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Influence of composition and additives on properties of set-type yoghurt from goat milk
Small Ruminant Research, 2 (1989) 35-45 Elsevier Science Publishers B.V., AmsterdAm - - Printed in The Netherlands
35
Influence of Composition and Additives on Properties of S e t - T y p e Yoghurt from Goat Milk C. KEHAGIAS, A. ZERVOUDAKI and C. PARLAMA
School of Food Technology, TEI of Athens, St. Spyridon Str., 122 10 Egaleo, Athens (Greece) (Accepted 15 June 1988)
ABSTRACT Kehagias, C., Zervoudaki, A. and Parlama, C., 1989. Influence of composition and additives on properties of set-type yoghurt from goat milk. Small Rumin. Res., 2: 35-45.
Composition of thirty samples of goat milk from indigenous, Saanen and Saanen-cross breeds was studied in relationto physical properties of set-type yoghurt. Penetration force and viscosity of yoghurt were correlatedbest with Ca, fat and totalsolidscontents. Milk from indigenous breeds had the richest composition, and yoghurt prepared from it gave the highest Instron penetration force and viscosity,compared with that from Saanen and cross breeds. From the organoleptic point of view, yoghurt prepared from Saanen goat milk was leastpreferred, while yoghurt from indigenous breeds approached, in preference, commercial brands of yoghurt from cow milk. Physicalcharacteristicsof yoghurt were improved by increasingsolids-non-fatcontents and by adding commercially available milk proteins (from cow milk). On the other hand, by increasing fat content and adding CaCl2, the physical characteristicsof yoghurt were not improved in spiteof strong relationships that were found for fat and Ca contents of goat milk with penetration force and viscosityof yoghurt.
INTRODUCTION
Goat milk has played an important role in human nutrition, especially in countries where difficult climatic conditions are not suitable for large ruminants like cows. In such countries, much goat milk remains on farms for processing, especially into cheese. Distribution is limited to markets of urban areas. Dairy products from goat milk, with the exception of some cheeses, are rarely commercially available to many people. However, in order to insure that goat farming will continue to play an important economical role in the future, milk should be utilized in a more profitable way by manufacturing products that will be readily available in the market. Although, there are many factors involved in the limited expansion of a wide range of dairy products from goat milk, certainly some of them are related to its technology. Goat milk behaves, in many aspects, in a different manner than 0921-4488/89/$03.50
© 1989 Elsevier Science Publishers B.V.
36 cow milk. Manufacture of goat cheese, because of a more fragile coagulum usually results in lower yield. In the production of yoghurt, texture of the product is poor and flavor weak (Abrahamsen and Holmen, 1981; Kehagias et al., 1986). Recently, some of the problems related to fermented products from goat milk have been reviewed (Kehagias, 1987). Existence of large gaps in our knowledge and need for collecting information about specific characteristics of products from goat milk, is widely recognized. The purpose of this work was to provide information on variation that exists in composition of milk of various goat breeds and their relationship with physical properties of set-type yoghurt. Furthermore, the influence of various additives on the physical properties of yoghurt was examined. MATERIALSAND METHODS
Milk samples and yoghurt preparation A total of 30 samples of goat milk were analyzed and used for preparation of yoghurt in a two-year period. Samples were collected from various flocks representing different breeds (12 from indigenous, 3 from Saanen and 6 from Saanen crossed with local breeds). For studying the effect of various additives on physical properties of yoghurt, milk from the same indigenous breed was used and collected during one lactation period. Milk for the preparation of settype yoghurt (fermented in a container) was heat-treated at 80-85°C for 30 min. Inoculation was made with 2.5% mixed culture of S. thermophilus and L. bulgaricus (Wiesby V1 ) at 43 ° C. Fermentation took place in a 250 ml plastic container for sensory evaluation and in a 200 ml beaker for measurement of physical properties. Incubation time was monitored so that the final pH should fall within 4.1-4.4. After fermentation, yoghurt was cooled at 5-7 °C prior to examination.
Utilisation of various additives Calcium content of milk was increased by adding various concentrations of CaC12. Sodium-caseinate, calcium-caseinate and whole milk protein, manufactured from cow milk (DMV, Veghel, Holland) were used at various levels for studying the effect of these proteins. For manipulating the variations in solidsnon-fat and fat contents, skim milk concentrated by vacuum evaporation and cream were used, both prepared from goat milk.
Chemical analysis Determinations of total solids, fat, protein and Ca contents were made in milk after heat-treatment with standard methods (APHA, 1967; IDF, 1962,
37 1966, 1982 ). Casein from heat-treated milk and yoghurt was separated according to Zittle and Custer (1966). In case of yoghurt, there was no adjustment of pH. Samples were centrifuged and the sediment used for electrophoresis. Sediments were treated according to Morr (1974) prior to electrophoresis. Samples were subjected to polyacrylamide gel electrophoresis by the method of Kiddy (1974).
Measurement of the physical properties of yoghurt Firmness of undisturbed yoghurt was evaluated with Instron, by penetrating a 36 m m diameter disc probe in 125 ml of sample in a beaker of 60 m m diameter. Following penetration, yoghurt was stirred manually (30 times across the diameter of the beaker with a spoon) and also evaluated on a Brookfield LVT viscometer. Volume of the expelled serum was measured after centrifugation of 40 g of yoghurt at 3500 rpm for 20 min.
Organoleptic evaluation of yoghurt Sensory evaluation of yoghurt was carried out by a consumer panel of 19 judges that were asked to rank samples of yoghurt according to their preference. Higher numbers indicate less acceptability. RESULTS AND DISCUSSION
Composition of goat milk and properties of yoghurt In Table 1 is the mean composition of milk of the three breeds, representing 30 samples of goat milk after heat-treatment, and physical properties of yoghurt prepared from them. It can be seen that milk from indigenous goat breeds had the richest composition, its yoghurt gave highest values for Instron penetration force and stirred viscosity, compared with Saanen and their crosses. Table 2 shows results of sensory evaluations of yoghurt by a consumer panel. Yoghurt prepared from Saanen goat milk was the least preferred while yoghurt from indigenous breeds approached in preference commercially prepared brands of yoghurt from cow milk. Instron penetration force and stirred viscosity of yoghurt represent two essential components of consistency and influence organoleptic properties. Lower acceptability of yoghurt from Saanen goat milk compared to indigenous breeds could be attributed to its poor consistency. It is known that casein has a strong influence on rheological properties of dairy products. Some authors {Storry et al., 1983; Remeuf and Lenoir, 1986) have found relationships between cheese coagulum strength and concentration of alpha casein, while others (Yun et al., 1982) related it to beta casein concentration. In this study we did not try to quantitate casein fractions of goat milk
38 TABLE 1 Mean compositionsof heat-treated goat milk and physical properties of their yoghurt Goat breeds Milk
Indigenous
Crosses ofindigenous with Saanen
Saanen
Fat (%) Total protein (%) Total solids (%) Ca (rag/100 g)
5.68 4.23 15.62 145.50
3.80 3.85 13.95 124.53
3.69 3.03 13.01 124.25
Yoghurt Instron penetration force (g) Stirred viscosity at 6 rpm (cps)
115.00
70.00
67.00
4960.00
4032.00
2795.00
TABLE 2 Sensory evaluationof yoghurt Sample
Mean score
Cow commercialA Cow commercialB Cow commercialC Goat from indigenousbreeds Goat from Saanen breeds
3.0a 1.8 b
2.3a 2.9a 4.9c
Mean scorewas determinedby assigningI to the most preferredsample and 5 to the leastpreferred. a,b,CMeansthat have been superscriptedwith differentlettersdiffer(P < 0.05). from the various breeds, however, electrophoretic patterns of caseins of yoghurt milk and of yoghurt from t he t hr e e flocks of Saanen and t he t hree indigenous breeds (Fig. 1) were quite similar. T hi s indicates t h a t no qualitative differences in caseins of h e a t - t r e a t e d milk from indigenous and Saanen breeds were found and any differences in physical properties of yoghurt might be related to concentrations of t he various casein fractions.
Relationship between goat milk composition and physical properties of yoghurt Correlations between concentrations of various components and physical characteristics of yoghurt are in Table 3. Penetration force of yoghurt was correlated best with Ca, fat and total-solids contents. W e a k e r was the correlation with total protein and the ratio of protein/fat. Correlation between vis-
39
a
b 1
a b
a
b 3
Fig. 1. Polyacrylamide gel electrophoresis of casein from goat milk after heat-treatment and of yoghurt prepared from it. Milk (a) and yoghurt (b) from three (1, 2, 3) indigenous breeds and from three samples (4, 5, 6) of Saanen breed.
cosity and various components was similarwith the correlationof penetration force with the same components, although levelsof significancechanged for Ca and totalsolids.Significantcorrelationsbetween Ca and casein and cheese
40 TABLE 3 Correlations between various components of goat milk and physical characteristics of yoghurt Components
Fat Protein Total solids Solids-non-fat Ca Protein/fat Protein/Ca
Correlation coefficient Penetration force
Viscosity
0.645c 0.433a 0.569c 0.222 0.708c 0.486~ 0.138
0.648¢ 0.419a 0.516b 0.126 0.493b 0.488~ 0.138
Significancelevels:aP<0.05; bP< 0.01; cP< 0.001 (30 samples).
coagulum strength have been found by other researchers (Storry et al., 1983; Remeuf and Lenoir, 1986). Weaker correlations between total protein and physical properties of yoghurt found here could be attributed to the fact that total protein includes also whey proteins which are known to play a less important role t h a n casein in the structure of the yoghurt gel.
Effect of various additives on physical properties of yoghurt In Fig. 2 is the Instron penetration force and volume of the expelled serum. In Fig. 3 the viscosity at various rpm. Although a strong relationship was found between Ca content and physical properties of yoghurt (Table 3 ), the results showed that by increasing Ca content these properties were not affected. Storry and Ford (1982) found that the addition of Ca to cow milk increased cheese coagulum strength. These experiments do not enable us to explain the reason for the different behaviour of CaC12 in yoghurt. However, yoghurt gel is formed in a different manner, has lower pH and more whey proteins t h a n rennet gel. These factors will not favour incorporation of Ca to gel that will increase gel strength. On the other hand, the relationship which has been found (Remeuf and Lenoir, 1986) between Ca content and casein might explain the behaviour of yoghurt gel towards Ca addition. In Fig. 4 is the effect of increasing solids-non-fat and fat contents of goat milk on Instron penetration force and viscosity of yoghurt. It can be seen that by adding increasing amounts of concentrated skim milk, the penetration force increased while viscosity remained practically unchanged. Increase of penetration force and of viscosity by increasing solids-non-fat is known for yoghurt of
41 25
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Fig. 2. Influence of various additives on Instron penetration force (circles) and volume of expelled serum (triangles).
cow m i l k ( S c h m i d t et al., 1980). W h e n f a t c o n t e n t was increased, p e n e t r a t i o n force a n d v i s c o s i t y r e m a i n e d t h e s a m e . T h e s t r o n g r e l a t i o n s h i p t h a t was ob-
42 30
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served between fat and penetration force and between fat and viscosity (Table 3) might be due, as in the case of Ca, to existing secondary relationships between fat and casein.
43 300
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Fig. 4. Influence of solids-non-fat and fat on Instron penetration force and viscosity (circles:3 rpm; triangles:6 rpm; squares: 12 rpm; crosses: 30 rpm). CONCLUSIONS
This study showed that consistency of yoghurt from milk of indigenous goat breeds was better compared to Saanen and cross breeds. Although, consumers
44 are n o t a c c u s t o m e d to y o g h u r t f r o m goat milk, y o g h u r t f r o m m i l k of i n d i g e n o u s goat b r e e d s was c o m p a r a b l e to c o m m e r c i a l b r a n d s . I n case t h a t f u r t h e r f i r m n e s s n e e d s to be i m p r o v e d , t h i s c a n b e s t be d o n e b y i n c r e a s i n g s o l i d s - n o n - f a t . A d d i n g c o m m e r c i a l cow m i l k p r o t e i n p r o d u c t s is effective, b u t affects t h e genuiness of t h e p r o d u c t . C o r r e l a t i o n s b e t w e e n m i l k c o m p o n e n t s a n d p h y s i c a l p r o p e r t i e s of y o g h u r t m i g h t be also a f f e c t e d b y s e c o n d a r y r e l a t i o n s h i p s w i t h milk components. ACKNOWLEDGEMENTS T h i s r e s e a r c h was s u p p o r t e d b y t h e M i n i s t r y of E d u c a t i o n of G r e e c e a n d E E C . T h e a u t h o r s w i s h to t h a n k M i s s P. K o n i d a r i a n d Mrs. E. X i r o g i a n n i for t h e i r t e c h n i c a l a s s i s t a n c e a n d M r s . H. K a r a f e r i for t y p i n g t h e m a n u s c r i p t .
REFERENCES Abrahamsen, R.G. and Holmen, T.R., 1981. Goat's milk yoghurt made from non-homogenised and homogenised milks, concentrated by different methods. J. Dairy Res., 50: 349-356. APHA, 1967. Standard methods for the examination of dairy products. Am. Public Health Assoc., 1740 Broadway, New York, NY, pp. 183-188. IDF, 1962. Determination of the total nitrogen content by the Kjeldahl method. International Dairy Federation. Standard No 20, Brussels. IDF, 1966. Determination of calcium content. International Dairy Federation. Standard No 36, Brussels. IDF, 1982. Milk, cream and evaporated milk-determination of the total solids content (reference method). International Dairy Federation. Standard 21A, Brussels. Kehagias, C.H., 1987. Fermented milk products in developing countries with emphasis on those produced from ewe's and goat's milk. In: Milk - The Vital Force. Reidel, The Hague, pp. 683696. Kehagias, C., Komiotis, A., Koulouris, S., Koroni, H. and Kazazis, J., 1986. Physicochemical properties of set-type yoghurt made from cow's, ewe's and goat's milk. Bull. International Dairy Federation, Document No 202, pp. 167-169. Kiddy, C., 1974. Gel electrophoresis in vertical polyacrylamide gel. In: American Dairy Science Association, Nomenclature and Methods of Milk Proteins, pp. 14-15. Molder, H.W., Larmond, M.E., Lin, C.S., Froehlich, D. and Emmons, D.B., 1983. Physical and sensory properties of yoghurt stabilised with milk proteins. J. Dairy Sci., 66: 422-429. Morr, C.W., 1974. Methods of gel electrophoresis of milk proteins. In: American Dairy Science Association, Nomenclature and Methods of Milk Proteins, pp. 12-13. Remeuf, F. and Lenoir, J., 1986. Relationship between the physicochemical characteristics of goat's milk and its rennetability. Bull. International Dairy Federation, Document No 202, Brussels, pp. 68-72. Schmidt, R.H., Sistrunk, C.P., Richter, R.L. and Cornell, J.A., 1980. Heat treatment and storage effects on texture characteristics of milk and yoghurt systems fortified with oilseed proteins. J. Food Sci., 45: 471-475. Storry, J.E. and Ford, G.D., 1982. Some factors affecting the post clotting development of coagulum strength in renneted milk. J. Dairy Res., 49: 469-473.
45 Storry, J.E., Grandison, A.S., Millard, D., Owen, A.J. and Ford, G.D., 1983. Chemical composition and coagulating properties of renneted milks from different breeds and species of ruminant. J. Dairy Res., 50: 215-229. Yun, S., Ohmiya, K. and Shimizu, S., 1982. Role of casein in milk curding. Agric. Biol. Chem., 46: 443-449. Zittle, C.A. and Custer, J.H., 1966. Identification of the casein among the components of whole goat casein. J. Dairy Sci., 49: 788-791.
35
Influence of Composition and Additives on Properties of S e t - T y p e Yoghurt from Goat Milk C. KEHAGIAS, A. ZERVOUDAKI and C. PARLAMA
School of Food Technology, TEI of Athens, St. Spyridon Str., 122 10 Egaleo, Athens (Greece) (Accepted 15 June 1988)
ABSTRACT Kehagias, C., Zervoudaki, A. and Parlama, C., 1989. Influence of composition and additives on properties of set-type yoghurt from goat milk. Small Rumin. Res., 2: 35-45.
Composition of thirty samples of goat milk from indigenous, Saanen and Saanen-cross breeds was studied in relationto physical properties of set-type yoghurt. Penetration force and viscosity of yoghurt were correlatedbest with Ca, fat and totalsolidscontents. Milk from indigenous breeds had the richest composition, and yoghurt prepared from it gave the highest Instron penetration force and viscosity,compared with that from Saanen and cross breeds. From the organoleptic point of view, yoghurt prepared from Saanen goat milk was leastpreferred, while yoghurt from indigenous breeds approached, in preference, commercial brands of yoghurt from cow milk. Physicalcharacteristicsof yoghurt were improved by increasingsolids-non-fatcontents and by adding commercially available milk proteins (from cow milk). On the other hand, by increasing fat content and adding CaCl2, the physical characteristicsof yoghurt were not improved in spiteof strong relationships that were found for fat and Ca contents of goat milk with penetration force and viscosityof yoghurt.
INTRODUCTION
Goat milk has played an important role in human nutrition, especially in countries where difficult climatic conditions are not suitable for large ruminants like cows. In such countries, much goat milk remains on farms for processing, especially into cheese. Distribution is limited to markets of urban areas. Dairy products from goat milk, with the exception of some cheeses, are rarely commercially available to many people. However, in order to insure that goat farming will continue to play an important economical role in the future, milk should be utilized in a more profitable way by manufacturing products that will be readily available in the market. Although, there are many factors involved in the limited expansion of a wide range of dairy products from goat milk, certainly some of them are related to its technology. Goat milk behaves, in many aspects, in a different manner than 0921-4488/89/$03.50
© 1989 Elsevier Science Publishers B.V.
36 cow milk. Manufacture of goat cheese, because of a more fragile coagulum usually results in lower yield. In the production of yoghurt, texture of the product is poor and flavor weak (Abrahamsen and Holmen, 1981; Kehagias et al., 1986). Recently, some of the problems related to fermented products from goat milk have been reviewed (Kehagias, 1987). Existence of large gaps in our knowledge and need for collecting information about specific characteristics of products from goat milk, is widely recognized. The purpose of this work was to provide information on variation that exists in composition of milk of various goat breeds and their relationship with physical properties of set-type yoghurt. Furthermore, the influence of various additives on the physical properties of yoghurt was examined. MATERIALSAND METHODS
Milk samples and yoghurt preparation A total of 30 samples of goat milk were analyzed and used for preparation of yoghurt in a two-year period. Samples were collected from various flocks representing different breeds (12 from indigenous, 3 from Saanen and 6 from Saanen crossed with local breeds). For studying the effect of various additives on physical properties of yoghurt, milk from the same indigenous breed was used and collected during one lactation period. Milk for the preparation of settype yoghurt (fermented in a container) was heat-treated at 80-85°C for 30 min. Inoculation was made with 2.5% mixed culture of S. thermophilus and L. bulgaricus (Wiesby V1 ) at 43 ° C. Fermentation took place in a 250 ml plastic container for sensory evaluation and in a 200 ml beaker for measurement of physical properties. Incubation time was monitored so that the final pH should fall within 4.1-4.4. After fermentation, yoghurt was cooled at 5-7 °C prior to examination.
Utilisation of various additives Calcium content of milk was increased by adding various concentrations of CaC12. Sodium-caseinate, calcium-caseinate and whole milk protein, manufactured from cow milk (DMV, Veghel, Holland) were used at various levels for studying the effect of these proteins. For manipulating the variations in solidsnon-fat and fat contents, skim milk concentrated by vacuum evaporation and cream were used, both prepared from goat milk.
Chemical analysis Determinations of total solids, fat, protein and Ca contents were made in milk after heat-treatment with standard methods (APHA, 1967; IDF, 1962,
37 1966, 1982 ). Casein from heat-treated milk and yoghurt was separated according to Zittle and Custer (1966). In case of yoghurt, there was no adjustment of pH. Samples were centrifuged and the sediment used for electrophoresis. Sediments were treated according to Morr (1974) prior to electrophoresis. Samples were subjected to polyacrylamide gel electrophoresis by the method of Kiddy (1974).
Measurement of the physical properties of yoghurt Firmness of undisturbed yoghurt was evaluated with Instron, by penetrating a 36 m m diameter disc probe in 125 ml of sample in a beaker of 60 m m diameter. Following penetration, yoghurt was stirred manually (30 times across the diameter of the beaker with a spoon) and also evaluated on a Brookfield LVT viscometer. Volume of the expelled serum was measured after centrifugation of 40 g of yoghurt at 3500 rpm for 20 min.
Organoleptic evaluation of yoghurt Sensory evaluation of yoghurt was carried out by a consumer panel of 19 judges that were asked to rank samples of yoghurt according to their preference. Higher numbers indicate less acceptability. RESULTS AND DISCUSSION
Composition of goat milk and properties of yoghurt In Table 1 is the mean composition of milk of the three breeds, representing 30 samples of goat milk after heat-treatment, and physical properties of yoghurt prepared from them. It can be seen that milk from indigenous goat breeds had the richest composition, its yoghurt gave highest values for Instron penetration force and stirred viscosity, compared with Saanen and their crosses. Table 2 shows results of sensory evaluations of yoghurt by a consumer panel. Yoghurt prepared from Saanen goat milk was the least preferred while yoghurt from indigenous breeds approached in preference commercially prepared brands of yoghurt from cow milk. Instron penetration force and stirred viscosity of yoghurt represent two essential components of consistency and influence organoleptic properties. Lower acceptability of yoghurt from Saanen goat milk compared to indigenous breeds could be attributed to its poor consistency. It is known that casein has a strong influence on rheological properties of dairy products. Some authors {Storry et al., 1983; Remeuf and Lenoir, 1986) have found relationships between cheese coagulum strength and concentration of alpha casein, while others (Yun et al., 1982) related it to beta casein concentration. In this study we did not try to quantitate casein fractions of goat milk
38 TABLE 1 Mean compositionsof heat-treated goat milk and physical properties of their yoghurt Goat breeds Milk
Indigenous
Crosses ofindigenous with Saanen
Saanen
Fat (%) Total protein (%) Total solids (%) Ca (rag/100 g)
5.68 4.23 15.62 145.50
3.80 3.85 13.95 124.53
3.69 3.03 13.01 124.25
Yoghurt Instron penetration force (g) Stirred viscosity at 6 rpm (cps)
115.00
70.00
67.00
4960.00
4032.00
2795.00
TABLE 2 Sensory evaluationof yoghurt Sample
Mean score
Cow commercialA Cow commercialB Cow commercialC Goat from indigenousbreeds Goat from Saanen breeds
3.0a 1.8 b
2.3a 2.9a 4.9c
Mean scorewas determinedby assigningI to the most preferredsample and 5 to the leastpreferred. a,b,CMeansthat have been superscriptedwith differentlettersdiffer(P < 0.05). from the various breeds, however, electrophoretic patterns of caseins of yoghurt milk and of yoghurt from t he t hr e e flocks of Saanen and t he t hree indigenous breeds (Fig. 1) were quite similar. T hi s indicates t h a t no qualitative differences in caseins of h e a t - t r e a t e d milk from indigenous and Saanen breeds were found and any differences in physical properties of yoghurt might be related to concentrations of t he various casein fractions.
Relationship between goat milk composition and physical properties of yoghurt Correlations between concentrations of various components and physical characteristics of yoghurt are in Table 3. Penetration force of yoghurt was correlated best with Ca, fat and total-solids contents. W e a k e r was the correlation with total protein and the ratio of protein/fat. Correlation between vis-
39
a
b 1
a b
a
b 3
Fig. 1. Polyacrylamide gel electrophoresis of casein from goat milk after heat-treatment and of yoghurt prepared from it. Milk (a) and yoghurt (b) from three (1, 2, 3) indigenous breeds and from three samples (4, 5, 6) of Saanen breed.
cosity and various components was similarwith the correlationof penetration force with the same components, although levelsof significancechanged for Ca and totalsolids.Significantcorrelationsbetween Ca and casein and cheese
40 TABLE 3 Correlations between various components of goat milk and physical characteristics of yoghurt Components
Fat Protein Total solids Solids-non-fat Ca Protein/fat Protein/Ca
Correlation coefficient Penetration force
Viscosity
0.645c 0.433a 0.569c 0.222 0.708c 0.486~ 0.138
0.648¢ 0.419a 0.516b 0.126 0.493b 0.488~ 0.138
Significancelevels:aP<0.05; bP< 0.01; cP< 0.001 (30 samples).
coagulum strength have been found by other researchers (Storry et al., 1983; Remeuf and Lenoir, 1986). Weaker correlations between total protein and physical properties of yoghurt found here could be attributed to the fact that total protein includes also whey proteins which are known to play a less important role t h a n casein in the structure of the yoghurt gel.
Effect of various additives on physical properties of yoghurt In Fig. 2 is the Instron penetration force and volume of the expelled serum. In Fig. 3 the viscosity at various rpm. Although a strong relationship was found between Ca content and physical properties of yoghurt (Table 3 ), the results showed that by increasing Ca content these properties were not affected. Storry and Ford (1982) found that the addition of Ca to cow milk increased cheese coagulum strength. These experiments do not enable us to explain the reason for the different behaviour of CaC12 in yoghurt. However, yoghurt gel is formed in a different manner, has lower pH and more whey proteins t h a n rennet gel. These factors will not favour incorporation of Ca to gel that will increase gel strength. On the other hand, the relationship which has been found (Remeuf and Lenoir, 1986) between Ca content and casein might explain the behaviour of yoghurt gel towards Ca addition. In Fig. 4 is the effect of increasing solids-non-fat and fat contents of goat milk on Instron penetration force and viscosity of yoghurt. It can be seen that by adding increasing amounts of concentrated skim milk, the penetration force increased while viscosity remained practically unchanged. Increase of penetration force and of viscosity by increasing solids-non-fat is known for yoghurt of
41 25
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cow m i l k ( S c h m i d t et al., 1980). W h e n f a t c o n t e n t was increased, p e n e t r a t i o n force a n d v i s c o s i t y r e m a i n e d t h e s a m e . T h e s t r o n g r e l a t i o n s h i p t h a t was ob-
42 30
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4'o
[ 0
s~
m g Ca / 1 0 0 m l
,.~
,~o
Na- caseinat
30
30
20
20
e
I.~ (,I.J
v~
o
10
0
0.5
1.0
C a - c a s e i n a t e (°1o)
1.5
~:
0.5
Whole
Jnu
1.0
;1-
I.S
m i l k protein~o/,)
Fig. 3. Influence of various additives on viscosity. Circles: 3 rpm; triangles: 6 rpm; squares: 12 rpm; crosses: 30 rpm.
served between fat and penetration force and between fat and viscosity (Table 3) might be due, as in the case of Ca, to existing secondary relationships between fat and casein.
43 300
300
.°200
200
Q 1,4
0
o I00
ioo i,-,i
50
,'o
8
Solids-
,'~
~'~
,;~
non-fat
ij8
50 0~
2
4 Fat
(°/o)
30
30
20
20
6
8
I0,
(°lo)
A m
v
I0
10 ¸
0 0 W .1=1
i,i
)#
0
,'o Solids
~
n
,2 - non
,', - fat
i(
,'+ ('1.)
1+
o
n
..
n
",
+ Fat
,~
I
('1#
Fig. 4. Influence of solids-non-fat and fat on Instron penetration force and viscosity (circles:3 rpm; triangles:6 rpm; squares: 12 rpm; crosses: 30 rpm). CONCLUSIONS
This study showed that consistency of yoghurt from milk of indigenous goat breeds was better compared to Saanen and cross breeds. Although, consumers
44 are n o t a c c u s t o m e d to y o g h u r t f r o m goat milk, y o g h u r t f r o m m i l k of i n d i g e n o u s goat b r e e d s was c o m p a r a b l e to c o m m e r c i a l b r a n d s . I n case t h a t f u r t h e r f i r m n e s s n e e d s to be i m p r o v e d , t h i s c a n b e s t be d o n e b y i n c r e a s i n g s o l i d s - n o n - f a t . A d d i n g c o m m e r c i a l cow m i l k p r o t e i n p r o d u c t s is effective, b u t affects t h e genuiness of t h e p r o d u c t . C o r r e l a t i o n s b e t w e e n m i l k c o m p o n e n t s a n d p h y s i c a l p r o p e r t i e s of y o g h u r t m i g h t be also a f f e c t e d b y s e c o n d a r y r e l a t i o n s h i p s w i t h milk components. ACKNOWLEDGEMENTS T h i s r e s e a r c h was s u p p o r t e d b y t h e M i n i s t r y of E d u c a t i o n of G r e e c e a n d E E C . T h e a u t h o r s w i s h to t h a n k M i s s P. K o n i d a r i a n d Mrs. E. X i r o g i a n n i for t h e i r t e c h n i c a l a s s i s t a n c e a n d M r s . H. K a r a f e r i for t y p i n g t h e m a n u s c r i p t .
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45 Storry, J.E., Grandison, A.S., Millard, D., Owen, A.J. and Ford, G.D., 1983. Chemical composition and coagulating properties of renneted milks from different breeds and species of ruminant. J. Dairy Res., 50: 215-229. Yun, S., Ohmiya, K. and Shimizu, S., 1982. Role of casein in milk curding. Agric. Biol. Chem., 46: 443-449. Zittle, C.A. and Custer, J.H., 1966. Identification of the casein among the components of whole goat casein. J. Dairy Sci., 49: 788-791.