It has long been assumed that dry food and ingredients, by virtue of their low water activity (aw), carry relatively little risk with regards to foodborne pathogens. It has become clear, however, that whilst pathogens are unlikely to grow in dry products, if present they can often survive.

Foods with a low aw tend to have a long shelf life because spoilage organisms grow slowly or are prevented from growing. Pathogens cannot grow in low aw products; however, if they are present in high numbers, or the infective dose is low, then the risk to the consumer still remains. If low aw ingredients are rehydrated during manufacture or preparation, growth can occur, thus increasing the risk to consumers.

Recent outbreaks have raised concerns regarding the safety of low moisture foods. According to the Centers for Disease Control and Prevention1, a recent USA outbreak in dry ingredients originated in organic sprouted chia powder. A total of 31 people infected with the outbreak strains of Salmonella Newport (20 people), Salmonella Hartford (seven people), or Salmonella Oranienburg (four people) were reported from 16 states. Another outbreak in June 2013 originated in tahini sesame paste and involved Salmonella Montevideo and Salmonella Mbandaka. This outbreak resulted in the infection of 146 people, with one fatality. In 2012, dry dog food (Salmonella Infantis) and peanut butter (Salmonella Bredeney) outbreaks occurred, infecting 49 and 42 people respectively. Peanut butter had already previously had a major outbreak in 2009, where 714 people were infected with Salmonella Typhimurium, which it is believed may have led to the deaths of nine individuals.

Traditional thermal processing methods are not designed for decontamination of low aw foods due to the increase in moisture content required and alteration to the quality of the products. Other available applications, such as fumigation with ethylene oxide, are restricted and even not permitted in some countries of the European Union. Irradiation, although approved for use with certain products, is another technology which is not used by manufacturers in Europe because of the negative image it has with consumers.



At Campden BRI we are interested in validation and verification of different technologies suitable for low water activity foods. A current research project is evaluating and validating those technologies, incorporating a range of emerging technologies including: a continuous UV-C tunnel, a large scale cold plasma machine and a cabinet for pulsed light processing. These are disinfection tools for manufacturers and processors of low water activity products and are available to companies that require independent practical evaluation of the technologies for their specific product applications

Some of the new commercial systems designed for the processing of low aw food products are described below.

Steam sterilisation units typically incorporate two metallic spirals: one for pasteurising and the other for drying. The pasteurising spiral is electrically heated and the product is moved along by a vibrating motion, so the particles are in constant movement. The vibrating motion of the product is said to prevent particles settling on hot surfaces and becoming scorched. The temperature of the spiral is usually set at 95–115°C for pasteurisation and can be set at temperatures up to 400°C for roasting and toasting.

For pathogen reduction purposes, the pasteurisation spiral possesses steam inlets, which provide the option to apply steam to the product as it passes through the spiral. This increases the kinetic energy to assist in the elimination of microorganisms on the product. The addition of steam increases the water activity on the surface of the product, so decreasing the heat resistance of organisms, which is known to be an issue in dry ingredients. The pasteurisation spiral is said to provide a controlled wet environment that is suitable for the destruction of microorganisms. The machine usually incorporates a second vibrating spiral tube that introduces cool filtered air onto the product after the heating process. This allows the process to operate in a continuous mode to obtain a dry stable product on exit from the machine. Depending on the size of the machine and the type of product, the flow rates can range from 200kg/h for the smallest units up to 7,000kg/h for the largest units.

Some examples of the use of the machine include: steam sterilisation (e.g. seeds, herbs, spices, medicinal plants, almonds and peanuts); special heat treatment of grains (e.g. soya); target volatile molecule extraction (e.g. cork); drying (e.g. spices and tea); roasting (e.g. sesame, peanuts, almonds, pistachio and onion slices); toasting (e.g. seeds, nuts, wood chips for wine and wheat flour); flavour extraction; and elimination of insect larvae and eggs.

Another unit from Imtech-Steri2 uses saturated steam under pressure to sterilise items. The company claims that the application of moist heat allows for shorter sterilisation times, lower temperatures and improved heat transfer and penetration compared to dry heat alternatives.

Steam sterilisation takes place in an autoclave chamber once air has been eliminated (air pockets can hinder inactivation of microorganisms). The treatment time depends on the type and size of the chamber, weight of the product, temperature applied and packaging. When loading the chamber it is important to conform to the validation guidelines, so as to not over pack the chamber, which could hinder the efficiency of the treatment.

Bühler-Barth’s3 steam pasteurising Controlled Condensation Process (CCP) system is said to be suitable for pasteurisation of variations of tree nuts, peanuts, seeds, herbs, grain and spices. The method applies moist heat under conditions that limit the condensation of moisture on the product surface. This allows a controlled water activity to be established and sufficient heat transfer to attain the necessary microbiological inactivation, resulting in a gentle treatment of sensitive dry foodstuffs.

Conditions with limited condensation of moisture on the product surface can be achieved when the product surface temperature is slightly below the evaporation temperature at the beginning of the thermal inactivation phase. The evaporation temperature can be controlled in a wide range between 80-120°C by either a vacuum or overpressure condition inside the pressure vessel. Very little moisture condenses on the product surface, thus quickly increasing the product temperature to the evaporation temperature, and the inactivation conditions at the evaporation temperature are maintained for a sufficient length of time. At the evaporation conditions, no additional moisture condenses on the surface and virtually no surface moisture is lost. After the treatment in the humid atmosphere is finished and excess moisture is removed from the surface by a vacuum phase in order to restore the product to its original state.

The Barth NR-CEP system is a combination of a hot air roaster and a steam pasteurisation system. It allows granular products like nuts and seeds to be pasteurised during the roasting process, which amongst other things saves on energy and space. Here pasteurisation is independent of the degree of roast so that even decontamination without roasting is possible.

The two step roasting process developed and patented by Bühler largely preserves the microstructure of the nuts and thus achieves an unparalleled shelf life. The partly fluidised roasting ensures that energy is distributed evenly between each individual nut, thus achieving a homogeneous and optimal aroma and taste for the entire batch. This pasteurisation technology achieves an inactivation of pathogenic microorganisms such as Salmonella of over five log, even without roasting the product. This is achieved by directing steam into the roasting chamber for a defined period of time, after a warming phase. Reproducible inactivation of pathogenic microorganisms is achieved thanks to good control and traceability during batch operation.

Campden BRI is also using equipment designed for more specific applications. For example, we are using a static unit designed for the pasteurisation of hygroscopic products with delicate surfaces. Short batch times allow for a high throughput, and this process is suitable for tree nuts, seeds, and dried fruit.

We are also investigating a relatively new semi-continuous process whereby products are gently moved throughout pasteurisation/sterilisation. The movement of the product avoids clumping and the high level of automation keeps the environment dust free. This system is suitable for powders, cereal mixes, seeds, animal feed, botanicals, and many raw materials for the cosmetic and pharmaceutical industries. This process exposes product to saturated steam in a partial vacuum; thus moisture pick up is minimal and no drying is necessary. The steam vacuum process aims to apply a uniform treatment throughout the product. As the product is not in contact with the autoclave the downtime for cleaning is reduced and cross contamination during product changeover is minimised. As such, the manufacturer claims that the product exits the system dry and ready to pack, with a minimal impact on appearance, colour and flavour.

Another process, provided by RF Biocidics4, uses radio frequency (RF) photons at specific frequencies to energise (activate) specific target molecules in the host commodity. In foods the absorbed RF energy is due to dielectric and electric polarisation effects in selected molecules, which usually make up a fraction of the whole molecular composition. This mechanism is significantly different to conventional surface heating technologies, in which all molecules are energised, and thus operates with higher power densities. This can induce controlled thermal and electronic effects that can lead to disinfection, disinfestation, enzyme inactivation and drying effects depending on the treatment applied. RF heats volumetrically and is able to penetrate packaging materials, thus providing opportunities for sanitation while minimising or avoiding the risks of re-contamination. The company claims that RF is a physical, chemical free (residue free, non-additive) new process applicable to conventional and organic commodities.

There are a number of challenges that the dry food industry needs to address to ensure the production of safe products. Firstly, some agriculture practices for raw material production (seeds, nuts, spices) may lead to high levels of pathogens entering dry food processing facilities. Secondly, many existing dry food factories were not designed for appropriate zoning, therefore it is difficult to apply appropriate segregation. Thirdly, the design of equipment to process low aw food is very often not designed for wet cleaning.

Some of the challenges facing food manufacturers listed above may potentially result in contaminated products reaching consumers, so it is important to ensure that an appropriate decontamination step is implemented. It is also important to reduce the contamination in raw materials by maintaining good agricultural practice (GAP) during production. When the control of the raw ingredients may be limited, a suitable decontamination process should be applied to the product to lower any contamination that may have occurred. The prevention of recontamination can be achieved by the use of good manufacturing practice (GMP). This may include producing food under hygienic conditions and incorporating the use of zoning within production to separate wet/dry and high/low risk items.

Low aw foods are known to be a risk for foodborne outbreaks, particularly if the products are not handled and treated in an effective manor to render them safe. Traditional thermal processes are not adequate for this challenge but more technologies are now on the market specifically focused on processing low aw food products. Campden BRI can provide more information and help with processing low aw food products and process validation in this challenging area.

Izabela Palgan is a Process Technologist in the Food Manufacturing Technologies Department at Campden BRI. She is responsible for managing activities relating to new technologies for the food industry (e.g. feasibility studies, scale up, validation, etc.), be it in research projects or carrying out contract work and consultancy. His current focus is on new technologies for food manufacturers to maintain and improve food safety, quality and shelf life.

Emma Maguire is a Hygiene Specialist in the Food Manufacturing Technologies Department at Campden BRI. Her main research interests have been in the field of emerging preservation technologies for the food industry and factory hygiene related issues, and her work focuses on the field of hygiene within food environments. This includes factory layout, equipment design and investigations into the various sources and vectors of microbial contamination and how to sample and control these. Emma has spent some time researching novel methods of decontamination in low moisture food products.

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