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

• During hydration, diffused pigments from the bran caused the rice discoloured.
• The colour change was different among different paddy varieties.
• Hydrating the grain above gelatinisation temperature caused more colour change.
• Breakage-prone and breakage-resistant grains had no difference in colour change.
• Page model could be used for predicting colour change during hydration.

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, ReiziqP and SherpaP), milling properties (SherpaP High Head Rice Yield and SherpaP 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.