KNIFE LORE NUMBER 60 JULY 1993 BERNARD LEVINE'S KNIFE LORE (c)1993 Bernard Levine, exclusive to the National Knife Magazine NKCA WOOD versus PLASTIC Strictly speaking this story is not about knives. It is about cutting boards. Yet cutting boards and knives are intimately associated. Furthermore, the implications of this story for knives bear serious contemplation. There are two basic types of cutting boards in use today, wood and plastic. Plastic boards are a good deal harder on the cutting edges of kitchen and butcher knives than are wooden boards, but plastic has been growing in popularity because it seems to be more sanitary. The crucial word in the preceding sentence is "seems." Scientists at the Food Research Institute, Department of Food Microbiology and Toxicology, University of Wisconsin at Madison, began research to try to find a way to make wooden cutting boards just as sanitary as plastic cutting boards *seem* to be. Later, a distributor of both wooden and plastic boards, concerned about the safety of his products, raised funds in his industry, to help underwrite the research. The scientists in charge of the investigation were Dr. Dean Cliver and Dr. Nese Ak, both microbiologists. Their findings were so startling that they made national headlines, through an Associated Press story written by Mary MacVean, and distributed on March 10, 1993. Subsequently Dr. Cliver sent me a copy of his and Dr. Ak's five-page preliminary report, which has not yet been published, and he explained to me over the telephone some of the details of their research. THE SAMPLES The first step was to make standardized test samples for comparing wood and plastic. The scientists cut 2 inch by 2 inch pieces from cutting boards made of the following listed materials. For some of these materials they also included pieces of cutting boards that had long been in use, either in homes, in grocery stores, or in biology laboratories. WOOD: hard maple (both face grain and end grain), American black walnut, ash, basswood, beech, birch, black cherry, butternut, oak. POLYMER: polyacrylic, polyethylene, polypropylene, polystyrene, hard rubber. Within the wood group there were no significant differences in performance among tree species, nor were there differences between bare wood boards and those treated with *mineral* oil, nor between rough boards and sanded boards. However, in one important set of tests, the previously used wooden boards were measurably superior to the brand new ones. Similarly, within the polymer group, there were no significant differences between types of plastic. However, in most of the tests, new plastic boards were significantly superior to used ones. The startling and dramatic differences in performance in all the experiments were between the wood group, generally, and the plastic group, generally. Those differences were both profound and unambiguous, but they were not the differences that anyone had anticipated. THE EXPERIMENTS In the first series of experiments, each sample board was washed down with culture medium infected with a known quantity of *Escherichia coli* (human intestinal bacteria). Then the boards were allowed to sit under a variety of controlled conditions, and for a variety of controlled times, after which they were rinsed down with sterile culture medium. Then this medium was examined for surviving bacteria. Three minutes after initial contamination, at room temperature, all the bacteria on the plastic boards were still alive and active. In that same brief time, more than 99.9% of the bacteria on the wooden boards had died. Twelve hours later, still at room temperature, the bacteria count on the plastic boards had increased. After the same twelve hours, no bacteria at all could be found on the wooden boards. The same experiments were also carried out under refrigeration (39 degrees Fahrenheit), and under both high and normal humidity. The results were all virtually identical. Next, and I quote, "To measure the bacteria-absorbing and holding capacity of wood under extreme conditions, high levels of inoculum (more than a million per milliliter) were applied three days in a row, with overnight incubation under high humidity conditions, and with no attempt to clean the contaminated surfaces. Although some bacteria were recovered in this set of experiments, the percent recovery was extremely low; but when the same experiment was repeated with plastic blocks, growth of the test microorganisms was observed under these conditions." [D. O. Cliver, personal communication.] RAISING THE STAKES After this, the scientists' next tactic was to try more virulent types of food poisoning bacteria. First they repeated the experiments with a hardier strain of *E. coli.* Then they tried *Listeria monocytogenes*, and finally *Salmonella typhimurium*. The *Listeria* had a slightly higher survival rate than the other organisms, but essentially the results were unchanged. Next they repeated the trials using a "natural" contaminant, the liquid from packages of fresh chicken purchased at a supermarket. In this test, the bacteria naturally present in fresh chicken juices multiplied on the *new* wooden cutting boards about 16-fold (four generations) after twelve hours. On the plastic boards (both new and used) the bacteria multiplied between 1,000-fold and 1,000,000-fold in the same overnight period. Meanwhile, on the *used* wooden cutting boards, all of the bacteria died. As a control to this part of the trial, chicken juice was filter sterilized, which removes all the microorganisms without altering the chemistry of the medium. This sterile juice was then contaminated with the known strains of bacteria. The results of these tests were essentially the same as those in the first series, which used laboratory culture medium. GREASE Next, to add further realism, all sample boards were rubbed down with fresh chicken fat. Then they were ultraviolet sterilized, to kill any bacteria present in the fat. Then the boards were contaminated as before. On those wooden blocks which absorbed all of the contaminant overnight, despite the fat coating, the bacteria count declined by more than 99.9%. By contrast, on all the plastic blocks, and on those wooden blocks where the contaminant remained pooled on the surface overnight, the bacteria count increased. THEORY This last result has an interesting implication. It suggests that the naturally absorbent quality of wood is a key factor in the inability of bacteria to survive on it. The result also suggests the importance of cleaning grease off of wooden cutting boards thoroughly and promptly. Drs. Cliver and Ak theorize that the pores in wood may trap and immobilize bacteria, and then the microorganisms would die rapidly as the wood dries. According to this theory, used wooden cutting boards outperform new ones perhaps because the numerous cuts on their surfaces would increase their natural ability to absorb juices, and thereby to speed evaporation. By contrast, grease on the surface of a wooden board would make that surface repel liquid, causing the greased wooden boards to perform similarly to plastic, which always repels liquids. Further research would be needed to test this theory, and to discover exactly how wooden cutting boards are so deadly to bacteria. Although all plastic materials naturally repel liquids, the cuts on the surfaces of *used* plastic cutting boards tend to trap liquids, and thereby to slow down their evaporation. This seems to be why plastic cutting boards, especially used ones, provide an ideal environment for bacteria to multiply. Indeed, the researchers found that it is very difficult to sterilize the cut surfaces of used plastic cutting boards. A strong penetrating bleach solution is necessary, or a full cycle in a commercial dishwasher, for complete sterilization of a plastic board. Hand washing in hot soapy water is not enough to sterilize a used plastic board, but it is more than enough for a wooden board. WHAT ABOUT KNIVES? I asked Dr. Cliver what might be the implications of his research for wood versus plastic on knife handles. After all, the government now requires butchers to use knives with plastic handles for "sanitary" reasons, despite the fact that slippery plastic handles cause many more accidents than traditional wooden handles. "Weren't those government regulations based on scientific research?" I asked him. Evidently they were not. Dr. Cliver has not been able to find any prior published research comparing wood with plastic in any area of food processing. As far as he can discover, the current knife and cutting board regulations were based on "common sense." Plastic is "obviously" more sanitary than wood. ENTER POLITICS, STAGE LEFT Now that regulations requiring plastic are in place, it would take an Act of Congress to change the rules. Don't hold your breath waiting for that to happen. Children will continue to die of *E. coli* and *Salmonella* poisoning, and millions of us will frequently be infected (vomiting within an hour of a meal, or diarrhea within six hours, are the usual symptoms. Flu-like symptoms and sometimes death can ensue). Government committees will continue to wring their hands and hunt for scapegoats in each new mass food poisoning case, but nothing will change, because government regulations, once enacted, are sacred. Look at the asbestos abatement rules. We now know that asbestos in buildings is only hazardous when it is being broken up during abatement. However, government rules still require that it be abated, even when it would be harmless if left in place. Associated Press correspondent Mary MacVean, who wrote the March 10 feature, completely missed this aspect of the story -- that government regulations may be contributing to outbreaks of food poisoning. This should not be a surprise. Canon VII-A of the Media Creed states that "All government regulations are good." Canon VII-B says that "More government regulations are better." Dr. Cliver was of course reluctant to extend his results beyond cutting boards, such as to knife handles or anything else. He knows better than most of us the difference between "common sense" and scientific results. Dr. Cliver's research was only on cutting boards, and should not be extrapolated further. If a knife company, or perhaps a meat packer, the butchers' union, or even OSHA or the USDA, really wants to find out the story on wooden versus plastic knife handles, he and his team would be willing to do the research. Me, I am not a scientist. I am willing to go out on a limb. I strongly suspect that the results would be similar with knife handles -- that clean plain wood would be vastly more sanitary than any plastic. Indeed wooden handles might prove to be more sanitary than steel blades. The greatest uncertainty regarding knives is what happens in the junctions between blade and handle. On modern professional butcher knives, with molded in place plastic handles, the junction is small and usually well sealed. On knives with slab handles, whether of wood, plastic, or metal (such as Gerber Balance Plus), the gaps are very long. However, wooden handles are the most likely type to shrink and warp over time, due largely to repeated cleaning in hot water, causing the gaps to widen, and to trap grease and other food residue. Given all the potential sources of infection that surround us, knife handles are probably pretty low on the list. One potential source that I am a lot more concerned about is the way fresh meat is now packed for retail sale. It comes in slick *plastic* trays, which are usually lined with absorbent pads that are soggy with blood and juices, and which are all wrapped up in airtight *plastic* wrap. I don't know about you, but at my house all plastic-wrapped fresh meat from grocery stores now gets cooked very well done. To a non-scientist those slick styrofoam meat trays look suspiciously similar to plastic cutting boards -- and of course the butchers in the back room now use plastic handled knives and plastic cutting boards. It's not their fault. They are only following government orders. * * * ***END**