Rice - the long and the short of it.

Tony Evers

Quality criteria for cereals are related to the end-use to which they are destined. In the case of rice, FAO data for the period 1995-1997 show that around 90% of all rice grown commercially is used directly for human food (Fig 1). 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 South-East Asia


Figure 1. World usage of rice 1995-1997 (FAO web site)

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 (FAO figures 1995-1997), indicating that most is consumed within the country where it is grown. Indeed it has been estimated that 50% of the world crop is consumed on the farms where it is grown. Within individual countries, quality criteria are appropriate to local preferences and financial constraints.

The rice plant
Rice is an annual grass 50-130cm tall (up to 5m in some deep water types). 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 50-500 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.
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Species
Twenty two species of rice (Oryza spp) are recognised 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 is also sold as an 'exotic' addition to rice meals.

Forms
Within the species Oryza sativa there is a wide range of types, each being suited originally to local preferences but increasingly enjoyed round 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 recognised 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) short grain (pearl) are used. The descriptions in parenthesis are also used to make the same distinctions but the basis of these descriptions is less consistent and hence less transparent.

Another basis of distinction is colour although the colour of 'normal' varieties is rarely referred to. It is those with exceptional colours that are described in this way. Thus 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 colour 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 indeed 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 as harvested, with hull intact is known as 'paddy'. After dehulling the remaining grain is known as 'brown rice' and 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, the equipment available to him and the requirements of customers. Crudely whitened rice grains may have rough surfaces, with minute scores in which bran and endosperm dust may lurk. The appearance is thus opaque and less attractive than polished grains, which have a smooth dust-free, translucent appearance. A perception of polishing can also 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, finished to a specified degree, are packaged in bags bearing the name and chosen designs of a hotel restaurant, retailer or other customer.

Nutrition
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 which, although it is the outer layer of the endosperm, 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 habit 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 remains of this also increase nutrient content of the white rice. Essential nutrients not originating in the grain at all may also be added to rice grains as a means of ensuring adequate levels of them 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 and hence sophisticated methods have to 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; already the pigment beta-carotene, the precursor of retinol (Vitamin A), has been introduced into to a new variety 'Golden rice'

Storage and transport
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 maximise 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 is also 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 savoury 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 types is particularly suited to suchi and some communities use it for all purposes. Successful breeding strategies whereby characteristics of one type have been introduced into other types 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 characterise 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 DenominaciŁn 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, probably the best known outside the country 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 recognised 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 UK have adopted an image analysis method, devised at Campden & Chorleywood Food Research Association (CCFRA), which are capable of detecting significant adulteration of Basmati with cheaper types.

Fig 2 Images of cooked grains of two types of long grain rice. Authentic Basmati grains are shown on the left image. (reproduced by kind permission of CCFRA, UK)

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 and it is this, and not the presence of either wax or gluten, as these unfortunate epithets suggest, that gives the endosperm of these 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 generally favoured vitreous endosperm, air spaces do not exist or are very much smaller. Degree of chalkiness increases as a result of high temperatures during growth.

In the US Grading system Long Medium and Short classes are defined for Rough, Brown and Milled Rice, in terms of aspect (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 is also 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, Parboiled milled rice.

Full details of the US grading system are posted on the USDA website: http://www.usda.gov/agency/gipsa/strulreg/standard/rice.htm

Further Reading