A quest to understand the spices of life: the antimicrobial powers of spices

Whether your taste buds draw you to a bowl of spicy Thai shrimp seasoned with cumin and paprika, or if you lean more towards the mild flavors of Swedish meatballs with dill and parsley, you have probably recognized that traditional menus from tropical climates tend to be spicier than those of cooler climates. But have you ever wondered why?

Figure 1. Paul Sherman: Spice Researcher. Spices were a change of pace for evolutionary biologist Paul Sherman, who usually focuses his research on natural selection and animal social behavior and is famous for his work on naked mole rats. Photo Courtesy of Frank DiMeo/Cornell University Photography.

Figure 1. Paul Sherman: Spice Researcher. Spices were a change of pace for evolutionary biologist Paul Sherman, who usually focuses his research on natural selection and animal social behavior and is famous for his work on naked mole rats. Photo Courtesy of Frank DiMeo/Cornell University Photography.

For thousands of years, people from various cultures around the world have added spices to their food for both preparation and preservation. At first, spice use was restricted to native plants, but by the Middle Ages, explorers braved the sea just to bring back highly treasured spices from other countries. Today, spices can be found everywhere, but one can still ask why we ever decided to use spices in our food in the first place. This idea sparked the interest of Paul Sherman, professor of neurobiology and behavior, and graduate student Jennifer Billings, both researchers at Cornell University, who poured over archived cookbooks containing traditional recipes from over 30 countries around the world to see if they could find a pattern in the use of spices in hopes that they would find an answer to why spice use evolved in human cultures.

Sherman and Billings opted to limit their search to meat-based recipes only, mainly because there are simply more recipes for meat dishes than vegetarian dishes, which would allow them to obtain adequate sample sizes. After counting the ingredients of over 4,500 recipes and summarizing the data, it became apparent that spice use around the world is not random. Recipes from countries with hot climates used, on average, more spices per dish than countries with cooler climates. Also, while the majority of spices were used infrequently, some were consistently used more often than others, such as black pepper, onion, garlic, chilies, lemon and lime juice, parsley, and ginger.

 Figure 2. Spices come from various woody shrubs and vines, trees, aromatic lichens, and the roots, flowers, seeds, and fruits of herbaceous plants. Cookbooks generally do not distinguish between herbs, which are usually used in their fresh state, and spices, which are generally dried before use. Photo courtesy of Thomas Neuhaus, Neuhaus Features.

Figure 2. Spices come from various woody shrubs and vines, trees, aromatic lichens, and the roots, flowers, seeds, and fruits of herbaceous plants. Cookbooks generally do not distinguish between herbs, which are usually used in their fresh state, and spices, which are generally dried before use. Photo courtesy of Thomas Neuhaus, Neuhaus Features.

So why do some like it hot? Sherman and Billings believe spice use is an evolutionary adaptation. The proximate (or immediate) reason we probably began adding spices to our food was simply because they added color, flavor, and made the food taste better. But the ultimate (evolutionary-based) reason is not that simple.

Before electrical refrigeration, food spoilage was a huge problem, especially in countries with hot climates. The same bacteria that attack meat and other foods and can make us sick also attack plants, including the very herbs and spices we consume. These plants have evolved mechanisms for inhibiting bacteria and their poisonous toxins, which include producing phytochemicals as a defense. Laboratory experiments have shown that phytochemicals from some spices have extremely powerful antibacterial abilities; garlic, onion, allspice, and oregano inhibited or killed every bacterium they were faced with; 15 of the 30 different spices tested inhibited at least 75% of the bacteria. Sherman and Billings found that as the mean annual temperature of a country increased, so did the number of spices per recipe. More specifically, the use of the most inhibitory spices (garlic, onion, chilies, etc.) increased with temperature. So without realizing it, people who began adding tasty spices to their meat were actually preventing food borne illnesses by exploiting the antimicrobial properties of spices.

 Figure 3. Garlic and onion have powerful antimicrobial properties and grow in every country that was sampled in the study by Sherman and Billings. (Courtesy Stock.XCHNG).

Figure 3. Garlic and onion have powerful antimicrobial properties and grow in every country that was sampled in the study by Sherman and Billings. (Courtesy Stock.XCHNG).

"Traits that are beneficial are transmitted both culturally and genetically, and that includes taste receptors in our mouths and our taste for certain flavors. People who enjoyed food with antibacterial spices probably were healthier, especially in hot climates. They lived longer and left more offspring. And they taught their offspring and others: 'This is how to cook a mastodon.' We believe the ultimate reason for using spices is to kill food-borne bacteria and fungi," says Sherman.

Can you add spices at any point during the cooking period and still get the antimicrobial benefits? Research suggests that "thermostable" spices, those added at the beginning of cooking (e.g., garlic, pepper, and onion) don't lose their flavor when heated (in fact, their flavor is enhanced), and probably are able to fight bacteria in heat also. Conversely, "thermolabile" spices (parsley, cilantro, etc) break down when heated and are usually added as a garnish at the end of cooking; presumably their beneficial properties function after heating but before food is served.

 Figure 4. Relationship between mean annual temperature of the countries studied and the average number of spices called for per recipe of that country. As these data show, spice use increases with temperature. (Data modified from Billing and Sherman, 1998).

Figure 4. Relationship between mean annual temperature of the countries studied and the average number of spices called for per recipe of that country. As these data show, spice use increases with temperature. (Data modified from Billing and Sherman, 1998).

Two spices that don't fit into either of the above categories are pepper and lemon juice; they are the most frequently added spices, but their use does not increase based on climate temperature like other spices and they aren't very good at inhibiting bacteria. This fact does not weaken the antimicrobial hypothesis, however; Sherman and Billings believe they act as "synergists", working to increase the antimicrobial effects of other spices using their own special characteristics. Pepper increases the rate at which bacteria absorb the phytochemicals of plants. The citric acid from lemon and lime juice, with its low pH, breaks down the cell membranes of bacteria, similar to the effects of heating. This means that foods to which lemon and lime juice are added don't need to be heated as much to obtain the same antibacterial effects.

 Figure 5. Antimicrobial properties of 30 spices. The left-most spices represent the greatest inhibition of bacteria, with garlic, onion, allspice, and oregano all killing 1.0 (100%) of bacteria. The top line indicates all spices above that point kill at least 75% of bacteria, the next line indicating at least 50% of all bacteria inhibited. (Sherman and Billing, Bioscience).

Figure 5. Antimicrobial properties of 30 spices. The left-most spices represent the greatest inhibition of bacteria, with garlic, onion, allspice, and oregano all killing 1.0 (100%) of bacteria. The top line indicates all spices above that point kill at least 75% of bacteria, the next line indicating at least 50% of all bacteria inhibited. (Sherman and Billing, Bioscience).

By taking advantage of plants' defensive compounds and putting them to use in our cooking, early human cultures were able to kill bacteria in their food and live longer, healthier lives. While not everyone is convinced by this evidence, the idea that natural selection played a role in selecting for human cultures that came to use spices in their food to prevent food poisoning is an intriguing one, to say the least.

"Everything we do with food -- drying, cooking, smoking, salting or adding spices -- is an attempt to keep from being poisoned by our microscopic competitors." says Sherman, "They're constantly mutating and evolving to stay ahead of us. One way we reduce food-borne illnesses is to add another spice to the recipe. Of course that makes the food taste different, and the people who learn to like the new taste are healthier for it."

Further reading:

Takikawa, Akiko , et al. Antimicrobial activity of Nutmeg against Escherichia coli O157. Journal of Bioscience & Bioengineering; Oct 2002, Vol. 94 Issue 4, p315.

Hunter, Beatrice. Food for Thought. Consumers' Research Magazine, Dec2000, Vol. 83 Issue 12, p8.

Sherman, Paul W.; Billing, Jennifer. Darwinian Gastronomy: Why We Use Spices. Bioscience, Jun99, Vol. 49 Issue 6, p453.

Chemical evidence for the apparent health benefits of herbs and spices] by Carolyn Fisher, a quality insurance manager at the Global Industrial Group of McCormick. Cultured Inquiry: students learn about the scientific process and gain critical skills through disc diffusion assays.
JYI staff members in the Research Department help student authors every step of the writing, editing, and peer-review process.
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