The heating rate in a microwave oven depends on the molecular composition of the substance being heated and its ability to absorb and interact with microwave radiation. In the case of a cake and milk, their heating rates can vary due to differences in their compositions and water content.
Water content: As you mentioned, microwaves primarily interact with water molecules in food items. Cakes typically contain some amount of water in their ingredients, such as eggs, milk, and butter. The presence of water in the cake allows for efficient absorption of microwave radiation, leading to relatively faster heating.
Density and structure: The density and structure of the food also play a role in how efficiently it absorbs microwaves. Cakes are usually less dense and have an open structure, allowing the microwaves to penetrate deeper into the cake, leading to more uniform and quicker heating.
Homogeneity: Milk is a homogeneous liquid, whereas a cake is a solid with a mixture of different ingredients. The heterogeneous nature of the cake, with varying water content and other components, can result in multiple interactions with microwaves, leading to more efficient heating compared to a uniform liquid like milk.
Heat distribution: When heating a liquid like milk, the heat is primarily generated on the surface, and then it slowly conducts inward. In contrast, when heating a solid like a cake, microwaves can penetrate and generate heat throughout the mass simultaneously, leading to faster overall heating.
Stirring: Milk often requires stirring in a microwave to distribute the heat evenly throughout the liquid. Without stirring, some areas may become superheated while others remain cooler. Cakes, on the other hand, do not require stirring, as microwaves can penetrate and heat them more evenly.
In summary, cakes heat faster than milk in a microwave because they are solid, have a heterogeneous composition with varying water content, and their structure allows for deeper microwave penetration and more efficient absorption of microwave energy. On the other hand, milk, being a homogeneous liquid, heats more slowly and requires additional care, such as stirring, to ensure uniform heating.