SUMMARY
   Food consumption habits have recently changed due to the lack of time to cook and a decrease in the number of household members. The ready-to-eat meals sector has consolidated its position in the food market by offering products that are easily prepared. In this context, the microwave oven has become an essential appliance in every home because it is a quick and easy way to prepare such products. The peculiarities of microwave heating, where the own food formulation has a crucial impact, make it difficult to achieve a general prediction of microwave heating pattern in foodstuff and so it is necessary to study each particular case. Therefore, the current trend is to design microwaveable food starting with the formulation itself.
   In the present dissertation, model bechamel sauce systems were studied as an example of a marketed product which is also used in other ready-to-eat products. Initially, the effect of formulation of different model systems made with salt, water and different hydrocolloids (starches and carrageenans) on the essential microwave heating properties (dielectric properties), rheology and microstructure was analysed. Then, the microstructural and rheological properties of sauces made with different starches (native and modified) were studied in order to assess their behaviour under microwave heating. Moreover, the chemical and microstructural stability of sauces made with different proteins (milk and soy protein isolate) subjected to a freeze-thaw process in a microwave or conventional oven were also analysed. Finally, the microstructure and dielectric properties of sauces made with different fats (butter, and olive, sunflower and soy oils) and prepared at different agitation speeds were studied.
   The results showed a significant negative effect (P < 0.05) of starch on microwave heating. Therefore, a partial substitution of starch with carrageenan as the carbohydrate base of the sauces was proposed. On the other hand, there were significant differences (P < 0.05) in the rheological parameters values depending on the type of starch used, however these values remained generally stable when sauces were subjected to microwave heating. There were changes in the microstructure regarding fat globules size and protein aggregation when sauces were subjected to microwaves especially during prolonged heating. The chemical and microstructural stability of samples subjected to a freeze-thaw process depended on the type of protein used in the system, however, thawing in microwave oven has a lower impact in the parameters analysed in comparison with conventional oven. Finally, an increase in the agitation speed caused a significant decrease (P < 0.05) on the fat globule size regardless the type of fat used in the formulation, whilst the thawing method did not significantly affect the morphology and size of fat globules in this type of sauces.
   The preparation of microwavable sauces was possible using a mixture of starch and carrageenan. Moreover, the protein sources used in the formulation (skimmed milk powder and soy protein isolate) proved to be appropriate to obtain sauces stable to cold storage and reheating processes. In this context, the use of soy products (both protein and oil) could be appropriate to develop alternatives to dairy products in this type of foodstuff.