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Factors influencing the friction of raisins during processing and handling
0 PII:
SO260-8774(97)00037-X
Joumul qf Fovd Engineering 33 (1997) 38%3Y3 1997 Elsevicr Science Limited. All rights rescrvcd Printed in &eat Britam 0260-8774197 $17.00 +O.OO
ELSEVIER
Factors Influencing the Friction of Raisins During Processing and Handling A. E. Kostaropoulos,a * J. Mandala: & G. D. Saravacof “Department
W. E. L. Spies@
of Food Science and Technology, Agricultural University of Athens, 75, Votanikos-Athens 11855, Greece “Bundesforschungsanstalt fiir Erngrung, Karlsruhe, Germany (Received
10 October
1995; accepted
Iera Odes
18 May 1997)
ABSTRACT Our study on the friction of Sultana and Corinth raisins indicates that there is a linear increase of the static and the kinetic friction force when the sample was loaded with added weights. An increase in the moisture content, up to 18%, reduces the friction of Sultana raisins, but increases that of Corinth raisins. For moistures larger than 307G, the friction of both types of raisins remains constant. Anti-sticking substances, reduce the friction but increase the stick-slip effect. The friction of raisins increases with sugar loss. The friction coefficients of raisins, sliding on Teflon-coated surfaces, are reduced by more than 30910, compared to PVC surfaces. 0 1997 Elsevier Science Limited. All rights reserved
INTRODUCTION The friction and adhesion properties of solid and semi-solid foods are needed for the rational design of bins, silos, and material handling and processing equipment. They are also important in packaging and in evaluating texture in compression tests. Reduced friction means less energy in transport and less damage to the products. This is especially important in processes in which there is a relative motion between sensitive products and the contact surfaces of handling or processing equipment. In raisins, friction is important in handling, washing, eliminating the stem cups, cleaning and sorting processes, and packaging system design. Besides friction and adhesion, the angle of repose, is another material property involved in bulk storage estimations. In case of raisins it is about 34”. *To whom correspondence
should be addressed. 385
Tel.: (301) 529 4692; fax: (301) 684 7111.
386
A. E. Kostaropoulos et al.
There is not much work done in the field of food friction and adhesion. A review of the subject was given by Mohsenin (1980) Voisey and Reid (1974) investigated the influence of the applied normal force, the area of the contact surface, and the speed the test sample over the surface. This investigation was carried out for frozen/ thawed peas and drained canned products, such as baked beans, green beans, peaches and spinach. The reduction of friction through lubrication of the samples was investigated by Atkin and Sherman (1984) and Goh and Sherman (1987) who applied the analysis of stress relaxation, and Canet and Sherman (1988) who studied the influence of friction on the uniaxial compression of potato flesh. In most of the research work that has been reported, the adhesion forces were not significant, as the investigated products were not sticky. However, friction depends not only on the applied normal force, but also on the force by which a solid material is moved over another. The nature of the materials in contact, and especially adhesion and cohesion forces, are also involved (Mohsenin, 1980). For common applications, friction is defined by the empirical law: F = N xJ Where F is the force required to move a solid material over a surface, N is the applied normal force, and f is the coefficient of friction. We distinguish between the static coefficient of friction fs, and the kinetic coefficient of friction fk. The static coefficient is a measure of the resistance caused by friction when a solid starts moving over another. The kinetic coefficient takes into account the resistance due to friction after this motion has started. In products with a certain degree of elastic freedom, when sliding over a firm surface takes place, the ‘stick-slip’ effect is observed (Mohsenin, 1980). Raisins can be classified as sticky dried products. Their stickiness increases when damaged, as the sugar content on their surface increases. In the present work, we investigated the influence of several factors on the friction of raisins when sliding over ‘different flat surfaces. Such factors are the moisture content, the applied normal force, the use of anti-sticking substances, and the temperature of the product.
MATERIALS
AND METHODS
Materials The materials tested were commercial Sultana and Corinth raisins. Both raisin types were produced by sun drying of seedless grape berries. The Sultana grapes were pretreated by immersion in 2% K2C03 solution to which a small amount of olive oil had been added. This was done for destroying the natural wax coating, resulting in an increase in the drying rate. The Sultana raisins were treated with SO1 after drying. The Corinth raisins were not pre-treated. The Sultana raisins, according to the Greek classification, belonged to number 2 class. Their average length was about 13.6 mm, and the width approximately 4.8 mm. The Corinth raisins belonged to the Greek crop class, and the average length and width were approximately 6.0 and 3.7 mm, respectively. The raisins were conditioned by adjusting their moisture content to the desired level. The conditioning took place in glass jars, which contained different saturated salt solutions (Spiess & Wolf, 1983). The water content of the product was determined by the sorption isotherm of the sample and was checked by the oven drying method (24 h, 70°C). Measurements have been also carried out for
The friction of raisins
3x7
investigating the influence of anti-sticking substances on the friction of Sultana raisins. The commercial substances used were special anti-sticking oils (Delios and Durkex 550). Instrumentation
For measuring the friction coefficients, a box-like apparatus similar to that described by Voisey and Reid (1974) was used (Fig. 1). This apparatus was originally used by Clark and McFarland (1973) and referred to by Voisey and Reid (1974). The apparatus consisted of a PVC frame, with a cover that could be moved up and down for adjusting the height of raisins put in it. This way, no contact occurred between the frame and the surface on which the raisins moved. The four sides of the frame were each 4 cm in width. The weight of the empty frame was 234.52 g. The product put in it was 25 g. A fine steel wire was used to pull the frame along a horizontal contact surface. The wire passed under a pulley and was attached to a universal testing machine (Instron 1011). The machine was connected to a PC. The results were calculated automatically. The main material of the contact surface was PVC. However, measurements were also carried out with contact surface of stainless steel and Teflon. Methods
The box-like temperatures.
frame, filled with raisins was conditioned at various moistures and The standard testing weight was that of the frame plus the weight of
Fig. 1. Apparatus of measurement of friction.
388
A. E. Kostaropoulos et al.
the loaded material, i.e. a total of 259.52 g. In testing the influence of the applied normal force, additional weights were put on the frame. In all experiments, the sliding speed was 120 mm/min. Force-time diagrams were obtained for every test. Each experiment was repeated 12 times. The frame was refilled with new material of the same batch, after each test. Unless otherwise stated, the temperature of the samples was about 20°C and the material on which the raisins slipped was PVC. For investigating the influence of the anti-sticking substances, Sultana raisins were dipped for 1 s in the substances cited above. The raisins were then left at ambient temperature to dry off, before using them in friction experiments. For reducing the sugars content, the Sultana raisins were dipped in water at 35°C for selected times. The loss of sugars was measured gravimetrically. The average sugar content of the Sultana raisins before leaching was estimated to be approximately 70% (Payne, 1987).
RESULTS
AND DISCUSSION
Results The main results of the measurements are given in Table 1. The coefficient of variation of the values of the measurements for most cases was about 7%. Table 1 indicates the influence of added weight and moisture content on the coefficient of friction of Sultana raisins and Corinth raisins. The added weights were 100-400 g. Moisture content was 17% and 35%. At constant moisture content, the relationship between force and added weight is linear. As indicated by Mohsenin (1980), the static force of friction is larger than the kinetic one. For a certain moisture content the difference between the static and the kinetic force remains constant all over the range of added weights used, unless the sample was pre-treated with anti-sticking compounds. This difference decreased as the moisture content of the product increased. At constant added weight, the mean static or kinetic forces of samples of lower moisture content, were larger than those corresponding to samples of higher moisture content. The slope of the linear relationship: force-added weight does not depend on the moisture content of the products. The ratio of static to kinetic friction coefficient (fslf) at a constant moisture content and added weight is constant (fJ& = 1.1). However, at a constant moisture, the relationship between the friction coefficients and the added weight is not linear. It is remarkable that the friction coefficients at a moisture of 17% decrease when additional weights are added, while the coefficients of samples of 35% moisture increase under the same conditions. In both cases, at high added weight and constant moisture, the relationship between the friction coefficients and the added weight tends to become constant. This result is similar to the findings of Richter (1954) for foodstuffs, as referred to by Mohsenin (1980). Figure 2 shows the influence of moisture content on the static and the kinetic coefficients of friction of Sultana and Corinth raisins for a constant load of 260 g. For Sultana raisins, the coefficients of friction, in samples of low moisture content (lo-15%), decrease from an almost constant value of 0.38 for the static and 0.33 for the kinetic coefficient of friction, down to the minimum values of 0.27 and 0.24 respectively. At higher moisture, both coefficients increase logarithmically, up to the almost constant values 0.33 and 0.29 respectively, at moisture contents exceeding
The friction
389
of raisins
50%. Both forces have a minimum at 18% moisture content. In Corinth raisins, exactly the opposite happens. The static and the kinetic coefficients of friction, at moistures IO-15%, are 0.43 and 0.37 respectively. These values increase at 18% moisture to 0.57 and 0.51, and then reach almost constant values, 0.51 and 0.46 at moisture contents higher than 50%. An increase of the temperature of the sample, from 20 to 30°C was found to increase the static and the kinetic friction coefficients. The dipping of raisins in special anti-sticking oils, at constant moisture content (35%) and load 260 g, reduced the static friction force, but increased the stick-slip effect (Fig. 3). The reduction of friction of Sultana raisins due to anti-sticking substances is given in Table 2. The temperature of the samples was 25”C, and their moisture content 35%. Finally, the effect of sugar loss and the influence of the contact surface on the coefficients of friction of Sultana raisins was investigated. The sugar content of raisins was reduced up to 43%, after dipping the Sultana raisins in water. The results are given in Fig. 4. Both friction coefficients were found to increase as the sugar content was reduced. The results of the effect of surface on which raisins slide, are given in Table 3. The values are given as a percentage of the maximum value, which for both types of raisins is the value sliding on a surface of PVC (static friction). The moisture content of both types of raisins was about 14%.
Mean Values of Friction Coefficients Type of ra kin
s s s s s s s s S ii
s
S
c C
C
c’
Moisture content
Added weight“
@I
k)
17 17 17 17 35 35 35 9 17 18 35 5s 3s 9 14 18 35 55
100 200 300 400 203.5 305.5 404.4 -
-
TABLE 1 of Sultana (S) and Corinth (C) Raisins on PVC Surface
Temperature (“C)
Coefficients
of variation (S) -.-
SII 20 20 20 20 20 20 20 20
f30Il -
Coefficients qf fiction’
20 20 20 20 20
Remarks: “The standard load in all cases was: 260 g. ‘yfs,fk: coefficient of static and kinetic friction.
0.57 0.52 0.50 0.50 0.32 0.37 0.39 0.38 0.37 0.27 0.32 0.33 0.42 0.43 0.43 0.57 0.53 0.51
0.50 0.45 0.45 0.45 0.30 0.35 0.37 0.33 0.33 0.24 0.29 0.29 0.40 0.37 0.37 0.50 0.47 0.46
4.45-7.38
3.66-6.94
7.43-l 1.31
6.30-l 3.48
3.84-7.92
3.0-7.66
7.71
8.10
3.98-12.71
4.10-14.87
A. E. Kostaropoulos et al.
390
0.7 -
’ fs (S) + fk (S) x fs (K)
’ fk (K)
0.60' 'E 0 'i v- 0.575 E s 0.4E : 0 0.3--
0.2' 5
I
I
I
I
I
I
I
I
I
I
J
10
15
20
25
30
35
40
45
50
55
60
Moisture content Fig. 2. Influence
(%)
of moisture content on coefficients of friction (fs, fk) on PVC surface (S:
Sultana raisins, C: Corinth raisins).
Discussion At constant moisture content, the forces of friction increase linearly with the applied load. The slope of this curve does not depend on the moisture content of the product. The difference between the static and the kinetic force of friction decreases
Durkex
550
Without anti-sticking substances
Delios
036
zs
z
g a4 9
j
m
0,2
0
03
0.4
0.2
0
to
20
Displacement
30 (mm)
40
50
0
0
10
a
Displacement
30 (mm)
40
50
0
10
20
Displacement
30
40
50
(mm)
Fig. 3. Influence of anti-sticking oils (Delios, Durkex) on friction of Sultana raisins.
391
The fn‘cfion of raisins
Effect
TABLE 2 of Anti-Sticking Substances on the Coefficient of Friction (T = 25”C, Moisture Content: 35%, PVC Surface)
Anti-sticking substance
0.30 0.14 0.32
Raisins
Coeficienfs of variation (%I
fk
Delios Durkex 550 No Treatment
of Sultana
0.25 0.07 0.29
8.17 13.55 7.92
16.58 13.89 7.66
when the moisture of the raisins increases. However, at constant applied load, the difference between their mean values is constant (Table 1). The static and the dynamic friction coefficients at 17% moisture, decreased as the applied load increased, but these coefficients increased when the moisture of the product reached 35%. In all cases, at quite large applied loads, all friction coefficients of Sultana raisins tended to reach a constant final average value of about 0.45 (Table 1). For moisture contents below 15% and above 40%, the friction coefficients of raisins do not depend on the moisture content of the product. At constant applied load, an increase in the moisture content of Sultana raisins, reduces the static and
0,35 ’ 5
I
I
10
15
I 20
I
I
I
I
25
30
35
40
Sugar loss (%) Fig. 4. Influence
of sugar loss on coefficients
of friction (fq, fk).
45
392
A. E. Kostaropoulos et al.
TABLE3 Influence of Sliding Surface on Coefficient of Friction of Sultana and Corinth Raisins (% Coefficient of Friction According to the Maximum Value) Sliding surface
f?
.fk
100 90.91 63.69
90.91 81.82 49.09
100 98.52 62.55
92.62 91.88 52.58
Sultana raisins
PVC Steel Teflon Corinth raisins
PVC Steel Teflon
the kinetic coefficients of friction. This reduction reaches a minimum at moisture contents of about 18%. It then increases again, reaching constant values, which are lower than the values at moisture contents below 15%. This is probably due to the fact that moisture has a strong affect on the delicate skin of this type of raisin. By increasing the moisture content above 18%, the sugar, that is leached on the surface of the product, is the main factor influencing the friction behavior (Fig. 2). The moisture content of raisins during storage, must be initially less than 17-18% for quality and preservation reasons (e.g. preventing crystallization and the growth of molds). Increasing the moisture content of the Sultana raisins from this value to 18%, due to washing in the factory, can be considered as significant for the subsequent processing steps of the product up to redrying and packaging. For Corinth raisins, however, the friction coefficients reach a maximum value at 18% moisture content. The maximum values decrease to lower constant values as the moisture is increased. However, the lower final values remain higher than those at moistures below 18% (Fig. 2). This is probably due to the fact that the skin of the Corinth raisins is harder than that of the Sultanas and that this type of raisin loses more sugars when its moisture content increases. With the exception of raisins pre-treated with anti-sticking substances, the static friction force is always larger than the kinetic one. In raisins that have been treated with anti-sticking substances, the stick-slip effect is very intensive during the movement of raisins (Fig. 3). The average value between the sticking and the slipping forces of friction, may be lower, equal or larger than the static one. However, by using anti-sticking substances, the static friction force is 2-4 times lower than that of untreated. Therefore, the oiling of raisins, which is often used for commercial purposes (better appearance and prevention of sticking in the final packages), should be applied after the sieving process in the factory. Otherwise, the screens will operate less efficiently, which may lead to a product of lower quality. Increasing the temperature of the sample from 20 to 3O”C, increased the static friction coefficient of Sultana raisins by 24% and the kinetic by 27%. This affect may be attributed to the effect of temperature on the viscosity of sugars. However the ratio of the two coefficients remained almost constant: ~s~~F(2c~~c~/fs(30~c~ = 0.76 and fk(2O”C)f!&3O”C) = 0.73.
3K?
The ,fn’ction of raisins
Anti-sticking substances (Delios and Durkex 550) reduced the static friction coefficient by 9-58%, and the kinetic by 14-76%. A reduction of the sugar content by about 30% by leaching, results in an increase in the average friction coefficients by 31%. This may be due to the fact that, although leaching reduces the total sugar content of raisins, it also results in an increase of the sugar content on their surface. It can be concluded from the investigation of the influence of the contact surface (Table 3) that the static and the kinetic coefficients of friction are reduced by 36%’ and 46%, when the contact surface is Teflon instead of PVC. Both coefficients are reduced by 10% when the contact surface is stainless steel, instead of PVC. There is almost no difference between the coefficients of friction of both types of raisins for moisture contents below 15%.
ACKNOWLEDGEMENTS Part of this paper was presented in the IFT Annual Meeting 1995 (Anaheim/USA). The research was financed by the EU (Program CAMAR, Contract 8001CT900027). The assistance of the BFEiKarlsruhe (Germany) and especially Mr. W. Wolf is greatly appreciated.
REFERENCES Atkin, G. & Sherman, P. (1984) The influence of surface friction on food firmness evaluation by compression tests. In Proceedings IXth Int. Congr: Rheol. Applications, pp. 123-144. Canet, W. & Sherman, P. (1988). Influence of friction, sample dimensions and deformation rate on the uniaxial compression of raw potato flesh. Journal of Texture Studies, 19, 275-287.
H. A. (1973) Granular materials friction apparatus. In rltztzual Lexington, Kentucky. Goh, H. C. & Sherman, P. (1987). The influence of surface friction on the stress relaxation of Gouda cheese. Journal of Texture Studies, 18, 389-404. Mohsenin, N. N. (1980) Physical Properties of Plunt and Animal Materials, 3rd edition. Gordon and Beach, New York. Payne, T. J. (1987). The role of raisins in high-fiber muesli-style formulations. Cereal Foods Clark, R. L. & McFarland, Conference
Amer: Sot. Agr: Eng., pp. 73-544,
World, 32, 545-547.
Richter,
D. W. (1954).
Engineering,
Friction
coefficients
of some
agricultural
materials.
Agricultural
35(6), 411-413.
Spiess, W. E. L. CyrWolf, W. (1983) The results of the cost 90 Project on water activity. In Physicaf Properties of Foods, eds. R. Jowitt, F. Escher, B. Hallstrom, H. F. T. Meffert, W. E. L. Spiess & G. Voz, pp. 65-88. Applied Science Publishers, London. Voisey, P. W. & Reid, W. S. (1974). Effect of friction on the performance of texture test cells. Journal of Texture Studies, 5, 239-248.
SO260-8774(97)00037-X
Joumul qf Fovd Engineering 33 (1997) 38%3Y3 1997 Elsevicr Science Limited. All rights rescrvcd Printed in &eat Britam 0260-8774197 $17.00 +O.OO
ELSEVIER
Factors Influencing the Friction of Raisins During Processing and Handling A. E. Kostaropoulos,a * J. Mandala: & G. D. Saravacof “Department
W. E. L. Spies@
of Food Science and Technology, Agricultural University of Athens, 75, Votanikos-Athens 11855, Greece “Bundesforschungsanstalt fiir Erngrung, Karlsruhe, Germany (Received
10 October
1995; accepted
Iera Odes
18 May 1997)
ABSTRACT Our study on the friction of Sultana and Corinth raisins indicates that there is a linear increase of the static and the kinetic friction force when the sample was loaded with added weights. An increase in the moisture content, up to 18%, reduces the friction of Sultana raisins, but increases that of Corinth raisins. For moistures larger than 307G, the friction of both types of raisins remains constant. Anti-sticking substances, reduce the friction but increase the stick-slip effect. The friction of raisins increases with sugar loss. The friction coefficients of raisins, sliding on Teflon-coated surfaces, are reduced by more than 30910, compared to PVC surfaces. 0 1997 Elsevier Science Limited. All rights reserved
INTRODUCTION The friction and adhesion properties of solid and semi-solid foods are needed for the rational design of bins, silos, and material handling and processing equipment. They are also important in packaging and in evaluating texture in compression tests. Reduced friction means less energy in transport and less damage to the products. This is especially important in processes in which there is a relative motion between sensitive products and the contact surfaces of handling or processing equipment. In raisins, friction is important in handling, washing, eliminating the stem cups, cleaning and sorting processes, and packaging system design. Besides friction and adhesion, the angle of repose, is another material property involved in bulk storage estimations. In case of raisins it is about 34”. *To whom correspondence
should be addressed. 385
Tel.: (301) 529 4692; fax: (301) 684 7111.
386
A. E. Kostaropoulos et al.
There is not much work done in the field of food friction and adhesion. A review of the subject was given by Mohsenin (1980) Voisey and Reid (1974) investigated the influence of the applied normal force, the area of the contact surface, and the speed the test sample over the surface. This investigation was carried out for frozen/ thawed peas and drained canned products, such as baked beans, green beans, peaches and spinach. The reduction of friction through lubrication of the samples was investigated by Atkin and Sherman (1984) and Goh and Sherman (1987) who applied the analysis of stress relaxation, and Canet and Sherman (1988) who studied the influence of friction on the uniaxial compression of potato flesh. In most of the research work that has been reported, the adhesion forces were not significant, as the investigated products were not sticky. However, friction depends not only on the applied normal force, but also on the force by which a solid material is moved over another. The nature of the materials in contact, and especially adhesion and cohesion forces, are also involved (Mohsenin, 1980). For common applications, friction is defined by the empirical law: F = N xJ Where F is the force required to move a solid material over a surface, N is the applied normal force, and f is the coefficient of friction. We distinguish between the static coefficient of friction fs, and the kinetic coefficient of friction fk. The static coefficient is a measure of the resistance caused by friction when a solid starts moving over another. The kinetic coefficient takes into account the resistance due to friction after this motion has started. In products with a certain degree of elastic freedom, when sliding over a firm surface takes place, the ‘stick-slip’ effect is observed (Mohsenin, 1980). Raisins can be classified as sticky dried products. Their stickiness increases when damaged, as the sugar content on their surface increases. In the present work, we investigated the influence of several factors on the friction of raisins when sliding over ‘different flat surfaces. Such factors are the moisture content, the applied normal force, the use of anti-sticking substances, and the temperature of the product.
MATERIALS
AND METHODS
Materials The materials tested were commercial Sultana and Corinth raisins. Both raisin types were produced by sun drying of seedless grape berries. The Sultana grapes were pretreated by immersion in 2% K2C03 solution to which a small amount of olive oil had been added. This was done for destroying the natural wax coating, resulting in an increase in the drying rate. The Sultana raisins were treated with SO1 after drying. The Corinth raisins were not pre-treated. The Sultana raisins, according to the Greek classification, belonged to number 2 class. Their average length was about 13.6 mm, and the width approximately 4.8 mm. The Corinth raisins belonged to the Greek crop class, and the average length and width were approximately 6.0 and 3.7 mm, respectively. The raisins were conditioned by adjusting their moisture content to the desired level. The conditioning took place in glass jars, which contained different saturated salt solutions (Spiess & Wolf, 1983). The water content of the product was determined by the sorption isotherm of the sample and was checked by the oven drying method (24 h, 70°C). Measurements have been also carried out for
The friction of raisins
3x7
investigating the influence of anti-sticking substances on the friction of Sultana raisins. The commercial substances used were special anti-sticking oils (Delios and Durkex 550). Instrumentation
For measuring the friction coefficients, a box-like apparatus similar to that described by Voisey and Reid (1974) was used (Fig. 1). This apparatus was originally used by Clark and McFarland (1973) and referred to by Voisey and Reid (1974). The apparatus consisted of a PVC frame, with a cover that could be moved up and down for adjusting the height of raisins put in it. This way, no contact occurred between the frame and the surface on which the raisins moved. The four sides of the frame were each 4 cm in width. The weight of the empty frame was 234.52 g. The product put in it was 25 g. A fine steel wire was used to pull the frame along a horizontal contact surface. The wire passed under a pulley and was attached to a universal testing machine (Instron 1011). The machine was connected to a PC. The results were calculated automatically. The main material of the contact surface was PVC. However, measurements were also carried out with contact surface of stainless steel and Teflon. Methods
The box-like temperatures.
frame, filled with raisins was conditioned at various moistures and The standard testing weight was that of the frame plus the weight of
Fig. 1. Apparatus of measurement of friction.
388
A. E. Kostaropoulos et al.
the loaded material, i.e. a total of 259.52 g. In testing the influence of the applied normal force, additional weights were put on the frame. In all experiments, the sliding speed was 120 mm/min. Force-time diagrams were obtained for every test. Each experiment was repeated 12 times. The frame was refilled with new material of the same batch, after each test. Unless otherwise stated, the temperature of the samples was about 20°C and the material on which the raisins slipped was PVC. For investigating the influence of the anti-sticking substances, Sultana raisins were dipped for 1 s in the substances cited above. The raisins were then left at ambient temperature to dry off, before using them in friction experiments. For reducing the sugars content, the Sultana raisins were dipped in water at 35°C for selected times. The loss of sugars was measured gravimetrically. The average sugar content of the Sultana raisins before leaching was estimated to be approximately 70% (Payne, 1987).
RESULTS
AND DISCUSSION
Results The main results of the measurements are given in Table 1. The coefficient of variation of the values of the measurements for most cases was about 7%. Table 1 indicates the influence of added weight and moisture content on the coefficient of friction of Sultana raisins and Corinth raisins. The added weights were 100-400 g. Moisture content was 17% and 35%. At constant moisture content, the relationship between force and added weight is linear. As indicated by Mohsenin (1980), the static force of friction is larger than the kinetic one. For a certain moisture content the difference between the static and the kinetic force remains constant all over the range of added weights used, unless the sample was pre-treated with anti-sticking compounds. This difference decreased as the moisture content of the product increased. At constant added weight, the mean static or kinetic forces of samples of lower moisture content, were larger than those corresponding to samples of higher moisture content. The slope of the linear relationship: force-added weight does not depend on the moisture content of the products. The ratio of static to kinetic friction coefficient (fslf) at a constant moisture content and added weight is constant (fJ& = 1.1). However, at a constant moisture, the relationship between the friction coefficients and the added weight is not linear. It is remarkable that the friction coefficients at a moisture of 17% decrease when additional weights are added, while the coefficients of samples of 35% moisture increase under the same conditions. In both cases, at high added weight and constant moisture, the relationship between the friction coefficients and the added weight tends to become constant. This result is similar to the findings of Richter (1954) for foodstuffs, as referred to by Mohsenin (1980). Figure 2 shows the influence of moisture content on the static and the kinetic coefficients of friction of Sultana and Corinth raisins for a constant load of 260 g. For Sultana raisins, the coefficients of friction, in samples of low moisture content (lo-15%), decrease from an almost constant value of 0.38 for the static and 0.33 for the kinetic coefficient of friction, down to the minimum values of 0.27 and 0.24 respectively. At higher moisture, both coefficients increase logarithmically, up to the almost constant values 0.33 and 0.29 respectively, at moisture contents exceeding
The friction
389
of raisins
50%. Both forces have a minimum at 18% moisture content. In Corinth raisins, exactly the opposite happens. The static and the kinetic coefficients of friction, at moistures IO-15%, are 0.43 and 0.37 respectively. These values increase at 18% moisture to 0.57 and 0.51, and then reach almost constant values, 0.51 and 0.46 at moisture contents higher than 50%. An increase of the temperature of the sample, from 20 to 30°C was found to increase the static and the kinetic friction coefficients. The dipping of raisins in special anti-sticking oils, at constant moisture content (35%) and load 260 g, reduced the static friction force, but increased the stick-slip effect (Fig. 3). The reduction of friction of Sultana raisins due to anti-sticking substances is given in Table 2. The temperature of the samples was 25”C, and their moisture content 35%. Finally, the effect of sugar loss and the influence of the contact surface on the coefficients of friction of Sultana raisins was investigated. The sugar content of raisins was reduced up to 43%, after dipping the Sultana raisins in water. The results are given in Fig. 4. Both friction coefficients were found to increase as the sugar content was reduced. The results of the effect of surface on which raisins slide, are given in Table 3. The values are given as a percentage of the maximum value, which for both types of raisins is the value sliding on a surface of PVC (static friction). The moisture content of both types of raisins was about 14%.
Mean Values of Friction Coefficients Type of ra kin
s s s s s s s s S ii
s
S
c C
C
c’
Moisture content
Added weight“
@I
k)
17 17 17 17 35 35 35 9 17 18 35 5s 3s 9 14 18 35 55
100 200 300 400 203.5 305.5 404.4 -
-
TABLE 1 of Sultana (S) and Corinth (C) Raisins on PVC Surface
Temperature (“C)
Coefficients
of variation (S) -.-
SII 20 20 20 20 20 20 20 20
f30Il -
Coefficients qf fiction’
20 20 20 20 20
Remarks: “The standard load in all cases was: 260 g. ‘yfs,fk: coefficient of static and kinetic friction.
0.57 0.52 0.50 0.50 0.32 0.37 0.39 0.38 0.37 0.27 0.32 0.33 0.42 0.43 0.43 0.57 0.53 0.51
0.50 0.45 0.45 0.45 0.30 0.35 0.37 0.33 0.33 0.24 0.29 0.29 0.40 0.37 0.37 0.50 0.47 0.46
4.45-7.38
3.66-6.94
7.43-l 1.31
6.30-l 3.48
3.84-7.92
3.0-7.66
7.71
8.10
3.98-12.71
4.10-14.87
A. E. Kostaropoulos et al.
390
0.7 -
’ fs (S) + fk (S) x fs (K)
’ fk (K)
0.60' 'E 0 'i v- 0.575 E s 0.4E : 0 0.3--
0.2' 5
I
I
I
I
I
I
I
I
I
I
J
10
15
20
25
30
35
40
45
50
55
60
Moisture content Fig. 2. Influence
(%)
of moisture content on coefficients of friction (fs, fk) on PVC surface (S:
Sultana raisins, C: Corinth raisins).
Discussion At constant moisture content, the forces of friction increase linearly with the applied load. The slope of this curve does not depend on the moisture content of the product. The difference between the static and the kinetic force of friction decreases
Durkex
550
Without anti-sticking substances
Delios
036
zs
z
g a4 9
j
m
0,2
0
03
0.4
0.2
0
to
20
Displacement
30 (mm)
40
50
0
0
10
a
Displacement
30 (mm)
40
50
0
10
20
Displacement
30
40
50
(mm)
Fig. 3. Influence of anti-sticking oils (Delios, Durkex) on friction of Sultana raisins.
391
The fn‘cfion of raisins
Effect
TABLE 2 of Anti-Sticking Substances on the Coefficient of Friction (T = 25”C, Moisture Content: 35%, PVC Surface)
Anti-sticking substance
0.30 0.14 0.32
Raisins
Coeficienfs of variation (%I
fk
Delios Durkex 550 No Treatment
of Sultana
0.25 0.07 0.29
8.17 13.55 7.92
16.58 13.89 7.66
when the moisture of the raisins increases. However, at constant applied load, the difference between their mean values is constant (Table 1). The static and the dynamic friction coefficients at 17% moisture, decreased as the applied load increased, but these coefficients increased when the moisture of the product reached 35%. In all cases, at quite large applied loads, all friction coefficients of Sultana raisins tended to reach a constant final average value of about 0.45 (Table 1). For moisture contents below 15% and above 40%, the friction coefficients of raisins do not depend on the moisture content of the product. At constant applied load, an increase in the moisture content of Sultana raisins, reduces the static and
0,35 ’ 5
I
I
10
15
I 20
I
I
I
I
25
30
35
40
Sugar loss (%) Fig. 4. Influence
of sugar loss on coefficients
of friction (fq, fk).
45
392
A. E. Kostaropoulos et al.
TABLE3 Influence of Sliding Surface on Coefficient of Friction of Sultana and Corinth Raisins (% Coefficient of Friction According to the Maximum Value) Sliding surface
f?
.fk
100 90.91 63.69
90.91 81.82 49.09
100 98.52 62.55
92.62 91.88 52.58
Sultana raisins
PVC Steel Teflon Corinth raisins
PVC Steel Teflon
the kinetic coefficients of friction. This reduction reaches a minimum at moisture contents of about 18%. It then increases again, reaching constant values, which are lower than the values at moisture contents below 15%. This is probably due to the fact that moisture has a strong affect on the delicate skin of this type of raisin. By increasing the moisture content above 18%, the sugar, that is leached on the surface of the product, is the main factor influencing the friction behavior (Fig. 2). The moisture content of raisins during storage, must be initially less than 17-18% for quality and preservation reasons (e.g. preventing crystallization and the growth of molds). Increasing the moisture content of the Sultana raisins from this value to 18%, due to washing in the factory, can be considered as significant for the subsequent processing steps of the product up to redrying and packaging. For Corinth raisins, however, the friction coefficients reach a maximum value at 18% moisture content. The maximum values decrease to lower constant values as the moisture is increased. However, the lower final values remain higher than those at moistures below 18% (Fig. 2). This is probably due to the fact that the skin of the Corinth raisins is harder than that of the Sultanas and that this type of raisin loses more sugars when its moisture content increases. With the exception of raisins pre-treated with anti-sticking substances, the static friction force is always larger than the kinetic one. In raisins that have been treated with anti-sticking substances, the stick-slip effect is very intensive during the movement of raisins (Fig. 3). The average value between the sticking and the slipping forces of friction, may be lower, equal or larger than the static one. However, by using anti-sticking substances, the static friction force is 2-4 times lower than that of untreated. Therefore, the oiling of raisins, which is often used for commercial purposes (better appearance and prevention of sticking in the final packages), should be applied after the sieving process in the factory. Otherwise, the screens will operate less efficiently, which may lead to a product of lower quality. Increasing the temperature of the sample from 20 to 3O”C, increased the static friction coefficient of Sultana raisins by 24% and the kinetic by 27%. This affect may be attributed to the effect of temperature on the viscosity of sugars. However the ratio of the two coefficients remained almost constant: ~s~~F(2c~~c~/fs(30~c~ = 0.76 and fk(2O”C)f!&3O”C) = 0.73.
3K?
The ,fn’ction of raisins
Anti-sticking substances (Delios and Durkex 550) reduced the static friction coefficient by 9-58%, and the kinetic by 14-76%. A reduction of the sugar content by about 30% by leaching, results in an increase in the average friction coefficients by 31%. This may be due to the fact that, although leaching reduces the total sugar content of raisins, it also results in an increase of the sugar content on their surface. It can be concluded from the investigation of the influence of the contact surface (Table 3) that the static and the kinetic coefficients of friction are reduced by 36%’ and 46%, when the contact surface is Teflon instead of PVC. Both coefficients are reduced by 10% when the contact surface is stainless steel, instead of PVC. There is almost no difference between the coefficients of friction of both types of raisins for moisture contents below 15%.
ACKNOWLEDGEMENTS Part of this paper was presented in the IFT Annual Meeting 1995 (Anaheim/USA). The research was financed by the EU (Program CAMAR, Contract 8001CT900027). The assistance of the BFEiKarlsruhe (Germany) and especially Mr. W. Wolf is greatly appreciated.
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H. A. (1973) Granular materials friction apparatus. In rltztzual Lexington, Kentucky. Goh, H. C. & Sherman, P. (1987). The influence of surface friction on the stress relaxation of Gouda cheese. Journal of Texture Studies, 18, 389-404. Mohsenin, N. N. (1980) Physical Properties of Plunt and Animal Materials, 3rd edition. Gordon and Beach, New York. Payne, T. J. (1987). The role of raisins in high-fiber muesli-style formulations. Cereal Foods Clark, R. L. & McFarland, Conference
Amer: Sot. Agr: Eng., pp. 73-544,
World, 32, 545-547.
Richter,
D. W. (1954).
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Friction
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of some
agricultural
materials.
Agricultural
35(6), 411-413.
Spiess, W. E. L. CyrWolf, W. (1983) The results of the cost 90 Project on water activity. In Physicaf Properties of Foods, eds. R. Jowitt, F. Escher, B. Hallstrom, H. F. T. Meffert, W. E. L. Spiess & G. Voz, pp. 65-88. Applied Science Publishers, London. Voisey, P. W. & Reid, W. S. (1974). Effect of friction on the performance of texture test cells. Journal of Texture Studies, 5, 239-248.