Though enthusiasts will forever wax poetic about the flavors of their favorite frozen treats — Super Chocolate Orgasmo Death-in-a-Cup or Triple Ripple Vanilla Road Monkey — it’s really texture that defines a great ice cream. That is why Ben & Jerry’s was happy to sell ten dollar “cookbooks” detailing the components of their top flavors a few years ago. The equipment you’d need to turn those ingredients into Ben & Jerry’s quality ice cream costs another fifty grand.
Why? Well, because it’s the freezing process that dictates more than anything else the texture of the ice cream. The gritty sensation you get on your tongue from most home made ice cream is caused by ice crystals. And while all ice cream — even Ben & Jerry’s — has them, it’s the size of those crystals that makes the difference between silky smoothness and that ever-so-slight (yet noticeable) cookie-crumbs-in-the-bed sensation you get from less well-made product.
Here again it helps to back up and talk a little bit about water. Water makes up about 85% of milk, and anywhere from 55-65% of cream. Fully crystallized, that’s enough to turn even heavy whipping cream hard as a rock were you to just plunk a carton of it down in the freezer. The water molecules stack up on each other in neat little rows, creating big, brittle masses. The challenge for the ice cream maker: how to freeze milk and cream in such a way that all those little crystals stay separated like flowing sand instead of uniting into giant boulders?
Part of the answer, as I mentioned earlier, is molecular debris. Sugar is a great way to spoil the water crystallization party. But then why stop there? Fat blobs will get between water molecules, and egg yolks are loaded with “emulsifiers”, a scientific-sounding word for bits of molecular flotsam and jetsam that keep like molecules apart. But since even all that isn’t always enough to keep the graininess out of a finished ice cream, there are all sorts of other non-dairy molecules (what are collectively known in the ice cream industry as “stabilizers”) you can toss in for good measure: finely ground flours (starches) of various kinds, gums, proteins — anything to keep those water molecules, or small collections of crystallized molecules, from finding each other. It’s like a bad production of Wutherine Heights, with Catherine and Heathcliff as water molecules, desperately trying to find one another on the British moors. The ice cream maker is a God-like Hindley, first hurling wind at them, then rain, then sleet, then hail — and when all that fails, finally bricks, old refrigerators and sub-compact cars.
Yet for all of that it’s the speed at which ice cream freezes that really determines the size of ice crystals in a finished ice cream. The molecular junk certainly helps, but lightning-fast freezing creates smaller crystals regardless of what’s in (or not in) the mixture.
Industrial ice cream mixer/freezers consist of long tubes surrounded by a refrigerant. Inside the tubes are long, skinny agitators that scrape thin layers of frozen ice cream off the tube’s interior as the liquid mix is pumped through, exposing more of the super-cooled inside surface of the tube to unfrozen mix. Armed with this kind of technology, big ice cream manufacturers can freeze up to 50% of the water in a mix in less than 30 seconds (which is amazingly fast). The partially frozen ice cream emerges from the machine with the consistency of mashed potatoes, at which point it’s flowed into containers, then sent to a minus 50-degree blast freezer for hardening. Not even a top-of-the-line Subzero will do that for you.
It is, sadly, a cruel fact of life that without tens of thousands of dollars worth of professional machinery you’ll never make ice cream as smooth as the big boys. But then a good home ice cream maker can still turn our some mighty wicked stuff.