A comparative analysis on fruit characteristic aroma compounds in eight strawberry varieties (strains)
[Objective]Aroma substances play a key role in the flavor formation of strawberry fruits.Pleasant aroma is one of the most important characteristics of high-quality fresh strawberry and makes the berry more attractive and competitive. An analysis of aroma compounds in strawberry helps us to understand the pathway of aroma formation and identify the special compounds related to particular aroma. The results can be used in strawberry breeding aimed to improving fruit quality.[Methods]In this study, aroma compounds of eight strawberry varieties were analyzed, including six varieties bred by Beijing Academy of Forestry and Pomology Sciences, one from the USA and one from Japan, of which6 were red fruit type and 2 were white fruit type. The domestic varieties were'Jintaoxiang'Rose Princess' (tentative name) (10-30-47), 'Xiangshan Princess' (tentative name) (09-30-20), 'Yuquan Princess' (Tentative name) (09-30-36), 'Snow White-15' (tentative name, white fruit type) (w74-15) and'Snow White-6' (tentative name, white fruit type) (w74-6). The other two varieties, 'Benihoppe'from Japan and'Sweet Charlie'from the USA, are currently the most widely cultivated strawberry varieties in Chi-na. All varieties were cultivated employing routine field management and pest and disease control.Fresh fruit samples were collected at maturity. Samples were analyzed using immersion stir bar absorptive extraction-gas chromatography quadrupol mass spectrometry (Imm-SBSE-GC-q MS), combined with the sensory evaluation by sniffing and tasting. The aroma type and the core compounds of each variety were identified and the relationship among these varieties were analyzed using Principal Component Analysis (PCA).[Results]According to the scores evaluated by sensory test, the 8 strawberry varieties could be divided into three grades: grade with extraordinarily strong aroma, grade with strong aroma and aromatic grade. The results of GC-q MS analysis showed that totally 46 types of volatile substances were detected in the tested varieties. Among these compounds, 23 were esters (50％), 7 alcohols (15.22％), 6 ketones, 6 aldehydes (10.87％) and 4 acids (8.70％). 36-40 types of aroma compounds were detected for each variety and 23 were common in all. The relative concentration of γ-decalactone was extremely high in'Jingtaoxiang' (259.25), which accounted for 79.52％ of the total relative concentration of esters. In'Rose Princess' (10-30-47), linalool (relative concentration 30.36) was the main ingredient. Among all the varieties, 'Rose Princess' (10-30-47) had the strongest rose fragrance, which was related to the ester type and content, fragrance alcohol (especially linalool) and furan class substrates.The relative concentrations of linalool and DMMF accounted for 82.08％ and 79.54％ of the alcohols and the ketones, respectively, while the two volatile aroma substances exceeded 50％ of the total concentration of the main aromatic compounds important for the fragrance of rose. The two white fruit selections, 'Snow White-15' (w74-15) and'Snow White-6' (w74-6), had unique compounds of methyl 2-methyl butyrate and methyl benzoate, the former having a pineapple aroma. In'Xiangshan Princess' (09-30-20) and'Yuquan Princess' (09-30-36), which were the F1 progenies of'Yanxiang'×'Benihoppe', the volatile compounds were similar to those in'Benihoppe'. They all had the highest relative concentrations of acids, accounting for 36.47％-44.44％ of the total relative concentration of volatile substances. The distance interval of'Yuquan Princess' (09-30-36) and'Benihoppe'was smaller in the scores plot of PCA, and both had flower fragrance. Their aroma compounds were similar, and the aroma heredity was thus relatively stable. The ethyl 2-methylbutyrate and 2-heptanol were undetected in'Benihoppe', which were the unique volatile compound in'Xiangshan Princess' (09-30-20) found in this study. It might be influenced by the parent'Yanxiang'. There might also be a genetic variation after hybridization responsible for new aroma substances.[Conclusion]In conclusion, γ-decalactone, methyl butanaoate, ethyl butanaoate, methyl hexanaoate, 2, 5-dimethyl-4-methoxy-3 (2 H)-furanone (DMMF), linalool and (E)-2-hexenal were the main aroma substances in the eight strawberry varieties (strains). Esters were the most active aroma compounds in strawberry.'Jingtaoxiang'showed the highest total volatile compounds concentration, and γ-decalactone contributed a lot to its aroma formation.The aroma of'Xiangshan Princess' (09-30-20) was not only influenced by the male parent'Benihoppe'with flower fragrance, but also affected by the female parent'Yanxiang', which should be further studied. The pineapple aroma of'Snow White-15' (w74-15) and'Snow White-6' (w74-6) was supposed to be formed by the interaction of characteristic compounds of methyl butanoate, ethyl butanaoate, methyl 3-methylbutanoate, methyl 3-methylbutanoate, methyl hexanoate, ethyl hexanaoate, DMMF, (E)-2-hexenal and especially the unique substance of methyl 2-methylbutanoate. Furthermore, it is suggested that γ-decalactone, DMMF and (E)-2-hexenaldehy play important roles in strawberry aroma, which should be further studied.
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