Research of the Gelling Properties of Milk Powder with Rapid Viscosity Analyzer and Textural Analyzer

Wu Weidu*, Zhu Hui, Wang Yaqiong, Li Yanjun Research Institute of Hangzhou Wahaha Group Company Limited,No.14 Street, Xiasha

Economic and Technology Development Zone, Hangzhou 310018, Zhejiang, China

E-mail: wwd@wahaha.com.cn

Abstract: In this paper, the viscosity change of milk powder during the period of heat-holding-cooling is studied with rapid viscosity analyzer and through viscosity curve we could know whether if milk powder has experienced low or high heat treatment during processing of milk powder. In addition, the evaluation method for gelling properties of milk powder with textural analyzer is developed as well, where each parameter relating to the gelatinization is characterized in a detailed way. The experiments show that in characterization of gel properties of milk powder rapid viscosity analyzer and textural analyzer have a higher correlation, and there also exists high degree of correlation between parameters obtained in the gelatinization test with texture analyzer. Keywords: Rapid viscosity analyzer; Textural analyzer; Milk powder; Gelling properties 1. Introduction

The milk protein has the ability to form a gel that can hold water and fat and helps to the formation of organizational structure and improvement of taste (Zhang, 1997; Guo et al., 2003). Further, both casein and albumin could form a gel network by acidification or rennin. Therefore, phenomenon of gelation in favor of the structure and texture of food is formed resulting from heat-induced gelling of whey protein (Chen et al., 2006; Liu et al., 2007; Carunchia et al., 2005; De J N. Wit., 1990). RVA gelling properties of milk powder reflects the viscosity change according to the heating-holding-cooling profile, showing the denaturation degree of whey protein in the process of milk powder classified into low heat, medium heat and high heat. Meanwhile, it is often necessary of assessing the milk powder in the form of reconstituted milk to obtain some potential characteristics for gel strength. It has been reported that milk powder is reconstituted into milk with concentration of up to 50% (w / w), heated to a higher temperature like 80 as stirring, cooled in a refrigerator for 10 to 12℃ hours for later analysis of gel strength with specific analytical instrument. In this study, the method for evaluation of gelling properties of milk powder is established with textural analyzer. Based on this method and method with rapid viscosity analyzer, gelling properties of nearly 30 batches of milk powder are evaluated and correlation analysis is made between two methods as well.

2. Materials and methods

2.1 Materials Milk powders including skimmed milk powder and whole milk powder have been supplied by several manufactures in China and New Zealand. The moisture content of milk powder has been measured from oven drying tests before use. BTP (Bis-tris-propane) 6.5 Buffer has been prepared by making BTP (Sigma) 10 mmol concentration adjust to pH 6.5 using NaOH/lactic acid. 2.2 Methods 2.2.1 Gelling properties with rapid viscosity analyzer

A rapid viscosity analyzer (Model is RVA-IV, Newport Corporation, Australia) is used to observe the gelling properties. Weigh 15.56 g ± 0.01g sample (dry weight basis) and X g BTP pH 6.5 Buffer respectively to make total weight of 35.00 g. Cover the canister with rubber stopper and shake vigorously to disperse. According to the profile in Table 1 the curve for gelling properties has been obtained during the process of heating-holding-cooling, where the peak viscosity and final viscosity are recorded respectively.

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Table 1 Profile for assessing of gelling properties with rapid viscosity analyzer

Time Type Value

00:00:00 Temp 50℃

00:00:00 Speed 1000rpm

00:01:00 Speed 160rpm

00:01:00 Temp 50℃

00:04:30 Temp 90℃

00:09:30 Temp 90℃

00:15:00 Temp 30℃

Note: Idle temperature of 50±1 . Test ℃ time is 20min. Time between readings is 4s.

2.2.2 Gelling properties with textural analyzer After gelation with rapid viscosity analyzer the gel is sealed and refrigerated overnight for further test with

textural analyzer (Model is TA.XT plus and test probe is P/0.5R, SMS Co. Ltd., UK). The test parameters for evaluation of gelling properties are set as follows. Pre-test speed, test speed and post-test speed is 1mm/s respectively. The compression distance is set 2cm. The trigger force is 5g and the data acquisition rate is 400poins per second.

3. Results and discussion 3.1 Gelling properties of milk powder with rapid viscosity analyzer

The tests for gelling properties are conducted on nearly 30 batches of milk powder during the period of heating-holding-cooling and the results are obtained as Table 2.

Table 2 Gelling properties of milk powder with rapid viscosity analyzer and textural analyzer

Batch Gelling properties with rapid

viscosity analyzer Gelling properties with textural analyzer

Peak-1

cP

Peak-2

cP

Final viscosity

cP

A2-3

g.s

A3-4

g.s

Gelling strength

at 4mm

g

Area by 4mm

g.s

Slope

g/s

Sample-1 / / 470 1191.3 -611.1 44.5 109.1 10.68

Sample-2 / / 477 872.6 -494.1 31.4 24.7 5.92

Sample-3 / / 45 202.3 -85.5 8.3 25 0.785

Sample-4 / / 132 159.5 -54.2 4.1 18.1 0.379

Sample-5 / / 339 1121.9 -617.5 37.3 88.5 8.71

Sample-6 / / 379 1195.5 -678.7 37.2 86.9 9.14

Sample-7 396 / 1056 2092.5 -1022 78.9 199.7 26.74

Sample-8 / / 586 343.5 -147.9 12.1 29.9 1.354

Sample-9 714 / 1467 2688 -1171.1 106.8 251.6 29.31

Sample-10 987 638 1233 2884.5 -1182.5 123.9 262.5 29.7

Sample-11 848 648 1296 2295.3 -957.7 89.8 201.2 23.8

Sample-12 647 590 1129 1998 -793.2 84.2 197.4 24.5

Sample-13 629 / 1112 1491.3 -611.8 53.3 127.9 13.6

Sample-14 747 993 1899 4173.2 -1675.3 172.7 406.7 52.6

Sample-15 1150 / 2098 6432.7 -2226.6 265.3 608.7 81.1

Sample-16 879 / 1378 3344.9 -1307.3 134.5 320.2 39.17

Sample-17 211 / 907 1668.1 -764.6 64.7 159.9 17.33

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Sample-18 191 / 876 1873.3 -797.2 76.5 186.6 21.4

Sample-19 1140 / 1262 3979.4 -1648.6 160.9 388.9 48.8

Sample-20 814 657 1471 4400.1 -1703.6 184.3 451.9 58.7

Sample-21 1900 1111 1829 5553.5 -2128 223 465.3 57.5

Sample-22 1883 1147 1899 5850.4 -2422.1 231 469.5 58.9

Sample-23 727 804 1656 3352.1 -1487.6 137.4 354.2 47

Sample-24 721 863 1561 3681.9 -1445.9 157.6 386 50.4

Sample-25 1003 / 1712 5716.8 -2347.9 236.2 629.3 90.1

Sample-26 921 / 1582 5139 -2325 214.3 556 77.5

Sample-27 1593 1313 1706 5217.6 -1820.5 195.9 456.3 58.27

Sample-28 433 / 1219 2343 -1094.6 96.8 245 30.9

Seen from Table 2, the milk powders show significant differences in the characteristics of the gelling

properties with rapid viscosity analyzer, some samples with two peaks, some with only one peak and some with no peak at all. In addition, on the final viscosity for the samples there also existed significant differences. 3.2 Gelling properties of milk powder with textural analyzer

After gelation with rapid viscosity analyzer the gel is sealed and refrigerated overnight for further test with textural analyzer. The gelling characteristics of milk powder are shown in Table 2. The typical curve for gelling properties of milk powder with textural analyzer is shown in Figure 1.

Once the compression force reaches 5g of the trigger force, the probe proceeds down 2cm, then rises up again. The change of force as time or distance is recorded during the period of compression and rising of the probe. By data process from the curve, we obtain the characteristics like A2-3, A2-5, A3-4, F4mm, G1-2, which could be used to characterize the gelling properties of milk powder respectively as follows. A2-3: The compression work done by probe downward 2cm, which could characterize the toughness or firmness of the gel. A3-4: After testing the work done by probe while completely out of the surface, which could be used as characterization of stickiness or consistency of the gel. A2-5: The compression work done by probe downward 4mm, which could also characterize the toughness or firmness of the gel. F4mm: The compression force while the probe moves downward 4mm, characterizing the gel strength of the gel. G1-2: Making a slope at the initial stage of the curve close to a straight line, characterizing the stiffness or deformability of the gel. Seen from Table 2, each milk powder shows distinctly different gelling properties with the textural analyzer. In Figure 2, three batches out of 28 are selected for comparison, representing high, medium and low gel strength from top to bottom respectively. Further, if the milk powder has been treated high-heat, there is no peak occurring on the curve with rapid RVA and the corresponding weak gel strength lower than 50g or less with textural analyzer. If the first peak appears on the RVA viscosity curve even with low value of peak viscosity, the gel strength can also be significantly increased. Gel strength is mainly affected by the first peak on the RVA viscosity curve, while if there is a second peak occurring on the curve, the gel strength can be significantly enhanced synergism.

3.3 Correlation analysis of gelling properties with RVA and textural analyzer

With two methods by RVA and textural analyzer, the gelling properties of milk powder are characterized. In order to associate the two methods, the correlation analysis is made on the final viscosity with firmness, stickiness, gel-strength and stiffness, as shown in Table 3.

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Figure 1 A typical curve for evaluation of gelling properties of milk powder with textural analyzer

Figure 2 Curves for evaluation of gelling properties of obviously differentiated three batches of milk

powder, representing high, medium and low gel strength from top to bottom respectively.

Table 3 Correlation analysis of gelling properties with RVA and textural analyzer

A2-3 A3-4 F4mm A2-5 G1-2

Final viscosity 0.915362 -0.89974 0.918751 0.902525 0.882596

A2-3 / 0.968115 0.997546 0.981104 0.968115

A3-4 / / -0.98404 -0.97486 -0.96698

F4mm / / / 0.987325 0.975629

A2-5 / / / / 0.995742

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From the literatures, when the sample number is 28, if the correlation coefficient is more than 0.37389 on the level of significance (α) at 0.05, then both variables show correlation, while on the level α = 0.01 if the correlation coefficient is greater than 0.47851, then both show a strong correlation. There exist strong correlations between the final viscosity with RVA and gelling properties obtained by textural analyzer like firmness, stickiness, gel strength and stiffness with the correlation coefficient of 0.88 or more. Higher final viscosity, higher firmness, stickiness, gel strength and stiffness accordingly. In addition, the correlation coefficients between each parameter obtained by textural analyzer reaches 0.96 or above, showing high degree of correlation. While, the results show that there is no obvious correlation between the peak viscosity from the RVA curve and gelling properties obtained by textural analyzer. For different milk powder, the gelling properties and texture of whey proteins are not only subjected to internal factors such as the protein concentration, but also to external factors such as heating temperature, pH, ionic strength and other present food ingredients like fat, sugar, starch and the like. 3. Conclusions

With rapid viscosity analyzer and textural analyzer the history of the heat treatment of milk powder can be evaluated, knowing of the heat treatment intensity in the early process and protein denaturation of milk powder, mastering gelation properties and gel strength characteristics for constructive specific guidance on the end use. 4. References [1]Zhang, J.-Ch., Thermal denaturation of whey protein and its application on yogurt manufacture[J]. Food Science,1997,18(2):14-16. 3. [2]Guo, B.-H., Modern processing technology series of dairy milk-milk powder[M]. Chemical Industry Press,200 [3]Chen, W.-L., Sun, K.-J., He, J., Study on heat stability of whey protein. Journal of Dairy Science and Technology[J]. 2006,(6),276-278. [4]Liu, J., Han, Q.-B., Characteristics and application of whey protein[J]. Food Science,2007, 28(7):535-537. [5]Carunchia Whetstine M E, Croissant A E, Drake M A, Characterization of dried whey protein concentrate and isolate flavor[J]. J Dairy Sci.,2005,88:3826-3839. [6]De J N. Wit, Thermal stability and functionality of whey protein[J]. J Dairy Sci.,1990,73:3602-3619.

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