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Graininess and roughness of stirred yoghurt made with goat's, cow's or a mixture of goat's and cow's milk
Small Ruminant Research 96 (2011) 173–177
Contents lists available at ScienceDirect
Small Ruminant Research journal homepage: www.elsevier.com/locate/smallrumres
Short communication
Graininess and roughness of stirred yoghurt made with goat’s, cow’s or a mixture of goat’s and cow’s milk A. Küc¸ükc¸etin ∗ , M. Demir, A. As¸ci, E.M. C¸omak Department of Food Engineering, Faculty of Engineering, Akdeniz University, 07059 Antalya, Turkey
a r t i c l e
i n f o
Article history: Received 23 April 2010 Received in revised form 30 November 2010 Accepted 9 December 2010 Available online 14 January 2011 Keywords: Stirred yoghurt Goat’s milk Cow’s milk Graininess Roughness
a b s t r a c t The aim of this work was to study how the type of milk and the storage time affect the physicochemical characteristics, including graininess and roughness, of stirred yoghurt. Stirred yoghurt was produced from goat’s, cow’s or a mixture of goat’s and cow’s milk and was stored for 15 days at 4 ◦ C. Yoghurt produced from goat’s milk was characterized by its lower number of grains, mean perimeter of grains, roughness, viscosity and water holding capacity in comparison to that of the yoghurt developed using cow’s milk. Yoghurt with half and half cow’s/goat’s milk had a higher viscosity and water holding capacity than that only containing goat’s milk and also had a lower number of grains, mean perimeter of grains and roughness than that only containing cow’s milk. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The use of goat’s milk as an excellent food source is undeniable (Ribeiro and Ribeiro, 2010). Goat’s milk is designated for direct consumption or for the manufacture of cheese, fermented milks (e.g. yoghurt) and milk powder (Domagala, 2009). Nowadays, the production of goat’s milk and its processing constitutes an economic activity of increasing importance due to high nutritional interest of goat’s milk, as it provides high-quality protein, fat, carbohydrates, vitamins, and several minerals, such as iron, calcium, and phosphorus (Medeiros et al., 2009). The average composition of goat’s milk does not differ remarkably from that of cow’s milk. However, essential differences are present with regards to the structure, composition and size of the casein micelles, the proportion of individual protein fractions and higher content of nonprotein nitrogen and mineral compounds in goat’s milk (Domagala, 2009). All these differences could lead to the
∗ Corresponding author. Tel.: +90 242 3106569; fax: +90 242 2274564. E-mail address: [email protected] (A. Küc¸ükc¸etin). 0921-4488/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2010.12.003
milk behaving differently during the gelation process and gel formation; thus, affecting the final quality of dairy products obtained from the goat’s milk. In this context, yoghurt obtained from goat’s milk differs in some physicochemical properties such as the firmness of the coagulum, which tends to be soft and less viscous, from yoghurt obtained from cow’s milk. Additionally, yoghurt obtained from goat’s milk shows a weaker gel in comparison to yoghurt obtained from cow’s milk (Vargas et al., 2008). Textural defects of stirred yoghurt like graininess (particle) or roughness (irregularities in the yoghurt matrix) are undesirable, as consumers expect smooth, uniform and fine-bodied products. Although several works have been published determining the physicochemical and sensory properties of yoghurt made from goat’s milk and goat’s–cow’s milk mixtures (Malek et al., 2001; Uysal et al., 2003; Kavas et al., 2004; Vargas et al., 2008; Domagala, 2009), textural defects of yoghurt made from goat’s milk have not been studied in detail. The objective of this research was to determine the physicochemical properties, including the graininess and roughness, of stirred yoghurts containing goat’s, cow’s and a mixture of goat’s and cow’s milk.
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2. Materials and methods
3. Results and discussion
2.1. Milk processing and yoghurt preparation
After 1 day of cold storage, the yoghurt obtained from goat’s milk reached the lowest pH value, which remained relatively constant throughout the rest of the storage period (results not shown). This result was similar to that found by Vargas et al. (2008). The rate of the acid development due to Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in goat’s milk was faster than that observed within cow’s milk and the mixture of goat’s and cow’s milk, as has also been reported by Tamime and Robinson (1999). This different behavior could be explained by the enhancement of the microbial growth, acidity progress and peptidase activity of L. delbrueckii subsp. bulgaricus in goat’s milk (Vargas et al., 2008). The pH values decreased significantly (p < 0.01) in each yoghurt sample during storage at 4 ◦ C for 15 days. The pH values of the samples ranged from 4.06 to 4.22 by day 15. The viscosity values of the day 1 and day 15 samples varied from 0.79 to 1.66 Pa s and from 1.12 to 2.38 Pa s, respectively, according to the milk type (Fig. 1). The viscosity was significantly affected by the milk type and storage time (p < 0.001) (Table 1). The viscosity of the yoghurt obtained from goat’s milk was lower than that of the yoghurt obtained from the cow’s milk and the mixture of goat’s and cow’s milk. These results are in agreement with those obtained by Vargas et al. (2008). Malek et al. (2001) reported that concentrated yoghurts obtained from cow’s milk are more sticky than those obtained from goat’s milk. Martín-Diana et al. (2003) found that although goat’s and cow’s milk had similar total solids content and besides goat’s milk had higher protein content than that of cow’s milk, the viscosity of cow’s milk coagulum was about 30% higher than that of goat’s milk. The authors reported that the differences in casein content and micelle structure between species had high influence on the texture of fermented milk products. Additionally, ␣s1 -casein content of goat’s milk was lower than that of cow’s milk reported by Vargas et al. (2008). The authors pointed out that ␣s1 casein plays a very important role during gel formation, since a lower ␣s1 -casein content leads to weaker texture. The yoghurt texture is highly dependent on the total solids content, as well as on protein content and type. Goat’s milk has a slightly lower casein content than cow’s milk, with a very low proportion or the absence of ␣s1 -casein while having a higher degree of casein micelle dispersion (Herrero and Requena, 2006). These characteristics could be responsible for the differences observed in the viscosity values among yoghurt samples. However, it should be noted that the mineral compounds in milk and milk products play an important role in the stability of the proteins and in their some characteristics (Tsioulpas et al., 2007). Although protein is a significant contributor to development of texture in yoghurts, changes in minor components of milk that mediate changes in protein and mineral equilibria of milk may also alter yoghurt properties (Cheng et al., 2002). During storage at 4 ◦ C, viscosity values increased for all yoghurt samples, which is in agreement with those obtained by Karademir et al. (2002). The number and the mean perimeter of grains of the yoghurt samples varied from 18 to 25 per 3 mL of the sam-
Cow’s milk was obtained from the dairy farm of the Akdeniz University (Antalya, Turkey). Goat’s milk was collected from goat farms located in the Korkuteli district of Antalya Province. The goat’s and cow’s milk used in the production of stirred yoghurt had the following compositions, respectively: a total solids content of (g/L) 134.3 ± 0.71 and 116.55 ± 1.06, a total protein content of (g/L) 35.60 ± 1.13 and 32.00 ± 1.56, a fat content of (g/L) 41.45 ± 0.49 and 29.75 ± 0.35, ash content (g/L) 7.35 ± 0.21 and 7.10 ± 0.28, and the pH of 6.68 ± 0.02 and 6.50 ± 0.07. Three different formulations were prepared using the following combinations: (i) 100% cow’s milk, (ii) 50% goat’s milk and 50% cow’s milk and (iii) 100% goat’s milk. The milk was skimmed to a 1.0 g/L fat content at about 55 ◦ C. For the yoghurt preparation, skimmed milk powder (954 g/L total solids, 352 g/L protein, 11 g/L fat, 79 g/L ash; Izi Dairy Inc., Konya, Turkey) was added to the milk to give a final total solids content of 150 g/L. The standardized milk was heated at 90 ◦ C for 5 min and, then, subsequently cooled to 42 ◦ C. After cooling, 0.1 g/L of frozen pellets (starter culture DI-PROX TY 973, Bioprox, France) were added according to the manufacture’s reference, and the milk was incubated at 42 ◦ C until the pH decreased to 4.60. Fermentation was stopped by rapid cooling to the temperature of 4 ◦ C in an ice-water bath. At the beginning of the cooling process in an ice-water bath, the yoghurt was manually stirred for almost 60 s by up and down movements with a stainless-steel bored disk. After setting the stirred product in 200 mL cups (top diameter 65 mm, bottom diameter 53 mm and 80 mm height), the stirred yoghurt samples were stored at 4 ◦ C for 15 days. The physicochemical characteristics of the samples were analyzed on day 1, 7 and 15 of storage. The entire experimental process was performed in triplicate.
2.2. Physicochemical properties measurements The total solids, total protein, fat and ash contents of goat’s and cow’s milk were determined according to the Association of Official Analytical Chemist methods (AOAC, 1997). The pH was measured by a WTW pH meter (Inolab pH level 2, WTW GmbH, Weilheim, Germany). The viscosity was measured using a Brookfield DVII + Pro viscosimeter (Brookfield, Middleboro, MA, USA) with a Helipath (T spindle, type D) rotated at 5 rpm following the method described by Rasmussen et al. (2007). The sample temperature was 4 ◦ C. The water-holding capacity was determined by the procedure described by Sodini et al. (2005). A sample of about 20 g of yoghurt (Y) was centrifuged for 10 min at 1250 × g at 4 ◦ C. The whey expelled (W) was removed and weighed. The water-holding capacity (WHC) was calculated as: WHC (%) = 100 ×
(Y − W ) Y
The number and mean perimeter (diameter calculated from the measured boundary length) of grains, used as a measure of the graininess, were measured by image analysis according to the protocol described by Küc¸ükc¸etin et al. (2009). An image of the transmitted sample was taken with a built-in CCD camera (TFP-M/WL, LTF-Labortechnik, Wasserburg, Germany) having a resolution of 768 × 576 pixels and a 256 grayscale color depth effectively corresponding to 74-pixels/cm. The image analysis was performed with Optimas 6.1 software (Media Cybergenetics, Silver Spring, MD, USA). The software was set for the evaluation of 3 mL sample. The number of grains indicating a perimeter greater than 1.0 mm per 3 mL of yoghurt and the mean perimeter of grains were evaluated. The measurement of the visual roughness (Rvis ) was based upon that described by Küc¸ükc¸etin et al. (2009). The Rvis of the sample was taken as an additional descriptive parameter of the digital image made for graininess. All measurements were performed in duplicate.
2.3. Statistical analyses All statistical calculations were analyzed using SAS Statistical Software (released for Windows, SAS Institute Inc., Cary, NC, USA). The Duncan’s multiple range test was conducted to detect differences among the treatment means.
A. Küc¸ükc¸etin et al. / Small Ruminant Research 96 (2011) 173–177
175
Viscosity (Pa s)
2
1
day 1
0
day 7
day 15
day 7
day 1
CM yoghurt
day 15
day 1
MM yoghurt
day 15
day 7
GM yoghurt
Fig. 1. Viscosity of yoghurt samples obtained from cow’s milk (CM), a mixture of goat’s and cow’s milk (MM) and goat’s milk (GM) during storage at 4 ◦ C for 15 days. The bars represent mean values, and the error bars represent the standard deviation of the mean. Table 1 Effect of the milk type (MT) and storage time (ST) on some selected characteristics of stirred skim milk yoghurt. ANOVA
pH
Water holding capacity (%)
Viscosity (Pa s)
Number of grains/3 mL
PGb (mm)
MT ST MTxST
***
***
***
**
***
*
***
***
***
n.s.a
n.s.
n.s.
n.s. n.s.
n.s. n.s.
n.s. n.s.
** *** a b c
Significantly different at p < 0.05. Significantly different at p < 0.01. Significantly different at p < 0.001. n.s., not significant. PG, perimeter of grains. Rvis , visual roughness; AIDm , mean absolute intensity deviation.
ple and from 1.9 to 4.6 mm, respectively, depending upon the milk type and the storage time (Fig. 2). The number and the mean perimeter of grains were influenced by the milk type (p ≤ 0.01) (Table 1). The effect of storage was not
significant (p > 0.05). There were significant differences in the number and the mean perimeter of grains between the yoghurts obtained from cow’s and goat’s milk. Furthermore, the number and the perimeter of grains in yoghurt 6
30
Numbers of grains/ 3mL
5
4
20
3
2
10
Mean perimeter of grains (mm)
*
Rvis (unit of AIDm )c
1
day 1
day 7
day 15
day 1
day 7
day 15
day 1
day 7
day 15
0 CM yoghurt
MM yoghurt
0
GM yoghurt
Fig. 2. Number ( ) and mean perimeter () of grains in yoghurt samples obtained from cow’s milk (CM), a mixture of goat’s and cow’s milk (MM), and goat’s milk (GM) during storage at 4 ◦ C for 15 days. The bars represent mean values, and the error bars represent the standard deviation of the mean.
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5
4 40 3
2 20
Rvis (unit of AIDm)
Water holding capacity (%) (w/w)
60
1
0
day 1
day 7
day 15
day 1
CM yoghurt
day 7
day 15
day 1
MM yoghurt
day 7
day 15
0
GM yoghurt
Fig. 3. Water holding capacity ( ) and visual roughness (Rvis ) () of grains in yoghurt samples obtained from cow’s milk (CM), a mixture of goat’s and cow’s milk (MM), and goat’s milk (GM) during storage at 4 ◦ C for 15 days. The bars represent mean values, and the error bars represent the standard deviation of the mean.
obtained from cow’s milk was significantly (p < 0.01) higher than those from goat’s milk and the mixture of goat’s and cow’s milk. However, no differences were found in the number of grains between the yoghurt samples obtained from the goat’s milk and the samples obtained from the mixture of goat’s and cow’s milk. Vargas et al. (2008) reported that yoghurt made from goat’s milk had no grains. The difference in the graininess among the yoghurts may be due to a difference in the composition of the protein within the milk type that is used for yoghurt manufacture. The water holding capacity of the samples varied from 39.3 to 51.2% (Fig. 3). As shown in Table 1, the milk type and the storage time significantly (p < 0.01) affected the water holding capacity of the yoghurt. The water holding capacity of yoghurt obtained from the goat’s milk was significantly (p < 0.01) lower than that obtained from the cow’s milk and that of the mixture of goat’s and cow’s milk. These results are in agreement with those obtained by Domagala (2009). Malek et al. (2001) reported that concentrated yoghurts obtained from cow’s milk exuded less water than those obtained from goat’s milk, despite the total solids content of the yoghurt samples being similar. During storage at 4 ◦ C, the water holding capacity decreased for all the yoghurt samples, with the difference between the day 7 and day 15 samples not being of significance (p > 0.05). The visual roughness of the samples is shown in Fig. 3. The visual roughness was influenced by milk type (p < 0.05). No effect of storage was detected (p > 0.05). The visual roughness of yoghurt obtained from goat’s milk was significantly (p < 0.01) lower than that of the yoghurt produced from cow’s milk. This is probably due to differences in the graininess among yoghurt samples. Moreover, no significant differences were observed between the yoghurt samples obtained from goat’s milk and the yoghurt samples containing the mixture of goat’s and cow’s milk with regards to their visual roughness.
4. Conclusions This study has shown that the milk type and the storage time affect the physicochemical properties of yoghurt. Yoghurt obtained from cow’s milk showed higher viscosity and water holding capacity when compared with that containing goat’s milk. Yoghurt obtained from goat’s milk had the lowest number of grains, mean perimeter of grains and visual roughness. Yoghurt produced by mixing goat’s and cow’s milk in the same ratio had a higher viscosity and water holding capacity than that only containing goat’s milk. It also had a lower number of grains, mean perimeter of grains and visual roughness than that of cow’s milk. For practical applications, the addition of an amount of goat’s milk to cow’s milk can be optimized for the purpose of improving the quality or to develop fermented dairy products with different physicochemical properties. Acknowledgements The authors would like to thank The Scientific Research Projects Coordination Unit of Akdeniz University (Turkey). The assistance of Zeynep Atamer on this topic justifies grateful acknowledgement. References AOAC, 1997. Official methods of analysis. Association of Analytical Chemist International, 16th ed. Washington. Cheng, L.J., Clarke, P.T., Augustin, M.A., 2002. Seasonal variation in yogurt properties. Aust. J. Dairy Technol. 57, 187–191. Domagala, J., 2009. Instrumental texture, syneresis and microstructure of yoghurts prepared from goat, cow and sheep milk. Int. J. Food Prod. 12, 605–615. Herrero, A.M., Requena, T., 2006. The effect of supplementing goats milk with whey protein concentrate on textural properties of set-type yoghurt. Int. J. Food Sci. Technol. 41, 87–92. Karademir, E., Atamer, A., Tamucay, B., Yaman, S., 2002. Some properties of goat milk yoghurts produced by different fortification methods. Milchwissenschaft 57, 261–263.
A. Küc¸ükc¸etin et al. / Small Ruminant Research 96 (2011) 173–177 Kavas, G., Uysal, H., Kilic, S., Akbulut, N., Kesenkas, H., 2004. Production and selected properties of bioghurt made from goat milk and cow–goat milk mixtures by ultrafiltration and addition of skim milk. Int. J. Food Prod. 7, 473–482. Küc¸ükc¸etin, A., Weidendorfer, K., Hinrichs, J., 2009. Graininess and roughness of stirred yoghurt as influenced by processing. Int. Dairy J. 19, 50–55. Malek, A., Shadarevian, S., Toufeili, I., 2001. Sensory properties and consumer acceptance of concentrated yogurt made from cow’s, goat’s and sheep’s milk. Milchwissenschaft 56, 687–690. Martín-Diana, A.B., Janer, C., Peláez, C., Requena, T., 2003. Development of a fermented goat’s milk containing probiotic bacteria. Int. Dairy J. 13, 827–833. Medeiros, U.K.L., Medeiros, M.F.D., Passos, M.L., 2009. Goat milk powder production in small agra-cooperatives. In: Passos, M.L., Ribeiro, C.P. (Eds.), Innovation in Food Engineering. New Techniques and Products. CRC Press, Boca Raton, pp. 539–578. Rasmussen, M.A., Janhoj, Th., Ipsen, R., 2007. Effect of fat, protein and shear on graininess, viscosity and syneresis in low-fat stirred yoghurt. Milchwissenschaft 62, 54–58.
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Ribeiro, A.C., Ribeiro, S.D.A., 2010. Specialty products made from goat milk. Small Rumin. Res. 89, 225–233. Sodini, I., Montella, J., Tong, P.S., 2005. Physical properties of yogurt fortified with various commercial whey protein concentrates. J. Sci. Food Agric. 85, 853–859. Tamime, A.Y., Robinson, R.K., 1999. Yoghurt Science and Technology, 2nd ed. Woolhead Publishing Limited, Cambridge, p. 619. Tsioulpas, A., Lewis, M.J., Grandison, A.S., 2007. Effect of minerals on casein micelle stability of cows’ milk. J. Dairy Res. 74, 167–173. Uysal, H., Kilic, S., Kavas, G., Akbulut, N., Kesenkas, H., 2003. Some properties of set yoghurt made from caprine milk and bovine–caprine milk mixtures fortified by ultrafiltration or the addition of skim milk powder. Int. J. Dairy Technol. 56, 177–181. Vargas, M., Chafer, M., Albors, A., Chiralt, A., Gonzalez-Martinez, C., 2008. Physicochemical and sensory characteristics of yoghurt produced from mixtures of cows’ and goats’ milk. Int. Dairy J. 18, 1146–1152.
Contents lists available at ScienceDirect
Small Ruminant Research journal homepage: www.elsevier.com/locate/smallrumres
Short communication
Graininess and roughness of stirred yoghurt made with goat’s, cow’s or a mixture of goat’s and cow’s milk A. Küc¸ükc¸etin ∗ , M. Demir, A. As¸ci, E.M. C¸omak Department of Food Engineering, Faculty of Engineering, Akdeniz University, 07059 Antalya, Turkey
a r t i c l e
i n f o
Article history: Received 23 April 2010 Received in revised form 30 November 2010 Accepted 9 December 2010 Available online 14 January 2011 Keywords: Stirred yoghurt Goat’s milk Cow’s milk Graininess Roughness
a b s t r a c t The aim of this work was to study how the type of milk and the storage time affect the physicochemical characteristics, including graininess and roughness, of stirred yoghurt. Stirred yoghurt was produced from goat’s, cow’s or a mixture of goat’s and cow’s milk and was stored for 15 days at 4 ◦ C. Yoghurt produced from goat’s milk was characterized by its lower number of grains, mean perimeter of grains, roughness, viscosity and water holding capacity in comparison to that of the yoghurt developed using cow’s milk. Yoghurt with half and half cow’s/goat’s milk had a higher viscosity and water holding capacity than that only containing goat’s milk and also had a lower number of grains, mean perimeter of grains and roughness than that only containing cow’s milk. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The use of goat’s milk as an excellent food source is undeniable (Ribeiro and Ribeiro, 2010). Goat’s milk is designated for direct consumption or for the manufacture of cheese, fermented milks (e.g. yoghurt) and milk powder (Domagala, 2009). Nowadays, the production of goat’s milk and its processing constitutes an economic activity of increasing importance due to high nutritional interest of goat’s milk, as it provides high-quality protein, fat, carbohydrates, vitamins, and several minerals, such as iron, calcium, and phosphorus (Medeiros et al., 2009). The average composition of goat’s milk does not differ remarkably from that of cow’s milk. However, essential differences are present with regards to the structure, composition and size of the casein micelles, the proportion of individual protein fractions and higher content of nonprotein nitrogen and mineral compounds in goat’s milk (Domagala, 2009). All these differences could lead to the
∗ Corresponding author. Tel.: +90 242 3106569; fax: +90 242 2274564. E-mail address: [email protected] (A. Küc¸ükc¸etin). 0921-4488/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2010.12.003
milk behaving differently during the gelation process and gel formation; thus, affecting the final quality of dairy products obtained from the goat’s milk. In this context, yoghurt obtained from goat’s milk differs in some physicochemical properties such as the firmness of the coagulum, which tends to be soft and less viscous, from yoghurt obtained from cow’s milk. Additionally, yoghurt obtained from goat’s milk shows a weaker gel in comparison to yoghurt obtained from cow’s milk (Vargas et al., 2008). Textural defects of stirred yoghurt like graininess (particle) or roughness (irregularities in the yoghurt matrix) are undesirable, as consumers expect smooth, uniform and fine-bodied products. Although several works have been published determining the physicochemical and sensory properties of yoghurt made from goat’s milk and goat’s–cow’s milk mixtures (Malek et al., 2001; Uysal et al., 2003; Kavas et al., 2004; Vargas et al., 2008; Domagala, 2009), textural defects of yoghurt made from goat’s milk have not been studied in detail. The objective of this research was to determine the physicochemical properties, including the graininess and roughness, of stirred yoghurts containing goat’s, cow’s and a mixture of goat’s and cow’s milk.
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2. Materials and methods
3. Results and discussion
2.1. Milk processing and yoghurt preparation
After 1 day of cold storage, the yoghurt obtained from goat’s milk reached the lowest pH value, which remained relatively constant throughout the rest of the storage period (results not shown). This result was similar to that found by Vargas et al. (2008). The rate of the acid development due to Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in goat’s milk was faster than that observed within cow’s milk and the mixture of goat’s and cow’s milk, as has also been reported by Tamime and Robinson (1999). This different behavior could be explained by the enhancement of the microbial growth, acidity progress and peptidase activity of L. delbrueckii subsp. bulgaricus in goat’s milk (Vargas et al., 2008). The pH values decreased significantly (p < 0.01) in each yoghurt sample during storage at 4 ◦ C for 15 days. The pH values of the samples ranged from 4.06 to 4.22 by day 15. The viscosity values of the day 1 and day 15 samples varied from 0.79 to 1.66 Pa s and from 1.12 to 2.38 Pa s, respectively, according to the milk type (Fig. 1). The viscosity was significantly affected by the milk type and storage time (p < 0.001) (Table 1). The viscosity of the yoghurt obtained from goat’s milk was lower than that of the yoghurt obtained from the cow’s milk and the mixture of goat’s and cow’s milk. These results are in agreement with those obtained by Vargas et al. (2008). Malek et al. (2001) reported that concentrated yoghurts obtained from cow’s milk are more sticky than those obtained from goat’s milk. Martín-Diana et al. (2003) found that although goat’s and cow’s milk had similar total solids content and besides goat’s milk had higher protein content than that of cow’s milk, the viscosity of cow’s milk coagulum was about 30% higher than that of goat’s milk. The authors reported that the differences in casein content and micelle structure between species had high influence on the texture of fermented milk products. Additionally, ␣s1 -casein content of goat’s milk was lower than that of cow’s milk reported by Vargas et al. (2008). The authors pointed out that ␣s1 casein plays a very important role during gel formation, since a lower ␣s1 -casein content leads to weaker texture. The yoghurt texture is highly dependent on the total solids content, as well as on protein content and type. Goat’s milk has a slightly lower casein content than cow’s milk, with a very low proportion or the absence of ␣s1 -casein while having a higher degree of casein micelle dispersion (Herrero and Requena, 2006). These characteristics could be responsible for the differences observed in the viscosity values among yoghurt samples. However, it should be noted that the mineral compounds in milk and milk products play an important role in the stability of the proteins and in their some characteristics (Tsioulpas et al., 2007). Although protein is a significant contributor to development of texture in yoghurts, changes in minor components of milk that mediate changes in protein and mineral equilibria of milk may also alter yoghurt properties (Cheng et al., 2002). During storage at 4 ◦ C, viscosity values increased for all yoghurt samples, which is in agreement with those obtained by Karademir et al. (2002). The number and the mean perimeter of grains of the yoghurt samples varied from 18 to 25 per 3 mL of the sam-
Cow’s milk was obtained from the dairy farm of the Akdeniz University (Antalya, Turkey). Goat’s milk was collected from goat farms located in the Korkuteli district of Antalya Province. The goat’s and cow’s milk used in the production of stirred yoghurt had the following compositions, respectively: a total solids content of (g/L) 134.3 ± 0.71 and 116.55 ± 1.06, a total protein content of (g/L) 35.60 ± 1.13 and 32.00 ± 1.56, a fat content of (g/L) 41.45 ± 0.49 and 29.75 ± 0.35, ash content (g/L) 7.35 ± 0.21 and 7.10 ± 0.28, and the pH of 6.68 ± 0.02 and 6.50 ± 0.07. Three different formulations were prepared using the following combinations: (i) 100% cow’s milk, (ii) 50% goat’s milk and 50% cow’s milk and (iii) 100% goat’s milk. The milk was skimmed to a 1.0 g/L fat content at about 55 ◦ C. For the yoghurt preparation, skimmed milk powder (954 g/L total solids, 352 g/L protein, 11 g/L fat, 79 g/L ash; Izi Dairy Inc., Konya, Turkey) was added to the milk to give a final total solids content of 150 g/L. The standardized milk was heated at 90 ◦ C for 5 min and, then, subsequently cooled to 42 ◦ C. After cooling, 0.1 g/L of frozen pellets (starter culture DI-PROX TY 973, Bioprox, France) were added according to the manufacture’s reference, and the milk was incubated at 42 ◦ C until the pH decreased to 4.60. Fermentation was stopped by rapid cooling to the temperature of 4 ◦ C in an ice-water bath. At the beginning of the cooling process in an ice-water bath, the yoghurt was manually stirred for almost 60 s by up and down movements with a stainless-steel bored disk. After setting the stirred product in 200 mL cups (top diameter 65 mm, bottom diameter 53 mm and 80 mm height), the stirred yoghurt samples were stored at 4 ◦ C for 15 days. The physicochemical characteristics of the samples were analyzed on day 1, 7 and 15 of storage. The entire experimental process was performed in triplicate.
2.2. Physicochemical properties measurements The total solids, total protein, fat and ash contents of goat’s and cow’s milk were determined according to the Association of Official Analytical Chemist methods (AOAC, 1997). The pH was measured by a WTW pH meter (Inolab pH level 2, WTW GmbH, Weilheim, Germany). The viscosity was measured using a Brookfield DVII + Pro viscosimeter (Brookfield, Middleboro, MA, USA) with a Helipath (T spindle, type D) rotated at 5 rpm following the method described by Rasmussen et al. (2007). The sample temperature was 4 ◦ C. The water-holding capacity was determined by the procedure described by Sodini et al. (2005). A sample of about 20 g of yoghurt (Y) was centrifuged for 10 min at 1250 × g at 4 ◦ C. The whey expelled (W) was removed and weighed. The water-holding capacity (WHC) was calculated as: WHC (%) = 100 ×
(Y − W ) Y
The number and mean perimeter (diameter calculated from the measured boundary length) of grains, used as a measure of the graininess, were measured by image analysis according to the protocol described by Küc¸ükc¸etin et al. (2009). An image of the transmitted sample was taken with a built-in CCD camera (TFP-M/WL, LTF-Labortechnik, Wasserburg, Germany) having a resolution of 768 × 576 pixels and a 256 grayscale color depth effectively corresponding to 74-pixels/cm. The image analysis was performed with Optimas 6.1 software (Media Cybergenetics, Silver Spring, MD, USA). The software was set for the evaluation of 3 mL sample. The number of grains indicating a perimeter greater than 1.0 mm per 3 mL of yoghurt and the mean perimeter of grains were evaluated. The measurement of the visual roughness (Rvis ) was based upon that described by Küc¸ükc¸etin et al. (2009). The Rvis of the sample was taken as an additional descriptive parameter of the digital image made for graininess. All measurements were performed in duplicate.
2.3. Statistical analyses All statistical calculations were analyzed using SAS Statistical Software (released for Windows, SAS Institute Inc., Cary, NC, USA). The Duncan’s multiple range test was conducted to detect differences among the treatment means.
A. Küc¸ükc¸etin et al. / Small Ruminant Research 96 (2011) 173–177
175
Viscosity (Pa s)
2
1
day 1
0
day 7
day 15
day 7
day 1
CM yoghurt
day 15
day 1
MM yoghurt
day 15
day 7
GM yoghurt
Fig. 1. Viscosity of yoghurt samples obtained from cow’s milk (CM), a mixture of goat’s and cow’s milk (MM) and goat’s milk (GM) during storage at 4 ◦ C for 15 days. The bars represent mean values, and the error bars represent the standard deviation of the mean. Table 1 Effect of the milk type (MT) and storage time (ST) on some selected characteristics of stirred skim milk yoghurt. ANOVA
pH
Water holding capacity (%)
Viscosity (Pa s)
Number of grains/3 mL
PGb (mm)
MT ST MTxST
***
***
***
**
***
*
***
***
***
n.s.a
n.s.
n.s.
n.s. n.s.
n.s. n.s.
n.s. n.s.
** *** a b c
Significantly different at p < 0.05. Significantly different at p < 0.01. Significantly different at p < 0.001. n.s., not significant. PG, perimeter of grains. Rvis , visual roughness; AIDm , mean absolute intensity deviation.
ple and from 1.9 to 4.6 mm, respectively, depending upon the milk type and the storage time (Fig. 2). The number and the mean perimeter of grains were influenced by the milk type (p ≤ 0.01) (Table 1). The effect of storage was not
significant (p > 0.05). There were significant differences in the number and the mean perimeter of grains between the yoghurts obtained from cow’s and goat’s milk. Furthermore, the number and the perimeter of grains in yoghurt 6
30
Numbers of grains/ 3mL
5
4
20
3
2
10
Mean perimeter of grains (mm)
*
Rvis (unit of AIDm )c
1
day 1
day 7
day 15
day 1
day 7
day 15
day 1
day 7
day 15
0 CM yoghurt
MM yoghurt
0
GM yoghurt
Fig. 2. Number ( ) and mean perimeter () of grains in yoghurt samples obtained from cow’s milk (CM), a mixture of goat’s and cow’s milk (MM), and goat’s milk (GM) during storage at 4 ◦ C for 15 days. The bars represent mean values, and the error bars represent the standard deviation of the mean.
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5
4 40 3
2 20
Rvis (unit of AIDm)
Water holding capacity (%) (w/w)
60
1
0
day 1
day 7
day 15
day 1
CM yoghurt
day 7
day 15
day 1
MM yoghurt
day 7
day 15
0
GM yoghurt
Fig. 3. Water holding capacity ( ) and visual roughness (Rvis ) () of grains in yoghurt samples obtained from cow’s milk (CM), a mixture of goat’s and cow’s milk (MM), and goat’s milk (GM) during storage at 4 ◦ C for 15 days. The bars represent mean values, and the error bars represent the standard deviation of the mean.
obtained from cow’s milk was significantly (p < 0.01) higher than those from goat’s milk and the mixture of goat’s and cow’s milk. However, no differences were found in the number of grains between the yoghurt samples obtained from the goat’s milk and the samples obtained from the mixture of goat’s and cow’s milk. Vargas et al. (2008) reported that yoghurt made from goat’s milk had no grains. The difference in the graininess among the yoghurts may be due to a difference in the composition of the protein within the milk type that is used for yoghurt manufacture. The water holding capacity of the samples varied from 39.3 to 51.2% (Fig. 3). As shown in Table 1, the milk type and the storage time significantly (p < 0.01) affected the water holding capacity of the yoghurt. The water holding capacity of yoghurt obtained from the goat’s milk was significantly (p < 0.01) lower than that obtained from the cow’s milk and that of the mixture of goat’s and cow’s milk. These results are in agreement with those obtained by Domagala (2009). Malek et al. (2001) reported that concentrated yoghurts obtained from cow’s milk exuded less water than those obtained from goat’s milk, despite the total solids content of the yoghurt samples being similar. During storage at 4 ◦ C, the water holding capacity decreased for all the yoghurt samples, with the difference between the day 7 and day 15 samples not being of significance (p > 0.05). The visual roughness of the samples is shown in Fig. 3. The visual roughness was influenced by milk type (p < 0.05). No effect of storage was detected (p > 0.05). The visual roughness of yoghurt obtained from goat’s milk was significantly (p < 0.01) lower than that of the yoghurt produced from cow’s milk. This is probably due to differences in the graininess among yoghurt samples. Moreover, no significant differences were observed between the yoghurt samples obtained from goat’s milk and the yoghurt samples containing the mixture of goat’s and cow’s milk with regards to their visual roughness.
4. Conclusions This study has shown that the milk type and the storage time affect the physicochemical properties of yoghurt. Yoghurt obtained from cow’s milk showed higher viscosity and water holding capacity when compared with that containing goat’s milk. Yoghurt obtained from goat’s milk had the lowest number of grains, mean perimeter of grains and visual roughness. Yoghurt produced by mixing goat’s and cow’s milk in the same ratio had a higher viscosity and water holding capacity than that only containing goat’s milk. It also had a lower number of grains, mean perimeter of grains and visual roughness than that of cow’s milk. For practical applications, the addition of an amount of goat’s milk to cow’s milk can be optimized for the purpose of improving the quality or to develop fermented dairy products with different physicochemical properties. Acknowledgements The authors would like to thank The Scientific Research Projects Coordination Unit of Akdeniz University (Turkey). The assistance of Zeynep Atamer on this topic justifies grateful acknowledgement. References AOAC, 1997. Official methods of analysis. Association of Analytical Chemist International, 16th ed. Washington. Cheng, L.J., Clarke, P.T., Augustin, M.A., 2002. Seasonal variation in yogurt properties. Aust. J. Dairy Technol. 57, 187–191. Domagala, J., 2009. Instrumental texture, syneresis and microstructure of yoghurts prepared from goat, cow and sheep milk. Int. J. Food Prod. 12, 605–615. Herrero, A.M., Requena, T., 2006. The effect of supplementing goats milk with whey protein concentrate on textural properties of set-type yoghurt. Int. J. Food Sci. Technol. 41, 87–92. Karademir, E., Atamer, A., Tamucay, B., Yaman, S., 2002. Some properties of goat milk yoghurts produced by different fortification methods. Milchwissenschaft 57, 261–263.
A. Küc¸ükc¸etin et al. / Small Ruminant Research 96 (2011) 173–177 Kavas, G., Uysal, H., Kilic, S., Akbulut, N., Kesenkas, H., 2004. Production and selected properties of bioghurt made from goat milk and cow–goat milk mixtures by ultrafiltration and addition of skim milk. Int. J. Food Prod. 7, 473–482. Küc¸ükc¸etin, A., Weidendorfer, K., Hinrichs, J., 2009. Graininess and roughness of stirred yoghurt as influenced by processing. Int. Dairy J. 19, 50–55. Malek, A., Shadarevian, S., Toufeili, I., 2001. Sensory properties and consumer acceptance of concentrated yogurt made from cow’s, goat’s and sheep’s milk. Milchwissenschaft 56, 687–690. Martín-Diana, A.B., Janer, C., Peláez, C., Requena, T., 2003. Development of a fermented goat’s milk containing probiotic bacteria. Int. Dairy J. 13, 827–833. Medeiros, U.K.L., Medeiros, M.F.D., Passos, M.L., 2009. Goat milk powder production in small agra-cooperatives. In: Passos, M.L., Ribeiro, C.P. (Eds.), Innovation in Food Engineering. New Techniques and Products. CRC Press, Boca Raton, pp. 539–578. Rasmussen, M.A., Janhoj, Th., Ipsen, R., 2007. Effect of fat, protein and shear on graininess, viscosity and syneresis in low-fat stirred yoghurt. Milchwissenschaft 62, 54–58.
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Ribeiro, A.C., Ribeiro, S.D.A., 2010. Specialty products made from goat milk. Small Rumin. Res. 89, 225–233. Sodini, I., Montella, J., Tong, P.S., 2005. Physical properties of yogurt fortified with various commercial whey protein concentrates. J. Sci. Food Agric. 85, 853–859. Tamime, A.Y., Robinson, R.K., 1999. Yoghurt Science and Technology, 2nd ed. Woolhead Publishing Limited, Cambridge, p. 619. Tsioulpas, A., Lewis, M.J., Grandison, A.S., 2007. Effect of minerals on casein micelle stability of cows’ milk. J. Dairy Res. 74, 167–173. Uysal, H., Kilic, S., Kavas, G., Akbulut, N., Kesenkas, H., 2003. Some properties of set yoghurt made from caprine milk and bovine–caprine milk mixtures fortified by ultrafiltration or the addition of skim milk powder. Int. J. Dairy Technol. 56, 177–181. Vargas, M., Chafer, M., Albors, A., Chiralt, A., Gonzalez-Martinez, C., 2008. Physicochemical and sensory characteristics of yoghurt produced from mixtures of cows’ and goats’ milk. Int. Dairy J. 18, 1146–1152.