The use of wood in contact with food
In all food preparation, whether in private kitchens or in the food industry, good hygiene is one of the most important ways of preventing the spread of illness through food.
It is also important that equipment and surfaces that come into contact with the raw materials or the finished product are clean. This means that equipment and surfaces must be easy to clean.
Food producers and kitchen suppliers have had various traditions in the types of material used for things like chopping boards, worktops and shelves. While it used to be common to use wood, in recent years there has been an increase in the use of steel, stone and plastic.
Norway does not yet have any specific regulations for the use of wood in contact with food. Denmark’s food safety authority – Fødevarestyrelsen – has concluded that the use of the Nordic types of wood, such as beech, oak, pine and ash, in contact with food is without risk. The food hygiene regulations state that surfaces that come into contact with food shall be smooth and washable and that it must be possible to disinfect them. Exemptions can be made in the production of traditional foods, so that coarse surfaces like woodwork can be used in contact with food. But the main prerequisite us that the food produced should be safe to eat. All kinds of woodwork must therefore be of a satisfactory hygienic standard and must be cleaned and disinfected regularly.
In 2009 Nofima Mat and SIFO, the national institute for consumer research, carried out a major survey among Norwegian consumers. One of the topics covered was the use of chopping boards in private kitchens for preparing meat and chicken. This found that 47% of Norwegian consumers use only plastic chopping boards, while 23% prefer wooden boards when preparing meat and chicken. 27% showed no preference and used both wood and plastic boards.
De Jong et al. found that cross infection from chopping boards could be effectively reduced by cleaning them with hot water (68 ºC), but they also found that the way in which the normal consumer cleaned chopping boards was generally not good enough to prevent cross infection.
In 1991, when the United States Department of Agriculture recommended students to avoid the use of wooden chipping boards in the kitchen, their grounds were that it was healthy common sense. But is it really the case that the risk of spreading infection is greater with wooden materials than with plastic or steel?
What counts against wood
The main argument for not using wood is that it is difficult to clean. Wooden boards tolerate less than plastic ones and will be more quickly ruined in the dishwasher, for example. Unlike plastic and steel, wood is also a porous material. This means that bacteria can get into the wood and survive there until perhaps being released and infecting food the next time the board is used. Several studies have confirmed that pathogenic bacteria can survive in wood. It has also been demonstrated that bacteria that have been absorbed into the wood can be released again when the wood becomes damp.
Other studies have also shown that the effect of certain disinfectants (including quaternary ammonium compounds) is lower on wood than on plastic and that the bacteria count remains high, even after washing. The study by Deza et al. also showed that the bacteria count on wood is not reduced by rinsing with water, while the same procedure can reduce the bacteria count on plastic by 99%. It has never been demonstrated that bacteria that lie hidden in the wood can infect knives that cut into the board.
What counts in wood’s favour
If we ignore the hygienic properties, wood has a number of obvious advantages compared with plastic or steel. Wood absorbs impacts better than many other materials and knives slip less, which increases safety and reduces wear on the knives. Wooden shelving has the property of being able to absorb and release moisture, which is of great significance when maturing cheese, for example. There are also arguments in favour of wood from the hygiene point of view. The main argument is that bacteria die more quickly on wood than on other surfaces. The first scientific studies to show that wood is not necessarily less hygienic than plastic came in 1994. The studies showed that fewer bacteria were demonstrated from wooden boards than from plastic ones. There has since been speculation that the reduced survival of bacteria in wooden materials may be due to drying out. Several studies have showed that bacteria die quickly in wood, while surviving well on other materials like plastic or glass. However other studies found no difference in survival rates on wood and other materials. In contrast to Dhaliwal, who wrote that the effect of disinfectants is lower on wood than on plastic, another group found later that bacteria on wood can be more easily killed with disinfectants than bacteria on glass or plastic. The hygienic properties of veneer were also investigated and found to be similar to those of plastic and stainless steel. The transfer of bacteria from wood to food has also been tested, and the studies showed less transfer from wood than from plastic.
Whether wood has an antimicrobial effect in itself has long been a subject for discussion. Some studies have compared different types of wood, and some indications have been found that bacteria survive less well on some types of wood, especially pine, than on others and that this effect is regardless of the age of the wood. Extracts of some hardwoods (especially white ash and also to some extent cherry and oak, but not many others) have also shown an antimicrobial effect in model trials. Other studies showed that bacteria have a better survival rate on hardwood than on softwood types, but it is not known whether this is due to the wood itself or to the pretreatment that the softwood has been exposed to.
Causes of uncertainty regarding the use of wood
The reason why different studies arrive at different answers when judging the risk of using wood is that different test conditions and analysis methods are used. A number of studies have looked closely at how external conditions affect bacterial survival on different materials. In general, we can say that bacterial survival is highest at low temperature and high humidity. At the same time, it has been shown that bacteria can be more easily washed off damp wood than dry wood. This is because damp wood is less able to absorb more liquid and thereby bacteria. Different washing methods can also affect the wood’s bacteria absorbing properties.
Another important factor that not many scientific publications have taken into account is which way the wood has been cut. The survival rates of bacteria are very different in wood that has been cut against the grain or with the grain. A study showed that liquid and bacteria penetrate much more deeply into wood that has been cut across the grain. The same study showed that wood that has been cut along the grain is easier to clean and shows almost the same cleaning properties as new plastic.
It has also been shown that bacteria survive better on used wood and plastic boards and shelves than on new ones. This is because the chopping boards get damaged and cut over time. Microscopic images have shown that bacteria accumulate in the cracks and unevenness in the material and are thus less easy to remove effectively with washing and disinfection.
Even though a good deal of research has been done in this area, there is still no easy answer to the question of whether wood is well suited for use in the food industry or in consumers’ kitchens. There are clearly many factors that affect the survival of bacteria in wood. This means it is not possible to give a final answer to the question of whether wood is suitable as a contact material for food. In some cases, conditions will be such that wood can represent an infection risk, while in other cases this risk can be as little or even less than using other materials. Whatever the case, quick, correct and regular cleaning of all materials is the most important prerequisite for keeping the risk of transferring infection low.
Contact the co-author of this article, Bjørn Schirmer, for a reading list.
Nofima Mat is currently working on two projects designed to provide more information about the use of wood in food production and handling. One of these projects, “Listeria in small scale cheese production” (Listeria i småskala osteproduksjon) is intended to tell us more about the risk of Listeria infection from small farmhouse cheesemakers. The use and significance of different materials is a central topic here and samples are being taken from producers with different traditions of material use.
The other project is aimed at the consumer and food safety in home kitchens. Our knowledge is currently limited regarding Norwegian consumers’ level of knowledge about handling food, their attitude to hygiene and their perceptions of the home as a possible source of food-borne infection. Our knowledge of hygiene levels and the incidence of pathogenic bacteria in Norwegian kitchens is also limited. The project “Better food safety in the home” (Bedre mattrygghet i hjemmet) will gather and analyse scientifically based data about consumer knowledge, attitudes and behaviour and hygiene levels in Norwegian homes. One of the things being focused on is the use and cleaning of chopping boards in private kitchens.