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Flavour qualities of new Australian fragrant rice cultivars
by Kirstin Wilkie and Associate Professor Michael Wootton November 2004
RIRDC Publication No 04/160 RIRDC Project No UNS-12A
Identification of the effects of nitrogen fertiliser on the aroma volatiles of rice has demonstrated the impact of nitrogen fertiliser level on the aroma of rice. From data over three seasons, it was concluded that nitrogen fertiliser level had a significant effect on the concentration of volatile compounds in rice plant material at several stages during growth. Plants fertilised at the lower nitrogen rate (50kg N/ha) had a lower concentration of volatile compounds compared to the plants fertilised at the normal nitrogen rate (150kg N/ha). In particular this was the case for the compound 2-AP, which contributes significantly to the aroma of rice. In addition, at tillering, compounds including, (E)pent-2-enal; (E)hex-2-enal; hexanal; (E)pent-2-en-1-ol; hexanol and methyl salicylate, which contribute green/grassy aromas, were present in lower concentrations in the plants fertilised at the lower nitrogen level. There were many significant effects on the concentration of individual volatile components in the rice grain. Over two seasons it was found that the majority of important volatile compounds were significantly higher in grain fertilised at the lower nitrogen level for both fragrant and one non-fragrant rice variety, but lower in another non-fragrant variety.
The volatile aroma components of Australian fragrant rices were compared with those of their imported counterparts. Australian Basmati breeding line did not contain several volatiles present in the imported samples, but contained others, including 2-AP, in higher amounts. Caution must be exercised in interpreting these results as the Australian sample was analysed soon after harvest while imported Basmati rice traditionally undergoes storage for up to 12 months. Of those compounds in higher amounts in the imported Basmati, several of these are probably derived from lipid breakdown and may be a reflection of this storage period, the conditions of which are unknown. Similarly, the higher 2-AP levels in the Australian samples may simply indicate that this compound had broken down during the storage encountered by the imported material.
Commercial Australia Jasmine rice was very similar to imported varieties both qualitatively and quantitatively with respect to volatile aroma compounds. However, there were some obvious quantitative differences, which distinguished the rices. The close resemblance of the Australian Jasmine rice to imported varieties suggests its acceptance in the international market. Some concern over the aroma potential of an Australian Jasmine breeding line YRF207 arose since it contained significantly less 2-AP than the commercial Kyeema variety over two seasons of testing.
Substantial changes in volatile components occurred during storage of Australian Basmati breeding lines with new compounds forming and existing ones decreasing. Most importantly, 66% of 2-AP was lost during a 3 month storage period. This underscores the importance off storage protocols for Basmati rices developed within Australia.
Sensory evaluation of rice aroma provided valuable insight into consumer attitudes and perceptions to the aroma of Australian and imported Jasmine and Basmati rices as well as Australian non-fragrant rice. Consumers, as a group, could distinguish between the aroma of fragrant and non-fragrant rice and between Australian and imported Jasmine and Basmati fragrant rice. There were differences with respect to age of subject and ability to distinguish between Australian fragrant Jasmine and nonfragrant rice and between Australian fragrant and imported fragrant Jasmine rice. In addition it was found that consumers as a group could distinguish between the aroma of Australian fragrant Basmati and imported fragrant Basmati rice. Age, gender and cultural background had no effect on the subject’s ability to distinguish between the aroma of these two rices. Differences in consumer perception between the three rice samples and aroma descriptions could be explained in the context of differences in their volatile components.
Aroma descriptions of the Australian fragrant Jasmine rice compared to the imported counterpart revealed a greater number of positive descriptions of rice aroma and less negative aroma descriptions for the former. However, preference tests indicated no direct preferences for the aroma of either the Australian or imported Basmati fragrant rice.
An SPME technique was optimised for the adsorption of 2-AP using pandan leaves and these optimum extraction conditions were utilised for the adsorption of 2-AP in rice plants. It was concluded that the 3-leaf growth stage of rice plants was the best to extract 2-AP from rice plants. Therefore SPME can be used for the screening of fragrance in rice plants at the early growth stages without waiting for the mature grain allowing the rapid detection of fragrance. The technique can be used on-farm for collection of volatile components for later GC/MS analysis in the laboratory Plants and grain fertilised at 150kg N/ha had a greater concentration of total amino acids compared to plants and grain fertilised at 50kg N/ha. In particular, levels of proline were significantly lower in rice plants and grain fertilised at the lower nitrogen rate. Proline was specifically targeted in this work because it has been implicated as a precursor of 2-AP. The levels of proline were greater in the nonfragrant plant compared to the fragrant plants throughout growth, but the levels of proline in the grain were similar. Since fragrant and non-fragrant plants had similar levels of free proline, level of free proline is not indicative of the fragrance potential of the plant.
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