The change in the texture of cheese on a frozen pizza when heated in the oven is primarily due to the nature of cheese and the cooking process involved. Several factors contribute to the cheese becoming hard and chewy:
Moisture loss: As the pizza cooks in the oven, the heat causes the moisture in the cheese to evaporate. When moisture is lost, the cheese can become drier and firmer.
Protein denaturation: Cheese is composed of proteins, fats, and water. Heating causes the proteins in the cheese to denature, which means they unfold and reconfigure. This process can lead to a change in the cheese's texture, making it firmer and potentially chewy.
Fat separation: As the pizza cooks, the fat in the cheese may start to separate from the rest of the cheese components. This can contribute to an uneven texture, with some parts of the cheese becoming hard or chewy.
Oven temperature and cooking time: If the oven temperature is too high or the pizza is cooked for too long, the cheese can overcook and become excessively hard and chewy.
Quality of the cheese: The type and quality of the cheese used on the frozen pizza can also play a role. Cheeses with higher moisture content, like fresh mozzarella, are more prone to becoming tough when exposed to high heat for an extended period.
Covering the pizza with foil may help to some extent by reducing the direct exposure of the cheese to the oven's heat, potentially slowing down the moisture loss. However, it may not completely prevent the cheese from becoming firmer during the cooking process.
If you find the cheese on your frozen pizza consistently turns too hard and chewy for your liking, you can try adjusting the cooking temperature and time. Experiment with slightly lower temperatures and shorter cooking times to see if it helps retain more moisture in the cheese. Additionally, using high-quality cheeses with better melting properties might lead to a more satisfying result.