Milling rice

by Emily Wilson
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Quality criteria for cereals are related to the end use to which they are destined. In the case of rice, data from the Food and Agriculture Organization for the period 1995-97 show that around 90% of all rice grown commercially is used directly for human food. This is the highest proportion dedicated to food use for any cereal, the equivalent values for wheat and maize being 70% and 10%, respectively.

Rice is the main staple food of 40% of the world's population and the main food throughout Southeast Asia.

In spite of the high proportion of rice consumed as human food, international trade in rice is relatively small, representing only about 5% of total production, indicating that most is consumed within the country where it is grown. Indeed, it has been estimated that 50% of the world rice crop is consumed on the farms where it is grown.

Within individual countries, quality criteria are related to local preferences and financial constraints.

THE RICE PLANT. Rice is an annual grass from 50 to 130 centimeters tall; some deep water types can grow up to 5 meters. The main stem branches at the base to produce a number of tillers that have the potential to bear inflorescences (ears). The inflorescence is known as a terminal panicle and each can bear 50 to 500 spikelets, each of which bears one fruit, known as a caryopsis or grain. (The term "kernel" is also applied.)

Like barley and oats, but unlike other cereals, rice grains as harvested are not fully separated from the floral structures of the ear. Structures known botanically as pales (lemma and palea) or inner glumes, and technically as hulls or husks, remain in close contact with the grain even after threshing.

Twenty-two species of rice are recognized, but the only one that is traded significantly is Oryza sativa L., or common rice. The marsh grass Zizania aquatica, known as American wild rice or Indian rice, also is sold as an "exotic" addition to rice meals.

Within the species Oryza sativa there is a wide range of types, each being suited originally to local preferences but increasingly enjoyed around the world in different styles of cooking. Physical characteristics of the grains provide criteria by which the main types of rice can, to a large extent, be recognized and defined. But it is the cooking and eating qualities associated with the physical dimensions that provide the incentive for making the distinction.

The main physical characteristic employed in distinguishing type is grain length. Hence, the terms "long grain" (patna), "medium grain" (rose) and "short grain" (pearl) are used. The alternative descriptions — patna, rose and pearl — are also used to make the same distinctions, but the basis of these descriptions is less consistent and less transparent.

Another basis of distinction is color, although the color of "normal" varieties is rarely addressed. Only those rices with exceptional colors are described in this way. Varieties with dark brown pericarp are described as "red." Most are of inferior quality and excluded from graded samples but exceptions include one type that has become established as a speciality of the Camargue region of southern France. Varieties described as "black" are also highly regarded.

Somewhat confusingly, the term "brown rice" refers not to the natural color of the pericarp, but to the fact that the pericarp (and embryo) remains intact around the endosperm rather than having been removed, by milling, to produce "white rice."

While it is possible, and nutritionally beneficial, to consume brown rice (or whole rice, as it is also known), the majority of this cereal is consumed as the fully milled "white" product. Rather than being ground into flour or meal, most rice is consumed as cooked grains.

Because rice hulls do not become detached during threshing, grain can be traded in several forms according to how much milling has been performed. The form harvested with hull intact is known as "paddy." After dehulling, the remaining grain is known as "brown rice." After further abrasion, "white rice" results. This is not a well-defined product since whitening can take place to varying degrees, according to the skill of the miller, available equipment and customers' requirements.

Crudely whitened rice grains may have rough surfaces, with minute scores in which bran and endosperm dust may lurk. The appearance is opaque and less attractive than polished grains, which have a smooth, dust-free, translucent appearance. A perception of polishing also can be achieved by providing a surface coating of glycerol or talc, although this practice is less widespread than in the past.

Premiums are paid for grains that are polished, sometimes to a degree laid down by a customer. In Japan, premium products – those finished to a specified degree – are packaged in bags bearing the name and chosen designs of a hotel restaurant, retailer or other customer.

NUTRITIONAL PROPERTIES. The endosperm that constitutes the whitened grain comprises mainly starch and protein. As in all cereal grains, other valuable nutrients, including vitamins and minerals, are stored in the embryo, hull, pericarp and mainly the aleurone layer. Although it is the outer layer of the endosperm, the aleurone is removed during whitening.

Unless consumers of white rice take other sources of the vitamins and minerals, deficiency symptoms can occur. One means of avoiding deficiency without changing eating habits is to parboil the rice before milling. Parboiling leads to a partial redistribution of nutrients from the surrounding layers into the endosperm, where they are retained during and after milling. After parboiling, the aleurone layer is more difficult to remove cleanly and its remains increase nutrient content of the white rice.

Essential nutrients not originating in the grain at all also may be added to rice grains as a means of ensuring adequate levels among consumers. The nutrients have to be absorbed into the milled grains in such a way that they are not removed by contact with boiling water or steam, so sophisticated methods must be adopted. In one system, thiamine, niacin, riboflavin, pantothenic acid and pyridoxine may be added in one layer, followed by vitamin E, calcium and iron in a second layer. Finally, a protective coating is added.

As well as retaining the added nutrients, the coatings reduce leaching of the naturally occurring ones. Through biotechnology, other nutrients may be introduced into rice grains; the pigment beta-carotene, the precursor of retinol (vitamin A), has already been introduced into to a new variety called "Golden Rice."

Paddy rice can be stored for a year or more at moisture contents up to 14%. Signs of deterioration through excessive storage are yellowing and brittleness.

Well-milled white rice can be stored for even longer periods but rancidity develops in brown or roughly whitened grains, in which damaged aleurone cells are present. Aleurone cells, which are removed by whitening and polishing, contain both fat and enzymes that render them rancid. When the cells are damaged, the enzyme and substrate are brought into contact.

Hulls have little commercial value so to maximize the amount of high-value commodity in a limited volume, most exported rice is transported after the hulls have been removed. For this reason, the "brown" form also is known as "cargo rice."

QUALITY CRITERIA. In western countries, where rice is regarded as a healthy means of adding variety to the diet rather than as a staple, longer grain rices (long and medium) are used in savory dishes, while short grained types are used in sweet dishes. Short grained types are generally more sticky than long and medium types. In Japan, the sticky character of short grained rice is particularly suited to sushi, and some communities use it for all purposes. Successful breeding strategies whereby characteristics of one type have been introduced into others have led to a blurring of the association of grain length with stickiness and other properties.

While stickiness is the most obvious cooking property that distinguishes types of rice, it is not the only one. Some types, such as Thai Fragrant (jasmine), are valued for the aroma that emerges when they are cooked.

The combination of cooking and eating qualities that characterize a particular type of rice may be indefinable by analytical values, and definition depends upon a name related to the region of origin or cultivation. In rices of high quality, it is usual for the country of origin to be identified as an endorsement. The term "basmati" can be applied only to selected varieties that are grown in India or Pakistan. In Spain, the concept of regional identity is enshrined in a "Denominaci n de origen" system similar to that accorded to wines of high quality. Calasparra rice has been registered under this scheme and applications for others are pending.

Many premium rices are grown in Italy, the best known being arborio. This very large grained rice is particularly good as a base for risotto, a dish in which the rice absorbs the stock, wine and juices in which it is cooked.

Rice types with a recognized cachet of quality command premium prices but unscrupulous traders can adulterate them with inferior types. Responsible retailers ensure authenticity by dealing only with trusted suppliers and demanding audit trails. As a further safeguard, some retailers in the U.K. have adopted an image analysis method, devised at Campden & Chorleywood Food Research Association, which are capable of detecting significant adulteration of basmati with cheaper types.

Considerable variation exists in rice starch composition. The two major components, amylose and amylopectin, remain the same but the ratio of one to the other varies.

"Waxy" or "glutinous" rices have the highest proportions of amylopectin. It is this – and not the presence of either wax or gluten, as these unfortunate epithets suggest – that gives the endosperm of these types of rice their special sticky character when cooked.

The endosperm of glutinous rices have a chalky appearance. Chalkiness, an opaque white condition of parts of the endosperm, although characteristic of some rice types, leads to downgrading in other types. The condition arises as a result of light scattering at interfaces between air spaces and the solid endosperm components.

In the favored vitreous endosperm, air spaces do not exist or are very much smaller. Degree of chalkiness increases as a result of high temperatures during growth.

GRADING CRITERIA. In the U.S. grading system, long, medium and short classes are defined for rough, brown and milled rice, in terms of length to breadth ratio. Dehulling and milling tend to reduce length disproportionately so lower aspect ratio limits are set for processed grains than whole grains. In the brown rice category (rice for processing), there also is a class described as mixed.

Grades are defined in terms of levels of contamination with seeds of other species, damaged or red grains, heat damaged or objectionable grains, and chalky grains.

An important quality factor for all classes of rice is the ability of grains to withstand the milling process without breaking. For grading rough and brown classes, a standard milling system is applied to samples and the level of breakage is assessed. Ability to remain unbroken is a partly a varietal character and partly the result of environmental factors.

The presence of cracks or checks in the endosperm is an indication of a tendency to break during processing. A further three classes of milled rice are based on the percentage of whole kernels and of broken kernels of different size, the proportion of small particles increasing in the order second head milled rice, screenings milled rice and brewers milled rice. Special classes exist for coated milled rice, granulated brewers milled rice and parboiled milled rice.

Full details of the U.S. grading system are posted on the U.S. Department of Agriculture web site at www.usda.gov/gips/strulreg/standard/rice.htm.

Tony Evers, based in London, is a former head of the Cereals and Milling Department at Campden and Chorleywood Food Research Association (U.K.) Ltd. He is now a consultant for the cereals industry.

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