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Physicochemical and sensory characteristics of yoghurt produced from mixtures of cows' and goats' milk
International Dairy Journal 18 (2008) 1146–1152
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International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj
Physicochemical and sensory characteristics of yoghurt produced from mixtures of cows’ and goats’ milk Maria Vargas*, Maite Cha´fer, Ana Albors, Amparo Chiralt, Chelo Gonza´lez-Martı´nez Department of Food Technology, Institute of Food Engineering for Development Universidad Polite´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2008 Received in revised form 26 June 2008 Accepted 27 June 2008
Five formulations of yoghurt were prepared by mixing different proportions of non-homogenised cows’ and goats’ milk. Samples were analysed in terms of their pH, mechanical properties, flow behaviour, syneresis, colour and sensory properties throughout storage at 4 C. The addition of goats’ milk led to smaller changes in pH, a higher whiteness index, lower syneresis and a significant decrease in the firmness and consistence of the gel during storage. The physicochemical properties of yoghurts were correlated with gel microstructure. Sensory evaluation showed that incorporating goats’ milk had a significant impact on the whiteness, flavour, syneresis and lumpiness of yoghurts. In general, the higher the goats’ milk content, the greater the physicochemical and sensory differences with regard to the 100% cows’ milk yoghurt. Samples with half and half cows’/goats’ milk were preferred by the sensory panel. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Goats’ milk (GM) has special nutritional properties that make it attractive to some consumers. It is easier to digest than cows’ milk (CM) and may have certain therapeutic value (Haenlein, 2004). The use of GM becomes an opportunity to diversify the dairy market since it allows us to develop added value fermented products with particular characteristics, in comparison to CM. The major differences between CM and GM are related to the different proportions of the different kinds of casein (as1-casein, as2casein, k-casein, etc.), and also to the different structure and size of fat globules and protein micelles (Tziboula-Clarke, 2003). Caprine casein micelles have higher dispersion rate, higher mineralization and a lower hydration level than CM micelles (Trujillo, Guamis, & Carretero, 1997). In addition, GM is poorer in as1-casein, which is present in variable proportions depending on the individual goat breed (Remeuf, 1993; Tziboula-Clarke, 2003). As regards the lipid fraction, caprine milk contains a higher proportion of smaller fat globules (Attaie & Ritcher, 2000; Haenlein, 1996) than CM. All these differences could lead to the milk behaving differently during the gelation process and gel formation and thus, could affect the final quality of GM dairy products. In this sense, GM yoghurt differs from CM yoghurt in some important properties like the firmness of the coagulum, which tends to be soft and less viscous (Bozanic, Tratnik, & Maric, 1998; Karademir, Atamer, Tamucay, & Yaman, 2002). Moreover, GM yoghurt shows less syneresis, a weaker gel, and a sharper flavour, which is different from the typical flavour of CM yoghurt (Haenlein, 2004).
* Corresponding author. Tel.: þ34 96 387 7000x73642; fax: þ34 96 387 73 69. E-mail address: [email protected] (M. Vargas). 0958-6946/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2008.06.007
Different studies on fermented GM products can be found in the literature, especially from areas where GM has been originally used for the preparation of fermented milks, such as the Middle East, Balkans, Greece, Egypt, and Turkey (Karademir et al., 2002; Malek, Shadarevian, & Toufeili, 2001; Stelios & Emmanuel, 2004; Uysal, Kilic, Kavas, Akbulut, & Kesenkas, 2003). However, few of these studies provide information about the source of the goats’ milk used. The latter is of great importance considering the influence of genetic protein polymorphism (mainly associated with the as1-casein and the b-lactoglobulin content) on the manufacture and functionality of goats’ milk-based dairy products (Andre´n, Allmere, & Bjo¨rk, 1997; Creamer & Harris, 1997; Fitzgerald & Hill, 1997; Ng-Kwai-Hang, 1997). Moreover, to the best of our knowledge, there is a lack of available data regarding the quality aspects of yoghurts made from milk from Spanish breeds of goat. The aim of this work is to study the influence of using Murciano–Granadina GM as a component of set-type yoghurt by mixing CM and GM at different ratios. Different relevant characteristics of yoghurt such as pH, whey retention, gel firmness and consistency, product viscosity, and colour were analysed by instrumental techniques and some of these physicochemical properties were correlated with the results of the sensory evaluation and microstructure observations. 2. Materials and methods 2.1. Fresh milk and starter cultures Fresh raw caprine (Murciano–Granadina) milk and bovine (Friesian) milk obtained from organic production systems (Council
M. Vargas et al. / International Dairy Journal 18 (2008) 1146–1152
of the European Union, 1999), and lyophilized starter culture (MY900, RhodiaFood, Dange´ Saint Romain, France), containing Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus ready for direct vat inoculation were used for yoghurt preparation. 2.2. Yoghurt preparation Fresh raw caprine and fresh raw bovine milk were heated separately at 90 C for 5 min, before being cooled to inoculation temperature (42 C). Then, five different formulations were prepared, by mixing both types of milk in different proportions. Mass fraction (%) of goats’ milk in the mixtures of yoghurt formulations were 100, 75, 50, 25 and 0. The amount of goats’ milk (%) has been used for sample notation throughout the discussion of results. The lyophilized MY900 culture was suspended in 12.5% reconstituted skim milk powder, previously autoclaved at 120 C for 15 min. The mixtures of GM and CM were gently stirred, and the suspended starter culture was inoculated at a concentration of 0.08 U L1, as recommended by the manufacturer. Afterwards, the mixtures were transferred to sterilized containers, which were kept at 42 C in an incubator (Hot-Cold M4000668, P-Selecta Barcelona, Spain) until a pH value of approximately 4.6 was reached. After fermentation, yoghurts were stored at 4 C and physicochemical analyses of samples were carried out at 1, 14 and 28 days of cold storage. The entire experimental process was performed twice.
Hue ¼ hab ¼ tan1
Chroma ¼ Cab ¼
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a b
2
(1)
2
a þ b
0:5
(2)
Whiteness index (WI) was calculated through Eq. (3):
2 2 2 0:5 Whiteness Index ¼ WI ¼ 100 ð100 L Þ þa þ b
(3)
The mechanical characterization of samples was carried out by means of a back extrusion test performed using a Texture Analyser (TA.XT-plus, Stable Micro Systems, Surrey, UK). Samples of yoghurt (100 g) incubated in cylindrical glass cups (70 80 mm), were back extruded with a plunger (50 mm diameter) till 20% deformation was reached at 2 mm s1. Force and distance of the maximum force peak, as well as the area under the curve, were evaluated as representative mechanical parameters. Five replicates of each analysis were carried out for each formulation and storage time. Consistency was determined after stirring yoghurt samples for 1 min with a rotational vertical stirrer (MRVS-13, SBS, Rubı´, Spain). Bostwick consistency was expressed as the distance (in cm) over which the material flowed in a Bostwick consistometer in 30 s. Five replicates of each analysis were carried out for each formulation and storage time. 2.5. Microstructural analysis
2.3. Compositional analysis The total solids, fat, total protein, casein and ash contents of cows’ and goats’ milk were determined according to the Association of Official Analytical Chemist methods (AOAC, 1997). Lactose content was estimated as the difference between total solids and the sum of fat, total protein and ash contents. Analyses were performed in triplicate. 2.4. Physicochemical analyses The pH of yoghurt samples was measured at 1, 14 and 29 days of storage at 4 C by using a C831 pH-meter (Consort, Tumhout, Belgium). Five replicates of each measurement were carried out for each formulation and storage time. The syneresis index (SI) of yoghurts was calculated throughout the storage by gently removing and weighing the surface liquid phase, which corresponds to the whey separated during storage. Yoghurts were contained in cylindrical glass cups (70 80 mm) that were used for incubation and storage. SI was expressed as the mass of drained whey per 100 g of yoghurt. Five replicates were carried out for each formulation and storage time. In order to determine the centrifugally separated whey (CSW), samples of yoghurt were incubated in centrifuge tubes. After fermentation, the tubes were weighed and centrifuged (MedifrigerBL, P-Selecta, Barcelona, Spain) using 350 g at 5 C for 30 min. Whey drained from the samples was removed and the tubes were weighed again. The value of CSW was expressed as the mass of drained whey per 100 g of yoghurt Colour was measured by using a spectrocolorimeter (CM-3600d, Minolta Co., Tokyo, Japan) in samples of yoghurt incubated in 20 mm depth transparent plastic containers. The containers were deep enough to avoid the influence of the background in colour measurements. Measurements were carried out in triplicate for each formulation and storage time. CIE L*a*b* coordinates and the psychometric parameters, hue (Eq. (1)) and chroma (Eq. (2)), were obtained using observer 10 and illuminant D65 (CIE, 1986).
Samples of formulations containing 0%, 50% and 100% GM, respectively, were prepared for electron microscopy observations, which were performed by Cryo Scanning Electron Microscopy (Cryo-SEM) in a microsocope JEOL JSM-5410 (Jeol Ltd., Tokyo, Japan), with a cryofixation external camera OXFORD CT-1500 (Oxford Instruments, Oxfordshire, UK). Samples were placed in the CryoSEM sample holder and plunged into slush nitrogen (at 210 C). The frozen samples were transferred to the cryo-stage and then freeze fractured, etched (at 90 C, 7.5 106 torr for 15 min), gold coated and observed at 145 C and 15 kV. Image analysis of the micrographs was performed by using Adobe Photoshop version 5.0. 2.6. Sensory analysis Using triangle tests, 8 judges were selected from a panel of 20, taking into account their capacity for discrimination and reproducibility. With the selected judges, a multi-comparison test with a seven point scale was performed (Meilgaard, Civille, & Carr, 1999). Sensory parameters were analysed in terms of lower () or higher (þ) intensity of the different sensory attributes with respect to a reference sample (0% GM yoghurt), which was scored as 0 (the centre of the scale). Yoghurt samples, contained in white plastic covered cups and coded randomly with three-digit numbers, were scored in terms of aroma, whiteness, presence of whey, consistency after breaking the gel with a spoon, presence of lumps after stirring with the spoon and its subsequent fluid consistency in the mouth, taste, acidity and overall preference. Each judge on the sensory panel performed the test three times, with an interval of 24 h between sessions. 2.7. Statistical analysis Results were analysed by multifactor analysis of variance with 95% significance level using StatgraphicsÒPlus 5.1. Multiple comparisons were performed through 95% least significant difference (LSD) intervals.
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3. Results and discussion The average composition of the GM and CM used to prepare the mixtures is shown in Table 1. These values are similar to those obtained by Hellı´n, Lo´pez, Jorda´n, and Laencina (1998) working with milk from the same breed of goat. On the other hand, the values obtained for cows’ milk were within the average range reported for Friesian CM (Walstra, Geurts, Noomen, Jellema, & Van Boekel, 1999). GM has higher total protein, lactose, ash and total solids contents than CM. The major differences between CM and GM were those concerning lipid and casein content, which was significantly higher for goats’ milk. The pH values reached by the different formulations immediately after removing the yoghurts from the incubator were between 4.4 and 4.6. After 1 day of cold storage samples containing 75 and 100% GM reached the lowest pH value (4.1), which remained constant during the whole period of storage. Formulations containing 50% or less GM showed a slowest pH decrease, reaching a practically constant value (between 3.9 and 4.1) after 14 days storage (results not shown). A faster acidification and lower pH values in GM yoghurt has been also reported by other authors (Rysstad & Abrahamsen, 1987; Bozanic et al., 1998). This different behaviour could be explained by the enhancement of the microbial growth, acidity progress and peptidase activity of L. delbrueckii ssp. bulgaricus in goats’ milk, as has been reported by Tamime and Robinson (1999). Moreover, Rogelj and Perko (1998) pointed out that the activity and growth rate of the starter cultures are strain dependent. These authors found that the acidification rate of lactic acid bacteria varied with the type of milk, being some yoghurt starters more active in GM while others in CM, regardless of the starter type. 3.1. Microstructural analysis Microstructural observations of samples containing 0, 50 and 100% GM were carried out by Cryo-SEM. Fig. 1 reveals a microstructure with fat globules embedded in a matrix of aggregated caseins, and voids filled with serum and bacterial cells. Microstructural differences among the different formulations were found. As shown in Fig. 1b, formulations 100% and 50% were characterized by a smaller number of junction points, which led to a more open structure and larger pores. This structure was more noticeable in pure GM yoghurt. The differences among the three formulations were quantified in terms of matrix porosity and fat globule size by analysing and performing measurements in the micrographs at different magnification levels. Measurements were carried out in at least five samples per formulation by quantifying the area that corresponds to pores. Porosity values were expressed as percentage over the total matrix area (casein network, fat globules and pores). Matrix porosity values were significantly higher (p < 0.05) in the 100% formulation, which showed average values (standard deviation) of 37% (7%), whereas the 50% and 0% formulations (pure CM) showed average porosity values of 30 (4%) and 32% (5%), respectively. The globules sizes were counted and measured in the micrographs and the obtained size distribution for formulations 0 and
Table 1 Composition of raw goats’ milk and cows’ milk used to prepare yoghurt formulationsa Milk Protein (%) Casein (%)
Fat (%)
Lactose (%) Ashes (%)
Total solids (%)
Goat 3.62 (0.03)a 3.36 (0.09)a 4.82 (0.02)a 5.02 (0.02)a 0.76 (0.03)a 14.22 (0.08)a Cow 3.1 (0.2)b 2.69 (0.03)b 3.65 (0.02)b 4.84 (0.02)b 0.69 (0.02)b 12.28 (0.30)b a Values are means with standard deviations given in parenthesis; values with different superscript letters are significantly different (p < 0.05).
100% is plotted in Fig. 2. As expected, the 100% formulation showed a significantly higher number of smaller fat globules than 0% formulation. However, the average size of fat globules of these formulations was slightly lower than the average values of 3.89 and 4.42 mm reported by Mehaia (1995) for non-homogenised GM and CM, respectively. 3.2. Mechanical properties, Bostwick consistency and syneresis The mechanical properties of yoghurts were evaluated through the maximum force (Fmax) and positive area (Aþ) at the gel rupture, which are related to the firmness and consistency of the gel, respectively. The values of these back extrusion parameters as a function of GM content (%) at different storage times are shown in Fig. 3. Storage time did not affect significantly the values of either parameter. The addition of GM significantly (p < 0.05) decreased gel firmness and gel consistency, even though the total solid content was higher in these formulations. The higher fat content contributes to diminish the gel firmness, by interrupting the threedimensional gel network (Walstra & Jenness, 1987), as yoghurts were prepared with non-homogenised milk. As regards the relationship between fat globule size and mechanical properties of the gel, Michalski, Cariou, Michel, and Garnier (2002) found a positive correlation between the diameter of fat globules and the storage modulus (G’) of the gel formed, therefore the lower firmness found in the 100% formulation could also be explained by the smaller fat globule size, especially if compared with the 0% formulation (Fig. 3a). The greater porosity of the protein network, as deduced from the microstructure analysis and the subsequent lower degree of micelle aggregation also contribute to the observed mechanical response, since a gel with a low porosity is typical from a more compact matrix, which would contribute to increasing the firmness and consistency of the gel network. Additionally, Murciano–Granadina goats’ milk has a lower as1casein content, (around 18.8 g 100 g1 casein, Serradilla et al., Dept. Produccio´n Animal, Universidad de Co´rdoba, Spain, personal communication) than Friesian cows’ milk, which has values of around 36.27 g 100 g1 casein (Summer, Malacarne, Martuzzi, & Mariani, 2002). This type of casein plays a very important role during gel formation, since a lower as1-casein content leads to a weaker texture (Michalski et al., 2002; Tamime, Kala´b, Davies, & Mahdi, 1991). The viscosity of the sol phase after gel rupture under controlled conditions was measured through the Bostwick consistency, which is plotted in Fig. 3c. The greater the Bostwick consistency values, the less viscous the product. The ANOVA showed a significant effect of GM content, storage time and the interaction between factors (p < 0.05). For all the formulations, the Bostwick consistency diminished with storage time, showing a significant increase in viscosity. Further aggregation of casein micelles during storage led to an increase in the size of the particles of the sol that results from gel rupture, and consequently, a greater viscosity (higher distance) was observed. On the other hand, there was no correlation between the Bostwick consistency and the total solid content or the ratio of fat to non-fat solids. The latter can be explained by the different firmness of the gel (see above), since significant correlations (p < 0.01) were found between mechanical parameters (maximum force and positive area) and the Bostwick consistency, obtaining Pearson correlation coefficients of 0.768 and 0.765, respectively. The weaker the gel before stirring, the smaller the diameter of the gel fragments, which leads to a less viscous product after stirring. In this sense, the Bostwick consistency reached the maximum value for 100% GM formulation, which was significantly less viscous (higher Bostwick consistency values) at all storage times (p < 0.05) and showed no lumps after stirring, which makes it particularly suitable for the production of liquid yoghurt.
M. Vargas et al. / International Dairy Journal 18 (2008) 1146–1152
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Fig. 1. Microstructural observations of formulations of 0% (a, d), 50% (b, e) and 100% (c, f) goats’ milk at (a, b, c) 5000 magnification (bar ¼ 10 mm) and (d, e, f) 1500 magnification (bar ¼ 20 mm).
Syneresis index (SI) development during storage is plotted in Fig. 3d. As expected, SI increased during storage because of the progressive strengthening of cross-links and bonds in the casein matrix, which led to a continuous contraction of the gel during storage (Tamime & Robinson, 1999). The addition of GM significantly diminished the syneresis index (SI) at all storage times (p < 0.05), reaching a reduction of 53.27% (as regards pure CM yoghurt) when the goats’ milk content was 50% or more. The higher total solids and fat content of GM, but mainly the less intense attractive forces among casein micelles, can explain their higher water holding capacity and lower gel contraction, thus maintaining the high porosity values in the gel matrix and reducing the natural syneresis. The lower gel contraction was clearly revealed from the sample microstructure, where a more open matrix of casein
micelles was observed in the formulations containing goats’ milk, which point out the less intense attractive forces among micelles. This can be explained by the higher negative net charge of k-casein in goats’ milk (Tziboula-Clarke, 2003), which affect the balance of interaction forces. Syneresis induced by centrifugation, determined as described above, showed average values (standard deviation) of CSW of 25.2 (1.1), 30 (3), 34.2 (1.9), 22.5 (1.2), and 19 (1) g 100 g1 for 0%, 25%, 50%, 75% and 100% formulations, respectively. Differences among formulations were significant (p < 0.05), and the average values at each storage time are not reported, since storage time has no significant effect on CSW. The non-influence of storage time is logical, considering that an increase in the gravitational field accelerates the gel network aggregation and syneresis, and thus
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3.3. Colour evaluation
Fig. 2. Distribution of the diameter of fat globules in the microstructural observations of formulations containing 0% and 100% goats’ milk. The number of fat globules (n), the average size in mm (X) and the standard deviation (s) were calculated to be 170, 3.21 and 1.09, respectively, for formulations containing 0% goats’ milk and 218, 2.81 and 0.99, respectively, for formulations containing 100% goats’ milk.
CSW gives information about the capacity of the gel to retain liquid phase at a hypothetical equilibrium status (Gonza´lez-Martı´nez et al., 2002). At this equilibrium state, the gel’s capacity to retain whey significantly increased when the proportion of GM was higher (formulations 75% and 100%), which coincides with the tendencies found for the SI parameter in formulations with a high concentration of goats’ milk. Nevertheless, the opposite trend was found for 25% and 50% formulations, which showed the maximum CSW values. The different intensity of syneresis, at the equilibrium status, which is caused by the changes in the level of aggregation and compaction of the micelles suggests that probably new interactions are established among caprine and bovine caseins when they are mixed. Thus, the differences in the organisation of the caprine and bovine casein micelle structure and/or an unfavourable spatial distribution of the fat globules packaged in the casein network could explain the observed trend. According to these results, only when the ratio of caprine:casein micelles is predominant (>50%), does this effect become negligible.
The luminosity (L*), chroma (C*ab), hue (h*ab) and whiteness index (WI) of all formulations at each storage time are shown in Fig. 4. The addition of GM caused an increase in L*, h*ab and WI and a decrease in chroma (C*ab) at all storage times. Nevertheless, the differences were only significant (p < 0.05) between the 0% and 100% formulations. Changes in colour coordinates can be attributed to the different level of gel opacity (Hutchings, 1999), which is related to the casein ratio and their aggregation level. The higher the luminosity values, the higher the opacity and the lower the chroma, in line with a higher whiteness index. The absence of b-carotene in GM (Alichanidis & Polychroniadou, 1996) together with its elevated proportion of small fat globules as compared to CM (Fig. 2), can explain the increase in WI of yoghurt samples when GM is added (Fig. 4b). Moreover, colour parameters (L*, C*ab and h*ab) increased and WI decreased throughout storage time. The compaction of the solid matrix and the increase in the syneresis index during storage would also explain these colour changes. 3.4. Sensory evaluation The average score for each sensory attribute and the overall preference of the samples are shown in Fig. 5. The score of all the sensory parameters significantly decreased (p < 0.05) after the addition of GM, except for whiteness and creaminess, which increased significantly (p < 0.05) when more GM was added. Goats’ milk yoghurt (formulation 100%) was evaluated as less consistent and more acid, with a non-typical yoghurt taste and flavour. This is in accordance with the lower level of acetaldehyde detected in pure GM yoghurts (Abrahamsen & Rysstad, 1991; Rysstad, Knutsen, & Abrahamsen, 1990) and their less characteristic taste (Alichanidis & Polychroniadou, 1996). In fact, GM contains a higher level of short chain fatty acids than CM, which explains the characteristic flavour of caprine dairy products (Karademir et al., 2002). However, formulation 100% GM was scored as whiter and creamier. The latter is one of the most important parameters for consumer preference
Fig. 3. Gel firmness (a), consistency (b), Bostwick consistency (c) and syneresis index (d) of yoghurts at 1 ( 95% LSD intervals.
), 14 (
), and 29 (
) days of storage. Mean values and
M. Vargas et al. / International Dairy Journal 18 (2008) 1146–1152
Fig. 4. Luminosity (a), whiteness index (b), hue (c) and chroma (d) of yoghurts at 1 (
in dairy products (Duboc & Mollet, 2001). In terms of overall preference, the formulation with 50% GM was given the highest score. In terms of syneresis (separated whey), there was a significant effect of the sensory evaluation session (p < 0.05), probably because the sensory assessment took place on three successive days, during which syneresis could have progressed. As regards acidity, there were no significant differences among formulations, which coincide with the few detected differences in pH values. Lumpiness and consistency results coincide with those obtained instrumentally by the Bostwick consistometer and the back extrusion test, respectively. Finally, whiteness scores obtained in the sensory analysis were correlated with the values of colour parameters obtained in the instrumental tests. A linear regression analysis for whiteness score as a function of whiteness index (WI) provided a significant equation (Eq. (4)), with a correlation coefficient of 0.995.
Whiteness score ¼ 149:52 þ 1:72684 WI
(4)
), 14 (
), and 29 (
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) days of storage. Mean values and 95% LSD intervals.
4. Conclusion The quality of yoghurts was markedly affected by the different proportion of goats’ milk in the mixture since the increase in the content of goats’ milk led to important differences in terms of the physicochemical properties of yoghurts, especially with regard to syneresis, flow properties, gel firmness and whiteness. The addition of GM decreased gel firmness and syneresis, especially when the ratio CM:GM was higher than 1:1. Regarding these parameters, the important role played by the different nature of caprine and bovine caseins in aggregation behaviour, whey retention capacity and the gel mechanical properties has been pointed out. The results were explained by the final gel microstructure, and were consistent with the sensory evaluation of the samples. The preferred set-type yoghurt was obtained by mixing GM and CM in the same ratio. At this proportion, no sensory differences were detected between the mixtures and cows’ milk yoghurt in terms of taste, flavour, whiteness, consistency and creaminess. Thus, Murciano–Granadina goats’ milk is an interesting raw material for the development of fermented dairy products. References
Fig. 5. Sensory attributes and overall preference of yoghurts as a function of goats’ milk content: d, 0% GM; *, 25% GM; —, 50% GM; B, 75% GM; 6, 100% GM.
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– Proceedings on the IDF seminar (pp. 22–35). Brussels, Belgium: International Dairy Federation. Remeuf, F. (1993). Influence du polymorphisme ge´ne´tique de la casein as1 caprine sur les characte´ristiques physico-chimiques et technologiques du lait. Lait, 73, 549–557. Rogelj, I., & Perko, B. (1998). Fermentation of goat and cow milk with different lactic ˜ a, & M. Espejo Dı´az (Eds.), Basis of acid starter cultures. In J. C. Flamant, D. Gabin the quality of the typical Mediterranean animal products – Proceedings of the international symposium on the basis of the quality of the typical Mediterranean animal products (pp. 262–267). Wageningen, The Netherlands: Wageningen Press. Rysstad, G., & Abrahamsen, R. K. (1987). Formation of volatile aroma compounds and carbon dioxide in yogurt starter grown in cow’s milk and goat’s milk. Journal of Dairy Research, 54, 257–266. Rysstad, G., Knutsen, W. J., & Abrahamsen, R. K. (1990). Effect of threonine and glycine on acetaldehyde formation in goats’ milk yogurt. Journal of Dairy Research, 57, 401–411. Stelios, K., & Emmanuel, A. (2004). Characteristics of set type yoghurt made from caprine or ovine milk and mixtures of the two. International Journal of Food Science and Technology, 39, 319–324. Summer, A., Malacarne, M., Martuzzi, F., & Mariani, P. (2002). Structural and functional characteristics of Modenese cow milk in Parmigiano-Reggiano cheese ` Medicina Veterinaria, Universita ` Parma, 23, 163–174. production. Annali Facolta Tamime, A. Y., Kala´b, M., Davies, G., & Mahdi, H. A. (1991). Microstructure and firmness of Labneh (high solids yogurt) made from cow’s, goat’s and sheep’s milks by a traditional method of ultrafiltration. Food Structure, 10, 37–44. Tamime, A. Y., & Robinson, R. K. (1999). Yoghurt science and technology. Cambridge, UK: Woodhead Publishing Ltd. Trujillo, A. J., Guamis, B., & Carretero, C. (1997). Las proteı´nas mayoritarias de la leche de cabra. Alimentaria, 258, 19–28. Tziboula-Clarke, A. (2003). Goat milk. In H. Roginnski, J. W. Fuquay, & P. F. Fox (Eds.), Encyclopedia of dairy sciences, Vol. 2 (pp. 1270–1279). London, UK: Academic Press. Uysal, H., Kilic, S., Kavas, G., Akbulut, N., & Kesenkas, H. (2003). Some properties of yoghurt made from caprine milk and bovine–caprine milk mixtures fortified by ultrafiltration or the addition of skim milk powder. International Journal of Dairy Technology, 56, 177–181. Walstra, P., Geurts, T. J., Noomen, A., Jellema, A., & Van Boekel, M. A. J. S. (1999). Dairy technology. Principles of milk properties and processes. New York, NY, USA: Marcel Dekker Inc. ´ gica. Zaragoza, Spain: Acribia Walstra, P., & Jenness, R. (1987). Quı´mica y fı´sica lactolo S.A.
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International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj
Physicochemical and sensory characteristics of yoghurt produced from mixtures of cows’ and goats’ milk Maria Vargas*, Maite Cha´fer, Ana Albors, Amparo Chiralt, Chelo Gonza´lez-Martı´nez Department of Food Technology, Institute of Food Engineering for Development Universidad Polite´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2008 Received in revised form 26 June 2008 Accepted 27 June 2008
Five formulations of yoghurt were prepared by mixing different proportions of non-homogenised cows’ and goats’ milk. Samples were analysed in terms of their pH, mechanical properties, flow behaviour, syneresis, colour and sensory properties throughout storage at 4 C. The addition of goats’ milk led to smaller changes in pH, a higher whiteness index, lower syneresis and a significant decrease in the firmness and consistence of the gel during storage. The physicochemical properties of yoghurts were correlated with gel microstructure. Sensory evaluation showed that incorporating goats’ milk had a significant impact on the whiteness, flavour, syneresis and lumpiness of yoghurts. In general, the higher the goats’ milk content, the greater the physicochemical and sensory differences with regard to the 100% cows’ milk yoghurt. Samples with half and half cows’/goats’ milk were preferred by the sensory panel. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Goats’ milk (GM) has special nutritional properties that make it attractive to some consumers. It is easier to digest than cows’ milk (CM) and may have certain therapeutic value (Haenlein, 2004). The use of GM becomes an opportunity to diversify the dairy market since it allows us to develop added value fermented products with particular characteristics, in comparison to CM. The major differences between CM and GM are related to the different proportions of the different kinds of casein (as1-casein, as2casein, k-casein, etc.), and also to the different structure and size of fat globules and protein micelles (Tziboula-Clarke, 2003). Caprine casein micelles have higher dispersion rate, higher mineralization and a lower hydration level than CM micelles (Trujillo, Guamis, & Carretero, 1997). In addition, GM is poorer in as1-casein, which is present in variable proportions depending on the individual goat breed (Remeuf, 1993; Tziboula-Clarke, 2003). As regards the lipid fraction, caprine milk contains a higher proportion of smaller fat globules (Attaie & Ritcher, 2000; Haenlein, 1996) than CM. All these differences could lead to the milk behaving differently during the gelation process and gel formation and thus, could affect the final quality of GM dairy products. In this sense, GM yoghurt differs from CM yoghurt in some important properties like the firmness of the coagulum, which tends to be soft and less viscous (Bozanic, Tratnik, & Maric, 1998; Karademir, Atamer, Tamucay, & Yaman, 2002). Moreover, GM yoghurt shows less syneresis, a weaker gel, and a sharper flavour, which is different from the typical flavour of CM yoghurt (Haenlein, 2004).
* Corresponding author. Tel.: þ34 96 387 7000x73642; fax: þ34 96 387 73 69. E-mail address: [email protected] (M. Vargas). 0958-6946/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2008.06.007
Different studies on fermented GM products can be found in the literature, especially from areas where GM has been originally used for the preparation of fermented milks, such as the Middle East, Balkans, Greece, Egypt, and Turkey (Karademir et al., 2002; Malek, Shadarevian, & Toufeili, 2001; Stelios & Emmanuel, 2004; Uysal, Kilic, Kavas, Akbulut, & Kesenkas, 2003). However, few of these studies provide information about the source of the goats’ milk used. The latter is of great importance considering the influence of genetic protein polymorphism (mainly associated with the as1-casein and the b-lactoglobulin content) on the manufacture and functionality of goats’ milk-based dairy products (Andre´n, Allmere, & Bjo¨rk, 1997; Creamer & Harris, 1997; Fitzgerald & Hill, 1997; Ng-Kwai-Hang, 1997). Moreover, to the best of our knowledge, there is a lack of available data regarding the quality aspects of yoghurts made from milk from Spanish breeds of goat. The aim of this work is to study the influence of using Murciano–Granadina GM as a component of set-type yoghurt by mixing CM and GM at different ratios. Different relevant characteristics of yoghurt such as pH, whey retention, gel firmness and consistency, product viscosity, and colour were analysed by instrumental techniques and some of these physicochemical properties were correlated with the results of the sensory evaluation and microstructure observations. 2. Materials and methods 2.1. Fresh milk and starter cultures Fresh raw caprine (Murciano–Granadina) milk and bovine (Friesian) milk obtained from organic production systems (Council
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of the European Union, 1999), and lyophilized starter culture (MY900, RhodiaFood, Dange´ Saint Romain, France), containing Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus ready for direct vat inoculation were used for yoghurt preparation. 2.2. Yoghurt preparation Fresh raw caprine and fresh raw bovine milk were heated separately at 90 C for 5 min, before being cooled to inoculation temperature (42 C). Then, five different formulations were prepared, by mixing both types of milk in different proportions. Mass fraction (%) of goats’ milk in the mixtures of yoghurt formulations were 100, 75, 50, 25 and 0. The amount of goats’ milk (%) has been used for sample notation throughout the discussion of results. The lyophilized MY900 culture was suspended in 12.5% reconstituted skim milk powder, previously autoclaved at 120 C for 15 min. The mixtures of GM and CM were gently stirred, and the suspended starter culture was inoculated at a concentration of 0.08 U L1, as recommended by the manufacturer. Afterwards, the mixtures were transferred to sterilized containers, which were kept at 42 C in an incubator (Hot-Cold M4000668, P-Selecta Barcelona, Spain) until a pH value of approximately 4.6 was reached. After fermentation, yoghurts were stored at 4 C and physicochemical analyses of samples were carried out at 1, 14 and 28 days of cold storage. The entire experimental process was performed twice.
Hue ¼ hab ¼ tan1
Chroma ¼ Cab ¼
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a b
2
(1)
2
a þ b
0:5
(2)
Whiteness index (WI) was calculated through Eq. (3):
2 2 2 0:5 Whiteness Index ¼ WI ¼ 100 ð100 L Þ þa þ b
(3)
The mechanical characterization of samples was carried out by means of a back extrusion test performed using a Texture Analyser (TA.XT-plus, Stable Micro Systems, Surrey, UK). Samples of yoghurt (100 g) incubated in cylindrical glass cups (70 80 mm), were back extruded with a plunger (50 mm diameter) till 20% deformation was reached at 2 mm s1. Force and distance of the maximum force peak, as well as the area under the curve, were evaluated as representative mechanical parameters. Five replicates of each analysis were carried out for each formulation and storage time. Consistency was determined after stirring yoghurt samples for 1 min with a rotational vertical stirrer (MRVS-13, SBS, Rubı´, Spain). Bostwick consistency was expressed as the distance (in cm) over which the material flowed in a Bostwick consistometer in 30 s. Five replicates of each analysis were carried out for each formulation and storage time. 2.5. Microstructural analysis
2.3. Compositional analysis The total solids, fat, total protein, casein and ash contents of cows’ and goats’ milk were determined according to the Association of Official Analytical Chemist methods (AOAC, 1997). Lactose content was estimated as the difference between total solids and the sum of fat, total protein and ash contents. Analyses were performed in triplicate. 2.4. Physicochemical analyses The pH of yoghurt samples was measured at 1, 14 and 29 days of storage at 4 C by using a C831 pH-meter (Consort, Tumhout, Belgium). Five replicates of each measurement were carried out for each formulation and storage time. The syneresis index (SI) of yoghurts was calculated throughout the storage by gently removing and weighing the surface liquid phase, which corresponds to the whey separated during storage. Yoghurts were contained in cylindrical glass cups (70 80 mm) that were used for incubation and storage. SI was expressed as the mass of drained whey per 100 g of yoghurt. Five replicates were carried out for each formulation and storage time. In order to determine the centrifugally separated whey (CSW), samples of yoghurt were incubated in centrifuge tubes. After fermentation, the tubes were weighed and centrifuged (MedifrigerBL, P-Selecta, Barcelona, Spain) using 350 g at 5 C for 30 min. Whey drained from the samples was removed and the tubes were weighed again. The value of CSW was expressed as the mass of drained whey per 100 g of yoghurt Colour was measured by using a spectrocolorimeter (CM-3600d, Minolta Co., Tokyo, Japan) in samples of yoghurt incubated in 20 mm depth transparent plastic containers. The containers were deep enough to avoid the influence of the background in colour measurements. Measurements were carried out in triplicate for each formulation and storage time. CIE L*a*b* coordinates and the psychometric parameters, hue (Eq. (1)) and chroma (Eq. (2)), were obtained using observer 10 and illuminant D65 (CIE, 1986).
Samples of formulations containing 0%, 50% and 100% GM, respectively, were prepared for electron microscopy observations, which were performed by Cryo Scanning Electron Microscopy (Cryo-SEM) in a microsocope JEOL JSM-5410 (Jeol Ltd., Tokyo, Japan), with a cryofixation external camera OXFORD CT-1500 (Oxford Instruments, Oxfordshire, UK). Samples were placed in the CryoSEM sample holder and plunged into slush nitrogen (at 210 C). The frozen samples were transferred to the cryo-stage and then freeze fractured, etched (at 90 C, 7.5 106 torr for 15 min), gold coated and observed at 145 C and 15 kV. Image analysis of the micrographs was performed by using Adobe Photoshop version 5.0. 2.6. Sensory analysis Using triangle tests, 8 judges were selected from a panel of 20, taking into account their capacity for discrimination and reproducibility. With the selected judges, a multi-comparison test with a seven point scale was performed (Meilgaard, Civille, & Carr, 1999). Sensory parameters were analysed in terms of lower () or higher (þ) intensity of the different sensory attributes with respect to a reference sample (0% GM yoghurt), which was scored as 0 (the centre of the scale). Yoghurt samples, contained in white plastic covered cups and coded randomly with three-digit numbers, were scored in terms of aroma, whiteness, presence of whey, consistency after breaking the gel with a spoon, presence of lumps after stirring with the spoon and its subsequent fluid consistency in the mouth, taste, acidity and overall preference. Each judge on the sensory panel performed the test three times, with an interval of 24 h between sessions. 2.7. Statistical analysis Results were analysed by multifactor analysis of variance with 95% significance level using StatgraphicsÒPlus 5.1. Multiple comparisons were performed through 95% least significant difference (LSD) intervals.
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3. Results and discussion The average composition of the GM and CM used to prepare the mixtures is shown in Table 1. These values are similar to those obtained by Hellı´n, Lo´pez, Jorda´n, and Laencina (1998) working with milk from the same breed of goat. On the other hand, the values obtained for cows’ milk were within the average range reported for Friesian CM (Walstra, Geurts, Noomen, Jellema, & Van Boekel, 1999). GM has higher total protein, lactose, ash and total solids contents than CM. The major differences between CM and GM were those concerning lipid and casein content, which was significantly higher for goats’ milk. The pH values reached by the different formulations immediately after removing the yoghurts from the incubator were between 4.4 and 4.6. After 1 day of cold storage samples containing 75 and 100% GM reached the lowest pH value (4.1), which remained constant during the whole period of storage. Formulations containing 50% or less GM showed a slowest pH decrease, reaching a practically constant value (between 3.9 and 4.1) after 14 days storage (results not shown). A faster acidification and lower pH values in GM yoghurt has been also reported by other authors (Rysstad & Abrahamsen, 1987; Bozanic et al., 1998). This different behaviour could be explained by the enhancement of the microbial growth, acidity progress and peptidase activity of L. delbrueckii ssp. bulgaricus in goats’ milk, as has been reported by Tamime and Robinson (1999). Moreover, Rogelj and Perko (1998) pointed out that the activity and growth rate of the starter cultures are strain dependent. These authors found that the acidification rate of lactic acid bacteria varied with the type of milk, being some yoghurt starters more active in GM while others in CM, regardless of the starter type. 3.1. Microstructural analysis Microstructural observations of samples containing 0, 50 and 100% GM were carried out by Cryo-SEM. Fig. 1 reveals a microstructure with fat globules embedded in a matrix of aggregated caseins, and voids filled with serum and bacterial cells. Microstructural differences among the different formulations were found. As shown in Fig. 1b, formulations 100% and 50% were characterized by a smaller number of junction points, which led to a more open structure and larger pores. This structure was more noticeable in pure GM yoghurt. The differences among the three formulations were quantified in terms of matrix porosity and fat globule size by analysing and performing measurements in the micrographs at different magnification levels. Measurements were carried out in at least five samples per formulation by quantifying the area that corresponds to pores. Porosity values were expressed as percentage over the total matrix area (casein network, fat globules and pores). Matrix porosity values were significantly higher (p < 0.05) in the 100% formulation, which showed average values (standard deviation) of 37% (7%), whereas the 50% and 0% formulations (pure CM) showed average porosity values of 30 (4%) and 32% (5%), respectively. The globules sizes were counted and measured in the micrographs and the obtained size distribution for formulations 0 and
Table 1 Composition of raw goats’ milk and cows’ milk used to prepare yoghurt formulationsa Milk Protein (%) Casein (%)
Fat (%)
Lactose (%) Ashes (%)
Total solids (%)
Goat 3.62 (0.03)a 3.36 (0.09)a 4.82 (0.02)a 5.02 (0.02)a 0.76 (0.03)a 14.22 (0.08)a Cow 3.1 (0.2)b 2.69 (0.03)b 3.65 (0.02)b 4.84 (0.02)b 0.69 (0.02)b 12.28 (0.30)b a Values are means with standard deviations given in parenthesis; values with different superscript letters are significantly different (p < 0.05).
100% is plotted in Fig. 2. As expected, the 100% formulation showed a significantly higher number of smaller fat globules than 0% formulation. However, the average size of fat globules of these formulations was slightly lower than the average values of 3.89 and 4.42 mm reported by Mehaia (1995) for non-homogenised GM and CM, respectively. 3.2. Mechanical properties, Bostwick consistency and syneresis The mechanical properties of yoghurts were evaluated through the maximum force (Fmax) and positive area (Aþ) at the gel rupture, which are related to the firmness and consistency of the gel, respectively. The values of these back extrusion parameters as a function of GM content (%) at different storage times are shown in Fig. 3. Storage time did not affect significantly the values of either parameter. The addition of GM significantly (p < 0.05) decreased gel firmness and gel consistency, even though the total solid content was higher in these formulations. The higher fat content contributes to diminish the gel firmness, by interrupting the threedimensional gel network (Walstra & Jenness, 1987), as yoghurts were prepared with non-homogenised milk. As regards the relationship between fat globule size and mechanical properties of the gel, Michalski, Cariou, Michel, and Garnier (2002) found a positive correlation between the diameter of fat globules and the storage modulus (G’) of the gel formed, therefore the lower firmness found in the 100% formulation could also be explained by the smaller fat globule size, especially if compared with the 0% formulation (Fig. 3a). The greater porosity of the protein network, as deduced from the microstructure analysis and the subsequent lower degree of micelle aggregation also contribute to the observed mechanical response, since a gel with a low porosity is typical from a more compact matrix, which would contribute to increasing the firmness and consistency of the gel network. Additionally, Murciano–Granadina goats’ milk has a lower as1casein content, (around 18.8 g 100 g1 casein, Serradilla et al., Dept. Produccio´n Animal, Universidad de Co´rdoba, Spain, personal communication) than Friesian cows’ milk, which has values of around 36.27 g 100 g1 casein (Summer, Malacarne, Martuzzi, & Mariani, 2002). This type of casein plays a very important role during gel formation, since a lower as1-casein content leads to a weaker texture (Michalski et al., 2002; Tamime, Kala´b, Davies, & Mahdi, 1991). The viscosity of the sol phase after gel rupture under controlled conditions was measured through the Bostwick consistency, which is plotted in Fig. 3c. The greater the Bostwick consistency values, the less viscous the product. The ANOVA showed a significant effect of GM content, storage time and the interaction between factors (p < 0.05). For all the formulations, the Bostwick consistency diminished with storage time, showing a significant increase in viscosity. Further aggregation of casein micelles during storage led to an increase in the size of the particles of the sol that results from gel rupture, and consequently, a greater viscosity (higher distance) was observed. On the other hand, there was no correlation between the Bostwick consistency and the total solid content or the ratio of fat to non-fat solids. The latter can be explained by the different firmness of the gel (see above), since significant correlations (p < 0.01) were found between mechanical parameters (maximum force and positive area) and the Bostwick consistency, obtaining Pearson correlation coefficients of 0.768 and 0.765, respectively. The weaker the gel before stirring, the smaller the diameter of the gel fragments, which leads to a less viscous product after stirring. In this sense, the Bostwick consistency reached the maximum value for 100% GM formulation, which was significantly less viscous (higher Bostwick consistency values) at all storage times (p < 0.05) and showed no lumps after stirring, which makes it particularly suitable for the production of liquid yoghurt.
M. Vargas et al. / International Dairy Journal 18 (2008) 1146–1152
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Fig. 1. Microstructural observations of formulations of 0% (a, d), 50% (b, e) and 100% (c, f) goats’ milk at (a, b, c) 5000 magnification (bar ¼ 10 mm) and (d, e, f) 1500 magnification (bar ¼ 20 mm).
Syneresis index (SI) development during storage is plotted in Fig. 3d. As expected, SI increased during storage because of the progressive strengthening of cross-links and bonds in the casein matrix, which led to a continuous contraction of the gel during storage (Tamime & Robinson, 1999). The addition of GM significantly diminished the syneresis index (SI) at all storage times (p < 0.05), reaching a reduction of 53.27% (as regards pure CM yoghurt) when the goats’ milk content was 50% or more. The higher total solids and fat content of GM, but mainly the less intense attractive forces among casein micelles, can explain their higher water holding capacity and lower gel contraction, thus maintaining the high porosity values in the gel matrix and reducing the natural syneresis. The lower gel contraction was clearly revealed from the sample microstructure, where a more open matrix of casein
micelles was observed in the formulations containing goats’ milk, which point out the less intense attractive forces among micelles. This can be explained by the higher negative net charge of k-casein in goats’ milk (Tziboula-Clarke, 2003), which affect the balance of interaction forces. Syneresis induced by centrifugation, determined as described above, showed average values (standard deviation) of CSW of 25.2 (1.1), 30 (3), 34.2 (1.9), 22.5 (1.2), and 19 (1) g 100 g1 for 0%, 25%, 50%, 75% and 100% formulations, respectively. Differences among formulations were significant (p < 0.05), and the average values at each storage time are not reported, since storage time has no significant effect on CSW. The non-influence of storage time is logical, considering that an increase in the gravitational field accelerates the gel network aggregation and syneresis, and thus
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3.3. Colour evaluation
Fig. 2. Distribution of the diameter of fat globules in the microstructural observations of formulations containing 0% and 100% goats’ milk. The number of fat globules (n), the average size in mm (X) and the standard deviation (s) were calculated to be 170, 3.21 and 1.09, respectively, for formulations containing 0% goats’ milk and 218, 2.81 and 0.99, respectively, for formulations containing 100% goats’ milk.
CSW gives information about the capacity of the gel to retain liquid phase at a hypothetical equilibrium status (Gonza´lez-Martı´nez et al., 2002). At this equilibrium state, the gel’s capacity to retain whey significantly increased when the proportion of GM was higher (formulations 75% and 100%), which coincides with the tendencies found for the SI parameter in formulations with a high concentration of goats’ milk. Nevertheless, the opposite trend was found for 25% and 50% formulations, which showed the maximum CSW values. The different intensity of syneresis, at the equilibrium status, which is caused by the changes in the level of aggregation and compaction of the micelles suggests that probably new interactions are established among caprine and bovine caseins when they are mixed. Thus, the differences in the organisation of the caprine and bovine casein micelle structure and/or an unfavourable spatial distribution of the fat globules packaged in the casein network could explain the observed trend. According to these results, only when the ratio of caprine:casein micelles is predominant (>50%), does this effect become negligible.
The luminosity (L*), chroma (C*ab), hue (h*ab) and whiteness index (WI) of all formulations at each storage time are shown in Fig. 4. The addition of GM caused an increase in L*, h*ab and WI and a decrease in chroma (C*ab) at all storage times. Nevertheless, the differences were only significant (p < 0.05) between the 0% and 100% formulations. Changes in colour coordinates can be attributed to the different level of gel opacity (Hutchings, 1999), which is related to the casein ratio and their aggregation level. The higher the luminosity values, the higher the opacity and the lower the chroma, in line with a higher whiteness index. The absence of b-carotene in GM (Alichanidis & Polychroniadou, 1996) together with its elevated proportion of small fat globules as compared to CM (Fig. 2), can explain the increase in WI of yoghurt samples when GM is added (Fig. 4b). Moreover, colour parameters (L*, C*ab and h*ab) increased and WI decreased throughout storage time. The compaction of the solid matrix and the increase in the syneresis index during storage would also explain these colour changes. 3.4. Sensory evaluation The average score for each sensory attribute and the overall preference of the samples are shown in Fig. 5. The score of all the sensory parameters significantly decreased (p < 0.05) after the addition of GM, except for whiteness and creaminess, which increased significantly (p < 0.05) when more GM was added. Goats’ milk yoghurt (formulation 100%) was evaluated as less consistent and more acid, with a non-typical yoghurt taste and flavour. This is in accordance with the lower level of acetaldehyde detected in pure GM yoghurts (Abrahamsen & Rysstad, 1991; Rysstad, Knutsen, & Abrahamsen, 1990) and their less characteristic taste (Alichanidis & Polychroniadou, 1996). In fact, GM contains a higher level of short chain fatty acids than CM, which explains the characteristic flavour of caprine dairy products (Karademir et al., 2002). However, formulation 100% GM was scored as whiter and creamier. The latter is one of the most important parameters for consumer preference
Fig. 3. Gel firmness (a), consistency (b), Bostwick consistency (c) and syneresis index (d) of yoghurts at 1 ( 95% LSD intervals.
), 14 (
), and 29 (
) days of storage. Mean values and
M. Vargas et al. / International Dairy Journal 18 (2008) 1146–1152
Fig. 4. Luminosity (a), whiteness index (b), hue (c) and chroma (d) of yoghurts at 1 (
in dairy products (Duboc & Mollet, 2001). In terms of overall preference, the formulation with 50% GM was given the highest score. In terms of syneresis (separated whey), there was a significant effect of the sensory evaluation session (p < 0.05), probably because the sensory assessment took place on three successive days, during which syneresis could have progressed. As regards acidity, there were no significant differences among formulations, which coincide with the few detected differences in pH values. Lumpiness and consistency results coincide with those obtained instrumentally by the Bostwick consistometer and the back extrusion test, respectively. Finally, whiteness scores obtained in the sensory analysis were correlated with the values of colour parameters obtained in the instrumental tests. A linear regression analysis for whiteness score as a function of whiteness index (WI) provided a significant equation (Eq. (4)), with a correlation coefficient of 0.995.
Whiteness score ¼ 149:52 þ 1:72684 WI
(4)
), 14 (
), and 29 (
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) days of storage. Mean values and 95% LSD intervals.
4. Conclusion The quality of yoghurts was markedly affected by the different proportion of goats’ milk in the mixture since the increase in the content of goats’ milk led to important differences in terms of the physicochemical properties of yoghurts, especially with regard to syneresis, flow properties, gel firmness and whiteness. The addition of GM decreased gel firmness and syneresis, especially when the ratio CM:GM was higher than 1:1. Regarding these parameters, the important role played by the different nature of caprine and bovine caseins in aggregation behaviour, whey retention capacity and the gel mechanical properties has been pointed out. The results were explained by the final gel microstructure, and were consistent with the sensory evaluation of the samples. The preferred set-type yoghurt was obtained by mixing GM and CM in the same ratio. At this proportion, no sensory differences were detected between the mixtures and cows’ milk yoghurt in terms of taste, flavour, whiteness, consistency and creaminess. Thus, Murciano–Granadina goats’ milk is an interesting raw material for the development of fermented dairy products. References
Fig. 5. Sensory attributes and overall preference of yoghurts as a function of goats’ milk content: d, 0% GM; *, 25% GM; —, 50% GM; B, 75% GM; 6, 100% GM.
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