Introduction
Recently, the popularity of dairy products made of goat milk has increased since there is an ever-increasing demand for healthy diets (Singh et al., 2024). Goat milk has anti-inflammatory, anti-microbial, and therapeutic qualities that can help cure malabsorption syndrome. It also has the ability to prevent cardiovascular diseases, cancer, anemia, poor mineral bone density and cholesterol disorders (Akshit et al., 2024). Globally, fermented foods made of goat milk are gaining favor because fermentation by various microbes eliminates the characteristic "goaty" taste (Pulina et al. 2018).
With various physicochemical characteristics like casein content, αs-casein ratio and micelle size, goat milk results in formation of a loose-textured gel structure in yoghurt (Guo, 2003; Park et al., 2007). Besides nutritional benefits, the loose texture, low viscosity and serum separation issues of goat yoghurt should also be addressed to enhance consumer acceptance. Improvement of these aspects is essential for increasing the consumption of goat dairy products with various positive effects on human health. Therefore, this study focuses on textural and consistency issues of goat yoghurt. The investigation will primarily look into the potential use of milk protein-based additives such as skim milk powder (SMP), milk protein concentrate (MPC), and whey protein concentrate (WPC) to enhance the physicochemical, biochemical, textural and sensory traits of Saanen goat milk yoghurts.
In general, dried milk products are divided into two groups. Casein-based dried products include sodium caseinate, skimmed milk powder (SMP) and calcium caseinate. Whey protein-based products include whey protein isolate (WPI) and whey protein concentrate (WPC). Both casein-based and whey protein-based products are used as additives in yoghurt production. As compared to non-additive control yoghurts, casein-based milk additives increased curd tension and reduced whey separation of set yoghurts and increased viscosity of stirred-type yoghurts with broken curd (Amatayakul et al., 2006). Bhullar et al. (2002) reported that 2 % whey protein concentrate supplementation increased viscosity, reduced whey separation, but reduced curd tension as compared to the control samples. Previous studies reported improved physical, textural and sensory properties of goat yoghurts with milk protein-based additives. Martin-Diana et al. (2003) used 3 and 5 % whey protein concentrate in production of set-type fermented dairy products with the use of commercial probiotic culture (ABT-2) and reported that WPC supplementations increased protein, potassium and magnesium content, apparent viscosity and gel firmness and reduced whey separation of the final product and 3 % WPC-supplemented product had superior sensory attributes (appearance, texture, taste and aroma) and general acceptance over the other products. Gürsel et al. (2016) produced goat yoghurts with 2 % (wt/vol) skim goat milk powder (SGMP), whey protein concentrate (WPC), sodium caseinate (NaCn), whey protein isolate (WPI) and yoghurt texture improver (YTI) supplementations and indicated that WPI yielded the hardest structure in yoghurt, leading to higher syneresis values. For the investigated parameters, milk protein-based products such as sodium caseinate (NaCn) or whey protein concentrate (WPC) yielded promising outcomes and thus were considered as suitable ingredients for goat yoghurt production. Wang et al. (2012) developed a probiotic goat yoghurt containing Lactobacillus casei, Lactobacillus acidophilus and Bifidobacterium spp. with the use of polymerized whey protein (PWP, 0.4 %) and pectin (0.3 %) as gelling agents and indicated that PWP might be a novel protein-based thickening agent for improving the consistency of goat yoghurts and the other products.
Despite the extensive studies on milk protein additives in yoghurt production, there is a lack of research comparing various physicochemical, biochemical (fatty acid composition, volatile compounds) and textural properties of yoghurts made of Saanen goat milk and fortified with milk-based proteins during storage. The specific aroma and flavor profiles of yoghurt rely on a thorough understanding of the fatty acid composition and volatile compounds. This study aims to bridge this gap by examining these attributes over a 14-day storage period, providing insights into the fatty acid composition, volatile profile, textural and sensory properties, all of which significantly influence consumer preferences and the unique aroma of yoghurt. Therefore, this research aims to improve the textural and compositional properties of goat yoghurt by using milk protein-based additives, addressing key issues that affect consumer acceptance. By addressing the existing research gap, the study will contribute to the broader knowledge of goat milk yoghurt production, potentially boosting its popularity and consumption.
Materials and methods
Materials
The primary material of the present study, goat milk, was supplied from Saanen race goats housed in Livestock Facility of Aydın Adnan Menderes University Agricultural Faculty. Yoghurt production from goat milk was performed in Integrated Livestock Research and Implementation Center of Aydın Adnan Menderes University Agricultural Faculty, Türkiye. Commercial Yo-Flex R Express 1.0 Thermophilic yoghurt culture (Batch no: 3141385) including Streptococcus thermophilus and L. delbrueckii subsp. bulgaricus microorganisms were used as the starter culture in yoghurt production. MPC and WPC to be used in present experiments were supplied from Maysa Food Co. (İzmir) and skim milk powder (SMP) was supplied from Akova Food Trade Co. (Konya). Composition of these milk-based powders (MPC, WPC, SMP) is provided in Table 1. Experimental samples were stored at 5±1 °C and passed through the relevant tests in subsequent day. All analyses of milk and yoghurt samples were conducted in ADÜ-TARBİYOMER (Aydın) laboratory.
Table 1. Composition of SMP, MPC and WPC utilized in the present study*
| Protein (%) | Moisture (%) | Fat (%) | Lactose (%) | Ash (%) | |
|---|---|---|---|---|---|
| SMP | ≥34 | ≤5.0 | ≤1.5 | 52.0 | 9.00 |
| MPC 70 | ≥69.5 | ≤6.0 | ≤2.0 | ≤19.0 | ≤7.5 |
| WPC 35 | ≥34 | ≤4.0 | ≤2.0 | ≤52.0 | ≤9.00 |
*SMP: Skim milk powder; MPC: Milk protein concentrate; WPC: Whey protein concentrate
Yoghurt manufacture and sampling
All milk was processed into yoghurt in accordance with the production technology specified by Tamime and Robinson (1999) (Figure 1). Gross chemical composition was assayed 1 day after storage; however, pH, titratable acidity (% LA), whey separation, hardness, FFAs, volatiles were determined 1, 7 and 14 days after storage. Productions were performed in 3 replicates and analyses were conducted in two parallels. Texture analysis was conducted in 3 parallels to get homogeneous data.
Figure 1. A flow chart for production of goat milk yoghurts
Physicochemical analyses of raw milk and yoghurt samples
Titration acidity, pH, total dry matter, fat, ash, N and protein content of goat milk to be processed into yoghurt were determined. Goat milk pH was measured with a pH meter (Adwa, Romania) equipped with a combined glass electrode. Titratable acidity (%) was analyzed with the use of 0.1N NaOH and phenolphthalein and expressed as the percentage of lactic acid. The total solids and fat content of the raw goat milk were analyzed in accordance with Turkish standards (TS 1018; Anonymous 1994). Protein content was determined with Kjeldahl method and ash content was analyzed by ashing in a muffle furnace at 550 °C (Anonymous 1993).
The pH value of yoghurt samples was measured with the use of pH meter equipped with a combined glass electrode. Titratable acidity (% Lactic acid) was determined in accordance with TS 1330 yoghurt standard (TS 1330; Anonymous 1999). Total dry matter content was analyzed using gravimetric method. Fat content was analyzed with the use of Gerber method. Total nitrogen content was determined with the Kjeldahl method (Anonymous, 1993). Resultant total N content was multiplied by a coefficient of 6.38 to get protein content (%). For whey separation, 10 g yoghurt sample at 20 ºC were centrifuged at 3500 rpm for 20 minutes and resultant serum was analyzed. Textural properties of the samples were analyzed in Zwick/Roel Z0.5TH texture analysis device (Akgül and Karaman, 2019).
Determination of free fatty acids
A gas chromatography (GC) system was used for free fatty acid analyses in accordance with the method specified by Deeth at al. (1983). About 2.5 g yoghurt sample was mixed with 2.5 g Na2SO4, 5 mL internal standard (C7) and 300 μL H2SO4. Resultant mixture was then supplemented with 5 mL hexane. The liquid phase was extracted from Biorad column with deactivated alumina after 1 h resting period. Sample elution was performed twice in each column. Columns were then washed twice with 5 mL hexane/diethyl ether (1:1), dried under 5 psi air pressure, placed into test tubes and supplemented with 2mL 6% formic acid. Resultant mixtures were centrifuged at 2000×g for 10 min. Supernatant was transferred into vials and stored at -18 °C until the time of analysis. Samples were analyzed for free fatty acids in a GC device equipped with an FID detector (260 °C) and a capillary column (30 m×320 μmi.d. with 0.25 μm film thickness). Injection mode was split (1/10) and injection volume was 5 μL. Flow rates were set as 33:370:30 mL min-1 (H2:air:N2). Temperature gradients were arranged as: Initial temperature of 120 °C, gradually increased to 200 °C with 10 °C min-1 rate, held at 200 °C for 2 min, gradually increased to 205 °C with 10 °C min-1 rate, held at 205 °C for 2 min, gradually increased to 210 °C with 10 °C min-1 rate, held at 210 °C for 2 min, gradually increased to 215 °C with 10 °C min-1 rate, held at 215 °C for 3 min, gradually increased to 230 °C with 10 °C min-1 rate, held at 230 °C for 3 min.
Determination of volatile compounds
Characterization of flavors and identification of volatile components and determination of residence times were performed by injecting the sample into a gas chromatography-mass spectrometry (GC/MS) system (Whetstine et al. 2003). The volatile components, separated using a DB Wax column (model 122-7032 from Agilent Technologies, USA; 30 meters length and 0.25 millimeters inner diameter, and a film thickness of 0.25 micrometers), were analyzed by scanning within the mass-to-charge ratio (m/z) range of 30-300 using a GC5975 C MSD mass spectrometer. Before analysis, the vials were sealed and kept in a freezer at -25 °C. The vials were allowed to equilibrate volatile components for half an hour at 40 °C in the SPME extraction equipment before being injected into the gas chromatography apparatus (Stashenko and Martinez, 2007). The following equation was used to determine the amounts of volatile components, taking into account the areas acquired for volatile components in gas chromatography as well as the internal standard utilized during sample extraction:
Relative quantity (µg/kg) = (Area of volatile compound / Area of internal standard) x Area of internal standard x correction factor (0.81)
Sensory properties
Sensory traits of manufactured yoghurts were assessed by ten panelists (five female and five male) selected from the staff members of Faculty of Agriculture who were familiar with food and dairy products. Thirty percent of panelists’ ages ranged from 40 to 55, thirty percent of ranged from 30 to 40 and forty percent of ranged from 20 to 30 years old. Results were recorded on a score sheet described by Bodyfelt et al. (1988). Panelists evaluated samples in terms of taste, consistency and general evaluation, using an eight-point scale (1=not present to 8=very intense). The panelists were trained before each evaluation session, introducing some sensory terms for aroma and yoghurt flavors. For this sensory evaluation, panelists held training sessions where they assessed goat yoghurts with noticeable defects to identify specific deficiencies. The excellent quality for this type of goat product was clearly defined, and potential defects were introduced. Additionally, panelists were instructed on how many points to deduct from the maximum score of eight for any deviations from the defined quality standards. Approximately 100 g of yoghurt was given to each panelist. The panelists were provided with a glass of water and an unsalted cracker alongside the samples. Samples were presented on white plates with 3-digit blinding codes. Sensory assessment of the samples was conducted twice.
Statistical analysis
Experiments were conducted in randomized blocks design. Data were subjected to analysis of variance (ANOVA) with the use of SPSS 20 software package (SPSS Inc., Chicago, IL). Variations between the yoghurt samples and storage periods were assessed using Duncan’s Multiple Range Test at a significance level of 0.05. In this study, all yoghurt productions and analyses were performed in triplicate with three parallel samples.
Results and discussion
Nutritional and chemical values of milk
Compositional profile of raw Saanen goat milk of the present study was identified as 14.21 % dry matter, 4.9 % fat, 3.31 % protein, 0.81 % ash and 6.8 pH. The nutritional composition of goat milk differed among goats from various geographic regions, seasons, locations, breeds, feeding systems, lactation stages, diets, environmental conditions and other factors (Haenlein 2004). Averages of Saanen goat milk composition originated from Turkiye were between 9.19-14.44 % dry matter, 1.8-5.50 % fat, 2.12-5.03% protein and 6.51-7.84 pH (Kesenkaş et al., 2010). Present composition values were in line with those reported by Kesenkaş et al. (2021) but higher than those reported by Zubeir et al. (2012) for goat milk in the Sudan. However, Terzioğlu and Bakırcı (2023) reported that goat milk obtained from local farms in east of Turkiye had total solids content of 12.99 %, fat content of 4.22 %, protein content of 3.73 %, ash content of 0.83 % and pH of 6.68. Differences in compositional profiles were mostly attributed to differences in breeds, feeding systems and environmental conditions.
Physicochemical characteristics of yoghurts
Table 2 shows the chemical composition of control yoghurt and the yoghurts prepared with three milk-based substrates. There were significant differences in total solids, protein and ash contents of the yoghurt samples (p<0.05). Total solids content was significantly (p<0.05) higher (13.60±0.06 %) with higher protein content (4.66±0.09 %) in MPC yoghurt as compared to other yoghurts. Ash content was significantly lower (p<0.05) in the control yoghurts. These differences were caused by the various compositional fortification substrates (Table 1) used for preparation of goat milk yoghurts.
Table 2. Chemical composition of yoghurt samples at day one post-manufacture (n= mean, ±sd)*
a-dMeans indicated with different letters in the same column are significantly different (p<0.05)
The pH and titration acidity values of yoghurt samples during the storage period (1, 7 and 14 days) are provided in Table 3. The pH values of yoghurt samples varied between 4.00 - 4.44 with the greatest value from SMP-supplemented samples at the beginning and the end of storage. The differences in average pH values of storage durations and average pH values of the samples were found to be significant (p<0.05). Samples containing whey powder (WP) showed a slower decrease in pH in the first 7 day than the others. This behavior is explained by the fact that the presence of whey protein (WP) increased the ionic strength of the liquid phase of the product, which affects the fermentation process. Starter cultures at lower lactic acid concentrations may become less active due to the higher ion concentration, which might lead to the process self-regulating. This may be advantageous as it could help increase the product's shelf life, which is usually restricted by high acidity (Gonzalez-Martinez et al., 2002).
Decreasing pH values are expected in fermented products with the development of acidity. Lactic acid bacteria also reduce pH values of fermented products throughout the storage (Kailasapathy, 2006). Since different protein concentrates were used in each sample, average pH values were different. This result agrees with what was found by Rahi et al. (2023), who showed the decrease in pH values of yoghurt when compared to the control treatment indicating the effect of the added proteins.
Table 3. Physicochemical properties of yoghurt samples during storage for 14 days (n= mean, ±sd)*
*C, control; SMP: Skim milk powder; MPC: Milk protein concentrate; WPC: Whey protein concentrate. a-fMeans indicated with different letters in the same row are significantly different (p<0.05). A-DMeans indicated with different letters in the same column are significantly different (p<0.05), sd: Standard deviation
Lactic acid values of the yoghurt samples varied between 0.67-1.12 % (Table 3). The titratable acidity was greater in the samples with MPC than the samples with SMP and WPC (p<0.05). Such a case was linked to the effect of higher protein levels (Table 1), thereby increasing soluble complexes of yoghurt milk could possibly provide more points of attachment during acidification (Jørgensen et al., 2019). The differences between average of storage days and average of samples were found to be significant (p<0.05). Lactic acid ratios increased throughout the storage and such an increase was found to be significant in each day of storage. It was expected that lactic acid contents increased based on activity of starter culture used in yoghurt production and different values were normal throughout the storage. Lactic acid content averages of the samples were also different. The greatest lactic acid content was obtained from MPC-supplemented samples at the last day of storage. Besides lactic acid, titration acidity also includes total acidy resulted from proteins, phosphates and citrates. Therefore, it is expected to have greater lactic acid contents on the 7th and 14th days of storage. Heterogenous results were observed since different protein concentrates were used in each sample. Lee and Lucey (2010) reported that whey proteins increased total solids and lactic acid contents of yoghurt samples. According to Wang et al. (2023), in yoghurt-making, the rate of acidification of milk through the production of lactic acid content, with a consequent decrease in pH, is related to the number of live bacteria.
Whey separation values of the yoghurt samples are provided in Table 3. Present values varied between 3.58-7.17 mL with the greatest value from the 1stday of control samples and the lowest value from the 1stday of SMP-supplemented samples. The differences in whey separation values of the samples were found to be significant (p<0.05). Since dry matter and protein contents of the samples were different (Table 2), whey separation values were also different. Compared to control samples without any additives, protein concentrate-supplemented samples had lower whey separation values because of water holding capacity of the proteins.
Yoghurts produced by the addition of MPC had higher protein content and lower lactose content, and thus, higher firmness with high whey separation than the yoghurts produced by SMP and WPC. Similarly, Tamime et al. (2014) reported that the use of membrane-manufactured powders containing less lactose and more protein than SMP could enhance the composition and textural characteristics of high-protein yoghurts. In the literature, Çelik and Temiz (2022) found that the occurrence of syneresis in yoghurt was linked to the dry matter content, particularly the protein content. In fermented milk gels, the reduced syneresis comes from adding casein or whey proteins that apparently due to their higher water-holding sites (Singh et al., 2024). Similarly, Gürsel et al. (2016) observed an increase in the syneresis of goat yoghurt made from WPC and WPI as compared to milk powder-supplemented yoghurts. The differences in whey separation values of the storage days were not found to be significant (p>005).
The Fi3 values indicate the maximum force applied to the product. Maximum force then indicates stiffness, so called as hardness or firmness of the product. Hardness indicates the product bearing resistance to an applied impact force. In other words, it indicates the strength required for a foodstuff to resist the pressure exerted between the palate and the tongue and between molar teeth (Szczesniak 2002). The Fi3 values of yoghurt samples varied between 0.06±0.01 - 0.55±0.02 N with the greatest value from the last day of 3 % MPC-supplemented samples (Table 3). There were significant differences both between samples and between storage days (p<0.05). WPC-supplemented samples constituted the weakest texture group with weaker curd formation even than the control samples. Li and Guo (2006) indicated that goat yoghurts had lower viscosity than cow yoghurts probably because of lower αs1-CN level of goat milk. Similarly, since MPC had greater casein content than WPC and SMP, MPC-supplemented yoghurts had the greatest hardness values in the present study. Similarly, UF concentration of goat milk to 1.5-2.0 folds expanded hardness and extrusion force with simultaneous decrease in whey syneresis as compared to non-concentrated milk in set-type goat yoghurt (Domagała, 2012). A general increase was observed in hardness values with the progress of storage days and the differences in hardness values of the 7th and 14th days were not found to be significant. Hardness of goat yoghurts supplemented with different proteins did not show notable changes on the 21st and 28th days of storage (Herrero et al., 2006; Gürsel et al., 2016).
Free fatty acid profiles of yoghurts
Free fatty acids (FFAs from C4 to C18:3) of yoghurt samples were determined and mean values are given in Table 4. The most abundant FFAs in yoghurts were oleic acid, palmitic acid and stearic acid. These same FFAs were also found to be the most abundant ones in salted goat yoghurts (Güler 2007). The lowest fatty acid was caprylic acid (C8:0) with values of between 8.73-11.18 ppm and it was respectively followed by caproic acid (C6, 9.77-11.72) and capric acid (C10, 11.01-16.37). Lowest fatty acid values were detected in control yoghurts, while the highest values were determined in SMP-supplemented yoghurts (Table 4). The addition of proteins led to significantly higher FFA profiles and related to higher protein and bacterial activity. Higher profiles were significantly observed on day 1 of storage in SMP and generally similar values on day 14 of storage with WPC. This could be explained by higher FFA profiles which might have resulted from the release of free fatty acids from triglycerides continue to occur during the process of lipolysis. Certain short-chain saturated fatty acids, such as capric, caprylic and caproic acids, are primarily responsible for the unique flavor and aroma of goat milk and its fermented products (Sumarmono et al., 2015). Yoghurt contains numerous free fatty acids (FFAs) as a result of the activity of microbiological starter cultures during processing. Additionally, the addition of lactic acid bacteria in dairy products might promote the production of FFAs through lipolysis of milk fat (Santos Júnior et al., 2012).
Table 4. Free fatty acid profiles of yoghurt samples during storage for 14 days (n= mean ±sd)*
*C: Control; SMP: Skim milk powder; MPC: Milk protein concentrate; WPC: Whey protein concentrate. a-fMeans indicated with different letters in the same row are significantly different (p<0.05). A-DMeans indicated with different letters in the same column are significantly different (p<0.05), sd: standard deviation
Butyric acid (C4) was identified as the most predominant short chain FFA in yoghurt samples, myristic acid (C14) as the major medium chain FFA and oleic acid (C18:1) as the dominant long chain FFA. Control yoghurts had much lower concentrations of these three fatty acids as compared to milk protein-based yoghurts (p<0.05). Contrary to present findings, Güler and Gürsoy-Balcı (2011) reported ethanoic acid (C2) as the dominant short chain FFA and palmitic acid (C16) as the dominant long chain FFA. This may be due to the type of milk and starter culture used in yoghurt production. This was consistent with the results of Şenel et al. (2011), who stated that butyric, capric, myristic, palmitic, stearic and oleic acids were predominant free fatty acids in both set and strained goat milk yoghurts.
The concentrations of short chain FFA (C4:0-C8:0) increased while long chain FFA (C16:0-C18:3) decreased at the end of the storage. An opposite trend was reported on goat and cow milk mixture yoghurts by Güler and Gürsoy-Balcı (2011). The beginning and mean concentrations of FFAs during storage were significantly affected by protein concentrates with mean levels in the control yoghurts being significantly lower than those in the other yoghurts, except for C18:3. Different protein concentrates significantly influenced (p<0.05) short FFA and C4-C18:2 levels. Güler and Park (2011) reported similar FFA levels for 10 brands of Turkish commercial set-type yoghurts. Myristic acid (C14:0) was the dominant fatty acid (23.95 - 32.56) among the short and medium chain fatty acids. There were no significant differences in short-chain FFA concentrations of the yoghurts made of different proteins (p>0.05). In terms of long-chain fatty acids (C16-C18:3), change in stearic acid (C18) and linoleic acid (C18:2) content of the control samples was found to be significant (p<0.05). However, both on the 1st and 14th days of storage, no significant differences were detected in palmitic (C16), oleic (C18:1) and linoleic acid (C18:3) contents of all yoghurt samples. The greatest palmitic and oleic acid contents were obtained from SMP-supplemented samples during the storage. Shortly, long-chain FFAs showed an opposite relationship with short-chain FFAs during the storage. Similarly, Abd Rabo et al. (1992) found that fatty acids ranging from C6 to C12 decreased, while those from C14 to C18:2 increased throughout fermentation in the production of goat yoghurt. Recently, FFAs in yoghurts fermented with different starter cultures have been studied and storage of yoghurt resulted in higher concentrations of short-chain FFAs and lower concentrations of saturated and medium-chain FFA contents (Gu et al., 2021).
Caproic acid (C6) contents of all yoghurts significantly decreased throughout the storage (p<0.05) and the lowest value was observed on the 14th day of storage. Generally, the amount of individual free fatty acids (C12-C18:3) in samples increased during the storage. Additionally, the decrease in lauric acid of SMP and WPC, palmitic acid of SMP, linoleic acid of the control samples at the end of the storage were not significant. Linoleic acid (C18:2) content slightly decreased in control, SMP, WPC samples on 7th day of storage, but increased again on 14th day of storage. Generally, FFA contents were significantly (p<0.05) influenced by protein addition and storage time. Therefore, control yoghurts had significantly different FFAs from the protein-based yoghurts, except for palmitic acid, oleic acid and linolenic acids. Similar findings were obtained by Güler (2007). The changes in butyric acid contents of SMP and control samples during the storage showed different trends from the others (Table 4). First, there was a slight increase and then a decrease occurred. Although butyric acid content of control yoghurts decreased on 14th day of storage, mean values increased (from 19.22 to 20.28).
Volatile compounds of yoghurt samples
Volatile compounds of yoghurt samples are provided in Table 5. Totally 22 components including acids (6), esters (2), terpenes (6), hydrocarbons (4) and others (4) were identified in all yoghurt samples by SPME/GC-MS technique. According to volatile flavor compounds, control yoghurts were similar to ones made of goat milk with SMP, MPC and WPC. In a study (Bennato et al., 2020), 26 aroma components containing acid, alcohol, ketone, aldehyde, ester, lactones were identified using SPME/GC-MS technique for Saanen goat yoghurts.
Table 5. Concentrations volatile compounds of yoghurt samples during storage for 14 days (ppm, mean±std error)*
*C: Control; SMP: Skim milk powder; MPC: Milk protein concentrate; WPC: Whey protein concentrate. a-fMeans indicated with different letters in the same row are significantly different (p<0.05).A-DMeans indicated with different letters in the same column are significantly different (p<0.05). sd: standard deviation
Major components were determined as acid (octanoic acid), ester (arsenous acid tris ester), terpen (D-limonene), hydrocarbon (benzene 1.3 dimethyl) and others (hexanal) for all yoghurt samples. Terpenes (in particular, D-limonene) were the most abundant volatile compounds in the present study. Hexanal and nonanal were detected as aldehydes in the samples. In previous studies, the characteristic aroma compounds of yoghurt were identified as acetaldehyde, diacetyl and lactic acid along with acetone, acetoin having a distinct goaty flavor as compared to bovine milk yoghurt (Terzioğlu and Bakırcı, 2023; Akshit et al., 2024). However, the main components as acetaldehyde (Bennato et al., 2020) and diacetyl (Gürsel et al., 2016) were not found in goat milk yoghurts (Table 5). The difference of main components may be explained by variations in analysis and extraction methods of the authors (Güler and Park 2011). Moreover, this may be also related to the amount of these volatile compounds affected by various factors, such as the type of milk, enzyme activities of starter cultures, culture rate and the production method (Terzioğlu and Bakırcı, 2023).
Acids are crucial for the aroma and flavor characteristics of yoghurt, serving as the main contributor to its fresh and sour taste (Panagiotidis and Tzia, 2001). Butanoic, hexanoic, heptanoic, octanoic, n-decanoic and tetra decanoic acids were quantified in all samples. Control yoghurts had insignificantly higher concentrations of these acids as compared to protein-based yoghurts (p>0.05). However, the lowest acids were detected in MPC yoghurts as compared to other protein concentrate-supplemented samples (p>0.05). Different protein concentrates and storage durations did not influence acid concentrations (p>0.05). The above results were consistent with the findings of Wang et al. (2023) in which octanoic acid, dodecanoic acid and n-decanoic acid were the prominent goat mutton flavor in goat milk yoghurts.
Esters primarily result from the hydrolysis of fatty acids and the bacterial activity (Rincon-Delgadillo et al., 2012). When present in high quantities, ethyl esters of fatty acids can cause a taste defect known as "fruity." At low concentrations, they improve the scent of dairy products (Liu et al., 2004). In this study, there were 2 different esters in low concentrations for all yoghurt samples. However, both between on the 1st and 14th days of storage and between yoghurt samples no significant differences were detected in ester concentrations of control and protein concentrate-supplemented yoghurt samples (p>0.05). Wang et al. (2023) observed the lowest ester concentrations in goat milk yoghurt as compared to cow milk and sheep milk yoghurts.
Terpenes, identified as secondary plant metabolites, are directly transferred to milk through feeding and play a crucial role in establishing the geographical origin (Bontinis et al., 2012). Terpene concentrations were generally lower in the control yoghurts. Limonene was the dominant compound in all yoghurt samples, which gives a fruity and citrus flavor and its concentrations changed from 22.672 (SMP sample on the 1st days of storage) to 7.313 (SMP sample on the 14st days of storage). Particularly, terpenes like limonene are a known component of citrus fruit-derived essential oils (Condurso et al., 2008). However, different protein concentrates and 14 day storage generally created no significant effect on terpene concentrations (p>0.05). At the beginning, terpene concentrations were highest, declining by lowest after storage for 14 days. Contrary to present findings, most of the authors did not find terpenes in goat milk yoghurts (Wang et al., 2023; Gürsel et al., 2016; Bennato et al., 2020). This is probably due to the differences in the composition of milk affected by animal type, breed, feeding and milking methods (Terzioğlu and Bakırcı, 2023). According to Sahingil and Hayaloglu (2022), rosehip addition had inducible effect on the presence of terpene compounds such as limonene and β-pinene in the yoghurts.
Hydrocarbons, the secondary products of lipid autoxidation, are not aromatic compounds (Bintsis and Robinson 2004). The concentrations of hydrocarbons, particularly benzene 1,3-dimethyl and benzene 1,2 dichloro, were significantly quite high in control yoghurts (p<0.05). A general decrease was observed in hydrocarbon concentrations with the storage days and the differences in four hydrocarbons of the 1th and 14th days were not found to be significant (p>0.05). A similar trend was observed in the other volatile compounds such as 2-nonanal. Similarly, Wang et al. (2023) determined hexanal and 2-nonenal as an aldehyde in goat milk yoghurts. Dairy products include unsaturated fatty acids that can undergo non-enzymatic intrachain oxidation in the presence of free radicals, which can result in the formation of hydro peroxides. Then, these hydro peroxides decompose rapidly to form unsaturated aldehydes, propanal, hexanal, heptanal, octanal and nonanal (Thierry et al., 2017; Mc. Gorrin et al., 2007).
Sensory properties
Figure 2 displays the mean sensory scores of yoghurts on the 1st, 7th and 14th day of storage. The effect of protein concentrates of the yoghurts on taste/flavor, consistency and acceptability scores was found to be significant (p<0.05). However, the effect of storage duration of the yoghurts on taste/flavor, consistency and acceptability was not significant (p>0.05). In terms of all sensory scores, it was noticed that both SMP and MPC yoghurts had the similar scores. The treatments involving MPC and SMP additions outperformed the control treatment, with the added protein types contributing positively to desirable attributes such as texture, taste and flavor. Higher acidity (0.91 % and 0.98 LA on the 1st day to 1.12 and 1.08 % LA on day 14, MPC and SMP respectively) of both goat milk yoghurts reduced the survivability of lactic acid bacteria. Therefore, the release of metabolites by lactic acid bacteria and the increase in acidity somewhat concealed the characteristic goaty flavor in yoghurt (Akshit et al., 2024). However, all sensory scores of control and protein-based yoghurts had the same trends and decreased during the storage (p<0.05). Additionally, at the beginning and the end of storage, the highest taste/flavor, consistency and acceptability scores (p<0.05) were identified in MPC yoghurts as compared to other treatments. Control yoghurts had much lower organoleptic scores as compared to protein-based SMP and MPC yoghurts but higher scores than WPC-supplemented samples (p<0.05). The WPC yoghurts exhibited significantly lower (p<0.05) taste/flavor, consistency and acceptability scores as compared to SMP and MPC yoghurt samples on the 1st days of storage. Moreover, the SMP and MPC yoghurts were perceived as having the highest acceptability in terms of all organoleptic properties of the 7th and 14th days. Despite the differences in total solids, protein and FFA profiles, the sensory similarity of milk-based concentrates (SMP and MPC yoghurts) may be related to the perception thresholds of FAs. In addition, the effect of FAs on acceptability may be less significant, especially in yoghurts with high pH levels. All yoghurt samples had acceptable scores at the end of storage. Although extremely weak consistency was perceived in WPC-supplemented samples at the last day of storage by some panelists, they did not refer off flavor in any samples. In this sense, off flavor was not also reported in strained goat milk yoghurt until the 45th day of storage (Şenel et al., 2011). Present findings on sensory properties comply with the results of Attia et al. (2023) who reported a decrease in organoleptic scores of goat yoghurts fortified with WPC during the storage for 14 days.
Yoghurt acceptance is closely related to its composition, which is important for improving its flavor, texture and appearance (Singh et al., 2024). The MPC-supplemented samples had a higher score for these sensory properties than the other yoghurts. A possible reason for the lower SMP and WPC flavor score than 3 % MPC-supplemented samples may be attributed to higher concentration of octanoic acid (C8, caprylic acid), which is related to flavor notes such as "waxy", "soapy", "goaty", "musty", "rancid" and "fruity" (Thierry et al., 2017). This is also correlated to the octanoic acid content of MPC yoghurts, which is lower than others in present study (Table 5). Rahi et al. (2023) reported the highest total sensory evaluation scores for MPC supplemented goat yoghurts at the end of 14-day storage period. Similar with the present findings, Singh et al. (2024) also reported improved scores for sensory properties of fortified yoghurts.
Chart representing mean scores of the sensory properties of yoghurts on the 1st day (solid lines) and 14th day (dashed lines) of storage. C: Control; SMP: Skim milk powder; MPC: Milk protein concentrate; WPC: Whey protein concentrate. Flavor, taste, consistency and acceptability were evaluated over a scale ranging from 1 to 8.
Figure 2. Sensory properties of yoghurt samples
Conclusion
Physicochemical composition of the Saanen goat milk yoghurts was greatly influenced by type of milk proteins supplemented into the yoghurts. Protein and dry matter contents varied with the SMP, MPC and WPC supplementations and the greatest protein and dry matter contents were obtained from 3 % MPC supplementations. The highest whey separation was observed in control yoghurts and the least in SMP yoghurts. MPC supplementations yielded the hardest structure in yoghurt, leading to higher syneresis values than other protein-based yoghurts. Additionally, milk proteins resulted in an increase generally in the concentration of short and medium-chain FFAs of the MPC and WPC yoghurts, which is favorable to the flavor of goat milk yoghurt. Higher FFA profiles were noted in SMP samples, linked to higher protein and bacterial activity. Differences in FFA content were significant (p<0.05), with short-chain FFAs increasing and long-chain FFAs decreasing by the end of storage. Totally 22 volatile compounds including acids, esters, terpenes, hydrocarbons and other compounds were quantified in all yoghurt samples. In general, acids and terpenes were the most important volatile compounds in all samples. Furthermore, dominant volatile compounds were identified as octanoic acid, arsenous acid tris ester, D-limonene, benzene 1.3 dimethyl and hexanal. Additionally, MPC and SMP yoghurts received higher sensory scores as compared to control and WPC samples, likely due to higher acidity reducing the goaty flavor. Control yoghurts had lower organoleptic scores but remained acceptable throughout storage. MPC-supplemented samples received the highest sensory scores, likely due to higher octanoic acid content contributing to favorable flavor notes. It is important to note that there are other components in milk and yoghurt that were not examined in this study but have an impact on some aspects of their properties. Further research is recommended to take these parameters into account. It was concluded based on present findings that milk based proteins, especially MPC with higher textural and compositional properties and sensory evaluation scores, could be used to improve physicochemical quality traits of goat yoghurts.
References
Rincon-Delgadillo, M.I., Lopez-Hernandez, A., Wijaya, I., Rankin, S.A. (2012): Diacetyl levels and volatile profiles of commercial starter distillates and selected dairy foods. Journal of Dairy Science 95, 1128-1139.https://doi.org/10.3168/jds.2011-4834.