Abstract
Strawberry (Fragaria×ananassa Duch.) is a relevant source of micronutrients, such as minerals, vitamin C and phenolic substances, most of which are natural antioxidants and contribute to the high nutritional quality of the fruit. All these compounds are essential for health and, in particular, strawberry phenolics are best known for their antioxidant and anti-inflammatory action, and possess antimicrobial, anti-allergy and anti-hypertensive properties (1). Therefore, increased consumption of fruits has been recommended for maintaining good health. However, strawberries are perishable soft fruits exhibiting an extremely short postharvest shelf-life. Freeze-drying is used as an alternative to preserve fruit, it allows obtaining highly stable and high-quality products. Nevertheless this technology might influence the antioxidant properties of the samples processed. To improve the quality and stability of the freeze dried fruit, the addition of high molecular weight solutes as, gum arabic (GA), with encapsulating and anticaking effect can be used. In this work, the effects of freeze-drying, gum arabic addition (1.2 g/100 g fruit) and extraction solvents (metanol, water or metanol-water 70/30 v/v) on strawberry amount of total phenolics and antioxidant activity were investigated. Total phenolics were analyzed spectrophotometrically using the modified FolinCiocalteu colorimetric method (2) with some modifications. Three different assays, DPPH (3), ABTS (4) and FRAP (5) were used to test the antioxidant capacity of the samples. Results showed that recovery of phenolic compounds and antioxidant activity was dependent on the solvent used in their extraction and its polarity. Total phenol content in the fresh fruit ranged from 5 to 20 mg gallic acid/g dry weight, depending on the extraction solvent. Methanol/water extract displayed the highest content of phenols. In general strawberry showed significantly (p<0.05) higher antioxidant activity with ABTS test in methanol extract 7.5 M Trolox/g dry weight. Total phenol content and antioxidant activity of the extracts from freeze-dried strawberry were not significantly different from the fresh fruit. Moreover gum arabic exerted a protective role on the phenolics and antioxidant activity of freeze-dried strawberry samples.
