In a September 22 story on fresh ingredients, selinene, a component of celery, was incorrectly identified as an effective insecticide. It is not an insecticide.

Fresh frozen, fresh and natural, freshly baked -- such are the idioms of the 20th-century supermarket. But like good and bad, fresh is a relative term.

Freshness can be measured by degrees: a cup of five-minute-steeped orange pekoe tea has a fresher taste than a tea made with powder and water, but just-made instant tea tastes fresher than day-old steeped tea because more of the instant tea's original aroma and flavor are retained.

Freshness shouldn't be confused with quality, however. A fresh orange can have unshriveled skin and unfermented flesh and still taste dry or watery.

Perception is as important as any concrete measurement of freshness since each person has his own awareness threshold for flavors and aromas. For example, some people who are hypersensitive to rancidity, if they are eating aged meat, prefer it overcooked until all traces of rancidity have evaporated. Other people can munch a two-year-old bag of potato chips while watching Howard Cosell without once feeling sick.

Yet freshness isn't just a matter of flavor and aroma. It's also related to cultural expectations of color, texture and taste. Most of us would consider a brown piece of beef to be less fresh than a red piece, one week its senior. A bunch of limp, day-old celery bought from a local farmer would be considered less fresh though tastier than another bunch shipped 2,000 miles and stored two weeks at high humidity to retain crispness. And a week-old loaf of watery, American sandwich bread would seem fresher to us than a day-old loaf of french bread.

But high in the Himalayas, Tibetans eat rancid yak butter and love it. Eskimos frequently bury meat out in the Tundra, eagerly anticipating its gamier reincarnation. Even many Americans prefer milk chocolate and salted butter that are slightly rancid.

Acculturation aside, aroma and flavor are the two scientific taste dimensions of freshness. Aroma is produced by evaporating highly volatile compounds (such as alcohols, esters, aldehydes and ketones), which are wafted around by passing air currents. Heating, processing and cooking cause the highly volatile compounds to evaporate; the aroma fades, leaving mostly flavor compounds, which are composed of volatile protein and fat breakdown products. For example, there are over 75 components of vanilla bean aroma. But when vanilla extract is baked, those compounds evaporate, leaving behind vanillin and other non-volatile components of vanilla flavor.

Hydrogen sulfide and ammonia are breakdown products of proteins, and most of us would recognize the distinctive odor of hydrogen sulfide, a poisonous gas at higher concentrations, as one of the primary aromas of cooked corn, boiled milk, baked bread and hard-cooked eggs.

Freshness, then, is really the degree to which cooked or uncooked aromas and flavors have been retained. And what degree of that retention is acceptable is defined by cultural expectations. Fresh Tibetan yak butter is supposed to be rancid.

Here are ways to gauge the freshness of various foods. Apples

Apples vary widely in texture and flavor. During the fall, the apples you buy are probably fresh, although waxed apples can be kept up to one year in controlled-atmosphere storage. These apples usually have a mealy texture and bland flavor owing to slow chemical changes. The flesh may be discolored, but this is usually due to freezing. A fresh apple, with the exception of a rome and a mcintosh, is crisp and firm. Beans -- Dried

There's no harm in eating two-year-old beans although they may have lost some nutritional value and appear unappetizingly yellow. When buying fresh black-eyed peas, lima beans or flageolets in supermarket plastic pouches, look closely to make sure they're not swimming in fluid which spoils them. Supermarkets sometimes mishandle these kinds of beans, which must be kept refrigerated. Beans -- Green

Green beans don't spoil easily because they have a relatively low water content, but they will sometimes develop mold spots. Don't buy spotty beans since the mold spreads rapidly. Old beans are lumpy and stringy. To retain freshness, beans should be cooked quickly. Beef

Beef doesn't spoil as easily as chicken or pork. As slaughtered beef ages, its cells lose moisture and shrink, flavors concentrate, enzymes gradually break down protein molecules into amides--very smelly compounds--and fats into fatty acids, some of which are pleasant at low concentrations but unpleasant at higher concentrations. Bacterial spoilage occurs on the surface, usually apparent as a viscous slime. Butter

Unsalted butter becomes rancid more slowly than salted butter. This type of rancidity, which is caused by the reaction of oxygen with some of the chemical bonds holding fats together, causes salted butter's characteristic harsh flavor. In areas close to butter processors, salted butter has a delicate flavor. Beware of restaurant butter patties; they are sometimes rancid.

Unsalted butter does turn rancid, but in a different way. Its lipolytic rancidity is caused by enzymes present in the cream. The butter, after prolonged storage, starts to taste more acid (this is the flavor of Tibetan yak butter). If butter has a spot of mold growing on its surface, or if it's unpleasant, take it back to the store. Carrots

Carrots lose less of their aroma than many other vegetables; they also stay fresh a long time, particularly if kept refrigerated. If kept too long, however, carrots begin to grow little roots, become wooden and acquire a musty taste. Carrots cooked in fat or steamed keep for weeks refrigerated. They may eventually become slimy, owing to the growth of one of the lactic acid bacteria, which are harmless. Candy Bars

Many candy bars are rancid when you buy them because the nuts they contain don't keep as well as the manufacturers and distributers would like. Eating rancid nuts isn't bad for your health unless you make a habit of it. Celery

Celery retains its flavor indefinitely and remains crisp if wrapped in a wet paper towel and put in a plastic bag that is left open. There are only a few spoilage bacteria or molds that attack celery. Selinene, the main flavor component, is an effective insecticide. Cereal

Dried cereals keep a very long time since they won't spoil from the growth of bacteria, molds or yeasts. Sometimes they do contain hydrogenated fats that will eventually turn rancid, although this process may take months, even years, owing to the presence of antioxidants. Cereals containing wheat germ, which is high in fat, will spoil more quickly. This is particularly true of the granola types, which are rich in fats. Chicken

In France, chickens are often sold with just the feathers removed. They fast for several days prior to slaughter in order to shrink the intestines and lengthen shelf life. In the U.S., chickens also fast, but they're eviscerated and dragged through a tank of ice-cold water as well. This removes a lot of flavor and adds water, thus hastening bacterial growth. Plastic wrap also keeps the moisture content high so that chicken, especially in summer, is sometimes nearly spoiled when you buy it. Spoilage occurs on all cut surfaces and between skin and flesh, but it's impossible to see. Only smelling will tell the tale. Coffee

Roasted coffee beans lose their flavor and aroma very quickly. Coffee merchants know this, and the more conscientious ones roast the beans themselves. Ground coffee loses its aroma within days; once open, a can or bag of ground coffee should be kept in the freezer to slow evaporation of its highly fragrant and volatile aroma. Cookies

Since cookies are high in sugar and low in water, they don't spoil from the growth of bacteria, yeast or molds. But their fats oxidize, giving them a rancid flavor. Cookies made with shortening or margarine turn rancid slowly, partly because these fats have been mixed with antioxidants (BHA and BHT) during their manufacture. But butter begins to turn rancid within 24 hours. Unfortunately, there's no fat that tastes as good as butter. To keep cookies fresh, store them in tightly sealed containers and away from sunlight. Airflow will cause rapid oxidation of the fats, as will sunlight's ultraviolet rays. Crackers

Like cookies, crackers don't spoil but they will turn rancid, although much more slowly since they have a lower fat content. British crackers in particular are very low in fats. But because there are still some fats present in flour (about one percent), they do eventually go. Cream

Today most dairies ultrapasteurize cream, a severe treatment that lengthens shelf life but also damages the proteins to such an extent that the cream whips poorly, producing a watery, unstable foam. The cream also smells like boiled milk due to the presence of hydrogen sulfide gas. There are a few dairies that still pasteurize cream the old way (at lower temperatures), but poor quality control on the farm and at the dairy may make the cream spoil within days of purchase. Eggs

Theoretically, a fresh egg could be kept a thousand years without spoiling because the interior is and remains sterile. Freshness, however, doesn't depend on sterility but on the loss of carbon dioxide during storage. The egg shell is perforated with thousands of holes that enable the chick to breathe. As carbon dioxide is lost, the yolk's membrane becomes fragile and the white becomes watery. In many European bakeries, eggs are not kept in the refrigerator and some spoilage sometimes does occur because bacteria are able to enter the shell through minute cracks. Fish

Fish often spoil enzymatically before bacterial growth can even occur since the flesh is very watery and its proteins are easily split, liberating ammonia. Some fish smell of ammonia only hours after being caught, making the smell an unreliable way of gauging freshness. The signs of a freshly caught fish are firm, protruding eyes; pink-to-red gills and firm flesh. Fish must be gutted immediately after being caught; otherwise, the gut enzymes soften the tissue within hours. Frozen fish rarely spoils from bacterial growth but develops off-flavors from the oxidation of fats, which are more easily oxidized than other animal fats. Fruit Juice

Freshly pressed fruit juices lose their freshness in two days. First, they gradually lose volatiles, some of which evaporate within minutes of pressing. (If you can smell it, you're detecting loss of volatiles). Second, they ferment. Almost instantly, bacteria and yeast begin to convert sugars into acids and alcohols, which are further oxidized by oxygen into aldehydes and ketones, the fetid-smelling and nasty flavors of spoiled fruits.

Frozen juices, though made with sophisticated machinery capable of trapping many of the aromas and flavors, still do not contain the most volatile compounds. That's why they don't have the depth of flavor of a freshly pressed juice. Garlic

Like tulip bulbs, garlic buds keep for a very long time, especially if refrigerated. When freshly picked, the parchment around garlic bulbs is soft and moist and the inside has a delicate flavor. As the bulbs age, moisture evaporates and the flavors concentrate. Freshly picked garlic is eaten raw in Italy and southern France. Herbs -- Dried

Dried herbs are usually grown in the U.S. or in Central America, thus enabling the spice merchants to better control freshness than if the herbs were shipped longer distances. Still, dried herbs are usually months old and shouldn't be kept more than a year. Otherwise, they begin to resemble cattle fodder. Herbs -- Fresh

Parsley, cilantro, watercress and other herbs should be washed after purchase, wrapped in moist paper towels and put loosely in plastic bags. If they're kept in closed bags, bacteria rapidly reduce them to pungent mush, which, though not dangerous, tastes like wet lawn. To preserve the most flavor, wash parsley under running water, vigorously shake it dry and chop fine with a sharp knife. Many French restaurant cooks and chefs have acquired the bad habit of squeezing the chopped parsley dry in a cloth towel, depriving it of flavor as well as many of its vitamins and minerals. Freshly chopped herbs should be kept cold--in an ice bath or refrigerator. Don't keep them in the chopped state for more than one day. Jam and Jelly

Preserves never spoil from the growth of bacteria or yeast, and they retain their aroma and flavor for years. However, they will mold if the jars aren't filled high enough or if the jam or jelly isn't covered with a tight paraffin seal. The mold is harmless and should be spooned off. Never store these in direct sunlight since the sun's rays will cause the plant pigments to break down, a frequent fate of strawberry jam. Meat -- Cooked

Pan-fried pork, beef and lamb don't keep well when stored in the refrigerator because they have a high fat content and turn rancid quickly. Their protein also picks up refrigerator odors quickly. Roasted cuts keep better because they're thicker and less fat is exposed to oxygen. Sliced roast beef, however, turns rancid quickly. Chicken, once cooked, keeps best of all the meats because it has very little fat. Meat -- Ground

Whether beef, lamb or chicken, ground meat spoils at a very rapid rate. The bacterial count of ground meat is usually about 100,000 per gram at time of purchase and can quickly multiply to 1,000,000 per gram by the time the meat reaches the refrigerator. It isn't wise to freeze ground meat or keep it more than one day. You can salt and spice the ground meat to slow bacterial growth, but you can only kill bacteria by rapid cooking. Ground meat begins to smell and taste funky somewhere between 1,000,000 and 10,000,000 bacteria per gram. This level can be reached as soon as two hours after purchase if the store's meat is old, the day is warm and your refrigerator is full and struggling. Assuming that your hamburger meat has 100,000 bacteria per gram, a six-ounce hamburger patty contains 170,250,000 living, breathing bacteria just before being cooked. Milk

In the olden days, milk was spoiled by bacteria introduced by the cow's udder and the milker's hands. Usually the lactic acid bacteria predominated, turning the milk sour. As a result, many recipes evolved using sour milk and baking soda. Now milk is stored in giant stainless steel tanks. It can be days, even a week old before the milk truck comes. Refrigeration kills the lactic acid bacteria, allowing cold-tolerant bacteria (the psychrophiles), to predominate. These bacteria break down the protein into bitter compounds. Thus, when milk spoils now, it's usually bitter and not much good for anything. Today's milk is also much higher in fat-splitting enzymes that will eventually create a rancid flavor. Onions

Hot onions keep much better than sweet, and they keep longest in the hydrator, shortest when left in their plastic bags at room temperature. When onion cells are cut open, a chemical reaction occurs instantly, producing a number of sulfur-containing compounds, some of which rapidly evaporate while others form onion oil, an essential oil that contains 16 fragrant compounds. Onion oil makes up less than 1 percent of the onion's weight and possesses a strength 4,000 times an equivalent amount of chopped onion. Pickles

Today's pickles are made from cucumbers stored in a concentrated brine (salt-stock pickles), then soaked in water to remove much of the salt and mixed with a spiced brine or marinade. They rarely ferment a second time in this marinade. If they do, the liquid becomes cloudy. There's no change in flavor, nor is there any danger of food poisoning. Pork

Pork spoils almost as easily as chicken, especially from today's lean hogs. In the days when we were blissfully unaware of saturated fats and midriffs were kept under control because we walked to work, pork was a much fattier meat. Since the protein content of fat or lean meat is the same, the water content of pork was much lower and the meat, consequently, didn't spoil quite as fast.

The plastic wrap around pork also hastens spoilage by retaining moisture, keeping the surface wet and preventing the contact of oxygen. On the other hand, because plastic wrap keeps out oxygen, it also slows the development of rancidity. Roots

Turnips, rutabagas and beets all keep for a long time because of their relatively low moisture contents and tough skins. Even if they shrivel a little, they still retain most of their original flavors. They can be stored at room temperature or in the refrigerator. Soup

Homemade soups are reservoirs of wonderful aromas. They're also very hospitable to guest bacteria. A freshly prepared soup that is to be kept for another meal should be cooled first in a two-or-three-inch-deep bath of running cold water, then tightly covered and stored in the back of the refrigerator. Soups do not lose much aroma or flavor during storage and usually give off the same magnificent olfactory and taste sensations when reheated. Spices

These are usually highly complex mixtures of aromatic compounds, most of which don't evaporate that quickly. Most spices, if kept tightly sealed, retain much of their flavor and aroma for a year after purchase. Cloves, nutmeg and most seeds retain their flavor and aroma much longer. Of course, really fresh spices are best. Small spice merchants who do enough business to order directly from importers usually sell the freshest spices. The big companies carefully monitor the freshness of what they put in jars but, in order to obtain the best prices, they often keep a year's supply on hand. Spinach

Like fresh herbs, spinach and other leafy greens must be removed from plastic before storing in the hydrator. Even though it's less convenient, loose spinach is usually fresher than bagged spinach. When it's cooped up inside a plastic bag, the leaves can't respire as easily and some of the cells die and begin to rot. The flavor of spinach, which seems to intensify when cooked (because of the concentration of flavors owing to evaporation of moisture), is best preserved by cooking it in the little bit of water clinging to the leaves.