Some further digging turned up this research paper from the African Journal of Biotechnology, which details the activity of alliinase relative to pH. Here’s a graph of the data:
Turns out alliinase is highly active at a neutral pH, with peak activity at a very slightly acidic pH of 6.5. As you get more and more acidic, its activity drops off precipitously. Lemon juice has a pH of just around 2. The study’s data goes down only to a pH of 3, but extrapolating that graph, we can guess that at pH 2, allicin’s activity is reduced to a quarter or less of its peak activity.

That’s what keeps garlic from becoming too harsh. Once enzymatic activity has stabilized, you can then incorporate that garlicky-but-not-harsh lemon juice mixture into your tahini sauce and hummus without fear.

I’ve since used the garlic-in-lemon trick in a number of applications, ranging from baba ganoush to a vinaigrette for some simple roasted cauliflower. Any time you have lemon and garlic in a recipe, it’s good to consider whether or not you might gain some advantage by upping the garlic flavor without upping its harshness.
This is my favorite kind of recipe: one that tastes great, but also teaches you an entirely new technique that has applications well beyond the scope of the original recipe.

Sorry, I’ll be right back. I’m heading off to purée some garlic. (After having a bite or two of hummus, that is.)

Bingo. Blending those chickpeas in the food processor turned out a hummus that was as smooth as store-bought.

How to Make Your Hummus Extra Smooth
I was happy with my results and all set to publish my recipe, when my buddy Chef John Fraser dropped a brand-new technique on me. I was having dinner at his new place, Nix (which, for the record, is the best vegetarian/vegan restaurant I’ve ever been to), and was amazed at how incredibly smooth his hummus was. Compared to his, mine felt like the sludge water in the bottom of a Death Star detention-level trash compactor. I asked him how he got it so darn smooth.

“We do it in the blender,” he told me. Because of their vortex action and high-power, low-torque blade motion, blenders can purée foods much more efficiently than a food processor can.

The problem is that they don’t work very well for pasty, viscous things like hummus; the hummus sticks to the side of the blender jar and never really comes in contact with the blade. So what was John’s secret?

Blending it hot.

It seems so obvious in retrospect. Starchy, viscous liquids get firmer as they cool. Up to this point, I’d been cooking my chickpeas and cooling them before adding them to the food processor. Transferring them to a regular blender straight from the pot, along with plenty of their cooking liquid, makes it easy to blend them into a thick, smooth paste with the texture of a milkshake.

I actually ended up taking another piece of John’s advice: adding some of the mirepoix in his cooking liquid to the blender along with the chickpeas. A little extra carrot and garlic in the blender add depth of flavor without taking away from the chickpeas.
Once the chickpeas are blended, I whisk in my tahini and season to taste with salt, cumin, and olive oil. The hummus is then ready to be chilled and served. I like serving it at just about room temperature, drizzled with olive oil, dusted with paprika or za’atar, and piled with some warm chickpeas or chopped parsley.

The Science of Garlic Flavor
One thing was still bugging me: that garlic. How was it that I was adding a whole head of garlic, but getting only aroma, without any of the hot, pungent garlic flavor I’d expect? I’m the kind of guy who likes to know where his sausage is coming from, which means that I really wanted to get to the bottom of this garlic mystery, but I wasn’t sure where to start. The answer came to me while I was trying to streamline the recipe.

The hummus was fantastic—smooth and flavorful—but I thought to myself, If I’m going to be puréeing my chickpeas anyway, why bother making the tahini paste separately? Can’t I just dump everything in the blender and hit go? I tried it out, placing the cooked chickpeas with their liquid, tahini, cumin, lemon juice, salt, and peeled garlic cloves into the blender, all at once.

I tasted the new batch and nearly had to spit it out: The sharp, hot garlic flavor was overwhelming. It tasted just like you’d expect five cups of hummus with a full head of garlic in it to taste. What the heck? How could this batch, which used the exact same ingredients as the previous batch, taste so darn different?

I knew that the hot flavors in garlic develop when the enzyme alliinase converts a mild compound called alliin into a more pungent one called allicin, and I also knew that this reaction doesn’t take place until the garlic is sliced open and cells are ruptured. It’s for this reason that you can drastically alter the flavor of garlic just by cutting it in different ways. But in my hummus, the garlic was getting fully puréed either way, so what was up?