood. Goat milk kefir, which might have advantages of its own, has not gotten as much attention as cow’s milk [2]. Future investigations on the impacts of fruit phenolics should combine compositional analysis, microbiological testing, and statistical modelling.

Tropical fruits like mulberry, jamblang, and jaboticaba have been added to dairy drinks in recent studies. According to de Oliveira Galdino et al. [2], adding tropical fruits to goat milk kefir promotes microorganisms, aids in fermentation, and stabilizes pigments, all of which enhance antioxidant levels and flavor. Additionally, Kandhan et al. [6] showed that amla and jamun extracts improve ice cream’s antidiabetic and antioxidant qualities, indicating that these fruits may be useful in functional dairy products. A relatively recent processing technique called thermosonication helps preserve phenolic compounds and their advantages during dairy fermentation, increasing the stability of these compounds [11]. Utilizing agricultural byproducts as sources of polyphenols promotes by reducing food loss and waste the food industry with both practical and financial advantages [1, 19].

Despite these developments, little is known about how different concentrations of S. cumini affect the microbiological, physicochemical, and bioactive characteristics of goat milk kefir. The aim of this study was to identify the ideal polyphenol concentration that maintains a balanced microbial composition, offers health benefits, and retains flavor and texture. It is essential to comprehend how phenolic compounds, bacteria, and milk components interact to make bioactive chemicals in order to create goods that are dependable, simple to manufacture, and satisfy consumer expectations as well as nutritional criteria [8, 10].

This study investigates how the physical, chemical, microbiological, and bioactive characteristics of goat milk kefir are affected by the addition of S. cumini extract. The objective is to measure the antioxidant activity, microbial growth, amino acid and fatty acid content, and structure of kefir in order to determine the best method to enhance its nutrition and function. Multivariate analysis is used in the study to comprehend S. cumini’s overall effects, particularly how its polyphenols interact with the bacteria in kefir. The results are intended to increase understanding of fruit and milk fermentation, which will aid in the creation of regional dairy products that promote global nutrition, sustainability, and health objectives.

S. cumini fruit extract showed a compositional profile typical of tropical fruits, with high moisture content as the dominant fraction, which may influence texture, shelf life, and suitability for fermentation. Its low lipid content and moderate protein level suggest that the extract serves mainly as a carbohydrate-rich substrate, potentially supporting microbial growth during kefir fermentation. The total soluble solids value further indicates the presence of fermentable sugars required for microbial metabolism. The acidic pH (4.10) and titratable acidity reflect the fruit’s natural organic acid composition and may contribute to fermentation stability. The total phenolic content (149 ± 2 mg GAE/100 g extract) confirms the presence of bioactive compounds, although at lower levels than those reported for other fractions such as seeds or peels [25, 27]. This is consistent with the relatively high EC50 value, indicating moderate antioxidant capacity. Overall, the extract may serve as a natural bioactive ingredient, although specific phenolic contributions remain unclear.

The present study showed that S. cumini extract significantly affected selected physicochemical properties of goat milk kefir, particularly protein content and colour attributes (Table 1). The increase in protein content in T2 and T3 suggests that supplementation modified the compositional characteristics of the kefir matrix. This increase may be related to the contribution of soluble solids from the extract and to possible interactions between milk proteins and phenolic compounds during fermentation. In dairy systems, such interactions may alter matrix organisation and protein distribution, although the present study did not directly evaluate binding processes [34]. Therefore, this explanation should be regarded as a plausible interpretation rather than a confirmed mechanism.

Color parameters (L*, a*, and b*) were also strongly influenced by treatment, indicating that S. cumini extract markedly altered the visual properties of kefir. This effect is consistent with the natural pigmentation of S. cumini, which is rich in anthocyanin-containing phenolic compounds [35, 36]. The observed changes in colour values therefore, likely reflect incorporation of these pigments into the fermented dairy matrix. By contrast, fat content, pH, and viscosity were not significantly affected, despite numerical variation among treatments. This indicates that S. cumini supplementation did not consistently affect these parameters under the conditions tested. Although viscosity tended to increase at higher extract concentrations, particularly in T3, the lack of statistical significance suggests that this trend should be interpreted cautiously. Similarly, the numerical decrease in pH with increasing supplementation does not support a definitive conclusion regarding enhanced acidification or fermentation intensity. Overall, the results show that the main effects of S. cumini extract were observed in protein content and colour-related properties, while other physicochemical changes remained limited or variable.

The significant increase in TEAC values with more S. cumini supplementation shows that the extract enhanced goat milk kefir’s antioxidant capacity. This matches the known effects of S. cumini’s phenolic compounds, particularly its anthocyanin components, which enhance radical-scavenging activity [37]. The steady TEAC rise from the control to T3 shows a dose-dependent effect. The higher antioxidant capacity in the 1.5% and 2% treatments may come directly from added phenolic compounds in the extract or from changes during fermentation. In fermented dairy systems, plant phenolics can be modified by microbes or their interaction with milk [38]. These changes may alter their stability and antioxidant activity. The dairy matrix may also affect the extent of antioxidant activity retained or expressed by influencing the accessibility of the compounds [39]. Still, this study did not characterize individual phenolic compounds or identify specific antioxidant metabolites after fermentation. S. cumini supplementation improved the antioxidant potential of goat milk kefir. The biochemical basis should be clarified in future studies.

The microbiological results indicate that S. cumini supplementation supported the growth of kefir-associated microorganisms, particularly LAB. The significant increase in LAB counts from the control to T3 suggests that the extract did not inhibit the bacterial population and may have provided conditions favorable for bacterial maintenance during fermentation. This response may be related to the presence of fermentable components or bioactive compounds in the extract that were compatible with the kefir microbiota. In fermented dairy systems, LAB are essential for acid production, matrix development, and overall microbial stability; therefore, the higher LAB counts observed in the supplemented treatments may reflect an active fermentation environment [40]. Yeast and mold counts also showed significant overall differences, although post hoc analysis indicated that the supplemented treatments did not differ significantly from one another. This pattern suggests that the main distinction was between the control and the extract-containing kefirs, while increasing the extract concentration beyond 1% did not yield a clearer separation among the supplemented groups. Because yeasts are important members of the kefir consortium and contribute to carbon dioxide, ethanol, and flavor-related metabolite production, their maintenance at relatively high levels indicates that the symbiotic microbial balance of kefir was preserved [41]. Taken together, these findings suggest that S. cumini extract was microbiologically compatible with goat milk kefir fermentation and may have contributed to a more favorable environment for microbial persistence. However, since the present study did not directly evaluate microbial metabolism or substrate utilization, the mechanisms underlying these changes remain to be clarified.

S. cumini supplementation changed the amino acid profile of goat milk kefir (Table 3). Both non-essential and essential amino acids increased, raising free amino acid availability in the matrix. This likely results from fermentation. The kefir microbiota breaks down milk proteins into peptides and free amino acids [42]. As extract levels rise, amino acid concentrations increase—especially Glu + Gln and Lys—indicating that the fermentation environment is changing. Glutamic acid and glutamine are key to protein metabolism and flavor. Lysine is vital for nutrition [43]. Higher levels in T2 and T3 suggest supplementation ≥ 1.5% causes greater biochemical changes. T1 showed intermediate values, pointing to a gradual response. In fermented dairy products, amino acid changes result from interactions between microbes and matrix components [44]. Plant compounds in S. cumini extract likely affect microbial metabolism or substrate availability, and thus protein breakdown [45]. Since we did not directly measure proteolysis or enzymes, this interpretation is inferential. S. cumini supplementation increased free amino acids, likely improving kefir's nutritional profile. Further studies should clarify the roles of microbial proteolysis and phenolic-milk protein interactions during fermentation.

The observed changes in the fatty acid profile (Table 4) indicate that S. cumini selectively influenced specific fatty acids in goat milk kefir, rather than all lipids equally. For example, significant increases were noted in certain saturated fatty acids (C4:0, C12:0, C14:0, C16:0, C18:0) and select unsaturated fatty acids (C15:1n-9, C16:1n-7, C18:2n-6, C18:2c9t11), supporting that only these particular fatty acids, and not the broader lipid composition, were affected during fermentation. The increase in fatty acids such as C16:0 and C18:2n-6, and the progressive rise in C15:1n-9 and C18:2c9t11 (a Conjugated Linoleic Acid (CLA)-related fraction), may be linked to changes in lipid metabolism within the kefir matrix. Microbial activity in fermented dairy systems, such as lipolysis and fatty acid transformation, can influence fatty acid composition [46]. The higher values observed in T2 and T3 may reflect a shift in the balance between lipid hydrolysis and microbial utilization. The presence of plant-derived compounds in S. cumini extract may have contributed to these changes [47]. However, supplementation did not affect all fatty acids equally. Specifically, no significant differences were observed in C17:0, C20:4n-6, C18:3n-3, and C18:2c10t12, indicating they are unaffected. This pattern indicates that S. cumini extract selectively influenced only certain lipid fractions, leaving others—such as those listed—unchanged. This supports the notion of targeted, rather than broad, alterations during fermentation. It is also important to note that the present study did not directly assess lipolytic activity or enzymatic mechanisms. The changes in fatty acid composition may be related to microbial metabolism and matrix interactions, but these explanations remain interpretative. Overall, the results indicate that S. cumini supplementation influenced certain fatty acids, especially at higher extract concentrations. Others remained relatively stable.

S. cumini supplementation changed the kefir matrix microstructure. The control sample had a more open, less compact structure. In contrast, the supplemented treatments, especially T2 and T3, formed a denser, more homogeneous network [48]. Adding S. cumini extract changed the matrix arrangement during fermentation. However, because this study did not directly measure molecular-level interactions, these interpretations should be considered tentative and speculative rather than definitive conclusions. The supplemented samples show a more uniform distribution of microstructural elements, indicating improved matrix homogeneity compared with the control [33]. This matches the numerical viscosity trends. However, these differences were not statistically significant. While the microstructural data show a shift toward a more compact network, interpret the implications for rheological properties with caution [48]. The images were obtained under consistent magnification, enabling direct comparison of structural features across treatments. The lack of distinct layered structures indicates that the observed differences result mainly from changes in density and spatial distribution, rather than from the formation of new structural phases [49]. Overall, the microstructural analysis supports the conclusion that S. cumini supplementation was associated with a more compact and homogeneous kefir matrix at higher concentrations [33]. Nevertheless, further studies are needed to directly assess the mechanisms underlying these changes. These mechanisms may include protein–polyphenol interactions and their impact on gel formation [50].

PCA results provide an overview of the relationships among variables and the distribution of treatments. These findings should be regarded as exploratory rather than confirmatory. Principal component 1 (PC1) accounts for 86.0% of the variance, indicating that most variability follows a common pattern across treatments. Principal component 2 (PC2) contributes an additional 9.7%. The control group is separated from T2 and T3 along PC1, suggesting that S. cumini supplementation resulted in coordinated changes in the measured parameters. Protein, TEAC, viscosity, lactic acid, and LAB are associated with T2 and T3, which exhibit higher values for these attributes. In contrast, pH is positioned on the opposite side, indicating an inverse relationship [50]. This observation is consistent with earlier results, in which higher protein content and antioxidant capacity corresponded to lower pH. However, PCA does not establish causality or identify optimal treatments. The grouping of T2 and T3 indicates profile similarity but does not define an ideal formulation. Furthermore, the observed associations among variables reflect statistical co-variation rather than direct mechanistic relationships. In summary, the PCA demonstrates that S. cumini supplementation altered several attributes of goat milk kefir, particularly at higher concentrations. These findings also underscore the importance of cautious interpretation of multivariate patterns.

All of these results demonstrate that adding S. cumini extract to goat milk kefir promotes a variety of biochemical, structural, and microbiological improvements, demonstrating its potential as an excellent functional dairy formulation. S. cumini extract-enriched kefir is an example of how ancient fermentation methods can be combined with contemporary functional food innovation by promoting microbial metabolism, strengthening protein and lipid matrix, and optimizing compositional features through sophisticated statistical modeling.