Antioxidant activity and sensory attributes of tomatoes dehydrated by combination of microwave and convective heating
Tomato halves were dried in a combined microwave-hot air system. Drying experiments were carried out by applying the microwave energy (0.5 W/g) at the beginning of the dehydration process (for 2 and 3.5 hours); simultaneously combining microwave (0.25 W/g) with convective hot-air drying (70°C); and by applying only convective hot air at 70°C. The combined microwave-hot air heating has reduced significantly the drying time (up to 50 %), if compared with the convective system alone. Simultaneous convection and microwave heating allowed to best preserving antioxidant activity of dried tomatoes, by minimizing lycopene and phenols losses, while ensuring satisfied texture characteristics and less percentage shrinkage. Moreover, the same samples better preserved the colour characteristics of fresh tomato.
Tomatoes (Lycopersicum esculentum) contain a number of health functional constituents, such as red-coloured carotenoid lycopene, flavonoids and other phenolic compounds, especially chlorogenic acids, in addition to basic nutritional compounds (1). High levels of antioxidants present in tomatoes and tomato products help prevent oxidative damage that is hazardous for humans. Thus, consumption of tomato products has been associated with decreased risk of some cancer types plus heart disease; various epidemiological findings confirm the observed health effects due to the presence of varied antioxidants in tomato (2, 3). Dehydration is commonly used to preserve the product and extend its shelf-life, although the quality of the dehydrated product is often poor: collapse of the structure, discoloration, change in nutritional properties and antioxidant activity are frequent quality problems. Generally the tomatoes are preliminarily sectioned and dried at high temperatures in the presence of oxygen, so, after drying the tomato products show the highest sensitivity to oxidative damage (4). Combining properly hot-air convective HA and microwave MW in a ...