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Wear of Rice in an Abrasive Milling Operation, Part 1: Prediction of Degree of Milling
ARTICLE IN PRESS Biosystems Engineering (2004) 88 (3), 337–342 doi:10.1016/j.biosystemseng.2004.02.011 PH}Postharvest Technology
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Wear of Rice in an Abrasive Milling Operation, Part 1: Prediction of Degree of Milling Debabandya Mohapatra; Satish Bal Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur 721302, India; e-mail of corresponding author: [email protected] (Received 11 June 2003; received in revised form 28 February 2004; published online 7 June 2004)
The abrasion coefficient of rice was determined at different degrees of milling levels, for three varieties of rice differing in slenderness ratio. The abrasion coefficient was found to decrease with the degree of milling, indicating the removal of asperities and progressive hardness of the core of the rice. The slenderness ratio was found to be negatively related with the value of the abrasion coefficient. The average values of the abrasion coefficient of rice on abrasive element (synthetic emery) varied between 003 and 005 for three varieties of rice differing in hardness and slenderness values. The average value of abrasion coefficient was used to predict the degree of milling using the principles of adhesive wear. The predicted values of the degree of milling fitted the experimental values adequately, with relative deviation modulus varying between 14 and 23% and good values for the coefficient of determination (R2 > 090). # 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction Milling any material is basically a phenomenon of wear (Sarkar, 1976). Wear is defined as either mass or volume of material, removed or displaced from a solid surface, which is repeatedly subjected to mechanical stresses by rubbing with another solid surface or surfaces. This involves removal of material from a solid surface by mechanical action, which may be sliding, rolling, impact or combination of these operations (Robinowicz, 1965). The wear processes may be classified into different modes depending on the kinematics and mechanisms (Zum Gahr, 1998). In abrasive wear, material is displaced or detached from the solid surface by the following: (i) hard particles or the presence of hard protuberances on a counter face in a relative motion as in the case of two-body abrasion; and (ii) hard particles between two surfaces or embedded in one of the two surfaces in relative motion, as occurs in three-body abrasion. In practice, when a single asperity slides against a softer counter face, the deformation mode can be entirely elastic, provided the asperity is sufficiently obtuse; in such a case the wear rate will be negligible (Xie & Williams, 1996). In polymers, strong adhesion occurs at the points of contact of the asperities; 1537-5110/$30.00
when sliding occurs, fragments are torn from the softer surface and are left deposited on the harder one (Meredith & Hearle, 1959). According to the literature (Zum Gahr, 1998), the rate of material removal from a solid body is 10% of the total time and 90% of the time it spent rolling. However, wear has no satisfactory quantitative laws. This is because any small change in one of the parameters, for example, speed, area of contact, load, amplitude, dimension of the abrasive as well as the abrading material, can completely alter the contribution of each factor to any overall wear equation (Bowden & Tabor, 1964). Rice milling operations, in particular, are of two types: (i) abrasion milling, in which brown rice is abraded by a hard abrasive surface at high speed and low pressure between two surfaces; and (ii) friction milling, in which two or three body wear take place due to the rubbing of two bodies of similar nature, under high pressure and at relatively low speeds. However, there is no pure form of abrasive or friction milling in rice polishing. It is a combination of adhesive and abrasive wear. When the rice grains revolve inside the milling chamber, the parts of rice grain coming in contact with an emery surface undergo an abrasion type of wear, whereas when the grains 337
# 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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Notation Ap d0 D DM Dobs Dpre k l L mb Mg n N p
total area of conical surface, m2 depth of penetration of the conical emery particle in the rice grain, m diameter of the emery wheel, m degree of milling, % observed degree of milling, % predicted degree of milling, % wear coefficient length of rice grain, m normal load, N bran mass, kg bulk mass of grain, kg number of data points rotational speed, min1 hardness of the grain, N m2
rub with each other, an adhesive type of wear takes place. The specific objectives of this investigation are: (i) to determine the coefficient of abrasion in rice grain varying in the degree of milling using laws of wear and (ii) to predict the degree of milling based on the coefficient of abrasion.
2. Theoretical considerations When a rice grain rubs against an abrasive surface, i.e. emery surface, there will be formation of wear particles at the junction of the point of contact between the asperities. This would result in mass loss from the grain surface. The volume V of the material removed in m3 or the mass loss mb in kg in the process of abrasion is dependent on the dimensions (slenderness ratio of the grain l=w, where l and w are the length and width of the grain in m, respectively) and hardness of the grain p in N m2, length of cut or sliding distance x in m, and normal load L in N. 2.1. Abrasion coefficient For developing a mathematical model to estimate the amount of bran (polish), the quantitative laws governing both adhesive and abrasive wear types were considered. The general laws of wear are as follows (Robinowicz, 1965): (i) The amount of wear is generally proportional to the load and sliding distance. (ii) The amount of wear is inversely proportional to the hardness of surface to be worn.
pw P r R2 t V w x y w2 rb DL
milling pressure, Pa relative deviation modulus, % radius of the emery cone, m coefficient of determination milling time, s volume of the material removed due to abrasion, m3 width of rice grain, m sliding distance, m angularity of the emery grain, deg chi-square bulk density of bran, kg m3 small load acting on the grain surface, N
Based on these laws, Holm (1946) (as cited by Robinowicz, 1965) developed an equation for the evaluation of the volume of wear material removed as in adhesive wear (Ravikiran & Jahanmir, 2001; Ling et al., 2002): V¼
kLx p
ð1Þ
where k is the wear coefficient. Archard (1953) postulated the adhesive wear law and presented the wear coefficient for the area of contact between the asperities. Another model of abrasive wear (Appendix A) suggested that the coefficient of abrasion may be calculated on the basis of the angularity y of the emery particles. However, the variation in angularity may be so much that it would be difficult to get a representative value of tan y. Moreover, it was not possible to measure values of angularity for the emery particles in bonded condition or on an emery wheel. Mulhearn and Samuels (1962) tried to determine the number of effective emery particles in a bonded silicon carbide surface. They found out that only 1/8 of the total number of grains on a particular wheel was in the cutting position. Assuming that all the effective emery grains are cutting the bran on rice grain, throughout the rotation of the polishing emery wheel, of diameter D in m and rotational speed N in min1, the bran removed by them operating through a sliding distance given by (pDNt/60) for an operating duration of t in s would be: L mb ¼ krb x p
ð2Þ
where rb is the bulk density of the bran in kg m3. Properties of the materials are given in Table 1.
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Table 1 Properties of rice used in the mathematical model
2
Hardness p, N m Load L, N Bulk density of bran rb , kg m3 Rotational speed N, min1 Diameter of the emery wheel D, m
Now Eqn (2) can be written as mb p k¼ rb Lx
Pusa Basmati
Swarna
ADT37
0015 196 420 1360 0155
0011
0018
ð3Þ
Using Eqns (2) and (3), the degree of milling DM , which is defined as the mass of bran removed per unit mass of brown rice and expressed in percentage, can be expressed as kr L DM ¼ b x 100 ð4Þ pMg where Mg is the mass of brown rice fed to the polisher in kg. Experimental results are validated using Holm– Archard’s equation of wear.
3. Materials and methods Freshly harvested Pusa Basmati, an aromatic, long and slender variety (procured from Sonepat, Haryana, India), Swarna, a medium grain variety (procured from local market), and ADT37, a short and round grain variety (procured from Tamilnadu, India), were selected for this study. The varieties were dehusked using a Satake rice dehusker (Type THU, Satake Engineering Co., Tokyo, Japan) and stored in double-sealed polythene bags at 58C in a refrigerator (Quick freezer, 200 l capacity, Remi Equipments, India) until the experimentation. Samples were removed from the refrigerator 24 h before the experiments to equilibrate temperature to room conditions. Moisture content was determined using standard air oven method by keeping 5 g of grain in an oven at 1088C for 24 h and then noting the weight. The moisture content was expressed in percent wet basis. Three principal diameters, viz length, width and thickness of brown rice of each variety were measured manually by Satake Grain Shape Tester (Model-MK 100, Japan) having 0001 mm precision. Measurement was made on 50 well-distributed, randomly drawn grains from the test samples of each variety. Each grain was manually adjusted between the spring-loaded measuring head of the tester to determine the three axial diameters of the kernel. The bulk density of bran was determined by weighing 1 l of bran in the USDA
test weight apparatus, as specified by the equipment manufacturers (Ohaus, USA, precision 01 kg h l1) in triplicate. The average value was then expressed in kg m3. Hardness of the samples was measured using a texture analyser (TA-XT2, Texture Technologies Corp., UK) with a 25 kg load cell using single compression (Perez et al., 1997). The brown rice samples i.e. long-slender Pusa Basmati (l=w ¼ 443), medium– grain Swarna (l=w ¼ 255) and short-bold ADT37 (l=w ¼ 19), after cleaning and grading were polished in abrasive polisher (Model: Satake Pearler- TM05) for 15–180 s. The emery grit size chosen was 36 and at a rotor speed of 1360 min1. The samples were aspirated (Bates Aspirator, USA) and mass loss was noted for three varieties to calculate the degree of milling. The data were analysed using ORIGIN 60 software to determine the coefficient of determination R2 and standard error, and the w2 test was applied to find the goodness of fit (Panse & Sukhatme, 1967). Model suitability was measured by the percent relative deviation modulus P using the following formula (Madamba et al., 1996): n jDpre Dobs j 100 X P¼ ð5Þ n 0 Dobs where: Dpre is the predicted degree of milling of the grain; Dobs is the observed value of the degree of milling of the grain; and n is the number of data points.
4. Results and discussion 4.1. Determination of abrasion coefficient and prediction of degree of milling It was found out that the value of k for the rice varieties varied from 005 to 0022, 007 to 004 and 0051 to 0024, average values being 003, 005 and 003, for Swarna, ADT37 and Pusa Basmati, respectively. The decrease in the values for k as the value for DM increased indicated that with progressive milling (Fig. 1), as starchy endosperm was exposed, less of bran was removed, suggesting the existence of harder core in the
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25
0.06 Degree of milling, %
Abrasion coefficient k
0.07
0.05 0.04 0.03 0.02 0.01 0.00 5
10 15 Degree of milling, %
Fig. 1. Variation in abrasion coefficient in relation to degree of milling for three rice varieties: -*-, Pusa Basmati; -}-, Swarna; -n-, ADT37
rice caryopsis. This observation agrees well with the earlier studies by Nagato and Kono (1963) (cited by Juliano & Bechtel, 1985), where they had indicated a tough interior for the indica variety. Hence, according to these findings, the power consumption to remove bran during initial whitening process would be less when compared to successive whitening processes. It appeared that the bran, consisting of lignin, hemicelluloses, cellulose, and oil bodies form weak bonds and were removed easily. The abrasion coefficient values were found to be highest in the case of ADT37. Being the coarse, round variety, it lost more bran material during milling. Moreover, the rise in temperature during the milling operation may have affected the abrasion coefficient value as temperature alters the starch property. A higher value of the abrasion coefficient implied that the material was comparatively harder, thus more power was required to remove bran from the caryopsis surface. It was revealed that although ADT37 had the highest value of hardness, the abrasion coefficient was also affected by the volume of bran removed for an effective length of travel. The volume removed per effective length was not only affected by hardness but also by the shape and constituents of the materials as discussed earlier. The harder grains tend to lose less material. In the case of ADT37, the hardness was imparted to it by its thickness but due to its round shape and surface resistance it lost more material, influencing the value of abrasion coefficient. Brown rice has an undulating surface and the asperities of the emery surface when in contact with the ridges and furrows of the grain surface tend to scour away the softer materials, till the grain surface becomes smoother. This results in reduction in mass loss, resulting in a decrease in the values for the abrasion coefficient.
15 10 5 0
20
0
50
100 Milling time, s
150
200
Fig. 2. Predicted (line and symbol) and observed (symbols) values of degree of milling for different varieties of rice: -*-, Pusa Basmati; -}-, Swarna; -n-, ADT37
25
Observed degree of milling, %
0
20
20
15
10
5
0 0
5
10 15 20 Predicted degree of milling, %
25
Fig. 3. Comparison of observed and predicted values of degree of milling by Holm–Archard’s wear model: * Pusa Basmati; } Swarna; n ADT37
The model developed to predict DM was validated using the average value of abrasion coefficient k for the three varieties of rice. The predicted values are compared with the observed data in Fig. 2. The model was found to be a moderate fit to the experimental results, with values for the relative deviation modulus P of 14–23%. These high errors seem to arise from the initial and final deviation of predicted result from the observed data. There was a high rate of bran removal during initial phase of milling and lower bran removal at the later stage attributing to kernel hardness. The abrasion coefficient as already mentioned is a function
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of hardness; hence there was variation in the value of the abrasion coefficient with milling degree. On considering the average value of the abrasion coefficient, initial results were underestimated, whereas they were overestimated during the final stage. Fig. 3 presents the comparison between predicted and actual values of the degree of milling. The linear nature of the curve, at 458 slope from the origin indicates that, the predicted model is a good fit for the actual/observed degree of milling values with values for R2 of more than 090, and with standard error values varying between 05–401, 023–163 and 006–23 for Swarna, Pusa Basmati and ADT37, respectively, at 95% confidence level. The calculated values of w2 lie between 0.8 and 0.99 for the three varieties under consideration, indicating a quite satisfactory goodness of fit of the predicted model with the experimental results (Panse & Sukhatme, 1967).
5. Conclusions Holm–Archard’s equation was used to predict the value of coefficient of abrasion when the rice grains were abraded in a randomly hard abrasive surface. This provided the information how the wear occurs at different stages of rice milling and also quantified it. The abrasion coefficient decreased as milling progressed, indicating a tougher core of the rice caryopsis compared to the outer bran layer for the indica varieties. It appeared that bran layer comprising of oil bodies, cellulose, hemicelluloses and fibre offers less resistance to abrasion compared to the inner starchy endosperm. Prediction of the degree of milling using the average value of abrasion coefficient moderately fitted the experimental results. The model indicated that wear rate and coefficient of abrasion invariably depend on the material hardness and its shape.
Meredith R; Hearle J W S (1959). Physical Methods of Investigating Textiles. Interscience Publishers, New York Mulhearn T O; Samuels L E (1962). The abrasion of metal-a model of the process. Wear, 5, 478–498 Panse V G; Sukhatme P V (1967). Statistical Methods for Agricultural Workers. ICAR, India Ravikiran A; Jahanmir S (2001). Effect of contact pressure and load on wear of alumina. Wear, 251, 980–984 Robinowicz E (1965). Friction and Wear of Materials. Wiley, New York Sarkar A D (1976). Wear of Metals. Pergamon Press, Oxford Xie Y; Williams J A (1996). The prediction of friction and wear when a soft surface slides against a harder rough surface. Wear, 196, 21–34 Zum Gahr K H (1998). Wear by hard particles. Tribology International, 31(10), 587–596
Appendix A To derive a quantitative expression for the mass of the abrasive wear taking place during the milling operation of rice, a simple model consisting of a conical asperity of emery carrying a load DL may be considered to be penetrating the soft surface of rice and then ploughing a groove in it and removing the bran along the length of the rice as shown in Fig. 4. If L is the maximum permissible load and pw is the intensity of working pressure in the milling chamber: DL r2 ¼ ðA1Þ ppw The projected area of the penetrated portion of the cone in the vertical plane is rd 0 if the depth of penetration is d 0 (Fig. 5). Thus, if the cone, of angle y, moves through a distance dx, which is equal to a length of one rice grain, it would sweep out a volume of dv of the bran in one pass: dv ¼ rd 0 dx
ðA2Þ
and where d 0 ¼ r tan y
ðA3Þ
dv ¼ r2 tan y dx
ðA4Þ
∆L (load carried by emery particle) Hard surface (emery particle)
References Archard J F (1953). Contact and rubbing of flat surfaces. Journal of Applied Physics, 24(8), 981–988 Bowden F P; Tabor D (1964). Friction and Lubrication of Solids Part II. Oxford Clarendon Press, London Juliano B O; Bechtel D B (1985). The rice grain and its gross composition in Rice: Chemistry and Technology, (Juliano B O, ed) (2nd Ed.), pp 17–57. American Association of Cereal Chemists, St. Paul, MN, USA Ling F F; Bryant M D; Doelling K L (2002). On irreversible thermodynamics for wear prediction. Wear, 253, 1165–1172 Madamba P S; Driscoll R H; Buckle K A (1996). Thin-layer drying characteristics of garlic slices. Journal of Food Engineering, 29, 75–97
θ
Ap
θ r
Soft material surface (rice grain)
Radius of the emery cone
Fig. 4. Schematic illustration of a abrasive cone pressed into a soft grain surface: r, radius of the emery particle; DL normal load or pressure intensity working on the harder material; Ap , projected area of the harder indentation; y, angularity of the emery particle
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Emery grain at position 2
Direction of emery particle
2r Emery grain at position 1
d′ 2
1
Rice grain dx
Fig. 5. Abrasive wear model of rice grain: dx, small sliding distance (length of single rice grain); d0 , depth of penetration of the emery cone on the rice grain; r, radius of the emery cone in contact with the rice grain at positions 1 and 2
Substituting the value of r2 from Eqn (A1) in Eqn (A4) L tan y dx ðA5Þ dv ¼ pp If the contribution made by all the emery grains passing one after the other is taken into consideration the total volume of bran swept in one pass, dv ¼
L tan y dx pp
ðA6Þ
On integration the equation becomes V¼
L tan y x pp
ðA7Þ
where tan y is the weighted average of the tan y values of all the individual cones. Eq (A7) is similar to Eqn (1), with the term [tan y=p] replacing k.
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Wear of Rice in an Abrasive Milling Operation, Part 1: Prediction of Degree of Milling Debabandya Mohapatra; Satish Bal Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur 721302, India; e-mail of corresponding author: [email protected] (Received 11 June 2003; received in revised form 28 February 2004; published online 7 June 2004)
The abrasion coefficient of rice was determined at different degrees of milling levels, for three varieties of rice differing in slenderness ratio. The abrasion coefficient was found to decrease with the degree of milling, indicating the removal of asperities and progressive hardness of the core of the rice. The slenderness ratio was found to be negatively related with the value of the abrasion coefficient. The average values of the abrasion coefficient of rice on abrasive element (synthetic emery) varied between 003 and 005 for three varieties of rice differing in hardness and slenderness values. The average value of abrasion coefficient was used to predict the degree of milling using the principles of adhesive wear. The predicted values of the degree of milling fitted the experimental values adequately, with relative deviation modulus varying between 14 and 23% and good values for the coefficient of determination (R2 > 090). # 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction Milling any material is basically a phenomenon of wear (Sarkar, 1976). Wear is defined as either mass or volume of material, removed or displaced from a solid surface, which is repeatedly subjected to mechanical stresses by rubbing with another solid surface or surfaces. This involves removal of material from a solid surface by mechanical action, which may be sliding, rolling, impact or combination of these operations (Robinowicz, 1965). The wear processes may be classified into different modes depending on the kinematics and mechanisms (Zum Gahr, 1998). In abrasive wear, material is displaced or detached from the solid surface by the following: (i) hard particles or the presence of hard protuberances on a counter face in a relative motion as in the case of two-body abrasion; and (ii) hard particles between two surfaces or embedded in one of the two surfaces in relative motion, as occurs in three-body abrasion. In practice, when a single asperity slides against a softer counter face, the deformation mode can be entirely elastic, provided the asperity is sufficiently obtuse; in such a case the wear rate will be negligible (Xie & Williams, 1996). In polymers, strong adhesion occurs at the points of contact of the asperities; 1537-5110/$30.00
when sliding occurs, fragments are torn from the softer surface and are left deposited on the harder one (Meredith & Hearle, 1959). According to the literature (Zum Gahr, 1998), the rate of material removal from a solid body is 10% of the total time and 90% of the time it spent rolling. However, wear has no satisfactory quantitative laws. This is because any small change in one of the parameters, for example, speed, area of contact, load, amplitude, dimension of the abrasive as well as the abrading material, can completely alter the contribution of each factor to any overall wear equation (Bowden & Tabor, 1964). Rice milling operations, in particular, are of two types: (i) abrasion milling, in which brown rice is abraded by a hard abrasive surface at high speed and low pressure between two surfaces; and (ii) friction milling, in which two or three body wear take place due to the rubbing of two bodies of similar nature, under high pressure and at relatively low speeds. However, there is no pure form of abrasive or friction milling in rice polishing. It is a combination of adhesive and abrasive wear. When the rice grains revolve inside the milling chamber, the parts of rice grain coming in contact with an emery surface undergo an abrasion type of wear, whereas when the grains 337
# 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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Notation Ap d0 D DM Dobs Dpre k l L mb Mg n N p
total area of conical surface, m2 depth of penetration of the conical emery particle in the rice grain, m diameter of the emery wheel, m degree of milling, % observed degree of milling, % predicted degree of milling, % wear coefficient length of rice grain, m normal load, N bran mass, kg bulk mass of grain, kg number of data points rotational speed, min1 hardness of the grain, N m2
rub with each other, an adhesive type of wear takes place. The specific objectives of this investigation are: (i) to determine the coefficient of abrasion in rice grain varying in the degree of milling using laws of wear and (ii) to predict the degree of milling based on the coefficient of abrasion.
2. Theoretical considerations When a rice grain rubs against an abrasive surface, i.e. emery surface, there will be formation of wear particles at the junction of the point of contact between the asperities. This would result in mass loss from the grain surface. The volume V of the material removed in m3 or the mass loss mb in kg in the process of abrasion is dependent on the dimensions (slenderness ratio of the grain l=w, where l and w are the length and width of the grain in m, respectively) and hardness of the grain p in N m2, length of cut or sliding distance x in m, and normal load L in N. 2.1. Abrasion coefficient For developing a mathematical model to estimate the amount of bran (polish), the quantitative laws governing both adhesive and abrasive wear types were considered. The general laws of wear are as follows (Robinowicz, 1965): (i) The amount of wear is generally proportional to the load and sliding distance. (ii) The amount of wear is inversely proportional to the hardness of surface to be worn.
pw P r R2 t V w x y w2 rb DL
milling pressure, Pa relative deviation modulus, % radius of the emery cone, m coefficient of determination milling time, s volume of the material removed due to abrasion, m3 width of rice grain, m sliding distance, m angularity of the emery grain, deg chi-square bulk density of bran, kg m3 small load acting on the grain surface, N
Based on these laws, Holm (1946) (as cited by Robinowicz, 1965) developed an equation for the evaluation of the volume of wear material removed as in adhesive wear (Ravikiran & Jahanmir, 2001; Ling et al., 2002): V¼
kLx p
ð1Þ
where k is the wear coefficient. Archard (1953) postulated the adhesive wear law and presented the wear coefficient for the area of contact between the asperities. Another model of abrasive wear (Appendix A) suggested that the coefficient of abrasion may be calculated on the basis of the angularity y of the emery particles. However, the variation in angularity may be so much that it would be difficult to get a representative value of tan y. Moreover, it was not possible to measure values of angularity for the emery particles in bonded condition or on an emery wheel. Mulhearn and Samuels (1962) tried to determine the number of effective emery particles in a bonded silicon carbide surface. They found out that only 1/8 of the total number of grains on a particular wheel was in the cutting position. Assuming that all the effective emery grains are cutting the bran on rice grain, throughout the rotation of the polishing emery wheel, of diameter D in m and rotational speed N in min1, the bran removed by them operating through a sliding distance given by (pDNt/60) for an operating duration of t in s would be: L mb ¼ krb x p
ð2Þ
where rb is the bulk density of the bran in kg m3. Properties of the materials are given in Table 1.
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Table 1 Properties of rice used in the mathematical model
2
Hardness p, N m Load L, N Bulk density of bran rb , kg m3 Rotational speed N, min1 Diameter of the emery wheel D, m
Now Eqn (2) can be written as mb p k¼ rb Lx
Pusa Basmati
Swarna
ADT37
0015 196 420 1360 0155
0011
0018
ð3Þ
Using Eqns (2) and (3), the degree of milling DM , which is defined as the mass of bran removed per unit mass of brown rice and expressed in percentage, can be expressed as kr L DM ¼ b x 100 ð4Þ pMg where Mg is the mass of brown rice fed to the polisher in kg. Experimental results are validated using Holm– Archard’s equation of wear.
3. Materials and methods Freshly harvested Pusa Basmati, an aromatic, long and slender variety (procured from Sonepat, Haryana, India), Swarna, a medium grain variety (procured from local market), and ADT37, a short and round grain variety (procured from Tamilnadu, India), were selected for this study. The varieties were dehusked using a Satake rice dehusker (Type THU, Satake Engineering Co., Tokyo, Japan) and stored in double-sealed polythene bags at 58C in a refrigerator (Quick freezer, 200 l capacity, Remi Equipments, India) until the experimentation. Samples were removed from the refrigerator 24 h before the experiments to equilibrate temperature to room conditions. Moisture content was determined using standard air oven method by keeping 5 g of grain in an oven at 1088C for 24 h and then noting the weight. The moisture content was expressed in percent wet basis. Three principal diameters, viz length, width and thickness of brown rice of each variety were measured manually by Satake Grain Shape Tester (Model-MK 100, Japan) having 0001 mm precision. Measurement was made on 50 well-distributed, randomly drawn grains from the test samples of each variety. Each grain was manually adjusted between the spring-loaded measuring head of the tester to determine the three axial diameters of the kernel. The bulk density of bran was determined by weighing 1 l of bran in the USDA
test weight apparatus, as specified by the equipment manufacturers (Ohaus, USA, precision 01 kg h l1) in triplicate. The average value was then expressed in kg m3. Hardness of the samples was measured using a texture analyser (TA-XT2, Texture Technologies Corp., UK) with a 25 kg load cell using single compression (Perez et al., 1997). The brown rice samples i.e. long-slender Pusa Basmati (l=w ¼ 443), medium– grain Swarna (l=w ¼ 255) and short-bold ADT37 (l=w ¼ 19), after cleaning and grading were polished in abrasive polisher (Model: Satake Pearler- TM05) for 15–180 s. The emery grit size chosen was 36 and at a rotor speed of 1360 min1. The samples were aspirated (Bates Aspirator, USA) and mass loss was noted for three varieties to calculate the degree of milling. The data were analysed using ORIGIN 60 software to determine the coefficient of determination R2 and standard error, and the w2 test was applied to find the goodness of fit (Panse & Sukhatme, 1967). Model suitability was measured by the percent relative deviation modulus P using the following formula (Madamba et al., 1996): n jDpre Dobs j 100 X P¼ ð5Þ n 0 Dobs where: Dpre is the predicted degree of milling of the grain; Dobs is the observed value of the degree of milling of the grain; and n is the number of data points.
4. Results and discussion 4.1. Determination of abrasion coefficient and prediction of degree of milling It was found out that the value of k for the rice varieties varied from 005 to 0022, 007 to 004 and 0051 to 0024, average values being 003, 005 and 003, for Swarna, ADT37 and Pusa Basmati, respectively. The decrease in the values for k as the value for DM increased indicated that with progressive milling (Fig. 1), as starchy endosperm was exposed, less of bran was removed, suggesting the existence of harder core in the
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25
0.06 Degree of milling, %
Abrasion coefficient k
0.07
0.05 0.04 0.03 0.02 0.01 0.00 5
10 15 Degree of milling, %
Fig. 1. Variation in abrasion coefficient in relation to degree of milling for three rice varieties: -*-, Pusa Basmati; -}-, Swarna; -n-, ADT37
rice caryopsis. This observation agrees well with the earlier studies by Nagato and Kono (1963) (cited by Juliano & Bechtel, 1985), where they had indicated a tough interior for the indica variety. Hence, according to these findings, the power consumption to remove bran during initial whitening process would be less when compared to successive whitening processes. It appeared that the bran, consisting of lignin, hemicelluloses, cellulose, and oil bodies form weak bonds and were removed easily. The abrasion coefficient values were found to be highest in the case of ADT37. Being the coarse, round variety, it lost more bran material during milling. Moreover, the rise in temperature during the milling operation may have affected the abrasion coefficient value as temperature alters the starch property. A higher value of the abrasion coefficient implied that the material was comparatively harder, thus more power was required to remove bran from the caryopsis surface. It was revealed that although ADT37 had the highest value of hardness, the abrasion coefficient was also affected by the volume of bran removed for an effective length of travel. The volume removed per effective length was not only affected by hardness but also by the shape and constituents of the materials as discussed earlier. The harder grains tend to lose less material. In the case of ADT37, the hardness was imparted to it by its thickness but due to its round shape and surface resistance it lost more material, influencing the value of abrasion coefficient. Brown rice has an undulating surface and the asperities of the emery surface when in contact with the ridges and furrows of the grain surface tend to scour away the softer materials, till the grain surface becomes smoother. This results in reduction in mass loss, resulting in a decrease in the values for the abrasion coefficient.
15 10 5 0
20
0
50
100 Milling time, s
150
200
Fig. 2. Predicted (line and symbol) and observed (symbols) values of degree of milling for different varieties of rice: -*-, Pusa Basmati; -}-, Swarna; -n-, ADT37
25
Observed degree of milling, %
0
20
20
15
10
5
0 0
5
10 15 20 Predicted degree of milling, %
25
Fig. 3. Comparison of observed and predicted values of degree of milling by Holm–Archard’s wear model: * Pusa Basmati; } Swarna; n ADT37
The model developed to predict DM was validated using the average value of abrasion coefficient k for the three varieties of rice. The predicted values are compared with the observed data in Fig. 2. The model was found to be a moderate fit to the experimental results, with values for the relative deviation modulus P of 14–23%. These high errors seem to arise from the initial and final deviation of predicted result from the observed data. There was a high rate of bran removal during initial phase of milling and lower bran removal at the later stage attributing to kernel hardness. The abrasion coefficient as already mentioned is a function
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WEAR OF RICE IN AN ABRASIVE MILLING OPERATION
of hardness; hence there was variation in the value of the abrasion coefficient with milling degree. On considering the average value of the abrasion coefficient, initial results were underestimated, whereas they were overestimated during the final stage. Fig. 3 presents the comparison between predicted and actual values of the degree of milling. The linear nature of the curve, at 458 slope from the origin indicates that, the predicted model is a good fit for the actual/observed degree of milling values with values for R2 of more than 090, and with standard error values varying between 05–401, 023–163 and 006–23 for Swarna, Pusa Basmati and ADT37, respectively, at 95% confidence level. The calculated values of w2 lie between 0.8 and 0.99 for the three varieties under consideration, indicating a quite satisfactory goodness of fit of the predicted model with the experimental results (Panse & Sukhatme, 1967).
5. Conclusions Holm–Archard’s equation was used to predict the value of coefficient of abrasion when the rice grains were abraded in a randomly hard abrasive surface. This provided the information how the wear occurs at different stages of rice milling and also quantified it. The abrasion coefficient decreased as milling progressed, indicating a tougher core of the rice caryopsis compared to the outer bran layer for the indica varieties. It appeared that bran layer comprising of oil bodies, cellulose, hemicelluloses and fibre offers less resistance to abrasion compared to the inner starchy endosperm. Prediction of the degree of milling using the average value of abrasion coefficient moderately fitted the experimental results. The model indicated that wear rate and coefficient of abrasion invariably depend on the material hardness and its shape.
Meredith R; Hearle J W S (1959). Physical Methods of Investigating Textiles. Interscience Publishers, New York Mulhearn T O; Samuels L E (1962). The abrasion of metal-a model of the process. Wear, 5, 478–498 Panse V G; Sukhatme P V (1967). Statistical Methods for Agricultural Workers. ICAR, India Ravikiran A; Jahanmir S (2001). Effect of contact pressure and load on wear of alumina. Wear, 251, 980–984 Robinowicz E (1965). Friction and Wear of Materials. Wiley, New York Sarkar A D (1976). Wear of Metals. Pergamon Press, Oxford Xie Y; Williams J A (1996). The prediction of friction and wear when a soft surface slides against a harder rough surface. Wear, 196, 21–34 Zum Gahr K H (1998). Wear by hard particles. Tribology International, 31(10), 587–596
Appendix A To derive a quantitative expression for the mass of the abrasive wear taking place during the milling operation of rice, a simple model consisting of a conical asperity of emery carrying a load DL may be considered to be penetrating the soft surface of rice and then ploughing a groove in it and removing the bran along the length of the rice as shown in Fig. 4. If L is the maximum permissible load and pw is the intensity of working pressure in the milling chamber: DL r2 ¼ ðA1Þ ppw The projected area of the penetrated portion of the cone in the vertical plane is rd 0 if the depth of penetration is d 0 (Fig. 5). Thus, if the cone, of angle y, moves through a distance dx, which is equal to a length of one rice grain, it would sweep out a volume of dv of the bran in one pass: dv ¼ rd 0 dx
ðA2Þ
and where d 0 ¼ r tan y
ðA3Þ
dv ¼ r2 tan y dx
ðA4Þ
∆L (load carried by emery particle) Hard surface (emery particle)
References Archard J F (1953). Contact and rubbing of flat surfaces. Journal of Applied Physics, 24(8), 981–988 Bowden F P; Tabor D (1964). Friction and Lubrication of Solids Part II. Oxford Clarendon Press, London Juliano B O; Bechtel D B (1985). The rice grain and its gross composition in Rice: Chemistry and Technology, (Juliano B O, ed) (2nd Ed.), pp 17–57. American Association of Cereal Chemists, St. Paul, MN, USA Ling F F; Bryant M D; Doelling K L (2002). On irreversible thermodynamics for wear prediction. Wear, 253, 1165–1172 Madamba P S; Driscoll R H; Buckle K A (1996). Thin-layer drying characteristics of garlic slices. Journal of Food Engineering, 29, 75–97
θ
Ap
θ r
Soft material surface (rice grain)
Radius of the emery cone
Fig. 4. Schematic illustration of a abrasive cone pressed into a soft grain surface: r, radius of the emery particle; DL normal load or pressure intensity working on the harder material; Ap , projected area of the harder indentation; y, angularity of the emery particle
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D. MOHAPATRA; S. BAL
Emery grain at position 2
Direction of emery particle
2r Emery grain at position 1
d′ 2
1
Rice grain dx
Fig. 5. Abrasive wear model of rice grain: dx, small sliding distance (length of single rice grain); d0 , depth of penetration of the emery cone on the rice grain; r, radius of the emery cone in contact with the rice grain at positions 1 and 2
Substituting the value of r2 from Eqn (A1) in Eqn (A4) L tan y dx ðA5Þ dv ¼ pp If the contribution made by all the emery grains passing one after the other is taken into consideration the total volume of bran swept in one pass, dv ¼
L tan y dx pp
ðA6Þ
On integration the equation becomes V¼
L tan y x pp
ðA7Þ
where tan y is the weighted average of the tan y values of all the individual cones. Eq (A7) is similar to Eqn (1), with the term [tan y=p] replacing k.