46 DRY FOOD PRODUCTS AND MATERIALS – Food and Drink – Good Manufacturing Practice, 7th Edition



Dry food products or materials are those foods that have a low moisture content. Some materials are naturally dry. Alternatively, the removal of water from a food material is undertaken during manufacturing in order to inhibit the growth of bacteria, yeasts and moulds. Examples of manufacturing processes that will dry foods include desiccation, dehydration and evaporation processes such as freeze drying, spray drying, air drying and smoking. Dry goods production involves the same general hygiene considerations as other foods with regard to statutory requirements. These have been discussed in preceding chapters.


46.1 Most bacteria require a water activity (aw) of at least 0.95, and very few micro‐organisms can grow in foods around 0.6. Some bacteria, for example Bacillus cereus, can produce spores that will germinate on the reconstitution of the dry product. Yeasts and moulds can grow at lower levels of aw.

46.2 Dry foods and their ingredients can carry heavy microbiological loads undetected and without the dry food undergoing any noticeable change. This can be exacerbated by incubative temperatures, which can occur during some drying and mixing operations. Vegetative organisms or spores can proliferate and cause problems when the dry food is reconstituted. Dried materials used as the ingredients in the manufacture of canned products should have low bacterial spore counts. All ingredients, according to their origin and substance, should be subjected to a system of microbiological control by examination. Similarly, finished products should be sampled within a scheme of microbiological examination. When the food is to be consumed without further heat processing, it must be microbiologically safe. Particular care should be taken to prevent airborne dust from causing microbiological contamination of raw materials, finished product and plant. In areas that are dry‐cleaned, microbiological contamination can be very difficult to remove.

General Hygiene

46.3 Processes involving dry materials can have problems associated with dust, particularly those of cleaning, the possible creation of an explosive dusty atmosphere and the risks of cross‐contamination by dust particles. It is important therefore to contain dust as far as possible in an enclosed system and, with the aid of dust removal and extraction systems, to maintain a high standard of cleanliness. If allergens are utilised as raw materials, then dust control is critical and control measures need to be in place according to the physical state of the material, including particulate size and hardness of the particle.

46.4 The general environment of the plant and equipment, including ledges and girders, should be regularly cleaned and an effective air extraction system should be installed. Such a system should discharge through a filter and at a point situated so as to minimise the risks of the discharge being able to contaminate the other plant or products. Dust extraction systems should be properly maintained, cleaned and serviced; they become heavily coated inside duct‐work, and cleaning and filter changing can create a very dusty atmosphere. Dusty atmospheres should be considered as potentially dangerous explosion hazards. It may be desirable therefore to use flameproof motors and switches or ensure that they are situated in a relatively clean environment. Adequate protective clothing and other equipment should be provided for those involved physically in the cleaning operations, and during production if necessary.

46.5 Plant and equipment should be designed with easy access for cleaning in mind. When possible, manufacturing operations should be carried out in closed vessels or systems. Spray and fluid‐bed dryers should be fitted with efficient filter bags. Where closed systems are not practicable, it is usually possible to carry out an operation within a dust extraction system. Delicacy of handling in relation to product friability may also need special attention.

46.6 Particle size reduction of dry materials produces fine dust, and unless the process can be carried out in a closed system, including the discharge of the ground product, it should be operated within a dust extraction system. In flour mills where stone milling is undertaken, controls should be in place for the assessment of foreign body contamination risk. Further consideration should be given to the level of contamination present in wheat and the use of in‐line magnets and screens to remove foreign bodies. Care must be taken to ensure that the product does not ‘bridge’ at these screening points. An investigation should be implemented where necessary to identify the likely source of the contaminant so that appropriate corrective action can be identified and instigated. Mixing and sieving operations usually have similar dust control problems, and these tasks should be undertaken using appropriate dust control methods.

46.7 A particular problem occurs with hygroscopic materials that become sticky and so call for special attention with regard to cleaning. They are not always completely removed by dust extraction systems, but those particles that are removed can clog ducts and filters. If a vacuum system is used for dry cleaning, then care must be taken to ensure that all pipework and attachments are stored in suitable locations in each production area and routinely inspected for signs of wear and damage and replaced as necessary. This inspection should be formally recorded. Wet cleaning methods should be available in all areas where such dry goods are produced and handled, but precautions should be taken against the risk of creating humid conditions, which might allow microbial growth.


46.8 Dry materials require special attention concerning contamination and its detection. Particular attention should be given to examination for insect infestation, both past and present, and measures adopted to deal with either or both situations. At all times dry materials should be protected against attack by insects and rodents. Dry goods should be inspected on arrival for signs of pest infestation, including stored product insect contamination. There have been national recalls in the UK, for example, for both weevil and biscuit beetle contamination, but other pests such as psocids (booklice), rust red flour beetle, flour moths and their larvae are also of concern. Visual inspection will include observing the materials, delivery vehicles and subsequently storage areas for signs of pests or infestation. Where materials/premises are found to be infested, the pest contractor must be informed and appropriate action taken. Any contaminated material must be disposed of immediately to prevent the further spread of contamination.

While the use of magnets has been previously discussed, all dry materials should pass through a metal detector at least once.

Packaging and Storage

46.9 Consideration must be given to the potential minimum durability of dry products in the form in which they are offered for sale. Exposure of such a product to light, air and water vapour may cause physical or chemical or both types of change in the product, sometimes fairly rapidly, as may significant changes of temperature. Packaging can protect the product from these effects and can also protect it from insects and rodents; it should therefore be considered and designed with these objectives in mind.

46.10 Where dry foods are intended to be reconstituted for consumption, detailed instructions for preparation given on the package should, where applicable, have regard to safety considerations.