Botulism: the hard-boiled facts.

It's not unusual for families to show off their colorful and cleverly decorated Easter eggs in seasonal displays around the house. But depending on how those hard-boiled eggs were cooled, their unrefrigerated storage may pose a risk of botulism, according to researchers with the National Food Processors Association (NFPA) in Washington, D.C.

In its raw state, the egg has several antimicrobial defenses. The cuticle, or outside portion of the shell, "protects the eggs from bacterial invasion as long as this layer remains intact," note Linda Lubin, Dale Morton and Dane Bernard of NFPA in the July-August JOURNAL OF FOOD SCIENCE. Some researchers believe shell membranes may be an even more resistant bacterial barrier, they add. And the lysozyme enzyme in shell membranes and in egg white (albumen) destroys many bacteria.

However, the group notes, cooking not only inactivates the egg's lysozyme but also enlarges the shell's pores. But the most important breakdown in defense may occur when boiled eggs are cooled in water--a common practice. A natural contraction of the eggs during cooling creates an air pocket between the albumen and shell membranes "which apparently produces a vacuum which can draw in bacteria present in the cooling water," the researchers say.

In their experiment, they cooled hard-boiled eggs for 30 minutes in water incoulated with spores of Clostridium botulinum--strains that thrive at room temperature. Even water contaminated with as few as 10 spores per milliliter eventually resulted in botulism toxin being produced in the eggs.

Since these bacteria flourish in the absence of oxygen, it was not surprising that eggs stored anaerobically spoiled first--in two or three days. Those in tightly sealed plastic storage containers produced botulism toxin in about a week; eggs stored in the open spoiled about a day later. Though most contamination produced spoilage sufficiently obvious to warn off any unsuspecting eater, not all did. And the toxin contained in those seemingly harmless eggs was "sufficient to produce symptoms in adult humans," the researchers say.

The lesson in all this, say the researchers, is that air cooling is safer than water cooling. If eggs must be cooled in water, they should not be stored in airtight containers. Moreover, these eggs should be considered perishable and thus refrigerated.