Some analytical assays for the determination of bioactivity of exotic fruits

The consumption of new exotic fruits, with their high nutritional and sensory value, has significantly increased in the past few years. Among the tropical fruits durian (Durio zibethinus Murr.) is less known than mango (Mangifera indica L.) and avocado (Persea americana). It has been shown that durian, mango and avocado possessed high nutritional and bioactive properties, but these data were determined using different methods. In order to obtain reliable results we investigated samples of durian, mango and avocado of the same stage of ripeness and unified methods were used for determination of the antioxidant potential. As far as we know, no results of such comparative investigation of three tropical fruits (durian, mango and avocado) and the use of such tests for phytochemical control have been published.

Lyophilised durian, mango and avocado samples harvested in 2008 in Thailand and Israel were investigated.

The contents of crude protein, fat, carbohydrate, dietary fibre, total polyphenols, flavonoids, tannins and flavanols were determined by elemental analysis and UV spectroscopy. The presence of polyphenols (flavonoids and phenolic acids) in the investigated samples was studied by Fourier transform infrared (FT-IR) spectroscopy and three-dimensional fluorometry. Four complementary radical scavenging assays were used for antioxidant determination: ferric reducing antioxidant power (FRAP), 2, 2-azino-bis (3-ethyl-benzothiazoline-6-sulfonic acid) diamonium salt (ABTS(*+)), 1-diphenyl-2-picrylhydrazyl method (DPPH) and cupric reducing antioxidant capacity (CUPRAC). Chemometrical processing was used for statistical comparison of the fruits.

All spectrometric measurements were highly correlated. The contents of total fibre, proteins and fats were significantly higher (p < 0.05) in avocado, and carbohydrates were significantly lower in avocado (p > 0.05) than in the two other fruits. The wavelength numbers of FTIR spectra for three investigated fruits were in the same range (1700-600 cm(-1)) as for catechin and gallic acid, used as standards. One main peak could be easily observed at the approximate location of ex/em 275/305 nm and the other one at ex/em 350/430 nm in the methanol polyphenol extracts of investigated fruits in three-dimensional fluorescence, in contour and cross fluorescence maps. Similarity was found between durian, mango and avocado in polyphenols (9.88 +/- 1.0, 12.06 +/- 1.3 and 10.69 +/- 1.1, mg gallic acid equivalents/g dry weight, d.w.), and in antioxidant assays such as CUPRAC (27.46 +/- 2.7, 40.45 +/- 4.1 and 36.29 +/- 3.7, microM Trolox equivalent (TE)/g d.w.) and FRAP (23.22 +/- 2.0, 34.62 +/- 3.4 and 18.47 +/- 1.9, microM TE/g d.w.), respectively. The multisample median test between all possible pairs of groups is a Tukey-HSD type comparison and denotes the different groups in a case when a pair-wise test is significant and its q statistical value is greater than the table q parameter. The multisample median test of FRAP values were chosen from the compared fruits triplets as similar or homogenous subsets durian and avocado.

Nutritional and bioactive values of durian are comparable with these indices in mango and avocado. These fruits contain high, comparable quantities of basic nutritional and antioxidant compounds, and possess high antioxidant potentials. All fruits show a high level of correlation between the contents of phenolic compounds and the antioxidant potential. The methods used (three-dimensional fluorescence, FTIR spectroscopy, radical scavenging assays) are suitable for bioactivity determination of these fruits. In order to receive best results, a combination of these fruits has to be included in the diet. The methods used are applicable for bioactivity determination in phytochemical analysis in general.