Analysis of the main aroma substances in eight walnut accessions

Abstract:【Objective】Eight walnut accessions were used as materials for analyzing their main aroma substances, and the differences between these accessions were clarified.【Methods】The variety of ‘Kuixiang’was from Chongyukou walnut plantation, Lulong county, Qinhuangdao.‘Kuixiang’‘Liaoning No. 1’‘Qingxiang’‘Xiangling’‘Xiling’‘Yuanbao’and‘Zanmei’were from Qipanshan plantation, Lulong county.‘FN39’is a superior line selected from‘Shimen walnut’seedling populations. HSSPME/GC-MS was used to determine the aroma components in the kernels of these walnut varieties. ‘Kuixiang’was used as the test material for optimization of the extraction protocol. Then extraction temperature was set in four levels, 40, 50, 60 and 70 ℃, the extraction time was 40 min, then 230 ℃ for 5 min. For optimizing the extraction time, the extraction temperature was set as 70 ℃; the extraction time was set in three levels, 40, 50 and 60 min; and the solution was performed at 230 ℃ for 5 min. After optimization, the best conditions were applied to determine the aroma substances of the eight walnut accessions. The walnut kernels were crushed, and 3.0 g of the kernels powder were put into a 15 mL headspace injection bottle, sealed and adsorbed at 70 ℃ for 60 min. The extracted sample was then inserted into the gas chromatography with an injection port set at 230 ℃ for 5 min. The chromatographic conditions included an HP-5ms quartz elastic capillary column (30 m×250 μm×0.25 μm), carrier gas of He (99.999% ), a split ratio of 20∶1, a flow rate of 1.0 mL · min- 1 , an inlet temperature, which started with 40 ℃ for 5 min, increased to 70 ℃ at 5 ℃·min^-1 and maintained for 2 min, increased to 120 ℃ at 5 ℃·min^-1 and maintained for 6 min, and finally increased to 230 ℃ at 10 ℃·min^-1 and maintained for 5 min. The mass spectrum conditions included an EI source voltage of 70 eV, an ion source temperature of 230 °C, an interface temperature of 250 °C, a quadrupole temperature of 150 °C, and a mass scanning range of 30-500 Da. The content of aroma substance was calculated using 3-octanol the internal stan- dard.【Results】A total of 29 compounds were detected in the eight walnut accessions, including 11 alde- hydes, 4 alkanes, 3 alcohols, 3 esters, 1 phenols, 4 acids and 3 other substances. Among them, the main aroma substances were aldehydes and alkanes. There were remarkable differences in aroma components among different walnut varieties. Aldehydes were the main aroma substance in the kernels of‘Shimen walnut’‘Liaoning No.1’and‘Xiangling’, while alkane was the main substance in‘Qingxiang’. In ad- dition, 1-nonanal and (2E,4E)-undeca-2,4-dienalnonaldehyde were rich in the‘Shimen walnut’‘. Yuan- bao’hadthehighestcontentofaromasubstanceandaldehydes,whichwas484.75μg·g-1 and339.80μg · g- 1, respectively. 1- nonanal and gamma- caprolactone were detected in all the eight walnut acces- sions, with a content ranging from 56.72 to 168.81 μg · g- 1 and from 2.98 to 19.95 μg · g- 1, respectively. Except for‘Yuanbao’, 1-nonanal, ranging from 56.72 to 168.81 μg·g-1, was the most abundant aroma substance in all the accessions. Besides 1-nonanal, Kuixiang’was rich in (E)-2-octenal and tetradecane;‘Liaoning no.1’was rich in (2E,4E)-undeca-2,4-dienal and 2-pentylfuran.‘Qingxiang’had high con- tent of tetradecane and pentadecane;‘Xiangling’was high in (E)-2-octenal.‘Xiling’was rich in 2-un- decenal;‘Zanmei’was rich in 2-pentylfuran and (E)-2-octenal; and‘FN39’was rich in pentadecane. In general, the slight difference in aroma components led to different oder types of walnut. The scents of different aroma components were classified and described according to Chemical Dictionary and Per- flavoury. Most of aldehydes have very pleasant odor, such as 1-nonanal has a waxy aldehydic rose fresh orris orange fatty peely flavor, (2E, 4E)-undeca-2,4-dienal has a green oily caramellic pungent citrus buttery baked flavor. Meanwhile, most of alkanes have waxy odor.‘Kuixiang’and‘FN39’with high content of 1-nonanal (114.11 and 168.81 μg·g-1, respectively) belongs to the odor type of aldehydic. Be- cause‘Qingxiang’was rich in alkanes, it had a waxy flavor. (2E,4E)-undeca-2,4-dienal was the highest aromatic substance in‘Yuanbao’, it tasted a little different from‘Kuixiang’, the odor type of‘Yuan- bao’is green.【 Conclusion】The optimal extraction protocol for the determination of aroma components of walnut kernel by headspace solid-phase microextraction combined with GC-MS was established, which included 50/30 μm DVB/CAR/PDMS extraction head, 3.0 g walnut kernels powder, adsorption at 70 °C for 60 min, and analysis at 230 °C for 5 min. A total of 29 compounds were detected in the eight walnut accessions, including 11 aldehydes, 4 alkanes, 3 alcohols, 3 esters, 1 phenols, 4 acids and 3 other substances. There were significant differences in aroma substances among the accessions. The content of aromatic substances in‘Yuanbao’was the highest (484.75 μg·g-1) with rich varieties of aro- matic substances (16 kinds).‘Xiling’had the lowest amount of aromatic substances (200.59 μg · g- 1) and the least varieties (13 kinds). The main aromatic substances in the eight walnut accessions were al- dehydes and alkanes, followed by alcohols and esters. Aldehydes play the most prominent role and are the most diversified aroma components, resulting in a strong fatty, green and fruity flavor.