Final answer:
Collagen is the connective tissue protein that breaks down into gelatin when cooked with moist heat, giving slow-cooked meats their tender and gel-like consistency.
Step-by-step explanation:
The protein in connective tissue that breaks down into gelatin when cooked with moist heat is collagen. Connective tissues are made up of cells, ground substance, and protein fibers such as collagen, elastic, and reticular fibers. Collagen is responsible for providing tensile strength to the connective tissue, which prevents it from tearing. When collagen is subjected to moist heat during cooking, it breaks down into gelatin, which is why slow-cooked meats become tender and have a gel-like consistency. Collagen is a key ingredient in broths and stews, contributing to their rich texture.