Underlying mechanisms behind adhesion of fermented milk to packaging surfaces

Journal of Food Engineering 130 (2014) 52–59

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Underlying mechanisms behind adhesion of fermented milk to packaging surfaces Carolina Cragnell a,⇑, Kristina Hansson b, Thorbjörn Andersson b, Bengt Jönsson c, Marie Skepö a,* a

Division of Theoretical Chemistry, Lund University, Lund, Sweden Material Design, Tetra Pak Packaging Solutions AB, Lund, Sweden c Division of Biophysical Chemistry, Lund University, Lund, Sweden b

a r t i c l e

i n f o

Article history: Received 28 August 2013 Received in revised form 22 January 2014 Accepted 26 January 2014 Available online 3 February 2014 Keywords: Interaction Packaging surface Filmjölk Phase separation Dewatering Adhesion

a b s t r a c t Fermented milk is commonly eaten during breakfast or as a snack between meals either in one-portion cups or from a bowl with a spoon. Approximately 5–10% of the fermented milk remains in the packages upon pouring, which during the last years, has received attention in public media as newspapers and television. This is a problem, not only from an economical point of view, but also from an ecological and unethical perspective. In this study, we have investigated the influence of rheology, surface characteristics, and storage time; on product adhesion, and this study is a continuation of an earlier study published in this journal. The physical properties of the surface as roughness and morphology were determined by Scanning Electron Microscopy, Interferometry, contact angle measurements, and Fourier Transform Infrared/Attenuated Total Reflectance Spectroscopy. After incubation, the adhered amount was determined by gravimetrical studies. The results have shown that there are two mechanisms over different time scales that predominate. Initially (seconds–minutes) the excess product is drained of the surfaces until the gravity is opposed by the rheological strength such as the yield stress and the apparent viscosity. The second mechanism occurs on a longer time scale (minutes–hours) and it includes dewatering of the product residue at the surface. Concluded in this work is that the rheological properties of fermented milk is of major importance and that the problem with fermented milk remaining in the packages could be regarded as a flow and dewatering problem rather than an adsorption problem. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Adhesion of food and food residues in packages are a problem not only from an economical point of view (Michalski et al., 1999; Saikhwan et al., 2006; Santos et al., 2004), but also from an ethical and an ecological perspective. Widely used complex fluid foods, such as emulsions, cause this problem because they contain fat, as well as hydrocolloids as stabilizers, that are likely to adsorb to surfaces. The definition of adhesion is that two materials are sticking together, and the forces that cause adhesion can be divided into several categories for example chemical adhesion, dispersive adhesion, and diffusive adhesion. The surface tension of the liquid is known to play a key role in wetting, in ⇑ Corresponding authors. Tel.: +46 46 222 33 66; fax: +46 70 962 88 22. E-mail addresses: [email protected] (C. Cragnell), marie.skepo@ teokem.lu.se (M. Skepö). http://dx.doi.org/10.1016/j.jfoodeng.2014.01.021 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved.

general, the lower the surface tension the better the liquid will wet a surface. Fermented milk products, also known as cultured dairy foods, cultured dairy products, or cultured milk products, are dairy foods that have been fermented with lactic acid bacteria such as Lactobacillus, Lactococcus, and Leuconostoc, and there are evidence that fermented milk products have been produced since around 10,000 BC. Products as yoghurt, sour milk, and sour cream, based on bovine milk, are very popular products in Northern Europe. In comparison with ordinary milk, many people believe that these products have a more appealing taste and firmness, but it is also easier for the body to digest. Fermented milk products belong to the category fat in water emulsions and they have a thixotropic, pseudo and bingham plastic character (Walstra and Jenness, 1990; Hellinga et al., 1986). The latter means that it is a shear thinning system where the viscosity decreases not only with the shear rate but also with time at constant shear rate. In addition,

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fermented milk exhibits yield stress. This type of flow is typically shown by gel-forming systems. Fermented milk contains approximately 87% water and 13% solids, and it contains both hydrophilic and hydrophobic components such as proteins and triglycerides. The production of fermented milk products is increasing on a yearly basis and today approximately 260 tonnes are produced in Sweden only. Unfortunately, approximately 10% of the product remains in the one-litre family package due to adhesion to the inner package surface. The rheology of the product as well as the solid surface free energy of the packaging material is known to affect the adhered amount (Michalsky et al., 1998; Adhikari et al., 2007; Schmidt et al., 2012a; Loibl et al., 2012). Regarding adhesion of fermented milk products, several different mechanisms have been suggested, but still no general theory has been presented: to the authors knowledge all presented results in the literature are system dependent (Schultz and Nardin, 2003). In these studies, thermodynamic and surface roughness based models are the most frequently used (Saikhwan et al., 2006; Michalsky et al., 1998; Bobe et al., 2007; Michalski et al., 1997; Karbowiak et al., 2007; Keijbets et al., 2009), and it has also been observed that the rheological properties of the product are of relevance for the degree of adherence (Michalski et al., 1998; Adhikari et al., 2007; Adhikari et al., 2003; Schmidt et al., 2012b). Hansson et al. (2012), have studied adhesion of fermented milk to materials having different surface properties, such as polarity and relative oxidation. The materials were incubated in fermented milk and other dairy products varying in fat concentration between a few minutes to 1 week. The later time slot was chosen due to the fact that the product is stored in the package for approximately 5– 7 days before purchased by the consumer. In this investigation it was concluded that the initial adhesion (the first 24 h) is depending on material and fat content, whereas on longer time scales (days), the effect was levelled off. It was also shown that the frequently used polyethylene packaging material inner surface seems to be the best option as well as glass. Due to the fact that 87% of the product is water and the product is a fluid, the question is if it is possible to minimize the adhesion by lowering the surface free energy and thereby increasing the dewatering velocity (promoting dewetting) or by inducing a viscosity gradient? Here we present a thorough characterization of the frequently used low density polyethylene package surface (Ref), three potential fermented milk packaging surfaces - two low surface free energy surfaces (Variants 1 and 2) and a biocoated surface (Variant 3), along with a gravimetrical study evaluating the effect of storage time, surface characteristics, and rheology, on the product residue.

2. Materials and methods Initially a thorough characterization of the four surfaces was done by employing techniques such as SEM, Interferometry, Fourier Transform Infrared/Attenuated Total Reflectance Spectroscopy (FT IR/ATR), and contact angle measurements (dynamic and static). The effects of storage time, surface characteristics, and rheology on the product residue were determined gravimetrically. In addition, in order to determine if surface induced phase separation occurred at the surface–product interface during incubation and due to the fact that the consumer is encouraged to shake the packages with product prior pouring, two parallel investigations were conducted, shaken and not. In the shaking procedure the beakers with product were shaken prior sample withdrawal and weighing. Moreover, surfaces incubated in fermented milk were evaluated by using Fourier Transform Infrared/Attenuated Total Reflectance Spectroscopy, FT IR/ATR, and contact angle measurements in order to determine whether the surfaces were changed or not during storage time.

2.1. Surface morphology The surfaces were examined with a JEOL JSM-6700F FEG SEM, in lens detector, acceleration voltage 10 kV. Prior investigation the surfaces were sputtered with Au/Pd for 150 s (Ref) and 120 s (Variants 1–3) respectively, using a Cressington Sputter Coater 108 Auto, giving an Au/Pd deposited layer of 13 nm. The samples were tilted 19° during investigation. Five samples of each surface were randomly chosen for investigation. The surfaces were examined at a magnification of 100, 1000, and 50,000. The most representative (Ref) is shown in the Results section. 2.2. Surface roughness The roughness of the surfaces were determined by MicroXam; ADE Phase Shift, Inc. (Tucson, AZ, USA) interferometer. Three samples of each surface type and three positions on each sample were investigated. The scan height was set to 100 lm and a Gaussian 50 lm  50 lm filter was utilized. The measurement area was 200 lm  250 lm. The following roughness parameters were determined: Sa the arithmetic deviation relative the surface mean plane. Sdr the enlargement induced by the roughness relative a plane surface. Ssk the skewness of the height distribution. Sci the fluid retention parameter. 2.3. Surface free energy The surface free energy, including polar and dispersive contribution, was determined by measuring the static equilibrium contact angles of milli-Q water (Buchi Frontvapor 259), diiodmethane (99% Sigma Aldrich) and employing the harmonic mean Eq. (1). According to Wu (1982), the harmonic mean equation gives consistent results for interactions between low energy systems, such as the present system and combines contact angle measurements of two liquids with different characteristic (polar and non-polar).

cp cpsv cd cd ð1 þ cosHC Þclv ¼ 4 d lv svd þ p lv p clv þ csv clv þ csv

! ð1Þ

where HC is the equilibrium contact angle, clv the surface tension of the fluid (tabulated), cplv the polar surface tension component of the fluid (tabulated), cdlv the dispersive surface tension component of the fluid (tabulated), cpsv the polar surface energy component of the substrate (calculated), and cdsv is the dispersive surface energy component of the substrate (calculated). Five randomly chosen samples of each surface variant were cut into 1.5  12 cm pieces. Ten droplets of each liquid were placed on top of the surfaces and the contact angle measured utilizing a DAT 110 FIBRO system. The average of the measured angles was used in the calculation. Moreover, the contact angle hysteresis was determined from measuring the dynamic contact angles using an Attension Theta system. Ten samples of each surface variant were used in the measurements. 2.4. Product residues vs incubation time The product residues vs various incubation times were determined gravimetrically. The Ref surface was cut in pieces of 5.5  12 cm, double folded such that the inner polyethylene surface was facing the product and the material edges were sealed with tape (3 M) in order to prevent product uptake during incubation. The samples were weighed (using XS204 from Mettler

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Toledo) and individually immersed in beakers containing fermented milk (Skånemejeriers Filmjölk 3.0% fat). The beakers with sample were further covered with Parafilm (Sigma Aldrich) and stored in 5 °C for different time slots ranging from 1 to 400 h. The samples were vertically withdrawn in a velocity of 0.02 m/s. Two parallel procedures were conducted, shaking and not. In the shaking procedure the beakers were shaken for 3 s prior withdrawal and weighing. Five replicas of each procedure and incubation time were evaluated. 2.5. Product residues vs package material Five replicas of each surface variant and procedure (shaken and not) were incubated for 96 h at 5 °C in fermented milk. 96 h incubation was chosen as a representative time required for the product to reach the consumers. The samples were vertically brought up from the fermented milk reservoir, weighed, and kept vertical during 2 min under subsequent weighing. 2.6. Effect of rheology The effect of rheology on the product residue was evaluated by incubating five replicas of the Ref surface in a mixture of fermented milk and milk (Skånemejeriers mjölk 3.0% fat) ranging from pure milk to pure fermented milk for 96 h. Fermented milk, which is a high viscous product exhibiting yield stress while milk could be considered as a low viscous Newtonian fluid (for reasonable shear rates). The beakers with product were shaken for 3 s prior sample take up and weighing.

Figs. 2 and 3, depicting the Ref surface at 1000 and 50,000 magnification respectively, reveal irregularities at three different length scales – cavities of approximately 200 lm, roughness at 10 lm, and a waviness at 500 nm. The rather large scale of the asperities indicates that fermented milk most likely penetrates all cavities and asperities. The observed corrugation of the surfaces, are thought having their origin in the cellulose fibres beneath the polymer layers as well as in the micro roughness of the blasted rolls during the laminating process. In Variants 1 and 2 (not shown), on the other hand, a polymer coating covers the asperities, resulting in a more smooth appearance. The appearance of Variant 3 (not shown) seems to have the same morphological characteristic as Variant 1. 3.2. Surface roughness The arithmetic mean deviation relative the surfaces mean plane, Sa, as well as the developed surface area, Sdr, along with Sci and Ssk, were determined, utilizing Phase shift Interferometry. Additionally, images of the four surfaces were reconstructed from the obtained data, see Fig. 4. The results are shown in Table 1 and Figs. 5 and 6. The asperities, voids, and irregularities are partly covered by the polymer layer in Variant 1 and Variant 2, resulting in flatter surfaces. Variant 2 is smoother than Variant 1. Moreover, the motif and topographical characteristic of Variant 3 resembles

3. Results and discussion The rheological properties of fermented milk in comparison to ordinary milk is most likely the main reason for why a thick layer of fermented milk is stuck to the inner surface of package material upon pouring. In order to address this hypothesis, Ref was incubated in a mixture of fermented milk and milk, ranging from pure milk to pure fermented milk. Fig. 1, depicting the adhered amount of product left in a 1 l package, manifests this statement explicitly. 3.1. Surface morphology The SEM images reveal that the Ref surface structure is in the range between nano to micro scale.

Fig. 1. Incubation in milk-fermented milk mixture ranging from pure fermented milk to pure milk. The Ref surface was incubated for 96 h and the beakers were shaken prior to material up take. The depicted amount of product residue, is the percent of the total amount stored in a 1 l family package.

Fig. 2. SEM image Ref 1000 the image is acquisitioned with a JEOL JSM-6700F FEG SEM, LEI inlens detector. The scale bare refers to 10 lm.

Fig. 3. SEM image Ref 50,000, utilizing JEOL JSM-6700F FEG SEM, SEI inlens detector. The scale bare refers to 200 nm.

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Fig. 4. (A) Ref, (B) Variant 1, (C) Variant 2, and (D) Variant 3 reconstructed by utilizing a MicroXam ADE Phase Shift Inc. (Tucson, AZ, USA). Measurement area: 200 lm  250 lm.

Table 1 ISO roughness parameters measured with an interferometer, three measurements per surface and three surfaces per surface type. The depicted values refer to the mean and the P values. The standard deviation is shown within brackets. Sa (lm)

Sdr (%)

Ssk

Sci

0.50 0.24 0.12 0.36

34.9 (6.03) 5.32 (2.74) 0.71 (0.48) 11.63 (4.31)

18.56 (2.27) 19.32 (13.91) 1.58 (2.24) 19.90 (5.30) P ¼ 0:90

0.99 1.37 1.37 1.12

Ref V1 V2 V3 Ref vs V1

(0.05) (0.04) (0.02) (0.06)

P ¼ 5:2  109

P ¼ 3:1  108

Ref vs V2

P ¼ 2:6  10

9

7

Ref vs V3

P ¼ 6:4  105

P ¼ 8:1  108

V1 vs V2

P ¼ 9:6  106

P ¼ 9:2  104

V1 vs V3

P ¼ 1:7  104

P ¼ 2:5  103

P ¼ 4:9  103 P ¼ 0:91

P ¼ 9:4  102

V2 vs V3

P ¼ 3:0  107

P ¼ 5:8  105

P ¼ 1:35  1006

P ¼ 7:4  103

P ¼ 1:3  10

P ¼ 3:1  10 P ¼ 0:50

11

(0.14) (0.37) (0.15) (0.19)

P ¼ 1:5  102 P ¼ 4:6  105 P ¼ 0:11 P ¼ 0:99

Variant 1. Regarding the determined roughness parameters, a oneway ANOVA statistical analysis, utilizing the software R, was done (Table 1). The differences of Sa and Sdr for all surfaces are highly significant ðP < 0:005Þ, whereas the differences between the Ref surface vs Variant 2 and Variant 2 vs Variant 3 for all parameters are significant. In general the roughness, established from Sa and Sdr, follows the trend Ref > Variant 3 > Variant 1 > Variant 2. An inconsistency in this trend is noticed in Ssk and Sci. The skewness of the height distribution is more pronounced for Variant 3 and least for Variant 1 and all surfaces exhibit more pits than peaks. Variants 1 and 2 have the greatest ability for fluid retention. This is probably due to the root mean square roughness normalization of Sci. The results from the Interferometry measurements, both the determined parameters and the image reconstructions, coincide with the obtained SEM images.

Fig. 5. Sa, the arithmetic mean deviation relative to the surfaces mean plane. The parameter was determined using a MicroXam ADE Phase Shift Inc. (Tucson, AZ, USA) interferometer. Three samples of each surface type and three positions on each sample were investigated. Measurement area: 200 lm  250 lm. The differences of all surfaces are determined to be highly significant, P < 0:005.

3.3. Surface free energy By measuring the static and dynamic contact angles, the surface free energies and the heterogeneities of the surfaces were assessed. Table 2 reveals the polar, dispersive, and the total surface free energy along with the contact angle hysteresis, DH. The total surface free energies follow the trend Variant 3 > Ref > Variant 2 > Variant 1. The same trend is observed in both the dispersive and the polar contribution. This coincides well with the H2O contact angles of the different surfaces. The H2O droplet almost completely wets Variant 3 and forms an almost spherical droplet at the Variant 1 surface. Further, the differences regarding the surface free energy,

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The foremost reason for this is the lower surface tension for fermented milk (50 mJ/m2 (Walstra and Jenness, 1990)) in comparison to water (72 mJ/m2). Moreover, the large hysteresis for the Ref surface (often seen in Wenzel state (de Gennes et al., 2002)) indicates that the fermented milk droplet is totally pinned at the surface, despite rather high advancing angle. 3.4. Product residues vs incubation time

Fig. 6. Sdr, enlargement area in comparance to the measuring area. The parameter was determined using a MicroXam ADE Phase Shift Inc (Tucson, AZ, USA) interferometer. Three samples of each surface type and three positions on each sample were measured. Measurement area: 200 lm  250 lm. The differences or all surfaces are determined to be highly significant, P < 0:005.

Table 2 Surface free energy measured with a Fibro Dat system, ten measurements per surface and three surfaces per surface type. The depicted values refer to the mean values. The standard deviation is shown within brackets. The differences regarding the surface free energy between the surfaces are determined to be highly significant P < 0:005 except for Variant 1 vs Variant 2. DH denotes the contact angle hysteresis.

Ref Variant 1 Variant 2 Variant 3

Polar part of surface free energy (mJ/m2)

Dispersive part of surface free energy (mJ/m2)

Total surface free energy (mJ/m2)

DH (°)

5 2 2.3 34.6

33.9 21.7 22.3 38.2

38.9 23.7 24.6 72.8

34 25 20 NA

(0.3) (0.8) (0.3) (0.4)

are determined to be highly significant, P < 0:005, except for Variant 1 vs Variant 2, for which the difference is not significant. The contact angle hysteresis is most prominent for the Ref surface and least for Variant 2. The total surface free energy of the Ref surface, determined from the contact angle measurements, is higher than expected from literature, indicating oxidation of the surfaces. Interestingly, the surface free energy of Variant 3, which binds a significant amount of water (determined from FT IR/ATR not shown), exhibits the same surface free energy as water. From the FT IR/ATR measurement of Variant 1 and Variant 2 (not shown), one could expect that Variant 2 would have lower surface free energy in comparison to Variant 1. However, the opposite was determined even though the difference is not significant. This could be due to the effect caused by rough and textured surfaces, affecting the contact angles and thus the apparent surface free energy (Burton and Bhushan, 2005). Shibuichi et al. (1996) conducted a notable series of experiments on this topic, where it was shown that the natural tendency of a surface is enhanced by the presence of a texture, which will make a hydrophilic material more hydrophilic and a hydrophobic material more hydrophobic. Besides the apparent static contact angles, roughness also allows an interval of angles instead of a specific angle, and thereby inducing contact angle hysteresis (de Gennes et al., 2002). Consequently, the contact angle hysteresis of the evaluated surfaces is following the same trend as the roughness, determined by the arithmetic mean deviation parameter. Notice that, chemical heterogeneity has the same effect on the contact angles. Thus, it is difficult to determine which effect that is responsible for the hysteresis (Israelachvili, 2011). The dynamic contact angles of fermented milk (not shown) are lower, both advancing and receding, for all surfaces in comparison to the water angles.

The effect of storage time and shaking procedure on the product residue is shown in Fig. 7. The inset depicts the product residues at the Ref surface after 1–3 h of incubation. The product residues at the not shaken samples incubated for 1 h, are more prominent than the shaken, although the reverse was observed already after 2 h of incubation. As revealed in Fig. 7, there is an apparent trend over time. The two different procedures prior to withdrawal of the surfaces from the incubation beakers are causing diverging results. The shaking procedure is unexpectedly giving rise to more product adhered to the surfaces. The results are unexpected due to the shear thinning and thixotropic behaviour of fermented milk, exhibiting reducing apparent viscosity upon shearing. The shear thinning and thixotropic behaviour of fermented milk is however observed after 1 h of incubation when the phenomenon of shrinkage, phase separation and network recover, are not yet observed. Thus, the thickness of product adhered to the shaken samples is less than the not shaken samples. As observed during the withdrawal and weighing procedure, the product tended to shrink over incubation time under exclusion of serum, i.e. syneresis. Additionally, the initial disturbed network seems to partly recover over time, forming a weak gel. This time-dependent observed phenomenon, often referred to as rebodying, is further confirmed in the literatures (Lee and Lucey, 2010; Walstra and Jenness, 1990). The observed phenomenon with a less viscous phase facing the material surface could either be the result of syneresis or a surface induced phase separation. During the experiments it was clearly shown that if the product texture was recovered less product adhered to the surface. This can be explained by the existence of a thin less viscous phase between the surface and the bulk, which gave rise to a slip effect. When the beaker was shaken, the less viscous product entered the bulk, which gave rise to a thicker layer of product residues. Excessive acidity, the so called post-acidification (Robinson et al., 2007), is another effect that one has to take into consideration as well as the contamination risk. However, this is most likely not the case in the presented study, since pH was determined to be constant during the incubation time. 3.5. Product residues vs package material Fig. 8 reveals the product remnant at the surfaces 2 min after surface withdrawal from the fermented milk reservoir, were they were shortly immersed. As clearly shown in the image, Variant 1 was dewetted and the product ran off the surface in a plug like characteristic, as expected from a Bingham fluid with slip boundary condition. Ref and Variant 3 gave rise to the most prominent amount of product residue and the flow characteristics coincided. The product residue on the upper part of the surfaces was thinner than the lower part, consequently wedge-shaped flow profile, with low contact angle (receding upper part), was observed. As a consequence, the surfaces were not dewetted. Thus, the upper contact line was totally pinned. Further, surface Variant 2 gave rise to the same flow characteristic as Variant 1, except for the run-off time, which was substantial shorter for the latter one, resulting in more residue at Variant 2. The dissimilar run-off profiles are thought being due to difference between the cohesive interaction holding the product together

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Fig. 7. The mean residues, on a weight basis prior excess run-off, at the Ref surface incubated in fermented milk for different time slots, ranging from 1 to 400 h. Half of the beakers were shaken prior to sample take up. The upper curve displays the shaken samples whereas the lower, the undisturbed fermented milk-containing beakers. The inset depicts the adhered amount after the first couple of hours of incubation.

Fig. 8. Dewetting of the surfaces – from the left, Variant 2, Variant 3, Ref, and Variant 1 with the remaining product, 2 min after sample take up. The red arrows indicate the initial product upper contact line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and the adhesive interaction between the product and surface. The main driving force is the imbalance between the three interfacial tension of the product–air, product–surface, and surface–air interfaces which meet at the three-phase contact line (Reiter et al., 2009; Israelachvili, 2011). The differences of the adhesive fermented milk-surface forces were further confirmed in dynamic contact angle measurements. For Variants 1 and 2, the failure of the fermented milk droplet occurred at the solid–liquid interface, upon withdrawal of the fluid through the syringe, while the break occurred within the droplet for Variant 3 and the Ref surface. Moreover, the large hysteresis and low receding angle for the Ref surface is thought being the origin to the totally pinned upper three-phase contact line.

Fig. 9. Shaken and not, 96 h gravimetrical study. The plot depicts the adhered amount of fermented milk on all surfaces after 96 h of incubation prior run-off. In half of the samples the beakers were shaken prior withdrawal of the surfaces. The differences between shaken and not, are highly significant, P < 0:005.

The results from the 96 h incubation study is shown in Fig. 9 and Table 3, depicting the fermented milk residues prior to excess run-off, the means and standard deviations of the residues after excess run-off, respectively. There is a larger difference between the shaken and not procedure than between the surfaces. Approximately twice as much products adhere to the shaken samples in comparison to the not shaken. The Ref surface is causing the largest amount of adhered product for the not shaken procedure and Variant 3 the least. The differences between those are the only significant difference observed prior to letting the excess drain off. Furthermore, the results from the shaking procedure are almost equal for all surfaces, and no significant differences were determined. Regarding the remaining product adhered to the surfaces after run-off the excess product, Variant 3 gave rise to significant less residues except compared to Variant 2, in the not shaken procedure. This is thought being due to a larger amount of transparent

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Table 3 Adhered fermented milk, all surfaces, shaken and not, 96 h incubation. The values presented are the average residues after 2 min run-off. The calculated residue (wt%) in a 1 l family package is depicted within the brackets. The differences are more prominent between shaken and not than between the surfaces. The stated P values confirm that there is significant less product adhered the Variant 3 ðP < 0:05Þ, except for compared to Variant 2, in the not shaken procedure. Mean (g/ dm2)

Std (%)

Mean, shaken (g/ dm2)

Std, shaken (%)

Ref Variant 1 Variant 2 Variant 3 Ref vs V3 Ref vs V1 Ref vs V2 V3 vs V1 V3 vs V2

4.8 (3.8) 5.3 (4.1) 4.3 (3.3) 3.2 (2.5) P = 0.005 P = 0.593 P = 0.442 P = 0.033 P = 0.192

14 26 34 11

8.8 (6.8) 8.5 (6.5) 8.8 (6.8) 6.8 (5.2) P = 0.008 P = 0.573 P = 0.963 P = 0.002

4 11 12 8

V1 vs V2

P = 0.392

P ¼ 6:0  104 P = 0.301

thin serum phase, induced at the interface (product–surface), not entering the bulk product upon shaking. However, the difference is more apparent between the shaken and not procedure than between the surfaces, indicating that rheology (foremost the viscosity gradient) has the major impact on the adhered amount over time. The results from the 2 min run-off, Fig. 10, further manifest the observation regarding the more prominent serum phase towards the surface of Variant 3. The resulting viscosity gradient from bulk product towards the surface and the low viscosity surface fluid, serve as a lubricant, letting the thicker product slide along the interface. Moreover, since no significant differences were observed between the other surfaces, one can conclude that the surface roughness did not affect the flow rate notably, which is opposing the widely accepted knowledge regarding important properties affecting adhesion and friction in dry/wet contacts (Jung and Bhushan, 2006; Michalsky et al., 1998). This is most likely due to the small pressure gradient (solely gravity) driving the flow and the fact that it is the rheology of the product rather than the surfaces, that has the foremost impact on the flow in this regime. Additionally, the evaluated surfaces exhibit rather equal characteristics despite confirmed significant differences regarding both roughness and surface free energy. Thus, even though Variant 1 and Variant 2 induce larger density and viscosity gradients, the effect of this could not be seen due to the higher ability for fluid withholding. In addition, the characteristics of the surfaces are partly lost over time due to adsorption of surface active species such as proteins, phospholipids and so fort (confirmed in Monte

Fig. 10. The excess fermented milk run-off from the material surfaces. The dotted lines serve as guidelines for the eyes.

Carlo simulations conducted by Evers et al. (2012), FTIR/ATR (not shown), and contact angle measurements), present in fermented milk. Fig. 10 displays the fermented milk run-off from the material surfaces. Variant 3-shaken before withdrawal, gave rise to the best flow. Although syneresis was observed along all inner surfaces, the most prominent phase separation was seen towards Variant 3, not entering the bulk upon shaking. Furthermore, the fermented milk flow profiles along the surfaces of Variants 1 and 2 incubated for 96 h were dramatically changed in comparison to the profiles observed for the surfaces shortly immersed. The most prominent change was observed for Variant 1, from being plug shaped with slip behaviour to a more wedge-shaped drainage characteristic with a smaller receding angle, illustrated in Fig. 11. The left sketch refers to the initial flow profile and the right after 96 h of incubation. Furthermore, in fluids exhibiting yield stress, as fermented milk, the yield value has to be exceeded by the gravitational stress in order to initiate a flow. Consequently, the characteristic of the flow is not parabolic as for Newtonian fluids, but rather plug like (Croll and Kisha, 1992; Chan and Venkatraman, 2006). If the yield value of the product is increased, the plug region is moved towards the surface and a steep shear region is giving rise to thinner fluid towards the surface. However, higher stress is required in order to initiate the flow. With that in mind it is obvious that the flow is more pronounced for the shaken samples due to thicker initial adhered product layer, until the drainage of the product is resulting in a thinner layer and the gravitational stress less significant.

Fig. 11. Illustrative sketch describing the flow profile. The left drawing describes the flow profiles of the residues shortly immersed in fermented milk (Variants 1 and 2), while the right refers to the 96 h incubated surfaces of all types.

Fig. 12. Water contact angles on incubated surfaces. The surfaces were incubated in fermented milk and water contact angles were measured after 24, 74 and 168 h of incubation.

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Since the residue flow-off profile, after 96 h of incubation, showed an altered characteristic in comparison to the residue on surfaces that were shortly immersed, water contact angle measurements of the incubated surfaces were conducted. Fig. 12 depicts the water contact angles measured at the surfaces after increasing incubation time. The contact angles for the Ref, Variant 1, and Variant 2 decrease over immersion time in contrary to Variant 3, which approaches the contact angle for the Ref surface. 4. Conclusion Fermented milk is a complex liquid and concluded from this work is that it is the rheological properties that play the most important role for product adhesion. The results have shown that there are two mechanisms over different time scales that predominate. Initially (seconds–minutes) the excess product is drained of the surfaces until the gravity is opposed by the rheological strength such as the yield stress and the apparent viscosity. The second mechanism occurs on a longer time scale (minutes–hours) and includes dewatering of the product residue at the surface. As was shown by diluting fermented milk by ordinary milk, the viscosity plays an important role for product residues. It was clearly shown that the adhered amount was decreased when a viscosity gradient was observed towards the package surface (the thinner product phase closest to the interface). Moreover both the storage time and the product rebodying affect the product residues as well. Hence the problem with fermented milk remaining in the packages should be regarded as a fluid flow and dewatering problem. Acknowledgements We acknowledge Vinnova, the Vinnmer program, and The Royal Physiographic Society in Lund, Per-Eric and Ulla Schybergs Foundation, for financial support. References Adhikari, B., Howes, T., Bhandari, B., Truong, V., 2003. In situ characterization of stickiness of sugar-rich foods using a linear actuator driven stickiness testing device. J. Food Eng. 58 (1), 11–22. Adhikari, B., Howes, T., Shrestha, A., Bhandari, B.R., 2007. Effect of surface tension and viscosity on the surface stickiness of carbohydrate and protein solutions. J. Food Eng. 79 (4), 1136–1143. Bobe, U., Hofmann, J., Sommer, K., Beck, U., Reiners, G., 2007. Adhesion – where cleaning starts. Trends Food Sci. Technol. 18, 536. Burton, Z., Bhushan, B., 2005. Hydrophobicity, adhesion and friction properties of nanopatterned polymers and scale dependence for mems/nems. Nano Lett. 5, 1607–1613. Chan, C.-M., Venkatraman, S., 2006. Coating rheology. In: Coatings Technology Handbook, third ed. Taylor & Francis.

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