How to Determine the Shelf Life of Food
Deterioration and spoilage:
Food is perishable by nature. Changes will take place naturally in all food while it is being handled and stored by the processor, retailer and the consumer. The changes can be rapid as with spoilage of raw meat and fish or spoilage and deterioration can take place over a period of days or weeks, e.g. bread becomes mouldy, biscuits become stale and soft, and processed meats become smelly and slimy. For some foods, e.g., retorted and very dry foods, the deterioration in the quality may not become apparent until after months or even years of storage. These foods are described as shelf-stable.
Factors that affect the rate at which food deteriorates and spoils:
There are many factors that may affect the shelf life of a product. Some factors relate to the food itself (intrinsic factors), such as moisture and pH, while others are external to the product (extrinsic factors), e g. the packaging conditions, materials and storage conditions. By understanding which are the most important factors impacting on the shelf life of a food, it may be possible to manipulate these factors to extend the shelf life.
Altering the composition, formulation, processing or packaging may inadvertently lead to a decrease in the shelf life or make the food more susceptible to the growth of spoilage or even pathogenic microorganisms. So, it is important to assess any changes proposed for their potential to have an adverse effect on shelf life. This will be especially important if the safety of a food relies on a number of interacting factors or hurdles to inhibit the growth of pathogenic bacteria.
Hazard Analysis and Critical Control Point (HACCP) systems:
It is essential to understand how the ingredients, process, final characteristics, packaging and storage conditions influence the safety, as well as deterioration and spoilage of a food. This will require an understanding of:
- what biological hazards are reasonably likely to be present; and
- the effectiveness of the control measures in place throughout the process at killing or inhibiting the growth of pathogenic bacteria in the food.
A review of the entire production process using hazard analysis will assist in identifying the potential for the hazards of concern to be present. In particular the four key cold-tolerant bacteria, which need to be controlled during the shelf life and the relevant control measures.
Control may be applied at critical control points (CCPs) which are steps at which a process is applied to prevent, eliminate or reduce a food safety hazard to an acceptable level, e.g., application of heat. Alternatively, it may come from the application of hurdle technology. Hurdle technology refers to the concept of achieving control by combining in series, a number of measures that would not individually be adequate for control. For example, uncooked fermented meats such as salami where the hurdles are the rapid development of a low pH along with the reduction in the water content by drying and the addition of inhibitory salts.
Effect of processing on survival of microorganisms:
The processing of a food may eliminate or at least reduce the number of microorganisms present. This will help to make food safe by controlling pathogenic bacteria and may extend the shelf life of the food by reducing the numbers of spoilage microorganisms.
Many processes applied to food, e.g. washing fresh produce or pasteurising, will not eliminate all the microorganisms present and a few may survive processing. Processed foods other than those that are retorted or receive an ultra-high temperature (UHT) treatment are not sterile. If the conditions during storage permit bacteria to grow, the food could become unsafe to consume. This may occur even if the bacteria in the food have been reduced to a safe level (even below the limit of detection of the laboratory method) following a validated process. Some pathogenic and spoilage bacteria produce spores that can be very resistant to heat and may not be eliminated during processing. If these spore-forming and/or cold-tolerant bacteria could be present after processing, this will need to be taken into account when establishing the shelf life. Spores from fungi (mould) on the other hand are easily destroyed by heat processing.
Processing using an appropriate validated heat treatment will decrease the numbers of pathogenic bacteria present in the raw materials and ingredients. Some food preservation techniques such as fermenting or coldsmoking, if not adequately controlled, have the potential to increase the number of pathogenic bacteria present and may result in a reduced shelf life.
Where there is an expectation that processing will reduce the number of microorganisms present, make the food safer and extend the shelf life. This can be compromised if re-contamination occurs, for example, cross-contamination during processing.
Effects of chilled storage:
The shelf life of many foods can be extended through chilled storage. Low temperatures slow down chemical changes and the growth of many moulds, yeasts and spoilage and pathogenic bacteria. However, there are some microorganisms including pathogenic bacteria that are able to grow readily at low temperatures. In some situations, while the levels of these cold-tolerant pathogenic bacteria may be considered safe at the end of processing if the duration of the shelf life is extended it provides a greater opportunity for these pathogenic bacteria to grow. This will have a major impact on the shelf life of the food. Section 10 provides information on the four key pathogenic bacteria that are a concern for chilled foods.
Loss of nutrients during storage:
Nutrient levels may decrease in a food over time. The rate of loss will depend on the stability of the particular nutrient. If the nutrient levels could decrease to below the expected level (i.e. as stated on the label), a ‘use by’ date is needed to indicate the point in time at which the nutrient levels will be less than the level stated on the label. This includes foods where the nutritional profile and the maintenance of these levels in the food are critical to the health of the intended consumer. This is important for foods which are a major nutritional source for a specific group of consumers.
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