Abstract/Summary

2-Acetyl-1-pyrroline (2-AP) is a popcorn-like aroma compound with low odour detection threshold. This compound is regarded as the key contributor of fragrant rice aroma. 2-AP can be detected in fragrant rice, whereas the content of this compound in non-fragrant rice is too low to be detected. Levels of 2-AP in raw fragrant rice can be affected by abiotic stress and growth conditions, such as climate and soil conditions. Whereas it was originally thought that this compound could not be enhanced post-harvest, including cooking. One previous study in our research group demonstrated that 2-AP could be increased in fragrant rice after baking at 180 °C for 20 min. Hence, the objective of this project was to evaluate the mechanism of 2-AP formation in rice during cooking. 2-AP content was evaluated in fragrant and non-fragrant rice prepared by different cooking methods. The results of sensory evaluation on boiled milled rice confirmed that 2-AP and its popcorn-like aroma is the most important factor to differentiate fragrant and non-fragrant rice. Popcorn-like aroma was also noticed in boiled non-fragrant rice and the results from GColfactometry confirmed the presence of 2-AP in raw non-fragrant rice. Levels of 2-AP increased on baking milled rice (without water addition) at 180 °C. As the high level of lipids and polyphenols in rice bran might affect 2-AP formation during baking, 2-AP was not observed in rice bran after baking. When milled rice was baked at a range of temperatures, formation of 2-AP in fragrant and nonfragrant rice showed different trends; maximum 2-AP formation was observed in fragrant rice at around 100 °C, whereas 2-AP formation in non-fragrant rice only took place when the baking temperature was higher than 140 °C. Addition of intermediate compounds confirmed that the 2- AP formation at 180 °C occurred through Maillard reaction; the addition of external 1-pyrroline and methylglyoxal was shown to reduce 2-AP generation yield at 100 °C. These results suggested that formation of 2-AP in both fragrant and non-fragrant rice is driven by Maillard reaction at 180 °C baking. However, 2-AP generation in fragrant rice during baking at 100 °C may follow another reaction pathway and this pathway is sensitive to 1-pyrroline and methylglyoxal. Further research should focus on the effect of biosynthesis on 2-AP generation at 100 °C baking.