DEAR DR. BLONZ: We recently had a close call with botulism and are lucky enough to be able to tell the tale. I have learned about its causes, symptoms and progress but have one unanswered question: How long is it safe to leave covered food in the refrigerator? I tend to cook one day a week, preparing soups and vegetables for later in the week. They are covered and refrigerated until it's time to eat them. Would it make a difference if they were uncovered? What other precautions should we take? Also, what risk is there to our dog from botulism? -- W.O., Albany, N.Y.

DEAR W.O.: First, some background for readers who are not as familiar with botulism as you now are. Botulism is a very severe type of food poisoning -- much worse than the flu or E. coli that we hear of periodically in the news. An amount of botulism toxin the size of a single crystal of salt can kill several people in less than hour.

To appreciate how botulism works, it helps to understand its three facets: the bacteria, its spores and the deadly toxin. The bacteria's name is Clostridium botulinum, it reproduces by giving off spores, and these spores are what produce the deadly toxin. Botulism is caused when an individual has the misfortune of ingesting foods contaminated by the toxin.

The bacteria are found everywhere and are usually present in soil. What is particularly insidious about these bacteria is that the spores can remain in a vegetative state for extended periods until they are either destroyed, or until conditions become right for them to grow.

The botulinum bacteria can grow in low-acid, oxygen-free environments, and they tend to be a particular problem with improperly processed canned foods, such as vegetables, fish and meat. One saving grace is that the toxin is not heat-stable. Food preservation techniques designed to control these bacteria must include a heat treatment. The botulism toxin can be inactivated when exposed to a temperature of 180 degree F for 10 minutes or longer. Eliminating the Clostridium botulinum bacteria entirely takes exposure to 212 degrees F for 10 minutes or longer. The spores are the most heat-tolerant, requiring exposure to 240 degrees F to destroy them.

The botulinum bacteria are also inhibited by an acidic environment, which is why acidic foods such as tomatoes, citrus and pickled foods tend to be safe. The bacteria are also thwarted by nitrite preservatives, which is why sodium nitrite is added to cured meat products.

In your situation, it becomes essential to follow good kitchen hygiene, scrubbing all foods before use and avoiding any cross-contamination with uncooked foods. Prepared foods should be immediately refrigerated (under 38 degrees F) in covered containers and used within three to five days.

Relying on a once-a-week cooking system is pushing it. You might also consider storing the later-in-the-week portion of your vegetables in the freezer, as toxin production from botulinum bacteria is inhibited at freezer temperatures (0 degrees F). As for your dog: Dogs are less at risk due to their more acidic digestive system. This allows them to drink, eat and lick items that are a bit less than sanitary.

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: One of my resolutions is to cut down on caffeine from all sources, but I'm finding that labels do not spell out how much caffeine is actually there. There are plenty of products claiming to be "99 percent caffeine-free." Does this mean the product still contains 1 percent of its caffeine, or that 99 percent of the ingredients listed are caffeine-free, or what? If the product had a bunch of caffeine and 99 percent was removed, doesn't this mean that there may still be enough to give a caffeine-sensitive person a minor jolt? -- N.O., Boston

DEAR N.O.: Let's go down your list of concerns to clear up the confusion. The "percent caffeine-free" claim relates to the caffeine in a product as a whole. Given the sensitivity of analytical devices, few ingredient-removal processes can claim to be 100 percent effective, and the decaffeination process is no exception. This means that irrespective of how much caffeine was originally present, there will always be a very small amount of it in a decaffeinated product.

A 6-ounce cup of coffee normally contains between 100 and 200 milligrams of caffeine (the finer the grind and longer the brew, the higher the number). For decaffeinated coffee, the amount drops to about 2 milligrams of caffeine -- hardly enough to cause even a minor case of the jitters.

Caffeine-free cola drinks make their claim because there was never any caffeine to start with. The caffeine gets added to the regular beverage, and is simply omitted for the caffeine-free version.

The ingredient statement on a food label will list caffeine whenever it is added to a product. This means you'll find it on caffeine-containing soft drinks, but it won't appear on the label for a product such as chocolate-covered espresso beans, where the caffeine is a natural component of the coffee bean. There is no requirement to state how much caffeine is present in a food; to get this information, you will have to consult the manufacturer or check the USDA Nutrient Database (ndb.nal.usda.gov), which lists the caffeine content for many foods.

DEAR DR. BLONZ: I am a vegetarian and would like some nutritional information on dates and raisins. Are they a good source of iron, or is it best to take a supplement? -- S.T., Chicago

DEAR S.T.: Dates and raisins do contain some iron, with a half cup containing 1 milligram and 1.5 milligrams of iron, respectively. If you enjoy dried fruits, you might consider figs, which contain just over 2 milligrams of iron per half cup. Other vegetarian sources of iron include dark green leafy vegetables, lentils and other legumes, blackstrap molasses, iron-fortified cereals and enriched breads. Assuming there are no health problems or absorption issues that would create a need to take a larger amount, a vegetarian should have no problem satisfying his or her iron requirement using foods alone. The National Institutes of Health has more information on iron in the diet at tinyurl.com/cksla.

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: You always speak about the importance of eating whole foods, but I keep seeing articles about the fact that our soils have become depleted of minerals. This depletion makes our foods less adequate to provide the nutrients we need than they were in decades past. In the survey course I took in agriculture when I was in college, I do recall that plants require certain nutrients to grow to their proper size, shape, color, form and taste. If this is the case, then it would seem that all the produce I see at the marketplace would have to have had their essential nutrients or they would not have grown -- or at least they would not look very good. Are those articles based on solid information, or is what I recall from the class more correct? -- O.F., Carlsbad, Calif.

DEAR O.F.: Your class recollections are on solid ground. Plants act as miners, pulling the mineral elements out of the soil that they need to grow. If a mineral needed by the plant is unavailable, the plant will not grow. It is incorrect to think that today's fruits, vegetables or grains will provide any less of their essential nutrients than the same varieties would have provided in the past. Plants synthesize their own vitamins, so these will be there as the plant grows.

Another layer to this question is the fact that nonessential minerals present in the soil can also end up in a plant. Whole foods grown in iodine-rich or selenium-rich soils, for example, can have more of these nutrients than the same type of food grown in soils with lesser amounts of these minerals.

Aside from the essential elements, what gets pulled in can vary from plant to plant, and mineral to mineral -- even among different varieties of the same fruit, vegetable or grain. The plant's overall nutrient content can also vary according to the time of the growing season and the length of time the plant has had to grow. This means that a plant picked green may not have the same total nutrient content as one allowed to ripen "on the vine."

It is difficult to speak with any statistical certainty, because we don't have records of nutrient content from the produce of a hundred years ago. Based on what we know, however, it's likely that the amounts would be comparable. Nutrient data tables, such as those at the USDA database (ndb.nal.usda.gov), present an average of what would be expected in a particular food.

DEAR DR. BLONZ: If the body can only absorb a limited portion of the minerals in a supplement, say 50 percent, and a supplement claims to supply 100 percent of the RDA, does that mean that you have to take two times the dose to actually absorb the RDA? -- S.M., San Francisco

DEAR S.M.: Dietary recommendations take the body's efficiency of absorption into account. They reflect the average amount an individual should be eating every day to satisfy the requirement. If, for example, science determines that the average body should have 100 milligrams of a certain mineral every day, and research indicated that we only absorb about 50 percent this mineral when consumed orally, then the recommended daily allowance (RDA) for that mineral would be 200 milligrams per day.

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.