Colour change in rice during hydration: Effect of hull and bran layers

The colour change (yellowing or reddening) that occurs during parboiling affects the consumer acceptance of parboiled rice. In order to understand the factors controlling colour change in rice during hydration, trials were performed that examined the effect of variety (four commercial varieties: Kyeema, Langi, Reiziq^P and Sherpa^P), milling properties (Sherpa^P High Head Rice Yield and Sherpa^P Low Head Rice Yield), degree of processing before hydration (paddy, brown rice, milled rice) and hydration temperature (below and above gelatinisation temperature: 60°C and 90°C respectively). The total colour difference (ΔE₀₀) was calculated and four different models (zero order, first order, fractional and Page) were evaluated in order to predict the ΔE₀₀. The Page model was found to be the best model for predicting ΔE₀₀. There was no difference in ΔE₀₀ between the breakage resistant and breakage prone grains however the varietal effect was observed. The degree of processing of rice before hydration affected the total colour change where the brown rice had the highest ΔE₀₀ followed by paddy and milled rice.