Rice: Scented and un-, old and new

Last week I wrote in the New York Times about the distinctive aroma of Himalayan basmati and Thai jasmine rices and the chemical that contributes it. I've collected several other interesting studies of rice over the last few months. They come from labs all over the world: not surprisingly Japan and Korea and Thailand, but also France, Switzerland, Italy, Belgium--and Arkansas. Here's some of their news; more tomorrow.

Rice Flavor: From Thailand, a study of the basmati/jasmine flavor molecule, 2-acetyl-1-pyrroline, 2AP for short, notes that 2AP is produced through the whole of the rice plant except for the roots. It's also found at very high levels in pandanus leaf, or screwpine (Pandanus amaryllifolius), an Asian herb that's often used to impart that same aroma to cooked dishes of various kinds. And in "bread flowers," flowers of Vallaris glabra, a tropical shrub, which form part of the traditional wedding potpourri in Malaysia. The Thai researchers found that the unmilled, brown fragrant rice had several times as much 2AP as it did after milling into white rice. And aged Indian basmatis had half the 2AP as fresh Thai jasmine. So you can get more intense basmati aroma from the brown rice and from newly harvested rice.

Rice texture: Interestingly, in the Himalayan region where basmati rice is produced, the people prefer their rice aged for several months--which apparently means that they lose some of the flavor for which basmati is prized. By contrast, Japanese consumers prize newly harvested rice, and don't like the cooking qualities of old rice. India and Pakistan prefer the grains to remain separate and firm, while Japan prefers them somewhat sticky and soft. As any rice sits for months, in the warehouse or in the kitchen cabinet, its cooked texture gets progressively firmer and less sticky.

These changes are probably the result of a number of factors, including the breakdown of grain oils into free fatty acids, which form very stable, hard complexes with starch molecules. A Japanese team looked at what happens to rice during storage, and found that normally dissolvable proteins at the rice surface become oxidized and undissolvable. They were able to reverse the increased firmness and decreased stickiness of the cooked texture by adding a chemical reducing agent, sodium sulfite, to the cooking water (to reverse the oxidation), or by abrading away the surface of the aged grain.

So apparently as a rice grain sits in storage, the surface proteins become oxidized, bond to each other, and form a thin skin on the grain, which limits water penetration into the grain, and limits the starch leakage outward that causes stickiness. That's at least part of the cause of the firmer cooked texture in aged rice.

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Sriseadka, T. et al. Rapid method for quantitative analysis of the aroma impact compound, 2-AP, in fragrant rice . . . . J. Agric. Food Chem. 2006, 54, 8183-89.

Ohno, T. and N. Ohisa. Studies on textural and chemical changes in aged rice grains. Food Sci. Technol. Research 2005, 11, 385-89.