The increasing global demand for functional foods reflects consumers’ growing awareness of the relationship between nutrition and health. Probiotic beverages such as kefir are recognized for their rich composition of nutrients, bioactive compounds and beneficial microorganisms that support gut health, enhance immune response and contribute to overall well-being. Regular consumption of kefir has been associated with antimicrobial activity, improved gastrointestinal balance, anticarcinogenic effects, regulation of serum glucose and cholesterol and better tolerance for lactose. These attributes position kefir as a promising functional food, offering additional health benefits for diverse consumers, including vegans and individuals intolerant to dairy products (Subbalakshmi et al., 2024). Kefir, a fermented milk beverage produced through the symbiotic action of lactic acid bacteria and yeasts, possesses numerous functional benefits, including antimicrobial activity, improved digestibility and antioxidant potential (Lubna et al., 2023).
       
However, the perishable nature of liquid kefir limits its distribution and shelf life. Converting kefir into powder form can enhance product stability, ease of storage and portability while maintaining its functional value (He et al., 2023). Spray-dried probiotic milk powders have been shown to improve nutrient utilization and health performance in animal models, demonstrating the viability of probiotic formulations processed into powdered form (Singh et al., 2019). Furthermore, integrating natural plant extracts into fermented dairy powders is a promising approach to improve their functional, antioxidant and sensory characteristics (Nguyen et al., 2024).
       
Torch ginger (Etlingera elatior), locally known as kecombrang, is an aromatic herb widely used in Indonesian cuisine and traditional medicine. Its flower and stem extracts have been reported to contain polyphenols, flavonoids and essential oils exhibiting strong antioxidant activity (Nuryanti et al., 2021; Naufalin et al., 2024). The incorporation of kecombrang tea as a natural antioxidant in functional dairy products could therefore provide added nutritional and health benefits. Recent studies have also demonstrated the potential of kecombrang in various food formulations, such as functional beverages, yogurt and soft cheese (Naufalin et al., 2023; Arkan et al., 2024).
       
In the food industry, optimizing formulation variables is essential to ensure both functionality and consumer acceptance. The Response Surface Methodology (RSM) provides a powerful statistical approach to determine the optimal combination of factors by analyzing interactions among multiple independent variables. This approach has been widely used in food product development to optimize formulation, processing and sensory outcomes (Mendes et al., 2022; He et al., 2023).
       
This study focuses on optimizing the concentration and type of kecombrang tea in kefir powder production using RSM to achieve an ideal balance of physicochemical, antioxidant and sensory properties. Specifically, three variations of kecombrang tea (original, lemongrass and lemon) and concentrations ranging from 5% to 10% were evaluated. The results are expected to provide valuable insights into the utilization of local Indonesian botanical resources in functional dairy innovation, supporting the diversification of health-oriented products and strengthening the national functional food industry.

Fresh cow milk was obtained from a local dairy producer in Banyumas, Central Java, Indonesia. Three types of kecombrang (Etlingera elatior) tea original, lemongrass and lemon were prepared according to the method described by Naufalin et al., (2024). Skim milk powder and sucrose were purchased from Merck (Germany). The kefir starter culture, consisting of lactic acid bacteria (Lactobacillus kefiri and Lactococcus lactis) and yeasts (Saccharomyces kefir), was provided by the Department of Food Technology, Universitas Jenderal Soedirman. All chemicals used were of analytical grade. Kecombrang flowers were cleaned, sliced and dried at 50°C for 12 h. Dried petals were ground into fine powder and blended with lemongrass or lemon peel (depending on treatment). The mixture was brewed in hot water (90°C) for 10 min, filtered and spray-dried (Naufalin et al., 2024) to produce fine kecombrang tea powder. The powders were stored in airtight containers at room temperature until use.
 
Making powdered kecombrang tea kefir
 
The main ingredient used was fresh cow’s milk obtained from the Experimental Farm of Faculty of Animal Husbandry, Jenderal Soedirman University, Purwokerto, Indonesia. The process began with pasteurizing cow’s milk at 71-72°C for 15 seconds, followed by cooling to room temperature (30°C). Kefir grains (5%) and kecombrang tea are added and the mixture was incubated at 30°C for 24 hours. The fermented kefir was then filtered, to remove the kefir grains and then dried using the foam mat drying method at a temperature of 60°C for 8 hours. The resulting dry sheets are then ground into powder using a chopper and sieved through a 60-mesh sieve to produce powdered kecombrang tea kefir.
 
Research design
 
The experiment was arranged in  a completely randomized design (CRD). The method was used  response surface methodology (RSM) with an experimental design using design expert software version 13 and optimization design uses optimal custom design. The optimization process consists of several stages, namely design, response model analysis, optimization, verification and validation and characterization.
       
The factors in this study included tea concentration as a numerical factor and tea type as a categorical factor. For the numerical factor ranged from 5% to 10%, with the lower and upper limits determined based on reliminary experiments and previous studies reporting similar trends (Maharani et al., 2020). The categorical factor onsisted of three levels of kecombrang tea type, namely original kecombrang tea, lemongrass kecombrang tea and lemon kecombrang tea. Optimization was performed based on antioxidant activity as the response variable, which was assigned a high level of importance (***). The optimum treatment was identified based on the highest desirability value. Verification and validation conducted by preparing the optimum formulation in triplicate, followed by re-measurement of antioxidant activity. Aftezr that, characterization was carried out to determine the typical characteristics sample kefir powder resulting from an optimized formula.
 
Physical characteristics
 
1. Viscosity analysis (Caesaron and Nintyas, 2015): The viscometer was turned on and the spindle was attached. Prepare a 100 ml sample of rehydrated kefir powder, dip the spindle to the specified limit and wait for the reading to stabilize.

2. Yield analysis (AOAC, 1995): Yield kefir powder calculated based on the ratio of the final weight of kefir powder after drying to the initial weight of liquid kefir before drying.The yield is calculated using the following formula:

 

3. Rehydration analysis (Munira et al., 2020): Rehydrate kefir powder using water at 40°C in a 1:3 ratio. Sample test was stirred and calculated the time for complete/even dissolution.

4. Syneresis analysis (Agustin and Putri, 2014): 10 ml sample of rehydrated powdered kefir was centrifuged at a speed of 6000 rpm for 15 minutes. The syneresis percentage is calculated using the following formula:

 

5. Color analysis (Wibawati and Rinawidiastuti, 2018): The powdered kefir sample is placed in a flat container, then the color reader is attached to the sample surface. The reading button is set to L* (lightness), a* (redness) and b* (yellowness), then press the target button above sample and wait for the reading to complete.
 
Chemical characteristics
 
Antioxidant activity analysis (Sheikh et al., 2009).
 
Sample test is obtained by mixing 1 gram kefir powder in 9 ml of methanol which is then centrifuged at 2500 rpm for 10 minutes until a precipitate forms. 2 ml of test sample is placed in a tube reaction which already covered with aluminum foil and add 2 ml of DPPH solution and then incubated it for 30 minutes. The decrease in absorbance of the sample and blank was measured using a UV-Vis spectrophotometer at a wavelength of 517 nm (As). Antioxidant activity was calculated based on the percentage of inhibition using the following formula:

pH Analysis (AOAC, 1995)
 
Calibrate the pH meter first with buffer pH 4 and pH 7. Kefir powder is rehydrated with distilled water and 20 ml is taken in a beaker glass then the pH meter electrode is dipped and wait until the pH reading of the sample is obtained stable.
 
Total titrated acid (TTA) (Suhaeni, 2018)
 
Mix 10 grams of kefir powder in 100 ml of distilled water. Sample was filtered with filter paper and 20 ml of filtrate was taken then add 2 drops of 1% phenolphthalein (PP) indicator.The sample was titrated using 0.1 N NaOH until it turned a constant pink color. The percentage of TTA was calculated using the following formula:

Water content (Nielsen, 2017)
 
1 gram of powdered kefir sample is placed in a halogen moisture analyser and wait until the water content percentage reading is complete.
 
Ash content analysis (AOAC, 2005)
 
A cup containing 2 grams of powdered kefir sample is placed in the furnace. The sample was heated at a temperature of 600! for 3 hours, then the resulting ash was weighed. The percentage of ash content was calculated by the formula is as follows:

 
Fat content analysis (AOAC, 2005)
 
Powdered kefir sample weighed 2 grams (Weight A) in filter paper and oven for 4 hours. The sample and filter paper weight was measured after being oven-dried (Weight B). Soxhlet reflux was carried out with 500 ml of diethyl ether solvent for 5 hours. The sample was oven-baked again for 1 hour and weighed (Weight C). The percentage of fat content was calculated by the formula is as follows:

 
Protein content analysis (AOAC, 1990)
 
Weigh a sample of 0.1-0.2 grams in a Kjeldahl flask, add 0.7-0.8 grams of protein catalyst and 3 ml of H₂SO₄ then digest until a clear green solution is formed. Distillate the sample for 10 minutes by adding NaOH thiosulfate or NaOH 40% 17.5 ml and sufficient distilled water. The distillate is collected in a 100 ml Erlenmeyer flask containing 5 ml of 0.1 N boric acid solution and 5 drops of MR indicator. Titrate the distillate with 0.02 N HCl until it turns pink constant. The percentage of protein content is calculated by the formula is as follows:

 

Protein content (%) = %N ´ conversion factor (6.25)

 
Carbohydrate content analysis (AOAC, 2005)
 
Percentage of carbohydrate content by difference method counted with the formula is as follows:
 

Carbohydrate content (%) = 100 % - % (Water + ash + fat + protein)

 
Microbiological characteristics
 
Total lactic acid bacteria (LAB) analysis (Hidayat et al., 2013).
 
Diluted sample kefir powder in solution NaCl physiological sterile with a ratio of 1:9 of dilution 10¹ to 10⁸. Cupping is carried out by taking 200 µl sample of the final dilution and add 8 ml of MRS agar liquid. Incubate at 37°C for 48 hours. Total Lactic acid bacteria colonies of the sample counted with the formula is as follows:

 
Sensory evaluation
 
Sensory analysis (Khoiria and Bahar, 2023)
 
A comparison was made between the assessments of 35 semi-trained panelists on control powdered kefir and optimum treatment of kefir powder includes color parameters, distinctive aroma of kefir, texture, distinctive taste of kefir and overall (preferences). The data was analyzed for normality at a significance level of 5%, then continued with the T test if the data was normal or the Mann Whitney test if the data was not normal.
 
Statistical analysis

All data were analyzed using Analysis of Variance (ANOVA) and differences among means were determined using duncan’s multiple range test (DMRT) at a 95% confidence level (p<0.05). Model adequacy, R² and lack-of-fit tests were used to evaluate the fitness of the regression models generated by RSM.

Model fitting and statistical analysis
 
The results of the optimization analysis using Response Surface Methodology (RSM) showed that both kecombrang tea concentration and tea type significantly influenced the antioxidant activity of kefir powder (p <0.05). The regression model generated from the Central Composite Design (CCD) exhibited high adequacy, with coefficients of determination (R²) ranging from 0.93 to 0.97 for all response variables. The lack-of-fit test was not significant (p>0.05), indicating that the model accurately represented the experimental data. These results confirm that RSM was an effective tool for optimizing the formulation of kefir powder containing kecombrang tea.

Effect of kecombrang tea concentration on antioxidant activity
 
The concentration of kecombrang tea added to powdered kefir has a positive effect on antioxidant activity (Fig 1). These results indicate that more higher the concentration of kecombrang tea added, more greater the antioxidant activity of kefir powder. Naufalin (2023) stated that the inhibition of free radicals by the given antioxidants will increase with increasing concentration, which will also have an impact on the increasing number of antioxidant compounds that donate their electrons to free radicals in the body, resulting in an increasing number of unreactive or unstable free radical molecules.


 
The effect of tea type on antioxidant activity
 
On average, the three types of kecombrang tea added to powdered kefir had a significant effect on the average antioxidant activity produced. The highest antioxidant activity was produced by lemongrass kecombrang tea (49.98±3.44%), followed by original kecombrang tea (48.27±3.04%) and the lowest antioxidant activity was produced by lemon kecombrang tea (47.95±1.57%). The main antioxidant content of lemongrass is citral with a complete composition contained in lemongrass essential oil including citronellal 32-45%, gereniol 12-18%, citronellol 11-15%, geranyl acetate 3-8%, citronellyl acetate 2-4%, citral, kavicol, augenol, elemol, cadonone, cadinene, vanillin, limonene and camphene (Rasyid et al., 2017).
 
Optimization process
 
The concentration and type of tea factors was observed an interaction between treatments on the antioxidant activity response of powdered kefir (Table 1).


       
The best model of the process of making kefir powder with factors of tea concentration (A) and tea type (B) on the response of antioxidant activity can be seen from the following mathematical model equation:
 

Y = 48.77 + 3.26(A) + 0.6333(B1) + 1.57(B2)

 
Information:
Y= Antioxidant activity (%).
A= Tea concentration.
B= Type of tea.
       
The equation was observed that the tea concentration factor has a positive constant A so that the tea concentration factor is directly proportional to the antioxidant activity as a response variable. If the value of A (tea concentration) increases by one unit, the value of Y (antioxidant activity) becomes Y = 48.77 + 3.26. The tea type factor also was observed a positive constant B so that the tea type factor is also directly proportional to the antioxidant activity as a response variable. If the value of B (tea type) increases by one unit, the value of Y (antioxidant activity) becomes Y = 48.77 + 0.6333 + 1.57. Based on the fit summary analysis, the linear model has a p-value (0.0015) <0.05 so that the model has met the requirements for ANOVA testing (Table 2).


       
The obtained p-value of the model is smaller than 0.05 (0.0015) so that the concentration and type of tea factors have a significant influence on antioxidant activity with a 95% confidence level. Lack of fit of 0.4611 indicates an insignificant value against pure errors related to research data. The R² value of 0.6823 indicates that 68.23% of the diversity of antioxidant activity responses is influenced by the diversity of concentration and type of tea factors. The difference between the adjusted R² and predicted R² values of 0.1639 (<0.2) indicates that the model is appropriate and the antioxidant activity value is a reasonable value. The AP value of 8.9913 (>4) indicates that the prediction model can be used to organize the design space determined by the RSM method with the optimal custom design model. The coefficient of diversity or coefficient of variance (CV%) value of the data is 3.85, meaning the data is homogeneous and the amount of deviant data can still be tolerated. A VIF value of 1.00 indicates that there is a relationship between factor A (tea concentration) and factor B (tea type) on antioxidant activity.
       
The optimization treatment according to design expert software version 13 is lemongrass kecombrang tea with a concentration of 10% which will produce an antioxidant activity of 53.59% with a desirability value of 0.771. The optimum formula was then verified and validated with the results presented in Table 3.


       
The average antioxidant activity of the verified samples was 52.39±1.87%, which was close to the predicted value (53.5924%) and was between the lowest prediction interval (50.72%) and the highest prediction interval (56.47%). Based on this verification, 10% concentration of lemongrass kecombrang tea was valid as the most optimal treatment.
 
Product characterization
 
The physicochemical characteristics of the samples were compared with reference standards CODEX Standard for Fermented Milk (CODEX STAN 234-2003) on kefir and SNI 01-2970-2006 on full- fat milk powder (Table 4).


 
pH
 
The pH values of the control kefir powder and the optimum formula kefir powder were 3.32±0.13 and 3.15±0.08. These pH values   are in accordance with the CODEX Standard for Fermented Milk (CODEX STAN 234-2003) reference standard, where the quality requirement for kefir is a pH below 4.6. Kefir has a distinctive sour taste due to the production of lactic acid by lactic acid bacteria (LAB) during the fermentation process. During kefir fermentation, LAB convert lactose into lactic acid, leading to a decrease in pH and the development of kefir’s characteristic sour taste. Similar pH ranges have been reported in cow’s milk kefir fermented using different starter cultures, where pH values typically fall between 3.0 and 4.5 (Sinurat et al., 2018). The slightly lower pH in the optimum formula may also be influenced by the addition of kecombrang tea, which contains organic acids and phenolic compounds that can contribute to acidity and potentially stimulate LAB activity. Comparable findings were reported by Maharani et al., (2020), who observed a decrease in pH in fermented dairy products supplemented with tea infusion, attributed to the interaction between bioactive compounds and microbial fermentation.
 
Viscosity
 
The viscosity values of the control kefir powder and optimum formula kefir powder were 58.50±3.65 cP and 61.71±4.21 cP. The slightly higher viscosity observed in the optimum formulation may be associated with differences in formulation composition, particularly the addition of liquid ingredients, which can influence the total solids content and rheological properties of fermented dairy products. Variations in viscosity of kefir powder have been reported previously and are influenced by formulation factors, filler concentration and interactions between milk components and added ingredients (Rizqiati et al., 2016; Nielsen, 2017). The viscosity measurement method itself may also contribute to variability in results depending on spindle type and rotational speed used during analysis (Caesaron and Nintyas, 2015).
 
Yield
 
The yield of control powdered kefir and optimum formula powdered kefir was 28.43±1.07% and 29.48±1.93%, respectively. The yield of powdered kefir is closely related to the total solids content of the product, as a higher proportion of solids contributes to greater powder recovery after the drying process. The slightly higher yield observed in the optimum formulation may therefore be attributed to its higher total solids content, which has been reported as a key factor influencing yield in dried fermented dairy products (Rizqiati et al., 2016; Nielsen, 2017).
 
Rehydration
 
The rehydration time of the control powdered kefir and the optimum formula powdered kefir was 25.26±0.39 seconds and 23.42±0.46 seconds, respectively. Faster rehydration was observed in the optimum formulation, indicating improved reconstitution properties. Rehydration behavior of powdered products is influenced by moisture content, particle structure and drying characteristics, where lower moisture content generally facilitates faster water absorption during reconstitution (Zainuddin, 2016; Nielsen, 2017). The addition of lemongrass kecombrang tea in the optimum formulation may have contributed to lower moisture content, thereby promoting easier water evaporation during drying and resulting in a shorter rehydration time.
 
Syneresis
 
The syneresis values of the control powdered kefir and the optimum formula powdered kefir were 87.15±1.12% and 86.06±1.03%, respectively. The slightly lower syneresis observed in the optimum formulation indicates improved water-holding capacity. Syneresis in fermented dairy products is influenced by factors such as pH, protein network structure and water-binding capacity, which determine the ability of the matrix to retain water (Hidayat et al., 2013; Sinurat et al., 2018). The incorporation of lemongrass kecombrang tea in the optimum formulation may have contributed to reduced syneresis by enhancing interactions between milk proteins and added bioactive compounds, thereby improving water retention in the kefir powder.
 
Color
 
Color characteristics of powdered kefir were evaluated using the CIE Lab* color system, where the L* value represents lightness ranging from 0 (black) to 100 (white), the a* value indicates the color spectrum from green (negative) to red (positive) and the b* value describes the spectrum from blue (negative) to yellow (positive). This color measurement system is widely used in food products to objectively evaluate color changes associated with formulation and processing conditions (Nielsen, 2017).
 
Lightness (L*)
 
The lightness (L*) values of the control powdered kefir and the optimum formula powdered kefir were 71.07±2.64 and 67.51±3.37, respectively. Both formulations exhibited relatively high L* values, indicating a color that tended toward white. The slightly lower lightness observed in the optimum formulation suggests a darker appearance compared to the control. This change in color may be associated with the addition of lemongrass kecombrang tea, which contains natural pigments that can influence the color attributes of food products. The incorporation of plant-based ingredients has been reported to affect the lightness of fermented dairy products due to the presence of bioactive compounds such as anthocyanins and other phenolic pigments (Safitri et al., 2018; Maharani et al., 2020).
 
Redness (a*)
 
The redness (a*) values of the control powdered kefir and the optimum formula powdered kefir were -1.45±2.12 and 0.01±3.04, respectively. The control sample exhibited a slightly greenish hue, whereas the optimum formulation showed a shift toward neutral to slightly reddish tones. This change in a* value may be associated with the addition of lemongrass kecombrang tea, which contains natural pigments capable of influencing the redness of food products. The presence of plant-derived bioactive compounds, including anthocyanins and other phenolic substances, has been reported to affect the color attributes of fermented dairy and plant-fortified products (Safitri et al., 2018; Maharani et al., 2020).
 
Yellowness (b*)
 
The yellowness (b*) values of the control powdered kefir and the optimum formula powdered kefir were 8.20±2.03 and 8.73±0.63, respectively. The slightly higher b* value observed in the optimum formulation indicates a more yellowish appearance compared to the control. This difference may be associated with the addition of lemongrass kecombrang tea, which contains natural pigments and phenolic compounds that can contribute to yellow color development in food products. The incorporation of plant-based ingredients has been reported to influence the yellowness of fermented dairy products due to the presence of flavonoids and other non-anthocyanin pigments (Safitri et al., 2018; Maharani et al., 2020).
 
Total titratable acid (TTA)
 
The Total Titratable Acidity (TTA) values of the control powdered kefir and the optimum formula powdered kefir were 1.38±0.46% and 1.56±0.43%, respectively. Both values met the requirements of the CODEX Standard for Fermented Milk (CODEX STAN 234-2003), which specifies a minimum acidity of 0.6% for kefir. The TTA value reflects the amount of organic acids produced during fermentation as a result of lactic acid bacteria activity, which is a characteristic feature of fermented dairy products. Similar acidity levels have been commonly reported in kefir and yogurt-based products and are associated with fermentation intensity and microbial metabolism (Sinurat et al., 2018; Suhaeni, 2018).
 
Water content
 
The water content of the control powdered kefir and the optimum formula powdered kefir was 3.94±1.87% and 3.89±1.71%, respectively. Both values complied with the Indonesian National Standard for full-fat milk powder (SNI 01-2970-2006), which specifies a maximum moisture content of 5%. The low moisture content observed in both samples can be attributed to the efficiency of the drying process applied, which effectively removed free water from the fermented kefir matrix. Low moisture content is a desirable characteristic in powdered fermented dairy products, as it contributes to improved shelf stability and reduced susceptibility to microbial growth (Nielsen, 2017; Rizqiati et al., 2016).
 
Ash content
 
The ash content of the control powdered kefir and the optimum formula powdered kefir was 1.08±0.03% and 1.15±0.09%, respectively. Ash content reflects the total mineral content of a food product and is influenced by the mineral composition of the raw materials used. A higher ash value generally indicates a greater concentration of minerals present in the product. Similar ash content ranges have been reported for fermented dairy and plant-fortified products, where variations are associated with differences in formulation and ingredient composition (AOAC, 2005; Nielsen, 2017).
 
Fat content
 
The fat content of the control powdered kefir and the optimum formula powdered kefir was 31.86±2.71% and 31.71±1.82%, respectively. Both values complied with the CODEX Standard for Fermented Milk (CODEX STAN 234-2003), which specifies a minimum fat content of 10% for kefir, as well as the Indonesian National Standard for full-fat milk powder (SNI 01-2970-2006), which requires a minimum fat content of 26%. The relatively high fat content observed in both samples is characteristic of full-fat dairy-based products and is influenced by the fat composition of the milk used as the primary raw material. Similar fat content ranges have been reported in powdered kefir and fermented milk products produced from whole milk (Rizqiati et al., 2016; Nielsen, 2017).
 
Protein content
 
The protein content of the control powdered kefir and the optimum formula powdered kefir was 27.96±1.62% and 29.54±1.27%, respectively. Both values complied with the CODEX Standard for Fermented Milk (CODEX STAN 234-2003), which specifies a minimum protein content of 2.7% for kefir and the Indonesian National Standard for full-fat milk powder (SNI 01-2970-2006), which requires a minimum protein content of 23%. The protein content of kefir powder is influenced by the protein composition of the raw materials and microbial activity during fermentation, as lactic acid bacteria contribute to protein transformation and the overall protein profile of fermented dairy products. Similar protein levels have been reported in powdered kefir and fermented milk products derived from whole milk (Rizqiati et al., 2016; Sinurat et al., 2018).
 
Carbohydrate content
 
The carbohydrate content of the control powdered kefir and the optimum formula powdered kefir was 35.16±2.11% and 33.71±1.51%, respectively. These values were lower than those reported by Rizqiati et al., (2016) for goat milk kefir powder with the addition of various dextrin concentrations (0-10%), which resulted in carbohydrate contents ranging from 40.83±1.42% to 71.00±3.68%. The lower carbohydrate content observed in the present study may be attributed to differences in formulation and processing conditions, particularly the absence of carbohydrate-based fillers such as dextrin. In addition, carbohydrate levels in kefir powder are influenced by lactose utilization during fermentation, as lactic acid bacteria metabolize carbohydrates as an energy source, thereby reducing the overall carbohydrate content of the final product (Rizqiati et al., 2016; Sinurat et al., 2018).
 
Total lactic acid bacteria (LAB)
 
The total lactic acid bacteria (LAB) count of the control powdered kefir and the optimum formula powdered kefir was 4.33×10w±3.51 cfu/g and 5.67×10w±6.43 cfu/g, respectively. Both values complied with the CODEX Standard for Fermented Milk (CODEX STAN 234-2003), which requires a minimum LAB count of 1×10w cfu/g for kefir. In addition, both samples met the probiotic quality criteria according to the Indonesian National Standard (SNI 7552:2018), which specifies that probiotic products should contain more than 10v  cfu/g of viable lactic acid bacteria at the time of consumption. The relatively high LAB counts observed in both formulations indicate that the drying process and formulation applied were able to maintain the viability of beneficial microorganisms in the powdered kefir.
 
Sensory characteristics
 
The normality test results (Table 5) indicated that all sensory parameters had significance values of less than 5%, demonstrating that the data were not normally distributed. Consequently, non-parametric analysis using the Mann-Whitney test was applied (Table 6) to evaluate differences between the two samples. All statistical analyses were conducted using IBM SPSS Statistics software.




 
Color
 
There is a significant difference between control powder kefir and optimum formula powder kefir in color parameters because it has an Asymp. Sig (2-tailed) value of 0.009 (<0.05). This is in accordance with the average value of the color parameter of control powder kefir of 4.03±0.66 (yellow-yellowish white) and optimum formula powder kefir of 3.03±0.86 (yellowish white-white). The addition of lemongrass kecombrang tea causes a slightly red color difference because it contains anthocyanin pigments (Safitri et al., 2018).
 
The distinctive aroma of kefir
 
There was a significant difference between the control powdered kefir and the optimum formula powdered kefir in terms of the distinctive kefir aroma attribute, as indicated by an Asymp. Sig. (2-tailed) value of 0.038 (p<0.05). This result was consistent with the mean sensory scores, where the control powdered kefir obtained a score of 2.69±0.76 (not distinctive to somewhat distinctive), while the optimum formula powdered kefir showed a higher score of 3.03±0.86 (somewhat distinctive to typical). The increase in aroma intensity in the optimum formulation may be associated with the addition of kecombrang flower tea, which is known to contain terpenoid-based essential oils that contribute to characteristic aromatic properties in plant-derived ingredients (Safitri et al., 2018; Kusumayanti et al., 2016).
 
Texture
 
There was a significant difference between the control powdered kefir and the optimum formula powdered kefir in terms of texture attributes, as indicated by an Asymp. Sig. (2-tailed) value of 0.007 (p<0.05). This finding was consistent with the mean sensory scores, where the control powdered kefir obtained a texture score of 2.40±0.81 (not thick [liquid] to slightly thick), while the optimum formula powdered kefir showed a lower score of 1.89±0.63 (very thin [very liquid] to not thick [liquid]). The more liquid texture observed in the reconstituted optimum formulation may be attributed to differences in rehydration behavior, as the rehydration process can reduce apparent viscosity and result in a thinner texture in fermented dairy powders (Nielsen, 2017; Rizqiati et al., 2016).
 
The distinctive taste of kefir
 
There was no significant difference between the control powdered kefir and the optimum formula powdered kefir in terms of the distinctive kefir taste attribute, as indicated by an Asymp. Sig. (2-tailed) value of 0.699 (p>0.05). This result was consistent with the mean sensory scores, where the control powdered kefir obtained a score of 3.00±0.69 and the optimum formula powdered kefir obtained a score of 3.06±0.94, both categorized as somewhat distinctive. The characteristic sour taste of kefir is primarily associated with the production of organic acids during fermentation by lactic acid bacteria, which contributes to similar taste perceptions in both formulations (Sinurat et al., 2018; Suhaeni, 2018).
 
Overall (Preferences)
 
There was no significant difference between the control powdered kefir and the optimum formula powdered kefir in terms of overall preference, as indicated by an Asymp. Sig. (2-tailed) value of 0.744 (p>0.05). This finding was consistent with the mean hedonic scores, where the control powdered kefir obtained a score of 2.63±0.88 and the optimum formula powdered kefir obtained a score of 2.69±0.99, both categorized as somewhat liked. Overall preference represents a combined evaluation of several sensory attributes, including color, aroma, texture and taste and is inherently influenced by the subjective perceptions of panelists. Similar trends in overall acceptance have been reported in fermented dairy products enriched with plant-based ingredients, where modifications in individual sensory attributes do not necessarily result in significant differences in overall liking (Khoiria and Bahar, 2023; Kusumayanti et al., 2016).

Each tea concentration and type of tea significantly affected the antioxidant activity of kefir powder. More higher tea concentration, more higher antioxidant activity. Kecombrang tea with a concentration of 10% produced kefir powder with the highest antioxidant activity. The type of kecombrang lemongrass tea produced kefir powder with the highest antioxidant activity. The optimum treatment of kefir powder was produced from a combination of kecombrang lemongrass tea with a concentration of 10% with a desirability value of 0.771 which could produce antioxidant activity. The physicochemical and sensory characteristics of the developed kefir powder differed from the control product, particularly in rehydration behavior, viscosity, color attributes and aroma profile. The addition of lemongrass kecombrang tea improved functional and sensory properties by contributing natural bioactive compounds and plant pigments, resulting in a product with a more distinctive aroma and acceptable texture while maintaining overall consumer preference comparable to the control.
       
Furthermore, the incorporation of lemongrass kecombrang tea provided added functional value without compromising product quality or probiotic viability. These results indicate that the developed kefir powder has potential for further industrial-scale production as a functional fermented milk powder utilizing locally sourced botanical ingredients, thereby enhancing product differentiation and market value.

The present study was supported by the chancellor’s decree number 014/C3/DT.05.00/2025 on Consortium Research Unggulan Berdampak in 2025 at Ministry of Education and Science and Tehnology.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.