EFFECT OF DEGREE AND PRESSURE OF MILLING ON THE STICKINESS OF MILLED PARBOILED RICE

ABDUL HALIM’.* and K. R. BHATTACHARYA”

Central Food Technological Research Institute Mysore 570 013, India

Received for Publication May 14, 1978

ABSTRACT

Parboiled brown rice was milled with a McGill miller to five series of milled rice, each series being milled with a particular milling load and comprising several samples of different degrees of milling (d.m.1. The flow time, angle of repose, porosity, adhering solids and reflectance o f the samples were determined as a measure of their “stickiness”. Stickiness of the rice increased sharply on milling, reaching a high peak in the range of 2 4 % d.m. depending on the load used, and then declined again with fu r ther milling. However the peak decreased somewhat in height and also shifted towards the right as the load used for milling was increased. Raw rice, in contrast, showed only a low and flat peak. The above behaviors seemed to be largely related to the surface fat contettts of the rice samples.

INTRODUCTION

It is well known among millers that the milling of parboiled brown rice presents some peculiarities compared to that of raw brown rice: The milled rice seems rather sticky and does not flow well; and it looks rather unattractive, seemingly due to bran particles sticking to it. Also, the screens of the mill tend to get clogged. To overcome these difficulties, millers normally add a little husk to the brown rice stream as it enters the

~

‘Work carried out in partial fulfillment of the M.Sc. Food Technology degree of the

’Present address: Directorate of Food Crop Economy, Directorate General of Food

TO whom all requests for reprints should be sent. ‘In rice milling, paddy (rice in husk) is first shelled (dehusked) to produce brown rice,

which is then whitened (milled. polished) to yield milled rice. If paddy is soaked in water, drained, steamed and dried, we get parboiled paddy which when milled gives parboiled rice. Rice that has not undergone parboiling is called raw rice.

Journal of Food Quality 1 (1977) 349-358. All Rights Reserved.

University of Mysore

Crop Agriculture, Jakarta, Indonesia

@Copyright 1978 by Food & Nutrition Press, Znc., Westport, Connecticut 349

350 ABDUL HALIM and K. R. BHATTACHARYA

whitening cone, and also replace the normal screen of the cone (usually 12 mesh) with one having a slightly larger mesh opening (10 mesh). Alter- natively, they mill the parboiled paddy directly, without initial shelling, in an Engelberg huller.

Raghavendra Rao et al. (1967). in a laboratory study on the milling characteristics of rice, noticed this difference between raw and parboiled rice and felt that the cause of it probably lay in the greater “oiliness” of parboiled as compared to raw rice. In a later study on physical properties of rice, we happened to observe (Bhattacharya et al. 1972) that the degree of milling (d.m.) of rice had a profound effect on its coefficient of friction and porosity, which were very high in the intermediate-d.m. range, especially in parboiled rice, and that this behavior was clearly related to the surface fat contents of the rice. This result would suggest that the above “stickiness” was probably related to the surface fat on the kernel, which in turn was a function of the d.m. Raghavendra Rao et al. (1967) mentioned further that the adhesion of bran could be eliminated by mill- ing the grain for a longer time under a low pressure. On the other hand, a miller during a personal discussion mentioned that he maintained a high pressure in the milling cone (less clearance between the cone and the screen) and thus avoided the trouble of screen clogging even without us- ing husk.

The a b o b considerations led to the following study. The aim was to collect some quantitative information on the extent of rice stickiness, its possible relation to the surface oil content, and the effects of the degree and pressure of milling on it.

MATERIALS AND METHODS

A local commercial lot of Coimbatore Sanna paddy, a fine-grain non- waxy tall indica variety, about a year old, was used.

Parboiling and Milling

About 25 kg of the paddy was parboiled by soaking overnight in warm water and steaming at 0 psig for 10 min, followed by drying in the shade (Bhattacharya and Indudhara Swamy 1967).

The paddy was shelled in a small commercial rubber-roller sheller (the unshelled paddy being separated and reshelled). Portions of the brown rice (125 g) were milled in a modified (Bhattacharya and Sowbhagya 1972) McGill miller No. 1, a metal-roller mill, using different loads (0 ,1 ,2 , 4 or 6 lb) and time periods (0-9 min) each. Five series of milled rice were prepared in this way, each corresponding to one particular milling load

STICKINESS OF MILLED PARBOILED RICE 35 1

and including samples having several degrees of milling (d.m.1 up to the highest attainable with that load (viz. 3.4% for 0 lb, 5.0% for 1 lb, 6.9% for 2 lb, 7.3% for 4 lb and 7.5% for 6 lb). To illustrate the range, samples of approximately 3% d.m. were produced by using a load of 0 lb for 300 sec, 1 lb for 120 sec, 2 lb for 40 sec, 4 lb for 27 sec, and 6 lb for 8 sec. These values of d.m. were a little too low, which was due to the sample being rather too dry (moisture content of brown rice, 10.2% on wet basis).

Raw rice was milled in a similar way, but in this case a single series of milled rice of 0-10% d.m. was produced by using optimal combinations of load and time each.

The d.m. was determined by collecting and weighing the bran, using a 16-mesh screen for parboiled rice and 18-mesh for raw rice (Bhattacharya and Sowbhagya 1972).

To study the effect of surface fat, all parboiled samples milled with a 6 lb load were also tested after surface defatting (stirring the rice under fresh petroleum ether in a beaker successively for 15,2, and 2 min respec- tively, decanting, and then air drying).

Methods

Density, bulk density, porosity, angle of repose, and total and surface fat contents were determined as described earlier (Bhattacharya et al. 1972), only whole grains freed from broken being used for the tests. As the quantities of rice were small, the angle of repose was determined by using the small rectangular box described in the above reference and the apparent coefficient of static friction, f’, was calculated from the formula (Barre 1954):

f’ = tan 0’ = tan (tan-’ hlb) = h/b

h = height of the rice column standing at the back wall of the box when its detachable front wall was flicked open allowing the rice to fall freely, and

Where 0’ = apparent angle of repose,

b = depth of the box at its base.

I t is called “apparent” because the angle of repose calculated as above was only approximate, the inclined top surface of the rice mass being slightly convex.

The amount of non-fat solids adhering to milled rice was determined at the same time as the surface fat content was determined. The petroleum ether-washed rice was quantitatively spread out on a double filter paper, rapidly stirred with a glass rod for 2 min (to prevent cooling) and then air

352 ABDUL HALIM and K. R. BHATTACHARYA

dried for 2 hr. It was then sieved in an l&mesh screen and weighed. The loss in weight, less the surface fat, gave the non-fat adhering solids.

Flow time of rice was determined with a device resembling a sand clock. Two glass tubes (50 mm diameter X 90 mm long) were tapered at one end and the tapered portions were connected by a short glass tube (12 mm internal diameter). Rice (50 g) was placed in the upper cup of the device, a ground glass rod stopper being previously placed into the cen- tral neck from the open top to prevent rice from flowing down. The device was then held firmly by the hand vertically on a Petri dish placed on a Syntron vibratory feeder (Type F 0.1, No. 6360) equipped with a Syntron Controller (Type CO, No. 24555). The Controller was set at speed regulator point 5 (to provide a gentle vibration and thus prevent clogg- ing), the stopper rod was lifted, and the time of flow was measured by a stop watch. The experiment was repeated three times and the mean taken.

The reflectance (lightness) of the samples was determined with a Photovolt reflection meter (model 610, standard search unit 610 Y) using a green tristimulus filter as described before (Bhattacharya and Subba Rao 1966).

RESULTS AND DISCUSSION

The results are shown in Fig. 1-6. The patterns of change of the coeffi- cient of friction, porosity and surface fat (Fig. 2, 3 and 6) as well as total fat, density and bulk density (not shown), with changing degree of milling (d.m.), seen here were largely similar to those observed earlier (Bhattacharya et al. 1972). The density results are not shown, for the gradual increase in density with increasing d.m. due to the progressive removal of fat, as observed before, occurred rather uniformly in all the series. Bulk density data are also not shown as the bulk density was af- fected secondarily by the changing density; the true effect of d.m. is brought out better in porosity where the effect of differences in density is eliminated. The total fat content also decreased progressively with in- creasing d.m. rather uniformly in all the series and hence is not shown.

Index of Stickiness of Milled Rice

Several criteria were tested for their suitability as a quantitative index of rice stickiness. As it turned out, all of them were satisfactory and, in retrospect, logically so: The flow time is actually a direct measure of the resistance to flow, i.e. stickiness - somewhat akin to measuring the viscosity of a fluid. Similarly the angle of repose, or its function, the

STICKINESS OF MILLED PARBOILED RICE 353

r t t t t t t 0 1 ,o 2 1 4

1 2 - I I

I

i 1 0 -

0 Y w

.- c.

0 1 2 3 4 5 6 7 8 9 to 01

Degree of milling , */*

FIG. 1. EFFECT OF THE DEGREE AND LOAD OF MILLING ON THE FLOW TIME OF RICE

In this and the subsequent figures, the numerals indicate the load (lb) used for mill- ing parboiled rice; R = raw rice; S = surface-defatted parboiled rice (milled with 6 Ib load). These symbols have been directly used to represent the respective experimen- tal values also. The arrows in this figure indicate infinite time (i.e. flow stopped)

coefficient of friction, is a measure of the resistance of the grains to slide over each other, i.e. their stickiness. Porosity again is a good index of the property, for the more free-flowing (i.e. the less sticky) the grains are, the better they pack in a container, in other words the less the void space or porosity, and vice versa. Finally, the relevance of adhering solids is ob- vious, for the solids adhere precisely because the rice (and/or the bran) is sticky.

Rice Stickiness and its Relation to Degree of Milling and Pressure of Milling

In each of the above four criteria, the values first increased sharply with increasing degree of milling and then declined again, as sharply, to near about the original values as the d.m. reached 7% or so (Fig. 1-4). These data thus confirm the common observation that milling of par- boiled brown rice, at any rate in metal-roller mills, gives rise to very sticky rice and/or bran in the intermediate milling range. The maximum stickiness lay in the range of 2 4 5 % d.m. depending on the load under which the sample was milled. Some of these samples were so sticky that

354 ABDUL HALIM and K.R. BHATTACHARYA

2 3 L 5 6 7 8 9 3 0 0.2

0 1

Degree of milling, *L

FIG. 2. EFFECT OF THE DEGREE AND LOAD OF MILLING ON THE STATIC FRICTION OF RICE GRAINS

they stopped flowing altogether in the flow-time test despite the vibra- tion provided (Fig. 1). Both unmilled f ie . brown) and fully milled par- boiled rice, on the other hand, had excellent flow and packing properties.

In contrast to the above, the curves for raw rice for all these properties showed only very small and flat peaks, denoting absence of appreciable stickiness even when medium milled.

The reflectance (lightness, i.e. whiteness as contrasted to darkness or grayness) data (Fig. 5 ) also agreed with the above results. In parboiled rice, some of the samples became actually slightly darker (less reflec- tance) on initial milling despite the removal of the (colored) bran - presumably because of the smearing of the bran particles all over the grain surface. The samples started becoming lighter only after 4-5% d.m., i.e. when the stickiness had decreased and adhering solids content had become low (Fig. 4). In raw rice, on the other hand, the adhering solids were never very high (Fig. 4) and indeed the reflectance increased gradually throughout with milling (Fig. 5).

The peak in the curves of parboiled rice in Fig. 1-4 persisted irrespec- tive of the load under which the samples were milled, indicating that

STICKINESS OF MILLED PARBOILED RICE 355

R -. R. -. -.

4 3 - //' , /'

0 1 2 3 b 5 6 7 8 9 {Q 41.

Degree of milling, *I.

FIG. 3. EFFECT OF THE DEGREE AND LOAD OF MILLING ON THE POROSITY OF RICE

Degree of milling X

FIG. 4. EFFECT OF THE DEGREE AND LOAD OF MILLING ON NON-FAT SOLIDS ADHERING TO RICE GRAIN

356 ABDUL HALIM and K.R. BHATTACHARYA

Degree of mill ing, *I.

FIG. 5. EFFECT OF THE DEGREE AND LOAD OF MILLING ON THE REFLECTANCE (LIGHTNESS) OF RICE

Degree of milling,%

FIG. 6. EFFECT OF THE DEGREE AND LOAD OF MILLING ON THE SUR. FACE FAT CONTENT OF RICE

STICKINESS OF MILLED PARBOILED RICE 357

stickiness cannot be altogether avoided in medium-milled parboiled rice in the milling system (metal-roller mill) employed here. However, there was a clear trend of the heights of the peaks being lowered as the pressure employed during milling increased, indicating that the stickiness can be somewhat reduced by milling under high abrasive force (and hence for less time). The reflectance results (Fig. 5) also support this conclusion in- asmuch as the medium-milled samples produced by a 1-lb load were slightly but definitely darker than those milled with a 4-lb load.

Another interesting result was that the peaks tended to shift to the right as the pressure applied during milling increased (Fig. 1-4). In other words the maximum stickiness occurred in samples of increasing degree of milling as the abrasive force increased. The meaning of this result is not clear at this time.

Relation to Surface Fat

That the surface fat content was responsible in a large measure for the observed behavior of milled parboiled rice is clear from the similarity of the shapes of the curves for surface fat (Fig. 6) and the other properties (Fig. 1-4). This is further testified by the fact that all the criteria of stickiness declined drastically after surface defatting of the samples (Fig. 1-4, curve S). The reflectance of the samples also greatly increased con- currently and the initial dip in the reflectance curve completely disap- peared (Fig. 5).

It has been noted by several workers that the oil content of parboiled rice bran was greater than that of raw rice bran (Raghavendra Rao et al. 1965; Padua and Juliano 1974). In fact Subrahmanyan et al. (1938) long back had stated that parboiling had the effect of pushing the fat out- wards in the kernel. Clearly the present phenomenon is related to this property.

However, it is to be noted that in raw rice also the surface fat curve showed a clear peak, though much lower in height than in parboiled rice (Fig. 6). Medium-milled raw rice nevertheless did not show appreciable stickiness. The greater oil content of the bran from parboiled as com- pared to raw rice may be responsible for this difference. Or else, there may be a difference in the surface characteristics of the raw and the par- boiled grain which react somewhat differently to oil.

Practical Implications

I t is clear that medium-milled parboiled rice, produced by metal-roller mills, always has poor flow and packing properties (and sticking of bran). However, these drawbacks can be partly mitigated by milling under high

358 ABDUL HALIM and K.R. BHATTACHARYA

pressure. No doubt it would be most convenient for storage and handling to have parboiled rice either in the form of brown or fully milled rice. These would flow a n d pack much better a n d would also probably attract less pests. I t should be noted that addition of husk during milling, as customary, is not necessary to produce a free-flowing parboiled rice pro- vided it is fully milled.

In view of the present results, it would be desirable to study the effect of (a) milling in emery whiteners (for it is often felt that the stickiness is more evident in metal whiteners), (b) moisture content of rice (the present sample was too dry, about 10% moisture), a n d (c) milling with t h e addi- t ion of husk (as customary with millers) on the flow properties of milled parboiled rice. Since all t h e criteria of rice stickiness employed here (flow time, angle of repose, porosity, adhering solids, color) gave nearly iden- tical results, angle of repose alone can be employed in future studies, be- ing the simplest to determine.

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BARRE, H.J. 1954. Country storage of grain. In Storage of Cereal Grains and Their Products (J.A. Anderson and A.W. Alcock, eds.) p. 318. Amer. Assoc. Cereal Chem., St. Paul., Minn.

BHATTACHARYA, K. R. and INDUDHARA SWAMY, Y.M. 1967. Conditions of drying parboiled paddy for optimum milling quality. Cereal Chem. 44,

BHATTACHARYA. K.R. and SOWBHAGYA, C.M. 1972. A colorimetric bran pigment method for determining the degree of milling of rice. J. Sci. Food Agric. 23, 161-169.

BHATTACHARYA. K.R.. SOWBHAGYA, C.M. and INDUDHARA SWAMY, Y.M. 1972. Some physical properties of paddy and rice and their interrela- tions. 23, 171-186.

BHATTACHARYA, K.R. and SUBBA RAO, P.V. 1966. Effect of processing conditions on quality of parboiled rice.

PADUA, A.B. and JULIANO, B.O. 1974. Effect of parboiling on thiamin, pro- tein and fat of rice. J. Sci. Food Agric. 25, 697-701.

RAGHAVENDRA RAO. S.N., ANANTHACHAR, T.K. and DESIKACHAR, H.S.R. 1965. Oil content of bran from rice milled to different degrees of polish- ing. J. Food Sci. Technol. 2, 115-116.

RAGHAVENDRA RAO, S.N., NARAYANA, M.N. and DESIKACHAR, H.S.R. 1967. Studies on some comparative milling properties of raw and parboiled rice.

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