After the canned tomato sauce is opened, if it can't be eaten once, it can be deteriorated easily after a period of time. If you can make tomato ketchup a mouthful, steam it and then eat it. The leftover ketchup can be degraded for a long time. There is another good way to prevent the deterioration of ketchup, that is, put a layer of plastic wrap at the opening of the ketchup and seal it well, and store it in the refrigerator for a long time. Scrape off the top layer when eating.