Non-Thermal UV-C Processing as an Alternative to Pasteurisation in Fermented Dairy Beverages: Ayran and Kefir

Abstract

This study investigated the microbiological, physicochemical, textural, and sensory characteristics of ayran and kefir samples produced from milk treated with different doses of UV-C radiation. For this purpose, raw milk was passed through a UV-C column at three different flow rates (15, 30, and 45 mL/min), and irradiated with doses of 72, 36, and 24 J/mL, respectively, corresponding to the flow rate. Samples produced from milk pasteurised by thermal treatment were used as the control group. This research indicated that UV-C treatment effectively reduced the microbial load in milk to a level comparable to that achieved through conventional pasteurisation. A reduction of 2.15 log cfu/mL in total aerobic mesophilic bacteria count was achieved, while total coliform group bacteria counts were decreased to an undetectable level. Samples produced from milk treated with UV-C showed lower pH and higher titration acidity (% lactic acid). Furthermore, the organic acid content was higher in these samples. Lactic acid, the main organic acid, levels in the ayran and kefir samples were measured at their highest as 11,951.51 mg/kg and 12,989.34 mg/kg, respectively, in the UV45 sample with a radiation dose of 24 J/mL. The treatment of UV-C resulted in a minor change in the colour and textural properties of the samples. Nonetheless, this change was not significant enough to influence consumer acceptance. The application of UV-C to raw milk, depending on the radiation level used, can enhance the fermentation process in the production of ayran and kefir. This study showed that the application of UV-C has improved the quality of drinkable fermented milk products. This research has shown that, while reducing nutritional losses caused by thermal processing, microbial safety is obtained at an approximate value similar to pasteurisation. As a result, UV-C application decreases the loss of dietary compounds and provides an alternative method for microbial inactivation.

1. Introduction

Milk and dairy products are an essential part of a balanced diet, containing not only macronutrients such as protein, fat, and carbohydrates, but also a rich content of vitamins and minerals [1]. Traditional dairy products, such as ayran and kefir, are an integral part of many cultures, providing a refreshing and nutritious alternative to conventional beverages [2]. Ayran, a fundamental component of Turkish cuisine, is a straightforward yet invigorating mixture of yoghurt, water, and salt, recognised for its refreshing properties and digestive benefits [3]. Kefir, which originates from the Caucasus region, is a fermented milk beverage recognised for its diverse probiotic microorganisms, intricate flavour profile, and potential health benefits [4]. Both beverages are traditionally produced through fermentation; this process not only gives them a distinct flavour and texture but also improves their nutritional value and digestibility [5,6]. Nevertheless, reliance on live microbial cultures renders ayran and kefir vulnerable to spoilage, thereby constraining their shelf life. This presents considerable obstacles for the production and distribution in the commercial sector [7].

Ayran and kefir, due to their rich nutritional composition, have a limited microbiological shelf life and are made safe through conventional heat treatments. Nonetheless, these treatments may adversely impact colour, aroma, specific vitamins, thermolabile constituents, and textural qualities [8]. Consequently, contemporary emphasis is placed on new technologies, including high hydrostatic pressure, microfiltration, cold plasma, and UV-C radiation.

UV-C irradiation has emerged as a promising alternative to conventional heat pasteurisation for prolonging the shelf life of diverse food products, all while reducing undesirable alterations in their sensory and nutritional characteristics. UV-C light, with wavelengths between 200 and 280 nanometres, exhibits its antimicrobial properties by disrupting the DNA and RNA of microorganisms, which inhibits their reproduction and leads to their degradation [9]. In contrast to thermal processing, UV-C treatment operates without high temperatures that may denature proteins, eliminate vitamins, and modify the flavour profile of foods. The effectiveness of UV-C treatment is influenced by several factors, such as the UV-C dose, exposure duration, distance from the UV-C source, and the composition and turbidity of the liquid being treated. UV-C technology has effectively been utilised to pasteurise fruit juices, milk, and other beverages, demonstrating its capacity to reduce microbial load and prolong shelf life without substantially affecting quality [10]. Additionally, UV-C light is recognised for its capacity to inactivate a range of microorganisms, including bacteria, viruses, and moulds, establishing it as a versatile tool for ensuring food safety and preservation [11].

This study aims to determine the changes in the techno-functional and sensory properties of ayran and kefir produced from milk subjected to varying doses of UV-C light. For this purpose, the microbiological, physicochemical, textural, and sensory characteristics of ayran and kefir samples produced from heat-treated pasteurised milk (control) and those produced from milk subjected to different doses of UV-C light were analysed.

2. Materials and Methods

2.1. Materials

The bovine milk used in the study was obtained from a local producer in Afyonkarahisar province. Upon arrival at the laboratory, the milk was exposed to UV-C radiation before being transferred to sterile glass bottles.

2.2. Methods

2.2.1. Experimental Design

Control samples were produced using milk that underwent heat treatment through pasteurisation for 15 s at 75 °C. Raw milk intended for ayran and kefir production was subjected to three distinct doses of light in a UV-C reactor by adjusting the milk flow rate. In the study, flow rates of 15, 30, and 45 mL/min were used to administer doses of 72, 36, and 24 J/mL, respectively. This essentially meant the following: UV15 = 72 J/mL, UV30 = 36 J/mL, UV45 = 24 J/mL. Subsequently, ayran and kefir were produced from these milk samples.

2.2.2. UV-C Treatment of Milk

The UV-C application was carried out according to the method described by Atik and Gümüş [12]. The UV-C reactor used in this study was manufactured by Defne Engineering Laboratory Equipment (Afyonkarahisar, Turkey). The reactor consists of a 2 L capacity stainless steel feed unit, a peristaltic pump, a stainless-steel column equipped with a UV-C lamp, and polyurethane connection pipes integrated into the main body. The column is double-walled to allow cooling water to pass through, ensuring a constant temperature during the process. The UV-C reactor is shown in Figure 1. A Lightech (Dunakeszi, Hungary) GPH846T5L/HO/4 model UV-C lamp was used in the study. The lamp emits radiation with a strength of 18 W at a wavelength of 253.7 nm. The UV-C reactor was activated 15 min before the experiment began, and the column was sterilised via UV-C radiation.

2.2.3. Calculation of UV-C Radiation Dose

The theoretical calculation of the UV-C dose applied during the transition through the UV-C reactor was conducted using Equation (1), as outlined by Uysal Pala [13] and Cilliers et al. [14]. The variables used included the flow rate of milk passing through the column and the total output power of the UV-C lamp, which was 18 W.

Dose (J/mL) = Total UV-C output power (W)/Flow Rate (mL/s)

(1)

W = J/s

2.2.4. Cleaning of UV-C Reactor

Following through to the experiment, the column, feeding unit, and fasteners were sanitised using the CIP system. The CIP process was applied as described by Gopisetty et al. [15] with certain modifications. This protocol includes passing 250 mL of sterile water, followed by 250 mL of 0.1 N NaOH, and concluding with another 250 mL of sterile water through the pump at the maximum flow rate. Following prewashing, the reactor parts were reassembled, and a final wash was conducted using 1 L of 0.1 N NaOH and 1 L of sterile water.

2.2.5. Production of Ayran and Kefir

The production of ayran was carried out in accordance with Atik et al. [16]. The control sample consisted of raw milk (Brix: 10.50%, acidity (°SH): 6.79, pH: 6.07) that was pasteurised at 75 °C for 20 s. The other samples were subjected to UV-C light exposure. Processed milk samples were diluted with sterile distilled water until the Brix value reached 6%. They were then heated to 47 °C, inoculated with yoghurt culture (Vivo Activ LLC, Brovary, Ukraine) and left to incubate until the pH reached 4.30. At the end of incubation, the samples were stirred and cooled to 25 °C, then stored in a refrigerator at 4 °C for analysis.

In kefir production, milk samples with the same characteristics as those supplied for ayran production were used. A mesophilic/thermophilic kefir cultures (Vivo Activ LLC) were added to the samples at a temperature of 25 °C. The culture used consists of the microorganisms Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis var. diacetylactis, Leuconostoc mesenteroides subsp. cremoris, Streptococcus thermophilus, Kluyveromyces marxianus subsp. marxianus, and Debaryomyces hansenii. The samples were then incubated at 25 °C for 22 h until they reached pH 4.5. At the end of incubation, the kefir samples were stirred until they became homogeneous and stored in a refrigerator at 4 °C [16].

The samples were analysed immediately upon reaching the desired temperature in the refrigerator, with no storage before analysis.

2.2.6. Microbial Analyses

Before microbiological analysis, 10 mL was measured from all samples and homogenised by adding 90 mL of sterile Ringer’s solution (Oxoid BR52; ¼ strength, Wesel, Germany). Subsequently, other decimal dilutions were prepared using this dilution. Microbiological analyses were performed using the spread plate technique on these serial dilutions. The total mesophilic aerobic bacteria (TMAB) count was determined using Plate Count Agar (PCA, Oxoid, CM0463) following incubation at a temperature of 30 ± 1 °C for a duration of 48 ± 2 h [17]. Total Coliform Group Bacteria Counts (TCGB) were performed on Violet Red Bile Agar (VRBA, Oxoid, CM0978) using the spread plate method after incubation at 37 ± 1 °C for 24 ± 2 h. At the end of the incubation period, colonies appearing as dark red in the centre were counted, and the results were calculated as log cfu/mL [18]. The total yeast and mould count (TYM) was determined on Dichloran Rose-Bengal Chloramphenicol agar (DRBC, Oxoid, CM1148) after 5 days of incubation at 25–28 °C. At the end of incubation, colony-forming units were counted, and the results were calculated as live log cfu/mL [18]. The counts of Lactobacillus spp. bacteria were determined using de Man, Rogosa and Sharpe (MRS) Agar (110660, Merck Millipore, Darmstadt, Germany), incubated at 30 °C for 48–72 h under anaerobic conditions, while the counts of Lactococcus/Streptococcus spp. bacteria were determined using M17 Agar (Merck, Millipore, 115108, Germany) after 24–48 h of incubation at 30 °C under aerobic conditions for 24–48 h using M17 Agar (Merck, Millipore, 115108, Germany) [19].

2.2.7. Proximate Analyses

The total dry matter, water-soluble dry matter (°Brix), pH, percentage of titratable acidity (based on lactic acid), and water activity (a_w) of the samples were determined using standardised procedures from AOAC [20]. The colour analysis of the ayran and kefir samples was performed using a colorimeter (Minolta Co., Osaka, Japan) to measure L*, a*, and b* values. Each sample was measured a minimum of three times [21].

2.2.8. Syneresis (%)

10 g from each sample was weighed and individually placed on filter paper into a funnel to allow for settling. Following a 10 min filtration at room temperature, the residual sample mass was measured, and the syneresis (%) values of the samples were determined in accordance with Akarca and Denizkara [22].

2.2.9. Texture Analysis

The texture parameters, including firmness, consistency, cohesiveness, and viscosity index, of the samples were determined using a TA.XT Plus Texture Analyser (Stable Micro Systems, Godalming, Surrey, UK) equipped with a back extrusion rig apparatus (TA-30A, 7.62 cm diameter, cylindrical acrylic, and 10 mm height) [22].

2.2.10. Organic Acid Content

The samples’ organic acid concentrations were quantified via HPLC (Shimadzu Prominence, Tokyo, Japan). 4 mL was taken from each ayran and kefir sample, to which 20 mL of 0.01 N H₂SO₄ was added to each. The mixture was vortexed, passed through a 0.45-μm filter, and the resulting filtrate was injected into the system [23]. The specifications of the system used for analysis were as follows: CBM: 20ACBM, Detector: DAD (SPD-M20A), Column Furnace: CTO-10ASVp, Pump: LC20 AT, Autosampler: SIL 20ACHT, Software: LC Solution (version: 1.23 sp1). Column: ODS 4 (250 mm × 4.6 mm, 5 μm) (GP Sciences, Inertsil ODS-4, Nakatsugawa, Japan), Mobile phase: ultrapure water, pH adjusted to 3 using orthophosphoric acid [24].

2.2.11. Sensory Analyses

Sensory evaluations of the ayran and kefir samples were performed on the qualities of colour, odour, consistency, taste and general acceptability. Sensory evaluations of the samples were performed using the method described by Ercan et al. [25]. A scale ranging from 1 to 10 points (1: dislike at all; 10: extremely like) was used for the evaluation. Sensory analysis was conducted at Afyon Kocatepe University, Afyon Vocational School, with a group of 12 volunteer panellists, consisting of non-smoking men and women aged between 25 and 45, in accordance with ISO standard guidelines [26,27]. Before the sensory analysis, the panellists were trained on the sensory qualities that ayran and kefir products should possess. The samples were presented to the panellists in 100 mL transparent glass cups at 4 °C. The panellists were supplied with filtered water and instructed to cleanse their palate between samples.

2.2.12. Statistical Analyses

This study design was set up as 4 (Control, UV15, UV30, UV45) * 2 (ayran, kefir) = 8. The data obtained in the study were evaluated in two parallel groups. The statistical analysis was conducted using the SPSS software program version 23.0.0 for variance analysis. Duncan’s multiple range tests revealed a significant difference (* p < 0.05).

3. Result and Discussion

3.1. Microbiological Analyses

Raw milk used for the production of ayran and kefir, with pasteurised milk serving as the control group, and milk samples subjected to varying dosages of UV-C were analysed for TMAB, TCGB, and TYM counts. It was observed that UV-C treatment rendered the milk microbiologically safe. The highest TMAB count was detected in raw milk samples at 5.46 log cfu/mL, while the lowest TMAB count was detected in pasteurised milk samples at 3.03 log cfu/mL (

Table 1. Raw milk microbiological analysis results (log cfu/mL).

Sample	TMAB	TCGB	TYM
Raw milk	5.46 ± 0.01 a	2.49 ± 0.03	5.18 ± 0.04 a
Pasteurised	3.03 ± 0.01 d	<1	<1
UV15	4.15 ± 0.03 b	<1	4.22 ± 0.03 b
UV30	3.83 ± 0.04 b	<1	3.68 ± 0.06 bc
UV45	3.31 ± 0.02 c	<1	3.32 ± 0.07 c

UV15: Flow rate: 15 mL/min and 72 J/mL UV-C dose, UV30: Flow rate: 30 mL/min and 36 J/mL UV-C dose, UV45: Flow rate: 45 mL/min and 24 J/mL UV-C dose, a–d: Values with the different letters in the same column for each analysis differ significantly (p < 0.05).

). On the other hand, UV-C application resulted in a 2.15 log reduction in TMAB counts (p < 0.05). The initial microbial load, the applied radiation dose, and the optical density of the product influence the microbial inactivation efficacy of UV-C treatment (Diesler et al., 2019) [28]. Similar studies have found that applying UV-C to raw milk reduces the total bacterial load by 2–2.5 log cfu/mL [12,29,30]. The TCGB count decreased to a level where it could not be counted using pasteurisation and UV-C application. This decrease is consistent with the literature. Koutchma [31] reported that UV-C application can achieve a reduction of 3–4 log cfu/mL. TYM is at a level of 5.18 log cfu/mL in raw milk. Pasteurisation reduced the TYM count to an undetectable level. UV-C application, on the other hand, achieved a reduction of 1.86 log cfu/mL, depending on the dose applied (p < 0.05). The most significant reduction was observed at the lowest treatment dosage of 24 J/mL UV-C (UV45). A comparable study previously conducted on raw milk likewise determined that the rate of microbial load reduction decreased as the flow rate slowed. This is thought to result from a residual quantity of milk fat adhering to the quartz glass during processing. As the flow rate slows down, the amount of fat adhering to the quartz glass increases, forming a thin layer on the glass. As the thickness of the fat layer formed on quartz glass increases, the transparency of the glass decreases. For this reason, it is thought that this fat layer negatively affects the effectiveness of UV-C, meaning that despite the treatment dose increasing at low flow rates, treatment effectiveness decreases [12].

The product type and the product type × treatment interactions on the TMAB and TYM counts in the ayran and kefir samples produced from UV-C-treated milk were very highly significant (p < 0.0001) (

Table 2. Microbiological count results of ayran and kefir samples (log cfu/mL).

	Sample	Ayran	Kefir
TMAB	Control	4.38 ± 0.04 c	6.85 ± 0.04 c
	UV15	4.96 ± 0.02 b	7.21 ± 0.09 b
	UV30	5.10 ± 0.03 a	7.37 ± 0.05 ab
	UV45	5.16 ± 0.07 a	7.48 ± 0.06 a
	Interaction	p Value	r
	Product type	<0.0001	0.972 **
	Treatment	0.001	−0.019
	Product type × Treatment	<0.0001	--
Lactobacillus spp.	Control	6.88 ± 0.04 b	5.38 ± 0.05 c
	UV15	6.84 ± 0.04 b	5.50 ± 0.07 bc
	UV30	6.90 ± 0.03 ab	5.57 ± 0.06 ab
	UV45	7.02 ± 0.06 a	5.72 ± 0.06 a
	Interaction	p Value	r
	Product type	<0.0001	−0.988 **
	Treatment	0.002	0.127
	Product type × Treatment	0.128	--
Streptococcus/Lactococcus spp.	Control	6.53 ± 0.03 c	6.64 ± 0.01 b
	UV15	6.59 ± 0.01 bc	6.70 ± 0.01 b
	UV30	6.62 ± 0.02 ab	6.75 ± 0.02 ab
	UV45	6.66 ± 0.04 a	6.86 ± 0.08 a
	Interaction	p Value	r
	Product type	<0.0001	0.688 **
	Treatment	<0.0001	0.655 **
	Product type × Treatment	0.282	--
TYM	Control	2.15 ± 0.05 a	5.08 ± 0.04 c
	UV15	2.05 ± 0.01 b	5.18 ± 0.04 bc
	UV30	2.03 ± 0.01 b	5.21 ± 0.03 ab
	UV45	1.99 ± 0.03 b	5.30 ± 0.04 a
	Interaction	p Value	r
	Product type	<0.0001	0.999 *
	Treatment	0.579	0.077
	Product type × Treatment	<0.0001	--

Control: Pasteurised milk; UV15: Flow rate: 15 mL/min and 72 J/mL UV-C dose; UV30: Flow rate: 30 mL/min and 36 J/mL UV-C dose; UV45: Flow rate: 45 mL/min and 24 J/mL UV-C dose. a–c: Values with the different letters in the same column for each analysis differ significantly (p < 0.05). Statistical significance: p < 0.0001, very highly significant; p < 0.01, highly significant; p < 0.05, significant; p > 0.05, not significant. Correlation significance is indicated by asterisks: **, significant at the 0.01 level (2-tailed); *, significant at the 0.05 level (2-tailed).

). It was found that, on the count of TMAB, the product type had a highly positive correlative effect. The product type interaction on the Lactobacillus spp. count was very highly significant (p < 0.0001), while the product type and the treatment interactions on the count of Streptococcus/Lactococcus spp. were very highly significant (p < 0.0001; Table 2). It was also found that, on the count of Lactobacillus spp. the product type had a highly negative correlative effect, while on the count of Streptococcus/Lactococcus spp. the product type and the treatment had highly positive correlative effects.

The TMAB count was found to be highest in ayran samples at 5.16 log cfu/mL (UV45) and in kefir samples at 7.48 log cfu/mL. It was determined that the TMAB count was higher in ayran and kefir samples made from milk treated with UV-C (p < 0.05). The lowest Lactobacillus spp. count was detected at 6.84 log cfu/mL in the application with the highest radiation dose of 72 J/mL (UV15). The highest Lactobacillus spp. count was determined to be 7.02 log cfu/mL in the sample with a radiation dose of 24 J/mL (UV45). In kefir samples, Lactobacillus spp. was determined to be 5.38 (Control)–5.72 (UV45). Streptococcus/Lactococcus spp. counts increased depending on UV-C treatment (p < 0.05). The lowest counts in the ayran and kefir samples were found in the control sample at 6.53 and 6.64 log cfu/mL, respectively. The highest counts in the ayran and kefir samples were found in the UV45 sample at 6.66 and 6.86 log cfu/mL, respectively. TYM counts in ayran samples ranged from 1.99 to 2.15 log cfu/mL. Lower TYM counts were detected in samples treated with UV-C (p < 0.05). In kefir samples, higher TYM counts were obtained in samples treated with UV-C (p < 0.05). TYM counts in the samples ranged from 5.08 to 5.30. It is thought that the changes caused by UV-C treatment in the carbohydrate and protein structure of milk are responsible for the higher counts of TMAB, Lactobacillus spp. and Streptococcus/Lactococcus spp. found in samples of ayran and kefir produced from milk treated with UV-C. Lactose, present in milk and serving as a principal nutrient for bacteria, can be hydrolysed by UV-C radiation. Hydrolysis yields glucose and galactose, hence facilitating the metabolism of lactose by bacteria during fermentation [32]. It is thought that this mechanism promotes the development of microorganisms that dominate the fermentation process.

3.2. pH, Water Activity (a_w), Dry Matter (%), Syneresis and Titratable Acidity (%)

The product type interaction on the syneresis values of the samples was very highly significant (p < 0.0001). Furthermore, the product type revealed a highly positive correlative effect on this value (

Table 3. pH, a_w, dry matter (%), syneresis (%), and titratable acidity (%) values of ayran and kefir samples.

	Sample	Ayran	Kefir
pH	Control	3.87 ± 0.01 a	3.90 ± 0.02 a
	UV15	3.83 ± 0.09 a	3.90 ± 0.01 a
	UV30	3.89 ± 0.01 a	3.89 ± 0.02 a
	UV45	3.79 ± 0.07 a	3.86 ± 0.03 a
	Interaction	p Value	r
	Product type	0.107	0.459
	Treatment	0.619	0.850
	Product type × Treatment	0.512	--
aw	Control	0.960 ± 0.01 a	0.955 ± 0.01 a
	UV15	0.945 ± 0.01 a	0.938 ± 0.01 b
	UV30	0.954 ± 0.01 a	0.944 ± 0.01 ab
	UV45	0.955 ± 0.01 a	0.949 ± 0.01 ab
	Interaction	p Value	r
	Product type	0.037	−0.474
	Treatment	0.070	0.008
	Product type × Treatment	0.439	--
Dry Matter %	Control	7.23 ± 0.35 a	11.70 ± 0.45 a
	UV15	7.79 ± 0.01 a	11.62 ± 0.35 a
	UV30	7.59 ± 0.51 a	11.24 ± 0.03 a
	UV45	7.23 ± 0.02 a	11.27 ± 0.04 a
	Interaction	p Value	r
	Product type	0.085	0.320
	Treatment	0.088	0.802
	Product type × Treatment	0.054	--
Syneresis %	Control	46.4 ± 7.91 b	20.6 ± 1.41 a
	UV15	47.6 ± 0.56 ab	18.0 ± 1.69 a
	UV30	58.2 ± 1.41 a	20.0 ± 1.13 a
	UV45	47.8 ± 0.58 ab	20.6 ± 0.85 a
	Interaction	p Value	r
	Product type	<0.0001	0.966 **
	Treatment	0.064	0.060
	Product type × Treatment	0.072	--
Titratable Acidity (% LA)	Control	2.10 ± 0.28 a	3.11 ± 1.19 a
	UV15	2.19 ± 0.46 a	3.21 ± 0.54 a
	UV30	2.23 ± 0.58 a	3.45 ± 1.73 a
	UV45	2.19 ± 0.85 a	3.96 ± 2.09 a
	Interaction	p Value	r
	Product type	0.570	0.602 *
	Treatment	0.938	0.507
	Product type × Treatment	0.959	--

Control: Pasteurised milk; UV15: Flow rate: 15 mL/min and 72 J/mL UV-C dose; UV30: Flow rate: 30 mL/min and 36 J/mL UV-C dose; UV45: Flow rate: 45 mL/min and 24 J/mL UV-C dose. a,b: Values with the different letters in the same column for each analysis differ significantly (p < 0.05). Statistical significance: p < 0.0001, very highly significant; p < 0.01, highly significant; p < 0.05, significant; p > 0.05, not significant. Correlation significance is indicated by asterisks: **, significant at the 0.01 level (2-tailed); *, significant at the 0.05 level (2-tailed).

).

UV-C treatments reduced the pH values of the samples (ayran-kefir) (p > 0.05). However, when the changes that occurred were examined statistically, they were found not to be significant. The samples with the most significant decrease in pH value were found to be those with the values 3.79 and 3.86, respectively, in the samples coded UV45. As the column flow rate increased, the pH value of the products decreased accordingly. The use of UV-C treatment in fermented milk products such as ayran and kefir may affect lactic acid production, which could alter microbial activity and contribute to the formation of an environment associated with a decrease in pH levels. Studies show that during kefir fermentation, lactic acid bacteria (LAB) generate organic acids, notably lactic acid, resulting in a decrease in pH. This phenomenon is crucial for ensuring product stability and safety, as it inhibits spoilage and pathogenic microorganisms [33].

The pH change during fermentation is shown in Figure 2. It was observed that applying UV-C to raw milk effectively reduces pH during fermentation. pH levels were measured hourly in the ayran samples left to ferment after adding the culture, and fermentation was stopped when the pH dropped below 4. It was detected that the pH decrease occurred more rapidly in samples treated with high doses (UV15) of UV-C. The pH decrease in the UV30 and UV45 samples progressed in parallel with the control sample. In the kefir samples, pH measurements were taken every three hours over the 24-h fermentation period. While the pH decrease was similar in the samples treated with UV-C, it was observed that the pH decreased more quickly in the control sample during the initial hours of fermentation. Although UV-C radiation is primarily known for its antibacterial properties, it can also induce photochemical alterations in organic molecules, including carbohydrates like lactose. The scope of these alterations is contingent upon several factors, including the UV-C dosage, duration of exposure, and the presence of other chemicals within the matrix. The direct exposure of lactose to UV-C radiation may cause it to be hydrolysed. This breakdown may affect increasing the capacity of microorganisms to metabolise lactose during fermentation [32].

Although UV-C treatments did not have a significant effect on a_w values in ayran samples (p > 0.05), they generally caused a decrease in kefir samples (p < 0.05). As the flow rate increased, the rate of decrease in the a_w values of the samples slowed down. The a_w values of the ayran samples ranged from a minimum of 0.945 (UV15) to a maximum of 0.965 (UV45). The lowest a_w value in kefir samples was 0.938 in the UV30 sample, while the highest a_w value was 0.955 in the control sample. The % dry matter content of the samples ranged from 7.23 to 7.79 in ayran samples and from 11.24 to 11.90 in kefir samples (p > 0.05). Although changes were observed among the examples, these changes were not statistically significant.

In ayran samples, the % syneresis value, expressed as the product’s water release amount, was found to be as low as 46.4% in the control sample and as high as 58.2% in the UV30 sample (p < 0.05). In kefir samples, the lowest syneresis value was determined to be 18% (UV15), and the highest syneresis value was determined to be 20.6% (Control and UV45) (p > 0.05). However, when the changes in values were examined statistically, no difference was observed. The applied treatment generally led to an increase in syneresis value. UV-C treatment may affect syneresis by altering protein structures, thereby influencing microbial dynamics and gel matrix stability over time. The changes caused by UV-C treatment in the protein network may result in a diminished cohesive structure, thereby enhancing the extent of whey separation. This structural alteration may result in higher syneresis levels in the samples [34].

The titratable acidity, expressed as lactic acid percentage, was determined to be between 2.10 and 2.23 in ayran samples and between 3.11 and 3.96 in kefir samples. A slight increase in titratable acidity was observed in products obtained from milk treated with UV-C (p > 0.05). The change in titratable acidity was not found to be statistically significant. The treatment of UV-C is thought to hydrolyse lactose in milk, which impacts the fermentation process and results in an increase in titratable acidity.

3.3. Colour Values

It was determined that UV-C treatment had no significant effect on the L* value, which is an indicator of brightness in foods, in both ayran and kefir samples (p > 0.05). The L* values in ayran samples ranged from 84.38 to 86.96, while the L* values in kefir samples ranged from 89.93 to 91.05 (Figure 3). It was observed that the a* value, which is an indicator of redness-greenness, increased in products made with milk treated with UV-C (p < 0.05). The lowest a* value was measured in the control group in both ayran and kefir samples. The highest a* value was detected in the UV30 sample in ayran samples and in the UV45 sample in kefir samples. Changes in b* values occurred depending on the UV-C dose applied to both product groups.

In the ayran samples, the highest b* value was detected in the UV15 sample, and the lowest b* value was detected in the UV30 sample. In the kefir samples, the highest b* value was determined in the control sample, and the lowest b* value was determined in the UV30 sample. It is also important to note that UV-C applications may cause changes in certain milk-specific vitamins, which may indirectly affect the colour and visual characteristics of the milk. In a study conducted by Cappozzo et al. [35] it was determined that the exposure of cow milk to UV-C light significantly decreased the levels of several vitamins. Vitamin A levels decreased by 8 to 13%, vitamin B2 by 3 to 10%, and vitamin C by up to 74%, while vitamin E contents dropped by 16 to 33% following UV irradiation [35]. The findings align with previous studies that demonstrate the sensitivity of these vitamins to UV light exposure, particularly highlighting vitamin C’s high susceptibility to degradation under such conditions, alongside vitamins E and A [36]. UV-C treatments are known to induce oxidative stress in milk, resulting in alterations to colour properties based on the level of oxidation and the formation of chemicals [37].

3.4. Textural Values

The product type interaction on the firmness and consistency values of the samples was very highly significant (p < 0.0001). Besides this, on both values, the product type had a highly positive correlative effect, while it had a negative correlative effect on the index of viscosity value (

Table 4. Firmness (g), consistency (g.sec), cohesiveness, and index of viscosity (g.sec) values of ayran and kefir samples.

	Sample	Ayran	Kefir
Firmness (g)	Control	12.01 ± 0.65 a	14.51 ± 0.14 b
	UV15	12.14 ± 0.81 a	14.97 ± 0.52 b
	UV30	11.71 ± 0.28 a	15.49 ± 0.91 b
	UV45	11.82 ± 0.31 a	18.77 ± 1.72 a
	Interaction	p Value	r
	Product type	<0.0001	0.835 **
	Treatment	0.029	0.283
	Product type × Treatment	0.018	--
Consistency (g.sec)	Control	110.14 ± 22.54 a	375.38 ± 49.76 a
	UV15	122.29 ± 21.29 a	393.19 ± 56.69 a
	UV30	122.83 ± 17.17 a	398.31 ± 69.26 a
	UV45	128.95 ± 22.55 a	495.57 ± 44.33 a
	Interaction	p Value	r
	Product type	<0.0001	0.957 **
	Treatment	0.183	0.153
	Product type × Treatment	0.337	--
Cohesiveness	Control	−5.89 ± 5.78 a	−9.37 ± 6.94 a
	UV15	−6.03 ± 5.93 a	−8.89 ± 7.13 a
	UV30	−4.82 ± 3.07 a	−9.70 ± 6.01 a
	UV45	−5.02 ± 5.64 a	−6.23 ± 4.06 a
	Interaction	p Value	r
	Product type	0.309	−0.349
	Treatment	0.955	0.564
	Product type × Treatment	0.974	--
Index of Viscosity (g.sec)	Control	−1.12 ± 0.82 a	−2.46 ± 1.69 a
	UV15	−1.07 ± 0.88 a	−2.19 ± 1.50 a
	UV30	−0.70 ± 0.25 a	−2.93 ± 1.92 a
	UV45	−0.80 ± 0.47 a	−1.21 ± 0.16 a
	Interaction	p Value	r
	Product type	0.057	−0.563 *
	Treatment	0.734	0.214
	Product type × Treatment	0.740	--

Control: Pasteurised milk; UV15: Flow rate: 15 mL/min and 72 J/mL UV-C dose; UV30: Flow rate: 30 mL/min and 36 J/mL UV-C dose; UV45: Flow rate: 45 mL/min and 24 J/mL UV-C dose. a,b: Values with the different letters in the same column for each analysis differ significantly (p < 0.05). Statistical significance: p < 0.0001, very highly significant; p < 0.01, highly significant; p < 0.05, significant; p > 0.05, not significant. Correlation significance is indicated by asterisks: **, significant at the 0.01 level (2-tailed); *, significant at the 0.05 level (2-tailed).

).

The UV-C treatment slightly reduced the firmness value in the ayran samples. This decrease was not statistically significant (p > 0.05). The lowest value was determined as 11.71 (UV30) and the highest value as 12.14 (UV15). In the kefir samples, an increase in the firmness value was observed in the samples exposed to UV-C (p < 0.05). The lowest firmness value was detected in the control sample (14.51 g), while the highest firmness value was detected in the UV45 sample (18.77 g). Although not statistically significant, the application of UV-C increased the consistency value in the ayran and kefir samples (p > 0.05). The lowest consistency in the ayran and kefir samples was measured in the control group, at 110.14 and 375.38 g.sec, respectively. The highest values were determined at 128.95 and 495.57 g.sec, respectively, in the UV45 sample.

UV-C application had no significant change in cohesiveness and index of viscosity values (p > 0.05). In ayran samples, the lowest value was determined as −6.03 in the UV15 sample, and the highest value was determined as −4.82 in the UV30 sample. In kefir samples, the lowest value was −9.70 (UV30) and the highest value was −6.23 (UV45). The viscosity index values ranged from −0.70 to −1.12 g.sec in ayran samples and from −1.21 to −2.46 g.sec in kefir samples.

In the context of kefir fermentation, the alteration of proteins through UV-C exposure affects texture characteristics. As proteins denature, their interactions may lead to different viscoelastic properties that can change the texture profile. Research indicates that UV-C exposure influences the rheological properties of food, demonstrating that the use of UV can modify features like hardness and springiness [38]. Although the specific effects on kefir require further empirical studies, parallels drawn with similar fermented milk products suggest that UV-C may increase or decrease texture properties depending on the dose and exposure time. Furthermore, although studies on the application of UV-C specifically to kefir products are limited, research indicates that UV-C application, when combined with certain product formulations or processing techniques, can enhance desired textural qualities while controlling spoilage [39]. For example, adding kefir grains can influence the final texture, and UV treatment can help regulate these interactions, thereby maintaining the creaminess and consistency that are typical of high-quality kefir.

3.5. Organic Acid Content

The product type interaction was very highly significant on oxalic, formic and shikimic acid values of the samples (p < 0.0001). The product type × the treatment interaction on tartaric, malic, ascorbic and citric acids was very highly significant (p < 0.0001). The product type, the treatment, and the product type × treatment interactions on lactic acid and succinic acid were very highly significant (p < 0.0001). It was also found that product type had a highly positive correlative effect on formic, malic, ascorbic, shikimic, succinic and fumaric acids, while it had a highly negative correlative effect on oxalic and tartaric acids. Furthermore, treatment had a highly positive correlative effect on lactic and citric acids (

Table 5. Organic acid profile of ayran and kefir samples (mg/kg).

	Sample	Ayran	Kefir
Oxalic Acid	Control	20.51 ± 0.87 b	3.26 ± 0.30 b
	UV15	22.91 ± 0.80 a	4.38 ± 0.49 ab
	UV30	23.56 ± 0.54 a	4.51 ± 0.43 a
	UV45	24.40 ± 0.56 a	4.26 ± 0.30 ab
	Interaction	p Value	r
	Product type	<0.0001	−0.992 **
	Treatment	0.001	0.736
	Product type × Treatment	0.042	--
Tartaric Acid	Control	9.55 ± 0.25 c	28.11 ± 0.30 c
	UV15	10.19 ± 0.41 bc	29.45 ± 0.57 b
	UV30	10.99 ± 0.46 ab	30.24 ± 0.48 b
	UV45	11.64 ± 0.34 a	31.53 ± 0.45 a
	Interaction	p Value	r
	Product type	<0.0001	−0.994 **
	Treatment	<0.0001	0.105
	Product type × Treatment	0.256	--
Formic Acid	Control	772.15 ± 25.17 b	1394.32 ± 31.19 a
	UV15	860.86 ± 28.61 a	1417.02 ± 31.06 a
	UV30	896.72 ± 23.03 a	1440.79 ± 27.18 a
	UV45	906.46 ± 19.82 a	1460.18 ± 16.82 a
	Interaction	p Value	r
	Product type	<0.0001	0.988 **
	Treatment	0.003	0.128
	Product type × Treatment	0.205	--
Malic Acid	Control	120.27 ± 1.71 b	219.09 ± 0.19 c
	UV15	122.51 ± 0.65 ab	220.39 ± 0.47 bc
	UV30	123.13 ± 0.42 ab	222.30 ± 0.98 ab
	UV45	124.62 ± 0.76 a	224.10 ± 0.84 a
	Interaction	p Value	r
	Product type	<0.0001	0.999 **
	Treatment	<0.0001	0.035
	Product type × Treatment	0.613	--
Ascorbic Acid	Control	9.23 ± 0.14 ab	29.96 ± 0.92 c
	UV15	9.915 ± 0.27 b	31.65 ± 0.62 bc
	UV30	10.62 ± 0.51 a	32.46 ± 0.77 ab
	UV45	10.99 ± 0.65 a	34.18 ± 0.61 a
	Interaction	p Value	r
	Product type	<0.0001	0.994 **
	Treatment	<0.0001	0.099
	Product type × Treatment	0.115	--
Lactic Acid	Control	9857.72 ± 73.95 c	9050.23 ± 78.07 d
	UV15	10,215.19 ± 49.83 b	11,794.03 ± 77.71 c
	UV30	11,810.52 ± 84.19 a	12,034.91 ± 55.12 b
	UV45	11,951.51 ± 75.13 a	12,989.34 ± 35.41 a
	Interaction	p Value	r
	Product type	<0.0001	0.203
	Treatment	<0.0001	0.888 **
	Product type × Treatment	<0.0001	--
Citric Acid	Control	309.29 ± 8.93 b	305.66 ± 16.89 c
	UV15	394.07 ± 9.32 a	329.19 ± 12.30 bc
	UV30	397.79 ± 6.09 a	358.10 ± 10.00 b
	UV45	402.80 ± 0.81 a	395.43 ± 13.54 a
	Interaction	p Value	r
	Product type	<0.0001	−0.364
	Treatment	<0.0001	0.821 **
	Product type × Treatment	0.011	--
Shikimic Acid	Control	8.99 ± 0.30 b	11.75 ± 0.41 b
	UV15	9.59 ± 0.36 ab	12.84 ± 0.29 ab
	UV30	9.93 ± 0.38 ab	13.31 ± 0.59 a
	UV45	10.48 ± 0.49 a	13.69 ± 0.44 a
	Interaction	p Value	r
	Product type	<0.0001	0.912 **
	Treatment	0.003	0.359
	Product type × Treatment	0.753	--
Succinic Acid	Control	6716.49 ± 45.30 d	7997.56 ± 15.65 d
	UV15	6928.51 ± 41.88 c	8764.37 ± 29.80 c
	UV30	7293.52 ± 17.02 b	8970.95 ± 43.76 b
	UV45	7913.14 ± 25.46 a	9137.83 ± 59.82 a
	Interaction	p Value	r
	Product type	<0.0001	0.860 **
	Treatment	<0.0001	0.485
	Product type × Treatment	<0.0001	--
Fumaric Acid	Control	1.60 ± 0.62 a	2.56 ± 0.56 a
	UV15	1.57 ± 0.39 a	2.72 ± 0.60 a
	UV30	1.69 ± 0.43 a	2.81 ± 0.45 a
	UV45	1.75 ± 0.70 a	2.79 ± 0.59 a
	Interaction	p Value	r
	Product type	0.005	0.799 **
	Treatment	0.956	0.679
	Product type × Treatment	0.994	--

Control: Pasteurised milk; UV15: Flow rate: 15 mL/min and 72 J/mL UV-C dose; UV30: Flow rate: 30 mL/min and 36 J/mL UV-C dose; UV45: Flow rate: 45 mL/min and 24 J/mL UV-C dose. a–d: Values with the different letters in the same column for each analysis differ significantly (p < 0.05). Statistical significance: p < 0.0001, very highly significant; p < 0.01, highly significant; p < 0.05, significant; p > 0.05, not significant. Correlation significance is indicated by asterisks: **, significant at the 0.01 level (2-tailed).

).

It was determined that the oxalic acid content was higher in both ayran and kefir samples produced from milk treated with UV-C (p < 0.05). The highest oxalic acid content in ayran and kefir samples was detected in the UV45 (24 J/mL radiation dose) sample, at 24.40 and 4.26 mg/kg, respectively. Tartaric acid levels ranged from 9.55 to 11.64 mg/kg in ayran samples and from 28.11 to 31.53 mg/kg in kefir samples. Tartaric acid levels were higher in ayran and kefir samples produced from milk treated with UV-C compared to control samples (p < 0.05). The lowest formic acid content in the ayran samples was found in the control sample at 772.15 mg/kg, while the highest formic acid content was found in the UV45 sample at 906.46 mg/kg. The amount of formic acid increased depending on the applied radiation dose (p < 0.05). The amount of formic acid in kefir samples ranged from 1394.32 to 1460.18 mg/kg. Although not statistically significant, the amount of formic acid was higher in kefir samples produced from milk treated with UV-C (p > 0.05). Malic acid levels in the ayran samples were determined to be between 120.27 and 124.62 mg/kg and 219.09–224.10 mg/kg. The lowest malic acid level was detected in the control sample, while the highest malic acid level was detected in the UV45 sample (p < 0.05). Similarly, the amount of ascorbic acid was also measured at higher levels in samples produced from milk treated with UV-C (p < 0.05). The ayran and kefir samples produced from milk treated with a radiation dose of 24 J/mL (UV45) had the highest ascorbic acid content, at 10.99 and 34.18 mg/kg, respectively.

Lactic acid is the predominant organic acid in ayran and kefir. An increase in lactic acid content was observed in samples treated with UV-C (p < 0.05). The lowest lactic acid content in ayran samples was 9857.72 mg/kg in the control sample, while the highest lactic acid content was 11,951.51 mg/kg in the UV45 sample. In kefir samples, lactic acid was determined to be between 9050.23 (Control) and 12,989.34 (UV45) mg/kg. Citric acid was found at higher levels in samples produced from milk treated with UV-C in both ayran and kefir samples (p < 0.05). The highest ascorbic acid content was measured in both ayran and kefir samples in the UV45 sample, at 402.80 and 395.43 mg/kg, respectively. Similarly, an increase in shikimic acid and succinic acid levels was observed in samples treated with UV-C (p < 0.05). Fumaric acid levels ranged from 1.60 to 1.75 mg/kg in ayran samples and from 2.56 to 2.79 mg/kg in kefir samples. The change in the amount of fumaric acid was not found to be statistically significant (p > 0.05).

Fermented milk beverages like ayran and kefir have notable alterations in their organic acid compositions throughout production, primarily attributable to the metabolic processes of lactic acid bacteria and yeasts [40]. The fermentation process entails the breakdown of lactose, the principal sugar in milk, into many organic acids, predominantly lactic acid [3]. Nonetheless, other organic acids, including acetic acid, propionic acid, and butyric acid, may be produced in lesser amounts, contingent upon the specific microbial strains and fermentation conditions present [41]. The fermentation temperature, duration, and specific strains of microorganisms can substantially affect the types and amounts of organic acids produced in ayran and kefir [42]. The initial composition of milk, encompassing its lactose, protein, and fat content, might influence the milk’s buffering ability, the final acidity of the fermented product, and, therefore, the organic acid profile. UV-C treatment can modify the physical properties of milk, including changes in carbohydrate and protein structures, as well as fat globules, which may later influence fermentation kinetics [43,44]. Such changes may influence the bacteria’s capacity to produce lactic acid and create an environment favourable for additional metabolic activities, potentially yielding a wider variety of organic acids than those typically produced from conventionally unprocessed milk [45,46]. Consequently, the production of kefir from UV-C-treated milk is expected to modify its organic acid profile, notably by enhancing the lactic acid ratio and potentially influencing the concentrations of acetic acid and other metabolites.

3.6. Sensory Analysis

The sensory characteristics of fermented milk beverages significantly impact consumer preferences, comparable to their physiological advantages. The methods used in milk pasteurisation, the fermentation process, and the starter cultures used significantly influence the sensory qualities of ayran and kefir. The application of UV-C-treated milk in ayran and kefir samples significantly affected the sensory properties of the products (p < 0.05). It was observed that the radiation dose applied affects colour, odour, consistency, taste and general acceptability criteria (Figure 4). The control group samples received the highest scores for the criteria in question. The lowest scores were observed in the UV15 sample for both ayran and kefir samples. The UV45 samples (24 J/mL radiation dose) obtained the scores closest to the control group. As the column flow rate decreases, the UV-C dose applied to the milk increases, leading to oxidation-related changes in proteins and fats. Consequently, an oxidative aroma develops in the milk [14]. According to the results of a study on the application of UV-C to raw milk, it has been reported that as the application dose increases, the amount of carbon disulfide, a protein oxidation product, also increases. However, it has been noted that 3-methyl butanal and 2-methyl butanal, which are particularly associated with fat oxidation, are formed [47]. In the first stages of oxidation, free radicals are formed first, followed by hydroperoxides. In the next stages, volatile hydrocarbons such as aldehydes and ketones are formed [48]. These hydrocarbons and sulphur compounds formed indicate that UV-C treatment causes oxidation in milk and adversely affects its sensory properties. Research indicates that UV-C application is capable of preserving antimicrobial proteins like lactoferrin and lysozyme, which are reduced in conventional heating methods [49]. This factor may positively contribute to the functional properties of fermented milk drinks while preserving their sensory characteristics throughout their shelf life. Nonetheless, excessive exposure to UV-C may result in undesirable sensory defects. The interaction of UV rays with milk components can lead to oxidative reactions that adversely impact critical sensory characteristics like flavour and aroma [50]. Therefore, the radiation dose to be applied is of critical importance. The changes in milk components caused by UV-C application affected the sensory qualities of the ayran and kefir samples.

4. Conclusions

This study assessed the microbiological, physicochemical, textural, and sensory aspects of ayran and kefir samples produced from milk subjected to varying levels of UV-C treatment, in comparison to standard pasteurisation.

The use of UV-C treatment on raw milk demonstrated comparable effectiveness to traditional pasteurisation methods. A decrease of 2.15 log cfu/mL in the microbial load of raw milk was accomplished. The rate of pH decrease was found to be influenced by the radiation dose applied during fermentation. The content of organic acids was higher in samples derived from milk subjected to UV-C treatment. Lactic acid, the primary organic acid in ayran and kefir samples, was detected at the highest concentrations in the UV45 sample, measuring 11,951.51 and 12,989.34 mg/kg. The titration acidity, measured in lactic acid, was seen to be elevated in ayran and kefir samples derived from milk subjected to UV-C treatment. An increase in organic acids, particularly lactic acid, was observed as a result of changes in milk components linked to UV-C application. Furthermore, alterations in the colour and texture characteristics of the samples related to UV-C application were noted. The changes influenced the sensory characteristics of the samples based on the administered radiation dose. It was determined that an increase in radiation dose adversely impacted the sensory attributes of the ayran and kefir samples. The samples exhibiting sensory qualities most similar to the control group were those subjected to a radiation dose of 24 J/mL (UV45).

The safest method currently used in the dairy industry to ensure microbial quality is still thermal processing. Although many successful studies have been conducted on non-thermal methods, these methods have not gained industrial demand due to initial installation and operating costs, as well as traditional reasons. The results obtained demonstrate that UV-C application is sufficient in terms of both nutrient losses and microbiological safety when compared to conventional methods. Furthermore, this technique is significantly more environmentally friendly than other applications. Today’s consumers prefer natural, healthy, safe and minimally processed foods. This method is capable of meeting consumer expectations, and further research is needed to demonstrate its applicability on an industrial scale.

In conclusion, UV-C treatment improves the microbiological quality of milk and influences the fermentation process by altering specific milk components. Variations in milk constituents influence the evolution of the organic acid profile based on the applied radiation dose. Nonetheless, the radiation dose employed may induce unfavourable alterations in the colour and aroma profile; thus, additional research is deemed essential to ascertain the ideal parameters.