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Effects of different factors on stickiness of apple leathers
Journal of Food Engineering 149 (2015) 51–60
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effects of different factors on stickiness of apple leathers Catalina Valenzuela ⇑, José Miguel Aguilera Department of Chemical and Bioprocesses Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile
a r t i c l e
i n f o
Article history: Received 28 July 2014 Received in revised form 15 September 2014 Accepted 21 September 2014 Available online 30 September 2014 Keywords: Stickiness Fruit leathers T-peel test Experimental design
a b s t r a c t Apple leathers (ALs) are restructured food products made by dehydration of a thin layer of apple puree, resulting in a thin and flexible sheet. AL are composed mainly by low-weight carbohydrates which are highly hygroscopic, so AL become sticky when stored at ambient relative humidity (RH). Stickiness in food could be considered as a negative or positive aspect. In this work, four factors affecting stickiness of AL were studied (ingredients, RH, surface rugosity and compression time), both to decrease and to increase it. Surface rugosity of AL had the greatest impact on stickiness, being the smoothest side stickier than the roughest side. The RH at which AL were conditioned and the ingredients used in this work also had a great influence on the adhesion force. In this manner, stickiness can be modified (increasing or decreasing) according to the specific requirements for using AL in different applications. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Fruit leathers are products with an intermediate moisture content which have a flexible sheet form. They are consumed as snacks in many parts of the world (Torley et al., 2006). These products are light, pleasant to chew and tasty, becoming an attractive way to incorporate fruit to diet, especially for children and adolescents (Quintero Ruiz et al., 2012). The manufacturing process of fruit leathers consist on drying a thin layer of a fruit puree until a leathery consistence is achieved (Vatthanakul et al., 2010). Many kind of fruits can be used to make fruit leathers, like apple, papaya, mango, guava, durian, jackfruit, grape and kiwifruit, among others (Chan and Cavaletto, 1978; Irwandi et al., 1998; Vijayanand et al., 2000; Chen et al., 2001; Maskan et al., 2002; Gujral and Brar, 2003; Chowdhury et al., 2011; Quintero Ruiz et al., 2012). The fruit leather matrix (is composed mainly by) mainly consists of carbohydrates such as sugars, pectin and cellulosic substances (Torley et al., 2008). These hydrophilic compounds have affinity with the surrounding water vapor, making the matrix of these products highly hygroscopic (Mathlouthi and Roge, 2003; Tong et al., 2008). To prevent the fruit leather to absorb moisture from the environment after drying, the product must be packed properly and immediatly (Irwandi and Man, 1996; Man and Sin, 1997; Gujral and Khanna, 2002). Stickiness (also termed pressure sensitive adhesion or tack) can be described as the adhesion force between two different materials which are in contact with each other under a light pressure ⇑ Corresponding author. Tel.: +56 9 63424709. E-mail address: [email protected] (C. Valenzuela). http://dx.doi.org/10.1016/j.jfoodeng.2014.09.029 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved.
(Hoseney and Smewing, 1999). Some food materials such as spray-dried fruit juices and fruit leathers tend to adhere to the processing equipments, packaging materials, fingers, palate and teeth (Kilcast and Roberts, 1998). This feature of the material to stick on surfaces is affected by the amount of sugars with a low molecular weight as glucose, fructose and sucrose (Adhikari et al., 2003). On the other hand, it has been documented that maltodextrins improve the quality of dehydrated products by decreasing their stickiness and increasing their product stability (Roos and Karel, 1991). This is because of the ability of maltodextrins to absorb water and create a surface barrier between particles, and to increase the glass transition temperature (Tg) (Telis and Martínez-Navarrete, 2009). Stickiness can be considered both as a negative or a positive attribute. In food processing, stickiness slows the speed of the mechanized process and increases cleaning costs of equipments. The sticking of food to packaging results in product losses, packaging damage and product deformation. These parameters could have influence on the consumer acceptance, especially if they are finger food that stick to fingers while eating (Kilcast and Roberts, 1998; Hoseney and Smewing, 1999; Adhikari et al., 2003). Under other conditions, stickiness is desirable in products like toffees, sausages, and rice for risotto preparation (Michalski et al., 1997). Even more, stickiness is needed in the lamination based technology. This technique consists in assembling consecutive layers of a food base material fused with a bonding layer of the same or another material (Wegrzyn et al., 2012). The most important factors affecting stickiness are viscosity, relative humidity, temperature, compression, food ingredients (especially low molecular weight sugars) and the kind of material
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that food is in contact with. For example, inorganic materials have a higher surface tension than organic materials (polymers). These polymers which present a low energy surface will absorb strongly to a high-energy surface in order to decrease the energy of the system (Kilcast and Roberts, 1998; Hoseney and Smewing, 1999). When at least one of these materials bonded together is flexible, the T-peel test has been often used to measure the adhesion strength (Watts et al., 1988; Song and Yu, 2002; Zumelzu and Gipoulou, 2002; Hadavinia et al., 2006). This test measures the force needed to pull apart two materials bonded in T form. The objective of this study was to quantify the effects of ingredient type, the relative humidity, the surface rugosity and the compression time on stickiness in apple leathers. All these variables were measured in order to study their effect on the adhesion force, both to decrease and to increase it. 2. Materials and methods 2.1. Materials Canned apple puree (Conservera Pentzke S.A., San Felipe. Chile) were purchased from a local supermarket. Total soluble solids of apple puree were measured with a digital refractometer (Atago. PR-201). Apple puree had 20 g soluble solids/100 g puree. Glucose and maltodextrin were provided by Hela (Hela Especias Chile S.A., Santiago. Chile). According to the label information, the dextrose equivalent (DE) of the maltodextrin was between 17 and 19. Low density polyethylene (LDPE) pouches were supplied by HOMS (Plásticos HOMS Ltda., Santiago, Chile). 2.2. Moisture content The moisture content of apple puree and maltodextrin was determined by drying 5 g of sample (apple puree or maltodextrin) in a convection oven at 105 °C until constant weight was achieved. The initial moisture content of apple puree was found to be about 4.52 ± 0.16 g water/g dry solids. The initial moisture content of maltodextrin was found to be about 0.01 ± 0.001 g water/g dry solids. 2.3. Experiment division As was stated before, stickiness can be considered as a negative or positive attribute. Regarding to the process conditions and packaging materials, the aim of this work was to determine which were the factor that reduce stickiness, to prevent apple leathers from adhere to the surface. Otherwise, when a sticky material is required for some specific applications such as lamination technology, the aim of this work was to determine which were the factors that increase stickiness of apple leathers. Therefore, the experiments were divided in two sections, studying stickiness decrease (I) or increase (II). In experiment I (decreasing stickiness), maltodextrin was used as an additive, since it has been found that it contributes to decrease stickiness. Also, the adhesion of apple leathers to packaging material (LDPE) was studied. In experiment II (increasing stickiness), glucose was used as an additive, since its low molecular weight contributes to increase stickiness. The adhesion of apple leathers to itself was also studied. 2.4. Formulation of apple leathers For experiment I, apple puree and maltodextrin were weighted to obtain a final concentration of 0, 5 and 10 g maltodextrin/g total weight. For experiment II, apple puree and glucose were weighted to obtain a final concentration of 0, 5 and 10 g glucose/g total
weight. The nomenclature for each formulation was set as 0%, 5% and 10% maltodextrin or glucose respectively. The calculated amount of maltodextrin or glucose was first mixed with a small amount of puree (10% of total puree added in each formulation). This step was performed manually at 20 °C, taking care not to form bubbles, mixing the ingredients using a glass rod until there were no lumps and the maltodextrin or glucose was uniformly dispersed, forming a homogeneous paste. The paste was then added to the rest of the puree. 2.5. Apple leather formation Formulated apple puree was spread as a thin layer within the space of a 2-mm height frame placed in an aluminum tray. The puree was leveled using a glass rod to ensure the thickness of the puree was uniform. The aluminum tray was previously covered with a silicone sheet to prevent apple leather from sticking after drying. Drying was carried out in an air circulation oven (Köttermann 2736, Hänigsen, Germany). Apple leathers were dried at 60 ± 1 °C to a final moisture content of approximately 0.12 g water/g dry solids. 60 °C was determined as a suitable temperature to be used in the AL drying process due to it is high enough for a proper drying rate, and at the same time prevents the product to be burned (Valenzuela and Aguilera, 2013). 2.6. Thickness measurement of apple leathers and plastic material Apple leathers and LDPE pouches thickness were measured with Mitutoyo Absolute digital micrometer (Mitutoyo Corp, Kanogawa, Japan) at six random positions and average values were used in all calculations. The thickness of LDPE pouches was 0.0855 ± 0.005 mm. The thickness of LDPE pouches was considered constant, while the thickness of apple leathers varied between samples. 2.7. Conditioning of apple leather strips Apple leathers were cut into rectangular strips of 90 18 mm. These apple leather strips (ALS) were equilibrated at different relative humidity (RH) at 20 ± 1 °C for about 15 days in desiccators, according to the method described by Labuza et al. (1985). The desiccators contained the following saturated salt solutions: MgCl2 (RH = 33%), K2CO3 (RH = 44%), and Mg(NO3)2 (RH = 55%). This humidity range (33–55%) was found to be the optimum conditions to maintain the rubbery-like state of the leather strips. At humidity levels greater than 55%, ALS were infested with mold, while the ALS became brittle below 33% humidity level (Valenzuela and Aguilera, 2015). 2.8. Scanning Electron Microscopy (SEM) surface analysis of apple leather strips SEM was used to characterize qualitatively the ALS surface microstructure. Samples (at least 3 specimens per sample) were conditioned at 0% RH over P2O5, then sputter-coated with gold– palladium and examined with a scanning electron microscope (JEOL JSM 5300, Jeol Ltd., Tokyo, Japan), operated at an acceleration voltage of 20 kV. Images were taken within a specimen at different positions and magnifications. Selected images were reported. ADDA II was used as interface between the microscope and the computer and the images were analyzed with the software AnalySISÒ version 3.2 (Soft Imaging System GmbH. Münster. Germany). 2.9. Compression of apple leathers strips In experiment I, ALS were placed in contact with the LDPE packaging material. Both ALS and LDPE were previously cut into
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rectangular strips of 90 18 mm. For experiment II, ALS were placed in contact with another ALS. The two materials in contact with each other were compressed using a hydraulic press (MVP Superline), at 6894.76 kPa during 3 or 90 s, as indicated by the experimental design. 2.10. Mechanical testing apple leather strips After the compression of ALS to (I) LDPE strips and (II) another ALS, the adhesion force between the materials was determined using the T-peel test by ASTM-D-1876 in the Universal Texture Analyzer TA.XTplus (Stable MicroSystems, Godalming, Surrey, UK). The T-peel test was performed pulling the T ends apart at a rate of cross-head movement of 25 mm/min (Fig. 1). A graph of force (N) versus displacement (mm) was recorded. Adhesion force (N) was determined as the mean peel force from the recorded graph using Exponent Software (Stable Micro Systems Ltd.).
Table 1 Two-level assessment for each factor. Factor
Low level ( 1)
High level (+1)
A: Relative humidity (%) B: Ingredienta (%) C: Surface rugosity ( ) D: Compression time (s)
33 0 Rough 3
55 10 Smooth 90
a I: for decreasing assay maltodextrin was used as ingredient. II: for increasing assays glucose was used as ingredient.
independent variable was tested at two levels, a high level (+1) and a low level ( 1), as shown in Table 1. For the optimization step, the two most important variables were selected and a factorial design (22) was created, in which the effect of two factors was studied in 28 runs. These runs included four blocks of seven runs each, and three center points per block.
2.11. Weighing of plastic material to determine the type of failure
2.13. Differential scanning calorimetry measurements
The LDPE strips were cleaned with acetone and their weights were recorded before and after the T-peel test, to check if there was an adhesive or cohesive failure. True adhesive failure occurs if there was a clean failure of the bond between the apple leather and the plastic material, with no apple leather residue remaining on the plastic material surface. If it was a cohesive failure, some of the apple leather residue must be left on the plastic material after the separation (Kilcast and Roberts, 1998). The degree of cohesive failure (DCF) was expressed in mg of apple leather residue remained on each cm2 of the plastic material (mg/cm2).
The glass transition temperature (Tg) of samples selected as optimum in each experiment was measured using a differential scanning calorimetry (DSC), (DSC 821e, Mettler Toledo, Schwerzenbach, Switzerland) according to Torley et al. (2008). The instrument used a refrigerated cooling system to achieve temperatures of 80 °C and a nitrogen DSC cell purge at 60 ml/min. Hermetic aluminium pans of 30 ll capacity were used. Small size samples of 7–10 mg were cut from the ALS and placed into the aluminium pans. The preparations were cooled at a rate of 5 °C/min to 80 °C, then they were left there for thirty minutes, and the glass transition was determined from the midpoint of the heat capacity change observed at the same heating rate. An empty hermetically sealed aluminium DSC pan was used as a reference.
2.12. Experimental procedure The optimization of the assay procedure was carried out in two steps. A screening design was used to detect those variables having the highest influence on the stickiness of apple leathers, while a factorial design was applied to find the best parameter settings for the increase or decrease of stickiness. As a first step, a full factorial screening design at two levels with four factors (24) was performed involving the variables: the relative humidity, the added ingredient (maltodextrin in decreasing stickiness, experiment I, and glucose in increasing stickiness, experiment II), the surface rugosity (the apple leather side facing upward is rougher to touch than the side facing downward during drying), and the compression time. Each
2.14. Statistical analysis Statgraphics Centurion Software XV, 15.1.02 version (StatPoint Inc., VA, USA) was used to design the screening and factorial design experiments and to evaluate the obtained results. 3. Results and discussion 3.1. Surface morphology As the morphology of the surface has a high impact on the adhesion, SEM images of the surface of ALS were taken. As Fig. 2 shows, there were big differences between the side facing upward and the side facing downward of apple leathers during drying. The side facing upward during drying was very rough presenting many hills and valleys, while the side facing downward during drying had a smoother surface. A high surface roughness may reduce the intimacy degree in the area where the ALS are in contact with both the plastic material and itself (Gay, 2002). In the roughest side of this fruit leather material, the contact will be restricted to the top of the hills, resulting in small isolated contact regions, even more if these hills have different highs (Fig. 3). As a result, in the roughest side of ALS the actual area of contact will be less than the nominal area. 3.2. Decreasing stickiness experiments (I)
Fig. 1. Schematic diagram of T-peel test.
3.2.1. Screening design The results obtained for the adhesion force and degree of cohesive failure in experiment I are shown in Table 2. The adhesion force was higher when all factors were in their highest level (+1)
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0% D
0% U
300 μm
5%G, D
5%G, U
10%G, D
10%G, U
5%M, D
5%M, U
10%M, D
10%M, U
Fig. 2. SEM images of apple leather surface. (G: glucose; M: maltodextrin; D: side facing downward during drying; U: side facing upward during drying).
reaching a value of 1.66 N, as can be observed in the third row of Table 2, but this high value was a result of the interaction of all factors. The effects of each factor were presented in Table 3 with a 95% confidence level, where can be observed that the surface rugosity had the highest effect on the adhesion force, followed by RH, maltodextrin content and compression time. The results of this design indicate that in the studied levels, the variables surface rugosity and maltodextrin content had a significant statistical effect (p < 0.05) on adhesion force (Table 3). The adhesion force was mostly affected by the surface rugosity. A higher surface rugosity
showed a negative effect on the adhesion force. This means that the roughest side of ALS, which was facing upward during drying, was adhered in a weaker way than the smoother surface. The observations for the high rugosity were in agreement with what was discussed in Section 3.1. Since in this section it is intended to decrease the stickiness of apple leathers, in the optimization step the surface rugosity will be set to the roughest side of the surface. Moreover, the RH at which the ALS were conditioned at, and the interaction of RH with maltodextrin content had also a significant statistical effect (p < 0.05) on the adhesion force (Table 3). Because of this, the effect of these two factors on the adhesion
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contact area
contact sites
smooth surface
rough surface
Fig. 3. Diagram of the contact area depending on the roughness of the surface.
force will be studied on the optimization step. On the other side, compression time did not have a significant statistical effect on adhesion force. The effects of compression and compression time on the adhesion is closely related to the effect of the surface rugosity on the adhesion. As discussed in Section 3.1, the actual contact area will depend on the surface rugosity, but also on the applied load and the contact time (Hui et al., 2000). As discussed by Hui et al. (2000), under pressure of a rough surface, each surface hill
of one material (Fig. 3) will be in contact to the surface of the other material when the hill height exceeds the separation length between both materials. Only if the material is elastic enough, the applied pressure will displace the initial point reaching the top of another hill (lower than the first). In this manner, the area of real contact will increase with the contact time (Persson et al., 2004). If the material is highly deformable, a strong contact will be achieve and there will not be necessary to apply pressure. But if the material is too rigid the contact will remain weakly even under a reasonable pressure and contact time (Gay, 2002). In this work, the low effect that compression time had on the adhesion force of ALS may be related to the material rigidity. Fruit leathers are subjected to large drying periods, resulting in a considerable reduction of its thickness because of the water releasing. As discussed by Aguilera (2003), at low drying rates the final products are more compact and denser. Because of this, the molecules forming the network of the fruit leathers matrix are closely packed, which may lead to a low compressibility of the matrix. As a result, for the next step (the optimization factorial design), the compression time was set to constant value of 30 s. In relation to the DCF, maltodextrin content had a significant statistical effect (p < 0.05) on the cohesive failure between ALS and LDPE strip. Higher amounts of maltodextrin showed a negative effect on DCF (Table 3). That means that when maltodextrin was added it contributed to a true adhesive failure, with no residual apple leather remaining on the plastic surface. At low level of maltodextrin content fruit leather residues remained on the surface, indicating a cohesive failure within the materials. Moreover, the combined effect of maltodextrin content with RH had a significant statistical effect (p < 0.05) on the DCF. This could be related to the
Table 2 Screening design of 16 trials with adhesion force and DCF as response. Trial
Relative humidity
Malto-dextrin
Surface rugosity
Compression time
Adhesion force (N)
DCF (mg/cm2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+1 +1 +1 +1 1 1 1 1 +1 1 1 +1 1 1 +1 +1
1 +1 +1 +1 +1 +1 1 +1 +1 1 1 1 1 +1 1 1
1 +1 +1 1 +1 1 +1 1 1 1 +1 1 1 +1 +1 +1
1 1 +1 1 +1 1 +1 +1 +1 1 1 +1 +1 1 +1 1
0.095 0.393 1.656 0.174 0.336 0.000 0.166 0.000 0.321 0.115 0.398 0.077 0.188 0.253 0.427 0.198
33.97 0.00 18.10 35.16 8.65 0.00 0.00 0.00 0.00 99.60 78.25 94.44 16.90 87.54 306.98 75.79
Table 3 Estimated effects on adhesion force and DCF from results of screening design for decreasing stickiness experiments. Factor
Adhesion force Estimated effect
Mean A: Relative humidity B: Maltodextrin C: Surface rugosity D: Compression time AB AC AD BC BD CD *
Significant for 95% confidence level.
0.300 0.236 0.184 0.357 0.133 0.253 0.145 0.212 0.179 0.180 0.143
DCF p-Value 0.042* 0.232 0.046* 0.212 0.020* 0.333 0.177 0.243 0.24 0.339
Estimated effect 0.674 0.431 0.877 0.465 0.055 0.766 0.283 0.610 0.216 0.357 0.236
p-Value 0.403 0.022* 0.369 0.912 0.084* 0.575 0.146 0.666 0.483 0.639
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Table 4 Trials used in factorial design to determine adhesion force and DCF. Trial
Relative humidity
Maltodextrin
Adhesion force
DCF
1 2 3 4 5 6 7
1 +1 0 1 0 0 +1
+1 +1 0 1 0 0 1
0.000 ± 0.000 0.719 ± 0.005 0.287 ± 0.013 0.093 ± 0.007 0.164 ± 0.017 0.153 ± 0.012 0.057 ± 0.003
0.000 ± 0.000 0.093 ± 0.003 0.016 ± 0.001 0.000 ± 0.000 0.022 ± 0.007 0.015 ± 0.005 0.020 ± 0.009
AB
B: Maltodextrin
A: Relative humidity
effect of maltodextrin content on CDF for itself, or that the effect of RH and maltodextrin together enhanced the decrease of DCF. The selected factors for the optimization step were the ones previously mentioned: maltodextrin content and RH. 3.2.2. Factorial design The results of the factorial design for adhesion force and DCF are shown in Table 4. The effects of the variables on each response are presented on Table 5 with a 95% confidence level. The results of this design indicate that RH and the combination of RH with maltodextrin content had a significant statistical effect (p < 0.05) on the adhesion force (Table 5 and Fig. 4). These results agree with Hoseney and Smewing (1999) and Adhikari et al. (2001) who claim that the water surface tension and the interaction of water with the solids of the matrix are the main cause of stickiness in low moisture food. Thus, each factor that influences the water surface tension will have a large effect on the material surface stickiness. That is why when ALS were conditioned at low RH, the possibility of water to be adsorbed from the surface of the apple leather decreased, thus maintaining the surface dry and therefore no adhesion occurred. On the other side, the addition of maltodextrin to ALS enhanced the effect of RH. As stated by Roos and Karel (1991), high molecular ingredients, like maltodextrin; have a high Tg, increasing the Tg of the mixture. Maltodextrins have been used as drying aids in several studies, which have the property of reducing the stickiness and agglomeration of food powders during storage (Adhikari et al., 2001; Silva et al., 2006; Gabas et al., 2007; Telis and Martínez-Navarrete, 2009). In agreement with Ortega-Rivas et al. (2006), the effect of maltodextrin on the reduction of the adhesion force may be due to: it competes with sugars of the fruit leather matrix for moisture; it acts as physical barrier between particles or between particle and moisture; and it increases the Tg of the mixture, thus increasing the sticky point temperature (temperature higher than Tg, at which the product becomes sticky (Jaya and Das, 2009)). In relation to DCF, the results of the factorial design indicate that RH had a significant statistical effect (p < 0.05) on DCF (Table 5 and Fig. 5). When ALS were conditioned at low RH (33%), there were no particles of fruit leathers remaining on the plastic material surface, this means that there was a true adhesive failure between the ALS and the LDPE strip.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Standardized effect Fig. 4. Standardized Pareto Chart for adhesion force.
AB
B: Maltodextrin
A: Relative humidity
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Standarized effect Fig. 5. Standardized Pareto Chart for DCF.
To determine the most adequate operating conditions and to analyze the factors affecting the decrease of apple leathers stickiness, the estimated response surface and its contour were plotted. An increment on RH could lead to an increase in the adhesion force, whereas a rise on maltodextrin content could lead to a decrease in the adhesion force (Fig. 6). A similar effect occurred with DCF; ALS conditioned at high RH had higher DCF, while the DCF of ALS with high maltodextrin content tended to decreased (Fig. 7). The results of optimization for decreasing stickiness of apple leathers are summarized in Table 6. The adhesion force and DCF of ALS were minimum when ALS contained high levels of maltodextrin (10%), when they were conditioned at low RH (33%), and also when their roughest side was faced to the plastic material. 3.3. Increasing stickiness experiments (II) 3.3.1. Screening design As the obtained results for the adhesion force in experiment II show (Table 7), there are several combinations of factor that gave high values of adhesion force, reaching a value of 6.53 N. Evaluating
Table 5 ANOVA and effects for adhesion force and DCF. Adhesion force
A: Relative humidity B: Maltodextrin AB Average
DCF
Estimated effect
Standard error
p-Value
Estimated effect
Standard error
p-Value
0.342 0.285 0.378 0.210
0.148 0.148 0.148 0.056
0.031⁄ 0.068 0.019⁄
0.056 0.036 0.036 0.025
0.023 0.023 0.023 0.009
0.025⁄ 0.135 0.135
Standard errors are based on total error with 21 d.f. p < 0.05 means statistical significant differences.
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(b) 10
(a)
0.4 0.5
0.1 0.2
0,8
0.6
0.3
8
Adhesion force
0,0 0,2 0,4 0,6 0,8
2
(%)
35 0
0.3
4
0.2 0.1 0.2
hu
2 0.1 0.1
l at
4
extrin
0.4
Re
6
Malto d
0.5
0.3
0.2
ive
40
8
6
m
45
0,0
0.1
id i
ty
(%
55 50
0,2
)
0,4
Maltodextrin (%)
(N)
0.4
0,6
0.1
Adhesion force (N)
0 35
40
45
50
55
Relative humidity (%) Fig. 6. Estimated response surface (a) and contours of estimated response surface (b) for adhesion force.
(a)
(b)
10 0.02 0.06
0.10 0.00 0.02 0.04 0.06 0.08 0.10
0.06
idi ty (% )
0.04 0.02 0.00
Malto d
4
extrin
2
(%)
35 0
0.00
0.02 0.04
4 0.00
0.02
2
ive
40 6
0.06 0.04
6
lat
8
hu m
55 50 45
Maltodextrin (%)
0.02
0.08
0.00
Re
Cohesiveness
0.08
0.04
8
2
(mg/cm )
0.00
2
2 DCF (mg/cm (mg/cm ) Cohesiveness )
0.02
0 35
40
45
50
55
Relative himidity (%) Fig. 7. Estimated response surface (a) and contours of estimated response surface (b) for DCF.
Table 6 Optimization for decreasing stickiness experiments. Factor
Low
High
Optimum adhesion force
Optimum DCF
Relative humidity (%) Maltodextrin (%)
33 0
55 10
33 10
33 10
the estimated effect of each factor, the glucose content had the higher effect on the adhesion force (Table 8) followed by the RH, the surface rugosity and the compression time. Moreover, the results of this design indicate that in the studied levels the variables RH, glucose content and surface rugosity had a significant statistical effect (p < 0.05) on the adhesion force (Table 8). As discussed above (I), the observations for the surface rugosity were in agreement with what was discussed in Section 3.1. While smoother the surface is, the contact area will be greater and so the adhesion force. Since in this section it is intended to increase stickiness of apple leathers, the surface rugosity will be set to the smoother side of the surface in the factorial design of the optimization step. The surface rugosity and the compression time had a combined effect on the adhesion force. But this could be related to the effect of surface rugosity for itself, since de combination of the compression time with any other factor did not had a significant effect on adhesion force. These results could be explained with what was discussed on Section 3.2.1, about the compression time. The compression time did not have a significant effect on the adhesion force due to the low compressibility of fruit leathers.
The results obtained in this screening design made it possible to define the variables which had an effect on the adhesion force so that the complete factorial planning can be elaborated to determine the optimal conditions that affect the increase of stickiness. The variables selected for the optimization step were RH and glucose content. 3.3.2. Factorial design The results of factorial design for the adhesion force are shown in Table 9. The effects of the variables on each response are presented on Table 10 with a 95% confidence level. The results of this design, as Pareto chart demonstrate (Fig. 8), indicate that only the RH had a significant statistical effect on the adhesion force. These results agree with what was discussed above, in Section 3.2.2. The cited authors stated that the surface tension of water and the interaction of water with the solids of the matrix, and any other factor affecting one of these, are the main factors responsible on the stickiness of low moisture food (Hoseney and Smewing, 1999; Adhikari et al., 2001). In these results, conditioning of ALS at higher RH may be associated with the water solvation due to
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Table 7 Screening design of 16 trials with adhesion force as response. Trial
Relative humidity
Glucose
Surface rugosity
Compression time
Adhesion force (N)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 1 +1 +1 +1 1 +1 1 1 +1 1 1 +1 1 +1 +1
1 +1 1 +1 1 -1 +1 1 +1 +1 +1 1 1 +1 +1 1
+1 1 1 +1 +1 1 +1 +1 1 1 +1 1 1 +1 1 +1
1 +1 1 1 1 +1 +1 +1 1 1 1 1 +1 +1 +1 +1
1.068 0.356 1.004 0.335 0.57 0.671 1.199 6.527 0.538 0.555 1.729 7.891 0.123 1.735 0.762 6.034
Table 8 Estimated effects on adhesion force from results of screening design for increasing stickiness experiments. Factor
Estimated effect
p-Value
Mean A: Relative humidity B: Glucose C: Surface rugosity D: Compression time AB AC AD BC BD CD
1.944 1.24 2.085 0.912 0.465 0.864 0.511 0.949 0.215 0.241 1.484
0.091* 0.011* 0.080* 0.729 0.526 0.703 0.488 0.872 0.857 0.018*
AB
B: Glucose
A: Relative humidity
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Standardized effect *
Significant for 85% confidence level. Fig. 8. Standardized Pareto Chart for adhesion force.
Table 9 Trials used in factorial design to determine adhesion force. Trial
Relative humidity
Glucose
Adhesion force
1 2 3 4 5 6 7
1 +1 0 +1 0 0 1
+1 +1 0 1 0 0 1
0.450 ± 0.02 0.943 ± 0.02 1.345 ± 0.13 0.963 ± 0.00 1.042 ± 0.03 1.118 ± 0.0 0.549 ± 0.06
Table 10 ANOVA and effects for adhesion force. Adhesion force
A: Relative humidity B: Glucose AB Average
or condensed on capillaries can reduce the micro-roughness of the surface. This effect may also have contributed to an increase on the adhesion force of ALS conditioned at high RH. To determine the most adequate operating conditions and to analyze the factors affecting the increase of stickiness of apple leathers, the estimated response surface and contour of the estimated response surface were plotted (Fig. 9). As the levels of both RH and glucose increased, the adhesion force of ALS also increased, reaching a maximum in the middle and then decreased again. The results of optimization are summarized in Table 11. The adhesion force of ALS was maximum (1.35 N) when ALS contained 5% of glucose and were conditioned at 44% of RH, and also when their smoother side were faced with each other. 3.4. Glass transition temperature of optimized samples
Estimated effect
Standard error
p-Value
0.453 0.059 0.039 0.915
0.138 0.138 0.138 0.052
0.004⁄ 0.670 0.777
Standard errors are based on total error with 21 d.f. p < 0.05 means statistical significant differences.
the increased amount of water being adsorbed from the surface or absorbed into the bulk. Thus, changing the properties of the fruit leather matrix, like viscosity, and the surface tension, and therefore increasing the adhesion force of ALS. Furthermore Iveson and Litster (1998) states that the adsorbed water as mono/multilayer
The optimized samples in each experiment, presenting a minimum stickiness (I) and a maximum stickiness (II) respectively, were analyzed by DSC to observe how the glass transition temperature (Tg) is related to stickiness of samples. The ALS conditioned at 44% RH with no addition of glucose or maltodextrin were selected as control samples because at that RH fruit leathers are found to behave as a leathery material, pliable enough, not to soft, not too hard. Also, this RH was used to conditioned apple leathers in a previously study (Valenzuela and Aguilera, 2013). A control sample (44% RH; 0%) was used to compare the Tg with an optimized sample for a minimum stickiness (33% RH; 10% maltodextrin) and with an optimized sample for a maximum stickiness (44% RH; 5% glucose). The results of DSC analyses of optimized ALS
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C. Valenzuela, J.M. Aguilera / Journal of Food Engineering 149 (2015) 51–60
(a)
(b)
10
0.7
Adhesion 1.0 force (N)
0.8
0.6
0.9 1.0
0.7
0.8
8
1,4
1.1
1,0
0,4 0,6 0,8 1,0 1,2 50 12 14
0,8 0,6 0,4
idi ty (% )
55 50 45
0,2
4
se (%
35
2
0
)
0.7
1.1
0.8 1.1 0.9
1.0
1.2 1.1
4 0.7
0.8
1.1
0.6 1.0 0.9
2
1.1 1.0
0.7 0.5
0.8
1.0
0.9
0.6
0
Re
6
Gluco
l at ive
40
8
6
hu
m
0,0
1.1
1.0
Glucose (%)
(N) Adhesion force
0.9
1,2
35
40
45
50
55
Relative humidity (%) Fig. 9. Estimated response surface (a) and contours of estimated response surface (b) for adhesion force.
Table 11 Optimization for increasing stickiness experiments.
argue that there is a strong relation between the Tg and the stickiness of apple leathers.
Factor
Low
High
Optimum adhesion force
Relative humidity (%) Glucose (%)
33 0
55 10
44 5
4. Conclusions
0.5
44%RH; 0% 44%RH; 5%G
33%RH; 10%M
Heat Flow (mW)
0.0
-0.5
-1.0
-1.5 -80
-60
-40
-20
0
20
40
60
80
Temperature (ºC) Fig. 10. Heat flow variation as a function of temperature for optimized and control apple leather strips.
samples presented differences in the Tg in each case, as can be seen in Fig. 10. Fruit leathers are composed of high amounts of low molecular sugars (Chinnici et al., 2005). As was stated by Roos and Karel (1991), materials with high amounts of monosacharides exhibit low Tg values and sticky points. The Tg of ALS conditioned at 44% RH, with no addition of glucose or maltodextrin was 22.06 °C and the Tg of ALS conditioned at 44% RH and 5% glucose was 26.97 °C. Therefore, the very low Tg of this last sample was attributed to the plasticizing effect of glucose (a low molecular sugar). Such low Tg values are responsible for a good flexibility of the apple leathers, which is required when handling or folding of the product is desired for some specific applications (Azeredo et al., 2009). As was also stated by Roos and Karel (1991), since Tg increases as the molecular weight increase, maltodextrin are used to improve the dehydration features and to decrease stickiness. As it was expected, the samples conditioned at 33% RH with an addition of 10% maltodextrin had the higher glass transition temperature ( 11.87 °C). Accordingly, the results of the present investigation
Large differences were found between the side facing upward and the side facing downward of apple leathers during drying, regarding the surface rugosity, which had a great impact on stickiness. The smoothest side of apple leathers had a higher adhesion force than the roughest side. The RH at which apple leathers were conditioned at also had a great influence on the adhesion force and the type of failure. The amount of absorbed water on the apple leather surface affected significantly both the decrease and increase of stickiness. The added ingredients enhance the effects of RH on stickiness of apple leathers. When maltodextrin was added to the apple leathers and these were conditioned at low relative humidity there was a significant decrease on the adhesion force and on the degree of cohesive failure. Otherwise, when glucose was added to the apple leathers and these were conditioned at high RH there was a significant increase on the adhesion force. Compression time did not have a significant effect on stickiness; this was possible due to the low compression of fruit leathers. The experimental design was a useful tool to found and set the optimum levels of each factors both to decrease and to increase stickiness. Along these lines, stickiness of apple leathers can be diminished using maltodextrin to avoid problems during processing and packaging or can be increased using glucose in order to use apple leathers in preparations such as lamination technology. Acknowledgements The authors express their sincere appreciation to CONICYT (National Commission for Scientific and Technological Research) for their financial support. References Adhikari, B., Howes, T., Bhandari, B., Truong, V., 2001. Stickiness in foods: a review of mechanisms and test methods. Int. J. Food Prop. 4 (1), 1–33. 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. Aguilera, J.M., 2003. Drying and dried products under the microscope. Food Sci. Technol. Int. 9 (3), 137–143. Azeredo, H., Mattoso, L.H.C., Wood, D., Williams, T.G., Avena-Bustillos, R.J., McHugh, T.H., 2009. Nanocomposite edible films from mango puree reinforced with cellulose nanofibers. J. Food Sci. 74 (5), N31–N35. Chan, H.T., Cavaletto, C.G., 1978. Dehydration and storage stability of papaya leather. J. Food Sci. 43 (6), 1723–1725.
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Chen, X.D., Pirini, W., Ozilgen, M., 2001. The reaction engineering approach to modelling drying of thin layer of pulped kiwifruit flesh under conditions of small Biot numbers. Chem. Eng. Process. 40 (4), 311–320. Chinnici, F., Spinabelli, U., Riponi, C., Amati, A., 2005. Optimization of the determination of organic acids and sugars in fruit juices by ion-exclusion liquid chromatography. J. Food Compos. Anal. 18 (2), 121–130. Chowdhury, M., Bala, B., Haque, M., 2011. Energy and exergy analysis of the solar drying of jackfruit leather. Biosyst. Eng. 110 (2), 222–229. Gabas, A., Telis, V., Sobral, P., Telis-Romero, J., 2007. Effect of maltodextrin and arabic gum in water vapor sorption thermodynamic properties of vacuum dried pineapple pulp powder. J. Food Eng. 82 (2), 246–252. Gay, C., 2002. Stickiness—some fundamentals of adhesion. Integr. Comp. Biol. 42 (6), 1123–1126. Gujral, H.S., Brar, S.S., 2003. Effect of hydrocolloids on the dehydration kinetics, color, and texture of mango leather. Int. J. Food Prop. 6 (2), 269–279. Gujral, H.S., Khanna, G., 2002. Effect of skim milk powder, soy protein concentrate and sucrose on the dehydration behaviour, texture, color and acceptability of mango leather. J. Food Eng. 55 (4), 343–348. Hadavinia, H., Kawashita, L., Kinloch, A., Moore, D., Williams, J., 2006. A numerical analysis of the elastic-plastic peel test. Eng. Fract. Mech. 73 (16), 2324–2335. Hoseney, R., Smewing, J., 1999. Instrumental measurement of stickiness of doughs and other foods. J. Text. Stud. 30 (2), 123–136. Hui, C., Lin, Y., Baney, J., 2000. The mechanics of tack: viscoelastic contact on a rough surface. J. Polym. Sci., Part B: Polym. Phys. 38 (11), 1485–1495. Irwandi, J., Man, Y., Yusof, S., Jinap, S., Sugisawa, H., 1998. Effects of type of packaging materials on physicochemical, microbiological and sensory characteristics of durian fruit leather during storage. J. Sci. Food Agric. 76 (3), 427–434. Irwandi, J., Man, Y.B., 1996. Durian leather: development, properties and storage stability. J. Food Qual. 19 (6), 479–489. Iveson, S.M., Litster, J.D., 1998. Growth regime map for liquid-bound granules. AIChE J. 44 (7), 1510–1518. Jaya, S., Das, H., 2009. Glass transition and sticky point temperatures and stability/ mobility diagram of fruit powders. Food Bioprocess Technol. 2 (1), 89–95. Kilcast, D., Roberts, C., 1998. Perception and measurement of stickiness in sugarrich foods. J. Text. Stud. 29 (1), 81–100. Labuza, T., Kaanane, A., Chen, J., 1985. Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. J. Food Sci. 50 (2), 385–392. Man, Y.B.C., Sin, K.K., 1997. Processing and consumer acceptance of fruit leather from the unfertilised floral parts of jackfruit. J. Sci. Food Agric. 75 (1), 102–108. Maskan, A., Kaya, S., Maskan, M., 2002. Effect of concentration and drying processes on color change of grape juice and leather (pestil). J. Food Eng. 54 (1), 75–80. Mathlouthi, M., Roge, B., 2003. Water vapour sorption isotherms and the caking of food powders. Food Chem. 82 (1), 61–71.
Michalski, M.-C., Desobry, S., Hardy, J., 1997. Food materials adhesion: a review. Crit. Rev. Food Sci. Nutr. 37 (7), 591–619. Ortega-Rivas, E., Juliano, P., Yan, H., 2006. Food Powders: Physical Properties, Processing, and Functionality. Springer. Persson, B., Albohr, O., Creton, C., Peveri, V., 2004. Contact area between a viscoelastic solid and a hard, randomly rough, substrate. J. Chem. Phys. 120 (18), 8779–8793. Quintero Ruiz, N.A., Demarchi, S.M., Massolo, J.F., Rodoni, L.M., Giner, S.A., 2012. Evaluation of quality during storage of apple leather. LWT-Food Sci. Technol. 47 (2), 485–492. Roos, Y., Karel, M., 1991. Applying state diagrams to food processing and development. Food Technol. 45 (12), 66–68. Silva, M., Sobral, P., Kieckbusch, T., 2006. State diagrams of freeze-dried camu-camu (Myrciaria dubia (HBK) Mc Vaugh) pulp with and without maltodextrin addition. J. Food Eng. 77 (3), 426–432. Song, J., Yu, J., 2002. Analysis of the T-peel strength in a Cu/Cr/Polyimide system. Acta Mater. 50 (16), 3985–3994. Telis, V., Martínez-Navarrete, N., 2009. Collapse and color changes in grapefruit juice powder as affected by water activity, glass transition, and addition of carbohydrate polymers. Food Biophys. 4 (2), 83–93. Tong, H.H., Wong, S., Law, M.W., Chu, K.K., Chow, A.H., 2008. Anti-hygroscopic effect of dextrans in herbal formulations. Int. J. Pharm. 363 (1), 99–105. Torley, P.J., Bhandari, B.R., Halley, P.J., 2006. Composition-rheology-property relationships of commercial fruit leathers. In: 4th International Symposium on Food Rheology and Structure. Torley, P.J., De Boer, J., Bhandari, B.R., Kasapis, S., Shrinivas, P., Jiang, B., 2008. Application of the synthetic polymer approach to the glass transition of fruit leathers. J. Food Eng. 86 (2), 243–250. Valenzuela, C., Aguilera, J.M., 2013. Aerated apple leathers: effect of microstructure on drying and mechanical properties. Drying Technol. 31 (16), 1951–1959. Valenzuela, C., Aguilera, J.M., 2015. Effects of maltodextrin on hygroscopicity and crispness of apple leathers. J. Food Eng. 144, 1–9. Vatthanakul, S., Jangchud, A., Jangchud, K., Therdthai, N., Wilkinson, B., 2010. Gold kiwifruit leather product development using quality function deployment approach. Food Qual. Prefer. 21 (3), 339–345. Vijayanand, P., Yadav, A., Balasubramanyam, N., Narasimham, P., 2000. Storage stability of guava fruit bar prepared using a new process. LWT-Food Sci. Technol. 33 (2), 132–137. Watts, J., Castle, J., Hall, T., 1988. The effect of water on the failure mechanism of adhesive joints. J. Mater. Sci. Lett. 7 (2), 176–178. Wegrzyn, T.F., Golding, M., Archer, R.H., 2012. Food layered manufacture: a new process for constructing solid foods. Trends Food Sci. Technol. 27 (2), 66–72. Zumelzu, E., Gipoulou, G., 2002. A T-peel test to assess the adhesive properties of organic coatings applied to electrolytic tinplate. Surf. Coat. Int. B: Coat. Trans. 85 (1), 35–38.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effects of different factors on stickiness of apple leathers Catalina Valenzuela ⇑, José Miguel Aguilera Department of Chemical and Bioprocesses Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile
a r t i c l e
i n f o
Article history: Received 28 July 2014 Received in revised form 15 September 2014 Accepted 21 September 2014 Available online 30 September 2014 Keywords: Stickiness Fruit leathers T-peel test Experimental design
a b s t r a c t Apple leathers (ALs) are restructured food products made by dehydration of a thin layer of apple puree, resulting in a thin and flexible sheet. AL are composed mainly by low-weight carbohydrates which are highly hygroscopic, so AL become sticky when stored at ambient relative humidity (RH). Stickiness in food could be considered as a negative or positive aspect. In this work, four factors affecting stickiness of AL were studied (ingredients, RH, surface rugosity and compression time), both to decrease and to increase it. Surface rugosity of AL had the greatest impact on stickiness, being the smoothest side stickier than the roughest side. The RH at which AL were conditioned and the ingredients used in this work also had a great influence on the adhesion force. In this manner, stickiness can be modified (increasing or decreasing) according to the specific requirements for using AL in different applications. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Fruit leathers are products with an intermediate moisture content which have a flexible sheet form. They are consumed as snacks in many parts of the world (Torley et al., 2006). These products are light, pleasant to chew and tasty, becoming an attractive way to incorporate fruit to diet, especially for children and adolescents (Quintero Ruiz et al., 2012). The manufacturing process of fruit leathers consist on drying a thin layer of a fruit puree until a leathery consistence is achieved (Vatthanakul et al., 2010). Many kind of fruits can be used to make fruit leathers, like apple, papaya, mango, guava, durian, jackfruit, grape and kiwifruit, among others (Chan and Cavaletto, 1978; Irwandi et al., 1998; Vijayanand et al., 2000; Chen et al., 2001; Maskan et al., 2002; Gujral and Brar, 2003; Chowdhury et al., 2011; Quintero Ruiz et al., 2012). The fruit leather matrix (is composed mainly by) mainly consists of carbohydrates such as sugars, pectin and cellulosic substances (Torley et al., 2008). These hydrophilic compounds have affinity with the surrounding water vapor, making the matrix of these products highly hygroscopic (Mathlouthi and Roge, 2003; Tong et al., 2008). To prevent the fruit leather to absorb moisture from the environment after drying, the product must be packed properly and immediatly (Irwandi and Man, 1996; Man and Sin, 1997; Gujral and Khanna, 2002). Stickiness (also termed pressure sensitive adhesion or tack) can be described as the adhesion force between two different materials which are in contact with each other under a light pressure ⇑ Corresponding author. Tel.: +56 9 63424709. E-mail address: [email protected] (C. Valenzuela). http://dx.doi.org/10.1016/j.jfoodeng.2014.09.029 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved.
(Hoseney and Smewing, 1999). Some food materials such as spray-dried fruit juices and fruit leathers tend to adhere to the processing equipments, packaging materials, fingers, palate and teeth (Kilcast and Roberts, 1998). This feature of the material to stick on surfaces is affected by the amount of sugars with a low molecular weight as glucose, fructose and sucrose (Adhikari et al., 2003). On the other hand, it has been documented that maltodextrins improve the quality of dehydrated products by decreasing their stickiness and increasing their product stability (Roos and Karel, 1991). This is because of the ability of maltodextrins to absorb water and create a surface barrier between particles, and to increase the glass transition temperature (Tg) (Telis and Martínez-Navarrete, 2009). Stickiness can be considered both as a negative or a positive attribute. In food processing, stickiness slows the speed of the mechanized process and increases cleaning costs of equipments. The sticking of food to packaging results in product losses, packaging damage and product deformation. These parameters could have influence on the consumer acceptance, especially if they are finger food that stick to fingers while eating (Kilcast and Roberts, 1998; Hoseney and Smewing, 1999; Adhikari et al., 2003). Under other conditions, stickiness is desirable in products like toffees, sausages, and rice for risotto preparation (Michalski et al., 1997). Even more, stickiness is needed in the lamination based technology. This technique consists in assembling consecutive layers of a food base material fused with a bonding layer of the same or another material (Wegrzyn et al., 2012). The most important factors affecting stickiness are viscosity, relative humidity, temperature, compression, food ingredients (especially low molecular weight sugars) and the kind of material
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that food is in contact with. For example, inorganic materials have a higher surface tension than organic materials (polymers). These polymers which present a low energy surface will absorb strongly to a high-energy surface in order to decrease the energy of the system (Kilcast and Roberts, 1998; Hoseney and Smewing, 1999). When at least one of these materials bonded together is flexible, the T-peel test has been often used to measure the adhesion strength (Watts et al., 1988; Song and Yu, 2002; Zumelzu and Gipoulou, 2002; Hadavinia et al., 2006). This test measures the force needed to pull apart two materials bonded in T form. The objective of this study was to quantify the effects of ingredient type, the relative humidity, the surface rugosity and the compression time on stickiness in apple leathers. All these variables were measured in order to study their effect on the adhesion force, both to decrease and to increase it. 2. Materials and methods 2.1. Materials Canned apple puree (Conservera Pentzke S.A., San Felipe. Chile) were purchased from a local supermarket. Total soluble solids of apple puree were measured with a digital refractometer (Atago. PR-201). Apple puree had 20 g soluble solids/100 g puree. Glucose and maltodextrin were provided by Hela (Hela Especias Chile S.A., Santiago. Chile). According to the label information, the dextrose equivalent (DE) of the maltodextrin was between 17 and 19. Low density polyethylene (LDPE) pouches were supplied by HOMS (Plásticos HOMS Ltda., Santiago, Chile). 2.2. Moisture content The moisture content of apple puree and maltodextrin was determined by drying 5 g of sample (apple puree or maltodextrin) in a convection oven at 105 °C until constant weight was achieved. The initial moisture content of apple puree was found to be about 4.52 ± 0.16 g water/g dry solids. The initial moisture content of maltodextrin was found to be about 0.01 ± 0.001 g water/g dry solids. 2.3. Experiment division As was stated before, stickiness can be considered as a negative or positive attribute. Regarding to the process conditions and packaging materials, the aim of this work was to determine which were the factor that reduce stickiness, to prevent apple leathers from adhere to the surface. Otherwise, when a sticky material is required for some specific applications such as lamination technology, the aim of this work was to determine which were the factors that increase stickiness of apple leathers. Therefore, the experiments were divided in two sections, studying stickiness decrease (I) or increase (II). In experiment I (decreasing stickiness), maltodextrin was used as an additive, since it has been found that it contributes to decrease stickiness. Also, the adhesion of apple leathers to packaging material (LDPE) was studied. In experiment II (increasing stickiness), glucose was used as an additive, since its low molecular weight contributes to increase stickiness. The adhesion of apple leathers to itself was also studied. 2.4. Formulation of apple leathers For experiment I, apple puree and maltodextrin were weighted to obtain a final concentration of 0, 5 and 10 g maltodextrin/g total weight. For experiment II, apple puree and glucose were weighted to obtain a final concentration of 0, 5 and 10 g glucose/g total
weight. The nomenclature for each formulation was set as 0%, 5% and 10% maltodextrin or glucose respectively. The calculated amount of maltodextrin or glucose was first mixed with a small amount of puree (10% of total puree added in each formulation). This step was performed manually at 20 °C, taking care not to form bubbles, mixing the ingredients using a glass rod until there were no lumps and the maltodextrin or glucose was uniformly dispersed, forming a homogeneous paste. The paste was then added to the rest of the puree. 2.5. Apple leather formation Formulated apple puree was spread as a thin layer within the space of a 2-mm height frame placed in an aluminum tray. The puree was leveled using a glass rod to ensure the thickness of the puree was uniform. The aluminum tray was previously covered with a silicone sheet to prevent apple leather from sticking after drying. Drying was carried out in an air circulation oven (Köttermann 2736, Hänigsen, Germany). Apple leathers were dried at 60 ± 1 °C to a final moisture content of approximately 0.12 g water/g dry solids. 60 °C was determined as a suitable temperature to be used in the AL drying process due to it is high enough for a proper drying rate, and at the same time prevents the product to be burned (Valenzuela and Aguilera, 2013). 2.6. Thickness measurement of apple leathers and plastic material Apple leathers and LDPE pouches thickness were measured with Mitutoyo Absolute digital micrometer (Mitutoyo Corp, Kanogawa, Japan) at six random positions and average values were used in all calculations. The thickness of LDPE pouches was 0.0855 ± 0.005 mm. The thickness of LDPE pouches was considered constant, while the thickness of apple leathers varied between samples. 2.7. Conditioning of apple leather strips Apple leathers were cut into rectangular strips of 90 18 mm. These apple leather strips (ALS) were equilibrated at different relative humidity (RH) at 20 ± 1 °C for about 15 days in desiccators, according to the method described by Labuza et al. (1985). The desiccators contained the following saturated salt solutions: MgCl2 (RH = 33%), K2CO3 (RH = 44%), and Mg(NO3)2 (RH = 55%). This humidity range (33–55%) was found to be the optimum conditions to maintain the rubbery-like state of the leather strips. At humidity levels greater than 55%, ALS were infested with mold, while the ALS became brittle below 33% humidity level (Valenzuela and Aguilera, 2015). 2.8. Scanning Electron Microscopy (SEM) surface analysis of apple leather strips SEM was used to characterize qualitatively the ALS surface microstructure. Samples (at least 3 specimens per sample) were conditioned at 0% RH over P2O5, then sputter-coated with gold– palladium and examined with a scanning electron microscope (JEOL JSM 5300, Jeol Ltd., Tokyo, Japan), operated at an acceleration voltage of 20 kV. Images were taken within a specimen at different positions and magnifications. Selected images were reported. ADDA II was used as interface between the microscope and the computer and the images were analyzed with the software AnalySISÒ version 3.2 (Soft Imaging System GmbH. Münster. Germany). 2.9. Compression of apple leathers strips In experiment I, ALS were placed in contact with the LDPE packaging material. Both ALS and LDPE were previously cut into
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rectangular strips of 90 18 mm. For experiment II, ALS were placed in contact with another ALS. The two materials in contact with each other were compressed using a hydraulic press (MVP Superline), at 6894.76 kPa during 3 or 90 s, as indicated by the experimental design. 2.10. Mechanical testing apple leather strips After the compression of ALS to (I) LDPE strips and (II) another ALS, the adhesion force between the materials was determined using the T-peel test by ASTM-D-1876 in the Universal Texture Analyzer TA.XTplus (Stable MicroSystems, Godalming, Surrey, UK). The T-peel test was performed pulling the T ends apart at a rate of cross-head movement of 25 mm/min (Fig. 1). A graph of force (N) versus displacement (mm) was recorded. Adhesion force (N) was determined as the mean peel force from the recorded graph using Exponent Software (Stable Micro Systems Ltd.).
Table 1 Two-level assessment for each factor. Factor
Low level ( 1)
High level (+1)
A: Relative humidity (%) B: Ingredienta (%) C: Surface rugosity ( ) D: Compression time (s)
33 0 Rough 3
55 10 Smooth 90
a I: for decreasing assay maltodextrin was used as ingredient. II: for increasing assays glucose was used as ingredient.
independent variable was tested at two levels, a high level (+1) and a low level ( 1), as shown in Table 1. For the optimization step, the two most important variables were selected and a factorial design (22) was created, in which the effect of two factors was studied in 28 runs. These runs included four blocks of seven runs each, and three center points per block.
2.11. Weighing of plastic material to determine the type of failure
2.13. Differential scanning calorimetry measurements
The LDPE strips were cleaned with acetone and their weights were recorded before and after the T-peel test, to check if there was an adhesive or cohesive failure. True adhesive failure occurs if there was a clean failure of the bond between the apple leather and the plastic material, with no apple leather residue remaining on the plastic material surface. If it was a cohesive failure, some of the apple leather residue must be left on the plastic material after the separation (Kilcast and Roberts, 1998). The degree of cohesive failure (DCF) was expressed in mg of apple leather residue remained on each cm2 of the plastic material (mg/cm2).
The glass transition temperature (Tg) of samples selected as optimum in each experiment was measured using a differential scanning calorimetry (DSC), (DSC 821e, Mettler Toledo, Schwerzenbach, Switzerland) according to Torley et al. (2008). The instrument used a refrigerated cooling system to achieve temperatures of 80 °C and a nitrogen DSC cell purge at 60 ml/min. Hermetic aluminium pans of 30 ll capacity were used. Small size samples of 7–10 mg were cut from the ALS and placed into the aluminium pans. The preparations were cooled at a rate of 5 °C/min to 80 °C, then they were left there for thirty minutes, and the glass transition was determined from the midpoint of the heat capacity change observed at the same heating rate. An empty hermetically sealed aluminium DSC pan was used as a reference.
2.12. Experimental procedure The optimization of the assay procedure was carried out in two steps. A screening design was used to detect those variables having the highest influence on the stickiness of apple leathers, while a factorial design was applied to find the best parameter settings for the increase or decrease of stickiness. As a first step, a full factorial screening design at two levels with four factors (24) was performed involving the variables: the relative humidity, the added ingredient (maltodextrin in decreasing stickiness, experiment I, and glucose in increasing stickiness, experiment II), the surface rugosity (the apple leather side facing upward is rougher to touch than the side facing downward during drying), and the compression time. Each
2.14. Statistical analysis Statgraphics Centurion Software XV, 15.1.02 version (StatPoint Inc., VA, USA) was used to design the screening and factorial design experiments and to evaluate the obtained results. 3. Results and discussion 3.1. Surface morphology As the morphology of the surface has a high impact on the adhesion, SEM images of the surface of ALS were taken. As Fig. 2 shows, there were big differences between the side facing upward and the side facing downward of apple leathers during drying. The side facing upward during drying was very rough presenting many hills and valleys, while the side facing downward during drying had a smoother surface. A high surface roughness may reduce the intimacy degree in the area where the ALS are in contact with both the plastic material and itself (Gay, 2002). In the roughest side of this fruit leather material, the contact will be restricted to the top of the hills, resulting in small isolated contact regions, even more if these hills have different highs (Fig. 3). As a result, in the roughest side of ALS the actual area of contact will be less than the nominal area. 3.2. Decreasing stickiness experiments (I)
Fig. 1. Schematic diagram of T-peel test.
3.2.1. Screening design The results obtained for the adhesion force and degree of cohesive failure in experiment I are shown in Table 2. The adhesion force was higher when all factors were in their highest level (+1)
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0% D
0% U
300 μm
5%G, D
5%G, U
10%G, D
10%G, U
5%M, D
5%M, U
10%M, D
10%M, U
Fig. 2. SEM images of apple leather surface. (G: glucose; M: maltodextrin; D: side facing downward during drying; U: side facing upward during drying).
reaching a value of 1.66 N, as can be observed in the third row of Table 2, but this high value was a result of the interaction of all factors. The effects of each factor were presented in Table 3 with a 95% confidence level, where can be observed that the surface rugosity had the highest effect on the adhesion force, followed by RH, maltodextrin content and compression time. The results of this design indicate that in the studied levels, the variables surface rugosity and maltodextrin content had a significant statistical effect (p < 0.05) on adhesion force (Table 3). The adhesion force was mostly affected by the surface rugosity. A higher surface rugosity
showed a negative effect on the adhesion force. This means that the roughest side of ALS, which was facing upward during drying, was adhered in a weaker way than the smoother surface. The observations for the high rugosity were in agreement with what was discussed in Section 3.1. Since in this section it is intended to decrease the stickiness of apple leathers, in the optimization step the surface rugosity will be set to the roughest side of the surface. Moreover, the RH at which the ALS were conditioned at, and the interaction of RH with maltodextrin content had also a significant statistical effect (p < 0.05) on the adhesion force (Table 3). Because of this, the effect of these two factors on the adhesion
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C. Valenzuela, J.M. Aguilera / Journal of Food Engineering 149 (2015) 51–60
contact area
contact sites
smooth surface
rough surface
Fig. 3. Diagram of the contact area depending on the roughness of the surface.
force will be studied on the optimization step. On the other side, compression time did not have a significant statistical effect on adhesion force. The effects of compression and compression time on the adhesion is closely related to the effect of the surface rugosity on the adhesion. As discussed in Section 3.1, the actual contact area will depend on the surface rugosity, but also on the applied load and the contact time (Hui et al., 2000). As discussed by Hui et al. (2000), under pressure of a rough surface, each surface hill
of one material (Fig. 3) will be in contact to the surface of the other material when the hill height exceeds the separation length between both materials. Only if the material is elastic enough, the applied pressure will displace the initial point reaching the top of another hill (lower than the first). In this manner, the area of real contact will increase with the contact time (Persson et al., 2004). If the material is highly deformable, a strong contact will be achieve and there will not be necessary to apply pressure. But if the material is too rigid the contact will remain weakly even under a reasonable pressure and contact time (Gay, 2002). In this work, the low effect that compression time had on the adhesion force of ALS may be related to the material rigidity. Fruit leathers are subjected to large drying periods, resulting in a considerable reduction of its thickness because of the water releasing. As discussed by Aguilera (2003), at low drying rates the final products are more compact and denser. Because of this, the molecules forming the network of the fruit leathers matrix are closely packed, which may lead to a low compressibility of the matrix. As a result, for the next step (the optimization factorial design), the compression time was set to constant value of 30 s. In relation to the DCF, maltodextrin content had a significant statistical effect (p < 0.05) on the cohesive failure between ALS and LDPE strip. Higher amounts of maltodextrin showed a negative effect on DCF (Table 3). That means that when maltodextrin was added it contributed to a true adhesive failure, with no residual apple leather remaining on the plastic surface. At low level of maltodextrin content fruit leather residues remained on the surface, indicating a cohesive failure within the materials. Moreover, the combined effect of maltodextrin content with RH had a significant statistical effect (p < 0.05) on the DCF. This could be related to the
Table 2 Screening design of 16 trials with adhesion force and DCF as response. Trial
Relative humidity
Malto-dextrin
Surface rugosity
Compression time
Adhesion force (N)
DCF (mg/cm2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+1 +1 +1 +1 1 1 1 1 +1 1 1 +1 1 1 +1 +1
1 +1 +1 +1 +1 +1 1 +1 +1 1 1 1 1 +1 1 1
1 +1 +1 1 +1 1 +1 1 1 1 +1 1 1 +1 +1 +1
1 1 +1 1 +1 1 +1 +1 +1 1 1 +1 +1 1 +1 1
0.095 0.393 1.656 0.174 0.336 0.000 0.166 0.000 0.321 0.115 0.398 0.077 0.188 0.253 0.427 0.198
33.97 0.00 18.10 35.16 8.65 0.00 0.00 0.00 0.00 99.60 78.25 94.44 16.90 87.54 306.98 75.79
Table 3 Estimated effects on adhesion force and DCF from results of screening design for decreasing stickiness experiments. Factor
Adhesion force Estimated effect
Mean A: Relative humidity B: Maltodextrin C: Surface rugosity D: Compression time AB AC AD BC BD CD *
Significant for 95% confidence level.
0.300 0.236 0.184 0.357 0.133 0.253 0.145 0.212 0.179 0.180 0.143
DCF p-Value 0.042* 0.232 0.046* 0.212 0.020* 0.333 0.177 0.243 0.24 0.339
Estimated effect 0.674 0.431 0.877 0.465 0.055 0.766 0.283 0.610 0.216 0.357 0.236
p-Value 0.403 0.022* 0.369 0.912 0.084* 0.575 0.146 0.666 0.483 0.639
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Table 4 Trials used in factorial design to determine adhesion force and DCF. Trial
Relative humidity
Maltodextrin
Adhesion force
DCF
1 2 3 4 5 6 7
1 +1 0 1 0 0 +1
+1 +1 0 1 0 0 1
0.000 ± 0.000 0.719 ± 0.005 0.287 ± 0.013 0.093 ± 0.007 0.164 ± 0.017 0.153 ± 0.012 0.057 ± 0.003
0.000 ± 0.000 0.093 ± 0.003 0.016 ± 0.001 0.000 ± 0.000 0.022 ± 0.007 0.015 ± 0.005 0.020 ± 0.009
AB
B: Maltodextrin
A: Relative humidity
effect of maltodextrin content on CDF for itself, or that the effect of RH and maltodextrin together enhanced the decrease of DCF. The selected factors for the optimization step were the ones previously mentioned: maltodextrin content and RH. 3.2.2. Factorial design The results of the factorial design for adhesion force and DCF are shown in Table 4. The effects of the variables on each response are presented on Table 5 with a 95% confidence level. The results of this design indicate that RH and the combination of RH with maltodextrin content had a significant statistical effect (p < 0.05) on the adhesion force (Table 5 and Fig. 4). These results agree with Hoseney and Smewing (1999) and Adhikari et al. (2001) who claim that the water surface tension and the interaction of water with the solids of the matrix are the main cause of stickiness in low moisture food. Thus, each factor that influences the water surface tension will have a large effect on the material surface stickiness. That is why when ALS were conditioned at low RH, the possibility of water to be adsorbed from the surface of the apple leather decreased, thus maintaining the surface dry and therefore no adhesion occurred. On the other side, the addition of maltodextrin to ALS enhanced the effect of RH. As stated by Roos and Karel (1991), high molecular ingredients, like maltodextrin; have a high Tg, increasing the Tg of the mixture. Maltodextrins have been used as drying aids in several studies, which have the property of reducing the stickiness and agglomeration of food powders during storage (Adhikari et al., 2001; Silva et al., 2006; Gabas et al., 2007; Telis and Martínez-Navarrete, 2009). In agreement with Ortega-Rivas et al. (2006), the effect of maltodextrin on the reduction of the adhesion force may be due to: it competes with sugars of the fruit leather matrix for moisture; it acts as physical barrier between particles or between particle and moisture; and it increases the Tg of the mixture, thus increasing the sticky point temperature (temperature higher than Tg, at which the product becomes sticky (Jaya and Das, 2009)). In relation to DCF, the results of the factorial design indicate that RH had a significant statistical effect (p < 0.05) on DCF (Table 5 and Fig. 5). When ALS were conditioned at low RH (33%), there were no particles of fruit leathers remaining on the plastic material surface, this means that there was a true adhesive failure between the ALS and the LDPE strip.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Standardized effect Fig. 4. Standardized Pareto Chart for adhesion force.
AB
B: Maltodextrin
A: Relative humidity
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Standarized effect Fig. 5. Standardized Pareto Chart for DCF.
To determine the most adequate operating conditions and to analyze the factors affecting the decrease of apple leathers stickiness, the estimated response surface and its contour were plotted. An increment on RH could lead to an increase in the adhesion force, whereas a rise on maltodextrin content could lead to a decrease in the adhesion force (Fig. 6). A similar effect occurred with DCF; ALS conditioned at high RH had higher DCF, while the DCF of ALS with high maltodextrin content tended to decreased (Fig. 7). The results of optimization for decreasing stickiness of apple leathers are summarized in Table 6. The adhesion force and DCF of ALS were minimum when ALS contained high levels of maltodextrin (10%), when they were conditioned at low RH (33%), and also when their roughest side was faced to the plastic material. 3.3. Increasing stickiness experiments (II) 3.3.1. Screening design As the obtained results for the adhesion force in experiment II show (Table 7), there are several combinations of factor that gave high values of adhesion force, reaching a value of 6.53 N. Evaluating
Table 5 ANOVA and effects for adhesion force and DCF. Adhesion force
A: Relative humidity B: Maltodextrin AB Average
DCF
Estimated effect
Standard error
p-Value
Estimated effect
Standard error
p-Value
0.342 0.285 0.378 0.210
0.148 0.148 0.148 0.056
0.031⁄ 0.068 0.019⁄
0.056 0.036 0.036 0.025
0.023 0.023 0.023 0.009
0.025⁄ 0.135 0.135
Standard errors are based on total error with 21 d.f. p < 0.05 means statistical significant differences.
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C. Valenzuela, J.M. Aguilera / Journal of Food Engineering 149 (2015) 51–60
(b) 10
(a)
0.4 0.5
0.1 0.2
0,8
0.6
0.3
8
Adhesion force
0,0 0,2 0,4 0,6 0,8
2
(%)
35 0
0.3
4
0.2 0.1 0.2
hu
2 0.1 0.1
l at
4
extrin
0.4
Re
6
Malto d
0.5
0.3
0.2
ive
40
8
6
m
45
0,0
0.1
id i
ty
(%
55 50
0,2
)
0,4
Maltodextrin (%)
(N)
0.4
0,6
0.1
Adhesion force (N)
0 35
40
45
50
55
Relative humidity (%) Fig. 6. Estimated response surface (a) and contours of estimated response surface (b) for adhesion force.
(a)
(b)
10 0.02 0.06
0.10 0.00 0.02 0.04 0.06 0.08 0.10
0.06
idi ty (% )
0.04 0.02 0.00
Malto d
4
extrin
2
(%)
35 0
0.00
0.02 0.04
4 0.00
0.02
2
ive
40 6
0.06 0.04
6
lat
8
hu m
55 50 45
Maltodextrin (%)
0.02
0.08
0.00
Re
Cohesiveness
0.08
0.04
8
2
(mg/cm )
0.00
2
2 DCF (mg/cm (mg/cm ) Cohesiveness )
0.02
0 35
40
45
50
55
Relative himidity (%) Fig. 7. Estimated response surface (a) and contours of estimated response surface (b) for DCF.
Table 6 Optimization for decreasing stickiness experiments. Factor
Low
High
Optimum adhesion force
Optimum DCF
Relative humidity (%) Maltodextrin (%)
33 0
55 10
33 10
33 10
the estimated effect of each factor, the glucose content had the higher effect on the adhesion force (Table 8) followed by the RH, the surface rugosity and the compression time. Moreover, the results of this design indicate that in the studied levels the variables RH, glucose content and surface rugosity had a significant statistical effect (p < 0.05) on the adhesion force (Table 8). As discussed above (I), the observations for the surface rugosity were in agreement with what was discussed in Section 3.1. While smoother the surface is, the contact area will be greater and so the adhesion force. Since in this section it is intended to increase stickiness of apple leathers, the surface rugosity will be set to the smoother side of the surface in the factorial design of the optimization step. The surface rugosity and the compression time had a combined effect on the adhesion force. But this could be related to the effect of surface rugosity for itself, since de combination of the compression time with any other factor did not had a significant effect on adhesion force. These results could be explained with what was discussed on Section 3.2.1, about the compression time. The compression time did not have a significant effect on the adhesion force due to the low compressibility of fruit leathers.
The results obtained in this screening design made it possible to define the variables which had an effect on the adhesion force so that the complete factorial planning can be elaborated to determine the optimal conditions that affect the increase of stickiness. The variables selected for the optimization step were RH and glucose content. 3.3.2. Factorial design The results of factorial design for the adhesion force are shown in Table 9. The effects of the variables on each response are presented on Table 10 with a 95% confidence level. The results of this design, as Pareto chart demonstrate (Fig. 8), indicate that only the RH had a significant statistical effect on the adhesion force. These results agree with what was discussed above, in Section 3.2.2. The cited authors stated that the surface tension of water and the interaction of water with the solids of the matrix, and any other factor affecting one of these, are the main factors responsible on the stickiness of low moisture food (Hoseney and Smewing, 1999; Adhikari et al., 2001). In these results, conditioning of ALS at higher RH may be associated with the water solvation due to
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Table 7 Screening design of 16 trials with adhesion force as response. Trial
Relative humidity
Glucose
Surface rugosity
Compression time
Adhesion force (N)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 1 +1 +1 +1 1 +1 1 1 +1 1 1 +1 1 +1 +1
1 +1 1 +1 1 -1 +1 1 +1 +1 +1 1 1 +1 +1 1
+1 1 1 +1 +1 1 +1 +1 1 1 +1 1 1 +1 1 +1
1 +1 1 1 1 +1 +1 +1 1 1 1 1 +1 +1 +1 +1
1.068 0.356 1.004 0.335 0.57 0.671 1.199 6.527 0.538 0.555 1.729 7.891 0.123 1.735 0.762 6.034
Table 8 Estimated effects on adhesion force from results of screening design for increasing stickiness experiments. Factor
Estimated effect
p-Value
Mean A: Relative humidity B: Glucose C: Surface rugosity D: Compression time AB AC AD BC BD CD
1.944 1.24 2.085 0.912 0.465 0.864 0.511 0.949 0.215 0.241 1.484
0.091* 0.011* 0.080* 0.729 0.526 0.703 0.488 0.872 0.857 0.018*
AB
B: Glucose
A: Relative humidity
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Standardized effect *
Significant for 85% confidence level. Fig. 8. Standardized Pareto Chart for adhesion force.
Table 9 Trials used in factorial design to determine adhesion force. Trial
Relative humidity
Glucose
Adhesion force
1 2 3 4 5 6 7
1 +1 0 +1 0 0 1
+1 +1 0 1 0 0 1
0.450 ± 0.02 0.943 ± 0.02 1.345 ± 0.13 0.963 ± 0.00 1.042 ± 0.03 1.118 ± 0.0 0.549 ± 0.06
Table 10 ANOVA and effects for adhesion force. Adhesion force
A: Relative humidity B: Glucose AB Average
or condensed on capillaries can reduce the micro-roughness of the surface. This effect may also have contributed to an increase on the adhesion force of ALS conditioned at high RH. To determine the most adequate operating conditions and to analyze the factors affecting the increase of stickiness of apple leathers, the estimated response surface and contour of the estimated response surface were plotted (Fig. 9). As the levels of both RH and glucose increased, the adhesion force of ALS also increased, reaching a maximum in the middle and then decreased again. The results of optimization are summarized in Table 11. The adhesion force of ALS was maximum (1.35 N) when ALS contained 5% of glucose and were conditioned at 44% of RH, and also when their smoother side were faced with each other. 3.4. Glass transition temperature of optimized samples
Estimated effect
Standard error
p-Value
0.453 0.059 0.039 0.915
0.138 0.138 0.138 0.052
0.004⁄ 0.670 0.777
Standard errors are based on total error with 21 d.f. p < 0.05 means statistical significant differences.
the increased amount of water being adsorbed from the surface or absorbed into the bulk. Thus, changing the properties of the fruit leather matrix, like viscosity, and the surface tension, and therefore increasing the adhesion force of ALS. Furthermore Iveson and Litster (1998) states that the adsorbed water as mono/multilayer
The optimized samples in each experiment, presenting a minimum stickiness (I) and a maximum stickiness (II) respectively, were analyzed by DSC to observe how the glass transition temperature (Tg) is related to stickiness of samples. The ALS conditioned at 44% RH with no addition of glucose or maltodextrin were selected as control samples because at that RH fruit leathers are found to behave as a leathery material, pliable enough, not to soft, not too hard. Also, this RH was used to conditioned apple leathers in a previously study (Valenzuela and Aguilera, 2013). A control sample (44% RH; 0%) was used to compare the Tg with an optimized sample for a minimum stickiness (33% RH; 10% maltodextrin) and with an optimized sample for a maximum stickiness (44% RH; 5% glucose). The results of DSC analyses of optimized ALS
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C. Valenzuela, J.M. Aguilera / Journal of Food Engineering 149 (2015) 51–60
(a)
(b)
10
0.7
Adhesion 1.0 force (N)
0.8
0.6
0.9 1.0
0.7
0.8
8
1,4
1.1
1,0
0,4 0,6 0,8 1,0 1,2 50 12 14
0,8 0,6 0,4
idi ty (% )
55 50 45
0,2
4
se (%
35
2
0
)
0.7
1.1
0.8 1.1 0.9
1.0
1.2 1.1
4 0.7
0.8
1.1
0.6 1.0 0.9
2
1.1 1.0
0.7 0.5
0.8
1.0
0.9
0.6
0
Re
6
Gluco
l at ive
40
8
6
hu
m
0,0
1.1
1.0
Glucose (%)
(N) Adhesion force
0.9
1,2
35
40
45
50
55
Relative humidity (%) Fig. 9. Estimated response surface (a) and contours of estimated response surface (b) for adhesion force.
Table 11 Optimization for increasing stickiness experiments.
argue that there is a strong relation between the Tg and the stickiness of apple leathers.
Factor
Low
High
Optimum adhesion force
Relative humidity (%) Glucose (%)
33 0
55 10
44 5
4. Conclusions
0.5
44%RH; 0% 44%RH; 5%G
33%RH; 10%M
Heat Flow (mW)
0.0
-0.5
-1.0
-1.5 -80
-60
-40
-20
0
20
40
60
80
Temperature (ºC) Fig. 10. Heat flow variation as a function of temperature for optimized and control apple leather strips.
samples presented differences in the Tg in each case, as can be seen in Fig. 10. Fruit leathers are composed of high amounts of low molecular sugars (Chinnici et al., 2005). As was stated by Roos and Karel (1991), materials with high amounts of monosacharides exhibit low Tg values and sticky points. The Tg of ALS conditioned at 44% RH, with no addition of glucose or maltodextrin was 22.06 °C and the Tg of ALS conditioned at 44% RH and 5% glucose was 26.97 °C. Therefore, the very low Tg of this last sample was attributed to the plasticizing effect of glucose (a low molecular sugar). Such low Tg values are responsible for a good flexibility of the apple leathers, which is required when handling or folding of the product is desired for some specific applications (Azeredo et al., 2009). As was also stated by Roos and Karel (1991), since Tg increases as the molecular weight increase, maltodextrin are used to improve the dehydration features and to decrease stickiness. As it was expected, the samples conditioned at 33% RH with an addition of 10% maltodextrin had the higher glass transition temperature ( 11.87 °C). Accordingly, the results of the present investigation
Large differences were found between the side facing upward and the side facing downward of apple leathers during drying, regarding the surface rugosity, which had a great impact on stickiness. The smoothest side of apple leathers had a higher adhesion force than the roughest side. The RH at which apple leathers were conditioned at also had a great influence on the adhesion force and the type of failure. The amount of absorbed water on the apple leather surface affected significantly both the decrease and increase of stickiness. The added ingredients enhance the effects of RH on stickiness of apple leathers. When maltodextrin was added to the apple leathers and these were conditioned at low relative humidity there was a significant decrease on the adhesion force and on the degree of cohesive failure. Otherwise, when glucose was added to the apple leathers and these were conditioned at high RH there was a significant increase on the adhesion force. Compression time did not have a significant effect on stickiness; this was possible due to the low compression of fruit leathers. The experimental design was a useful tool to found and set the optimum levels of each factors both to decrease and to increase stickiness. Along these lines, stickiness of apple leathers can be diminished using maltodextrin to avoid problems during processing and packaging or can be increased using glucose in order to use apple leathers in preparations such as lamination technology. Acknowledgements The authors express their sincere appreciation to CONICYT (National Commission for Scientific and Technological Research) for their financial support. References Adhikari, B., Howes, T., Bhandari, B., Truong, V., 2001. Stickiness in foods: a review of mechanisms and test methods. Int. J. Food Prop. 4 (1), 1–33. 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. Aguilera, J.M., 2003. Drying and dried products under the microscope. Food Sci. Technol. Int. 9 (3), 137–143. Azeredo, H., Mattoso, L.H.C., Wood, D., Williams, T.G., Avena-Bustillos, R.J., McHugh, T.H., 2009. Nanocomposite edible films from mango puree reinforced with cellulose nanofibers. J. Food Sci. 74 (5), N31–N35. Chan, H.T., Cavaletto, C.G., 1978. Dehydration and storage stability of papaya leather. J. Food Sci. 43 (6), 1723–1725.
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