Volatile aroma compounds of Australian rice varieties

Abstract

The volatile aroma compounds in the green plant tissue and grain of five Australian rice varieties were studied in detail. Selected volatile aroma compounds, including 2-acetyl-1- pyrroline (AP), observed in three fragrant varieties, YRF 203, YRF 205 and Kyeema and two non-fragrant varieties, Pelde and Langi, were compared. The development of volatile aroma compounds in the rice plant was followed from early tillering to full maturity. The effect of nitrogen fertilisation level on the volatile aroma compound composition was evaluated. Volatile compounds were extracted by the Likens-Nickerson simultaneous distillation-extraction technique. GC-MS, GC-FID and GC-O were used to separate, identify and quantify the volatile aroma compounds in the rice grain and plant extracts. The 127 volatile compounds identified in rice plant tissue and grain included 28 aldehydes, 23 ketones, 20 alcohols, 8 phenolic compounds, 17 heterocyclic compounds, 7 hydrocarbons, 2 sulphur compounds, 5 terpenes, 11 acids and 6 esters. Most of the homologous series of aliphatic aldehydes, ketones and alcohols that were identified in cooked rice were also identified in plant tissue, but in different proportions. AP was the major volatile compound in fragrant rice plant and grain and was probably the major factor that contributed to the difference between fragrant and non-fragrant varieties. Relative levels of AP in rice plant correlated with the relative level of AP in the rice plant of the same variety. Therefore, AP concentration in rice plant tissue can be used as an early indicator of aroma in rice grain. The non-fragrant whole rice contained more pent-l-en-3- one, D-Limonene and hepta-2,4-dienal than fragrant rice, while fragrant rice contained more but-2-enal, hex-2-en-l-ol, pyridine and AP. Milled fragrant rice contained more but- 2-enal, pyridine, AP and pyrrole than non-fragrant rice. The increase in nitrogen fertilisation resulted in an increase of AP in rice plant tissue and grain in fragrant varieties. In addition, the increase in nitrogen fertilisation resulted in an increase in pyridine, hepta-2,4-dienal and 2-methoxyphenol in the mature rice plant and an increase of pentanal, hexanal, pyridine, heptan-2-one, pentan-l-ol, hexan-l-ol, oct-l-en-3- ol and furfural in milled rice. AP concentration was relatively high at the beginning of plant development and then decreased during plant development. Therefore, AP can be detected at the early stages of plant development, without having to wait until grain maturity.