Food Protection Trends - October 2010 - 577

TABLE 1. Recoveries (Relative Light Units) of E. coli from cutting board surfaces washed 60 min after inoculation (10) Comparison Among wood species Material Birch Maple Oak Among plastics Foamed polypropylene High-density polyethylene Polystyrene Wood versus plastic Maple High-density polyethylene Foamed polypropylene E. coli recovered1 2,753 2,863 1,785 7,605 4,621 3,117 3,200 2,277 5,315

Data are averages of triplicate samples from one trial. Number of cells applied, (1.0 × 106 CFU, or 5.0 × 104 RLU). Std. Error: ± 5%
2. When they penetrate into a wood matrix, bacteria do not grow, but rather die in a matter of hours (10). The hygroscopic nature of wood, as well as the presence of secondary metabolites (tannins, lignin, flavonoids and, in the case of the pine family, terpenoids) directly inhibits the growth of bacteria (11). 3. Bacteria within a lacunar network of scraped and gouged plastic do multiply (10); the inevitable crevices on the surface of a used plastic pallet could well act as miniature havens for bacterial culture. 4. Ground wood powder or shavings has been shown to inhibit bacterial growth; ground plastic powder did not (10, 11). 5. Pine wood has a significantly greater inhibitory effect than other wood species tested (11). 6. The overall message from published literature reviews is that there is no advantage of plastic over wood in regard to food safety (5); if there is a difference, wood is safer than plastic. with detergent, recovery of E. coli was slightly higher from plastic than from wood (Table 1). Blocks were washed with detergent 60 min after inoculation; levels of live bacteria in swab samples were measured by ATP bioluminescence, using a GEM Biomedicals BG-P Optocomp 1 luminometer (Hamden, CT) and GEM Biomedicals ATP Surface Hygiene Monitoring Kit (Sparks, NV). In another set of experiments, carving knives were used in their normal cutting function, to determine whether bacteria beneath the surface could be recovered in this way (Table 2). Some of the inoculated block surfaces had been washed before recovery was attempted. Ak (2, 3) and her colleagues found that following inoculation of bacteria onto wood surfaces, recoverability of the microbes decreased significantly over the course of 3 to 60 min. During these experiments, bacteria were found inside the wood but did not multiply, and they were affected by an apparent antibacterial action of the wood, common to all of the species tested. This result was basically the same for new and used wood. However, scratches and grooves in used plastic cutting boards tended to harbor microbes even after washing. Schönwälder (11) examined survival of Escherichia coli and Enterococcus faecium on wood of several tree species and on polyethylene. Using agar contact plates placed on the test surfaces at various times after inoculation, they found a significant difference in bacterial recoveries from wood versus plastic. The number of recoverable bacteria decreased over time on both surfaces, but the decrease was faster on wood. Pine was the fastest, followed by beech, and finally plastic. In a second set of experiments, wooden blocks were submerged for 15 min in bacterial suspensions. The inoculum was absorbed to different degrees by the different types of wood. When the blocks were cut and inner surfaces tested, it was found that the inner portions of pine reduced viability of the bacteria. After 7–8 h, no bacteria could be cultured from the inner surfaces of unwashed pine blocks. From blocks of beech and poplar, levels of bacteria could be cultured over a 24-h period. In a third set of experiments, the effect of wood age was tested with boards taken from pallets used since 1987, 1994, and 1996. The bacterial reduction effect over time was consistent and independent of wood age. The authors attributed the antibacterial properties of wood to two factors: First, the hygroscopic nature of dried wood lowered the amount of moisture available to the bacteria. Second, the fact that bacterial recovery from pine was lower than from beech, poplar, or spruce was interpreted as being due to tannins, which are natural wood preservatives.

RECOVERy
Direct comparisons between several types of used wood and plastic cutting boards (10) showed that after washing

OCTOBER 2010 | FOOD PROTECTION TRENDS

577

Food Protection Trends - October 2010

Table of Contents for the Digital Edition of Food Protection Trends - October 2010