FREEZERS ARE now at their fullest. The harvest is ending, the fishing is at its peak, and pound after pound of berries, fruit, vegetables, meat, fish and other bounties of the season are crammed into those 15 cubic feet in the cellar.

This represents a substantial investment; think, for a moment, about the cost of a few pounds of raspberries, or about the labor you invested in growing that broccoli. These valuable commodities deserve to be protected by a freezer that does the job for which it was designed.

Yet many people never give a thought to the functioning of their freezer -- they set the thermostat somewhere in the middle of the range, usually at "5", and assume that, since the food becomes hard after a few hours in the freezer, everything is fine.

It's not quite that simple, however. Food stored at temperatures higher than zero degrees Fahrenheit (-18 Celsius), which is generally considered to be the highest acceptable temperature for frozen food, will suffer a perceptible decline in quality. This isn't a matter of safety -- as long as the food remains frozen, the microbacterial activity is minimal -- but of avoiding those off-flavors associated with frozen foods.

A few quick steps, however, will enable you to make certain that your freezer -- whether a free-standing model or part of a refrigerator/freezer combination -- is doing its job. The first thing to check is the appliance's general condition. Most frost-free models (by far the majority these days) have a circulating fan right inside the food storage area; make sure that the space in front of this fan isn't blocked by packages of food. While you're at it, see if you can arrange the food on the racks so that all the internal walls of the freezer -- and the top and bottom as well -- allow for free air circulation.

Next, get a vacuum cleaner. Unplug the freezer, remove the grill -- almost always at the front and bottom of the appliance -- and vacuum the refrigrator coils, doubtless covered with several years' accumulation of greasy dust, especially if your freezer is in the kitchen. Plug the freezer in again.

Now check the temperature of your freezer. Get a thermometer with a low range of zero degrees or lower, and check its accuracy. You can do this by placing it in a glass or jar that has been filled with ice, then topped off with water. A thermometer left in this bath should read 32 degrees (zero degrees Celsius) within five minutes or so. If the variance is just a couple of degrees, you can make allowances. If the thermometer is off by 5-10 ten degrees, return it or throw it away.

Place the thermometer between two packages of food in the freezer, close the door, and go away for a few minutes. If, upon checking the thermometer, you find that your freezer is holding at zero degrees (-18 Celsius), great. If not, adjust the freezer's thermostat accordingly (there's not much advantage to storing food at temperatures lower than zero, and your electric bill will be increased needlessly).

Check again after a day or so. And while you're at it, check the temperature of your refrigerator -- it should be no lower than 40 degrees (5 degrees Celsius).