Cold-pressing is the process of separating the fiber from the cells of fruits and vegetables – you know, juicing – without the aid of any heat. It takes longer and produces less juice but, in theory, the juice is richer in nutrition.
There are, and they're called centrifugal juicers. More of the initial product makes it into the final juice through this process, but it is exposed to potentially degrading heat and air along the way.
Heat can destroy certain enzymes and nutrients, such as notoriously delicate vitamin C. However, some heat can be good. For example, cooking carrots actually results in increased levels of beta carotene.
It’s complicated. Some vegetables release higher levels of nutrients when exposed to heat via steam or boiling water. But heat isn’t the only thing cold-pressing is meant to protect produce from. There’s the matter of oxidation too.
Yes, as soon as the inner flesh of fruits and vegetables is exposed to air, degradation begins to occur. Centrifugal pressers kickstart the oxidation process by exposing newly liquid juice to air as it is forced through spinning blades.
Cold-press juicers still expose juice to air. They just aren’t pumping air into newly liquid juice.
Cold-pressed juice is fantastic for you if that’s the only way you’re otherwise getting these food groups into your diet. But if you’ve got the time, you’re better off just eating a salad.