Rice Production : Chapter 17 - Harvesting, threshing, drying, storage

Introduction

The final steps in producing a crop of rice -harvesting, threshing, drying, and storage -often are taken for granted by the extension agent. It is natural to assume that the major part of the work lies in growing the crop, and that farmers will know what to do once the grains have ripened. But this is a mistaken assumption. The fact is that a large percentage of the crop can be (and often is) lost through improper harvesting, threshing, drying, and storage practices. This chapter describes the traditional methods of processing and storing rice and includes ideas for improvement.

I. Harvesting

Manual harvesting

Most harvesting problems in Sierra Leone can be attributed to the fact that irrigated rice tends to ripen irregularly. At the end of the rainy season, when most varieties mature, frequent rains and a lack of continuous sunshine protract the grain ripening process and make difficult proper timing of harvesting, since grains even on the same panicle ripen at varying rates. If the crop is harvested too early, many green grains will be lost, because the high water content will lead to rotting. If, on the other hand, the crop is harvested too late, the rice will over-ripen and easily shatter. Furthermore, the unnecessary delay will expose the grains to increased bird attack, which can be devastating.

As harvest time approaches, the plants should be inspected daily, particularly the panicles on the most mature tillers. The proper time to harvest is when approximately 85% of the panicles are ripe. "Ripe, means that 90% of the spikelets are golden and hard, yet not so dried out as to shatter easily. The lowermost spikelets on each panicle will ripen last, but even they should be at least at the hard dough stage. The color of the leaves and stems should not be used as an indicator of the ripeness of the grains, since many varieties tend to have some green stems and leaves even when the grains are fully ripened.

Although grain ripening cannot be fully controlled by the farmer (climate and varietal characteristics will always be the major determinants), drain the plots 7-10 days before anticipated harvesting, i.e., when most of the grains are at the hard dough stage. This will contribute to even drying and facilitate harvesting by making it easier to walk inside the plots.

The traditional method of harvesting practiced throughout most of Sierra Leone involves panicle cutting. Harvesters grasp each stem several inches below the lowermost spikelets and cut it with a small straight knife. The panicle is retained and added to the growing bundle held in the hand. When the bundle becomes awkwardly large, it is tied together with a wisp of straw and carried out of the swamp. The chief advantage of panicle cutting is that the straw remains behind in the field. Bundles of rice produced by panicle cutting are easy to transport, easy to store, and easy to thresh. However, panicle cutting has several distinct disadvantages: it is very time-consuming, and since each panicle is handled frequently many grains shatter (fall to the ground).

In some areas of Sierra Leone, harvesting is done with the sickle. Sickle harvesting, popular in Asia, is very fast, since entire bunches of stems are grasped and cut in one swift motion. However, sickle harvesting requires the use of threshing machines - either the pedal thresher or the threshing table. At present few farmers possess threshing machines or know how to build them, so sickle harvesting remains relatively unpopular.

II. Threshing

Power Thresher

Rice that has been harvested by traditional panicle cutting is very easily threshed, since very little straw remains with the grains. The harvested bundles of rice are placed on a clean, hard surface and beat with sticks to separate the grains from the straw (dried leaves and stems). Frequently the threshers will walk over the bundles to speed the process. If any grains remain clinging to the straw, they are separated by pounding in wooden mortars.

Rice that has been harvested by sickle cutting must be threshed by other means, since the grains must be separated from a relatively large amount of straw. Usually a threshing machine is used to increase efficiency. The pedal thresher is a revolving drum (often an old oil drum) studded with nails or wire hoops which strip the grains from the panicles when bundles of rice are held against the moving drum, The pedal thresher is effective and fast, and it can be moved from plot to plot to eliminate extensive transportation of the cut plants. The mayor disadvantage of the pedal thresher is its price (about Le 150, or $150), which makes it too expensive for individual private ownership. However, farmers associations have been known to share pedal threshers with good results, since in most cases each individual's entire crop can be harvested and threshed in a few days. Another threshing device, the threshing table, is a slatted table constructed of bamboo, wood, or any suitable locally-available materials. Sheaves of rice are beaten against the table surface, dislodging the grains to drop through the slats into the collection area below. Threshing tables work best with those varieties in which the grains separate easily from the panicle, but the crop must be extremely dry for threshing to be effective.

III. Par-Boiling

Par-boiling 

In some parts of Sierra Leone, an intermediate step in the processing of rice is parboiling. Par-boiling improves storage quality of the grain and consequently is popular among farmers who produce rice for market. Although techniques vary, par-boiling usually involves soaking the grain in water and steaming over a slow fire in covered pots. Parboiling has several effects:

- cracks in the grain are "melted" together, leaving fewer broken grains and resulting in reduced loss during milling.

- several of the thin protein layers underlying the husk are boiled into the endosperm, thus increasing the protein content of the crop.

- the grain becomes significantly hardened, making par-boiled rice less susceptible to insect attack.

- the grain's tendency to absorb moisture from the air decreases, so that par-boiled rice keeps longer in storage.

- the husk soften and cracks during par-boiling, making it easier to mill.

But par-boiling also has disadvantages:

- par-boiling is labor-intensive

- par-boiling alters the sight, smell, taste, and texture of the rice and generally decreases its eating quality.

- par-boiled rice is harder and therefore takes longer to cook.

When it comes to the question of whether or not to parboil, there is no such thing as a "recommended" practice. Depending on the circumstances, par-boiling may or may not be economical and/or desirable. If grain loss during milling, protein content, or storage quality are important to the farmer, par-boiling may be preferred. If labor considerations, eating quality, or cooking time are important, the rice may best be left rough.

IV. Drying

Sun (solar) drying 

Before milling and storage, rice must be dried thoroughly. Rice which is not dried properly may crack during milling, or spoil during storage. At the time of harvesting, rice usually has a moisture content of 20-26%. This must be reduced to 12-14% before milling or storage can safely occur.

After threshing, the rice should be spread in the sun to dry. Constant turning is necessary to ensure slow, even drying and prevent cracking. Two to three days uninterrupted sunshine will suffice. Farmers often bite the grain to test for dryness (and usually they have a pretty good sense of what they're doing), but the extension agent may want to resort to this simple test'

1) Place in a small glass Jar (with a screw top) a handful of grains.

2) Add a spoonful of ordinary salt and seal the top.

3) Store 24 hours.

4) After 24 hours, examine the contents of the Jar. If the salt clumps together, the rice is too wet to mill or store. If the salt remains well dispersed, the rice has a moisture content of 15% or less and can safely be milled or stored in bags.

V. Storage

Rice storage facility

Rice harvested by panicle cutting traditionally is stored up in the rafters of the farmhouse or kitchen. The bundles are transported from the fields and stacked on reed mats high up off the ground, often over the cooking area so that smoke will sift in among the grains to assist drying and discourage insects. Where bags are readily available, the rice is often threshed and bagged for storage.

Rodent attack is generally a major problem. Since storage facilities are constructed of mud adobe and/or wood, rats easily gain access to feed on the rice. It is difficult to estimate accurately the annual crop loss to rodents, but 15% seems reasonable. In some instances, the loss is probably a good deal higher.

The key to effective grain storage lies in the construction of proper storage facilities. A good storage facility should be:

- plastered inside and out

- well ventilated

- well lit

- dry

- clean

- cool

- rat-proofed (heavy screen over windows and ventilation shafts)

After proper drying, threshed rice should be packed in bags and stored on wooden pallets in the storage facility. Instruct the farmer to stack the bags so as to allow good ventilation, since free air movement between the bags will prevent mildew and/or spoilage. Do not allow the bags to come into direct contact with the floor or walls, since moisture tends to condense where there is contact. If they are available, set rat traps (or get a cat!). Encourage the farmer to check the rice periodically for signs of spoilage and/or pest infestation.

Reference: FAO Rice Production