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Characterisation of improved foam aeration and rheological properties of ultrasonically treated whey protein suspension
International Dairy Journal 43 (2015) 7e14
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Characterisation of improved foam aeration and rheological properties of ultrasonically treated whey protein suspension M.C. Tan a, N.L. Chin a, *, Y.A. Yusof a, F.S. Taip a, J. Abdullah b a b
Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Plant Assessment Technology Group, Malaysian Nuclear Agency (Nuclear Malaysia), Bangi, 43000 Kajang, Selangor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2014 Received in revised form 18 September 2014 Accepted 21 September 2014 Available online 18 October 2014
Suspensions of 10, 15 and 20% (w/v) whey protein concentrate (WPC) were treated with 20 kHz ultrasound for 5, 15 and 25 min at an amplitude of 20, 40 or 60%. The treated suspensions were whipped into foam and the aeration and rheological properties were investigated. With increasing ultrasound amplitude and treatment time, whey protein foam at 15% concentration produced the highest foaming capacity, while foam stability, storage modulus, loss modulus, consistency index and viscosity of foam increased with protein concentration. Foam viscosity correlated with foam stability with R2 ¼ 0.7425 and significant at P < 0.001. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction Dairy proteins, in particular whey proteins, are widely applied in food industry due to their high nutritional values and versatile functional properties in food products. They are good for water binding, foaming, emulsifying, and gelling purposes (Jyotsna, Sai Manohar, Indrani, & Venkateswara Rao, 2007), making it an important foaming agent or ingredient. Whey protein is present as an ingredient in many foods, including pasta, flavoured milk, dessert pudding, muesli, savoury dip and ice cream (McIntosh et al., 1998). Whey proteins used as foaming agents in aerated foods, such as meringue, angel food cake or nougat, are considered to coexist in complexes with other ingredients and need to be stable when undergoing mixing, heating and drying processes (Foegeding, Luck, & Davis, 2006). Foaming properties of proteins are usually evaluated by aeration characterisation, i.e., foaming capacity and foam stability (Nicorescu et al., 2011; Raikos, Campbell, & Euston, 2007). Even though air is incorporated during foaming, aeration is only achieved by a stable network capable of retaining the air bubbles in the foam system. The network stability of an aerated system is primarily affected by its rheological properties (Campbell & Mougeot, 1999). Kinsella (1981) reported that high surface viscosity and high film yield were correlated with strong foams because it reflects strong cohesion between the film forming molecules. Films with
* Corresponding author. Tel.: þ60 3 89466353. E-mail address: [email protected] (N.L. Chin).
high viscosity and possessing viscoelasticity contribute to foam stability as it allows foam films to respond to stresses by compression and expansion (Kinsella, 1981). Whey protein is gaining popularity as a healthier source of protein in the food industry (Garcia-Gariabay, Jimenez-Guzman, & Hernandez-Sanchez, 2008) following demands of consumers with high health concerns. Due to the lower quality of whey protein for foaming functions (Yang & Foegeding, 2010), many trials have been undertaken in attempts to improve its functionality including using enzymatic hydrolysis treatment (Kuehler & Stine, 1974; Panyam & Kilara, 1996), dynamic pressure (Bouaouina, Desrumaux, Loisel, & Legrand, 2006), pH and heat treatment (Philips, Schulman, & Kinsella, 1990), and ultrasound treatment (Jambrak, Mason, Lelas, Herceg, & Herceg, 2008; Kresic, Lelas, Jambrak, Herceg, & Brn ci c, 2008). Ultrasound has been used in many studies of proteins, such as the effect of ultrasound on the resistance of protein-stabilised emulsions to environmental stresses (Pongsawatmanit, Harnsilawat, & McClements, 2006), mechanical strength changes of milk proteinbased edible films (Banerjee, Chen, & Wu, 1996), physical properties of soy proteins (Jambrak, Lelas, Mason, Kresi c, & Badanjak, 2009), and kinetic characterisation of hydrolysis of defatted wheat germ protein (Jia et al., 2010). Ultrasonication increased the surface activity of proteins and the rate of protein adsorption by lowering the surface tension of protein solutions (Gülseren, Güzey, Bruce, & ~ iga & Weiss, 2007; Güzey, Gülseren, Bruce, & Weiss, 2006; Zún Agulera, 2008). The application of high-intensity ultrasound changes proteins and makes it advantageous in bubble formation ~ iga & Agulera, 2008). Ultrasound is able to induce structural and (Zún
http://dx.doi.org/10.1016/j.idairyj.2014.09.013 0958-6946/© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
functional changes in proteins to bring potential applications in controlling the alterations of lipid and gas interfaces in emulsions, foams and gels in food products (Gülseren et al., 2007). The objective of this study was to characterise the improved aeration and rheological properties of whey protein foams made from ultrasound treated whey protein suspensions. 2. Material and methods 2.1. Preparation of protein suspension Whey protein concentrate (Textrion; PROGEL 800, DMV International, BA Veghel, Netherlands) used was in the form of protein powder. The composition of the protein powder (by weight) is 80% protein, 6.5% lactose, 6.3% fat, 4.4% ash, and 4.9% moisture. Aqueous suspensions of WPC at concentrations of 10, 15 or 20% (w/v) were prepared by dispersing 25.0, 37.5 or 50.0 g of dry matter with distilled water at 25 C into a total of 250 g in a 500 mL beaker and stirred using a mechanical stirrer (RW20 DZM.n S2, IKA Works (Asia) Sendirian Berhad, Malaysia) at 355 rpm for 20 min to increase protein hydration activity in water prior to further homogenisation using ultrasound treatment.
Aeration properties of foam were measured in terms of foaming capacity and foam stability. Foaming capacity was expressed in terms of percentage of overrun and calculated using Eq. (2) (Patel & Kilara, 1990). Foaming capacity was determined based on the mass of 100 mL of protein suspension using a 100 mL cylinder, and the mass of foam filled and levelled in a 100 mL plastic weighing boat. The average foaming capacity was taken from three samples from the same batch of whipped foam.
Overrun
foam
¼
Mass
suspension
Mass
Massfoam
foam
100%
(2)
Foam stability was determined from the foam drainage at room temperature of 25 C. A small plug of glass wool was placed at the top of the funnel stem of a filter funnel with dimension 7.5 cm inner top diameter, 0.4 cm inner stem diameter and 7.0 cm stem length, to retain the foam but allow drainage of the liquid. 100 mL of fresh whipped foam was then transferred into the filter funnel (Jambrak et al., 2009). The drainage (mL) of the liquid was recorded from the measuring cylinder after 20 min. The average drainage was taken from three samples from the same batch of whipped foam. 2.4. Measurement of rheological properties of foam
2.2. Ultrasound treatment of protein suspensions for foam preparation Protein suspensions in a 500 mL beaker were sonicated at 20, 40 or 60% amplitude for 5, 15 or 25 min using a 20 kHz, 400 W high intensity ultrasound probe (Digital Sonifier Model 450, Branson Ultrasonics Corporation, Danbury, CT, US). The probe had a 2.54 cm vibrating titanium tip and it was immersed to half level at the centre of the suspension medium. The control samples were those without ultrasonic treatment, zero min treatment time. The actual power inputs as illustrated by the ultrasonic device during each setting were recorded. The ultrasound energy densities of treated protein suspension at every concentration at a total volume of 250 mL for 5, 15 and 25 min at 20, 40 and 60% amplitude were calculated following Eq. (1) (Giordano, 2013). All the power inputs and energy densities are reported in Table 1. The temperature range increase recorded was from 43 to 68 C from all ultrasound treatment of protein suspension from 20 to 60% amplitude for 5e25 min was tested as independent factor of the experiment and found to be a non-significant factor (Tan, 2014). The treated and untreated whey protein suspensions was then whipped with a mixer (5K5SSS, Kitchen Aid Inc., St. Joseph, MI, US) for 15 min at 330 rpm at room temperature of 25 C.
PowerðWÞ TimeðsÞ Energy density W$sL1 @ JL1 ¼ VolumeðLÞ
2.3. Measurement of foam aeration properties
(1)
Table 1 Power input and energy density at each sonication time and ultrasound amplitude. Sonication time (min)
Ultrasound amplitude (%)
Power input (W)
Energy density (kJ L1)
5 5 5 15 15 15 25 25 25
20 40 60 20 40 60 20 40 60
22.5 50.5 78.0 22.5 50.5 78.0 22.5 50.5 78.0
27.0 60.6 93.6 81.0 181.8 280.8 135.0 303.0 468.0
Rheological properties of foam were measured as storage modulus, G0 , loss modulus G00 , consistency index, K, flow behaviour index, n, and viscosity, m. Foam rheological measurements were performed using a rheometer (AR-G2, TA Instruments, New Castle, DE, US) with a 40 mm diameter serrated plateeplate geometry set at a constant gap height of 2.4 mm at 25 C (Mleko, Kristinsson, Liang, & Gustaw, 2007). A sample from each batch of freshly prepared foam mix was used. Frequency sweep tests were performed on foams from 0.1 to 10 Hz within a similar linear viscoelastic region of 1% strain when analysing the texture of foams for its viscoelastic shear moduli, G0 (Pa) and G00 (Pa). Properties of K, n, m were obtained from another test where foams were sheared by linearly increasing rates of 0e100 s1 within 5 min to generate shear stress versus shear rate data. Data of shear stress, s (Pa) versus shear rate, g_ (s1) were fitted to a rheological model of Power law (Eq. (3)) to obtain rheological characteristics of consistency index, K (Pa sn) and flow behaviour index, n of the foam. K and n were obtained through curve fitting utilizing Generalized Reduced Gradient (GRG2) nonlinear optimisation using the solver function of Microsoft Excel. The best fitted line with minimum sum of square errors was used as the sole criteria during curve fitting to obtain the high yield of coefficient of determination, R2. Viscosity, m (Pa s) was determined as a function of shear rate at 5.5 s1 (Eq. (4)) because it approximates mixing speed of 330 rpm using found values of K and n (Chin, Chan, Yusof, Chuah, & Talib, 2009a).
s ¼ K g_ n
(3)
m ¼ K g_ n1
(4)
2.5. Experiment design and statistical analysis A 33 full factorial experiment which incorporated 3 factors, each at three levels, i.e., WPC concentration at 10, 15 and 20% (w/v), ultrasound amplitude at 20, 40 and 60%, and sonication time at 5, 15 and 25 min was designed using the Minitab software (Version 16, Minitab Statistical Software, State College, PA, US). Controls are protein suspension without ultrasound treatment at relevant
M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
concentrations. Statistical comparisons of analysis of variance (ANOVA) on the effect of factors, i.e., protein concentration, ultrasound amplitude and sonication time on the responses, i.e., overrun, drainage, storage modulus, loss modulus, consistency index and viscosity were performed using general linear model (GLM). The effect of levels of each factor was determined using Tukey test with 95% of confidence intervals. The ANOVA and Tukey test were performed on three samples for aeration measurements and one sample for rheological measurements for a repeated batch. 3. Results and discussion 3.1. Foaming capacity Fig. 1A shows the foam overrun of protein suspension at various levels of ultrasonic amplitude and sonication time with energy densities between 27 and 468 kJ L1 as reported in Table 1. Protein suspension treated at increasing amplitude and for a longer time had a higher overrun. Although overrun seemed to increase with protein concentration, where it was highest for the 15% WPC, there was a slight decrease in overrun for further increase in concentration at 20%. This is most probably due to the high viscosity in protein suspension, which creates high resistance force against the
▪
9
incorporation of gas bubbles thus restricted the formability of aerated protein suspension during whipping (Bals & Kulozik, 2003) and hence resulting in lower overrun (Glicksman, 1986; Nicorescu et al., 2011; Walstra, 2003). The highest foam overrun of 1348% was achieved when the highest amplitude of 60% and longest time of 25 min was applied on a 15% WPC suspension. Higher overrun implies greater incorporation of air bubbles and better foaming capacity of the protein (Patel & Kilara, 1990). The increased foam overrun in treated suspension is due the homogenising effect of ultrasound (Lomakina & , 2006). This is explained in the Knorr, Zenker, Heinz, and Míkova Lee (2004) study using liquid whole egg where ultrasound aided the dispersion of protein and fat particles in liquid whole egg to give more even distribution and hence improved its foaming capacity through the homogenization effect of ultrasound. According to Wu, Hulbert, and Mount (2000), the ultrasound wave does not directly induce vibration of the globules to cause homogenisation, but it is due to the intense cavitation produced by ultrasound. The acoustic cavitation disrupts the liquid bubbles by causing sudden formation and collapse of bubbles in liquid. The shockwaves given off in the protein suspension generate a powerful force, which suggested breaking up of dry powder agglomerates and better mixing with water molecules in order to produce a homogeneous
Fig. 1. Effect of amplitude (,, control; , 20% amplitude; , 40% amplitude; , 60% amplitude) and sonication time on (panel A) foam overrun and (panel B) foam drainage of ultrasound treated protein suspensions at 10, 15 and 20% (w/v) WPC. Values presented are mean values with standard errors.
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Table 2 P-values from ANOVA to indicate significance of whey protein concentration, sonication time and ultrasound amplitude individually and interactively on various foam properties.a Factor/Response
Overrun (%)
Drainage (mL)
Storage modulus, G0 (Pa)
Loss modulus, G00 (Pa)
Consistency index, K (Pa sn)
Viscosity, m (Pa s)
A: concentration (%) B: time (min) C: amplitude (%) AB AC BC ABC
*** *** *** 0.004 0.014 *** 0.810
*** *** *** *** 0.002 *** 0.002
*** *** *** 0.540 0.493 0.041 0.998
*** *** *** 0.612 0.634 0.012 0.970
*** *** *** 0.664 0.488 0.005 0.841
*** *** *** 0.943 0.992 0.012 1.000
a
Asterisks indicate significant effect (P < 0.0005).
suspension (Mancosky & Milly, 2011) where proteins structure unfold by the exposure of two opposite charged ends of the protein molecule, i.e., hydrophobic or water-repelling end, and hydrophilic or water-attracting end. This protein rearrangement at the airewater interface has reduced surface tension and enables air trapping which gives good foaming properties (Venugopal, 2008). Results of ANOVA in Table 2 show that foam overrun was significantly affected by all three factors, protein concentration, sonication time and ultrasound amplitude with P < 0.0005, and also its levels at P < 0.05 (Table 3). 3.2. Foam drainage Fig. 1B shows that the foam drainage of treated protein suspension decreased with increasing ultrasound amplitude, sonication time and protein concentration. The highest decrease was 45% when highest ultrasound amplitude of 60% was applied for 15 min. In general, a lower drainage indicates a better foam stability. The ultrasound treated WPC suspensions produced enhanced foam stability because of its effect in increasing surface activity at the solutioneair interface that helps stabilising foams with increased flexibility (Weiss, Kristbergsson, & Kjartansson, 2011). The increase in protein concentration increased the suspension viscosity and produced better texture of foam while the increase of ultrasonic wave amplitudes further increased the strength of protein films at the interface of the foam and led to higher stability (Weiss et al., 2011). The increase in protein concentration (Martínez-Padilla, ster, & Sosa-Herrera, 2014) and ultraGarcía-Mena, Cases-Alenca sound treatment intensity (Lim & Barigou, 2005a, b) that produced smaller bubbles in foam system (Fig. 1B), helped slow the rate of foam collapse. ANOVA test in Table 2 and Tukey test in Table 3 show that foaming stability was not only affected significantly by the two ultrasonic parameters and its level, i.e., sonication time and amplitude, but also concentration of protein.
3.3. Viscoelastic properties of foam Fig. 2A shows storage modulus, G0 of foam increased with protein concentration and ultrasound treatment with respect to its amplitude and duration of ultrasound treatment. A similar aspect was observed for loss modulus, G00 , as illustrated in Fig. 2B. For shorter sonication duration of 5 min, G0 and G00 did not vary as much at the different ultrasound amplitudes applied. The effect of ultrasound amplitude seemed to be more to foams at lower protein concentration while the absolute G0 and G00 values increased with protein concentration. An increase in G0 indicates that the foam was getting more solid elastic-like while increase of G00 means the foam was more to liquid viscous-like. With a higher absolute value of G0 over G00 in the same quantum of increase for both over the amplitude and time tested, the foam from treated WPC is said to be of solid elastic nature. It has been suggested that cavitation effect of ultrasound that induced hot spots of 4000 K and 100 MPa in protein suspension cause chemical changes in protein structure and lead to protein denaturation. During protein denaturation (Chen & Dickinson, 1998; Dickinson & Chen, 1999), protein unfolding caused protein aggregation through hydrophobic interactions and disulphide bonds. This contributes to formation of a gel network (Meza et al., 2011) in the continuous phase of foam system and increases solid elastic-like property of foam. Although all three factors had significant effect on both the moduli (Table 2), Tukey test's (Table 3) shows that only concentration levels have significantly effect at P < 0.05. 3.4. Foam consistency index and flow behaviour index Fig. 3A and B show the consistency index, K and flow behaviour index, n respectively with their values of coefficient of determination, R2, all above 0.9. K acts as an index that is related to strength and gives an indication of the capacity of the system for retaining
Table 3 Means of various foam properties and the associated probability (P-value) from multiple comparisons using Tukey test.a Factor/Response
Overrun (%)
Drainage (mL)
Storage modulus, G0 (Pa)
Loss modulus, G00 (Pa)
Consistency index, K (Pa sn)
Viscosity, m (Pa s)
Concentration (%) 10 15 20 Time (min) 0 5 15 25 Amplitude (%) 0 20 40 60
(n ¼ 48) 1127c 1174a 1143b (n ¼ 36) 1101d 1129c 1159b 1203a (n ¼ 36) 1101d 1124c 1164b 1203a
(n ¼ 48) 4.410a 3.527b 2.877c (n ¼ 36) 4.133a 3.858b 3.258c 3.169c (n ¼ 36) 4.133a 3.639b 3.411c 3.236d
(n ¼ 32) 165.6c 192.0b 227.5a (n ¼ 24) 186.0c 192.3bc 198.0ab 203.8a (n ¼ 24) 186.0c 193.0bc 197.7ab 203.5a
(n ¼ 32) 38.49c 46.22b 57.50a (n ¼ 24) 44.84c 46.43bc 48.46ab 49.89a (n ¼ 24) 44.84c 46.51bc 48.24ab 50.03a
(n ¼ 32) 22.67c 29.10b 39.15a (n ¼ 24) 29.31b 29.05b 30.82a 32.06a (n ¼ 24) 29.31c 29.46bc 30.71ab 31.75a
(n ¼ 32) 5.403c 6.922b 8.851a (n ¼ 24) 6.664c 6.918bc 7.175ab 7.478a (n ¼ 24) 6.664c 6.970bc 7.219ab 7.382a
a
Means in same column with different superscript letters are significantly different at P < 0.05.
M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
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▪
Fig. 2. Effect of amplitude (,, control; , 20% amplitude; , 40% amplitude; , 60% amplitude) and sonication time on (panel A) storage modulus and (panel B) loss modulus of ultrasound treated protein suspensions at 10, 15 and 20% (w/v) WPC. Values presented are mean values with standard errors.
mez, Moraleja, Oliete, Ruiz, & Caballero, 2010). K values air (Go increased significantly with protein concentration and ultrasound treatment and this trend is consistent with drainage decrease (Fig. 1B). Foam with lower drainage was more stable and had higher capability to retain trapped air in the system giving higher consistency index. Fig. 3A shows that at higher WPC concentration, ultrasound application in protein suspension can widen the range of K values of foam. The similar effect of ultrasound application to the consistency index of soy protein was found by Jambrak et al. (2009), where the largest increase in consistency index was observed after ultrasound treatment of 30 min at 40 kHz. These results suggest some useful correlations between consistency index and foam stability and also the potential of using ultrasound to improve the consistency index of protein foam was possible for whey proteins besides the soy protein. With n < 1, these aerated foams are classified as non-Newtonian and has a pseudoplastic or shear thinning behaviour. In general, the pseudoplasticity increases with protein concentration. The results show that highest K is paired with lowest n at highest amplitude of 60% and longest sonication time of 25 min. This inversed correlation between K and n was consistent with the findings in the studies of black gram flour
and starch dispersions (Changala Reddy, Susheelamma, & Tharanathan, 1989), batters for coating products (Salvador, Sanz, mez et al., 2010; Sakiyan, & Fiszman, 2003), and cake batter (Go Sumnu, & Sahin, 2004). 3.5. Foam viscosity Fig. 3C shows that the application of ultrasound at lower amplitude and shorter sonication time gave less effect on changes to the viscosity when compared with the control. In general, foam viscosity increased consistently with ultrasonic amplitude and treatment time due to the increased viscosity of protein suspension. It is suggested that ultrasound treatment may have led to protein unfold and became less compact (Weiss et al., 2011). This has altered the three-dimensional structure of proteins and caused the effective volume of proteins increase and as a consequence, viscosity of protein suspension also increased (Weiss et al., 2011). Viscosity of foam is a crucial parameter and is often assessed by its stiffness after the whipping process. It indicates stability of the foam and ensures trapped air bubbles enter safely into the subsequent processing step of producing cake batter. As expected,
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M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
▪
Fig. 3. Effect of amplitude (,, control; , 20% amplitude; , 40% amplitude; , 60% amplitude) and sonication time on (panel A) consistency index, (panel B) flow behaviour index and (panel C) viscosity of ultrasound treated protein suspensions at 10, 15 and 20% (w/v) WPC. Values presented are mean values with standard errors.
protein concentration increased foam viscosity effectively with P < 0.05 in Tukey test (Table 3), while the sonication effects of ultrasound amplitude and sonication time increased foam viscosity more significantly at P < 0.0005 as showed in Table 2. 3.6. Prediction of foam drainage from viscosity Foam viscosity is usually predicted manually by bakers through the observation on the peak condition of foam or by feel as to
estimate its stability. The correlation analysis in Fig. 4 provides the avenue to predict foam drainage from viscosity and vice versa using Eq. (5) for a relevant range of viscosity from 4.96 to 10.21 Pa s. Foam drainage was linearly correlated with foam viscosity at coefficient of determination, R2 of 0.7425 and the correlation was significant with P < 0.001. The decrease in foam drainage with increasing viscosity reinforces that a stable foam matrix which can hold or retain air is important for subsequent processing steps such as cake batter mixing plus other ingredients during cake making. Studies
M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
Fig. 4. Correlation between foam drainage and viscosity.
on the correlation between a rheological property with an aeration property were conducted on several food systems such as bread (Chin, Tan, Yusof, & Rahman, 2009b), cake batter (Sahi & Alava, 2003), and protein suspension (Glicksman, 1986; Nicorescu et al., 2011). This prediction could assist the manufacture and processing of highly aerated food through calibration either of the properties to achieve desired final quality as many have proven that viscosity and stability of bubbles are most essential parameters for final cake quality, especially cake volume (Gan, Ellis, & Schofield, 1995; Kinsella, 1981; Pernell, Luck, Foegeding, & Daubert, 2002).
Foam drainageðmLÞ ¼ 0:4775 ViscosityðPa sÞ þ 6:7715
(5)
4. Conclusions The increase in protein concentration and ultrasound applications had significantly improved aeration and rheological properties of protein foams. The highest foaming capacity increase of 8% was obtained when WPC concentration increased from 10 to 15% while the highest foam stability and all rheological properties increase was at protein concentration from 10 to 20%. At the optimal protein concentrations, with maximum ultrasound treatment for 25 min at 60% amplitude and energy density of 468 kJ L1, foam drainage decreased 35% while foaming capacity, storage modulus, loss modulus, consistency index and viscosity increased 22%, 17%, 26%, 18%, and 20% respectively. Acknowledgements This work was supported by the Ministry of Education (MOE), Malaysia, through the Fundamental Research Grant Scheme (FRGS/ 2/2013/TK05/UPM/02/5). References Bals, A., & Kulozik, U. (2003). Effect of pre-heating on the foaming properties of whey protein isolate using a membrane foaming apparatus. International Dairy Journal, 13, 903e908. Banerjee, R., Chen, H., & Wu, J. (1996). Milk protein-based edible film mechanical strength changes due to ultrasound process. Journal of Food Science, 61, 824e828. Bouaouina, H., Desrumaux, A., Loisel, C., & Legrand, J. (2006). Functional properties of whey proteins as affected by dynamic high-pressure treatment. International Dairy Journal, 16, 275e284. Campbell, G. M., & Mougeot, E. (1999). Creation and characterization of aerated food products. Trends in Food Science and Technology, 10, 283e296.
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International Dairy Journal, 8, 425e434. Meza, B. E., Chesterton, A. K. S., Verdini, R. A., Rubiolo, A. C., Sadd, P. A., Moggridge, G. D., et al. (2011). Rheological characterization of cake batters generated by planetary mixing: comparison between untreated and heattreated wheat flours. Journal of Food Engineering, 104, 592e602. Mleko, S., Kristinsson, H. G., Liang, Y., & Gustaw, W. (2007). Rheological properties of foams generated from egg albumin after pH treatment. LWT e Food Science and Technology, 40, 908e914. Nicorescu, I., Vial, C., Talansier, E., Lechevalier, V., Loisel, C., Della Valle, D., et al. (2011). Comparative effect of thermal treatment on the physicochemical properties of whey and egg white protein foams. Food Hydrocolloids, 25, 797e808. Panyam, D., & Kilara, A. (1996). Enhancing the functionality of food proteins by enzymatic modification. Trends in Food Science Technology, 7, 120e125.
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Patel, M. T., & Kilara, A. (1990). Studies on whey protein concentrates. 2. Foaming and emulsifying properties and their relationships with physicochemical properties. Journal of Dairy Science, 73, 2731e2740. Pernell, C. W., Luck, P. J., AllenFoegeding, E., & Daubert, C. R. (2002). Heat-induced changes in angel food cakes containing egg-white protein or whey protein isolate. Journal of Food Science, 67, 2945e2951. Philips, L. G., Schulman, W., & Kinsella, J. E. (1990). pH and heat treatment effects on foaming of whey protein isolate. Journal of Food Science, 55, 1116e1119. Pongsawatmanit, R., Harnsilawat, T., & McClements, D. J. (2006). Influence of alginate, pH and ultrasound treatment on palm oil-in-water emulsions stabilized by b-lactoglobulin. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 287, 59e67. Raikos, V., Campbell, L., & Euston, S. R. (2007). Effects of sucrose and sodium chloride on foaming properties of egg white proteins. Food Research International, 40, 347e355. Sahi, S. S., & Alava, J. M. (2003). Functionality of emulsifiers in sponge cake production. Journal of the Science of Food and Agriculture, 83, 1419e1429. Sakiyan, O., Sumnu, G., & Sahin, S. (2004). Influence of fat content and emulsifier type on the rheological properties of cake batter. European Food Research and Technology, 219, 635e638.
Salvador, A., Sanz, T., & Fiszman, S. M. (2003). Rheological properties of batters for coating products e effect of addition of corn flour and salt. Food Science and Technology International, 9, 23e25. Tan, M. C. (2014). Characteristics of ultrasound treated whey protein in foaming and cake batter for cake making (PhD thesis). Malaysia: Universiti Putra Malaysia. Venugopal, V. (2008). Marine products for healthcare: Functional and bioactive nutraceutical compounds from the ocean (pp. 51e102). Boca Raton, FL, USA: CRC Press. Walstra, P. (2003). Physical chemistry of foods (pp. 397e436). New York, NY, USA: Marcel Dekker, Incorporated. Weiss, J., Kristbergsson, K., & Kjartansson, G. T. (2011). Engineering food ingrenovas, & dientes with high-intensity ultrasound. In H. Feng, G. Barbosa-Ca J. Weiss (Eds.), Ultrasound technologies for food and bioprocessing (pp. 239e285). New York, NY, USA: Springer. Wu, H., Hulbert, G. J., & Mount, J. R. (2000). Effects of ultrasound on milk homogenization and fermentation with yogurt starter. Innovative Food Science and Emerging Technologies, 1, 211e218. Yang, X., & Foegeding, E. A. (2010). Effects of sucrose on egg white protein and whey protein isolate foams: factors determining properties of wet and dry foams (cakes). Food Hydrocolloids, 24, 227e238. ~ iga, R. N., & Agulera, J. M. (2008). Aerated food gels: fabrication and potential Zún applications. Trends in Food Science and Technology, 19, 176e187.
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International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj
Characterisation of improved foam aeration and rheological properties of ultrasonically treated whey protein suspension M.C. Tan a, N.L. Chin a, *, Y.A. Yusof a, F.S. Taip a, J. Abdullah b a b
Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Plant Assessment Technology Group, Malaysian Nuclear Agency (Nuclear Malaysia), Bangi, 43000 Kajang, Selangor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2014 Received in revised form 18 September 2014 Accepted 21 September 2014 Available online 18 October 2014
Suspensions of 10, 15 and 20% (w/v) whey protein concentrate (WPC) were treated with 20 kHz ultrasound for 5, 15 and 25 min at an amplitude of 20, 40 or 60%. The treated suspensions were whipped into foam and the aeration and rheological properties were investigated. With increasing ultrasound amplitude and treatment time, whey protein foam at 15% concentration produced the highest foaming capacity, while foam stability, storage modulus, loss modulus, consistency index and viscosity of foam increased with protein concentration. Foam viscosity correlated with foam stability with R2 ¼ 0.7425 and significant at P < 0.001. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction Dairy proteins, in particular whey proteins, are widely applied in food industry due to their high nutritional values and versatile functional properties in food products. They are good for water binding, foaming, emulsifying, and gelling purposes (Jyotsna, Sai Manohar, Indrani, & Venkateswara Rao, 2007), making it an important foaming agent or ingredient. Whey protein is present as an ingredient in many foods, including pasta, flavoured milk, dessert pudding, muesli, savoury dip and ice cream (McIntosh et al., 1998). Whey proteins used as foaming agents in aerated foods, such as meringue, angel food cake or nougat, are considered to coexist in complexes with other ingredients and need to be stable when undergoing mixing, heating and drying processes (Foegeding, Luck, & Davis, 2006). Foaming properties of proteins are usually evaluated by aeration characterisation, i.e., foaming capacity and foam stability (Nicorescu et al., 2011; Raikos, Campbell, & Euston, 2007). Even though air is incorporated during foaming, aeration is only achieved by a stable network capable of retaining the air bubbles in the foam system. The network stability of an aerated system is primarily affected by its rheological properties (Campbell & Mougeot, 1999). Kinsella (1981) reported that high surface viscosity and high film yield were correlated with strong foams because it reflects strong cohesion between the film forming molecules. Films with
* Corresponding author. Tel.: þ60 3 89466353. E-mail address: [email protected] (N.L. Chin).
high viscosity and possessing viscoelasticity contribute to foam stability as it allows foam films to respond to stresses by compression and expansion (Kinsella, 1981). Whey protein is gaining popularity as a healthier source of protein in the food industry (Garcia-Gariabay, Jimenez-Guzman, & Hernandez-Sanchez, 2008) following demands of consumers with high health concerns. Due to the lower quality of whey protein for foaming functions (Yang & Foegeding, 2010), many trials have been undertaken in attempts to improve its functionality including using enzymatic hydrolysis treatment (Kuehler & Stine, 1974; Panyam & Kilara, 1996), dynamic pressure (Bouaouina, Desrumaux, Loisel, & Legrand, 2006), pH and heat treatment (Philips, Schulman, & Kinsella, 1990), and ultrasound treatment (Jambrak, Mason, Lelas, Herceg, & Herceg, 2008; Kresic, Lelas, Jambrak, Herceg, & Brn ci c, 2008). Ultrasound has been used in many studies of proteins, such as the effect of ultrasound on the resistance of protein-stabilised emulsions to environmental stresses (Pongsawatmanit, Harnsilawat, & McClements, 2006), mechanical strength changes of milk proteinbased edible films (Banerjee, Chen, & Wu, 1996), physical properties of soy proteins (Jambrak, Lelas, Mason, Kresi c, & Badanjak, 2009), and kinetic characterisation of hydrolysis of defatted wheat germ protein (Jia et al., 2010). Ultrasonication increased the surface activity of proteins and the rate of protein adsorption by lowering the surface tension of protein solutions (Gülseren, Güzey, Bruce, & ~ iga & Weiss, 2007; Güzey, Gülseren, Bruce, & Weiss, 2006; Zún Agulera, 2008). The application of high-intensity ultrasound changes proteins and makes it advantageous in bubble formation ~ iga & Agulera, 2008). Ultrasound is able to induce structural and (Zún
http://dx.doi.org/10.1016/j.idairyj.2014.09.013 0958-6946/© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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functional changes in proteins to bring potential applications in controlling the alterations of lipid and gas interfaces in emulsions, foams and gels in food products (Gülseren et al., 2007). The objective of this study was to characterise the improved aeration and rheological properties of whey protein foams made from ultrasound treated whey protein suspensions. 2. Material and methods 2.1. Preparation of protein suspension Whey protein concentrate (Textrion; PROGEL 800, DMV International, BA Veghel, Netherlands) used was in the form of protein powder. The composition of the protein powder (by weight) is 80% protein, 6.5% lactose, 6.3% fat, 4.4% ash, and 4.9% moisture. Aqueous suspensions of WPC at concentrations of 10, 15 or 20% (w/v) were prepared by dispersing 25.0, 37.5 or 50.0 g of dry matter with distilled water at 25 C into a total of 250 g in a 500 mL beaker and stirred using a mechanical stirrer (RW20 DZM.n S2, IKA Works (Asia) Sendirian Berhad, Malaysia) at 355 rpm for 20 min to increase protein hydration activity in water prior to further homogenisation using ultrasound treatment.
Aeration properties of foam were measured in terms of foaming capacity and foam stability. Foaming capacity was expressed in terms of percentage of overrun and calculated using Eq. (2) (Patel & Kilara, 1990). Foaming capacity was determined based on the mass of 100 mL of protein suspension using a 100 mL cylinder, and the mass of foam filled and levelled in a 100 mL plastic weighing boat. The average foaming capacity was taken from three samples from the same batch of whipped foam.
Overrun
foam
¼
Mass
suspension
Mass
Massfoam
foam
100%
(2)
Foam stability was determined from the foam drainage at room temperature of 25 C. A small plug of glass wool was placed at the top of the funnel stem of a filter funnel with dimension 7.5 cm inner top diameter, 0.4 cm inner stem diameter and 7.0 cm stem length, to retain the foam but allow drainage of the liquid. 100 mL of fresh whipped foam was then transferred into the filter funnel (Jambrak et al., 2009). The drainage (mL) of the liquid was recorded from the measuring cylinder after 20 min. The average drainage was taken from three samples from the same batch of whipped foam. 2.4. Measurement of rheological properties of foam
2.2. Ultrasound treatment of protein suspensions for foam preparation Protein suspensions in a 500 mL beaker were sonicated at 20, 40 or 60% amplitude for 5, 15 or 25 min using a 20 kHz, 400 W high intensity ultrasound probe (Digital Sonifier Model 450, Branson Ultrasonics Corporation, Danbury, CT, US). The probe had a 2.54 cm vibrating titanium tip and it was immersed to half level at the centre of the suspension medium. The control samples were those without ultrasonic treatment, zero min treatment time. The actual power inputs as illustrated by the ultrasonic device during each setting were recorded. The ultrasound energy densities of treated protein suspension at every concentration at a total volume of 250 mL for 5, 15 and 25 min at 20, 40 and 60% amplitude were calculated following Eq. (1) (Giordano, 2013). All the power inputs and energy densities are reported in Table 1. The temperature range increase recorded was from 43 to 68 C from all ultrasound treatment of protein suspension from 20 to 60% amplitude for 5e25 min was tested as independent factor of the experiment and found to be a non-significant factor (Tan, 2014). The treated and untreated whey protein suspensions was then whipped with a mixer (5K5SSS, Kitchen Aid Inc., St. Joseph, MI, US) for 15 min at 330 rpm at room temperature of 25 C.
PowerðWÞ TimeðsÞ Energy density W$sL1 @ JL1 ¼ VolumeðLÞ
2.3. Measurement of foam aeration properties
(1)
Table 1 Power input and energy density at each sonication time and ultrasound amplitude. Sonication time (min)
Ultrasound amplitude (%)
Power input (W)
Energy density (kJ L1)
5 5 5 15 15 15 25 25 25
20 40 60 20 40 60 20 40 60
22.5 50.5 78.0 22.5 50.5 78.0 22.5 50.5 78.0
27.0 60.6 93.6 81.0 181.8 280.8 135.0 303.0 468.0
Rheological properties of foam were measured as storage modulus, G0 , loss modulus G00 , consistency index, K, flow behaviour index, n, and viscosity, m. Foam rheological measurements were performed using a rheometer (AR-G2, TA Instruments, New Castle, DE, US) with a 40 mm diameter serrated plateeplate geometry set at a constant gap height of 2.4 mm at 25 C (Mleko, Kristinsson, Liang, & Gustaw, 2007). A sample from each batch of freshly prepared foam mix was used. Frequency sweep tests were performed on foams from 0.1 to 10 Hz within a similar linear viscoelastic region of 1% strain when analysing the texture of foams for its viscoelastic shear moduli, G0 (Pa) and G00 (Pa). Properties of K, n, m were obtained from another test where foams were sheared by linearly increasing rates of 0e100 s1 within 5 min to generate shear stress versus shear rate data. Data of shear stress, s (Pa) versus shear rate, g_ (s1) were fitted to a rheological model of Power law (Eq. (3)) to obtain rheological characteristics of consistency index, K (Pa sn) and flow behaviour index, n of the foam. K and n were obtained through curve fitting utilizing Generalized Reduced Gradient (GRG2) nonlinear optimisation using the solver function of Microsoft Excel. The best fitted line with minimum sum of square errors was used as the sole criteria during curve fitting to obtain the high yield of coefficient of determination, R2. Viscosity, m (Pa s) was determined as a function of shear rate at 5.5 s1 (Eq. (4)) because it approximates mixing speed of 330 rpm using found values of K and n (Chin, Chan, Yusof, Chuah, & Talib, 2009a).
s ¼ K g_ n
(3)
m ¼ K g_ n1
(4)
2.5. Experiment design and statistical analysis A 33 full factorial experiment which incorporated 3 factors, each at three levels, i.e., WPC concentration at 10, 15 and 20% (w/v), ultrasound amplitude at 20, 40 and 60%, and sonication time at 5, 15 and 25 min was designed using the Minitab software (Version 16, Minitab Statistical Software, State College, PA, US). Controls are protein suspension without ultrasound treatment at relevant
M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
concentrations. Statistical comparisons of analysis of variance (ANOVA) on the effect of factors, i.e., protein concentration, ultrasound amplitude and sonication time on the responses, i.e., overrun, drainage, storage modulus, loss modulus, consistency index and viscosity were performed using general linear model (GLM). The effect of levels of each factor was determined using Tukey test with 95% of confidence intervals. The ANOVA and Tukey test were performed on three samples for aeration measurements and one sample for rheological measurements for a repeated batch. 3. Results and discussion 3.1. Foaming capacity Fig. 1A shows the foam overrun of protein suspension at various levels of ultrasonic amplitude and sonication time with energy densities between 27 and 468 kJ L1 as reported in Table 1. Protein suspension treated at increasing amplitude and for a longer time had a higher overrun. Although overrun seemed to increase with protein concentration, where it was highest for the 15% WPC, there was a slight decrease in overrun for further increase in concentration at 20%. This is most probably due to the high viscosity in protein suspension, which creates high resistance force against the
▪
9
incorporation of gas bubbles thus restricted the formability of aerated protein suspension during whipping (Bals & Kulozik, 2003) and hence resulting in lower overrun (Glicksman, 1986; Nicorescu et al., 2011; Walstra, 2003). The highest foam overrun of 1348% was achieved when the highest amplitude of 60% and longest time of 25 min was applied on a 15% WPC suspension. Higher overrun implies greater incorporation of air bubbles and better foaming capacity of the protein (Patel & Kilara, 1990). The increased foam overrun in treated suspension is due the homogenising effect of ultrasound (Lomakina & , 2006). This is explained in the Knorr, Zenker, Heinz, and Míkova Lee (2004) study using liquid whole egg where ultrasound aided the dispersion of protein and fat particles in liquid whole egg to give more even distribution and hence improved its foaming capacity through the homogenization effect of ultrasound. According to Wu, Hulbert, and Mount (2000), the ultrasound wave does not directly induce vibration of the globules to cause homogenisation, but it is due to the intense cavitation produced by ultrasound. The acoustic cavitation disrupts the liquid bubbles by causing sudden formation and collapse of bubbles in liquid. The shockwaves given off in the protein suspension generate a powerful force, which suggested breaking up of dry powder agglomerates and better mixing with water molecules in order to produce a homogeneous
Fig. 1. Effect of amplitude (,, control; , 20% amplitude; , 40% amplitude; , 60% amplitude) and sonication time on (panel A) foam overrun and (panel B) foam drainage of ultrasound treated protein suspensions at 10, 15 and 20% (w/v) WPC. Values presented are mean values with standard errors.
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Table 2 P-values from ANOVA to indicate significance of whey protein concentration, sonication time and ultrasound amplitude individually and interactively on various foam properties.a Factor/Response
Overrun (%)
Drainage (mL)
Storage modulus, G0 (Pa)
Loss modulus, G00 (Pa)
Consistency index, K (Pa sn)
Viscosity, m (Pa s)
A: concentration (%) B: time (min) C: amplitude (%) AB AC BC ABC
*** *** *** 0.004 0.014 *** 0.810
*** *** *** *** 0.002 *** 0.002
*** *** *** 0.540 0.493 0.041 0.998
*** *** *** 0.612 0.634 0.012 0.970
*** *** *** 0.664 0.488 0.005 0.841
*** *** *** 0.943 0.992 0.012 1.000
a
Asterisks indicate significant effect (P < 0.0005).
suspension (Mancosky & Milly, 2011) where proteins structure unfold by the exposure of two opposite charged ends of the protein molecule, i.e., hydrophobic or water-repelling end, and hydrophilic or water-attracting end. This protein rearrangement at the airewater interface has reduced surface tension and enables air trapping which gives good foaming properties (Venugopal, 2008). Results of ANOVA in Table 2 show that foam overrun was significantly affected by all three factors, protein concentration, sonication time and ultrasound amplitude with P < 0.0005, and also its levels at P < 0.05 (Table 3). 3.2. Foam drainage Fig. 1B shows that the foam drainage of treated protein suspension decreased with increasing ultrasound amplitude, sonication time and protein concentration. The highest decrease was 45% when highest ultrasound amplitude of 60% was applied for 15 min. In general, a lower drainage indicates a better foam stability. The ultrasound treated WPC suspensions produced enhanced foam stability because of its effect in increasing surface activity at the solutioneair interface that helps stabilising foams with increased flexibility (Weiss, Kristbergsson, & Kjartansson, 2011). The increase in protein concentration increased the suspension viscosity and produced better texture of foam while the increase of ultrasonic wave amplitudes further increased the strength of protein films at the interface of the foam and led to higher stability (Weiss et al., 2011). The increase in protein concentration (Martínez-Padilla, ster, & Sosa-Herrera, 2014) and ultraGarcía-Mena, Cases-Alenca sound treatment intensity (Lim & Barigou, 2005a, b) that produced smaller bubbles in foam system (Fig. 1B), helped slow the rate of foam collapse. ANOVA test in Table 2 and Tukey test in Table 3 show that foaming stability was not only affected significantly by the two ultrasonic parameters and its level, i.e., sonication time and amplitude, but also concentration of protein.
3.3. Viscoelastic properties of foam Fig. 2A shows storage modulus, G0 of foam increased with protein concentration and ultrasound treatment with respect to its amplitude and duration of ultrasound treatment. A similar aspect was observed for loss modulus, G00 , as illustrated in Fig. 2B. For shorter sonication duration of 5 min, G0 and G00 did not vary as much at the different ultrasound amplitudes applied. The effect of ultrasound amplitude seemed to be more to foams at lower protein concentration while the absolute G0 and G00 values increased with protein concentration. An increase in G0 indicates that the foam was getting more solid elastic-like while increase of G00 means the foam was more to liquid viscous-like. With a higher absolute value of G0 over G00 in the same quantum of increase for both over the amplitude and time tested, the foam from treated WPC is said to be of solid elastic nature. It has been suggested that cavitation effect of ultrasound that induced hot spots of 4000 K and 100 MPa in protein suspension cause chemical changes in protein structure and lead to protein denaturation. During protein denaturation (Chen & Dickinson, 1998; Dickinson & Chen, 1999), protein unfolding caused protein aggregation through hydrophobic interactions and disulphide bonds. This contributes to formation of a gel network (Meza et al., 2011) in the continuous phase of foam system and increases solid elastic-like property of foam. Although all three factors had significant effect on both the moduli (Table 2), Tukey test's (Table 3) shows that only concentration levels have significantly effect at P < 0.05. 3.4. Foam consistency index and flow behaviour index Fig. 3A and B show the consistency index, K and flow behaviour index, n respectively with their values of coefficient of determination, R2, all above 0.9. K acts as an index that is related to strength and gives an indication of the capacity of the system for retaining
Table 3 Means of various foam properties and the associated probability (P-value) from multiple comparisons using Tukey test.a Factor/Response
Overrun (%)
Drainage (mL)
Storage modulus, G0 (Pa)
Loss modulus, G00 (Pa)
Consistency index, K (Pa sn)
Viscosity, m (Pa s)
Concentration (%) 10 15 20 Time (min) 0 5 15 25 Amplitude (%) 0 20 40 60
(n ¼ 48) 1127c 1174a 1143b (n ¼ 36) 1101d 1129c 1159b 1203a (n ¼ 36) 1101d 1124c 1164b 1203a
(n ¼ 48) 4.410a 3.527b 2.877c (n ¼ 36) 4.133a 3.858b 3.258c 3.169c (n ¼ 36) 4.133a 3.639b 3.411c 3.236d
(n ¼ 32) 165.6c 192.0b 227.5a (n ¼ 24) 186.0c 192.3bc 198.0ab 203.8a (n ¼ 24) 186.0c 193.0bc 197.7ab 203.5a
(n ¼ 32) 38.49c 46.22b 57.50a (n ¼ 24) 44.84c 46.43bc 48.46ab 49.89a (n ¼ 24) 44.84c 46.51bc 48.24ab 50.03a
(n ¼ 32) 22.67c 29.10b 39.15a (n ¼ 24) 29.31b 29.05b 30.82a 32.06a (n ¼ 24) 29.31c 29.46bc 30.71ab 31.75a
(n ¼ 32) 5.403c 6.922b 8.851a (n ¼ 24) 6.664c 6.918bc 7.175ab 7.478a (n ¼ 24) 6.664c 6.970bc 7.219ab 7.382a
a
Means in same column with different superscript letters are significantly different at P < 0.05.
M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
11
▪
Fig. 2. Effect of amplitude (,, control; , 20% amplitude; , 40% amplitude; , 60% amplitude) and sonication time on (panel A) storage modulus and (panel B) loss modulus of ultrasound treated protein suspensions at 10, 15 and 20% (w/v) WPC. Values presented are mean values with standard errors.
mez, Moraleja, Oliete, Ruiz, & Caballero, 2010). K values air (Go increased significantly with protein concentration and ultrasound treatment and this trend is consistent with drainage decrease (Fig. 1B). Foam with lower drainage was more stable and had higher capability to retain trapped air in the system giving higher consistency index. Fig. 3A shows that at higher WPC concentration, ultrasound application in protein suspension can widen the range of K values of foam. The similar effect of ultrasound application to the consistency index of soy protein was found by Jambrak et al. (2009), where the largest increase in consistency index was observed after ultrasound treatment of 30 min at 40 kHz. These results suggest some useful correlations between consistency index and foam stability and also the potential of using ultrasound to improve the consistency index of protein foam was possible for whey proteins besides the soy protein. With n < 1, these aerated foams are classified as non-Newtonian and has a pseudoplastic or shear thinning behaviour. In general, the pseudoplasticity increases with protein concentration. The results show that highest K is paired with lowest n at highest amplitude of 60% and longest sonication time of 25 min. This inversed correlation between K and n was consistent with the findings in the studies of black gram flour
and starch dispersions (Changala Reddy, Susheelamma, & Tharanathan, 1989), batters for coating products (Salvador, Sanz, mez et al., 2010; Sakiyan, & Fiszman, 2003), and cake batter (Go Sumnu, & Sahin, 2004). 3.5. Foam viscosity Fig. 3C shows that the application of ultrasound at lower amplitude and shorter sonication time gave less effect on changes to the viscosity when compared with the control. In general, foam viscosity increased consistently with ultrasonic amplitude and treatment time due to the increased viscosity of protein suspension. It is suggested that ultrasound treatment may have led to protein unfold and became less compact (Weiss et al., 2011). This has altered the three-dimensional structure of proteins and caused the effective volume of proteins increase and as a consequence, viscosity of protein suspension also increased (Weiss et al., 2011). Viscosity of foam is a crucial parameter and is often assessed by its stiffness after the whipping process. It indicates stability of the foam and ensures trapped air bubbles enter safely into the subsequent processing step of producing cake batter. As expected,
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▪
Fig. 3. Effect of amplitude (,, control; , 20% amplitude; , 40% amplitude; , 60% amplitude) and sonication time on (panel A) consistency index, (panel B) flow behaviour index and (panel C) viscosity of ultrasound treated protein suspensions at 10, 15 and 20% (w/v) WPC. Values presented are mean values with standard errors.
protein concentration increased foam viscosity effectively with P < 0.05 in Tukey test (Table 3), while the sonication effects of ultrasound amplitude and sonication time increased foam viscosity more significantly at P < 0.0005 as showed in Table 2. 3.6. Prediction of foam drainage from viscosity Foam viscosity is usually predicted manually by bakers through the observation on the peak condition of foam or by feel as to
estimate its stability. The correlation analysis in Fig. 4 provides the avenue to predict foam drainage from viscosity and vice versa using Eq. (5) for a relevant range of viscosity from 4.96 to 10.21 Pa s. Foam drainage was linearly correlated with foam viscosity at coefficient of determination, R2 of 0.7425 and the correlation was significant with P < 0.001. The decrease in foam drainage with increasing viscosity reinforces that a stable foam matrix which can hold or retain air is important for subsequent processing steps such as cake batter mixing plus other ingredients during cake making. Studies
M.C. Tan et al. / International Dairy Journal 43 (2015) 7e14
Fig. 4. Correlation between foam drainage and viscosity.
on the correlation between a rheological property with an aeration property were conducted on several food systems such as bread (Chin, Tan, Yusof, & Rahman, 2009b), cake batter (Sahi & Alava, 2003), and protein suspension (Glicksman, 1986; Nicorescu et al., 2011). This prediction could assist the manufacture and processing of highly aerated food through calibration either of the properties to achieve desired final quality as many have proven that viscosity and stability of bubbles are most essential parameters for final cake quality, especially cake volume (Gan, Ellis, & Schofield, 1995; Kinsella, 1981; Pernell, Luck, Foegeding, & Daubert, 2002).
Foam drainageðmLÞ ¼ 0:4775 ViscosityðPa sÞ þ 6:7715
(5)
4. Conclusions The increase in protein concentration and ultrasound applications had significantly improved aeration and rheological properties of protein foams. The highest foaming capacity increase of 8% was obtained when WPC concentration increased from 10 to 15% while the highest foam stability and all rheological properties increase was at protein concentration from 10 to 20%. At the optimal protein concentrations, with maximum ultrasound treatment for 25 min at 60% amplitude and energy density of 468 kJ L1, foam drainage decreased 35% while foaming capacity, storage modulus, loss modulus, consistency index and viscosity increased 22%, 17%, 26%, 18%, and 20% respectively. Acknowledgements This work was supported by the Ministry of Education (MOE), Malaysia, through the Fundamental Research Grant Scheme (FRGS/ 2/2013/TK05/UPM/02/5). References Bals, A., & Kulozik, U. (2003). Effect of pre-heating on the foaming properties of whey protein isolate using a membrane foaming apparatus. International Dairy Journal, 13, 903e908. Banerjee, R., Chen, H., & Wu, J. (1996). Milk protein-based edible film mechanical strength changes due to ultrasound process. Journal of Food Science, 61, 824e828. Bouaouina, H., Desrumaux, A., Loisel, C., & Legrand, J. (2006). Functional properties of whey proteins as affected by dynamic high-pressure treatment. International Dairy Journal, 16, 275e284. Campbell, G. M., & Mougeot, E. (1999). Creation and characterization of aerated food products. Trends in Food Science and Technology, 10, 283e296.
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