How effective for preserving wine are reverse hand pumps that suck air from a partially empty bottle, leaving an airtight stopper in place?
In my estimation, they're not great. I have tried several (though not all) such brands and have found that they don't adequately protect wine from the bruising effects of oxygen, at least not for long.
So-called vacuum pumps or vacuum sealers are based on the theory that you can suck out oxygen from a partly empty bottle and secure the resulting vacuum by means of a rubber stopper. The manual pumping device fits over the bottleneck, much like a corkscrew. Pull the handle up and down repeatedly as though it were a bicycle pump or one of those dynamite-exploding triggers used by Wile E. Coyote on the old Road Runner cartoons. Presto, the air is gone, replaced by a sheer nothingness.
Smart in theory, but I'm not impressed by the reality. And I'm hardly the only one to give those hand pumps the thumbs down. Matt Kramer, the venerable columnist for Wine Spectator, long ago performed a controlled experiment on one popular brand, the Vacu-Vin, for the Wall Street Journal. He even enlisted the help of a chemistry professor at Portland State University in Oregon to measure the (rapidly declining) efficacy of the device. "Overnight – 12 hours – the vacuum is totally gone," he wrote.
Perhaps the devices have improved since then, but I'm not confident. The Sweethome, a product-review company owned by The New York Times, recently put various wine-preservation systems through rigorous testing, which you can read about here. It ranked vacuum sealers surprisingly low on the list, behind simply recorking the bottle and sticking it in the refrigerator, where cold temperatures slow down corrosive air reactions significantly. (Incidentally, The Sweethome researchers most strongly favoured Private Preserve, a spray can that dispenses inert gas through a small straw-like plastic tube into the bottle to create a protective gas "blanket" over the wine's surface. It's the method I've been using for years.)
But if you find all of this a bit pretentious and costly, don't worry too much. Just stick that half-finished merlot in the fridge. Cold makes a huge difference, especially in summer. And don't keep that opened wine around forever.