*4.1.4. Quality of microwave-dried food products*

In general, the quality is somewhere between air-dried and freeze-dried products. The reduction of drying times can be quite beneficial for the colour and the aroma. Venkatesh and Raghavan (2004) dried rosemary in a household microwave oven with good aroma retention. Krokida and Maroulis (1999) measured colour and porosity of microwave-dried apples, bananas, and carrots. Khraisheh et al. (2004) compared air-dried and microwavedried potatoes and found a reduction of shrinkage and improved rehydration for the latter. Venkatesh et al (2004) reported on chicken products, seafood, and vegetables of good quality. He used air at 10±20 0C to cool the product during microwave drying. Quality can often be improved further by the use of vacuum. This reduces thermal as well as oxidative stress during processing.

For instance, Yongsawatdigul and Gunasekaran (1996) showed that colour and texture of microwave-vacuum-dried cranberries were better than those of air-dried samples. If we look specifically at the retention of aroma, it becomes necessary to distinguish between two basic cases. In most foods the aroma molecules are present in very small amounts, so that they are likely to be dissolved in the water phase. In this situation, the volatility of the aroma molecule in water is essential.

Considering the fact that we perceive aroma -as opposed to taste - with our noses, it is quite clear that aroma molecules are normally volatile; otherwise they would stay in the food during eating and not contribute to the aroma. In other words, if there is an interface between a water phase (i.e. a food) and a gas phase, the aroma molecules tend to choose the gas phase. In air drying, the surface where the aroma molecules can escape is mainly the outer surface of the particles. This is also where the water molecules evaporate. So the surface of the food particle will be depleted of aroma, but the losses cannot be higher than those that come with the capillary water flow from within. As a result, the losses of water and aroma are coupled.
