It doesn’t seem very energy efficient. They also only use them for part of the frozen food section. Why is that? My guess is that dairy items are popular and the doors would be open a lot, but I rarely have to stand in line to get my milk or yogurt so they can’t be in that much demand. And what about at night when the store is closed and they are just stocking shelves. Wouldn’t it make sense to have them closed? It just seems very wasteful to me.
I am a veteran of the supermarket industry. Leaving the dairy case open is purely a business decision. Leaving it open and shopper-friendly outweighs any money that would be saved from making it more energy efficient.
The basic problem is that enclosed cases aren’t as easy to casually browse as open ones. That makes it not a viable idea from a business standpoint. The dairy section tends to be fairly high margin so the companies want the products as easily accessible as possible.
For stores that aren’t open 24 hours, there are covers to put over the cases after hours. These are often thick plastic blankets that provide good insulation.
Another part of it may be where you live. ISTR that there are far more open dairy cases back home in Michigan than here in the deep South. Here, it probably adds more to the cost of warm weather (far more than just summer, near the Gulf) cooling than to cold weather heating costs.
Shagnasty’s comment about accessibility is a matter of critical importance to sellers, but people who use milk at all will probably not forget to get it while they’re in the “grocery store”. I don’t think there’s any supermarket chain here that doesn’t have its milk behind doors. OTOH, Walgreen’s has their milk section open, in the one closest to me. Buying milk is far more of an impulse decision in a "drug"store than it is in a supermarket, so they gain more spur-of-the-moment milk buyers - and I suspect such chains have analyzed this thoroughly - by having it more noticeable.
As for those open-topped cases where you see cheese and frozen foods, don’t forget that heat rises, and cold falls (same reason why using a ceiling fan in the winter is efficient). There is less, um, cold-loss on an open-top frozen food case. The opposite example is the cases where ready-to-eat barbecue, etc. I don’t think I’ve ever seen one that didn’t have a little roof over it. That’s to minimize heat-loss. 
Also, the heat loss from the coolers and freezers reduces the load on the air conditioners, so it’s not really all that wasteful.
Actually, there is a small chain of what I’d call “ultramarkets” called Highland Farms here in Ontario. They’re like an ordinary supermarket, but about three or four times the size. (Think Costco, but only for food and sundry items normally carried by a supermarket, then add in a cafe, flower shop, wine shop, gift shop, ice cream parlor and candy/information counter) For whatever reason they keep their milk and cream behind closed doors in one section, and their pre-packaged cheese, butter, yogurt and other such products in open refrigerators in another section.
Doesn’t really bother me though. I work right near one so I’m in there daily buying my morning fruit bowl, muffin and coffee. It’s not much of a bother to have to go to the back, open a door, and get some milk when I need it.
It’s not milk, but I’ve been in a few Albertsons that have walk-in beer coolers. Of course, they’re usually stocked with mediocre beer, but the concept is pretty, uh, cool.
The Fairway Market on the upper West side of Manhattan got it right. The store looks like an old warehouse from the outside but it’s an amazing place that does a huge amount of business. If you want refrigerated goods (dairy, meat, seafood) you grab a jacket off the hook and roll your shopping cart into the cooler. Walk up and down the aisles and put what you want in you shopping cart. The butchers and seafood specialists work right in the cooler.
That’s how you should shop!
That’s what most of the supermarkets around here have converted to. Milk and perhaps creams behind doors, but eggs, cream cheese, cottage cheese, butter, yogurt, sweet rolls, pudding, juices, etc in the open cases.
I think this was for show – as it makes the milk seem like it’s being kept colder, and thus is fresher.
The open-front reach-in coolers have an “air curtain” that serves to keep the cold air inside the cooler. It’s a flat stream of fast-moving air across what would otherwise be a door frame that prevents air currents and normal convection from taking cold air out of the cooler.
As **Shagnasty ** said, there’s a business decision to make the coolers appear open and inviting. There’s no particular consensus which is better. Air curtains allow open coolers, but if the blowers fail, you’re in trouble. Doors get abused - hit by carts, swung open and slammed by kids, and they need washing pretty often or they look nasty.
The big open-top bins are called “dumps” and they work by simply being a box of cold air. The cold isn’t going to climb out of the box, and warm air isn’t going to fall into the box.
My father had a small store that sold primarily milk and the cases had glass doors because there was no way he could afford the big coolers that supermarkets had. But a convenience store just has people coming and going quickly and not browsing around for 30 minutes or so.
If someone hung around that store for 30 minutes, they were likely casing the place and going to rob it.
Another consideration would be traffic flow, i.e., narrow aisles with lots of shopping carts are going to have thier progress impeded by the open doors. The open doors will also tend to build condensation on the inside, so if you can’t see what’s inside the doors will be opened more often, blocking the aisles, and building condensation inside the doors…
You get the picture.
Cecil covered this a while back but I can’t seem to find the article in the archive. The gist was that supermarkets trade off between efficiency and displaying their wares in a pleasing and accessable manner, and the forced-air curtain effect is good enough for high-traffic items like milk and whatnot.
My local Safeway keeps milk and ice-cream behind doors, however yogurt, cheese, butter, etc. are in open-faced coolers.
The Andronico’s keeps all the milk in open-faced coolers along with the other dairy products.
My corner grocery store keeps all of it behind glass doors.
Here – in Michigan – “my” local Farmer Jack and Kroger (and if it counts, 7-Eleven) all have doors on milk displays. Lots of other stuff is in the open displays, though. And to be contrary with the southern people, the Sorianas in Mexico where I just lived for a year all have open displays – and the temperature where I was at regularly reached over 100F outside. Who knows?