Abstract
Antioxidants are gaining increasing attention from the food industry because of their ability to improve the quality and shelf life of food products. South Tyrolean food products, such as apples, small fruits, officinal herbs, and spices, are rich in bioactive compounds displaying antioxidant behavior, e.g., vitamin C, vitamin E, carotenoids and polyphenols. The methods generally used in the literature to assess the antioxidant activity and capacity of a complex matrix are numerous and involve several reaction mechanisms, from the single H-atom transfer to electron transfer. However, there is a lack of a simple, fast, robust, and standardized chemical method able to assess both antioxidant activity and capacity of foods. One of the most widely used approaches to describe the antioxidant behavior is the DPPH• assay. Nevertheless, in its most popular protocol, the assay is based on a two-point measurement that neglects the velocity of the reaction, and therefore the antioxidant activity. Thus, the aim of this thesis Is to: (i) develop a standardized and comprehensive kinetic method based on DPPH• radicals that considers the reaction mechanism; (ii) validate the method using analytical techniques such as mass spectrometry (MS) and electrochemistry; (iii) describe the antioxidant activity and capacity of typical South Tyrolean food products to valorize their intrinsic quality. The final goal of this thesis is to achieve a better understanding of the quality of South Tyrolean food products by studying their antioxidant behavior and their composition. The research successfully developed a simple, fast, and robust DPPH• kinetic method able to describe the antioxidant activity and capacity of fast reacting molecules, such as ascorbic acid, and of complex food matrices, such as apple extracts, officinal herbs and fruit juices from South Tyrol. The method was optimized by employing a stopped-flow system to achieve a better acquisition of signals in the first seconds of the reaction, crucial to properly describe the reactivity. Results reported R 2 values between 0.990 and 1, highlighting the ability of the model to successfully fit the experimental data. The antioxidant activity was described by the rate constant k1 (M-1 s -1 ) that represented the main reaction, and the rate constant k2 (M-1 s -1 ) that represented a side reaction. The rate constant k1 varied from 110 M-1 s -1 for ferulic acid to 21500 M-1 s - 1 for ascorbic acid, that displayed the highest antioxidant activity among all the compounds and extracts tested. Gallic acid instead had the highest value of k2 among the analytical standards (145 ± 8 M-1 s -1 ), while “Majda” among the food products tested (150 ± 5 M-1 s -1 ). Among the extracts, the highest antioxidant activity was reported by the apple “Majda” (5540 ± 390 M-1 s -1 ), followed by strawberry juice (4480 ± 100 M-1 s -1 ). The least reactive extract was red plum juice having a k1 value of 134.0 ± 5.1 M-1 s -1 .Moreover, the reaction mechanism was validated using MS by finding reaction products. Finally, a coulometric array detector was used to correlate the electron transfer ability determined with the stopped flow DPPH• method with electrochemical properties of individual antioxidants as well as apple extracts.