GB2173986A

GB2173986A - Par boiling rice

Info

Publication number: GB2173986A
Application number: GB08510498A
Authority: GB
Inventors: David Crawford Mcilroy
Original assignee: Mars Inc
Current assignee: Mars Inc
Priority date: 1985-04-25
Filing date: 1985-04-25
Publication date: 1986-10-29
Also published as: GB8510498D0

Abstract

A process for par-boiling starchy grains such as rice comprising treating the starchy grains with water at a temperature of 120 to 180 DEG C under a pressure at least sufficient to retain the water substantially in its liquid state for a time sufficient to gelatinise at least the surface layer of the starch in the starchy grains. The grains may be treated under vacuum (e.g. 0.8 bar) before the hot water treatment. As shown, rice is fed from slot 1 into screw 7, which may be under vacuum supplied by pump 11, before passing into container 14 filled with hot water to level 19, the water being recirculated under pressure by pump 23 via water-treatment section 21. <IMAGE>

Description

SPECIFICATION Par-boiling Rice The present invention relates to a process for par-boiling rice.

Processes for par-boiling rice have been known for hundreds of years, and have been applied commercially for at least fifty years.

Present commerciai par-boiling processes involve: 1) an optional vacuum step for removing airfrom the rice; 2) a wetting step which involves raising the moisture content of the rice from about 12% to between 33 and 36%; 3) a steam treatment step carried out at 100 to 2000C to gelatinise the starch in the rice kernel thereby giving the par-boiled rice its characteristic non-sticking properties; and 4) drying the rice to microbiological stability.

During par-boiling a significant proportion of the fats and vitamins in the bran layer are transferred to the rice kernel, thereby increasing the nutritional content of the rice kernel.

Generally the wetting and steaming steps take from three to six hours to complete.

Such a commercial process can only be carried out on paddy rice, i.e. rice comprising a kernel surrounded by a bran layer and contained in its husk. The husk serves to prevent the rice kernel from breaking into pieces during the processing thereof.

This is disadvantageous as it is necessary to wet and steam both the husk and the bran layer and also to separate the husk and the bran layer from the kernel prior to packaging of the par-boiled rice kernels. This clearly adds to the cost of production.

Par-boiling has also been applied, in a similar manner to that described above, to other starchy cereal grains and pulses. These processes suffer from the disadvantages and long processing times referred to above.

It is an aim of the present invention to provide an improved par-boiling process for rice and other starchy cereal grains and pulses (hereinafter referred to generically as 'starchy grains').

Therefore, according to the present invention, there is provided a process for par-boiling starchy grains comprising treating the starchy grains with water at a temperature of 120 to 180"C under a pressure at least sufficient to retain the water substantially in its liquid state for a time sufficient to gelatinise at least the surface layer of the starch in the starchy grains.

The pressure may exceed the pressure needed to retain the water in its liquid state, but generally should not exceed 15 bar.

The treatment time will vary with the temperature, pressure and type of starchy grain being treated.

However, it will be readily possible for a skilled person to determine this by trial and error experimentation on the basis of the results given hereafter. Generally, however, thetreatmenttime will be between 1 and 10 minutes.

Preferably, the treatment time will be sufficient to raise the moisture content of the starchy grains to between 30 and 40% and to gelatinise a significant proportion of the starch in the grains. It will, of course, be appreciated that the degree of gelatinisation can be controlled by varying the treatment time, temperature and pressure as necessary for each particular type of starchy grain being treated.

Advantageously, a vacuum is applied to the starchy grains before treatment with the hot, pressurised water to facilitate penetration of the water into the starchy grains. Conveniently, the pressure is reduced by about 0.2 bar for about five minutes, after which the starchy grains are immediately treated with the hot, pressurised water. It is however, not necessary to include a vacuum step as the water is readily able to penetrate into the starchy grains during the treatment with the hot pressurised water.

The starchy grains, after treatment with the hot, pressurised water, may be subjected to conventional further treatment steps, such as drying, dehusking or rolling to remove bran layers.

An advantage of the process of the present invention is that it can be applied not only to paddy rice but also to cargo (i.e. de-husked) rice and white (i.e. dehusked and de-branned) rice without there being any significant breakage of the rice kernel.

The treatment with hot, pressurised water may be carried out in any of the commercially available pressure treatment apparatus. For instance, the water may be sprayed onto a moving conveyor belt on which the starchy grains are supported. Alternatively, the starchy grains may be transported through a bath of hot, pressurised water on an elevator or by use of a screw conveyor.

The treatment may in further alternatives be carried out in an upwardly angled drum provided with lifting plates for transporting the starchy grains therethrough, a downwardly angled drum provided with baffle plates for controlling the descent of the starchy grains therethrough, a pressure lock batch treatment steeper, with or without mixing means, or a toothed wheel system rotating in a tightly fitting cylindrical chamber.

It will be appreciated that the treatment vessel will need to be provided with pressure locks, hoppers and supply lines for hot, pressurised water. However, these and the treatment vessels are well known in the art and therefore need no further description.

If a vacuum is to be applied to the starchy grains prior to the treatment with the hot pressurised water, this is preferably applied in a sealed screw conveyor of the type known in the art. However, the vacuum may be applied to the starchy grains in another continuous type treatment stage or in a batch treatment container.

It can be seen that the process of the present invention is advantageous in that it can be carried out considerably quicker than prior art processes, it is much more energy efficient, and it eliminates one entire process step.

One embodiment of the process of the present invention is now described, by way of example only, with reference to the accompanying drawing which shows schematically apparatus for carrying out the process.

Referring now to the drawing, the apparatus comprises a silo 1 for feeding starchy grains 3 through a pressure lock valve 5 into a screw conveyor 7 driven by motor 9. Vacuum may be applied to the interior of the screw conveyor 7 by means of vacuum pump 11.

The end of the screw conveyor 7 remote from the silo 1 is connected through a second pressure lock valve 12 to a closed container 14 which is sealed at its outlet side by a third pressure lock valve 15. An elevator system 17 is located in the container 14.

In use, the container is filled with water to the level shown at 19 in the drawing. The water is continuously recirculated through water treatment section 21 which removes contaminants therefrom and heats itto the desired temperature. The water is pumped into the container under pressure by the pump 23.

The apparatus shown in the drawing was used to treat various types of rice according to the process of the present invention. A series of experiments were carried out and the conditions of the experiments and types of rice treated are shown in the following table. The table also shows the moisture content of the rice as it emerges from the third pressure lock valve 15.

TABLE

Pressure in Screw Temperature in Residence Time Moisture Conveyor (7) Pressure in Container Container (14) in Container Content Experiment Type of Rice (Bar Absolute) (14) (Bar Absolute) ( C) (14) (Seconds) (%) 1 cargo 1 7 155 60 28.3 2 " 1 7 155 90 30.0 3 " 1 7 155 120 32.8 4 " 1 7 155 150 35.6 5 paddy 1 7 155 180 28.8 6 " 1 7 155 240 30.9 7 " 1 7 155 300 32.7 8 paddy 0.8 7 155 120 29.9 9 " 0.8 7 155 150 31.3 10 " 0.8 7 155 180 33.3 11 cargo 1 6 139 180 36.0 12 " 1 6 139 210 38.0 13 " 1 6 139 240 40.0 14 " 1 6 139 270 43.3 15 cargo 1 5 130 180 33.2 16 " 1 5 130 240 37.5 17 " 1 5 130 360 44.0 TABLE (contd.)

Pressure in Screw Temperature in Residence Time Moisture Conveyor (7) Pressure in Container Container (14) in Container Content Experiment Type of Rice (Bar Absolute) (14) (Bar Absolute) ( C) (14) (Seconds) (%) 18 (control) carge 1 2 110 240 35.6 19 " " 1 2 110 300 38.3 20 " " 1 2 110 360 39.5 21 " " 1 1 90 240 33.8 22 " " 1 1 90 480 37.4 23 " " 1 1 90 540 34.4 After the experiments with paddy rice, it was subjected to dehusking and de-branning in conventional fashion to produce the rice kernels.

In Experiments 1 to 17 above the rice kernels all had at least a discernible translucent surface layer of gelatinised starch. If the gelatinised starch layer was only a surface layer, the interior of the kernel was white. However, in some cases the whole of the kernel was converted to a white translucent state. This is characteristic of rice kernels treated according to the invention and distinguishes such rice kernels from those produced by conventional par-boiling processes.

The rice kernels produced in Experiments 1 to 17 were all non-sticking in character on cooking and it is believed that this is due to the layer of gelatinised starch on their surface.

In contrast, the rice kernels of the control Experiments 18 to 23 did not show any discernible surface layer. It thus appeared that these kernels had only been hydrated and had not been in any way hydrolysed.

These kernels did not have the non-sticking characteristics of par-boiled rice.

It can therefore be seen that the process of the present invention produces an acceptable par-boiled rice in an economical, effective and quick manner. It is believed that it will be readily apparent to the skilled person how the above process can be applied to other starchy grains, such as cereal grains and pulses.

It will also be appreciated that the present invention is not limited to the details set out above, and that other arrangements and conditions can be used without departing from the scope of the present invention.

Claims

1. A process for par-boiling starchy grains comprising treating the starchy grains with water at a temperature of 120 to 1 80"C under a pressure at least sufficient to retain the water substantially in its liquid state for a time sufficient to gelatinise at least the surface layer of the starch in the starchy grains.

2. The process of claim 1, wherein the water pressure does not exceed 15 bar.

3. The process of claim 1 or claim 2, wherein the treatment time is between 1 and 10 minutes.

4. The process of any one of claims 1 to 3, wherein the treatment time is sufficient to raise the moisture content of the starchy grains to between 30 and 40% and to gelatinise a significant proportion of the starch in the grains.

5. The process of any one of claims 1 to 4, wherein a vacuum is applied to the starchy grains before treatment with the hot, pressurised water to facilitate penetration of the water into the starchy grains.

6. The process of claim 5, wherein the pressure is reduced by about 0.2 bar for about five minutes, after which the starchy grains are immediately treated with the hot, pressurised water.

7. The process of any one of claims 1 to 6, further including drying, dehusking and/or rolling step following the treatment with the hot pressurised water.

8. The process of any one of claims 1 to 7, wherein the starchy grains are rice grains.

9. A process for par-boiling starchy grains, substantially as herein before described with reference to the accompanying drawing.