In recent years, there has been increasing interest in dietary phytochemicals or bioactive compounds, in view of their potential protective action against cardiovascular disease and certain types of cancer. These compounds include phenolic compounds such as isothiocyanates, flavonoids, isoflavones and lignans, together with other compounds such as saponins and cumestrol [1] (AICR, 2015).

The antioxidant capacity of bioactive compounds in foods is interesting from two standpoints: food technology and nutrition. Phenols and phenolic compounds are secondary metabolites, of vegetable origin, that are among the main antioxidant components in food and, in addition, the most abundant antioxidant compounds in the human diet. Many of the beneficial effects associated with the consumption of foods of vegetable origin are attributed to these phenolic compounds [2] (Rinaldo et al., 2010). Phenolic compounds also influence the sensory properties, such as flavour and colour, and contribute to the aroma and taste of many foodstuffs of vegetable origin. Consequently, they are of great importance in the food industry.

Spain is one of the main suppliers of tropical fruit to other European countries, and exported 115,129 tonnes in 2012 (latest available data), corresponding to 0.95% of all its fruit and vegetable exports [3] (MAPAMA, 2017). The Spanish Mediterranean coast presents environmental characteristics typical of a Mediterranean subtropical climate, which makes the coastal zones of Granada and Málaga provinces the only areas suitable in Andalusia for this type of crop [4] (CMAOT, 2015).

The present study was conducted to evaluate the bioactive compounds of Spanish-grown tropical fruits, especially their antioxidant capacity. The data obtained on these tropical fruits can be usefully incorporated into tables of nutritional composition.

In addition to the above study goals, ultra-high performance liquid chromatography (UPLC), coupled with mass spectrometry, was used to quantify individual phenolic compounds of nutritional importance in these home-grown subtropical fruits.

The following tropical fruits are the types most consumed in Spain and have been analysed in this study: carambola (starfruit) (Averrhoa carambola, n = 10) cherimoya (custard apple) (Annona cherimolla, Mill., n = 10), kiwi (Actinidiadeliciosa cv Hayward, n = 10) (Mangifera indica L., n = 10), papaya (Carica papaya L., n = 10), persimmon (Diospyros kaki L.) and avocado (Persea Americana Mill., Hass and Bacon varieties). Samples of Spanish-grown and imported fruits were analysed (except for some phenolic species of which, due to limited seasonal availability, only Spanish-grown samples were analysed). The samples were obtained from retailers in the cities of Granada and Málaga and were immediately taken to the laboratory to be processed. All non-edible parts―skin, seeds and/or pits―were removed; the separated edible part was then triturated in a blender and the juice extracted from each fruit. The processed samples were stored at 4˚C until needed for analysis, which was performed as soon as possible. Each sample consisted of two randomly-chosen pieces of fruit from the items contained in the sales unit, usually a basket or mesh bag. All analyses were conducted in triplicate.

The following reagents were obtained from Panreac Química SL (Barcelona, Spain): gallic acid (GA), p-coumaric acid, caffeic acid, vanillic acid, ferulic acid, sinapic acid, chlorogenic acid, ellagic acid, p-hydroxybenzoic acid, protocatechuic acid, pyrogallic acid, 3,5-dimethoxybenzoic acid, quercetin, naringenin, diethyl ether, acetic acid (glacial), anhydrous sodium sulphate and methanol. Sigma-Aldrich (Steinheim, Germany) supplied gallic acid, Folin-Ciocalteu reagent, sodium carbonate, 2,4,6-tri (2-pyridyl)-s-triazine (TPTZ), ferric chloride hexahydrate, sodium acetate, 6-hydroxy-7,8-tetramethylchroman-2-car-carboxylic acid (Trolox), chlorogenic acid, hydrochloric acid, 2,20-Azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS), sodium persulphate, sodium phosphate monobasic, sodium nitrite and aluminium chloride. All reagents were of analytical grade.

Individual phenolic compounds. The UPLC-ESI-MS/MS method was used to identify the different phenolic compounds [5] (Rueda et al., 2016) in fruits with modifications.

Treatment of the sample. A liquid-liquid extraction with final concentration was performed. Then, 20 mL of diethyl ether was added and the resulting solution was frozen at −20˚C for 24 hours, after which it was centrifuged for 10 minutes at 9000 rpm. The supernatant was transferred to a separatory funnel and three extractions made with 20 mL diethyl ether. A spatula tip of anhydrous sodium sulphate was added to the organic extract; this was then filtered and the filtrate was passed to a heart-shaped flask for rotary evaporation at 30˚C to the smallest possible volume. The extracts were collected with 1 mL methanol/water mixture (1:1), filtered through a 0.20 μm membrane filter and passed to a chromatography vial for analysis.

Material, equipment and operating conditions for UPLC-ESI-MS/M-QTOF Analysis: Chromatograph: Acquity UPLC System. Column: Waters ACQUITY UPLC™ HSS T3 2.1 × 100 mm, 1.8 μm. Column temperature: 400˚C. Injection volume: 10 μL. Mobile phase: Channel A: water with 0.5% acetic acid; Channel B: acetonitrile. Gradient: initial: 5% B, T15: 95% B, T15.1: 95% B. Analysis time: 18 min. Flow rate: 0.4 mL/min. Detector: SYNAPT G2 HDMS Q-TOF high resolution spectrometer. Waters. Ionisation source: electrospray ionisation. Ionisation mode: negative. Measurement range: 50 - 1200 uma.

Identification and quantification: Phenolic compounds were identified by comparing the negative masses recorded in previous research, using MassLynx V4 software (Waters Laboratory Informatics, Waters, 2010). Individual phenolic compounds were quantified by obtaining a series of solutions, with a concentration of 5 - 40 moles, of standard phenolic compounds with different retention times. For each phenolic compounds selected, a calibration curve with R2 ≥ 9.997 was performed, to ensure the linearity of the method. The standards were analysed under the same working conditions as the samples. As the retention times and the spectrum areas were known, the phenolic compounds in the different samples could be identified and quantified.

Analytical validation. Table 1 and Table 2 show the main analytical validation parameters of the method used.

Figure 1 shows a chromatogram obtained for a sample of mango.