The volatile composition and aroma profile of ‘Unique’, ‘Triumph’ and ‘Anatoki’ feijoa fruits (Acca sellowiana) during ripening

Abstract

Feijoa (Acca sellowiana) is a climacteric fruit which is widely planted in the Mediterranean region, Tunisia, USA, Australia and New Zealand. Although the functional and biochemical properties of feijoa fruits have been well-recognised, the volatile composition and aroma profile are less studied. This study focused on the analysis of the free and bound volatiles, as well as identification of aroma active compounds of feijoa fruits. Factors include fruit ripening stages and cultivar difference could influence the synthesise of fruit volatiles. Therefore, feijoa fruits from different cultivars and ripening stages were selected to give a deeper insight of aroma profile of feijoa fruits. Feijoa fruits from the ‘Unique’, ‘Triumph’ and ‘Anatoki’ cultivar were collected at four ripening stages (four weeks before ripening, two weeks before ripening, ripe and overripe). Their volatile was analysed by headspace solid phase microextraction (HS-SPME), gas chromatography mass spectrometry (GC-MS) and olfactory tests (GCO). A total of 164 free volatiles with 60 terpenes, 52 esters, 20 alcohols, 9 ketones, 6 aldehydes, 2 hydrocarbons and 15 unknowns were identified. Terpene was the dominant group in the unripe fruits, whereas ester was the dominant class in the ripe and overripe fruits. During ripening, the concentration of free terpenes, alcohols and ketones decreased while that of ester largely increased. A total of 26 aroma active compounds were identified. At unripe stages, terpene dominated the free aroma and released a ‘herbal and grassy’ odour. At ripe and overripe stages, esters dominated the free aroma profile and gave a ‘fruity, sweet and floral’ aroma. The key aroma active compounds were found to be ‘sweet, floral and fruity’ ethyl butanoate and ‘fruity, feijoa-like’ methyl benzoate and ethyl benzoate, ‘floral and fresh’ linalool, ‘fruity and green’ α- cubebene and ‘grass and woody’ caryophyllene. A total of 84 bound volatiles with 28 terpenes, 19 alcohols, 9 aldehydes, 8 ketones, 3 esters, 1 hydrocarbon and 16 unknowns were identified and 52 of them co-existed as free and bound volatiles. Bound alcohol, terpene and ketone were the main groups in the 4 weeks before ripening fruits and aldehyde was the dominant class in the 2 weeks before ripening fruits. In the ripe and overripe fruits, the major groups varied among the three cultivars. Comparable with the free terpene and ester, the bound terpene decreased while the bound ester increased. Both the number and concentration of bound volatiles were much lower than those of the free volatiles. The importance of bound volatiles to its whole volatile composition in feijoa fruits could be limited due to their lower number and insignificant concentrations. To sum up, the present thesis is the first study that focused on the analysis of free and bound volatile compounds in the whole feijoa fruit at different ripening levels. The study of feijoa volatile composition and aroma profile of feijoa fruits at different ripening stages could bring substantial insight into this fruit and give deeper understanding into fruit ripening and fruit aroma.