One of the world's great cheese-producing nations, Italy makes hundreds of varieties, ranging from mild, sweet bocconcini to sharp, pungent Pecorino Romano. Pecorino Romano is an aged, dry sheep's milk cheese, grated sparingly into pasta and other dishes as a potent flavor enhancer. If you don't have Romano on hand, other dry grating cheeses are usable -- if inexact -- substitutes.
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The Grate Taste Comparison
The major varieties of hard grating cheese share a broadly similar texture, brittle and very dense. Their flavors, on the other hand, vary widely depending on the milk used and the cheesemaker's preferences. Pecorino Romano is the sharpest of the common varieties, very pungent and salty. Good-quality Parmesan, or its close cousin Grana Padano, is nearly as salty but milder and nuttier in flavor. Italian or domestic Asiago, American Dry Jack and aged Spanish manchego have less salt, and sweeter, nuttier flavors.
Of the readily available alternatives, a well-aged Parmesan -- not the kind from a shaker, but a block of the good stuff -- is the closest substitute. It's got a subtler, more complex flavor, and you'll need to use slightly more to replace the saltiness of the Romano. Dry Jack, manchego and Asiago aren't as assertive, so you'll need to use more than the recipe calls for. Their flavor is equally enjoyable on pizza or cream sauces, though they won't stand up to a tomato sauce as notably as Romano. Where it's practical, adding a small quantity of pungent goat's cheese to your dish can help simulate Romano's sharp edge.