Introduction
Yoghurt is a fermented dairy product with an oil-in-water emulsion character formed by lactic acid bacteria, namely Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. In addition to caseins, fat globules, and whey proteins, primarily β-lactoglobulin (β-Lg), undergo numerous biochemical changes during yoghurt production and have key roles in the formation of yoghurt gel. The standardization stage involves the regulation of the fat content of milk which is crucial for the occurrence of 3D yoghurt gel (Asaduzzaman et al., Reference Asaduzzaman, Mahomud and Haque2021). Yoghurt can be classified based on fat content, i.e., ≤0.5% as skim yoghurt, 1.5 ≤ fat < 2.0% as semi-skimmed, and ≥3.8% as full-fat yoghurt (Anonymous, 2009). Fat globules mostly take on the role of filler in this network and an increase in fat content is usually associated with a higher G’ (storage modulus) value, so a more solid structure (Xu et al., Reference Xu, Emmanouelidou, Raphaelides and Antoniou2008). On the other hand, reduction in fat negatively affects the sensory characteristics, particularly texture and flavor (Kim et al., Reference Kim, Hyeonbin, Lee and Kim2020). A decline in water retention arises, causing phase separation and syneresis, resulting in a quasi-stable structure lacking sensory appeal to consumers (Kim et al., Reference Kim, Hyeonbin, Lee and Kim2020). Standardized milk goes through a heat treatment which enables destruction of pathogens, inactivation of enzymes, and denaturation of whey proteins, which is crucial for development of yoghurt texture (Tamime and Robinson, Reference Tamime and Robinson2007). On the other hand, excess heat treatment may result in formation of off-flavors and loss of nutritive value thus requires an optimization. Correspondingly, an increase in the storage modulus was reported with increasing fat content and severity of the heat treatment (Lucey et al., Reference Lucey, Munro and Singh1998). Also, regarding sensory acceptance, a balanced heat treatment is needed to prevent the formation of undesirable flavors and obtain the desired textural properties. In summary, both the fat content of milk and the applied heat treatment will influence the physical and biochemical quality hence sensory acceptance of yoghurt.
Fortification of foods with vitamin D is one of the three strategies addressed by WHO/FAO (2006) to improve the vitamin D status of society, especially in regions where sunlight is not available. Adequate vitamin D provides a healthy bone status by enhancing both calcium and phosphorous absorption from the intestine while its deficiency is associated with skeletal abnormalities in kids and osteoporosis and osteomalacia in adults (Ilgaz et al., Reference Ilgaz, Yardim, Çimen, Kanbur, Özmert, Satman, Tavil, Tayfur and Ulman2020). The USA, Canada, and Finland have been the pioneers of Vitamin D fortification of foods, mainly milk, margarine, and yoghurt, due to their emulsion characteristics and great acceptance. Gahruie et al. (Reference Gahruie, Eskandari, Mesbahi and Hanifpour2015) reported 0.03 μg/100 mL and trace amounts of vitamin D for full-fat and non-fat milk, respectively stating that milk is not a rich source of vitamin D. In this context, Aksu and Özbey (Reference Aksu and Özbey2021) suggested the fortification of yoghurt with vitamin D to enhance serum vitamin D levels and reduce insulin resistance. However, until recently, studies on the effect of vitamin D on yoghurt properties have been limited.
This study aims to assess the physical, textural, microbiological, and sensory traits of yoghurts made from cow milk varying in fat content. Additionally, it compares the quality features of yoghurts produced from milks with similar compositions but subjected to different heat treatments [Ultra-high temperature (UHT) sterilization (138–140°C for 4 s) and pasteurization (90°C for 16–20 s)]. Finally, it examines the impact of adding vitamin D on yoghurt quality.
Materials and methods
Yoghurt production
For yoghurt production, six types of homogenised cow milk (0.1% fat & UHT-sterilized; 1.5% fat & UHT-sterilized; 3% fat & UHT-sterilized; 3.5% fat & UHT-sterilized; 3% fat & pasteurised; 1.5% fat UHT-sterilized with vitamin D addition) were purchased from a grocery market (İçim, Ak Gıda Inc., Türkiye). The gross compositions stated on the labels of each milk type used for yoghurt making are presented in Supplementary Table S1. Thus, six types of yoghurt were produced using 2% inclusion of a lyophilized commercial yoghurt starter culture (YC-350, Chr. Hansen, Denmark) containing L. bulgaricus and S. thermophilus according to Celik and Temiz (Reference Celik and Temiz2022) and coded as following; SF-UHT: Yoghurt made from UHT-sterilized skimmed milk, HF-UHT: Yoghurt made from UHT-sterilized half-skimmed milk, 3 F-UHT: Yoghurt made from UHT-sterilized 3% fat milk, FF-UHT: Yoghurt made from UHT-sterilized full fat (3.5%) milk, 3 F-P: Yoghurt made from pasteurised 3% milk, HF-D-UHT: Yoghurt made from UHT-sterilized half-skimmed vitamin D-added milk. All the yoghurt samples were kept at room temperature for 1 h, then stored under refrigerator conditions (4°C) for 21 days. Yoghurt production was carried out in duplicate from two different batches of milk on separate days.
Gross composition and titratable acidity
Dry matter and ash contents were determined gravimetrically according to the methods by IDF/ISO (2004) and Metin and Öztürk (Reference Metin and Öztürk2008), respectively. The fat content of yoghurt samples was determined following the Gerber-Van Gulik method. The nitrogen content was determined using a Kjeldahl system (IDF/ISO, 2014) and multiplied by a factor of 6.38 to determine the crude protein content of yoghurt samples.
The compositional analyses mentioned so far have been performed in duplicate per production only once, at the beginning of storage. The titratable acidity (lactic acid %) was determined in duplicate during each storage period and determined by titration of diluted yoghurt (1:1) with a standardized 0.1 N NaOH (Sigma-Aldrich, MO) solution in the presence of phenolphthalein (Kurt et al., Reference Kurt, Cakmakci and Caglar1996). A new container of yoghurt was opened for analysis in each period.
Textural analysis and syneresis
The textural analysis included the parameters of firmness, consistency, cohesiveness, and index of viscosity (work of cohesion) and was carried out as previously described by Balpetek Külcü et al. (Reference Balpetek Külcü, Koşgin, Öf and Turabi Yolacaner2021). The syneresis was determined following the procedure of Temiz and Dağyıldız (Reference Temiz and Dağyıldız2017). Briefly, 10 g of yoghurt was weighed and centrifuged at 2,500 rpm and 4 °C for 10 minutes (NF-400 R, Nüve AŞ, Ankara, Türkiye). The syneresis value was determined by proportioning the supernatant weight to the initial sample weight and expressed as a percentage.
Enumeration of yoghurt bacteria
Lactobacilli and lactococci were enumerated according to IDF/ISO (2003) using MRS and M17 agars, respectively. Briefly, 10 g yoghurt was weighed into sterile bags aseptically. Then, 90 mL sterile NaCl solution (0.85%, w/v) was added, and the bag was homogenised for 30 seconds in a stomacher (BagMixer 400P, Interscience Co., France). Following a serial dilution (1:9, in 0.85% NaCl solution) of this homogenate, appropriate dilutions were plated, and an incubation period of 48 hours at 37°C under anaerobic and aerobic conditions was applied for L. bulgaricus and S. thermophilus, respectively (Celik and Temiz, Reference Celik and Temiz2020). A new container of yoghurt was opened for analysis in each period.
Whiteness index
The color values were measured in triplicate using a color analyzer (Minolta, Chromameter CR-400, Japan) once at the beginning of storage. After taking the yoghurts out of the refrigerator, measurements were made by spreading them smoothly to a depth of 10 mm in a 70 mm diameter glass petri dish and then touching the colour analyzer probe (Ø8 mm measurement and Ø11 mm illumination area, 2° observer, D65 illuminant) to the yoghurt surface. The Whiteness Index (WI) was calculated using Equation (1) below using the L*, a*, and b* values corresponding to brightness, redness and yellowness, respectively (Pan et al., Reference Pan, Liu, Luo and J-P2019).
\begin{equation}WI = 100 - \,\sqrt {{{\left( {100 - {L^*}} \right)}^2} + {a^{*2}} + {b^{*2}}} \end{equation}Sensory evaluation
Sensory evaluation was carried out only on the first day of storage according to Temiz and Ersöz (Reference Temiz and Ersöz2023) with minor modifications. The sensory evaluation of yoghurt samples was performed by a panel consisting of ten non-smoking healthy panelists aged between 18 and 45 who had previously participated in the sensory evaluation of dairy products with regards to colour & appearance, smell, consistency, flavor, and overall acceptability on a 10-point hedonic scale (1: the lowest and 10: the highest score). Before serving, yoghurt samples were randomly coded with three-digit numbers, and a cup of water was provided for mouth rinsing for the transition between yoghurts.
Statistics
The data of the analyzes were evaluated using the Minitab 17.0 statistical package program (Minitab, 2010). Statistical differences between yoghurt varieties were analyzed with one-way ANOVA, and differences between groups were assigned with Tukey multiple comparison test at 95% confidence interval (α = 0.05). Sensory results were expressed in median form and other data were expressed as mean ± standard deviation.
Results and discussion
Gross composition
The compositional data of yoghurt samples are presented in Table S2. While all yoghurt samples had a dry matter content >9%, FF-UHT (12.45%) and SF-UHT (9.47%) had the highest and lowest dry matter contents, respectively. The protein content of yoghurt samples varied between 3.19% and 3.51% with no significant difference (p > 0.05). The highest fat content was determined in FF-UHT, followed by 3 F-UHT and 3 F-P, and HF-UHT and HF-D-UHT (p < 0.05) while no fat was detected in the SF-UHT. The applied Gerber-Van Gulik method was probably insufficient to determine the stated fat content of 0.1% in SF-UHT on the label (Table S2). The ash content of the yoghurt samples ranged between 0.74% and 0.81%, and the highest ash content was determined in the SF-UHT sample (p < 0.05). Overall, it would be proper to state that the compositional data on the milk labels and the values obtained in the analyzes are somewhat consistent. Saleem et al. (Reference Saleem, Khan, Hussain, Khan, Al-Asmari, Khan, Zafar, Rahim, Eliasse and Ramadan2024) indicated no effect of vitamin D fortification on the proximal composition of yoghurt samples confirming the current findings.
Titratable acidity
The titratable acidity of all yoghurt samples showed an increasing trend during the 21-day storage period (Fig. 1). Similarly, Aamir et al. (Reference Aamir, Arshad, Afzaal, Rakha, Jalel Mahsen Oda, Nadeem, Saeed, Ahmed, Imran and Ateeq2023) determined increasing acidity values during 14-day storage of ginger fortified yoghurts. The acidity values of all samples varied between 0.80% and 1.16% and are in compliance with the 0.60–1.50% titratable acidity values specified in the Food Codex (Anonymous, 2009). The difference in fat contents and heat treatments has no significant effect on the acidity values of the yoghurt samples (p > 0.05). The rise in acidity during storage is a consequence of continuing conversion of lactose into lactic acid, even at low temperatures, by starter cultures and is known as post-acidification. The post-acidification causes flavor issues as well as rheological and textural defects such as syneresis (Deshwal et al., Reference Deshwal, Tiwari, Kumar, Raman and Kadyan2021). In parallel, although not significant, this post-acidification process also affected the number of yoghurt bacteria and could be especially effective in reducing the number of L. bulgaricus (Fig. S1a). Similar to the current findings, a higher viability of S. thermophilus compared to L. bulgaricus was reported by Akalın et al. (Reference Akalın, Fenderya and Akbulut2004). In a study by Helal et al. (Reference Helal, Rashid, Dyab, Otaibi and Alnemr2018), it was found that low-fat (0.1%) and full-fat (3.2%) yoghurt showed similar acidity during storage period of 14 days. Furthermore, Célia et al. (Reference Célia, da Silva, de Oliveira, Souza, de Moura Silva, Nicolau and Gonçalves2017) compared the pH and acidity yoghurts prepared from pasteurised and UHT treated milks during a storage period of 29 days and, in line with the current findings, found no significant difference.
Figure 1. Titratable acidity values of yoghurt samples during storage. Different lowercase letters indicate the periodical differences in the same sample (p < 0.05). UHT, ultra-high temperature; SF-UHT, yoghurt made from uht-sterilized skimmed milk; HF-UHT, yoghurt made from UHT-sterilized half-skimmed milk; 3 F-UHT, yoghurt made from UHT-sterilized 3% fat milk; FF-UHT, yoghurt made from uht-sterilized full fat (3.5%) milk; 3 F-P, yoghurt made from pasteurised 3% milk; HF-D-UHT, yoghurt made from UHT-sterilized half-skimmed vitamin d-added milk.
Texture and syneresis
Textural characteristics of yoghurt samples during storage are presented in Fig. 2. Firmness, consistency, cohesiveness, and work of cohesion were all affected significantly by yoghurt type (p < 0.05). There was no significant difference between the hardness, consistency and viscosity indices of each yoghurt sample during storage (p > 0.05).
Figure 2. Changes in textural characteristics of yoghurt samples during storage; (a) firmness, (b) consistency, (c) cohesiveness, (d) index of viscosity. UHT, ultra-high temperature; SF-UHT, yoghurt made from UHT-sterilized skimmed milk; HF-UHT, yoghurt made from uht-sterilized half-skimmed milk; 3 F-UHT, yoghurt made from uht-sterilized 3% fat milk; FF-UHT, yoghurt made from UHT-sterilized full fat (3.5%) milk; 3 F-P, yoghurt made from pasteurised 3% milk; HF-D-UHT, yoghurt made from UHT-sterilized half-skimmed vitamin D-added milk. Different capital letters on the columns represent the differences between the samples of the same period, and different lowercase letters indicate the periodical differences in the same sample (p < 0.05).
In general, increasing fat content resulted in increasing firmness values as indicated by Sfakianakis and Tzia (Reference Sfakianakis and Tzia2014). The highest average firmness values were determined in 3 F-P (1.77 N) and 3 F-UHT (1.61 N) samples, respectively. Considering that they had similar fat contents of 3%, the higher firmness of 3 F-P could be attributed to the pasteurization process applied to 3 F-P versus the UHT treated 3 F-UHT. Xu et al. (Reference Xu, Emmanouelidou, Raphaelides and Antoniou2008) stated that the higher the heating temperature applied and the higher the fat content, the more rigid the gel structure is. The lowest firmness values were found in HF-D-UHT (0.75 N) and SF-UHT (0.78 N), respectively. Considering that HF-D-UHT and HF-UHT have the same fat contents of 1.5%, the lower firmness value observed in HF-D-UHT can be associated with vitamin D addition. However, this statement has to be questioned and studied in detail for clarification.
Consistency values of yoghurt samples varied from 8.79 to 22.54 Ns during the storage of 21 days, with a slight increase in general (p > 0.05). The highest average consistency was determined in 3 F-P (22.03 Ns) and the lowest were in HF-D-UHT (9.69 Ns) and SF-UHT (9.81 Ns). Cohesiveness is the negative force required to pull back the probe during the back-extrusion analysis hence a higher absolute value of cohesiveness means a more cohesive yoghurt. Similarly, a higher value of index of viscosity refers to a thicker and more viscous yoghurt. Both cohesiveness and viscosity index showed a decreasing trend during storage confirming their generally known high correlation. The highest and the lowest cohesiveness and viscosity index values were observed for 3 F-P (−0.95 N, −1.02 Ns) and SF-UHT (−0.40 N, −0.38 Ns), respectively, indicating that increasing fat content results in higher viscosity and cohesiveness. Similarly, Gómez et al. (Reference Gómez, Odériz, Ferreiro, Már and Vázquez2020) reported higher cohesiveness, consistency, and index of viscosity in commercial full-fat yoghurt than low-fat yoghurt. When 3 F-P and 3 F-UHT with the same fat content and subjected to different heat treatments were compared, it was determined that pasteurization provided higher viscosity index, cohesiveness, and consistency compared to UHT. In parallel, Schmidt et al. (Reference Schmidt, Vargas, Smith and Jezeski1985) reported higher viscosity values for HTST (High temperature short time pasteurization at 82°C for 20 min) yoghurt than UHT (138°C, 3s and 6s) yoghurt samples.
Syneresis in yoghurt is a defect defined as the release of whey from the structure because of the shrinkage of yoghurt gel, and it affects both consumer preference and the physical quality of yoghurt (Rani et al., Reference Rani, Unnikrishnan, Dharaiya and Singh2012). Syneresis rate is not only affected by the composition (dry matter, protein and fat contents) but also the processing parameters (heat treatment, homogenization, fermentation, etc.) (Arab et al., Reference Arab, Yousefi, Khanniri, Azari, Ghasemzadeh-Mohammadi and Mollakhalili-Meybodi2022). The syneresis generally showed a trend of increase during storage (p > 0.05). The highest rates of syneresis were found in SF-UHT (55.08% on the 21st day), HF-D-UHT (43.31% on the 21st day), and HF-UHT (38.37% on the 21st day) samples, respectively. The lowest syneresis rates were detected in the 3 F-UHT, 3 F-P and FF-UHT samples with relatively higher fat content (ranged between 20.15% and 22.48% on day 21).
Although the syneresis values of HF-UHT and HF-D-UHT, yoghurts with the same fat content of 1.5%, are similar, the syneresis values determined in HF-D-UHT are slightly higher than HF-UHT (Fig. 3). Although vitamin D is fat-soluble, it could bind to casein and whey milk proteins in the emulsion form (Forrest et al., Reference Forrest, Yada and Rousseau2005; Tippetts et al., Reference Tippetts, Martini, Brothersen and McMahon2012). As vitamin D occupies the binding sites of milk proteins, it is possible that the yoghurt structure, consisting of casein, B-lactoglobulin, and fat globules, is not strong due to less interaction of proteins and cannot retain water in the network. This may explain the weak structure and higher syneresis of yoghurt made from vitamin D fortified milk.
Figure 3. Syneresis values of yoghurt samples during storage. UHT, ultra-high temperature; SF-UHT, yoghurt made from uht-sterilized skimmed milk; HF-UHT, yoghurt made from uht-sterilized half-skimmed milk; 3 F-UHT, yoghurt made from uht-sterilized 3% fat milk; FF-UHT, yoghurt made from UHT-sterilized full fat (3.5%) milk; 3 F-P, yoghurt made from pasteurised 3% milk; HF-D-UHT, yoghurt made from UHT-sterilized half-skimmed vitamin D-added milk. Different capital letters on the columns represent the differences between the samples of the same period (p < 0.05).
Viability of yoghurt bacteria
The alteration in the counts of yoghurt bacteria during storage is presented in Fig. S1. The number of viable bacteria for both L. bulgaricus and S. thermophilus were over the minimum required level of 107 cfu/g throughout the storage period of 21 days. Regarding L. bulgaricus, a downward trend of viable counts can be noted during storage, while no significant difference was determined between yoghurt samples (p > 0.05). Although a fluctuating pattern of viable counts was observed for S. thermophilus, overall, the differences between either yoghurt samples or storage periods were not remarkable. In this context, the fat content and heat treatment did not have a significant effect on yoghurt bacteria. Similarly, Helal et al. (Reference Helal, Rashid, Dyab, Otaibi and Alnemr2018) reported similar viabilities (>8 log cfu/mL) for S. thermophilus and L. bulgaricus in low- and full-fat yoghurt samples during storage.
Whiteness index
The whiteness index is an important criterion for the sensory acceptance of dairy products. The whiteness index (WI) values of yoghurt samples are shown in Fig. 4 (see Table S3 for complete data of L*, a*, and b* values). HF-D-UHT (84.60) and SF-UHT (80.77) owned the highest and the lowest WI values, respectively (p < 0.05). It would be logical to attribute the low WI value of SF-UHT to the fact that it does not contain fat. Considering that HF-D-UHT and HF-UHT had similar fat contents and composition other than vitamin D, and they were subjected to same heat treatments, it may be suggested that vitamin D has a positive effect on whiteness, but this has to be confirmed with an in-depth study. Although no significant difference was detected between 3 F-UHT, 3 F-P, and FF-UHT, it is reasonable to say that the WI value increases with the increase in fat content, excluding HF-D-UHT. The colour of yoghurt depends mainly on the composition (particularly fat, carotene, vitamins A and B2 contents, and additives if present), the processing conditions, i.e., homogenization, thermal treatment, and storage conditions. Fat globules play a major role in the bright colour of milk, hence L* value, due to its ability to scatter light together with casein micelles (Milovanovic et al., Reference Milovanovic, Djekic, Miocinovic, Djordjevic, Lorenzo, Barba, Mörlein and Tomasevic2020).
Figure 4. Whiteness index of yoghurt samples. UHT, ultra-high temperature; SF-UHT, yoghurt made from UHT-sterilized skimmed milk; HF-UHT, yoghurt made from UHT-sterilized half-skimmed milk; 3 F-UHT, yoghurt made from uht-sterilized 3% fat milk; FF-UHT, yoghurt made from UHT-sterilized full fat (3.5%) milk; 3 F-P, yoghurt made from pasteurised 3% milk; HF-D-UHT, yoghurt made from UHT-sterilized half-skimmed vitamin D-added milk. Different capital letters indicate significant differences (p < 0.05).
The severity of thermal treatment affects the browning due to the formation of melanoidins as products of the Maillard reaction. Cheng et al. (Reference Cheng, Barbano and Drake2019) reported increased L* values decreased and b* values with pasteurization. Approximately 3 times more browning rate was reported for 138°C, 4s compared to 95°C, 60s heat treatment by Deeth (Reference Deeth2021). Higher L* and b* values in ultra-pasteurised (138°C, 2s) milk compared to HTST (72°C, 15s) milk were determined in a study by Lee et al. (Reference Lee, Barbano and Drake2017). When WI values of 3 F-UHT and 3 F-P are compared to understand the effect of heat treatment on colour, it is plausible to state that the applied heat treatment did not affect the WI value significantly (p > 0.05).
Sensory evaluation
The sensory scores for yoghurt samples with varying fat levels and heat treatments are displayed in Fig. S2. Reducing the fat content of foods has become a trend due to health concerns, but with that comes some defects. Considering the fat content of yoghurt samples, it is clearly seen that SF-UHT, the sample with the lowest fat content, received the lowest score in all the parameters assessed. Brauss et al. (Reference Brauss, Linforth, Cayeux, Harvey and Taylor1999) attributed this phenomenon to the fact that the release of volatile substances in lower-fat yoghurts is intense and rapid but less persistent compared to those containing higher fat. Regarding flavor, the highest score was possessed by the HF-UHT sample, likely because half-fat (1.5%) yoghurt is the most consumed yoghurt in Türkiye. None of the panelists identified any off flavors, such as cooked flavor, or off-colours, such as browning, in the yoghurt samples. Among all UHT treated samples, FF-UHT and HF-UHT were the most preferred followed by HF-D-UHT and 3 F-UHT.
When 3 F-UHT (3% fat, UHT) and 3 F-P (3% fat, pasteurised) samples were compared, 3 F-P had higher scores in all categories. Furthermore, 3 F-P has even higher consistency and odor scores than the UHT-treated FF-UHT sample, which has a higher fat content. These results suggest better sensory acceptability could be achieved using pasteurised milk instead of UHT milk in yoghurt production. Although, Petrova et al. (Reference Petrova, Lapteva, Laricheva and Osipova2020) stated no noticeable effect of Vitamin D fortification on the organoleptic characteristics of drinking milk, the reason why vitamin D fortification reduced the consistency and flavor scores should be revealed in future studies. Similarly, in a study by Jalal Aghdasian et al. (Reference Jalal Aghdasian, Alizadeh and Soofi2022), vitamin D3 fortified yoghurt samples received lower scores of taste, texture and odor, and overall acceptability however, no significant difference was found between fortified and unfortified yoghurt samples. In a recent study by Saleem et al. (Reference Saleem, Khan, Hussain, Khan, Al-Asmari, Khan, Zafar, Rahim, Eliasse and Ramadan2024), no negative effect of vitamin D fortification (15 and 20 μg) on organoleptic properties of yoghurt was observed.
Conclusion
This study demonstrated the significant impact of milk fat content, heat treatment, and vitamin D fortification on the physical, textural, microbiological, and sensory properties of yoghurt. Yoghurts with higher fat content displayed superior textural qualities, including increased firmness, consistency, cohesiveness, and lower syneresis, contributing to enhanced sensory appeal. However, it has no significant effect on the post-acidification process and the viability of yoghurt bacteria. The effect of heat treatment is evident only when texture is considered; pasteurised milk yielded yoghurts with better textural attributes than UHT-treated milk, highlighting the importance of moderate heat treatment in preserving yoghurt structure. While vitamin D fortification improved the whiteness index, it slightly compromised textural integrity, resulting in increased syneresis and lower firmness, suggesting that vitamin D may interact with milk proteins in ways that weaken the gel matrix. In summary, this study underscores the critical role of milk fat and processing parameters in determining yoghurt's quality and consumer acceptance. Further studies may focus on the effect of fat content on the rheological properties of yoghurt. Also, the effects of both Vitamin D and, if any, its interaction with the protein structure on the texture of yoghurt gel and colour of yoghurt should be examined at the micro level to be enlightened.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0022029925101003.
Acknowledgements
This study was supported by “The Scientific and Technological Research Council of Türkiye (TUBITAK)” (Project #1919B012106161) under the “Research Project Support Program for Undergraduate Students (2209-A)”.
Competing interest
The authors declared that they have no conflict of interest.
Author contributions
Ömer Faruk Çelik: Conceptualization, supervision, investigation, methodology, data analysis, writing – original draft, review & editing. Bahadir Karakus: Investigation, data analysis, writing – original draft.
Data availability statement
Data is available on request from the authors.
