**8.3. Cooking**

36 The Development and Application of Microwave Heating

**8.2. Baking** 

(Moyano et al., 2002).

steaming). Their results suggested that with the exception of steam-blanched purple hull peas, ascorbic acid retention was not affected by the blanching method. Drake et al. (1981) studied the influence of blanching method on the quality of selected vegetables. Water-and steam-blanched asparagus and green beans had similar ascorbic acid concentrations and both were superior in ascorbic acid to the microwave-blanched product. Microwave and steam-blanched green peas contained less ascorbic acid than water-blanched green peas.

Microwave baking has been the focus of much research and development since the 1950s, with variable success (Seyhun et al., 2003). All results point to the general rule that to achieve success, considerable product reformulation must be considered (Decareau, 1992). Baking with various emulsifiers, gums, starches, fat contents and enzymes has been widely investigated (Ozmutlu et al., 2001; Sumnu, 2001; Keskin et al., 2004). With adequate product formulation, microwave baking can offer good quality products with high convenience. Pillsbury® has a new line of frozen biscuits and dinner rolls specifically designed for the

The browning reactions in baked products are the result of heating of reducing sugars with proteins or nitrogen-containing substances to form compounds like melanoidins, and start at around 160 0C (Matz, 1960). When sugars such as fructose, maltose and dextrose are heated to around 171 0C, molecules are combined to form coloured substances called caramels. A relatively low food surface and low surrounding temperatures in microwave baking do not enable the browning reactions to occur. Moreover, in microwave ovens, evaporated water molecules from the food system directly interact with cold air around the product and condense, which prevents browning and crisping reactions (Schiffmann, 1994). Dough products which are expected to be crisp and brown become soggy after baking. When heated for a longer period, they become dry and brittle but never brown. Brown surfaces achieved by Maillard reactions and caramelization of sugars are a result of high temperature accompanied by dehydration (Burea et al., 1987). In addition, time is necessary for completion of these browning reactions. The kinetic rate constant of browning reaction increases with increased temperature (Ibarz et al., 2000) and decreased moisture content

Good browning and flavour development were attained when no water was added to the system. The characteristic baked/roasted aroma produced decreased as moisture content increased. Flavour development was still apparent in the 5% moisture system (GC/MS analysis of flavour compounds) despite very little browning. Electrolytes (0-0.5M NaCl, CaCl2, FeCl2, or NaSO3) enhanced both flavor production and browning intensity in an Lcysteine/D-glucose model system. NaCl promoted the development of the greatest amount of volatiles (seven times the control) and FeCl2 the least (three Microwave processing, nutritional and sensory quality 93 times the control). NaCl produced the most browning while FeCl2 produced the least. Browning treatments for breads were evaluated. When susceptors were used with MW baking, desired browning and hardness were obtained on

microwave to deliver warm, soft, ready-to-eat bread rolls in 25 s.

Microwave heating of food products is done in a relatively quick time period as compared to conventional oven cooking. The flavour of the final product can result from aroma

generated during microwave cooking. It can also be already contained in the food, *e.g.*, in a precooked meal. In any case, the release phenomenon is the same and it is the timing that may be different. If the flavour is only produced at the end of heating, losses due to volatilization will be diminished considerably. In microwave heating of a precooked product, the volatile aroma compounds are integral to its final aroma, and losses can imbalance the aroma. Aroma is defined as the volatile aroma compounds that contribute both to the orthonasal (sniffing) and retronasal (eating) smell of a food. This section will explain the theory and give examples of how aroma compounds present in microwave foods can be lost during cooking.

The impact of microwave cooking on the formation of early Maillard products was investigated and compared with the effect of conventional cooking, using milk as a test system. Experiments were carried out at controlled temperatures of 80°C and 900C, respectively, at holding times up to 420 min. Hydroxymethylfurfural (HMF) and lactulose, which are all established indicators to estimate heat damage, were determined. The concentrations of all the heating indicators increased with increasing heating time. For example in the 90°C test series the furosine values rose from 34 mg litre-1 (0.5 h) to 94 mg litre-1 (2 h holding time) in the milk heated by microwaves and from 35 mg litre-1 (0.5 h) to 96 mg litre-1 (2 h) in the conventionally heated milk. None of the reaction products showed significant differences as between the microwave heating and conventional cooking methods (Katz, 1994).

Flavour may be a problem in MW-cooked foods because flavour volatiles distill off, bind to proteins and other molecules or fail to develop at all. A number of methods have been developed to prevent or offset these flavor problems. Extraction process wherein substrates are mixed with MW-transparent solvent and exposed to MW which liberates target compounds from natural materials (*e.g.* spices). Selectivity can be varied by altering solvents/conditions. MW extraction in combination with liquid CO2 can be used as an alternative to supercritical fluid extraction (decaffeination of coffee, defatting of cocoa powder). A process to generate desirable aromas when a food and/or package is subjected to MW radiation. The aroma-generating material, consisting of a sugar alone or in combination with an amino acid source, and an effective amount of a MW susceptible material for conductive heat transfer sufficient to catalyze the desired chemical reactions

Vegetables are often cooked to increase palatability and digestibility, ascorbic acid content of MW-cooked, frozen peas was lower, retention of chlorophyll and organic acids (lactic, succinic, malic, citric……*etc*) was higher for peas cooked without water. Effects were smaller for carrots. Those cooked without water had higher flavor scores and carotene retention than those cooked with water. While MW cooking of vegetables generally results in better nutrient retention, there is no one method that produces overall superior sensory characteristics when considering color, flavor, texture, and moistness.

Cooking starchy tubers gelatinizes the starch softening the texture. After a lag of 4 min, water loss during MW cooking of potatoes was rapid and linear. Starch gelatinization began at the surface and in the center, and then spread throughout the tuber cross-section after 1 min. Results suggest that the MW cooking process is divided into two phases: (1) the MW energy input raises the internal temperature to about 100 0C, then (2) water is vaporized at a constant temperature. Immersing potatoes in boiling water after the first phase prolonged cooking time compared to MW heating, suggesting that MW treatment affects texture by a mechanism independent of the thermal profile induced by cooking.
