Cooked rice

Rice that has been cooked

Cooked rice is the whole or milled grain of Asian rice, Oryza sativa, heated in water until its starch has taken up water and swollen and the grains are soft enough to eat. Rice is about 79 per cent carbohydrate, 7 per cent protein and 2 per cent fat, and its pH is close to neutral. Cooked, it is a moist, mild, starchy food that stays tender while hot and, depending on the rice, firms up as it cools. Almost everything about how cooked rice looks and eats comes from its starch.

That starch is a mixture of two molecules, amylose and amylopectin, and their ratio sorts rice into classes. Waxy or glutinous rice has apparent amylose of 0 to 2 per cent: it cooks to a sticky, translucent mass in which the grains cling together. Rices with low amylose, roughly 10 to 19 per cent, cook soft and rather sticky; intermediate rices, roughly 20 to 25 per cent, cook firmer and less sticky; high-amylose rices cook dry and separate. Authors draw the boundaries in different places, so the percentages are guides, not standards. “Glutinous” describes the glue-like stickiness, not the protein gluten, which the National Institute of Diabetes and Digestive and Kidney Diseases places in wheat, barley and rye.

Eating habits follow the starch. Thai consumers in the north and northeast prefer rice that is sticky or quite soft and sticky, while those of the Central Plains prefer a firmer, less sticky rice. Across Thailand and the Philippines, rice that hardens when it cools is rated low quality. Aged rice expands more in the pot, a trait Thai buyers rate as premium. The temperature at which a rice’s starch gelatinizes is used as a rough guide to how long it will take to cook.

Where rice comes from

Rice is the second most-consumed food crop in the world, after wheat. Its domestication is traced to the lower Yangtze River region of China, one of the places where the crop was brought under cultivation. Archaeobotanists there study phytoliths, charred seed and the bases of spikelets from eight Neolithic sites spanning roughly 10,000 to 2,000 years ago. The share of domesticated-type remains grows step by step, from the Shangshan culture, through the Kuahuqiao culture of 8,000 to 7,000 years ago, to the early Hemudu culture, which indicates a slow process rather than a single event.

Two lineages of Asian rice are recognised: japonica in East Asia and indica in South Asia, thought to descend from two different wild progenitors, Oryza rufipogon and O. nivara, although the story is complicated by genes that cross between them. Rice was probably gathered from the wild by seed foragers for millennia before it was cultivated, and there is evidence of more targeted foraging by 8,000 years ago along the lower Yangtze. At Tianluoshan and Hemudu, some of the earliest evidence for cultivation, rice was still a minor part of a broader foraging economy. Cultivation reached the Sichuan Plains more than 4,000 years ago, and the Han historian Sima Qian, writing between 109 and 91 BC, reports that the envoy Zhang Qian found rice grown in Dayuan, usually identified with Ferghana in Central Asia.

From East Asia, rice spread west along two parallel routes: overland along the Silk Road through the southern mountains and the dry heart of Central Asia, and by sea, along the Indian Ocean trade to the Red Sea and the Mediterranean. Rice is a thirsty crop, and its arrival in western Asia has been linked with investment in irrigation.

What a buyer looks at

Buyers judge rice before it is cooked. Grain length and the ratio of length to width sort rice into classes, though the classification systems use different ranges, so no single scale applies everywhere. Chalkiness, the opaque patch in a grain caused by tiny air spaces in the starchy endosperm, is a defect in most markets. Head rice, grains that are at least three-quarters intact after milling, measures how well the rice was milled; broken grains lower the price, particularly for affluent buyers. Whiteness can mean two things, translucency or how fully the bran was removed in milling.

Taste differs by region. In Southeast Asia, premium rice is described as soft, shiny and aromatic. In South Asia, buyers look for long, slender, fragrant grains that cook non-sticky. After cooking, the three measurements breeders use to describe the eating quality of a rice variety are apparent amylose content, which predicts hardness, gel consistency, a measure of how soft the cooked rice is, and gelatinization temperature, the guide to cooking time. Two rices in the same class on all three tests can still taste different, because these tests do not capture aroma, sweetness or the other flavors a person notices.

Cooked rice as a fermentation ingredient: sai krok

Cooked rice is a deliberate ingredient of sai krok Isan, the sour pork sausage of northeastern Thailand, also known as sai krok prew or by the Thai name for sour sausage, ไส้กรอกเปรี้ยว. The sausage is minced pork and fat mixed with garlic, salt, seasonings and cooked rice, stuffed into casing and fermented for two to three days at room temperature under restricted oxygen. In a published formulation the rice made up 15 per cent of the mix, beside 55 per cent pork and 30 per cent backfat. The recipe used on this site lists 200 g of cooked rice for 700 g of lean pork and 300 g of back fat, with the rice worked into a paste of garlic, salt and curing salt before it is mixed into the ground meat.

The rice gives the lactic acid bacteria something to eat. In the closely related nham glucose and fructose from the cooked rice and the garlic were reported as the main carbohydrate that the bacteria used for lactic acid. In the study of sai krok E-san, lactic acid bacteria climbed from about 3 or 4 log CFU/g at the start to 9.4 to 11.4 log CFU/g after three days, titratable acidity rose to 1.0 to 1.3 per cent as lactic acid, and the pH fell to about 4.4 to 4.9. Yeasts and other organisms are listed too: the Thai fermented-food database names Candida krusei, Debaryomyces, Lactobacillus and Streptococcus faecalis. The acid sours the sausage. In the study Staphylococcus aureus, present at 1 to 2 log CFU/g at the start, was undetectable by day three, and Salmonella was not detected.

Neither the Thai database entry nor the recipe on this site says which kind of rice is meant. The recipe does not say whether the rice is sticky or jasmine, or whether it is cooled before mixing, and it gives two fermentation schedules: about 24 hours at 80 to 90 °F (27 to 32 °C) with a starter culture and 72 to 96 hours at room temperature without one. The northeastern sausage is stuffed into natural hog casing and hung so that some water drains out, and the finished product has at least 12 per cent protein and no more than 30 per cent fat; nham, its relative, is made from lean meat with pork rind, packed tightly into plastic casing, and has at least 20 per cent protein and no more than 8 per cent fat. The sausage is eaten roasted, with cooked rice or steamed glutinous rice, ginger, chilli and cabbage, so rice appears on both sides of the plate, in the sausage and under it.

Cooked rice in koji and rice ferments

Cooked rice is also the bed on which koji is grown. Spores of Aspergillus oryzae are added to boiled and cooled rice, or to barley or another grain, and the mold grows through the grains, secreting amylases that break starch into sugars and proteases that break protein into amino acids and peptides. The result is rice koji, the starter of miso, sake and amazake, a sweet non-alcoholic rice drink. These are made products, each with its own preparation record; the rice in them is a substrate that is cooked first so that the mold and its enzymes can reach the starch.

The common feature in all of these uses is that cooking makes rice a food for microbes as well as for people. Heated in water, starch granules swell and lose their order, a change called gelatinization, and the temperature at which a rice does this is used as a guide to its cooking time. Cooking also brings the grain to a water activity at which Bacillus cereus can grow, which is why cooked rice needs the care described below.

Safety: cooked rice and Bacillus cereus

Cooked rice carries a specific hazard. Bacillus cereus, a spore-forming soil bacterium, is found on raw rice. Its spores survive cooking: boiling rice for about 20 minutes cuts the number of spores by only two to three decimal reductions, a hundred- to thousand-fold, which is not enough to make the rice safe. Cooking brings the grain to a water activity at which they can grow, so spores that survive can germinate in warm cooked rice and multiply. In cooked rice held at 26 to 32 °C the bacterium has reached 10 million to a billion cells per gram within 24 hours. While growing it makes cereulide, an emetic toxin that withstands heating, so reheating does not make rice safe once the toxin has formed. About 95 per cent of the emetic form of B. cereus illness is linked to rice.

The control is cooling and cold. Rice should be cooled quickly and stored below 4 °C; cooked rice kept at 8 °C still reached 10,000 to 100 million cells per gram in ten days. B. cereus grows between pH 4.5 and 9.5 and from 4 to 48 °C. For rice that goes into a fermented sausage this matters. The sausage is held at 27 to 32 °C for a day or more, and in the sai krok study the pH reached 4.4 to 4.9 within three days, so the early part of the fermentation, before the pH has dropped, is when a rice-borne hazard has room to grow. The sources reviewed report no B. cereus counts for sai krok, so this page makes no claim about how large the risk is; it only notes that the cooling and cold-storage advice above applies to the rice.

Choosing, making and keeping it

Choose the rice for the dish. Waxy rice cooks sticky, low-amylose rice soft and slightly sticky, and intermediate rice firmer and less sticky. For a fermented sausage the recipe leaves the choice open, and a cook should record which rice was used. The cooked rice should be cooled quickly and used promptly, and leftover rice carries the risk described above unless it was cooled fast and kept below 4 °C.

The glutinous label can mislead: it describes stickiness, and the gluten that triggers celiac disease is a protein of wheat, barley and rye, grains the National Institute of Diabetes and Digestive and Kidney Diseases names and rice is not among.

What the sources leave open

The cited recipe and the Thai database give no rice variety, no cooling step and no water ratio, and no source reviewed describes the traditional preparation of the cooked rice used in sai krok. The history of rice in sausage making is not documented in the sources used here.