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Article

Transglutaminase Effects on Texture and Flow Behaviour of Fermented Milk During Storage Using Concentrated Kombucha Inoculum

by
Mirela Iličić
1,
Jovana Degenek
1,
Vladimir Vukić
1,
Ljubica Dokić
1,
Katarina Kanurić
1,
Rade Popović
2 and
Dajana Vukić
1,*
1
Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia
2
Faculty of Economics of Subotica, University of Novi Sad, Segedinski Put 9-11, 24000 Subotica, Serbia
*
Author to whom correspondence should be addressed.
Processes 2025, 13(11), 3598; https://doi.org/10.3390/pr13113598
Submission received: 14 October 2025 / Revised: 31 October 2025 / Accepted: 3 November 2025 / Published: 7 November 2025
(This article belongs to the Section Food Process Engineering)
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Abstract

This study investigated the effect of a concentrated kombucha inoculum and transglutaminase (TG) on the rheological and textural properties of fermented milk products and compared their average production costs to commercial yoghurt. Semi-skimmed milk was used, to which microbial TG was added at a level of 0.02% w/w. The kombucha inoculum, prepared from black tea, was concentrated to 55.6% total solids. Four samples were produced: two with TG and two without. The TG-containing samples showed significantly higher textural properties, including firmness and consistency, than the non-TG samples. They also exhibited the largest hysteresis loop area and the highest yield stress, indicating a stronger gel structure. The Herschel–Bulkley model successfully described the flow behaviour of all samples and confirmed their shear-thinning, non-Newtonian nature. Principal Component Analysis (PCA) showed that both TG addition and inoculum concentration significantly influenced the product properties. TG improved the rheological and textural properties and increased the stability during storage. However, the production costs for TG-treated samples were higher than those for non-TG-treated samples and commercial yoghurt. Nevertheless, the higher costs could be justified by the perceived additional nutritional benefits for consumers. Overall, the results show that the combination of concentrated kombucha inoculum with transglutaminase can improve the structural and rheological quality of fermented dairy products, which is potentially of commercial importance.

Graphical Abstract

1. Introduction

In recent years, interest in functional dairy products made with kombucha as a non-conventional starter culture has increased significantly [1,2,3]. Kombucha, an acidic and nutrient-rich beverage, contains dominant microorganisms—acetic acid bacteria, lactic acid bacteria, and osmophilic yeasts [4]. The composition of this culture directly influences the kinetics of fermentation and the final rheological properties of the protein gel, opening up possibilities for the development of new types of fermented dairy products.
Although the use of kombucha is promising, it faces some challenges. One important factor is the lower initial number of lactic acid bacteria (LAB) in kombucha compared to conventional yoghurt cultures. While standard yoghurt cultures have high concentrations, kombucha made from green and black tea typically contains 105 to 106 CFU/mL [5], a number that includes acetic acid bacteria and not exclusively LAB. Due to this difference in the concentration of these important fermenters, the lactic fermentation process with kombucha can take longer, increasing the production time compared to conventional methods [6,7]. As a result, recent research has focused on the optimization of fermentation conditions and the use of specific microbial strains to improve the quality, functionality, and sensory profile of the final product [8,9].
Improvement in the rheological properties of fermented milk products with low and medium fat content can be achieved by adding conventional ingredients such as milk powder, milk protein or whey protein concentrate, as well as functional ingredients like hydrocolloids. While hydrocolloids improve texture and stability by binding water and are often cost-effective [10], the enzyme transglutaminase (TG) offers a significant economic advantage by reducing the need for more expensive dairy components [8,11,12]. TG catalyzes the acyl transfer between the γ-carboxamide group of peptide-bound glutamine residues and primary amines, forming covalent cross-links between proteins that directly strengthen the gel network [13]. Incorporating TG into low-fat fermented milk products has been shown to enhance their elastic and viscous moduli, improving texture and stability during storage [6,14,15]. Similarly, the addition of TG has been reported to improve the texture, rheology, and sensory properties of set and stirred fermented milk beverages produced using concentrated kombucha inoculum [16]. Given that kombucha fermentation is slower compared to conventional yoghurt cultures and considering the lower microbial counts in kombucha inoculum, the use of TG can be beneficial [17]. TG supplementation may compensate for the slower fermentation kinetics by enhancing the gel network formation, leading to improved texture and stability of the final product [18]. This combination could offer a novel approach to producing functional dairy products with desirable sensory attributes and economic viability.
The aim of this study was to evaluate the use of concentrated kombucha inoculum as a non-conventional starter culture for set-style fermented milk with the addition of microbial transglutaminase. The effects of inoculum concentration (1.5% and 3.0% v/v) and transglutaminase treatment on fermentation kinetics, textural and rheological properties, and their relationship during storage were investigated. To provide a broader perspective, the cost structure of kombucha-based fermented milk was also compared with that of conventional yoghurt (2.8% fat). By integrating technological and economic evaluation, this study provides new insights into the application of kombucha inoculum concentrate in milk fermentation.

2. Materials and Methods

2.1. Materials

Homogenized and pasteurized semi-skimmed milk (1.5% (w/w) fat, 10.24% (w/w) total solids, 3.0% (w/w) total protein) from AD Imlek Dairy (Belgrade, Serbia) was used for the production of fermented milk products. Microbial transglutaminase (TG) with a reported activity of 100 units per gram of powder (U/g) was purchased from Ajinomoto Co. Inc. (Hamburg, Germany). TG was added to milk to concentrations of 0.02% w/w and activated at 40 °C for 2 h.
Kombucha inoculum was prepared from black tea according to the procedure fully described by Malbaša et al. [19]. Kombucha was cultivated on sucrose substrate (70 g/L) with the addition of 1.5 g/L black tea (oxidized Camelia sinensis). The black tea used to prepare the kombucha inoculum was a loose-leaf variety, purchased in bulk from a local health food shop. The tea was of domestic origin and not linked to any specific commercial brand or trademark. This choice ensured consistent availability and a typical composition for experimental reproducibility. The tea was heated at 100 °C for 5 min, after which the tea leaves were removed by filtration. The resulting solution was cooled to room temperature and inoculated with 10% (w/w) of the fermentation beverage from the previous kombucha fermentation (at a constant temperature of 29 ± 1 °C) for seven days. The solution was subjected to vacuum evaporation so that 200 mL of concentrate was obtained from 1000 mL of the original solution. The concentration of kombucha inoculum (8.5% total solids) was carried out in a vacuum rotary evaporator (ROTAVAPOR-R, Büchi, Flawil, Switzerland) at a temperature of 40 °C to a total solids content of 55.6%. The Kombucha inoculum concentrate was used to prepare the fermented milk samples in two concentrations: 1.5% and 3.0% v/v.

2.2. Manufacture of Samples

Fermented milk products used, obtained by concentrated kombucha inoculum without or with the addition of transglutaminase, were prepared according to the procedure described by Iličić et al. [16]. Two samples were prepared from milk with added TG, which was activated at 40 °C for 2 h. These samples were treated at 80 °C for 1 min for TG inactivation. Heating was followed by cooling to 43 °C and inoculation with a 1.5% concentrated kombucha inoculum (sample KTG1.5) or with a 3.0% v/v concentrated kombucha inoculum (sample KTG3.0). The samples of the second series were prepared by adding concentrated kombucha inoculum at concentrations of 1.5% (sample K1.5) and 3.0% (sample K3.0) and did not contain TG. A quantity of the inoculated milk was transferred into twelve sterilized 300 mL jars. The pH value of all samples was monitored during the fermentation process of the milk and stopped when a pH value of 4.5 was reached. The gels were cooled to 8 °C, packaged and stored in the refrigerator at 5 °C (±1 °C) until the rheological and textural analyzes were performed on days 1, 7 and 14. The samples were prepared in duplicate and the experiments were repeated twice on different days.

2.3. Methods

2.3.1. Monitoring of the Fermentation Process

The fermentation process was monitored during fermentation and storage period with a pH meter (EcoScan pH 6, Eutech Instruments, Landsmeer, The Netherlands).

2.3.2. Textural Characteristics

The properties of the texture samples, such as firmness, consistency, cohesiveness and index of viscosity of set produced samples, were analyzed with a Texture Analyser TA. HD plus (Stable Micro System, Godalming, UK) with a 5 kg load cell. The set samples were taken directly from the refrigerator (5 ± 1 °C) and subjected to a simple compression test using an A/BE disk (diameter 35 mm) (Stable Micro System, Godalming, UK). Texture analysis of the samples described by Iličić et al. [16]. The probe was moved to the surface of the sample at a test speed of 1 mm s−1. At a trigger force of 10 g, the probe penetrated at 1 mm s−1 to a depth of 30 mm. The probe then returned to its original position at a follow-up speed of 1 mm s−1. The maximum compressive force was determined using the Exponent software in version 6.2 (Stable Micro System, Godalming, UK) and this value represented the firmness. The measurements were carried out in duplicate each time after 1, 7 and 14 days of storage in new jars.

2.3.3. Rheological Analyses

The rheological properties of the fermented milk samples were measured at 5 °C using a HAAKE RheoStress 600HP viscometer (Thermo Fisher Scientific, Karlsruhe, Germany) equipped with a PP60Ti sensor (1 mm gap, Thermo Fisher Scientific, Karlsruhe, Germany) and described by Iličić et al. [16]. Replicate measurements were performed independently for each sample and data processing was performed using the RheoWin Pro software package (version 2.94, Thermo Haake, Karlsruhe, Germany).
The flow curves were obtained by registering a hysteresis loop in the measurement cycle: The shear rate increased from 0 to 200 s−1 within 180 s, was kept constant at 200 s−1 and decreased from 200 to 0 s−1 within 180 s. The obtained data were analysed using the Herschel–Bulkley model (Equation (1)):
τ = τ0 + K(γ)n
where τ is the shear stress (Pa), τ0 is the yield stress (Pa), K is the consistency index (Pasn) and n is the flow behaviour index.
Other rheological parameters considered were hysteresis loop area [20] and the coefficient of thixotropic breakdown (Kd). According to Dokić et al. [21], Kd is defined as the ratio of the hysteresis area to the area beneath the ascending shear curve (Equation (2)):
Kd = (Aup − Adown)/Aup
where Aup and Adown are the areas under ascending and descending flow curves, respectively.

2.4. Production Cost Analysis

The cost structure of kombucha yoghurt is largely based on the production of plain yoghurt with 2.8% milk fat in a dairy plant with a capacity of 150 tons of milk per day. The production costs of the kombucha inoculum concentrate were estimated based on the required raw materials and their average market prices.

2.5. Statistical Analysis

In this study, Duncan’s multiple range test was applied to compare mean values and identify differences in the rheological properties of fermented dairy products. Principal component analysis (PCA) was also performed to assess differences between samples based on textural and rheological parameters, using STATISTICA statistical software in version 13.5.0.17 (StatSoft Inc., Tulsa, OK, USA).

3. Results

3.1. Fermentation Development and pH Evolution

The change in pH during lactic fermentation with concentrated kombucha inoculum, with or without the addition of transglutaminase (TG), was monitored, and fermentation was stopped at pH 4.5 (Figure 1a). The addition of TG accelerated fermentation, resulting in the shortest time (615 min) in both kombucha–TG samples, while TG-free samples fermented in 700 min (K1.5) and 850 min (K3.0).
During storage, pH decreased significantly. After one week, values ranged from 4.10 (K3.0) to 4.25 (K1.5), with final pH values of 3.95 (K1.5, KTG1.5), 4.00 (KTG3.0), and 4.05 (K3.0).

3.2. Textural Properties

The addition of transglutaminase (TG) before fermentation significantly improved gel formation, resulting in a three- to fivefold increase in firmness and consistency immediately after production compared to TG-free samples (Figure 2). Among the samples tested, KTG1.5 (0.02% w/w TG, 1.5% w/w concentrated kombucha) exhibited the highest texture parameters both after production and during the seven-day storage period, while K3.0 without TG consistently showed the lowest values over 14 days. During storage, the firmness of the TG-containing samples increased by 18–45% and the consistency by 15–43%, indicating a strengthening of the gel over time (Figure 2a,b). TG-free samples showed only minor changes in firmness and consistency throughout storage. Samples prepared with concentrated kombucha inoculum, with or without TG, demonstrated better textural properties than those prepared with 10% w/w native kombucha inoculum with TG [6,12].

3.3. Rheological Properties

The flow curves of kombucha-fermented milk products with and without added TG are shown in Figure 3. All samples showed thixotropic behaviour characterized by a clear peak corresponding to the breakdown of a weak gel structure at low shear rates.
To evaluate the stability of different fermented milk products produced with and without the addition of TG, the rheological properties of the samples were monitored during a 14-day storage period. The rheological parameters of the fermented milk products with concentrated kombucha and added TG at 5 °C are shown in Table 1.
The correlation coefficients (R2) obtained for the Herschel–Bulkley model were equal to or greater than 0.94, confirming that this model was suitable to describe the flow behaviour of the fermented milk samples throughout storage. From Table 1, kombucha-fermented dairy products containing TG exhibited higher yield stress and initial viscosity compared to TG-free samples, suggesting that TG effectively enhanced the cross-linking of milk proteins before fermentation.
The flow behaviour index (n), determined using the Herschel–Bulkley model, consistently reflected liquid-like properties (n > 1), confirming that the weak gel structure was initially disrupted at low shear rates, allowing the samples to flow under applied stress. Comparison of the flow curves also showed that TG-enriched samples required greater shear force to initiate flow, indicating a stronger internal network and improved structural integrity.
The magnitude of gel thixotropic breakdown (Kd) is presented in Table 1. For TG-free samples, Kd values slightly increased during storage, suggesting minor structural rearrangement, with sample K3.0 requiring the lowest energy to disrupt its gel network. In contrast, TG-containing samples maintained similar or slightly higher Kd values throughout storage, indicating a more stable gel network.
The effect of the starter culture (concentrated kombucha inoculum) and the addition of TG on the hysteresis loop area (HLA) is shown in Figure 4. TG-containing samples displayed significantly larger HLA than TG-free samples, with the highest values observed for KTG1.5 (11,420 Pa s−1) and KTG3.0 (11,300 Pa s−1), whereas TG-free samples K1.5 (4247 Pa s−1) and K3.0 (3897 Pa s−1) showed markedly lower values. Over the 14-day storage period, HLA decreased by 14% for KTG1.5 and 16% for KTG3.0, while TG-free samples had HLA values consistently two to three times lower than TG-treated samples.

3.4. Cost Structure of Kombucha-Fermented Milk Products

Table 2 presents data obtained during research and from the market, which enables the estimation of kombucha concentrate at 1.574 EUR/l. The most significant component is overhead cost, which consists mainly of energy costs, followed by smaller shares of labour, water and other supplies. It is assumed that the kombucha inoculum is produced in-house during the yoghurt production process.
Taking into account differences in the technological process, production costs are based on a dairy plant with a milk capacity of 150 tons per day (Table 3). The estimated data indicate a higher production cost for kombucha yoghurt varieties, ranging from 3.8% to 12.4% above the cost of the same packaging of plain yoghurt.

3.5. Principal Component Analysis

Principal component analysis (PCA) was conducted to visualize the complex effects of kombucha inoculum concentration and the addition of transglutaminase (TG) on the textural, rheological, and cost parameters of the fermented dairy products (Figure 5). The first component (PC1) had the highest eigenvalue of 4.53 and accounted for 75.54% of the total variance, while the second component (PC2) had an eigenvalue of 0.688 and accounted for 11.47% of the variance. Together, these two components explained 87.01% of the total variability, indicating a robust model for describing the differences between the samples (Figure 5a). The primary factor, PC1, which alone accounts for 75.54% of the variance, highlights a significant balance in gel structure. PC1 was strongly and positively correlated with cohesiveness (R2 = 0.918), while also showing a strong negative relationship with parameters indicative of strength and resilience, such as firmness (R2 = −0.899), viscosity (R2 = −0.901), yield stress (R2 = −0.919) and hysteresis loop area (HLA) (R2 = −0.912) (Figure 5b). This inverse relationship suggests that higher cohesiveness, characterized by a softer and more homogeneous texture, is associated with lower firmness and viscosity, indicating weaker structural rigidity. The clustering of the samples in the score plot (Figure 5c) clearly shows that the addition of TG was the dominant factor influencing their positioning. TG-containing samples clustered into groups with higher firmness, yield stress, viscosity and HLA, suggesting the formation of stronger and more stable gel structures. In addition, PCA enabled the identification of sample KTG1.5 as particularly optimal, as its position along PC1 indicates favourable textural and rheological performance during the 14-day storage period.

4. Discussion

4.1. Fermentation Development and pH Evolution

Fermentation with concentrated kombucha inoculum was approximately 200 min longer than with native kombucha cultures [22,23] and about twice as long as in probiotic-inoculated systems [19], likely due to differences in inoculum concentration and metabolic activity. The lower initial bacterial load in kombucha, which includes acetic acid bacteria in addition to lactic acid bacteria [5], probably contributed to the extended fermentation times.
Transglutaminase-mediated acceleration aligns with reports of improved fermentation kinetics and texture in blended cow–soy milk yoghurt [24]. This supports the view that transglutaminase strengthens the protein network and enhances microbial activity. While the primary function of transglutaminase is protein cross-linking, which reinforces the gel network, the enhancement of microbial activity is considered an indirect effect: the enzyme’s known hydrolytic side activity can generate small, readily available peptides and amino acids from milk and soy proteins. These compounds serve as essential, rate-limiting nitrogen sources for the Streptococcus thermophilus and Lactobacillus bulgaricus starter cultures, thereby accelerating their proliferation and the overall fermentation rate (acidification) observed in the study by Lin et al. [24]. In contrast, the prolonged fermentation of kombucha-inoculated samples reflects the slower acidification typical of kombucha starters, which is strongly dependent on inoculum concentration [25], highlighting the importance of both microbial load and metabolic potential.
The post-acidification pattern observed during storage is consistent with previous findings that starter type influences pH decline [17] and mirrors results in kefir [26]. The decrease results from lactose fermentation to lactic and other organic acids, as well as secondary metabolite formation [7,25,26], in line with reports that non-conventional starters can exhibit variable post-acidification affecting product stability [27].

4.2. Textural Properties

Fluctuations in pH during fermentation directly affect the textural properties of kombucha-fermented dairy products, with firmness, consistency, cohesiveness, and viscosity index serving as key quality indicators that influence sensory perception and consumer acceptance [6,12].
The texture improvement observed after TG addition is primarily attributed to the enzymatic cross-linking of milk proteins, particularly caseins, through the formation of robust, covalent ε-(γ-glutamyl)lysine bonds [12,28].
This process fundamentally alters the structural integrity of the gel network. Unlike non-covalent interactions, such as electrostatic forces or hydrogen bonds, which stabilize standard acid gels, these covalent cross-links provide a strengthened protein backbone, directly enhancing gel rigidity and cohesiveness [12,29]. This stabilization is particularly critical in low-fat systems, where the lack of stabilizing fat globules necessitates enhanced protein–protein interaction to increase water-holding capacity and effectively reduce syneresis [30,31].
The observed increase in firmness and consistency during storage (18–45% and 15–43%, respectively) is consistent with previous findings for TG-treated cow’s milk yoghurt [29]. This is due to the synergistic effect of TG and the starter culture, where TG-induced cross-links create a pre-reinforced protein scaffold that resists structural breakdown over time [30,31]. Textural changes during storage are not static; they are associated with continuous pH fluctuations, changes in chemical composition, and evolving rheological behaviour [6,21]. The slight decrease in these parameters at the final stage of storage (day 14) is likely due to partial proteolysis and microstructural relaxation, leading to minor syneresis and reduced resistance to deformation. Mechanistically, such effects are consistent with observations of TG-induced gel formation and stability reported in set skim milk yoghurt [31].
The improved texture of products fermented with a concentrated kombucha inoculum is likely due to two factors. While a higher microbial load can accelerate initial acidification kinetics, the key contribution comes from a higher concentration of microbial metabolites and bioactive compounds. This may include exopolysaccharides (EPS), which can significantly enhance gel stability, viscosity, and cohesiveness by binding water and promoting entanglement within the protein network [32]. Differences in milk fat content, inoculum concentration, and acidification kinetics also influence gel network formation.
Overall, these results highlight the potential of combining TG with concentrated kombucha inoculum to produce low-fat fermented milk with improved texture and storage stability, a finding consistent with literature confirming TG’s role in texture enhancement and syneresis reduction [32,33].

4.3. Rheological Properties

The correlation coefficients obtained for the Herschel–Bulkley model (R2 ≥ 0.94) indicate an excellent model fit, consistent with literature on similar dairy systems [34]. Probiotic fermented milk produced from cow’s milk and soy drink also exhibited R2 > 0.93, confirming that this model accurately describes the non-Newtonian, shear-thinning flow behaviour typical of fermented milk products.
Comparable findings were reported by Darnay et al. [35], who observed that TG treatment increased the viscosity and protein network strength of kefir produced from different types of milk, yielding high model fit coefficients similar to those obtained in this study. The improved viscosity and consistency observed here suggest that microbial TG strengthened the milk protein network, maintaining a stable flow behaviour during storage.
The greater yield stress and viscosity of TG-containing samples confirm that TG effectively enhanced milk protein cross-linking before fermentation, in line with reports showing that enzymatic protein cross-linking promotes isoelectric coagulation and increases protein polymerization [25,36,37]. Bönisch et al. [36] demonstrated that a 19–22% increase in protein polymerization significantly enhanced the rheological properties of stirred yoghurt, attributing this to the stabilization of the gel network through non-covalent interactions rather than residual enzyme activity.
The observed decrease in hysteresis loop area (HLA) during storage reflects partial restructuring of the gel matrix but also demonstrates the resilience of the TG-cross-linked protein network. Similar behaviour was reported by Bönisch et al. [36], where cross-linked yoghurt exhibited higher HLA values (11,000 Pa s−1) than non-cross-linked samples (7300 Pa s−1). Furthermore, TG treatment reduces gel permeability, producing a denser and more cohesive microstructure [6,38].
Overall, the present findings confirm that TG markedly influences the rheological behaviour of kombucha-fermented dairy products, enhancing viscosity, yield stress, and thixotropic stability through the formation of covalent ε-(γ-glutamyl)lysine bonds and reinforcement of the protein network [28,39,40].

4.4. Cost Structure of Kombucha-Fermented Milk Products

Given that raw milk is a basic cost, accounting for almost 60% of the cost structure of plain yoghurt, the results obtained are consistent with previous research [41]. In line with Popović et al. [40], the higher production costs of kombucha yoghurt are primarily due to the additional steps required for inoculum preparation and controlled fermentation, which are reflected in the overheads, particularly energy consumption. The increase of 3.8–12.4% in production costs corresponds to the added functional value of kombucha-fermented products, including potential probiotic effects and bioactive compounds, as highlighted by Popović et al. [41]. This suggests that, despite the higher cost, kombucha yoghurt can justify a slightly higher market price due to its enhanced nutritional characteristics. Furthermore, the predominance of energy costs within overheads indicates an area where process optimization could reduce expenses, which also aligns with the recommendations from Popović et al. [41] for balancing production cost with product quality and market competitiveness.

4.5. Principal Component Analysis

These findings are consistent with previous research by Vukić et al. [17], who also observed a similar relationship between cohesiveness and parameters related to gel strength. The effect of TG in forming stronger and more stable gel structures is in line with the catalytic role of TG in cross-linking proteins, which directly enhances rheological and textural properties, as previously reported by Iličić et al. [16]. Overall, the PCA results confirm that both the addition of TG and the concentration of kombucha inoculum significantly influence the textural and rheological properties of fermented dairy products. These observations are consistent with the patterns observed in the individual HLA and Kd analyses, further supporting the conclusions. The PCA also provided a clear visual representation of how variations in formulation affect the final product, enabling the efficient identification of samples that exhibit an optimal balance of key textural and rheological properties.

5. Conclusions

The use of a concentrated kombucha inoculum with microbial transglutaminase (TG) is an extremely effective method for producing improved fermented dairy products. The addition of TG significantly accelerates the fermentation process and shortens the time by up to 235 min. This efficiency is associated with a significant improvement in the textural and rheological properties of the product, without undesirable structural changes during storage. The samples treated with TG showed a remarkable four-fold increase in firmness and a ten-fold increase in consistency compared to samples without TG. These improvements, along with a larger hysteresis loop and higher yield stress, are the result of the enzyme’s ability to cross-link milk proteins and create a stronger, more stable gel network. The flow behaviour of all samples, which was characterized by shear thinning, was accurately described by the Herschel–Bulkley model, with a high correlation coefficient (R2 ≥ 0.94). Although production costs for TG-treated products are slightly higher, ranging from 3.8% to 12.4% higher than regular yoghurt, this is not a significant market constraint. The additional cost is easily justified by the improved quality and perceived nutritional benefits that consumers associate with the product. Overall, this approach of combining concentrated kombucha with TG offers a viable and promising solution for the production of functional dairy products with superior properties for the commercial market.

Author Contributions

Conceptualization, M.I.; methodology, M.I., K.K., V.V., D.V. and L.D.; formal analysis, M.I., K.K. and D.V.; investigation, K.K., V.V. and D.V.; resources, K.K. and D.V.; data curation, R.P.; writing—original draft preparation, M.I.; writing—review and editing, J.D.; visualization, L.D.; supervision, M.I.; project administration, M.I.; funding acquisition, M.I. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for the financial support provided by the Ministry of Education and the Ministry of Science, Technological Development and the Innovations of the Republic of Serbia (grant numbers 451-03-136/2025-03/200134 and 451-03-137/2025-03/200134).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are grateful to the Faculty of Technology Novi Sad, University of Novi Sad, for providing laboratory space, equipment, chemicals, and administrative support during this study. The authors also thank Ajinomoto Company (Chuo City, Tokyo, Japan) for providing free samples of the enzyme transglutaminase.

Conflicts of Interest

The authors have no relevant financial or non-financial interests to disclose. The funders had no role in the design of the study, the collection, analysis, or interpretation of data, the writing of the manuscript or the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
TGTransglutaminase
K1.5Fermented milk sample with 1.5% kombucha concentrate
K3.0Fermented milk sample with 3.0% kombucha concentrate
KTG1.5Fermented milk sample with 1.5% kombucha concentrate and added transglutaminase
KTG3.0Fermented milk sample with 3.0% kombucha concentrate and added transglutaminase

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Processes 13 03598 g001
Figure 1. Fermentation kinetics and storage stability of low-fat milk (1.5% w/w fat) fermented with kombucha and modified with transglutaminase; (a) the fermentation time (acidification profile) required to reach the target pH for samples treated with transglutaminase; and (b) the pH value (storage stability) of the corresponding samples monitored throughout the storage period.
Figure 1. Fermentation kinetics and storage stability of low-fat milk (1.5% w/w fat) fermented with kombucha and modified with transglutaminase; (a) the fermentation time (acidification profile) required to reach the target pH for samples treated with transglutaminase; and (b) the pH value (storage stability) of the corresponding samples monitored throughout the storage period.
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Figure 2. Textural characteristics of kombucha-fermented milk samples prepared from milk with a fat content of 1.5% (w/w) with the addition of transglutaminase during storage: (a) firmness, (b) consistency, (c) cohesiveness, and (d) index of viscosity; Different letters (a–i) above the bars indicate statistically significant differences (p < 0.05) among samples at the same storage time.
Figure 2. Textural characteristics of kombucha-fermented milk samples prepared from milk with a fat content of 1.5% (w/w) with the addition of transglutaminase during storage: (a) firmness, (b) consistency, (c) cohesiveness, and (d) index of viscosity; Different letters (a–i) above the bars indicate statistically significant differences (p < 0.05) among samples at the same storage time.
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Figure 3. Flow curves of kombucha-fermented milk products during storage: (a) 1st day, (b) 7th day, (c) 14th day.
Figure 3. Flow curves of kombucha-fermented milk products during storage: (a) 1st day, (b) 7th day, (c) 14th day.
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Figure 4. Hysteresis loop area of kombucha-fermented milk products during storage; Different letters (a–f) above the bars indicate statistically significant differences (p < 0.05) among samples at the same storage time.
Figure 4. Hysteresis loop area of kombucha-fermented milk products during storage; Different letters (a–f) above the bars indicate statistically significant differences (p < 0.05) among samples at the same storage time.
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Processes 13 03598 g005
Figure 5. PCA of the effect of concentrated kombucha inoculum and transglutaminase addition on selected textural and rheological properties of kombucha-fermented dairy products during storage: (a) distribution of samples by scores; (b) correlation circle as a projection of the initial variables into the factor space; (c) position of the analyzed samples in the factor plane in coordinates of PCA factors 1 and 2; 1—K1.5 on the 1st day of storage; 2—K1.5 on the 7th day of storage; 3—K1.5 on the 14th day of storage; 4—K3.0 on the 1st day of storage; 5—K3.0 on the 7th day of storage; 6—K3.0 on the 14th day of storage; 7—KTG1.5 on the 1st day of storage; 8—KTG1.5 on the 7th day of storage; 9—KTG1.5 on the 14th day of storage; 10—KTG3.0 on the 1st day of storage; 11—KTG3.0 on the 7th day of storage and 12—KTG3.0 on the 14th day of storage.
Figure 5. PCA of the effect of concentrated kombucha inoculum and transglutaminase addition on selected textural and rheological properties of kombucha-fermented dairy products during storage: (a) distribution of samples by scores; (b) correlation circle as a projection of the initial variables into the factor space; (c) position of the analyzed samples in the factor plane in coordinates of PCA factors 1 and 2; 1—K1.5 on the 1st day of storage; 2—K1.5 on the 7th day of storage; 3—K1.5 on the 14th day of storage; 4—K3.0 on the 1st day of storage; 5—K3.0 on the 7th day of storage; 6—K3.0 on the 14th day of storage; 7—KTG1.5 on the 1st day of storage; 8—KTG1.5 on the 7th day of storage; 9—KTG1.5 on the 14th day of storage; 10—KTG3.0 on the 1st day of storage; 11—KTG3.0 on the 7th day of storage and 12—KTG3.0 on the 14th day of storage.
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Table 1. Parameters of kombucha-fermented milk products obtained by fitting the descending curve using Herschel–Bulkley model (τ = τ0 + K(γ)n) at temperatures of 5 °C, on storage days 1st, 7th and 14th.
Table 1. Parameters of kombucha-fermented milk products obtained by fitting the descending curve using Herschel–Bulkley model (τ = τ0 + K(γ)n) at temperatures of 5 °C, on storage days 1st, 7th and 14th.
SamplesDayτ0 a
(H.B.) (Pa)		Km b
(Pas n)		nm cR2 dτ0 e
(Pa)		η f
(Pas)		Kd g
K1.512.68 ± 0.05 c0.474 ± 0.009 f0.665 ± 0.013 a0.99698.43 ± 1.9 e33.95 ± 0.67 e0.633 ± 0.012
71.92 ± 0.03 ab0.001 ± 0.000 a2.103 ± 0.000 bd0.99638.65 ± 0.8 b15.16 ± 0.30 b0.734 ± 0.014
141.79 ± 0.03 ab0.133 ± 0.002 b0.813 ± 0.001 ac0.99877.93 ± 1.6 d28.01 ± 0.56 d0.684 ± 0.013
K3.01-0.297 ± 0.005 d0.381 ± 0.006 c0.99621.58 ± 0.5 a8.37 ± 0.15 a0.422 ± 0.007
72.02 ± 0.04 a0.392 ± 0.007 e0.606 ± 0.012 cef0.99697.87 ± 1.9 e33.58 ± 0.67 e0.629 ± 0.012
143.65 ± 0.07 b0.002 ± 0.000 a1.736 ± 0.035 df0.94245.27 ± 0.9 c17.51 ± 0.35 c0.705 ± 0.014
KTG1.5115.48 ± 0.31 h0.001 ± 0.000 a2.005 ± 0.040 bd0.992183.0 ± 3.7 h72.0 ± 1.44 h0.665 ± 0.013
79.64 ± 0.19 d0.151 ± 0.003 b1.030 ± 0.020 ae0.990130.4 ± 2.6 f49.71 ± 0.99 f0.644 ± 0.011
1410.68 ± 0.21 e0.091 ± 0.001 c1.142 ± 0.022 a0.988155.4 ± 3.1 g58.08 ± 1.16 g0.632 ± 0.012
KTG3.0111.8 ± 0.23 f0.148 ± 0.002 b1.069 ± 0.021 aef0.988181.1 ± 3.6 h74.64 ± 1.49 h0.636 ± 0.013
713.38 ± 0.26 g0.169 ± 0.003 b1.036 ± 0.020 aef0.988158.4 ± 3.2 g59.61 ± 1.19 g0.616 ± 0.012
1410.60 ± 0.21 e0.063 ± 0.001 a1.189 ± 0.023 bef0.996127.8 ± 2.6 f48.57 ± 0.97 f0.641 ± 0.013
* a yield stress τ0 (H.B.) (Pa); b consistency index Km (Pasn); c flow behaviour index nm; d correlation coefficient; e yield stress obtained at a start point of ascending curve τ0 (S.P.) (Pa); f initial viscosity η (Pas); g coefficient of thixotropic breakdown Kd. Means (n = 2) ± standard deviation with different letters (a–h) in the same column are significantly different (p < 0.05).
Table 2. Estimated cost in production of 0.2 litres of kombucha concentrate in EUR.
Table 2. Estimated cost in production of 0.2 litres of kombucha concentrate in EUR.
UnitQuantityPrice EUR/UnitAmount
Black Teakg0.00158.37590.0126
Sucrosekg0.070.61220.0429
Depreciation 10.02980.0298
Overhead cost 10.22960.2296
TOTAL: 0.3148
Table 3. Estimated costs in production 4 kombucha yoghurt variates and one plain yoghurt, packaged in 1l PET bottle, in EUR.
Table 3. Estimated costs in production 4 kombucha yoghurt variates and one plain yoghurt, packaged in 1l PET bottle, in EUR.
Kombucha Milk Fermented BeveragePlain Yoghurt with 2.8% Milk Fat
K1.5K3.0KTG1.5KTG3.0
Raw milk0.26790.26790.26790.26790.2679
Inoculum kombucha/
starter culture		0.02360.04720.02360.04720.0060
TG0.00000.00000.01700.01700.0000
Packaging0.09950.09950.09950.09950.0995
Depreciation0.02040.02040.02040.02040.0204
Overhead cost0.07650.07650.07650.07650.0765
TOTAL:0.48790.51150.50490.52850.4702
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MDPI and ACS Style

Iličić, M.; Degenek, J.; Vukić, V.; Dokić, L.; Kanurić, K.; Popović, R.; Vukić, D. Transglutaminase Effects on Texture and Flow Behaviour of Fermented Milk During Storage Using Concentrated Kombucha Inoculum. Processes 2025, 13, 3598. https://doi.org/10.3390/pr13113598

AMA Style

Iličić M, Degenek J, Vukić V, Dokić L, Kanurić K, Popović R, Vukić D. Transglutaminase Effects on Texture and Flow Behaviour of Fermented Milk During Storage Using Concentrated Kombucha Inoculum. Processes. 2025; 13(11):3598. https://doi.org/10.3390/pr13113598

Chicago/Turabian Style

Iličić, Mirela, Jovana Degenek, Vladimir Vukić, Ljubica Dokić, Katarina Kanurić, Rade Popović, and Dajana Vukić. 2025. "Transglutaminase Effects on Texture and Flow Behaviour of Fermented Milk During Storage Using Concentrated Kombucha Inoculum" Processes 13, no. 11: 3598. https://doi.org/10.3390/pr13113598

APA Style

Iličić, M., Degenek, J., Vukić, V., Dokić, L., Kanurić, K., Popović, R., & Vukić, D. (2025). Transglutaminase Effects on Texture and Flow Behaviour of Fermented Milk During Storage Using Concentrated Kombucha Inoculum. Processes, 13(11), 3598. https://doi.org/10.3390/pr13113598

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