Analysing the freshness of intact rice grains by colour determination of peroxidase activity

Freshness is an important palatability characteristic of rice grains. However, this characteristic deteriorates rapidly during dry storage. In the present study we have developed an indirect method to examine the freshness of rice grains harvested in different crop seasons or stored under conditions known to accelerate deterioration. Bulk samples of intact brown rice grains were soaked in dianisidine solution containing guaiacol and H2O2, and the optical absorbance of the incubation solution was determined colorimetrically. Alternatively, intact brown rice grains were incubated in a similar solution in the wells of a 96-well microplate, with one grain per well, and the optical absorbance of the incubation solution was measured by a microplate reader. The results indicated that cultivar, storage duration and temperature significantly affected the activities of grain peroxidase. Marked differences in the development of purple colour in the incubation solutions and in vitro peroxidase activity were also found among the brown rice grains sampled in different seasons. The changes in colour of the incubation solutions measured by either of the above methods paralleled the changes in grain activity of peroxidase assayed using an in vitro method. The proposed method can differentiate brown rice grains harvested in different seasons. Moreover, this method, if combined with suitable statistical clustering procedures, can detect the blending status of fresh/old brown grains in a sample. The advantages of this test method using A385 over the conventional enzyme activity assay are the rapidity and simplicity with which A385 can estimate the in vitro determination of peroxidase activity of tested rice grains