When heating Nutella in the microwave, it is common for the outside of the Nutella to become hot while the inside remains relatively cooler. This is due to the way microwaves heat food.
Microwaves work by emitting electromagnetic waves that are absorbed by water, fat, and sugar molecules present in the food. These molecules vibrate and generate heat, which cooks the food. However, Nutella, being a spreadable chocolate-hazelnut paste, has a relatively low water content compared to other foods.
During the microwave heating process, the waves penetrate the Nutella jar and primarily heat the water molecules present on the surface and the surrounding air. As a result, the outside of the Nutella jar becomes hot, which is transferred to the outer layer of the Nutella.
However, the heat generated on the outer layer does not easily penetrate the thicker, less moist interior of the Nutella. The relatively low water content and the high fat content in Nutella act as insulators, inhibiting the transfer of heat to the center. This is why the inside of the Nutella does not become as hot as the outside.
It's important to note that uneven heating can occur in any food when using a microwave. To achieve more uniform heating, you can try stirring the Nutella periodically while heating or using shorter intervals at lower power settings to allow the heat to distribute more evenly throughout the jar. Always exercise caution when handling heated food, as microwaves can create hot spots that may cause burns.