Microbiological Stability

The role of microbiology in military subsistence systems is to ensure the safety and microbiological integrity of food-delivery systems and rations. This responsibility must respond to conventional feeding systems as well as those, present and future, that are considerably less conventional. These rations are prepared and delivered to their customers in a wide variety of facilities (dining halls, hospitals, submarines, ships, airplanes, in the field, etc) at widely scattered geographical locations and are either perishable or microbiologically stabilized rations produced commercially or under contract.

The methods currently used to stabilize military rations from microbiological deterioration and to render them safe are thermal processing, water activity control (dehydration, lowering osmotic pressure with humectant, salts, freezing, etc), low pH, microbial growth inhibitors (nitrites, benzoate, acids, etc), and refrigeration.

Inoculated pack studies on complex military rations that have been developed in-house are conducted to determine their thermal processing requirements. For these studies a simple, biphasic technique, which yields extremely heat resistant and clean spores of Clostridium spo-rogenes strain PA 3679, has been developed.

The problem of thermophiles, which are highly heat resistant organisms that grow above 45°C, is significant in military rations due to the increased chance of temperature abuse of rations during distribution and storage. Therefore, commercial sterility, based on minimum heat input necessary to provide stability and an acceptable safety margin for foods handled in a carefully controlled distribution environment, may be inadequate for military rations.

Experience has shown that to control or destroy microbial populations in a ration system of diverse foods, it is necessary to study each food group. For example, beef snacks, which are low-moisture foods designed for use in the field, must be processed so as to attain a water activity that will ensure that the organism of main concern for safety, in this case Staphylococcus aureus, will not grow. Water activity is the ratio of the water vapor pressure of the product to the vapor pressure of pure water equilibrated at a given temperature and is a measure of biological moisture available for microbial growth. Similarly, the minimum water activities for cakes, formulated for the tray pack container configuration, were also determined. The organism of concern in this case being Clostridium botulinum.

It is often necessary to use combinations of microbial inhibitory agents and environmental conditions to achieve microbial stability. The three-year shelf life required for canned bread is attainable by the combined use of a sufficiently low water activity to prevent the growth of Clostridium botulinum and the inclusion in the package of an oxygen absorber to prevent spoilage by fungal growth.

The military is also concerned with being able to predict and monitor the shelf life of perishable rations. This effort has resulted in the issuance of a guide designed to prevent the unnecessary discarding of temperature abused, although still safe and edible, perishable rations in commissaries.

To further ensure the safety of perishable foods a medium for quantitating Listeria organisms was developed which is particularly efficient in recovering heat-injured cells. This was accompanied by the development of a technique that can identify pathogenic Listeria in less than eight hours.

A program to evaluate microbial inactivation kinetics for the aseptic processing of particulates for application for future ration design is being actively pursued. For this purpose extremely thermoresistant Bacillus stearothermophi-lus spores have been produced. The application of modified atmospheres (carbon dioxide, nitrogen, oxygen removal, etc) to inhibit microbial growth and extend the shelf life of perishable foods is also being studied.

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