Burning and browning of mozzarella is directly related to how much milk sugar is left or added back into the cheese. Since this batch was homemade, the milk sugar was not washed out as thoroughly as a commercial, low moisture batch of cheese. Mozzarella has a wide range of lactose values ranging from 0.1% to over 1% lactose dry weight. That is a low lactose level compared to cream cheese or fresh dairy butter or cream but still enough to brown or burn when exposed to pizza making oven temperatures. Since the milk sugar level can vary over a range of 1 to 10 from brand to brand, this is a major factor in how long a mozzarella cheese can sit in a pizza making oven before it burns on the surface exposed directly to oven heat. The milk protein will also brown and burn when exposed to high heat but sugars burn faster than proteins. Some important browning flavors are caused by reducing sugars reacting with protein under high heat conditions. Breads and grilled steaks have a desirable crust from this reaction. Cheese can also brown and create new desirable flavors from this heated chemical reaction.