Quality factors relate to grain length, stickiness, aroma, texture, and flavor. Nutritional content may also vary between different types of rice. There are many different types of rice with many different qualities to suit different consumer preferences. Asian rice, contains two broad groups: indica (long-grain) and japonica (short-grain). Other types of Asian rice include glutinous rice and aromatic rice. 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. 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. 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. 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. 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% compared to 10% to 30% in non-glutinous rice 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. 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. 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. 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.
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 Flavor is the impression perceived through the chemical senses from a product in the mouth, 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 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.