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I'll cut to the chase instead of burying the lede here: In many cases, freezing fresh basil before incorporating it into pesto will produce tastier results.
I know, I find it hard to believe myself. But hear me out, because there's a good explanation for all of this.
This whole idea came about because my refrigerator is broken. Every couple of weeks, it decides to kick its compressor into high gear, and everything on the bottom shelf ends up freezing solid. That's what happened recently to the ginger and lemongrass I was storing in there. I ended up deciding to use that frozen-then-thawed lemongrass and ginger in a Thai-style curry paste, and was frankly surprised at how easily they incorporated into the mix in my mortar and pestle. Then it occurred to me: Perhaps freezing aromatics is a good thing?
My reasoning went like this: In many cases, we want our vegetables and herbs to stay relatively bright and intact. We want crunch, we want bite, we want texture. But sometimes, we want the opposite: We want our vegetables and herbs to get completely pulverized. We want them bruised, beaten, and squeezed until every single one of their cells bursts open and releases its fragrant juices. This is why we've found that for things like pesto, curry paste, and guacamole, a good, heavy-duty mortar and pestle produces far better results than a food processor or blender.
Why is that? Well, it's because a mortar and pestle crushes cells rather than shearing them. Think of your vegetable as a stack of shipping containers, each filled with flavorful juices. A food processor has a whirling blade that knocks those containers apart. It may well cut open a few of them, but more often than not, it simply batters them so that they separate from each other while staying intact individually. A mortar and pestle, on the other hand, doesn't just separate them but totally crushes them, releasing their cargo.
So where does freezing come in? Freezing can accomplish a similar goal as the mortar and pestle. Rather than crushing from the outside, though, it causes cells to tear apart from the inside, as liquid water expands and forms jagged ice crystals. Vegetables and aromatics that have been frozen and then thawed are limp and bruised-looking.
So, I figured, if freezing causes cells to rupture, and rupturing cells is our goal when making curry pastes and pestos, could it actually improve the results? Moreover, could it help produce extra-tasty pesto in the food processor or mini chopper, no mortar and pestle required?
To test this, I first made three different batches of pesto using Daniel's fantastic recipe.
The first I made the traditional way, using fresh basil and a mortar and pestle alone. The second I made in a mini chopper, using fresh basil. The third I made in the mini chopper using basil that I'd placed in the freezer overnight and allowed to thaw for a few minutes at room temperature. (Basil has such a high surface-area-to-volume ratio that it freezes and defrosts in moments.)
Well, the theory held true. Somewhat.
The pesto made in the mortar and pestle was definitely the best of the three, boasting a creamy, emulsified texture and really bright, vibrant basil flavor. The batch made from fresh basil in the mini chopper was easily the worst, with a sort of gritty texture and muted flavor. I wouldn't kick it out of my pasta bed, but it wouldn't be my first choice for a garlic-breath date night, either. The third batch—the one made with frozen basil in the mini chopper—was somewhere in the middle. It had the same great, creamy texture as the pesto made with the mortar and pestle, and you can see from its somewhat lighter color that it actually formed a more stable emulsion than the pounded version. Its flavor was also a big improvement over that of its non-frozen counterpart, though it wasn't quite as flavor-packed as the version made with the mortar and pestle.
For completeness, I made another batch in which I froze the basil, then crushed it with the mortar and pestle. I didn't find any serious advantages there.
In retrospect, this shouldn't have surprised me: I use the exact same logic when freezing vegetables for my gazpacho, and it works wonders there.
So what are the practical applications here? Well, if you're willing to put in the elbow grease and pound your pastes with a mortar and pestle, then freezing aromatics won't make your stuff much better. It will, on the other hand, make storing things and the act of pounding itself a little easier on you, as those aromatics will already be limp to begin with. If you've been taking the lazy route and using the food processor for all your curry pastes and pestos (and I know you're out there!), try popping those aromatics in the freezer before you throw them in the food processor. You might just find that your food comes out a bit tastier, with almost no extra work required on your part.