Vegetable 'immune systems' protect better against food spoilage

July 09, 2001

ITHACA, N.Y. -- When Cornell University biologists claimed, in 1998, that the traditional use of spices has a function other than making food taste good -- namely to help protect against more and more dangerous forms of foodborne microbes -- one thing was missing from their antimicrobial hypothesis. Now, the Cornell researchers have stirred in that missing ingredient, showing why vegetable-based recipes in 36 countries around the world are less spicy than meat-based dishes in the same societies.

"Without spoiling your lunch, let's just say that the cells of dead plants continue to be better protected against bacteria and fungi than are the cells of dead animals, whose immune systems cease to function at time of death," says researcher Paul W. Sherman. "Meat-based recipes that were developed over the centuries in hot climates need all the help they can get from antimicrobial spices, whereas foodborne pathogens are less of a problem in plant-based foods. Indeed, meat products are more often associated with outbreaks of foodborne illness than vegetables, especially in hot climates."

Sherman, a professor of neurobiology and behavior, and Cornell undergraduate student Geoffrey A. Hash report their latest findings in an article, "Why Vegetable Recipes Are Not Very Spicy," in the June 2001 issue of Evolution and Human Behavior (Vol. 72, pp. 147-163).

Previously, Sherman and another Cornell undergraduate biologist, Jennifer Billing, were the authors of a March 1998 report in Quarterly Review of Biology analyzing 4,570 recipes from 93 cookbooks. The recipes represented traditional, meat-based cuisines of 36 countries, and their survey took into account the temperature and precipitation levels of each country, the horticultural ranges of 43 spice plants and the antibacterial properties of each spice. Billing and Sherman concluded that the original function of spices, particularly in hot climates before the advent of refrigeration, was to kill or inhibit food-borne bacteria and fungi.

"But plants don't need as much protection after we cook them," Sherman says. "Although plants do not have a true immune system, they are protected against microorganisms by natural chemical compounds, sturdy cell walls containing cellulose and lignin, and low pH [higher acidity]. These chemical and mechanical defenses continue to ward off infection for a longer time after we kill the plant. Animals do have an immune system, and it becomes defunct when they die."

To test their antimicrobial hypothesis, the biologists needed to see if traditional vegetable-based recipes from the same countries -- where meat-based dishes are hot and spicy -- use significantly fewer spices. Their analysis of 2,129 vegetable recipes from 107 traditional cookbooks of 36 countries showed that while spice use in vegetables increases somewhat with higher ambient temperatures, cooks across the temperature range used far fewer spices on vegetables than meats. Of 41 spices (from onion, pepper and garlic to fennel and savory), 38 are used more frequently in meat-based recipes (the exceptions are sesame, caraway and sweet pepper, which offer little antimicrobial protection). And in all 36 countries, vegetable dishes call for fewer spices than meat dishes

The latest finding is more than academic, Sherman comments, noting that the chemicals in spices can be allergens, carcinogens, mutagens and abortifacients (causing spontaneous abortions). "That's probably why kids and pregnant women tend to avoid spicy foods," he suggests.

"Humans have always been in a co-evolutionary race with parasites and pathogens in foods, and our cookbooks are the written record of that race," Sherman says. "We haven't had to 'run' as hard when we ate vegetables. We haven't had to use extra pharmaceuticals to make vegetables safe for consumption."

Cornell University

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