Assessment of Microwave versus Conventional Heating Induced Degradation of Olive Oil by VIS Raman Spectroscopy and Classical Methods

Abstract

Over the last few decades, the microwave heating process has experienced more common and routine use for both home and industrial applications. In industrial field, microwave heating has been used for many applications, including food processing and preservation, bleaching, pasteurization, and sterilization (Decareau, 1985; Farag et al., 2001; Knutson et al., 1987). Numerous advantages boosted the use of microwave heating making it in many cases a technique preferred to conventional heating. These advantages include precise timing, rapidity, and energy saving. The principle of microwave heating is based on the interaction of electromagnetic waves with the molecular constituents of food. Such interaction leads to heat generation in the entire volume at nearly the same rate due to internal thermal dissipation of the vibrational energy of the molecules in the food (Decareau, 1985; Kamel & Stauffer, 1993). On the contrary, conventional heating generates heat at the contact surface first, and then the heat diffuses inward. The effects of microwave heating and conventional heating on the food components are therefore expected to be completely different. Since processed foods by microwaves are heated as a result of molecular excitation (Stein, 1972), many researchers have been concerned with the evaluation of the effect of microwaves on food constituents, nutrient retention and the change of flavours and colours of heated food (Finot, 1995; Mudgett, 1982). Microwave heating of roasted seeds and beans shows a better retention of flavour and antioxidant compounds without any significant chemical changes of the lipids (Behera et al., 2004; H. Yoshida & Kajimoto, 1989, 1994). With respect to lipid components, microwave heating was studied to verify eventual heat induced effects on different oils and fats (Farag, 1994; Hiromi Yoshida et al., 1990; H. Yoshida et al., 1992). For this purpose, peroxide value, carbonyl value and conjugated diene and triene levels were assessed. Extra virgin olive oil that comes from the first pressing of the olive, without using heat or chemicals, contains natural antioxidants such as tocopherols, carotenoids, sterols, and phenolic compounds (Boskou, 1996). It should be mentioned that carotenoids play a significant role as antioxidants by scavenging free radicals, and as singlet oxygen quenchers (Burton & Ingold, 1984; Di Mascio et al., 1989). Since oil and fat have low specific heat constants and heat quickly (Jowitt, 1983), nowadays microwave frying of food has been