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Aroma loss in rice as affected by different conditions during postproduction operations
Dissertation Abstract:
One of the attributes that contribute to the overall rice quality is its aroma. Fragrant rice has become popular and continues to command higher price than ordinary rice because of its distinctive pleasant scent, but decreases with time and conditions and during, drying, storage, and milling. Of the many volatile compounds present in aromatic rice, 2-acetyl-1-pyrroline (2AP) is a major compound that gives rice its popcorn-like aroma.
The loss of 2AP in rice at various conditions during drying, storage, and milling were investigated. The mass diffusivity of 2AP in rice increased with increasing ambient air temperature. Rice dried at 55°C air temperature had greater aroma loss than those dried at lower air temperature. Samples stored at higher temperature (27°C) recorded higher 2AP loss than those kept at lower temperature (14°C).
The amount of 2AP in rice decreases with time. The free 2AP, being volatile, is lost due to diffusion. Experimental data for drying air temperature of 35°C exhibited increased aroma loss because of the long drying duration compared with higher drying air temperature set-up (45°C). Storage experiments indicated a rapid loss of 2AP during the first five weeks of storage and subsequently leveled off afterwards, attaining the level of starch bound 2AP.
As in most literature, the free 2AP are easily lost while bound 2AP are left, only to be released upon exposure to high temperature such as cooking. Bound 2AP are mostly found in the endosperm while free 2AP are in the bran. Removal of bran during milling reduces the 2AP level.
Some semi-empirical equations that explain the loss of aroma as affected by temperature and duration were also developed.
The study should be able to guide processors in understanding and controlling the parameters in drying, storage, and milling operations to produce high quality rice.