By Joy Gaze and Mariane Hodgkinson - 14 December 2015

It is well known that foodborne pathogens are unlikely to grow in dry products. However, if they are present, they can survive, both in food and in food processing environments. Studies have shown that levels of Salmonella artificially inoculated on nuts (for example, pecans and almonds) were largely unchanged after 18 months, even with storage at both 4 and -20ºC.

Herbs, spices, nuts, seeds, cereals and dried vegetables can contribute significantly to the nutritional quality of products. Many of these ingredients have been becoming more popular in recent years, with the growth in interest in healthy alternative ingredients and international food recipes. Many of these dry ingredients commonly contain a high microbiological load, including organisms that can cause food poisoning. Irrespective of the dose, if dry (low Aw) ingredients undergo any form of rehydration during manufacture, then the potential for growth can occur, thus increasing the risk to consumers.

Salmonella has been the cause of several outbreaks of illness, product recalls and alerts associated with low Aw foods worldwide. A recent outbreak in the USA involved organic sprouted chia powder. A total of 31 people were infected with three strains of Salmonella (Newport, Hartford and Oranienburg) across 16 States. In June 2013, Tahini Sesame paste was implicated in an outbreak involving Salmonella Montevideo and Mbandaka; there were 146 known cases of illness, with one death. The USA has also seen outbreaks involving peanut butter, with one in 2009 affecting 714 people and resulting in as many as nine deaths; Salmonella Typhimurium was the strain implicated.

Given that dry ingredients are clearly a potential source of infection, it is vital that steps are put in place to combat the risk. Fumigation with ethylene oxide, irradiation and thermal treatments are all potentially effective, but ethylene oxide is not permitted in the EU, and irradiation is still associated with perceived consumer resistance. Heat treatment is therefore the primary mechanism for reducing microbial load.

Food composition can have a large bearing on the heat resistance of microorganisms. Fat, protein and salt content, water activity level and pH can all have an effect. Water activity is particularly significant: when microorganisms are heated in dry foods they exhibit considerably more heat resistance than when heated in moist foods. In one particular piece of research here, we found that the decimal reduction time ( D value, a measure of heat resistance) of Salmonella in ground beef was 27 seconds at 60ºC, but in wheat flour it was over 14 hours at 62ºC. We have considerable expertise in devising microbiological experiments to determine heat resistance data under a range of conditions and food matrices.

We have a Microbiology Process Hall in which we have our own and clients’ equipment for evaluation. We devise protocols to fully assess the effectiveness of the treatments against known concentrations of pathogens which we incorporate as controlled inoculums. These may be needed to determine the lethality achieved by new equipment, new formulations or new configurations.

We also have our Food Manufacturing Halls in which we can use surrogate harmless microorganisms to evaluate processes. As one example, we have a processing system that decontaminates products such as nuts and seeds by injecting steam into the product stream and combining this with electrical heating in a continuous process. This procedure increases the water content on the surface of the product, potentially lowering the microbial heat resistance and subsequent ability to survive.

The system consists of two spirals, one for heating and one for cooling and drying. Steam is introduced into the heating spiral, and the ingredients are transported via unidirectional vibrations which control product flow through the machine, allowing steam to cover the surfaces more effectively. It can be used for any product that can be conveyed by vibration, that is not water-soluble and that does not become too sticky when brought into contact with steam. The moisture content of these products can then be reduced to its original dry state by the end of the process for safe storage.

Practical microbiology and food technology assessments and use of innovative technologies has been one of the cornerstones of our research for over 25 years. For more information on any details in this article, or for project proposals for confidential research please get in touch with us.