Parboiling of Rice: Processes and Reasons for Parboiling of Paddy Rice

Parboiling of rice entails partial boiling of the paddy before milling in order to maximize its nutrition value, to change the texture of cooked rice, and reduce the breakage in milling. Parboiling induces a gelatinization of the starch after the heating and during cooling the amylase molecules re-accociate with each other and form a closely packed arrangement. The kernels are softer and look glassier during the parboiling process. The parboiling process transfers micronutrients found in the bran, which is normally separated in the whitening process in the rice mill, to the endosperm. Parboiled rice is, therefore, more nutritious than white rice. Parboiling often mends little cracks that may have formed in the endosperm during postharvet processing and thus head rice recoveries of parboiled rice are better.

Procedures and Processes of Parboiling Paddy Rice

The operations involved in paddy rice parboiling processes are as shown below:

Sieving

This is done if there are broken grains in the paddy. A wire netting is used and a sieve to isolate the broken kennels so that they will not get cooked and stick the stuff together during parboiling. Thick viscous liquid resulting from cooked broken-kernels normally stack the paddy together and hampers efficient speedy drying. This could lead to fungal growth and spoilage of the rice if caution is not taken.

Washing, Floatation and Sedimentation

The paddy is immersed in water and stirred vigorously for the soil and other dirt to dissolve away. The lighter debris float whiles the heavier materials settle at the bottom. The floating particles (dead insects, weeds, unfilled seeds etc) are skimmed off while the paddy rice is scooped out leaving heavier materials (sand, stones, fragments of metals etc.) at the bottom and poured away. Washing is performed twice or thrice depending on the extent of soil in rice. Scientifically, the washing extracts soil, debris and fungal toxin present in the rice.

Boiling

The washed paddy is then immersed in water on a pot and partly boiled to a warm temperature of 35 to 40 ^oC. This increases the absorption of water by the paddy, and also deactivates certain enzyme activities. Some microbes and their cellular products may be destroyed or inactivated at this temperature.

Soaking

The paddy rice is left to remain in the warm water overnight (10-24 hours) for the paddy to imbibed water and become moisture soaked. The scientific concepts here are enzyme activation. A favorable environment is produced after the material is cooled down for enzymes and pigments transformation. Toxins are reduced and microbial contaminants may be inactivated or eliminated at certain high moisture levels. The saturated grains extend and fill the hull’s lemma letting it lose in the process.

Steaming

The soaked paddy is then steamed in a pot with little water filling the pot. The high moisture content in the grain is used to gelatinize the starch during the steaming process. The steam is allowed to enter all parts of the pot by covering it with jute sacks and polythene mats. The covering prevents the steam from escaping easily thus generating a partial pressure over the material that helps in the inward movement of molecules in each gelatinous kernel. When paddy starts to break open their husks and there is steam vapour appearing all over the pot there is a sign that steaming is enough. The heat also evaporates excess moisture in the grain leaving concentrated nutrients in the kernel. A high temperature of up to 80^oC is able to inactivate or destroy certain microorganisms and degrade toxins and other toxic substances found in the rice. Physico-chemical reaction processes e.g. gelatinization, starch retrograding, pigment transformations, enzyme deactivations etc. are all enabled by the pressurized steaming method. This brings in better taste, colour-change, and cooking characteristics of the rice.

Drying

The steamed paddy is laid out immediately at an airy spot to dry. Excessive sunlight is avoided otherwise non-uniform drying of kernel which results in breakage during milling. Other scientific benefits of the drying methods may be:

1. the art of drying evaporates the moisture and this concentrates solutes in the kernel
2. drying will also stop microbial pathogens from developing on steamed paddy.
3. it compresses the gelatinous amylose starch together in a compact mass and delocalizes it from the husk making de-husking (milling) easier.

Milling

The dried paddy is processed utilizing a processing machine or, at the local level is beat utilizing mortar and pestle to isolate the kennels from the husk. This process additionally can eliminate microorganisms and other debris that stick to them.

Winnowing

Winnowing is done either mechanically or manually. Where the processing is done manually utilizing mortar and pestle to pound the paddy rice, the rice is winnowed to isolate the husks and unwanted materials away. Winnowing help to eliminates soil, dead creepy crawlies, and different debasements present in the rice.

Sorting/Grading

Parboiling for commercial purposes does more than just handpicking of discoloured rice and debris before sorting into grades (grade 1, 2 and 3) as, very few, few and many broken grains respectively).

Principles or Reasons behind Parboiling of Rice

Parboiling reduced the breakages, fat, protein and amylose content of the rice also the water uptake and thiamine contents are also increased by parboiling. The pre-treatments given to the rice before the parboiling process helps remove factors that could antagonized the nutritional qualities of the rice.

Firstly, washing paddy rice in water removes dirt, microbes and toxins in the rice. The long hours of soaking in warm water do not only facilitate water absorption but also further reduces the substances like afflatoxins that are in the rice during storage (old stock). This is because the absorption of water to a saturation point would dilute and reduces the concentration of toxic substances in the rice.

The soaking might also cause certain enzymes or pigments transformation and their movement inwards to the endosperm. The steaming to a temperature of 80 ^oC supplies heat enough to kill pathogens still found in the rice. The activity of pressurized steaming enabled vitamins notably thiamine to move inward onto the endosperm. It further deactivates enzymes and makes available substances like vitamins that otherwise could have been inaccessible. The transformation and concentration of enzymes and pigments could be the reason why parboiled products such as millet, groundnuts, rice etc taste better than non-parboiled ones.

In rice parboiling, the important nutritional and physiological change is the active movement of soluble molecules. Much of the rice nutrients are concentrated at the embryo (the germ) and during the steaming process, the partial pressure created by covering directs solutes movement inwards onto the endosperm through the aleurone layer that surrounds the endosperm.

Other advantages are easy milling of parboiled paddy rice because the amylose starches retrograde and stack together in a compact manner that pulls away from the husk when dried. This makes the kernel becomes somewhat delocalized and now moves freely inside the husk. This makes it easier for husk to be removed when milled or pounded on mortar and pestle. Since the point of attachment between the husk and the endosperm is also loosened, the husks can easily get removed without breaking off the germ.

Non-parboil rice has its husk intimately attached to the embryo where all the nutrients are concentrated. Forceful removal of the husk will usually break away the germ with the husk, thereby losing all the vital nutrients including thiamine. And such rice will certainly not taste the same as rice that retains its germ or embryo after parboiling. Thus the process of parboiling inadvertently cures nutrient deficient-diseases or has the ability to supply certain nutrients that otherwise could have lost to consumers.