By Edyta Margas - 18 August 2011

There is growing concern for the potential presence and survival of Salmonella in low–water–activity ('dry') products. The problem could potentially affect a range of dry ingredients. Reports from the Food Standards Agency and the Rapid Alert System for Foods and Feeds (RASFF) concerning foods and ingredients such as: whole egg powder, coriander, soy bean meal, Tahini, minced dehydrated onion, sesame seeds, dried sage, peanut butter, black pepper, dried mushrooms and chocolate. In addition, there is concern that Salmonella can survive in dry environments for extended periods – potentially contaminating product from time to time.

Salmonella contamination in low aw (water activity) products could be better controlled by attention to three key areas:

• A comprehensive, validated, thermal reduction process for the raw materials.
• A knowledge of how Salmonella survives in the processing environment and how it can be controlled.
• The use of decontamination techniques as a final process step that could reduce the risk of Salmonella present as a result of cross-contamination from the process environment.

It is well known that as the water activity of foods becomes lower and the available moisture is decreased, the probability of growth is reduced; however, the ability of organisms to survive is greatly increased. The heat resistance of Salmonella was found to be 27 seconds when heated in ground beef at 60?C , but 14.6 hours when heated in wheat flour at 62?C . Joy Gaze here at Campden BRI continues to lead work on heat resistance and death kinetics of microorganisms in dry products, which complements our research and services into dry pasteurisation processing, and cleaning in 'dry' areas.

All manufacturers of low aw products would be expected to validate their thermal processes, but for some product types (e.g. nuts, seeds and powders) this may not be easy.

Whilst there is some information on the survival of Salmonella in dried products, there is little information on the survival of Salmonella in the environment, either in terms of the length of survival and of the possible factors contributing to such survival. Many dry foods manufacturers have adopted policies to keep the processing environment dry to control Salmonella growth, but what is dry? This could range from no use of water at all, through occasional wet cleaning, to managing the ambient relative humidity.

There is very limited data available that can be used to design suitable (thermal and non–thermal) decontamination processes in terms of combination of times and temperatures/exposure.

As mentioned above, we are currently investigating a whole range of issues regarding Salmonella survival in dry products, including the performance of novel technologies such as high pressure and steam injection systems for product decontamination. We have recently installed a processing system for heat treating dry ingredients, which is available to clients for trials on how the treatment affects their products – for example in terms of microbiology, chemistry and sensory quality – and how this is influenced by varying the process parameters.