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Effect of surface tension and viscosity on the surface stickiness of carbohydrate and protein solutions
Journal of Food Engineering 79 (2007) 1136–1143 www.elsevier.com/locate/jfoodeng
Effect of surface tension and viscosity on the surface stickiness of carbohydrate and protein solutions B. Adhikari b
a,*
, T. Howes a, A. Shrestha b, B.R. Bhandari
b
a School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australia School of Land and Food Sciences, The University of Queensland, Brisbane, Qld 4072, Australia
Received 28 September 2005; accepted 6 April 2006 Available online 19 April 2006
Summary The effects of surface tension and viscosity on the measured tensile strength (surface stickiness) of carbohydrate (fructose, lactose and maltodextrin) and protein (Whey protein isolate (WPI)) solutions were studied. The effect of the addition of WPI on the surface tension and surface stickiness of lactose solutions was measured. Surface tension was found to better correlate with surface stickiness compared to viscosity. Cohesive failure occurred in all the cases indicating that the energy required to create new surfaces within the drops was lower than the energy required to achieve a clean (adhesive) failure at the probe–drop interface. WPI behaves as a surfactant and sharply lowers both the surface tension and tensile strength. This study shows that the surface forces are more dominant than the rheological forces when the energy required to create new surface within the drop is less than the adhesive energy at the drop–probe interface. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Stickiness; Proteins; Carbohydrates; Surface tension; Viscosity; Cohesive strength
1. Introduction Adhesion of liquid foods and food emulsions to processing equipment, packages and the resultant residue causes significant economic loss (Michalsky, Desobry, Babak, & Hardy, 1999). Fouling and scale formation in processing equipment are also known as ubiquitous problems (Wilson, 2005; Xin, Chen, & Ozkan, 2005). Losses include material loss, cost of cleaning the equipment and disposal and treatment. In spray drying operations, atomized droplets can reach the dryer walls and rooftop and adhere there causing product loss, product quality degradation and fire hazards (Bhandari, Datta, & Howes, 1997). Michalsky and coworkers (Michalsky, Desobry, & Hardy, 1998, 1999) studied the adhesive behavior of edible oil and food emulsions to glass, Teflon (PTFE), low density polyethylene (LDPE), poly ethylene terephthalate (PET)
*
Corresponding author. Tel.: +61 7 33659058; fax: +61 7 33654199. E-mail address: [email protected] (B. Adhikari).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.04.002
and stainless steel. They allowed the food samples to flow down an inclined substrate surface and measured the amount of remaining sample on the surface after the flow ceased. It was found that surface roughness, the yield stress of the sample and solid surface tension were the key factors responsible for adhesion. Recently, Bhandari and Howes (2005) reviewed the importance of surface energetics of the food materials and surface of contact on the stickiness property. Kudra (2003) suggested that pasty materials exhibit stickiness across a region of temperature and moisture (sticky region) while being converted from solution to powders. The tendency of droplets and soft particles to stick to the dryer walls during spray drying is a major problem encountered in the powder industries (Langrish & Fletcher, 2003; Masters, 1996). Adhikari, Howes, Bhandari, and Truong (2003) studied the stickiness of drying droplets, in situ, using a probe tack testing instrument. They reported that to attain a state of non-adhesion at the drop–probe interface it is essential that the cohesive strength of the droplet be greater than the adhesive
B. Adhikari et al. / Journal of Food Engineering 79 (2007) 1136–1143
strength at the droplet–probe interface. They found that a state of non-adhesion was reached when the outer layer of the droplet was transformed into a glassy matrix. It was suggested that the non-sticky state could also be reached if the viscosity is raised high enough to prevent failure within the droplet itself. The studies described above have suggested that both the surface forces (surface tension related) and bulk forces (viscosity related) are responsible for food adhesion. However, the question of which of these forces dominate the adhesion (stickiness) process has not yet been answered. Hence, this paper aims at measuring the surface tension, viscosity and tensile strength of carbohydrate and protein solutions in order to relate them to adhesion (surface stickiness) on a stainless steel surface. 2. Materials and methods 2.1. Materials Spray dried lactose powder (Murray Goulburn Co-Ltd., Australia), whey protein isolate, (WPI, ALATAL 817TM, New Zealand Milk Powder, New Zealand), fructose (ADM Corn Processing, USA) and maltodextrin of dextrose equivalent 6 (Roquette Freres, France) were used without further purification. They were vacuum dried (70 °C 500 mbar) overnight and stored in desiccators over P2O5 prior to solution preparation. The carbohydrates (fructose, lactose and maltodextrin) and protein (WPI) were chosen because WPI is a known surface active material while carbohydrates are not. Both low (fructose and lactose) and high (maltodextrin) molecular weight carbohydrates are chosen to study the effect of viscosity on stickiness. Furthermore, these materials are widely used in food processing. 2.2. Methods 2.2.1. Surface tension This property indicates how strongly the surface molecules of a liquid/solution are attracted by the adjacent molecules and is a factor affecting stickiness. Solution surface tensions were determined using a tensiometer (ST9000, Nima Technology, UK) which operated using the Wilhelmy plate principle. Paper plates of 21 mm in perimeter were used in all the cases. Sample temperatures were recorded before and after each test. Surface tension was also determined using a custom fabricated load cell device. The surface tension values obtained by both the devices were used to cross check the values obtained from each other. Three replicates were taken and mean values are reported. 2.2.2. Viscosity This parameter gives information regarding the resistance offered by liquid/solution molecules to the motion and is a factor influencing stickiness. A Rheoscope 1
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Thermo-Electron Corporation, (HAAKE) with parallel plate (plate diameter = 6 cm) was used to determine the viscosity. The shear stress versus shear rate data was recorded and the viscosity was determined from the slope of the stress–strain curves. 2.2.3. Tensile strength This property was measured using an in situ stickiness testing instrument. This instrument works on the principle of tack, that is, it mimics the feel when one touches a drop surface. Details of this device and test protocols are given elsewhere (Adhikari et al., 2003). The schematic diagram (Fig. 1) and the test procedure are briefly presented as follows. The shaft of the linear actuator holds the drop-holder and the drop on the tip of its shaft. The stainless steel probe (2.5 mm in diameter) is attached to microbalance (±0.01 mg). A digital video camera and stereomicroscope (50 times magnification) imaging system monitors the approach, withdrawal and contact of the probe to the drop surface. During the experiment, the stepper motor is driven until the probe makes good contact with the drop surface. Once contact is established, the motor is subsequently withdrawn. The variation in the tensile force over time is recorded continuously to a personal computer (PC). Digital images during the approach, contact and withdrawal events are also recorded. The temperature of the drop is recorded by inserting micro-thermocouples (T-type, Omega Engineering USA) at the drop centre. 3. Results and discussion 3.1. Fructose solution The variation of surface tension of fructose solution with concentration at 24 ± 0.3 °C is given in Fig. 2. This figure includes the values obtained from the tensiometer as well as the load cell. Although there is a difference between the mean values of surface tension obtained from both devices, the difference is within experimental error. The close agreement in results obtained by both the devices indicates that the method applied was reliable. Furthermore, the measured surface tension value of water was 70.2 mN/m which agrees well with the literature value of 70.52 mN/m (extrapolated to 24 °C) (Rahman, 1995). The small discrepancy might be due to trace amounts of impurities in the water or variation of temperature during the tests. However, this discrepancy is also within the experimental error. It can be seen that the surface tension of the fructose solution increases with solid concentration. The surface tension value of 60 wt.% fructose solution is 75.33 mN/m and the difference compared to pure water (Dc) is 5.13 mN/m. Since surface tension data of fructose as a function of concentration are not available in literature, however Dc for 55 wt.% sucrose for 15–40 °C range is reported to be 3.7 ± 0.5 mN/m (International Critical Tables, 1930). Since sugars are known to be non-surface active, the increase in surface tension of fructose is in
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Monitor
to balance
video recorder
m ca
Sm
er
a
Ws P D Dh
Gs La
Ls Fig. 1. Schematic diagram of a stickiness testing device. (P): stainless steel probe, (D): drop, (Dh): drop holder, (Gs): guide shaft with peripherals for linear motion, (La): linear actuator, (Ls): lead screw, (Ws): glass working section, (Sm): stereo-microscope system with video camera.
76
Tensiometer Load cell
76
75
Surface tension (mN/m)
75 74 73 72 71 70 69 0
10
20
98
Surface tension Tensile strength
30
40
50
60
70
96 94
74 92 73
90
72
88 86
71
Tensile strength (Pa)
Surface tension (mN/m)
77
84 70
82
Fructose concentration (%, w/w) 69
Fig. 2. Variation of surface tension (mN/m) of fructose solution as a function of concentration at 24 ± 0.3 °C.
accordance with the Gibbs equation (Eq. 1) which indicates that the method used is reliable. Fig. 3 presents the variation of tensile strength (Pa) of fructose drops with concentration. This figure also includes the variation of surface tension with concentration for comparison. The tensile strength of fructose solution increases with concentration which indicates that the fructose molecule has affinity for water molecule. Since the cohesive strength of the water molecule is mainly due to hydrogen bonds (Tsujii, 1998) higher tensile strength of fructose solution compared to water indicates possible
80 0
10
20
30
40
50
60
70
Fructose concentration (%, w/w)
Fig. 3. Variation of surface tension (mN/m) and tensile strength (Pa) of fructose solution as a function of concentration, at 24 ± 0.3 °C and 20.5 ± 0.5 °C, respectively.
increase in number of hydrogen bonds in the solution. The tensile strength of pure water droplets is 87.02 ± 0.58 Pa while that of the 60% w/w fructose is 95.25 ± 0.12 Pa. The difference in tensile strength compare to pure water (DTst) is 8.23 Pa. The ratio of the tensile strength of 60 wt.% fructose to water is 1.09 which is close to the surface tension ratio of this solution to water which is 1.07.
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Fig. 4. Cohesive failure with creation of new surface within the drop. (a) Approach, (b) contact, (c) withdrawal and (d) complete withdrawal.
Fig. 4 shows the mode of failure of fructose drop during the withdrawal of the probe. It can be seen that the drop undergoes necking first and the subsequent breakage or failure takes place within the droplet itself leading to the formation of two new surfaces. A considerable amount of fructose solution remains adhered to the initially clean probe. This signifies that the cohesive strength of the 60 wt.% fructose drop is still smaller compared to the adhesive strength at the (stainless steel) probe–drop interface. 3.2. Maltodextrin solutions A similar trend is observed when the surface tension and tensile strength of maltodextrin solutions were determined (Fig. 5). This figure shows that the surface tension increased with concentration and Dc at 40 wt.% was 2.34 mN/m. The relationship between surface tension and concentration indicates that maltodextrin is not surface active. However, the surface tension value of 40 wt.% maltodextrin was less than the fructose solution of the same concentration by 0.1 mN/m. Since this difference is within the measurement error, it can be said that the surface tension values of fructose and maltodextrin solutions are very close to each other. The tensile strength of the maltodextrin solution increased with concentration. DTst value for 40 wt.% solution was 7.05 Pa which is slightly higher than that of the fructose solution (DTst = 3.7 Pa) of the same concentration. The ratio of surface tension and tensile
75
95
Surface tension Tensile strength
93 91
73
89 87
72
85
71
83
Tensile strength (Pa)
Surface tension (mN/m)
74
strength of this solution to water are 1.03 and 1.08, respectively. As with fructose, the increment of these two parameters is similar. The probe tack test showed that the mode of failure for these solutions was cohesive indicating that the energy required to create a new surface within the drop was lower compared to adhesive energy at the at probe– drop interface. It explains our daily experience that if a solution comes in contact with an equipment surface, it easily adheres to it and can not be removed easily without leaving substantial amount of residue. The viscosity of carbohydrate and protein solutions is shown in Table 1. Compared to water the viscosity of a 40 wt.% maltodextrin solution has increased by a factor of 170. This fact shows that, the degree of variation of viscosity of maltodextrin solutions is much greater compared to the variation of tensile strength and surface tension within this range. Furthermore, the mode of failure was cohesive and that the necking and stretching during the process of new surface formation were not different compared fructose and lactose solutions of the same concentration. Hence, in the case of a drop that fails cohesively (cohesive energy is lower than the energy at the interface) and that the formation of the new surface takes place within the drop itself, the bulk parameter (i.e., the viscosity) is unlikely be the dominant or major factor that controls the surface stickiness. 3.3. Lactose solutions The surface tension and tensile strength of 5, 10, 15 and 20% w/w lactose solutions are presented in Fig. 6. Both the surface tension and the tensile strength of the solution decreased rather than increased with an increase in concentration. Being a sugar and a carbohydrate, it was expected that the surface tension would increase with concentration. It is reported that Dc for a 10 wt.% lactose solution is 0.9 ± 0.4 mN/m at 15 °C (International Critical Tables, 1930). The relationship between the surface tension, concentration and adsorption of a solute (in a dilute solution) is given by Gibbs’ equation (Eq. (1)):
70 81 69
79 0
5
10
15
20
25
30
35
40
45
Maltodextrin concentration (%, w/w)
Fig. 5. Variation of surface tension (mN/m) and tensile strength (Pa) of maltodextrin solution as a function of concentration, at 24.7 ± 0.3 °C.
dðcÞ ¼ RT Cdðln CÞ
ð1Þ
where, c, C and C are surface tension (mN/m), amount of solute adsorbed (g moles/m2), concentration of the solute (g moles/m3) in solution, respectively. Similarly, R is the gas constant (g cm2 s2 g mole1 K1) and T is the
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Table 1 Viscosity of maltodextrin (DE 6), lactose and WPI solutions at various concentrationsa Maltodextrin (DE 6)
Lactose
Whey protein isolate (WPI)
Concentration (wt.%)
Viscosity (m Pa s)
Concentration (wt.%)
Viscosity (m Pa s)
Concentration (wt.%)
Viscosity (m Pa s)
0 10 20 30 40
0.80 ± 0.1 2.10 ± 0.00 7.63 ± 0.19 30.47 ± 0.40 137.07 ± 1.15
0 5 10 15 20
0.80 ± 0.10 1.01 ± 0.01 1.24 ± 0.04 1.43 ± 0.05 1.81 ± 0.05
1 2 3 4 5
1.00 ± 0.00 1.13 ± 0.05 1.33 ± 0.05 1.60 ± 0.00 2.03 ± 0.11
Mean values ± standard deviations of triplicate samples (at 25 ± 0.2 °C).
73
88
71
86
69
84
67
82
65
80
63
78
61
76
59
74
Surface tension
72
Tensile strength
55
70 0
5
10
15
to the conclusion that the surface forces rather than the viscous forces are dominant when adhesion or surface stickiness of droplet/soft particle is concerned. It has to be stressed here that, for surface forces to be dominant over viscous forces, a key criterion has to be met which is: The cohesive strength of the drop should be weak enough to allow the formation of new surface. The surface stickiness of a droplet in which cohesive failure is the dominant mode of failure, is the worst case of the stickiness scenario observed in practice (Adhikari et al., 2003). Surface tension appears to be a good indicator for this particular mode of stickiness.
20
Lactose concentration (%, w/w)
Fig. 6. Variation of surface tension (mN/m) and tensile strength (Pa) of lactose solution as a function of concentration, at 23.5 ± 0.5 °C.
Fig. 7 presents the variation of surface tension of WPI with concentration. It can be seen from this figure that the surface tension of water is reduced by 62.7% with addition of 1 wt.% WPI. Beyond this point, the fall in surface tension with increase in WPI concentration from 1 wt.% to 5 wt.% is only 6.7%. The surface tension values of 5 wt.% and 10 wt.% solutions are the same. This phenomena is explained with the aid of Gibbs equation (Eq. (1)) which explains that the rapid fall in surface tension (with addition of 1 wt.% or less WPI) is indicative of rapid coverage (surface adsorption) of WPI at the solution–air interface. At 5 wt.% of WPI the so called ‘critical micelle
75
90
70
85 80
65
Surface tension 75
Tensile strength
60
70
55
65
50
60 55
45
Tensile strength (Pa)
absolute temperature in Kelvin. According to this equation for a non-surface active substance such as sugars, the solute gradient at the surface layer compared to interior are usually negative because pure sugars have no affinity for the surface. Hence, the decrease in surface tension of the lactose solutions in Fig. 6 can only be due to the presence of surface active contaminants in the sample. The lactose sample contained about 0.5 wt.% proteins (supplier’s specification) which could have caused this effect. The surface tension of 20 wt.% lactose fell to 58.4 mN/m compared to 70.2 mN/m of pure water. The ratio of these values is 0.83. Fig. 6 also presents the variation of tensile strength of the lactose solution with concentration. The variation in the tensile strength of these solutions is interesting. It was expected that the tensile strength of the lactose solution would increase with concentration following the behavior of the fructose and maltodextrin solutions described previously. However, the tensile strength decreased rather than increasing. Interestingly this decrease in the tensile strength clearly follows the pattern shown by the surface tension of this solution. The viscosity of the lactose solutions exhibited a different trend (Table 1). The viscosity of the solution increased with an increase in the concentration and at 20 wt.%, the ratio of viscosity of this solution to water was only 2.26. If the viscous force was dominant or controlling factor then the tensile strength of the drop should exhibit the increasing trend as shown by the viscosity. On the contrary, it showed a decreasing trend. This fact leads
3.4. Whey protein isolate (WPI) solutions
Surface tension (mN/m)
57
Tensile strength (Pa)
Surface tension (mN/m)
a
50 40
45
35
40 0
1
2
3
4
5
6
7
8
9
10
11
12
WPI concentration (%, w/w)
Fig. 7. Variation of surface tension (mN/m) and tensile strength (Pa) of WPI solution as a function of concentration, at 22.5 ± 0.5 °C.
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The sharp reduction of surface tension of water by 1 wt.% or less WPI shows that this protein behaves as a surfactant. The presence of this protein in a food system will lower the contact angle and facilitate surface wetting. Furthermore, the presence of this surfactant in food system will lower the energy required to break the drop from within and facilitate its breakage. This means that if one wishes to obtain a clean failure at the drop–equipment interface, presence of surface active elements or proteins will make it more difficult. However, this does not automatically lead to the conclusion that the proteins do not facilitate the (desired) adhesive mode of failure to occur when the cohesive strength of the drop is close enough to the adhesive strength at the drop–probe interface. Hence, further study is needed to evaluate the effect of surfactants at the vicinity of this critical tensile strength. Furthermore, quite different results may be obtained if a protein drop is subjected to elevated temperatures because the proteins tend to form a smooth skin having a glassy exterior. 3.5. Lactose and WPI mixtures
Surface tension (mN/m)
The effect of addition of WPI on surface tension and the cohesive strength of the lactose solution is presented in Fig. 8. The amount of solids in all the solutions was 5 wt.%. This figure shows that the addition of 20 wt.% WPI lowers the surface tension of the lactose solution quite considerably which was expected. The rate of the reduction of the surface tension varies quite considerably within 40 wt.% of WPI in the mixture. The rate of the surface tension lowering after 40 wt.% of WPI remains constant. This fact indicates that 20–40 wt.% addition of WPI in lactose already brings about the optimum lowering of surface tension. The effect of addition of WPI on the tensile strength of the droplet is also presented in Fig. 8. The pattern of lowering of the tensile strength bears the similarity to that of the surface tension. The rate of lowering of the tensile strength is quite high within 20% addition of WPI. This rate falls sharply and beyond 40 wt.%, it is constant. These facts indicate that the surfactants are not only capable of
75
85
70
80
65
75
Surface tension Tensile strength
60
70 55 65 50 60
45
Tensile strength (Pa)
concentration (CMC)’ has been achieved beyond which further increment in WPI concentration fails to lower the surface tension further. The viscosity of 1–5 wt.% WPI solutions is presented in Table 1. The test solutions were Newtonian in nature. From this table it can be seen that the viscosity increases consistently with concentration indicating increase in viscous forces. The tensile strength of the drops, as shown in Fig. 7, decreased with increase in the concentration of the WPI. The pattern of decrease in the tensile strength is similar to that of the decrease in surface tension. Firstly, the tensile strength decreases rapidly and secondly, the cohesive strength of 1 wt.% WPI is only 71.7% that of pure water. Further increase in concentration from 1 wt.% to 5 wt.% brings about a small 6.6% decrease in tensile strength. Furthermore, the tensile strength of the 5 wt.% and 10 wt.% WPI solutions are practically the same. These findings raise questions such as: How does the addition of the WPI lowers the cohesive strength or attractive strength between the water molecules? Why the increment in WPI concentration beyond 5 wt.% fails to lower the cohesive strength of water molecules further? The answers to these questions are difficult to find with certainty due to the fact that the mode of failure was cohesive in all the tests. Since the formation of the surface took place within the drop and that the energy required to create new surfaces was much lower than that for the water, the tensile strength reduction is entirely due to surface (tension) effect. The sharp lowering of the tensile strength (or cohesive strength) of the drop up to addition of the 1% WPI may be due to the fact that the WPI molecules not only help lower the free excess energy (surface tension) at the surface but also weaken the intramolecular cohesive strength of water due to weakening of hydrogen bonds. This argument is further supported due to the fact that the necking takes place for remarkably shorter time for the WPI solution compared to water indicating that the work required to create the new surfaces is reduced. In other words, the drop of WPI solution breaks much easily compared to that of water. Then, why does the tensile strength required to create the new surfaces remains the same when the concentration of WPI is increased from 5 wt.% to 10 wt.%? This may be due to the fact that at 5 wt.% concentration, an optimum distribution of the WPI unimer is achieved leading to complete surface coverage and uniform distribution within water matrix. Further increase of WPI concentration may lead to formation isolated pockets or icebergs of pure WPI. Formation of these icebergs may be the reason which prevents further lowering of cohesive strength of the water–WPI system (Holmberg, Jonsson, Kronberg, & Lindman, 2003). An observation of trends in viscosity (Table 1) and tensile strength (Fig. 7) of WPI solutions indicates that the increase in viscous forces has failed to counter balance the effect of surface forces under these test conditions. This means that the magnitude of viscosity of these WPI solutions is too low to make its effect felt.
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55
40 35
50 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
WPI fraction [-]
Fig. 8. Variation of surface tension (mN/m) and tensile strength of WPI/ lactose mixtures as a function of concentration, at 24.7 ± 0.3 °C. Total solid content 5% (w/w).
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lowering the surface tension and the cohesive strength of water, they are also capable of lowering the surface tension and cohesive strength of the carbohydrate solutions. Although the lactose sample used in this study contained some surfactants itself, however this will not alter the observations made and conclusions drawn from this study. From these two plots it can be seen that the surfactants are capable of lowering the excess surface energy and the cohesive strength of the bulk carbohydrate solutions. They will facilitate the wetting and spreading of carbohydrate solutions to equipment surfaces which will lead to cohesive form of stickiness. 3.6. Generalization of surface and viscous force effects The effect of surface forces and viscosity on stickiness of carbohydrate and protein solutions, discussed in Sections 3.1–3.3, 3.5 can be generalized as shown in Fig. 9. The quantity DTst/T st;H2 O shows how much the tensile strength of a solution diverges from that of a pure water. This quantity is positive, if the cohesive strength of the solution is stronger than that of pure water and vice versa. Similarly, Dc/cH2O indicates how much the surface tension of a solution diverges from that of pure water. If this quantity is positive, the addition of the solids increases the surface tension and vice versa. The line representing [(DTst/T st;H2 O )/(Dc/cH2O)] = 1 and passing through the origin indicates that the variation of the surface stickiness is of the extent of variation in the surface tension. If the slope > 1 the viscous forces contribute to the stickiness leading to the increase of cohesive strength. If the slope < 1 then the viscous forces fail to contribute to the surface stickiness leading to the decrease in the cohesive strength. As shown in Fig. 9, the data points for fructose drops lie close to the line with slope 1 up to 30 wt.% fructose concentration and then fall above the line at higher concentrations. This is expected because the viscous forces would become effective at higher fructose concentrations. In case of maltodextrin, the points are much further up towards the DTst/T st;H2 O axis indicating greater effect of viscosity. The data points for lactose, WPI and the mixtures of WPI/Lactose fall within the quadrant hav-
ing both the axes negative. All of these data points have slope < 1 indicating that the viscosity has completely failed to show its effect on stickiness. 3.7. Effect of temperature on surface tension of lactose and WPI solutions The effect of temperature on the surface tension of dilute lactose and WPI solutions are shown in Figs. 10 and 11, respectively. The variation of the surface tension of lactose with temperature is linear with one slope for each temperature. There is no leveling off of the values within this concentration range indicating that the surfactant concentration is less than critical micelle concentration (CMC). For WPI the rate of reduction of surface tension is quite strong for dilute solutions below 0.2 wt.% concentration. Beyond this point, the lowering of surface tension is linear (R2 P 0.9). The surface tension data at 50 °C for both the lactose and WPI has greater scatter due to relatively poor temperature control. The greatly reduced surface tension of carbohydrate and proteins solutions at high temperature lowers the contact
72
25 °C 40 °C
71
Surface tension (mN/m)
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70
30 °C 50 °C
69 68 67 66 65 64 63 0
0.2
0.4
0.6
0.8
1
Lactose concentration (%, w/w)
Fig. 10. Variation of surface tension of lactose solution as a function of temperature.
75 70
ΔSt/St H2O
0.1
0 -0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
-0.1
Surface tension (mN/m)
0.2
Fructose Lactose Maltodextrin WPI WPI/Lactose (0 to 1)
25 °C 40 °C
65
30 °C 50 °C
60 55 50 45
-0.2
40 -0.3
35 0
ΔY/YH2O
-0.4
Fig. 9. Generalization of effect of viscous and surface forces on stickiness.
0.2
0.4
0.6
0.8
1
WPI concentration (%, w/w)
Fig. 11. Variation of surface tension of WPI solution as a function of temperature.
B. Adhikari et al. / Journal of Food Engineering 79 (2007) 1136–1143
angle and enhances the wetting and spreading of these solutions on equipment surfaces. Furthermore, the cohesive strength of the protein and carbohydrate solutions is weaker at elevated temperatures due to greater mobility of their molecules. Similarly, at high temperatures, they have weaker viscous forces due to lower viscosity. All of these factors facilitate the spreading and the wetting further. All of these factors help to lower the energy required for the creation of new surface within a drop (breakage). Hence, cohesive form of stickiness will be dominant at elevated temperature leading to aggravated adhesion, fouling and stickiness. 4. Conclusions The variation of surface tension and viscosity of fructose, maltodextrin, lactose and WPI with concentration was measured in order to relate them to surface stickiness, where tensile strength was taken as a measure of stickiness. Stickiness of carbohydrate/WPI mixture solutions was also measured. In all the observations, the tensile strength correlated well with the surface tension. Viscosity alone proved to be a poor indicator of stickiness. Within the concentration and temperature range studied, surface forces rather than viscous forces appear to dominate stickiness. Acknowledgements The authors acknowledge the help of Mr. Kraileark Kittisuriyanont in carrying out the surface tension related experiments and to the Dairy Ingredients Group of Australia (DIGA) for partial funding of this study.
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References Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2003). In situ characterization of stickiness of sugar-rich foods using a linear actuator driven stickiness testing device. Journal of Food Engineering, 58(1), 11–22. Bhandari, B. R., Datta, N., & Howes, T. (1997). Problems associated with spray drying of sugar-rich foods. Drying Technology, 15(2), 671–684. Bhandari, B. R., & Howes, T. (2005). Relating the stickiness property of foods undergoing drying and dried products to their surface energetics. Drying Technology, 23(4), 781–797. Holmberg, K., Jonsson, B., Kronberg, B., & Lindman, B. (2003). Surfactants and polymers in aqueous solutions. New York: John Wiley and Sons, Ltd. International Critical Tables, (1930). International critical tables of numerical data, physics, chemistry and technology. Edward Washburn, ed. National Research Council, USA. Kudra, T. (2003). Sticky region in drying-definition and identification. Drying Technology, 21(8), 1457–1469. Langrish, T. A. G., & Fletcher, D. F. (2003). Prospects for the modelling and design of spray dryers in the 21st century. Drying Technology, 21(2), 197–215. Masters, K., (1996). Deposit-free spray drying: Dream or reality? In Drying, 96—Proceedings of the 10th International Drying Symposium (IDS’96), 30 July–2 August, Krakow, Poland, Vol. 1, pp. 52–60. Michalsky, M. C., Desobry, S., Babak, V., & Hardy, J. (1999). Adhesion of food emulsions to packaging and equipment surfaces. Colloid Sand Surfaces (A. Physiochemical and Engineering Aspects), 149, 107–121. Michalsky, M. C., Desobry, S., & Hardy, J. (1998). Adhesion of edible oils and food emulsions to rough surfaces. Food Science and TechnologyLebensmittel-Wissenschaft & Technologie, 3(5), 495–502. Rahman, S. (1995). Food Properties Handbook. CRC Press. Tsujii, K. (1998). Surface activity: Principles, phenomena, and applications. Tokyo: Academic Press. Wilson, D. I. (2005). Challenges in cleaning: Recent developments and future prospects. Heat Transfer Engineering, 26(1), 54–59. Xin, H., Chen, X. D., & Ozkan, N. (2005). Removal of model foulant from metal surfaces. AIChE Journal, 5(8), 1961–1973.
Effect of surface tension and viscosity on the surface stickiness of carbohydrate and protein solutions B. Adhikari b
a,*
, T. Howes a, A. Shrestha b, B.R. Bhandari
b
a School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australia School of Land and Food Sciences, The University of Queensland, Brisbane, Qld 4072, Australia
Received 28 September 2005; accepted 6 April 2006 Available online 19 April 2006
Summary The effects of surface tension and viscosity on the measured tensile strength (surface stickiness) of carbohydrate (fructose, lactose and maltodextrin) and protein (Whey protein isolate (WPI)) solutions were studied. The effect of the addition of WPI on the surface tension and surface stickiness of lactose solutions was measured. Surface tension was found to better correlate with surface stickiness compared to viscosity. Cohesive failure occurred in all the cases indicating that the energy required to create new surfaces within the drops was lower than the energy required to achieve a clean (adhesive) failure at the probe–drop interface. WPI behaves as a surfactant and sharply lowers both the surface tension and tensile strength. This study shows that the surface forces are more dominant than the rheological forces when the energy required to create new surface within the drop is less than the adhesive energy at the drop–probe interface. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Stickiness; Proteins; Carbohydrates; Surface tension; Viscosity; Cohesive strength
1. Introduction Adhesion of liquid foods and food emulsions to processing equipment, packages and the resultant residue causes significant economic loss (Michalsky, Desobry, Babak, & Hardy, 1999). Fouling and scale formation in processing equipment are also known as ubiquitous problems (Wilson, 2005; Xin, Chen, & Ozkan, 2005). Losses include material loss, cost of cleaning the equipment and disposal and treatment. In spray drying operations, atomized droplets can reach the dryer walls and rooftop and adhere there causing product loss, product quality degradation and fire hazards (Bhandari, Datta, & Howes, 1997). Michalsky and coworkers (Michalsky, Desobry, & Hardy, 1998, 1999) studied the adhesive behavior of edible oil and food emulsions to glass, Teflon (PTFE), low density polyethylene (LDPE), poly ethylene terephthalate (PET)
*
Corresponding author. Tel.: +61 7 33659058; fax: +61 7 33654199. E-mail address: [email protected] (B. Adhikari).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.04.002
and stainless steel. They allowed the food samples to flow down an inclined substrate surface and measured the amount of remaining sample on the surface after the flow ceased. It was found that surface roughness, the yield stress of the sample and solid surface tension were the key factors responsible for adhesion. Recently, Bhandari and Howes (2005) reviewed the importance of surface energetics of the food materials and surface of contact on the stickiness property. Kudra (2003) suggested that pasty materials exhibit stickiness across a region of temperature and moisture (sticky region) while being converted from solution to powders. The tendency of droplets and soft particles to stick to the dryer walls during spray drying is a major problem encountered in the powder industries (Langrish & Fletcher, 2003; Masters, 1996). Adhikari, Howes, Bhandari, and Truong (2003) studied the stickiness of drying droplets, in situ, using a probe tack testing instrument. They reported that to attain a state of non-adhesion at the drop–probe interface it is essential that the cohesive strength of the droplet be greater than the adhesive
B. Adhikari et al. / Journal of Food Engineering 79 (2007) 1136–1143
strength at the droplet–probe interface. They found that a state of non-adhesion was reached when the outer layer of the droplet was transformed into a glassy matrix. It was suggested that the non-sticky state could also be reached if the viscosity is raised high enough to prevent failure within the droplet itself. The studies described above have suggested that both the surface forces (surface tension related) and bulk forces (viscosity related) are responsible for food adhesion. However, the question of which of these forces dominate the adhesion (stickiness) process has not yet been answered. Hence, this paper aims at measuring the surface tension, viscosity and tensile strength of carbohydrate and protein solutions in order to relate them to adhesion (surface stickiness) on a stainless steel surface. 2. Materials and methods 2.1. Materials Spray dried lactose powder (Murray Goulburn Co-Ltd., Australia), whey protein isolate, (WPI, ALATAL 817TM, New Zealand Milk Powder, New Zealand), fructose (ADM Corn Processing, USA) and maltodextrin of dextrose equivalent 6 (Roquette Freres, France) were used without further purification. They were vacuum dried (70 °C 500 mbar) overnight and stored in desiccators over P2O5 prior to solution preparation. The carbohydrates (fructose, lactose and maltodextrin) and protein (WPI) were chosen because WPI is a known surface active material while carbohydrates are not. Both low (fructose and lactose) and high (maltodextrin) molecular weight carbohydrates are chosen to study the effect of viscosity on stickiness. Furthermore, these materials are widely used in food processing. 2.2. Methods 2.2.1. Surface tension This property indicates how strongly the surface molecules of a liquid/solution are attracted by the adjacent molecules and is a factor affecting stickiness. Solution surface tensions were determined using a tensiometer (ST9000, Nima Technology, UK) which operated using the Wilhelmy plate principle. Paper plates of 21 mm in perimeter were used in all the cases. Sample temperatures were recorded before and after each test. Surface tension was also determined using a custom fabricated load cell device. The surface tension values obtained by both the devices were used to cross check the values obtained from each other. Three replicates were taken and mean values are reported. 2.2.2. Viscosity This parameter gives information regarding the resistance offered by liquid/solution molecules to the motion and is a factor influencing stickiness. A Rheoscope 1
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Thermo-Electron Corporation, (HAAKE) with parallel plate (plate diameter = 6 cm) was used to determine the viscosity. The shear stress versus shear rate data was recorded and the viscosity was determined from the slope of the stress–strain curves. 2.2.3. Tensile strength This property was measured using an in situ stickiness testing instrument. This instrument works on the principle of tack, that is, it mimics the feel when one touches a drop surface. Details of this device and test protocols are given elsewhere (Adhikari et al., 2003). The schematic diagram (Fig. 1) and the test procedure are briefly presented as follows. The shaft of the linear actuator holds the drop-holder and the drop on the tip of its shaft. The stainless steel probe (2.5 mm in diameter) is attached to microbalance (±0.01 mg). A digital video camera and stereomicroscope (50 times magnification) imaging system monitors the approach, withdrawal and contact of the probe to the drop surface. During the experiment, the stepper motor is driven until the probe makes good contact with the drop surface. Once contact is established, the motor is subsequently withdrawn. The variation in the tensile force over time is recorded continuously to a personal computer (PC). Digital images during the approach, contact and withdrawal events are also recorded. The temperature of the drop is recorded by inserting micro-thermocouples (T-type, Omega Engineering USA) at the drop centre. 3. Results and discussion 3.1. Fructose solution The variation of surface tension of fructose solution with concentration at 24 ± 0.3 °C is given in Fig. 2. This figure includes the values obtained from the tensiometer as well as the load cell. Although there is a difference between the mean values of surface tension obtained from both devices, the difference is within experimental error. The close agreement in results obtained by both the devices indicates that the method applied was reliable. Furthermore, the measured surface tension value of water was 70.2 mN/m which agrees well with the literature value of 70.52 mN/m (extrapolated to 24 °C) (Rahman, 1995). The small discrepancy might be due to trace amounts of impurities in the water or variation of temperature during the tests. However, this discrepancy is also within the experimental error. It can be seen that the surface tension of the fructose solution increases with solid concentration. The surface tension value of 60 wt.% fructose solution is 75.33 mN/m and the difference compared to pure water (Dc) is 5.13 mN/m. Since surface tension data of fructose as a function of concentration are not available in literature, however Dc for 55 wt.% sucrose for 15–40 °C range is reported to be 3.7 ± 0.5 mN/m (International Critical Tables, 1930). Since sugars are known to be non-surface active, the increase in surface tension of fructose is in
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Monitor
to balance
video recorder
m ca
Sm
er
a
Ws P D Dh
Gs La
Ls Fig. 1. Schematic diagram of a stickiness testing device. (P): stainless steel probe, (D): drop, (Dh): drop holder, (Gs): guide shaft with peripherals for linear motion, (La): linear actuator, (Ls): lead screw, (Ws): glass working section, (Sm): stereo-microscope system with video camera.
76
Tensiometer Load cell
76
75
Surface tension (mN/m)
75 74 73 72 71 70 69 0
10
20
98
Surface tension Tensile strength
30
40
50
60
70
96 94
74 92 73
90
72
88 86
71
Tensile strength (Pa)
Surface tension (mN/m)
77
84 70
82
Fructose concentration (%, w/w) 69
Fig. 2. Variation of surface tension (mN/m) of fructose solution as a function of concentration at 24 ± 0.3 °C.
accordance with the Gibbs equation (Eq. 1) which indicates that the method used is reliable. Fig. 3 presents the variation of tensile strength (Pa) of fructose drops with concentration. This figure also includes the variation of surface tension with concentration for comparison. The tensile strength of fructose solution increases with concentration which indicates that the fructose molecule has affinity for water molecule. Since the cohesive strength of the water molecule is mainly due to hydrogen bonds (Tsujii, 1998) higher tensile strength of fructose solution compared to water indicates possible
80 0
10
20
30
40
50
60
70
Fructose concentration (%, w/w)
Fig. 3. Variation of surface tension (mN/m) and tensile strength (Pa) of fructose solution as a function of concentration, at 24 ± 0.3 °C and 20.5 ± 0.5 °C, respectively.
increase in number of hydrogen bonds in the solution. The tensile strength of pure water droplets is 87.02 ± 0.58 Pa while that of the 60% w/w fructose is 95.25 ± 0.12 Pa. The difference in tensile strength compare to pure water (DTst) is 8.23 Pa. The ratio of the tensile strength of 60 wt.% fructose to water is 1.09 which is close to the surface tension ratio of this solution to water which is 1.07.
B. Adhikari et al. / Journal of Food Engineering 79 (2007) 1136–1143
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Fig. 4. Cohesive failure with creation of new surface within the drop. (a) Approach, (b) contact, (c) withdrawal and (d) complete withdrawal.
Fig. 4 shows the mode of failure of fructose drop during the withdrawal of the probe. It can be seen that the drop undergoes necking first and the subsequent breakage or failure takes place within the droplet itself leading to the formation of two new surfaces. A considerable amount of fructose solution remains adhered to the initially clean probe. This signifies that the cohesive strength of the 60 wt.% fructose drop is still smaller compared to the adhesive strength at the (stainless steel) probe–drop interface. 3.2. Maltodextrin solutions A similar trend is observed when the surface tension and tensile strength of maltodextrin solutions were determined (Fig. 5). This figure shows that the surface tension increased with concentration and Dc at 40 wt.% was 2.34 mN/m. The relationship between surface tension and concentration indicates that maltodextrin is not surface active. However, the surface tension value of 40 wt.% maltodextrin was less than the fructose solution of the same concentration by 0.1 mN/m. Since this difference is within the measurement error, it can be said that the surface tension values of fructose and maltodextrin solutions are very close to each other. The tensile strength of the maltodextrin solution increased with concentration. DTst value for 40 wt.% solution was 7.05 Pa which is slightly higher than that of the fructose solution (DTst = 3.7 Pa) of the same concentration. The ratio of surface tension and tensile
75
95
Surface tension Tensile strength
93 91
73
89 87
72
85
71
83
Tensile strength (Pa)
Surface tension (mN/m)
74
strength of this solution to water are 1.03 and 1.08, respectively. As with fructose, the increment of these two parameters is similar. The probe tack test showed that the mode of failure for these solutions was cohesive indicating that the energy required to create a new surface within the drop was lower compared to adhesive energy at the at probe– drop interface. It explains our daily experience that if a solution comes in contact with an equipment surface, it easily adheres to it and can not be removed easily without leaving substantial amount of residue. The viscosity of carbohydrate and protein solutions is shown in Table 1. Compared to water the viscosity of a 40 wt.% maltodextrin solution has increased by a factor of 170. This fact shows that, the degree of variation of viscosity of maltodextrin solutions is much greater compared to the variation of tensile strength and surface tension within this range. Furthermore, the mode of failure was cohesive and that the necking and stretching during the process of new surface formation were not different compared fructose and lactose solutions of the same concentration. Hence, in the case of a drop that fails cohesively (cohesive energy is lower than the energy at the interface) and that the formation of the new surface takes place within the drop itself, the bulk parameter (i.e., the viscosity) is unlikely be the dominant or major factor that controls the surface stickiness. 3.3. Lactose solutions The surface tension and tensile strength of 5, 10, 15 and 20% w/w lactose solutions are presented in Fig. 6. Both the surface tension and the tensile strength of the solution decreased rather than increased with an increase in concentration. Being a sugar and a carbohydrate, it was expected that the surface tension would increase with concentration. It is reported that Dc for a 10 wt.% lactose solution is 0.9 ± 0.4 mN/m at 15 °C (International Critical Tables, 1930). The relationship between the surface tension, concentration and adsorption of a solute (in a dilute solution) is given by Gibbs’ equation (Eq. (1)):
70 81 69
79 0
5
10
15
20
25
30
35
40
45
Maltodextrin concentration (%, w/w)
Fig. 5. Variation of surface tension (mN/m) and tensile strength (Pa) of maltodextrin solution as a function of concentration, at 24.7 ± 0.3 °C.
dðcÞ ¼ RT Cdðln CÞ
ð1Þ
where, c, C and C are surface tension (mN/m), amount of solute adsorbed (g moles/m2), concentration of the solute (g moles/m3) in solution, respectively. Similarly, R is the gas constant (g cm2 s2 g mole1 K1) and T is the
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Table 1 Viscosity of maltodextrin (DE 6), lactose and WPI solutions at various concentrationsa Maltodextrin (DE 6)
Lactose
Whey protein isolate (WPI)
Concentration (wt.%)
Viscosity (m Pa s)
Concentration (wt.%)
Viscosity (m Pa s)
Concentration (wt.%)
Viscosity (m Pa s)
0 10 20 30 40
0.80 ± 0.1 2.10 ± 0.00 7.63 ± 0.19 30.47 ± 0.40 137.07 ± 1.15
0 5 10 15 20
0.80 ± 0.10 1.01 ± 0.01 1.24 ± 0.04 1.43 ± 0.05 1.81 ± 0.05
1 2 3 4 5
1.00 ± 0.00 1.13 ± 0.05 1.33 ± 0.05 1.60 ± 0.00 2.03 ± 0.11
Mean values ± standard deviations of triplicate samples (at 25 ± 0.2 °C).
73
88
71
86
69
84
67
82
65
80
63
78
61
76
59
74
Surface tension
72
Tensile strength
55
70 0
5
10
15
to the conclusion that the surface forces rather than the viscous forces are dominant when adhesion or surface stickiness of droplet/soft particle is concerned. It has to be stressed here that, for surface forces to be dominant over viscous forces, a key criterion has to be met which is: The cohesive strength of the drop should be weak enough to allow the formation of new surface. The surface stickiness of a droplet in which cohesive failure is the dominant mode of failure, is the worst case of the stickiness scenario observed in practice (Adhikari et al., 2003). Surface tension appears to be a good indicator for this particular mode of stickiness.
20
Lactose concentration (%, w/w)
Fig. 6. Variation of surface tension (mN/m) and tensile strength (Pa) of lactose solution as a function of concentration, at 23.5 ± 0.5 °C.
Fig. 7 presents the variation of surface tension of WPI with concentration. It can be seen from this figure that the surface tension of water is reduced by 62.7% with addition of 1 wt.% WPI. Beyond this point, the fall in surface tension with increase in WPI concentration from 1 wt.% to 5 wt.% is only 6.7%. The surface tension values of 5 wt.% and 10 wt.% solutions are the same. This phenomena is explained with the aid of Gibbs equation (Eq. (1)) which explains that the rapid fall in surface tension (with addition of 1 wt.% or less WPI) is indicative of rapid coverage (surface adsorption) of WPI at the solution–air interface. At 5 wt.% of WPI the so called ‘critical micelle
75
90
70
85 80
65
Surface tension 75
Tensile strength
60
70
55
65
50
60 55
45
Tensile strength (Pa)
absolute temperature in Kelvin. According to this equation for a non-surface active substance such as sugars, the solute gradient at the surface layer compared to interior are usually negative because pure sugars have no affinity for the surface. Hence, the decrease in surface tension of the lactose solutions in Fig. 6 can only be due to the presence of surface active contaminants in the sample. The lactose sample contained about 0.5 wt.% proteins (supplier’s specification) which could have caused this effect. The surface tension of 20 wt.% lactose fell to 58.4 mN/m compared to 70.2 mN/m of pure water. The ratio of these values is 0.83. Fig. 6 also presents the variation of tensile strength of the lactose solution with concentration. The variation in the tensile strength of these solutions is interesting. It was expected that the tensile strength of the lactose solution would increase with concentration following the behavior of the fructose and maltodextrin solutions described previously. However, the tensile strength decreased rather than increasing. Interestingly this decrease in the tensile strength clearly follows the pattern shown by the surface tension of this solution. The viscosity of the lactose solutions exhibited a different trend (Table 1). The viscosity of the solution increased with an increase in the concentration and at 20 wt.%, the ratio of viscosity of this solution to water was only 2.26. If the viscous force was dominant or controlling factor then the tensile strength of the drop should exhibit the increasing trend as shown by the viscosity. On the contrary, it showed a decreasing trend. This fact leads
3.4. Whey protein isolate (WPI) solutions
Surface tension (mN/m)
57
Tensile strength (Pa)
Surface tension (mN/m)
a
50 40
45
35
40 0
1
2
3
4
5
6
7
8
9
10
11
12
WPI concentration (%, w/w)
Fig. 7. Variation of surface tension (mN/m) and tensile strength (Pa) of WPI solution as a function of concentration, at 22.5 ± 0.5 °C.
B. Adhikari et al. / Journal of Food Engineering 79 (2007) 1136–1143
The sharp reduction of surface tension of water by 1 wt.% or less WPI shows that this protein behaves as a surfactant. The presence of this protein in a food system will lower the contact angle and facilitate surface wetting. Furthermore, the presence of this surfactant in food system will lower the energy required to break the drop from within and facilitate its breakage. This means that if one wishes to obtain a clean failure at the drop–equipment interface, presence of surface active elements or proteins will make it more difficult. However, this does not automatically lead to the conclusion that the proteins do not facilitate the (desired) adhesive mode of failure to occur when the cohesive strength of the drop is close enough to the adhesive strength at the drop–probe interface. Hence, further study is needed to evaluate the effect of surfactants at the vicinity of this critical tensile strength. Furthermore, quite different results may be obtained if a protein drop is subjected to elevated temperatures because the proteins tend to form a smooth skin having a glassy exterior. 3.5. Lactose and WPI mixtures
Surface tension (mN/m)
The effect of addition of WPI on surface tension and the cohesive strength of the lactose solution is presented in Fig. 8. The amount of solids in all the solutions was 5 wt.%. This figure shows that the addition of 20 wt.% WPI lowers the surface tension of the lactose solution quite considerably which was expected. The rate of the reduction of the surface tension varies quite considerably within 40 wt.% of WPI in the mixture. The rate of the surface tension lowering after 40 wt.% of WPI remains constant. This fact indicates that 20–40 wt.% addition of WPI in lactose already brings about the optimum lowering of surface tension. The effect of addition of WPI on the tensile strength of the droplet is also presented in Fig. 8. The pattern of lowering of the tensile strength bears the similarity to that of the surface tension. The rate of lowering of the tensile strength is quite high within 20% addition of WPI. This rate falls sharply and beyond 40 wt.%, it is constant. These facts indicate that the surfactants are not only capable of
75
85
70
80
65
75
Surface tension Tensile strength
60
70 55 65 50 60
45
Tensile strength (Pa)
concentration (CMC)’ has been achieved beyond which further increment in WPI concentration fails to lower the surface tension further. The viscosity of 1–5 wt.% WPI solutions is presented in Table 1. The test solutions were Newtonian in nature. From this table it can be seen that the viscosity increases consistently with concentration indicating increase in viscous forces. The tensile strength of the drops, as shown in Fig. 7, decreased with increase in the concentration of the WPI. The pattern of decrease in the tensile strength is similar to that of the decrease in surface tension. Firstly, the tensile strength decreases rapidly and secondly, the cohesive strength of 1 wt.% WPI is only 71.7% that of pure water. Further increase in concentration from 1 wt.% to 5 wt.% brings about a small 6.6% decrease in tensile strength. Furthermore, the tensile strength of the 5 wt.% and 10 wt.% WPI solutions are practically the same. These findings raise questions such as: How does the addition of the WPI lowers the cohesive strength or attractive strength between the water molecules? Why the increment in WPI concentration beyond 5 wt.% fails to lower the cohesive strength of water molecules further? The answers to these questions are difficult to find with certainty due to the fact that the mode of failure was cohesive in all the tests. Since the formation of the surface took place within the drop and that the energy required to create new surfaces was much lower than that for the water, the tensile strength reduction is entirely due to surface (tension) effect. The sharp lowering of the tensile strength (or cohesive strength) of the drop up to addition of the 1% WPI may be due to the fact that the WPI molecules not only help lower the free excess energy (surface tension) at the surface but also weaken the intramolecular cohesive strength of water due to weakening of hydrogen bonds. This argument is further supported due to the fact that the necking takes place for remarkably shorter time for the WPI solution compared to water indicating that the work required to create the new surfaces is reduced. In other words, the drop of WPI solution breaks much easily compared to that of water. Then, why does the tensile strength required to create the new surfaces remains the same when the concentration of WPI is increased from 5 wt.% to 10 wt.%? This may be due to the fact that at 5 wt.% concentration, an optimum distribution of the WPI unimer is achieved leading to complete surface coverage and uniform distribution within water matrix. Further increase of WPI concentration may lead to formation isolated pockets or icebergs of pure WPI. Formation of these icebergs may be the reason which prevents further lowering of cohesive strength of the water–WPI system (Holmberg, Jonsson, Kronberg, & Lindman, 2003). An observation of trends in viscosity (Table 1) and tensile strength (Fig. 7) of WPI solutions indicates that the increase in viscous forces has failed to counter balance the effect of surface forces under these test conditions. This means that the magnitude of viscosity of these WPI solutions is too low to make its effect felt.
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55
40 35
50 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
WPI fraction [-]
Fig. 8. Variation of surface tension (mN/m) and tensile strength of WPI/ lactose mixtures as a function of concentration, at 24.7 ± 0.3 °C. Total solid content 5% (w/w).
B. Adhikari et al. / Journal of Food Engineering 79 (2007) 1136–1143
lowering the surface tension and the cohesive strength of water, they are also capable of lowering the surface tension and cohesive strength of the carbohydrate solutions. Although the lactose sample used in this study contained some surfactants itself, however this will not alter the observations made and conclusions drawn from this study. From these two plots it can be seen that the surfactants are capable of lowering the excess surface energy and the cohesive strength of the bulk carbohydrate solutions. They will facilitate the wetting and spreading of carbohydrate solutions to equipment surfaces which will lead to cohesive form of stickiness. 3.6. Generalization of surface and viscous force effects The effect of surface forces and viscosity on stickiness of carbohydrate and protein solutions, discussed in Sections 3.1–3.3, 3.5 can be generalized as shown in Fig. 9. The quantity DTst/T st;H2 O shows how much the tensile strength of a solution diverges from that of a pure water. This quantity is positive, if the cohesive strength of the solution is stronger than that of pure water and vice versa. Similarly, Dc/cH2O indicates how much the surface tension of a solution diverges from that of pure water. If this quantity is positive, the addition of the solids increases the surface tension and vice versa. The line representing [(DTst/T st;H2 O )/(Dc/cH2O)] = 1 and passing through the origin indicates that the variation of the surface stickiness is of the extent of variation in the surface tension. If the slope > 1 the viscous forces contribute to the stickiness leading to the increase of cohesive strength. If the slope < 1 then the viscous forces fail to contribute to the surface stickiness leading to the decrease in the cohesive strength. As shown in Fig. 9, the data points for fructose drops lie close to the line with slope 1 up to 30 wt.% fructose concentration and then fall above the line at higher concentrations. This is expected because the viscous forces would become effective at higher fructose concentrations. In case of maltodextrin, the points are much further up towards the DTst/T st;H2 O axis indicating greater effect of viscosity. The data points for lactose, WPI and the mixtures of WPI/Lactose fall within the quadrant hav-
ing both the axes negative. All of these data points have slope < 1 indicating that the viscosity has completely failed to show its effect on stickiness. 3.7. Effect of temperature on surface tension of lactose and WPI solutions The effect of temperature on the surface tension of dilute lactose and WPI solutions are shown in Figs. 10 and 11, respectively. The variation of the surface tension of lactose with temperature is linear with one slope for each temperature. There is no leveling off of the values within this concentration range indicating that the surfactant concentration is less than critical micelle concentration (CMC). For WPI the rate of reduction of surface tension is quite strong for dilute solutions below 0.2 wt.% concentration. Beyond this point, the lowering of surface tension is linear (R2 P 0.9). The surface tension data at 50 °C for both the lactose and WPI has greater scatter due to relatively poor temperature control. The greatly reduced surface tension of carbohydrate and proteins solutions at high temperature lowers the contact
72
25 °C 40 °C
71
Surface tension (mN/m)
1142
70
30 °C 50 °C
69 68 67 66 65 64 63 0
0.2
0.4
0.6
0.8
1
Lactose concentration (%, w/w)
Fig. 10. Variation of surface tension of lactose solution as a function of temperature.
75 70
ΔSt/St H2O
0.1
0 -0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
-0.1
Surface tension (mN/m)
0.2
Fructose Lactose Maltodextrin WPI WPI/Lactose (0 to 1)
25 °C 40 °C
65
30 °C 50 °C
60 55 50 45
-0.2
40 -0.3
35 0
ΔY/YH2O
-0.4
Fig. 9. Generalization of effect of viscous and surface forces on stickiness.
0.2
0.4
0.6
0.8
1
WPI concentration (%, w/w)
Fig. 11. Variation of surface tension of WPI solution as a function of temperature.
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angle and enhances the wetting and spreading of these solutions on equipment surfaces. Furthermore, the cohesive strength of the protein and carbohydrate solutions is weaker at elevated temperatures due to greater mobility of their molecules. Similarly, at high temperatures, they have weaker viscous forces due to lower viscosity. All of these factors facilitate the spreading and the wetting further. All of these factors help to lower the energy required for the creation of new surface within a drop (breakage). Hence, cohesive form of stickiness will be dominant at elevated temperature leading to aggravated adhesion, fouling and stickiness. 4. Conclusions The variation of surface tension and viscosity of fructose, maltodextrin, lactose and WPI with concentration was measured in order to relate them to surface stickiness, where tensile strength was taken as a measure of stickiness. Stickiness of carbohydrate/WPI mixture solutions was also measured. In all the observations, the tensile strength correlated well with the surface tension. Viscosity alone proved to be a poor indicator of stickiness. Within the concentration and temperature range studied, surface forces rather than viscous forces appear to dominate stickiness. Acknowledgements The authors acknowledge the help of Mr. Kraileark Kittisuriyanont in carrying out the surface tension related experiments and to the Dairy Ingredients Group of Australia (DIGA) for partial funding of this study.
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