Changes in Volatile Components of Paddy, Brown and White Fragrant Rice During Storage

Abstract

Paddy rice from an Australian fragrant variety, YRF9, was dehulled and milled. A total of three samples—the original paddy and the resulting brown and white rices obtained by dehulling and/or milling—were then stored for 3 months at 30°C and 84% relative humidity under atmospheric pressure, or under a vacuum of about 150 Pa. Before storage, a sample was dehulled and milled and the volatile components were isolated from the resulting white rice using a modified Likens–Nickerson simultaneous distillation–extraction unit (SDE). After storage, the paddy and brown rices were dehulled and/or milled as appropriate and the volatile components were isolated from the resulting white rices, as well as that stored in the white rice form, using the same SDE technique. Rice stored under both sets of conditions acquired a slightly musty, rancid odour, the more so for the sample stored in air. During storage in air, there was an increase in the level of total volatile compounds in all three forms of rice, which was far greater for the white rice. This increase was largely due to aldehydes and ketones commonly formed as a result of lipid oxidation processes. This indicates that, when stored in air, the fragrant aroma character was retained better in brown and paddy rice than in white rice, because the development of off-flavours was inhibited by the protective layers of bran and hull. However, during vacuum storage, total and neutral volatiles decreased in all stored rices, with acidic and basic fractions being relatively unchanged. The volatiles profile of white rice stored under reduced pressure was closer to that of the fresh rice than were those of the equivalent storage samples of either the brown or paddy rices. During storage, the content of 2-acetylpyrroline, a compound that contributes greatly to the favourable character of fragrant rices, decreased to the extent of around 40–50% no matter what the form of the rice, or the condition of storage. It is thus concluded that, while it was found possible to inhibit the development of off-flavours to some extent, no way was found to assist the preservation of the desirable flavour compound, 2-acetylpyrroline.