Yes, grated Parmesan cheese will go bad if left unrefrigerated after opening. Like many dairy products, grated Parmesan cheese is perishable and can spoil if not stored properly. Once the package is opened, it is essential to refrigerate it to extend its shelf life and maintain its quality.
When left at room temperature for an extended period, grated Parmesan cheese is vulnerable to bacterial growth and can develop mold, leading to spoilage. To prevent this, always tightly reseal the package or transfer the cheese to an airtight container before putting it in the refrigerator.
If you accidentally leave grated Parmesan cheese unrefrigerated for a short time, such as a few hours, it might still be safe to eat. However, if it has been left out for an extended period, especially in a warm environment, it is best to discard it to avoid the risk of foodborne illness. When in doubt, it's better to prioritize food safety and get a fresh package of grated Parmesan cheese.