## 1 Introduction  
Preserving food with a microwave field usually has a lesser impact on the deterioration of the nutritional and sensory properties of food compared to other methods of conventional heat treatment or reheating of food [1]. The impact of the microwave field in the food industry is usually reduced to processes such as blanching, cooking, drying, pasteurization, sterilization, thawing, baking, fat rendering or as an element of combined technological processes [2, 3]. A relatively common problem encountered when heating food with microwave ovens is an uneven temperature per unit volume of product, especially when the product is nonuniform. In order to minimize this phenomenon, the use of various parameters of microwave treatment is suggested, in relation to the same raw material, earlier preparation of product ingredients of optimal size and shape, use of the pulsed microwave treatment mode (which was used in our research), selection of precise process conditions dedicated to specific products, microwave ovens with a suitable shape of the treatment chamber, using lower power during microwave processing [1, 4]. The main preservation factor in the process of thermal treatment, also in the aspect of microwave fields, is the temperature of a certain value applied for a sufficiently long time. In the food industry, microwave pasteurization is often performed when processing juices,and more energetic sterilization is typically used for milk. In addition, the publications describe the effectiveness of the microwave field also in the technology of preserving cider, coconut water or sweet potato puree [5]. In these studies, particular attention is paid to the dielectric properties of the preserved raw materials, as well as to the selection of energy-optimal process parameters in order to achieve the assumed degree of microbial inactivation. The impact of the microwave field can take place both in the case of liquid egg raw materials and eggs in the shell while maintaining a properly selected energy dose [6]. Research on microwave pasteurization in relation to _Salmonella Typhimurium_ in egg yolk proved that the effectiveness of this technology depended on the time and conditions of interaction [7]. The preservation of food products is carried out not only by classical thermal methods, but more and more often using new thermal methods (ohmic heating or using radio frequencies above 70 kHz) or nonthermal methods (high hydrostatic pressure, exposure to ultraviolet light, pulsed electric fields, high-frequency ultrasounds, ionizing radiation, oscillating magnetic field). After using these methods, there are less changes in the color of the food product, its sensory characteristics and the nutritional value. In addition, the amount of energy needed to preserve the product can be reduced by up to 70% compared to classic thermal methods [8; 9; 10; 11; 12]. Classic food sterilization is increasingly being replaced not only by microwave sterilization, but also by other technologies, such as, e.g., ozonation, strong UV exposure or cold plasma [13]. Several less intense preservation techniques are also used, previously known as the \"hurdle theory\", and now the \"hurdle technology\", which lead to the appropriate microbiological purity of the final product with less energy input and less nutritional value loss [14; 15]. The mechanism of inactivation of microorganisms in food during exposure to the microwave field is not fully understood. There are several theories on this subject. One of them, called the theory of selective heating, assumes that microorganisms are selectively more heated by the microwave field compared to the surrounding environment, which leads to their inactivation. According to the theory of electroporation, the potential difference on both sides of the bacterial cell membrane causes damage and the formation of the so-called pores through which cellular content passes out, which leads to the death of the bacterial cell [16].