Surface free fatty acids and hydroperoxides production and measurement on milled rice

Partially and fully-milled rice were stored at 24, 37 and 50°C and 70% relative humidity and the surface lipid hydrolysis and oxidation determined by colorimetric methods. Milled rice surface were also scanned by mid-infrared diffuse reflectance Fourier transform spectroscopy (DRIFTS) and spectra collected. Products of rice lipid{09} hydrolysis were determined by reversed-phase HPLC. Volatiles from oxidation of milled rice lipid were determined by GC-MS.; A technique based on DRIFTS spectra and partial least squares (PLS) regression provided good prediction of free fatty acids (FFA) and conjugated diene (CD) contents of milled rice that were comparable to colorimetric methods. Changes in DRIFTS spectra of stored milled rice provided qualitative information on lipid chemical changes occurring on the milled rice. Formation of FFA exhibited three phases: a sharp increase, no net increase or decrease, and a gradual increase. The change in the CD values, linoleic acid and oleic acid levels corresponded with FFA development. Phospholipids were hydrolyzed more than triglycerides. A semi-empirical mathematical expression of FFA formation kinetics on the milled rice surface during early storage was proposed. Hydrolytic cleavage mechanisms of two successive reactions was proposed for rice phospholipids.; The concentrations of 2-nonenal, hexanal and octanal were high and changed significantly in milled rice during storage. The aroma values of 2-nonenal and octanal remained high throughout the duration of rice storage.; This study showed that the DRIFTS-PLS technique is viable for determining FFA and CD and for studying qualitative chemical changes on milled rice surface lipids. Quality of milled rice may be preserved by controlling temperature during storage and transport. Understanding lipid hydrolysis could help in predicting milled rice shelf life. Determination of lipid volatiles on milled rice is useful in understanding how milled rice odor may change over time and certain volatiles may be used as indicators of quality deterioration.