Key findings:

The study found that adding date juice to kefir significantly affected its fiber content, ash content, and hedonic properties. The optimal concentration was 30%, resulting in a fiber content of 3.038%, ash content of 4.36%, and high organoleptic acceptability, indicating improved taste and texture.

What is known and what is new?

It is known that kefir, a fermented dairy product, has a sour and slightly alcoholic taste, and can be combined with natural foods like fruit to enhance its flavor. This study introduces the new finding that adding date juice to kefir significantly improves its fiber content, ash content, and hedonic properties. Specifically, a 30% concentration of date juice was identified as optimal, enhancing the kefir's nutritional profile and making it more organoleptically acceptable.

What is the implication, and what should change now?

The implication of this study is that incorporating date juice into kefir production can enhance its nutritional value by increasing fiber and ash content while also improving its taste and texture. Kefir producers should consider adding date juice at a 30% concentration to optimize both the health benefits and consumer acceptability of their products. This finding suggests a shift towards using natural fruit additives in kefir to boost its appeal and nutritional profile.

Along with changes in people's lifestyles with the Covid-19 pandemic, awareness of people's healthy lifestyles and awareness of health problems is increasing. One of the efforts that can be made to improve the health of the body is to consume nutritious foods such as milk. Milk is one of the livestock products that has complete nutritional content. Milk is a primary product of livestock with a high nutritional value obtained from milking livestock such as cows. Cow's milk contains, among others, 3.3% protein, 4.8% lactose, 3.8% fat, 0.71% minerals, and 12.8% total solid [1].

Milk consumption in Indonesia based on Badan Pusat Statistik (2021) [2] in 2020 is 16.27 kg/capita/year. This figure shows that there is still a lack of milk consumption in Indonesia when compared to other countries in Southeast Asia such as Malaysia liters/capita/year, Singapore 46.1 liter/capita/year, and Vietnam 20.1 liter/capita/year [3]. Efforts that can be made to increase milk consumption are to make processed products from milk that have added value, both in terms of efficacy and taste. Milk can be processed into functional food products that are rich in benefits such as kefir. Kefir is a dairy product fermented by some microbes including Lactic Acid Bacteria (BAL), acetic acid-producing bacteria, and yeast [4]. The starter form of bacteria used in making kefir in the form of kefir grains is white or cream grains from a collection of bacteria such as Streptococcus sp., Lactobacilli, and several types of nonpathogenic yeasts [5]. Kefir has a sour, alcoholic, carbonated taste derived from microbes during fermentation. One that can be done to increase the added value of kefir products is the addition of flavors and natural coloring from fruits such as date juice.

Dates are fruits that come from a type of palm plant that is brown and tastes sweet. Various studies reveal the potential of dates as a good source of antioxidants and fiber. The total sugar content in sukkari dates reaches 70.68% with a fiber content of 9.11% [6]. The content of various minerals and vitamins in dates is also believed to have potential as an anti-cancer, anti-inflammatory, analgesic, and plays a role in kidney and liver protection [7]. In addition, the iron indates can help increase hemoglobin levels and prevent anemia [8].

Processing kefir with the addition of date juice can increase the functional effectiveness of kefir products. This is because dates are believed to be able to ward off free radicals, maintain digestive health, and increase endurance. The lack of research and analysis regarding the addition of date juice to the manufacture of kefir products is an interesting rationale for the author. Based on this description, the author is interested in conducting research to know the effect and know the best concentration of date juice on fiber content, ash content and liking for kefir products.

Research is carried out by experimental methods. The experimental design used was a Complete Randomized Design (CRD) with 4 kinds of treatments, namely the addition of 10% date juice (P1), 20% (P2), 30% (P3), and 40% (P4). Each treatment was repeated 5 times so that 20 experimental units were obtained. The data obtained from the results of the study were analyzed by ANOVA (analysis of variants) and if the treatment had a real effect on the variables, it was continued with Duncan's Multiple Range test (DMRT). To determine the difference in the organoleptic properties of kefir, the test result data was analyzed statistically non-parametric, namely by Kruskal-Wallis test analysis. If the results of the analysis show a real difference or are significantly different, then the data will be tested further with the Mann-Whitney test to see where the difference.

1.            Research Materials

The main ingredient used is kefir grain obtained from Kefir Bandung store, fresh cow's milk obtained from KPS Eka Putra Jaya, West Java Indonesia, and sukkari dates obtained from the Kurma Alif Bandung store. Another additional ingredient used in the analysis is water for the manufacture of date juice.

2.           Research Methods

Making date juice is done by the modified method of Al-Farsi (2003) [9]. Making begins with the separation of dates with seeds and then the dates are cut into smaller sizes. After that, soaked using water with a temperature of 50-60°C at a ratio of 1: 1 for 30 minutes, then the dates are ground to form juice and filtered to separate with the pulp. The filtered juice is then pasteurized at 60°C for 30 minutes. The manufacture of kefir products is carried out by the modified Bahar (2008) [10] method. Making begins with fresh cow's milk measured in volume and preheated at a temperature of 80°C for 15 minutes. After that, date juice is added according to the treatment and the temperature is lowered to reach a temperature of 30°C. Then inoculated kefir grain as much as 10% of the volume of milk, then stirred and incubated at 30°C for 24 hours. After incubation, the kefir is stirred again and filtered to separate the kefir product from the kefir grain.

3.           Variable Measurement

3.1         Fiber Content

Crude Fiber Analysis is carried out using the Gravimetric method based on AOAC (2005) [11] referring to the modified dry matter analysis. The procedure involves a porcelain dish being put in the oven at 105°C for several days until the sample dries then cooled into a desiccator for 30 minutes and weighed to a constant sample weight. If the sample weight is constant, smooth the sample using a pestle and mortar until the sample weight reaches 5 g. After that, the sample is tested for fiber content using a Fiber Analyzer.

3.2         Ash Content

Ash content analysis was carried out using the Furnace method based on AOAC (1995) [12]. The experiment was carried out by oven drying porcelain dishes at a temperature of 100-105°C until the weight of the porcelain dishes was constant. Dry the wet sample for several days until the sample dries then cooled into the desiccator for 30 minutes and weighed until the sample weight is constant Next, put the cup containing the dried sample into the kiln at a temperature of 300 ° C until the material turns black then with a temperature of 550 ° C until the sample becomes white ash with a burning time of about 3-6 hours. Then cool the cup in the desiccator for about 30 minutes and weigh the sample and also the saucer. The ash content of experimental kefir samples was measured using a mathematical model as follows:

3.3          Hedonic

Organoleptic testing of kefir products with the addition of different concentrations of date juice includes the level of preference for color, taste, aroma, texture, and overall. According to Sofiah & Achsyar (2008) [13], the favorability test or hedonic test is a test where panelists are asked to give personal responses through the level of liking or disliking of a product.  The test will be conducted on 20 semi-trained panelists consisting of students of Animal Husbandry, Faculty of Animal Husbandry, Padjadjaran University who have approved the panelists' approval sheet. The test was carried out by presenting three samples of 20 ml each in a plastic cup, which had been given a 3-digit code. The assessment of each panelist is reported by filling out an organoleptic test questionnaire sheet including an assessment of color, taste, aroma, texture, and overall. The score used uses scales with a numerical scale of 1= Dislike extremely; 2= Dislike very much; 3= Dislike moderately; 4= Disliked slightly; 5= Neutral; 6= Like slightly; 7= Like moderately; 8= Like very much; 9= Like extremely [14].

The results of the study regarding the effect of adding date palm juice on fiber content, ash content, and hedonic for kefir products are presented in Table 1.

Table 1. Summary of Research Results on the Effect of Various Treatments Adding Dates Juice to Fiber Content, Ash Content and hedonic of Kefir Products

Description: The average value followed by lowercase letters (subscripts) that differ in the direction of the line indicates significantly different (P<0.05)

1.  Fiber Content

 The results of the analysis of variance showed that the addition of date palm juice with different concentrations in the manufacture of kefir had a significant effect (P<0.05) on fiber content. Based on the data obtained in Table 1, it shows that kefir with the addition of date juice has the lowest average fiber content, namely P1 (1.44%), while the highest is P4 (3.41%). The results showed that the kefir fiber content with addition of date juice at P1, P2, P3, and P4 had significant differences (P<0.05). This shows that the higher the addition of date juice causes the fiber content of kefir to increase. The increase in fiber content is in line with the increase in the addition of date juice which contains a lot of fiber. This is in accordance with the opinion of Pambudi, et al., (2022) [15] which states that dates contain large amounts of fiber, which is around 6.5–11.5% of total fiber content consisting of 84–94% insoluble fiber and 6–16% soluble fiber. The fiber content of the Sukkari dates, which is the type of dates used in this study, has a fiber content of 8.2% (Al-Shahib & Marshall, 2003) [16]. Based on SNI 7552:2009 regarding flavored fermented milk drinks, kefir does not contain fiber content so the fiber content in the product can be an added value to the resulting kefir product.

The fiber in dates has high antioxidant activity due to its lignin content. This is in accordance with the opinion of Pambudi, et al., (2022) [15] that fiber in dates consists of cellulose, hemicellulose, pectin, and lignin. Dates contain many antioxidants which can help boost the body's immune system and reduce the risk of cancer [17]. Soluble fiber is useful in the formation of gel in the digestive tract which can slow down intestinal absorption of anti-nutritional factors, namely cholesterol. Meanwhile, insoluble fiber will remain intact with food when it passes through the digestive tract so that it helps smooth bowel movements [18].

1.  Ash Content

The results of the analysis of variance showed that the addition of date palm juice in the manufacture of kefir had a significant effect (P<0.05) on the ash content. Based on the data obtained in Table 1, it shows that kefir with the addition of date juice has the lowest average ash content at P4 (4.03%), while the highest is at P1 (5.53%). The results showed that the kefir ash content in P1 had the same effect as P2, then the kefir ash content in P3 also had the same effect as P4. This can happen because the difference between the concentrations is not much different, so the difference in ash content is not significantly different. Riansyah, et al., (2013) [19] stated that the ash content value shows the total minerals of a food ingredient. Based on the research conducted, it was found that the ash content of date palm juice in this study was 2.25%. In line with the opinion of Ghnimi, et al., (2017) [18] that the ash content in dates is around 2%. It is supported by research from Ismail & Altuwairki (2016) [20] that the mineral content of fresh dates consists of 4.50% potassium, 3.50% sodium and 0.61% phosphorus. According to Mainasara, et al., (2017) [21] other mineral content was detected in small concentrations such as calcium and magnesium.

The ash content of kefir tends to decrease, namely at P2, P3, and P4. This can happen because the minerals in dates and also milk can be used by bacteria during the fermentation process. This statement is supported by Yuniati (2012) [22] who argued that the decreased ash content in fermented milk with bacteria could be caused by minerals used by microorganisms in milk as nutrients for their growth, so the ash content tended to decrease. Ash content is a mixture of inorganic or mineral materials present in foodstuffs which are organic residues from burning or oxidation of organic matter [23]. Meanwhile, according to Styawati (2014) [24], a decrease in ash content can occur because in the fermentation, process there will be an increase in organic matter due to the process of degradation of the material (substrate) by microbes. 

1. Hedonic

   1. Color

Color is an assessment of the level of preference that can give the first impression of the product and the first parameter to be assessed by the panelists. The results of the analysis showed that the colors at P1, P2, P3, and P4 with the Kruskal Wallis test were not significantly different (P>0.05) from the color of kefir in each treatment, with a numerical scale of 6-7 with the criteria of like slightly to like moderately.

The color of kefir obtained at P1, P2, P3, and P4 was brownish white. Fermentation Bacteria in kefir grains will degrade lactose into lactic acid which will not affect the color of the kefir produced [25]. According to Srianta & Trisnawati (2015) [26] the sensory characteristics of kefir products are that they have a white or yellowish color like yogurt. So that the brown color of the kefir product is due to the use of brown Sukkari dates in the manufacture of kefir. Fandi, et al., (2020) [27] stated that the sukkari date has a light brown color with slightly ripe yellow edges. It is supported by the research of Purwanto, et al., (2018) [28], who examined date yogurt with the addition of date juice which produces a brownish yogurt color. According to Agustin & Judiono (2018) [29], the dyes in dates which are brown in color come from the tannin content in dates.

1. Taste

Taste is an assessment of the level of preference received by the sense of taste, namely the tongue. Taste can be divided into four main tastes, namely salty, sour, sweet, and bitter [30]. The results of the analysis showed that the flavors at P1, P2, P3, and P4 based on the Kruskal Wallis test were significantly different (P<0.05) to the kefir color in each treatment, with a numerical scale of 4-6 with criteria from disliked slightly to like slightly.

Based on the results of the Man Whitney test, a significant value was obtained between the treatments of adding date juice to the taste of kefir. Table 12 shows that the taste of kefir P1 compared to P2 has the same effect because the concentration difference is not that much different. P1 compared to P3 and P4 has a different effect because the difference in the concentration given is quite large. P2 compared to P3 has the same effect because the difference in concentration is not that much different. P2 compared to P4 has a different effect because the difference in the concentration given is quite large. P3 compared to P4 has the same effect because the difference in concentration is not that much different.

Table 1 shows that the more concentration of date palm juice is given, the panelists' preference for kefir increases. Kefir P1 has a hedonic scale somewhat disliked by the panelists. This could be due to the sour taste in the 10% addition treatment which is still very dominant compared to the 20%, 30%, and 40% addition. According to Ginting, et al., (2022) [31], the sour taste in kefir is formed from kefir grains due to the activity of lactic acid bacteria (Lactobacillus bulgaricus and Streptococcus thermophilous) and yeast which biodegrade lactose into glucose, then it is broken down into pyruvic acid which will be broken down again into lactic acid to produce a sour taste. In addition, the panelists were still unfamiliar with the taste of kefir which contains alcohol and is carbonated (stinging). According to Haliem, et al., (2017) [32], the characteristic feature of kefir is that there is CO2 arising from yeast activity which gives a sparkling (carbonated) impression, contains lactic acid, and also contains alcohol which is one of the differences between kefir and yogurt.

The panelists preferred the sweet taste of date juice on P4 with the hedonic scale like slightly because it can reduce the sour taste of kefir. Based on research on goat milk yogurt conducted by Balia, et al., (2011) [33] stated that the addition of date juice can give a sweet taste to yogurt products. The sweet taste is because 100 g dry weight of Sukkari date meat contains 52.3 g glucose, 48.2 g fructose, and 3.2 g sucrose [34]. Even though the sugar content in dates is quite high, the glycemic index value of dates is relatively low compared to granulated sugar, because the sugar in dates is a simple sugar that is easily absorbed by the body [35].

1. Aroma

Aroma is an assessment of the level of preference received by the sense of smell, namely the nose. Aroma can arise due to chemical stimuli that are smelled by the olfactory nerves in the nasal cavity (Negara, et al., 2016). The results of the analysis showed that the aromas at P1, P2, P3, and P4 based on the Kruskal Wallis test were not significantly different (P>0.05) from the aroma of kefir in each treatment, with a numerical scale of 6-7 having the criteria of like slightly to like moderately.

The aroma emanating from kefir products is the typical aroma of kefir, which is like fermented tape caused by the metabolic activity of bacteria in kefir grains. As stated by Rumeen, et al., (2018) [5] which stated that the aroma of sour kefir resembles tape due to the presence of high alcohol and esters due to the activity of kefir bacteria. Apart from being caused by lactic acid bacteria, the aroma that arises in fermented milk such as kefir is also caused by the presence of volatile compounds such as acetaldehyde and diacetyl. According to Lestari, et al., (2018) [37], the number of volatile compounds such as lactic acid, acetic acid, and ethanol that are formed during fermentation can also affect how sharp the kefir aroma is so that it affects hedonic results.

The aroma of the resulting kefir has a relatively similar aroma in each treatment, namely the sour aroma. The relatively similar aroma of kefir can occur because the use of kefir starter in the fermentation process is the same, namely 10% and also because the sweet aroma of dates cannot cover the sour smell of kefir. According to Audina, et al., (2020) [38], adding more substrate results in a stronger sour smell produced by lactic acid bacteria. Hidayat, et al., (2013) [39] added that sugar content can stimulate growth and increase the activity of lactic acid bacteria in producing lactic acid. So the natural sugar content in dates can be a source of energy for bacteria in the fermentation process. This can explain why P1 has a higher aroma hedonic scale than P4, because panelists prefer P1, which has a less pungent aroma than P4.

1. Texsture

Texture is an assessment of the level of preference received by the senses of touch and taste, namely the mouth and sight. Texture that arises in a material as a result of a combination of several physical properties which include size, shape, number and elements of the material formation (Midayanto & Yuwono, 2014). The results of the analysis showed that the textures at P1, P2, P3, and P4 based on the Kruskal Wallis test were not significantly different (P> 0.05) from the texture of kefir in each treatment, with a numerical scale of 6 with criteria of like slightly.

The texture of kefir is related to its thickness or viscosity. The texture of the kefir with the addition of the resulting date juice is slightly thick. According to Kinteki, et al., (2019) [41] Kefir has a slightly thick texture because the acidic conditions of lactic acid make milk protein, namely casein, agglomerate, increasing its viscosity. The addition of date juice will add texture to kefir because dates contain fiber. As stated by Agustin & Judiono (2018) [29], who examined the texture of kefir ice cream, according to him, the fiber content in the added puree of dates will cause the texture of the kefir ice cream to become more fibrous which is felt in the mouth so that the softness of the ice cream is reduced. This can explain why P1 has a higher aroma hedonic scale than P4), because panelists prefer P1 which has a softer texture than P4.

1. Overall

Overall is an assessment of the panelist's level of preference based on the color, taste, aroma and texture of the product that has been tested. The results of the analysis showed that overall at P1, P2, P3, and P4 based on the Kruskal Wallis test the results were not significantly different (P>0.05) to the total acceptance of kefir in each treatment, with a numerical scale of 6 with criteria of like slightly.

Kefir P1, P2, P3, and P4 have a hedonic scale that is somewhat liked by the panelists. The resulting hedonic scale tends to be acceptable to the panelists based on the total acceptance of kefir. This is because the addition of date palm juice will cause color, taste, aroma, and texture which tend to be acceptable to the panelists. The value of the total acceptance of kefir showed that the more date juice was added to the kefir, the more acceptable it was to the panelists. This is also supported by the research of Hardiansyah, et al., (2023) [42] which states that the total acceptance value indicates that the more ajwa date palm juice is added to goat's milk kefir, the more acceptable the kefir is to the panelists with the most preferred treatment being P6 (addition of ajwa dates 40%). David & David (2020) [43] added that the assessment at the level of preference or hedonic is subjective depending on the response from the panelists.

The addition of date juice with different concentrations in the kefir-making process affected fiber content, ash content and preference (taste), but had no effect on color, aroma, texture, and total acceptability of kefir. The addition of 30% of palm juice is the best concentration with 3.038% fiber content, 4.36% ash content, and organoleptically acceptable to the panelists.

Conflict of Interest:

The authors declare that they have no conflict of interest.

Funding: No funding sources 

Ethical approval: The study was approved by the Institutional Ethics Committee of Universitas Negeri Makassar

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