DEAR DR. BLONZ: My cooking class instructor mentioned that broths or gravies should never be covered with an airtight lid until they are cold, because botulism toxins can form in anything lukewarm and covered tightly. How should one handle hot broth that you want to keep for a day or two? -- F.C., Sacramento, California

DEAR F.C.: Botulism is an often-fatal form of food poisoning. It's caused by eating foods that contain a toxin from the bacteria C. botulinum. These bacteria multiply by giving off spores, which produce the botulism toxin.

C. botulinum differ from other bacteria, such as E. coli, listeria and salmonella, in that they can grow without air (oxygen). In addition, the botulism-producing spores are heat-resistant. Food preservation techniques such as canning must include a heat treatment that lasts long enough to kill C. botulinum, knocking out the ability to produce spores. Ten minutes of boiling will destroy the toxin that causes the disease. Outbreaks of botulism from canned foods are usually traced to inadequate heating times or temperatures.

Placing food in an airtight container does not increase the risk of botulism unless the bacteria are already present. It's doubtful that these particular bacteria would be a problem in cooked foods when good sanitary practices have been followed. In fact, leaving the lid off increases the chances that other airborne bacteria can enter any nutrient-rich food or broth. It also increases the likelihood of picking up errant room or refrigerator odors.

It's a good practice to refrigerate leftovers immediately in a sealed container. By rapidly cooling leftovers to refrigerator temperature (40 degrees F.), you help ensure that the food will remain safe. Consider cooling your broth in a covered container with a large surface area, such as a shallow baking dish. One technique I use is to have several flexible, reusable ice packs at the ready in my freezer. That way, when some food needs to be chilled rapidly, the ice packs can be placed under, around or on top of the container.

Other sensible approaches are never eating or tasting from containers with bulges, or from any that spurt liquid when opened. (Note that this is different from vacuum-packed items, which suck in air when opened for the first time.) Find out more about botulism at b.link/hv67cc. For a general reference on food poisoning, you can download the second edition of the FDA's Bad Bug Book at b.link/yg7nrk.

Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 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: Do you know if there is any special value in drinking those "green powder" supplements, other than supplying antioxidants? The company says the nutrients in their greens are more efficiently absorbed than other supplements because they are in a powder form. After two months of taking the product, I realized there was no effect I could notice.

I want to know if this is the best thing to take if I'm not eating enough vegetables in my daily/weekly diet. -- R.R., Tulsa, Oklahoma

DEAR R.R.: I don't support the use of supplements as a substitute for good eating. Most of those powders are made from dried grasses, although there are many types on the market, and they tend to supply a modicum of nutrients and phytochemicals. If essential nutrients, including dietary fiber, are present in measurable amounts, they must be listed on the Nutrition Facts (or Supplement Facts) label. Check the details on your product's ingredient statement; make sure you know what you are paying for.

Aside from testimonials, there is little clinical evidence to support any special benefits -- such as preventing or treating disease, or being significantly more effective due to quicker absorption -- from powdered supplements. There is likely to be a grassy taste from such products, but they should be safe to consume if made following the extensive good manufacturing practices of the FDA.

However, such supplements do not "fill in the blanks" for one who routinely comes up short on healthful eating. You may want to rethink that "if I'm not eating enough" stance as acceptable for your well-being.

Then there's the fact that supplements are usually costly. You are paying for the manufacturer to use energy to dehydrate and process real food into a powder, perhaps add some vitamins, minerals and herbs, then package, ship and market the finished product. Why not simply eat real, whole foods from your local produce market?

You ask about "the best thing to take," and here is my answer: The best thing is a plant-focused (but not necessarily vegetarian) whole-foods diet, with daily servings of physical activity. Think of what you have to gain from such an approach, then begin gradually and set interim goals. I don't know where you are in life, but isn't this a value you would want to pass on to your next generation? It is all a matter of priorities, and the payoff is one you will have an opportunity to appreciate as you get older.

Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 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 concerned about using aluminum cookware and foil, as they present a danger of aluminum leaching into my food and causing health problems like Alzheimer's or autism. -- H.G., Chicago

DEAR H.G.: Are we in some danger zone where we must avoid using aluminum cookware and even aluminum foil? First things first: Any suggestion that aluminum exposure through our food can cause serious health issues like Alzheimer's, autism, dementia or cancer is not based on known facts.

Let's address this oft-aired concern with a bit of rigor, covering the following: 1. the amount of aluminum naturally present in foods; 2. how efficiently the aluminum we ingest is absorbed (its bioavailability); 3. the established safety tolerance for aluminum, and 4. what would be the weekly intake of aluminum if all foods were cooked in aluminum cookware, stored in aluminum containers and wrapped using aluminum foil. How would that level of exposure compare with the established safety tolerance limits?

Let's address those points in order:

1. Aluminum is the third-most abundant of all elements in the Earth's crust and is naturally present in foods. You cannot avoid exposure, whether your food is organic or conventionally grown. The daily intake of naturally present aluminum is about 3 milligrams per day, and processed foods and medications containing aluminum additives can add to that total. Another concern, of course, would be for any exposures at work or where one lives, but that is a separate issue.

2. The aluminum in our food has minimal bioavailability; less than one-tenth of 1% (0.01%) of ingested aluminum gets absorbed, most passing in the feces. Any absorbed will get handled by the kidneys, so those with impaired kidney function will be at greater risk.

3. Safe levels of aluminum intake for most individuals, referred to as the upper safety tolerances, have been established by various agencies and vary by age, sex and health status. The Environmental Protection Agency has its reference dose set at 1 milligram per kilogram of body weight per day. The joint U.N./World Health Organization expert committee came up with similar findings. This level equates to a safety tolerance of about 3.2 milligrams of aluminum for every pound of body weight per week. For a 150-pound individual, the safety tolerance would be 480 mg of aluminum per week.

4. The classic study published in the journal Food Additives and Contaminants (Jan./Feb. 1995) reported that if you were to take an entire week's food supply, store it in aluminum containers, prepare it in aluminum cookware and wrap it in foil, the total aluminum intake from these exposures would be approximately 42 milligrams of additional aluminum per week. Even after adding the aluminum naturally present in the food, you would still be at about one-tenth of the tolerable limit.

These numbers tell the story that the levels of aluminum in the food -- and that which would be added if one were to use aluminum cookware, containers and foil -- are well below the level at which any harm would be anticipated.

Finally, never lose sight of the message that an active lifestyle complemented by a whole-food, plant-based (not necessarily vegan) diet facilitates our overall health in so many ways -- one of which is to keep systems involved with disposing of unwanteds like aluminum on their game.

Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 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.