Sticks and Stones

Wood wins in the dispute on hygiene

After having been consistently branded as unhygienic for many years, new research now shows that wood can be used in the food industry. These new results come from German laboratory experiments and practical research. The positive qualities of wood provide a basis for product development of packaging, and a Scandinavian project will possibly form part of that development.
For a long time, opinions have been divided when discussing the use of wood with food, for example, in packaging and in the preparation of food. The knowledge that our forebears had regarding the natural qualities of wood, i.e. its ability to prevent infection, has sunk into oblivion and in the meantime people have focused on the ability of steel and plastic products to be washed and disinfected.
For a while, people’s prejudices were so strong that, apart from other things, wood became excluded from the regulations and guidelines for the food industry, which recommended plastic products instead. Wood, which throughout history has been the natural choice for the packaging and processing of food, had largely fallen out of use.
Hygienic pinewood pallets

Many surveys have branded wood as less hygienic than plastic. Bacteria are kept on the surface of plastic (and steel), from which they can be removed by thorough cleaning and disinfecting. This is not possible with wood, which has a porous structure that makes cleaning more difficult. The question is whether wood has other qualities that compensate for this. In order to investigate this, it is important to base the research on the practical use of packaging.
In a promising laboratory experiment, pallets made out of pinewood were for four weeks compared to plastic pallets in normal use at a meat production factory. The wood pallets were not cleaned, but the pallets made out of synthetic materials were used in the normal way, which included routine cleaning.
After the four weeks, considerably fewer live bacteria were found on the wooden pallets than on the plastic pallets, even though there were traces of protein and fat on the wooden pallets due to their not having been cleaned.
After the four weeks, the pallets were stored for a few days and measured again. This time, the number of bacteria on the wooden pallets had further decreased whereas the number was the same on the plastic pallets. Consequently, bacteria are able to survive longer on synthetic material than they are on wood.
The great advantage with wooden pallets is that they can perform much better even if they are not cleaned immediately after use and are occasionally put into storage for some time.
Laboratory experiments

The main intention of the initial laboratory experiments was to repudiate the contention that wood was unhygienic and difficult to clean. In addition to this, the aim was to investigate what sort of wood functioned best and under which conditions. The future perspective is to find the best ways of using wood in the long term, but at the present moment, it is also important to overturn the current beliefs within the food sector and amongst legislators.
The experiments divide the test samples into categories: Pine performed best, then came oak, larch, spruce, followed by poplar, beech and maple (all of which are typical mid-European/Scandinavian species of tree). Next to this came the plastic product, polyethylene, which also came last.
The experiments also showed that the anti-bacterial effect was dependant upon the tree species, moisture content, number and types of bacteria and the room temperature.
The first experiments were carried out on fine wood shavings where the surface is very big. After that, compact surfaces were examined (consistent with practical use) to see whether the results were equally clear and whether the porous structure of the wood and its ability to absorb water was an influencing factor.
One recurrent assertion has been that bacteria in wood was just absorbed and continued living inside the wood. Particularly with pinewood, there was no evidence that this was true: Bacteria were certainly absorbed into the wood, but after a short time, the bacteria were destroyed on the surface and inside the pinewood. With the other wood species, the process of reducing the amount of bacteria took longer, but there was no increase in the amount of bacteria. None of the wood species appeared to have lesser hygienic qualities than plastic.
The anti-bacterial properties of wood: The causes and the connections

The constituents of wood are very important with regard to how effectively it will be able to destroy bacteria. Of the species that were examined, pine, oak and larch definitely contain antibacterial substances, whereas the number of viable bacteria remains constant for a longer period on poplar, maple and beech – and on synthetic substances.
The concentration of the wood’s constituents is less important than the type. So, heartwood of pine was more effective than sapwood, but in the heartwood the concentration of constituents was also greater.
Different types and quantities of bacteria are reduced with different speed: The greater the concentration of bacteria, the longer it takes to destroy them. In this case they are so to say forming layers and consequently only part of them come into contact with the disinfecting substances inside the wood at a time.
Constant exposure to germs was a particular problem for the plastic products. Even though pinewood had bacteria applied ten times to the same surface, the total number of germs fell. In contrast to this, the bacteria just accumulated on the plastic.