CC BY
12
Jacek Domagala, Maria Czernicka, Ilona Wieteska, Dorota Najgebauer-Lejko ©
Department of Animal Product Technology, Food Technology Faculty, University of Agriculture in Krakow, Poland
FLOW PROPERTIES OF YOGHURT FROM GOAT MILK MODIFIED BY
TRANSGLUTAMINASE DEPENDING ON pH OF THE MILK
Abstract
Set yoghurt from goat milk with adjusted pH to 6,4, 6,3, 6,2 and 6,1 and than modified by microbial transglutaminase (TGase) were produced. Control yoghurt was produced from goat milk with pH 6,4 but without modification by TGase. In yoghurt determined sensory quality, pH and rheological properties (aparent viscosity and flow curves) Obtained flow curves were described by three rheological models: Ostwald de Waele, Herschel-Bulkley and Casson. Modification of goat's milk by TGase caused an increase in apparent viscosity of yoghurt. Among rheological parameters consistency coefficient K and yield stress increased, but flow index generally decreased in comparison with the value of these parameters in yoghurt without modification by TGase. The pH of the milk incubated with TGase in the pH range 6.4-6.1 has no influence on apparent viscosity and values of rheological parameters calculated from rheological models. During cold storage pH of yoghurts decreased significantly, however the value of yield stress increased in comparison with values of these parameters in fresh yoghurt.
Key words: goat milk yoghurt, pH, rheological properties, transglutaminase
Introduction
Transglutaminase (TGase) [EC 2.3.2.13] catalyses the formation of covalent cross-links between glutamine and lysine residues in many food proteins e.g. in casein. Crosslinking of food proteins by TGase modifies the hydration ability, the gelation and the rheological properties of food products as well as yoghurt. Optimum pH of microbial TGase is in the range of 6-7, however the enzyme is stabile at pH 5 - 9. Formation of cross-links in casein depends on pH of incubation with the enzyme and at pH 6.5 is negligible because of the electrostatic repulsion of the micelles [3,5,15]. Rheological properties of yoghurt and other fermented dairy products like apparent viscosity and flow properties play very important role in sensory evaluation and in consumer acceptance of these products. Too low viscosity or too liquid consistency are the most important defects of these dairy products [1]. According to Lauber et al. [10] and Lorenzen et al. [11] cross-linking of casein in milk by microbial TGase increase apparent viscosity of cow milk yoghurt gel. Faergemand et al. [7] have found, that addition of TGase to cow milk enable the production of yoghurt with proper rheological properties without addition of milk powder. TGase can be also used to improve the
© Jacek Domagala, Maria Czernicka, Ilona Wieteska, Dorota Najgebauer-Lejko, 2008
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rheological properties of goat milk yoghurt, which is characterised by lower viscosity than cow milk yoghurt.
In the previous paper the changes in texture of yoghurt from goat milk modified by TGase depending on pH of the milk were described [6]. The aim of presented work was to determine the influence of pH of goat milk incubated with TGase on flow properties of yoghurt.
Materials and methods
Raw milk was obtained from Polish White Improved breed of goats during the middle of lactation. Milk for yoghurt production was pasteurised at 85°C for 15 min., cooled down to 40°C and pH of the milk was adjusted using 10% solution of lactic acid to 6,4, 6,3, 6,2 and 6,1. Milk with regulated pH was incubated with microbial transglutaminase from Ajinomoto, Japan, given in the amount of 2 u (unit) per 1g protein for 2 hours at 40°C. After incubation the enzyme was inactivated at 80°C for 1 minute. Next the milk was cooled down to 44°C and inoculated with starter culture YC-180 from Chr. Hansen, Denmark. The starter was added at 5 u level to 25 litter of milk. The yoghurt was incubated at 44°C, until the pH 4.8 was reached. Then the products were cooled to 5-8°C, stored at this temperature for 14 days. Control yoghurt was produced from goat milk with pH 6,4 but without modification by TGase. Total solids, total protein, casein, whey protein, non protein nitrogen, fat, lactose, ash content as well as density, viscosity, titratable acidity and pH were determined in goat milk used for yoghurt production [2,14,18]. Yoghurt was analysed after 14 h (fresh yoghurt) and 14 days (stored yoghurt) of cold storage. For yoghurt the following analysis was done: sensory evaluation in 5-point scale performed by trained panel of 5 judges of verified sensory acuity, pH, apparent viscosity and flow curves using rotary viscometer Rheotest RV2 (VEB MLW Medingen. Germany). Three rheological models described obtained flow curves: Ostwald de Waele, Herschel-Bulkley and Casson.
Results and discussion
The chemical composition and main physicochemical properties of raw goat milk for yoghurt preparation are presented in Table 1. Total solids and total protein content in processed milk was lower than mean content of this component in goat milk given in literature. According to Pelczynska [12] the total solids content in goat milk ranged from 11,50 to 13,63% and total protein content from 2,9 to 3,76%. Also Kudelka [9] have found, that the mean total solids in goat milk is 12.06% and total protein content 3,2%.
The mean content of main components in analysed goat milk was also generally lower than the mean content of these components in goat's milk given by Ziarno and Truszkowska [17].
The results of sensory evaluation, pH and apparent viscosity of fresh and cold stored yoghurts from goat milk with adjusted pH, modified by transglutaminase and of control yoghurt are presented in Table 2. Results of statistical analysis concerning effects of pH of goat milk incubated with transglutaminase and time of storage on
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sensory properties, pH and apparent viscosity of produced yoghurts are presented in Table 3.
Table 1.
Chemical composition and physicochemical properties of raw goat milk for
Parameter Mean value
+ standard error
Total solids [%] 10,74 + 0,24
Total protein [%] 2,78 + 0,07
Casein [%] 2,28 + 0,07
Whey protein [%] 0,31 + 0,02
Non protein nitrogen [%] 0,29 + 0,02
Fat [%] 3,2 + 0,06
Lactose [%] 4,72 + 0,06
Ash [%] 0,82 + 0,02
Density [g/cm3] 1,0300 + 0,000
Viscosity [mPa-s] 1,87 + 0,02
Acidity [°SH] 6,5 + 0,06
pH 6,61 + 0,02
Table 2.
Quality properties of fresh and stored yoghurts from goat milk with adjusted pH and modified by transglutaminase in comparison with control yoghurt
Control yoghurt Yoghurts from milk
Parameter pH 6,4 modified by transglutaminase
pH 6,4 pH 6,3 pH 6,2 pH 6,1
Fresh yoghurts
Sensory 3,83 3,82 3,97 3,76 3,82
evaluation +0,06 +0,21 +0,02 +0,20 +0,09
[points]
pH 4,32 4,42 4,40 4,46 4,41
+0,03 +0,03 +0,05 +0,05 +0,05
Apparent 0,30a,b,C,D 0,53a 0,58B 0,51C 0,50d
viscosity [Pa-s] +0,06 +0,03 +0,03 +0,05 +0,04
Stored yoghurts
Sensory 3,58 3,99 3,96 3,27 3,89
evaluation +0,16 +0,08 +0,09 +0,53 +0,03
[points]
pH 4,25 4,30 4,21 4,22 4,22
+0,03 +0,02 +0,03 +0,02 +0,01
Apparent 0,28aaqd 0,52a 0,53B 0,49C 0,49d
viscosity [Pa-s] +0,06 +0,03 +0,03 +0,05 +0,04
A-D - statistically highly significant difference (p < 0,01) between means marked with the same letters in the rows
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Modification of goat milk by microbial transglutaminase caused highly significant increase in apparent viscosity, of fresh and cold storage yoghurts. According to Faaergemand et al. [7], Lauber et al. [10] and Lorenzen et al. [11] modification of cow milk by microbial TGase increase the apparent viscosity of yoghurts. The values of apparent viscosity for yoghurts made from goat milk incubated with TGase at pH in the range 6,4-6,1 were similar. Statistical analysis showed, that pH of milk incubated with TGase in the range of 6,4-6,1 has no influence on sensory quality, and pH of yoghurts Time of storage significantly influenced only pH of analysed yoghurt. The pH of stored yoghurts was lower than of fresh yoghurts.
Table 3.
Mean squares of deviation from variance analysis concerning effects of pH of
goat milk incubated with transglutaminase and time of storage on sensory _quality, pH and apparent viscosity of yoghurts_
SOURCE OF 1. pH of milk 2. Time of Interaction 1x2 Error
VARIATION storage
Degrees of 3 1 3 16
freedom
Sensory 0,2422 0,0267 0,1304 0,1455
quality
pH 0,0016 0,2501** 0,0012 0,0039
Apparent 4804,27 2799,79 359,63 2809,97
viscosity
** - statistically highly significant (p<0,01) effect of study factor
Flow curves of produced fresh yoghurts and yoghurts after 14 days of cold storage are presented on figure 1.
All curves have a shape of hysteresis loop. In comparison to control yoghurt, flow curves for yoghurt from goat milk modified by transglutaminase in adjusted pH have a higher slant angle, and higher values of shear stress for the same value of shear rate. It means, that these yoghurts were more resistant to action of shear forces. After storage the hysteresis loop area of flow curves was lower than of flow curves of fresh yoghurts. According to Schramm [16] hysteresis loop area is a measure of energy, which is needed for yoghurts gel destruction.
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Fresh yoghurts
Yoghurts after 14-days cold storage
25 T-
200 250 300
Shear rate [1/s]
Figure 1. Flow curves of yoghurts from goat milk with modified pH incubated with transglutaminase in comparison to flow curves of control yoghurt
20
15
10
5
0
In Table 4 are presented values of rheological parameters of produced yoghurts, calculated from models used for describing of their flow curves. Generally, modification of goat milk by TGase caused an increase in consistency coefficient K, yield stress t0 and in deviation from Newtonian flow (decrease in flow index n), and decrease in Cassons viscosity. After storage the values of consistency coefficient K calculated form Ostwald model were higher, and flow index lower than in fresh yoghurts. The values of these parameters calculated from Herschel-Bulkley model showed opposite changes. The yield stress of stored yoghurts were higher and
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Cassons viscosity lower than these of fresh yoghurts. Obtained data have a good fit to rheological models of Ostwald de Waele and Herschel-Bulkley and a little worse to Cassons model. Obtained values of consistency coefficient K, flow index n and Cassons viscosity were generally higher, and yield stress lower, than the values of these parameters obtained by Fortuna et al. [8] for market yoghurts from cow milk.
Table 4.
Values of rheological parameters of fresh and stored yoghurt from goat's milk
with regulated pH, modified by transglutaminase, and control yoghurt, calculated from models used for describing the flow curves of study products
Type of Model
yoghurt of Ostwald de Waele of Herschel-Bulkley of Casson
K n R2 T0 K n R2 T0 n R2
[Pas11] H [Pal [Pas11] [-l [Pal [mPas]
Fresh yoghurts
Control
pH 6,4 0,98 0,48 0.9943 0,00 1,08 0,46 0,9959 0,91 34,96 0,8910
With TG
pH 6,4 2,02 0,39 0,9974 0,31 1,76 0,42 0,9966 2,12 34,90 0,9168
With TG
pH 6,3 1,19 0,40 0,9965 0,06 2,13 0,40 0,9963 2,30 38,50 0,9065
With TG
pH 6,2 1,80 0,42 0,9966 0,00 1,91 0,41 0,9972 1,85 37,90 0,9048
With TG
pH 6,1 1,90 0,40 0,9980 0,44 1,55 0,43 0,9986 1,99 34,20 0,9421
Yoghurts after 14 days cold storage
Control
pH 6,4 1,07 0,43 0,9995 0,00 1,09 0,43 0,9994 1,05 27,00 0,9307
With TG
pH 6,4 2,53 0,34 0,9965 1,16 1,58 0,42 0,9980 2,77 26,60 0,9493
With TG
pH 6,3 2,28 0,37 0,9983 0,86 1,58 0,43 0,9992 2,44 31,90 0,9475
With TG
pH 6,2 2,18 0,36 0,9953 1,31 1,13 0,47 0,9975 2,35 28,20 0,9544
With TG
pH 6,1 2,02 0,37 0,9941 1,16 1,10 0,47 0,9992 2,16 28,20 0,9642
Conclusions
1. Modification of goat milk by TGase caused an increase in apparent viscosity of fresh and storage yoghurts.
2. Among rheological parameters consistency coefficient K and yield stress increased, but flow index generally decreased in comparison with the value of these parameters in yoghurt without modification by TGase.
3. The pH of the milk incubated with TGase in the pH range 6.4-6.1 has no influence on apparent viscosity and values of rheological parameters
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calculated from rheological models.
4. During cold storage pH decreased and the value of yield stress increased in comparison with values of this parameter in fresh yoghurt.
References
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7. Faergemand M., Sorensen M.V., Jorgensen U., Budolfsen G., Qvist K.B. 1999. Transglutaminase; effect on instrumental and sensory texture of set style yoghurt. Milchwissenschaft 54, 10, 563-566.
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Summary
Set yoghurt from goat milk with adjusted pH to 6,4, 6,3, 6,2 and 6,1 and than modified by microbial transglutaminase (TGase) were produced. Control yoghurt was produced from goat milk with pH 6,4 but without modification by TGase. In yoghurt determined sensory quality, pH and rheological properties (aparent viscosity and flow curves) Obtained flow curves were described by three rheological models: Ostwald de Waele, Herschel-Bulkley and Casson. Modification of goat's milk by TGase caused an increase in apparent viscosity of yoghurt. Among rheological parameters consistency coefficient K and yield stress increased, but flow index generally decreased in comparison with the value of these parameters in yoghurt without modification by TGase. The pH of the milk incubated with TGase in the pH range 6.4-6.1 has no influence on apparent viscosity and values of rheological parameters calculated from rheological models. During cold storage pH of yoghurts decreased significantly, however the value of yield stress increased in comparison with values of these parameters in fresh yoghurt.
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