Making and Preserving Pesto alla Genovese

It is that time of summer when the basil is ready to be picked and when there is a new crop of garlic at the markets. That means it is time for the annual summer rite of making pesto alla Genovese (basil pesto). As with many crops, basil comes all at once and it is highly perishable. In addition to a decent basil crop, we have an excellent crop of flat leaf parsley, which can be used on its own to make pesto or can be added to the classic recipe for basil pesto. We follow the recipe for basil pesto in Marcella Hazan's Essentials of Classic Italian Cooking. (This book is a critical resource for anyone interested in Italian cooking.) The problem always becomes how to preserve the pesto so that we can eat it year around.

fresh basil from the garden

Pesto alla Genovese is made from pine nuts, garlic, salt, basil, olive oil, and Parmesan cheese. Marcella recommends if you freeze the pesto for storage to wait and add the cheese until you're ready to use it, as the cheese doesn't freeze well. So, we make her recipe up to the point of stirring in the cheese, freeze it in individual portions (ca. 1/4 cup) in zip-lock bags from which all the air has been pushed out. This consumes a significant number of plastic bags, as we usually make 15-25 portions, and it's messy.

freshly picked flat-leaf parsley

This year we tried something different. We have a vacuum food sealer that has an attachment for wide-mouth Ball jars, and will suck the air right out of pint or quart jars. The jars end up with the lids very tightly sealed. We use quart jars to store arborio or carnaroli  rice for risotto so it doesn't go stale and we vacuum freeze salmon, meats, and nuts in plastic bags. The basil pesto is prepared in a food processor, doubling the recipe in the cookbook (4 cups basil, 1 cup oil). Turns out, this produces just about a pint of pesto, perfect for a wide-mouth pint jar. The jars were filled to about 1/2" of the rim, the lid was carefully placed on the jar, and the whole thing was hooked up to the machine and vacuum sealed before putting the band on the jar. (One thing I do differently from Marcella is to make a paste out of the pine nuts, garlic and salt before putting the basil in the processor, and then make a paste of that before slowly adding the olive oil.)

three pints of basil pesto and one pint of parsley pesto

Now most people know that you can't take a glass bottle or jar filled with water and freeze it, because water expands upon freezing and will shatter the container. (Don't believe me? Take a glass bottle of Coke or beer and put it in your freezer.) As it turns out, water is somewhat unique in that the solid phase is less dense than the liquid phase, whereas most other liquids contract upon freezing. Oil will contract by about 10%. Hopefully, freezing the pesto, whose components are about half oil and half water-based components, will result in little to no expansion, and the jars won't burst. Being the experimentalist that I am, I only froze one of four jars of pesto in case I'm wrong.

The plan is that when we want pesto, we open the jar, scoop out the amount we need, and re-seal the jar with the vacuum sealer and put it back in the freezer. Then we add the Parmesan cheese and toss it with pasta. The nice part is that we can re-use the jars.