Food under microwave processing: dielectric behavior and effects on phenolic costituent

Processed products that keep more of their original characteristics are the main goal of the current researches in food technology. This policy is carried out focusing on the technological progress in reduction of running costs by increasing the process efficiency, and in processing of new products with a high final quality. In others terms, development of unit operations performed with novel technologies that minimize the adverse effects of processing is required. In the particular case of food, this means low losses of volatiles, flavors and nutritional values, reduced changes in color and texture. Furthermore, the possibility of defining and controlling residual enzymatic and microbial activities in processed foods is required because they affect product quality and shelf-life [1]. Microwave technology is proposed to respond to the needs required to a innovative thermal technology. As a matter of fact, benefits in microwave heating applications can be found, such as reductions in manufacturing costs due to energy saving and shorter processing times, improved product uniformity and yields. These particular features depend on the direct interaction between electromagnetic field and matter. Indeed, while in conventional heating processes, the energy is transferred to the material by convection, conduction and radiation phenomena through the external materials surface, in presence of thermal gradients, microwave energy is delivered directly to materials through molecular interactions with electromagnetic field via conversions of electromagnetic energy into thermal energy. The presence of natural dipoles such as water molecules and charges (ionic groups) enhances the interaction with the electromagnetic field [2], [3]. Foodstuffs usually contain 50-97 % moisture and components such as salts (i.e. water molecules and ions) so that they are very well suited for heating with microwave energy, to obtain, under the technological point of view, the features before reported [1], [2], [4]. If microwaves treatments are suitable for foods processing in terms of nutritional factor preservation is a topic of great research interest. In particular, thermal treatments of fruit and vegetable is one of the most challenge in food processing because of their content of antioxidant phenolic metabolites and the protective effects against oxidant related processes of these last. As a matter of fact, vegetal foods accumulate a great variety of secondary metabolites, including phenolic and many other phytochemical compounds, as a protection against the adverse effects of mechanical bruising, light, and injury by predators such as beetles, fungi, and insects [5], [6]. Phenolic compounds have long been associated with flavor and color characteristics of fruits and vegetables. These compounds attract great interest due to their reported health protecting properties. Therapeutic effects such as antibacterial, anti-inflammatory, antiallergic, antimutagenic, antiviral, antineoplastic, antithrombotic, and vasodilatory activity have been attributed to phenolic compounds [7]. Many of these effects result from their potent antioxidant and free radical scavenging properties [8]. It has been hypothesized that an increase in dietary antioxidants can reduce oxidative stress and prevent chronic diseases. Microwave ovens are widely used for cooking and food processing [9], [10], [11]. Extensive studies [12], [13] have shown equal or better retention of some vitamins (B1, B2, B6, C, and folic acid) after microwave heating compared with conventional heating. There is, however, little information on how much loss of phenolic compounds occurs in foods during microwave heating. In the present study, the influence of different power input of microwave baking on the amount of phenolic marker compounds resting in potatoes Agria cultivar was assessed. A high-performance liquid chromatographic (HPLC) method with diode-array detection (DAD) was used to identify and quantify phenolic compounds. Dielectric behavior of the irradiated samples was also investigated to emphasize the role of water content and temperature during the baking process. Potato is chosen as vegetable food model because is representative of a large range of high moisture food and have several features such as small and homogeneous cells and structurally is less complex than other vegetables.