Rice – Quality Factors
There are many different types of rice with many different qualities to suit different consumer preferences. Quality factors relate to grain length, stickiness, aroma, texture, and flavor. Nutritional content may also vary between different types of rice.
Oryza sativa, or Asian rice, contains two broad groups: indica (long-grain) and japonica (short-grain). Other types of Asian rice include glutinous rice and aromatic rice. Oryza glaberrima, or African rice, includes long- and short-grain varieties. All varieties of rice can be processed post-harvest as either white or brown rice, affecting flavor, texture and nutritive value. Milling of rice post-harvest always leads to some grains being broken; a higher proportion of broken grains decreases the price since the quality is generally acknowledged to be reduced.
Indica varieties of Asian rice are long-grain and usually grown in hot climates, whereas japonica varieties of Asian rice are short-grain and include both temperate and tropical varieties. African rice and glutinous rice (a variety of Asian rice) also come in long- and short-grain varieties.
In short-grain rice varieties, including japonica varieties of Asian rice, grains tend to stick together when cooked. This is not to be confused with glutinous (or ‘sticky’) rice, descibed later on this page. Japanese rice (uruchimai or ‘sushi rice’) is a short-grain variety. Another popular short-grain variety is Arborio. Short-grain rice refers to rice with grain length up to 5.2 mm.
Long-grain rice does not stick together when cooked, but tends to remain separate and ‘fluffy’. Most of the rice produced in southern Asia, including India and Thailand, is Indica (long-grain) rice. Basmati rice (mainly grown in India and Pakistan) and Jasmine rice (only grown in Thailand) are two popular varieties of long-grain rice, and both are aromatic or fragrant, described in more detail later on this page. Long-grain rice refers to rice with grain length over 6.0 mm.
Medium-grain rice refers to rice with grain length above 5.2 mm up to 6.0 mm.
Glutinous rice varieties originate from Lao PDR and northeast Thailand, where they are the staple food (Almanac 2012; Chaudhary, 2003). Among glutinous rice varieties, physical characteristics, quality and environmental adaptations vary widely. Some glutinous rices are aromatic, colors include white, purple and black, and grain size varies. Glutinous rice is opaque when raw, unlike most non-glutinous rice varieties, which are somewhat translucent when raw. With regards to starch, amylose content is low, ranging from 2.6% to 4.8% (Chaudhary, 2003) compared to 10% to 30% in non-glutinous rice (knowledgebank.irri.org), but amylopectic content is high, accounting for the glue-like stickiness of glutinous rice. Glutinous rice does not contain dietary gluten (i.e. it does not contain glutenin and gliadin), and is thus safe for gluten-free diets. See also information on cooking methods. Glutinous rice can be cooked as grains or ground into flour and cooked as a paste or gel.
Aromatic rice is another variety of Asian rice, with medium to long grains and a light, fluffy texture and nutty or popcorn-like aroma when cooked. Aromatic rice is also generally said to have a nutty flavor, which is more pronounced in brown (unpolished) aromatic rice. The most internationally well-known types of aromatic rice are basmati and jasmine.
Basmati rice, grown mostly in India and Pakistan, is renowned for its long, slender shape that elongates rather than expands in width when it is cooked. The word ‘basmati’ means ‘queen of fragrance’, and the rice is distinguished by its aroma. There are hundreds of other aromatic varieties grown and consumed locally, but basmati is the only one that is exported (Chaudhary, 2003).
Jasmine rice, grown only in Thailand, is distinguished by its fragrance and a water milling process that leaves the grains silken to the touch. The grains are similar in size to long-grain rice but cook moist and tender like a medium-grain rice. In Thailand there are many other aromatic rice varieties, but jasmine and KDML 105 are the only ones exported (Chaudhary, 2003).
In the United States, domestically grown aromatic rice varieties include Texmati (a cross between ‘American’ long-grain rice and basmati rice), Wehani (developed by Lundberg Family Farms in California, using basmati seeds), and wild pecan rice (another basmati hybrid developed in Louisiana).
Aroma is detected when the volatile compounds of the rice enter the nasal passage. A good perfumer can reportedly differentiate 150—200 odorous qualities and rice aroma is typically described by trained panelists using a lexicon with 10—12 descriptors (Champagne, 2008).
The aroma of rice is mainly caused by the presence of the chemical compound 2-acetyl-1-pyrroline. But it is likely that many oils, phenolics and organic compounds are involved, such that hundreds of unique varieties of aromatic rice exist, in addition to many hybrids (Chaudhary, 2003).
Flavor of rice differs by type of rice (variety, grain length, stickiness, color, etc.) and also depends on whether or not it has been polished (i.e. brown or white rice) and, of course, cooking methods. Those considerations are obvious to most of us. But flavor may also vary by genetics, the growing environment, type of fertilizer and cultural practices (which affect amylose and protein content), the timing of draining and harvesting the field (affecting maturity and moisture content, and also amylose and protein content), harvest moisture content, rough rice drying conditions, final moisture content, storage conditions (temperature and length of time), degree of milling, and also finally also washing and soaking practices and serving temperature of the cooked rice (Champagne, 2008).
As explained in a review by Champagne (2008): “Flavor is the impression perceived through the chemical senses from a product in the mouth (Caul 1957). According to Meilgaard et al (2007), when defined in this manner, flavor includes aromatics (olfactory perceptions caused by volatile substances released from a product in the mouth through the posterior nares); tastes (gustatory perceptions [salty, sweet, sour, bitter] caused by soluble substances in the mouth); chemical feeling factors that stimulate nerve ends in the soft membranes of the buccal and nasal cavities (astringency, spice heat, cooling, bite, metallic flavor, umami taste).”
“Descriptive sensory analysis has identified over a dozen different aromas and flavors in rice. Instrumental analyses have found over 200 volatile compounds present in rice. However, after over 30 years of research, little is known about the relationships between the numerous volatile compounds and aroma/flavor. A number of oxidation products have been tagged as likely causing stale flavor. However, the amounts of oxidation products, singly or collectively, that need to be present for rice to have stale or rancid flavor have not been established. Only one compound, 2- acetyl-l-pyrroline (2-AP; popcorn aroma) has been confirmed to contribute a characteristic aroma. Furthermore, 2-AP is the only volatile compound in which the relationship between its concentration in rice and sensory intensity has been established.” (Champagne, 2008).
Specialty rice types
In some parts of the world, especially in North America and Europe, rice is developing a new market niche as a staple and as a gourmet food. This trend appears to be related to the arrival of large numbers of immigrants from Southeast Asia, who introduced aromatic rice to markets where it was previously unknown. It has been adopted by a food-quality-conscious public over the past several years.
There are a number of ‘speciality’ rices available, including colored rice and aromatic rice (already described above) and wild rice.
Black, purple and red rice
Wild rice (not really rice!)
Twenty-two wild species of rice are found in Asia, Africa, Australia, and the Americas. Only a few are closely related to Oryza sativa (Asian rice) and Oryza glaberrima (African rice). However, ‘wild rice’ is usually used to refer to the grain harvested from four species of grass that form the genus Zizania, including both wild and domesticated varieties. Historically, wild rice was gathered and eaten in North America and China. Currently it is a delicacy in North America, due to it’s taste and nutritional value, and it is now cultivated there, mainly in California, Minnesota and Saskatchewan. It is also produced in Hungary and Australia. In China, although Manchurian wild rice was once an important grain, now the plant’s stem is used as a vegetable and the grain is less commonly eaten.
Industrial grades of rice
For more information, please see Grain quality and nutrition.