DEAR DR. BLONZ: Can you tell me how long eggs are still usable, after the best-by date on the carton? Are they safe to use in recipes? What about eggs purchased at a farmers' market, where there is no date on the carton? -- T.D., Lafayette, California

DEAR T.D.: Eggs that have a U.S. Department of Agriculture grading need to display the day of the year (from 1 to 365) on which the eggs were packed. Egg cartons often display an expiration date, as well -- the month and day after which they can no longer be sold, but are still safe to eat. Assuming they have been under refrigeration, you have about four to five weeks after the pack date during which the eggs are considered fresh and safe.

The expiration date will pass during this period. If purchasing eggs at a farmers' market, be sure to ask the vendor for the pack date. It is likely that there will be a dating system on any container for a perishable item such as eggs. Then make sure you note this date (or the date you purchased the eggs) on the carton you take home.

As time passes, eggs will gradually lose some of their qualities. In a fresh egg, the yolk is compact and stands high, and the white stays close to the yolk. As the weeks pass, the yolk and the egg white begin to spread, and the yolk sac becomes more fragile. Anyone who eats eggs sunny-side-up or over easy will notice that it doesn't take much to rupture the yolk of an older egg. These are the results of subtle changes in the egg albumin, the protein in the egg, but this does not have a significant impact on the egg's nutritional value. Over time, though, these changes can affect how an egg performs in certain recipes.

Use the freshest eggs for poaching or frying, as older ones will be more runny. If you want your egg to be whipped for volume, such as in a meringue or a cake, it's OK to use a "middle-aged" egg. Older eggs will perform fine if they are going to be used in a batter. If you are making hard-boiled eggs, older ones are preferred, as it is easier to take the shell off an older egg than a fresh one.

If the expiration date is long gone and you have concerns, it is always best to discard the eggs. If you do end up using eggs near, or just after, their expiration date, be sure to cook them well. That is always the best way to eliminate any bacteria that might have taken up residence. The USDA has a booklet on how to buy eggs, and there is more detailed information at eggsafety.org.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: I am interested in your thoughts about my new method of storing sour cream in the fridge. After it has been opened, I put the top on tightly, but store it upside-down to prevent air from getting in. I keep the inverted container on a small plate to keep drips from making a mess on the refrigerator shelf. I can't tell you exactly how much this increases the storage life, but I haven't had to throw out any sour cream since I started doing this. -- L.S., San Francisco

DEAR L.S.: Air exposure is what needs to be controlled. Lids should provide a good seal after the inner liner has been removed prior to the first use. Turning the container upside-down in the manner you describe might make a better air seal, but there is that increased risk of leakage, and that could represent a tradeoff. Odors or other contaminants could be picked up and passed back to the sour cream using the seeped liquid as its conduit. The priority should be buying the right size container for your typical use pattern.

DEAR DR. BLONZ: Are canned beans as nutritious as dry beans? The canned kind is more expensive, but less work. -- C.C., Baton Rouge, Louisiana

DEAR C.C.: Canned beans are just as nutritious as the dry kind. Many commercial products have added salt, but giving the beans a fresh-water rinse will remove a lot of that sodium. There are also low- and no-salt varieties available.

DEAR DR. BLONZ: Isn't it a bit misleading for foods to claim to be "nonfat" when they can contain large amounts of sugar? Doesn't sugar just become fat in the body, anyway? -- T.T., Sacramento, California

DEAR T.T.: The body has a constant need for energy, but we eat only a few times per day. We typically take in more energy and nutrients than we need at the moment, so our human machine has a finely tuned ability to put excess calories away until needed. A very small amount of carbohydrate is stored in the form of glycogen, but the vast majority gets stored as fat -- the most concentrated form. Food labels deal with what is in the food, not what happens once it is in the body.

DEAR DR. BLONZ: I am a cat owner and know that taurine is essential for cats. But I keep seeing it in energy drinks, and wanted to know why. -- S.H., via email

DEAR S.H.: Taurine is a compound known as a sulfonic acid. You are correct that it is needed by cats, but it's not essential for healthy human adults, as our bodies are able to make what they need. (One exception is that there is preliminary evidence that high levels of taurine may provide some benefit for individuals suffering from congestive heart failure.) The presence of taurine in energy drinks lacks scientific justification, as it is the caffeine in these drinks that provides the kick. It is unclear what taurine provides aside from "curb appeal," especially given the small amounts present.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: A local natural foods store recently held a seminar on cooking oils. The talk favored olive oil over alternatives, but said that when you open any bottle of cooking oil, such as corn, safflower, canola or peanut, the oxygen in the air immediately begins spoiling the oil. They said this was a "hidden cause of disease." They advised refrigerating all cooking oils after opening. However, product labels make no mention of any need to refrigerate. What do you recommend? -- B.T., San Diego

DEAR B.T.: Of course it is true that air gets in when you open the bottle, and that the oxygen in the air participates in the oxidation reactions associated with spoilage. However -- and this is the key -- it's a slow process under normal conditions, and there is no basis or evidence to fear that your oil, or your body, is at risk.

You can keep oils at room temperature, but you should follow some simple guidelines: Stored oils should be kept out of the sunlight and away from heat. And always keep the container sealed when not in use. Avoid buying amounts in sizes greater than you typically use in three to four months. But if you do, consider splitting the large size into smaller portions, refrigerating the unused bottles until needed.

There's no problem with storing oil in the refrigerator, but if you decide to go that route, be sure to have a tight seal on the container to keep the oil from picking up any undesirable refrigerator odors. Some oils may become cloudy when refrigerated, but this disappears when they return to room temperature, and it's no reflection on the oil's wholesomeness.

Oils can go rancid if they are mistreated or stored in the wrong way. Rancidity does indeed occur when an oil reacts with oxygen, and aside from giving foods an "off" taste, the consumption of oxidized oil does represent a health risk. The greater the degree of unsaturation (double bonds), the greater the tendency to oxidize. Omega-3 oils, such as flax or fish oil, have more double bonds than other oils, so they are very susceptible to spoilage. This helps explain the nasty aroma of fish left at room temperature for an extended period of time.

Our body has to deal with unwanted oxidation on a regular basis, and we have a series of systems designed to handle it. There are antioxidants produced in our body, for example, which are supplemented by those we consume in our diet. Eating well makes it so that we don't have to sweat the small stuff.

DEAR DR. BLONZ: Since I began a new multivitamin, my urine has become very yellow. Is this normal? I drink plenty of fluids, so I am not dehydrated. Does the yellow urine indicate an excess of bilirubin, and is taking the vitamins a strain on my liver? -- T.F., Butte, Montana

DEAR T.F.: I will have to assume that you are in otherwise good health. You should be comforted by the fact that it is quite common for B vitamins to increase the yellowness of urine. It happens all the time. B vitamins are yellow, and they color urine as they pass out of the body via the kidneys. This is not a sign of strain on the liver or the production of excess bilirubin (a waste product from the breakdown of hemoglobin).

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.