San Marzano tomatoes are the most famous plum tomato to come out of Italy. They are grown in the rich volcanic soil at the base of Mount Vesuvius, which gives them a sweet flavor and low acidity and they are coveted for their firm pulp, deep red color, easy to remove skin and low seed count. In fact while regular round tomatoes usually have four or five locules or seed pockets, plum tomatoes like those from San Marzano have only two.
In San Marzano, the tomato harvest begins in August and runs through September. The crops are very delicate and all the tomatoes are picked by hand at the peak of their ripeness. Because of the close attention to quality, many cooks consider San Marzano tomatoes to be among the best in the world to use in a sauce.
The tomatoes are grown under very specific and strict rules and as such, authentic San Marzano tomatoes will have an official DOP (Denominazione d' Origine Protetta) on the can or jar, although there are domestically grown versions using the same seed varieties. It’s comparable to the difference between French Champagne and sparkling wine from California. Because of their great flavor and texture, very little has to be done to them upon opening then can.