Rice: Old and new flavors; cooking; parboiled

A last installment (for the time being) of rice news.

Old and new Japanese rice As I've mentioned, Japan generally prefers their rice as fresh as possible; new-harvest rice is prized. Japanese scientists looked at the flavor changes that take place during aging, and found some pretty significant changes.

The acidity of the grain increases, probably because oils are broken down into free fatty acids--the same molecules that also cause a firmer, dryer texture. This effect can be minimized if the rice is washed before cooking. The sucrose sugar in the grain is broken down into a mixture of glucose and fructose, which is slightly sweeter, and which is faster to undergo browning reactions and discolor if the grains dry out slightly at the end of cooking. And the mouth-filling umami sensation, caused by natural monosodium glutamate and ribonucleotides, declines with time. So for real rice connoissseurs, people who eat it every day and notice the nuances of its flavor and texture, it makes sense to buy rice labelled "new harvest" and use it quickly.

Cooking water and times From France and Switzerland, a study of the basic cooking properties of different milled rice types. The scientists looked at how much water the rices absorbed, how long they took to cook, and what might account for the differences.

They found that the kernel size and shape are what determine cooking times: not surprisingly, the thicker the longer. (There are three dimensions to a rice kernel: the length; the height, from back to belly; and the thickness through the sides). They found that both short- and long-grain rices absorbed about the same amount of water to give an al dente texture, with 95% of the grain starch soft and gelated; the workable ratio was 2.55 parts water to 1 of rice by weight, giving a grain that's about 75% water, 25% solids (dry rice is about 10% water). Short-grain rices were somewhat better at a ratio of 2.0, and long-grain at 3.0. The usual cooking instructions on packages and in books call for more water, probably to compensate for losses to steam that escapes around the edges of the lid.

The cooking experiments were done in Volvic mineral water with 0.35% salt, about a tenth the concentration of seawater. The rice was added to the boiling water and cooked just until all the rice starch had gelated. The cooking times ranged from 9.5 to 15.4 minutes. Smaller grains generally took less time to cook. But there were deviations from this rule, and they probably have to do with the effects of aging on rice, which slow the penetration of water into the grain and then into the starch granules. The scientists also looked at the microstructure of the grains, and suspect that the rice cell walls and their carbohydrates are arrayed differently in different varieties and may influence water penetration during cooking.

The takehome lesson from this paper is that long- and short-grained rices take about the same amount of water to develop a fully cooked texture, but cooking times vary by 50% or more, and there's no way to predict that. Longer cooking times may require more water to replenish the steam losses during prolonged cooking.

Parboiled rice: color and flavor From Belgium, a study of the qualities of parboiled rice, which is soaked, steamed, and dried before the bran is removed. (Normally the brain is milled away first, and the rice packaged raw.) Parboiling allows some of the bran nutrients to penetrate into the grain; and it results in a firmer rice that doesn't get as soft or leak sticky starch as much as standard rice, so the cooked grains are more tolerant to overcooking and remain separate and loose. They have their own distinctive flavor, with overtones of vanilla and nuttiness that come from the cooking and drying processes. Parboiling goes back thousands of years in India and is also used to manufacture packaged rices.

The Belgian scientists looked at the the typical yellowish color and the flavor of parboiled rice. They found that despite the high water content and moderate temperature, the rice bran underwent browning reactions. The soaking step encouraged rice enzymes to release reactive reducing sugars, and the steam heat encourages them to combine with amino acids from the proteins, generating brown pigments and toasty flavors. Both the native bran pigments and the browning-reaction products diffused into the rice gain during steaming and drying, giving it both it yellowish color and added flavor.

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Tran, T.U. et al. Detection in changes in taste of japonica and indica brown and milled rice during storage . . . . J Agric. Food Chem. 2005, 53, 1108-18. 

Vidal, V. et al. Cooking behavior of rice in relation to kernel physicochemical and structural properties. J Agric. Food Chem. 2006, 54, 336-46.

Lamberts, L. et al., Impact of browning reactions and brand pigments on color of parboiled rice. J Agric. Food Chem. 2006, 54, 9924-29.