The effect of induced yellowing on the physicochemical properties of specialty rices

Authors: Bryant, Rolfe; Yeater, Kathleen; Miller, Helen

Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2012
Publication Date: 6/20/2012
Citation: Bryant, R.J., Yeater, K.M., Miller, H.B. 2012. The effect of induced yellowing on the physicochemical properties of specialty rices. Journal of the Science of Food and Agriculture. 93(2):271-275.

Interpretive Summary: Postharvest yellowing, also called heat damage or stackburn, is a major problem in the rice industry and occurs when the rice is improperly dried or stored. These discolored kernels are discarded due to appearance. Profit lost due to postharvest yellowing is greatest for rice intended for niche markets, e.g. aroma, risotto, sushi, due to their increased value. If the processing quality of the discolored kernels are not significantly different from the non-yellowed kernels they may still be used. Therefore, the objective of this project was to determine the effect postharvest yellowing had on the processing quality of selected specialty rices. Arborio (used for making risotto), Calrose (used for making sushi), Basmati and Jasmine (Aromatic rices) were yellowed using a laboratory process. Factors associated with cooking quality of the yellowed rices were compared to non-yellowed control samples. The apparent amylose content was higher in the yellowed rices than the non-yellowed rice. Components of starch pasting properties of the yellowed rice were different from the non-yellowed rices. Also, the on-set and gelatinization temperatures of the cooked rice were increased due to yellowing. Although the various specialty rices responded somewhat differently, the processing quality of all was affected by yellowing. However, moisture and protein concentration of the yellowed rices were unchanged. The results of this study showed that yellowing of specialty rices significantly alters their processing quality, therefore, making them unsuited for their intended purpose.

Technical Abstract: Postharvest yellowing of rice kernels, commonly referred to as stackburn or heat damage, can be a major problem in the rice industry. This is especially true with high-valued specialty rice because the discolored grains are removed prior to packaging resulting in a greater profit loss. Specialty rice cultivars include aromatics and those that have unique cooking properties suitable for sushi and risotto dishes. If postharvest yellowing changes the characteristics of these kernels they should be discarded. However, if the characteristics are similar to the non-yellowed rice, they may be reclaimed and used. Therefore, the objective of this project was to determine whether a significant change occurs in the physicochemical properties (apparent amylose and protein concentrations, viscosity profile, gelatinization temperature) due to postharvest yellowing. In this study a set of specialty rices (Basmati, Jasmine, Arborio, and Sushi) were yellowed using a laboratory method. Yellowing significantly altered the processing quality of the specialty rices. Apparent amylose concentration and gelatinization temperature increased while peak, breakdown, and setback viscosities decreased. Moisture and protein concentrations were unchanged by the yellowing process. Each of the specialty rices underwent changes in processing quality caused by the yellowing process which may make the rice unsuitable for its intended purpose. The changes differed for each of the rice types.