Persistance and Survival of Pathogens in Dry Foods

ILSI Europe Report – 52 Page Full Report

Low-moisture foods and food ingredients, i.e., those appearing to be dry or that have been subjected to a drying process represent important nutritional constituents of human diets. Some of these foods are naturally low in moisture, such as cereals, honey and nuts, whereas others are produced from high-moisture foods that were deliberately submitted to drying (e.g., egg and milk powders). The addition of large amounts of salt or sugar can also be regarded as a ‘drying’ process by reducing the amount of water available for microbial growth.

Drying (removal of water) has been used since ancient times to preserve food. Although many pathogens and spoilage microorganisms can survive the drying process, this preservation technology is very effective because microbial growth will cease if water is no longer available for biological reactions. The water activity (aw) necessary to prevent growth of microorganisms, i.e., to inhibit physiological activities necessary for cell division, is 0.60 or less. If more water is available, some species of xerophilic spoilage moulds and osmophilic yeasts can grow at aw0.60 – 0.70; however, the minimum aw  for mycotoxin production by moulds is 0.80 with the majority not producing mycotoxins below a w 0.85 (Cousin et al., 2005). The minimum aw for growth of most bacteria is 0.87, although halophilic bacteria can grow at aw as low as 0.75. Among the pathogenic microorganisms, Staphylococcus aureus is particularly well-adapted to reduced moisture environments. Under optimal conditions it can grow at aw as low as 0.83 but in most foods the minimum is aw 0.85 (ICMSF, 1996). With this exception aside, in the context of this monograph, all foods and food ingredients that have an aw that prevents the growth of bacterial foodborne pathogens, i.e., with an aw of 0.85 or lower, are considered. These foods and ingredients are referred to as having low moisture or low aw. A wide range of products falls in this category: animal feeds such as fishmeal and pet foods, cereals, chocolate, cocoa powder, dried fruits and vegetables, egg powder, fermented dry sausage, flour, meal and grits, herbs, spices and condiments, honey, hydrolysed vegetable protein powder, meat powders, dried meat, milk powder, pasta, peanut butter, peanuts and tree nuts, powdered infant formula, rice and other grains, and seeds (e.g., sesame, melon, pumpkin, linseed). Although low moisture foods have some clear advantages with respect to food safety, there are nevertheless some major concerns:

• Many microorganisms, including pathogens, are able to survive drying processes. Once in a dried state, metabolism is greatly reduced, i.e., there is no growth but vegetative cells and spores may remain viable for several months or even years. They can often persist longer in low moisture foods and in dry food processing environments than in high-moisture foods and wet environments.

• It is often difficult or even impossible to eliminate pathogens from foods with low moisture by processes such as application of mild heat treatment (e.g., pasteurisation) or high hydrostatic pressure that work very well for high-moisture foods.

• Food processing environments, in which dried foods are handled, must be maintained at low humidity and kept dry, and this can give rise to problems in cleaning and sanitising, which are usually ‘wet’ procedures.

• Finally, it is of concern that consumers sometimes wrongly believe that low-moisture foods are sterile, which may lead to dangerous practices such as keeping reconstituted infant formula at ambient temperature for prolonged periods, thereby creating growth opportunities for pathogens such as Bacillus cereus and Cronobacter species.