Shelf Life

The stated shelf life of a food is the period of time for which it remains safe and suitable for consumption, provided the food has been stored in accordance with any stated storage conditions. This means that the food:

• must remain safe to consume, i.e. should not cause food-poisoning because of the growth of pathogenic bacteria, or the production of toxins (bacterial and fungal ) in the food during storage;
• has not deteriorated in quality or spoiled in any way that the consumer would find unacceptable;
• Has not lost significant amounts of any nutrients listed on the label.

Changes that may occur during processing and storage

• 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 purchaser. 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 relate to the food itself (intrinsic factors), such as moisture and pH, while others are external to the product, 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

 It should be kept in mind that 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, the effectiveness of the control measures in place throughout the process at killing or inhibiting the growth of pathogenic bacteria in the food.

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.

• Effect of processing on survival of microorganisms

The processing of a food may eliminate or at least reduce the number of microorganisms (bacteria, yeasts and moulds) present. This will help to make food safe by controlling pathogenic bacteria (those able to cause illness) and may extend the shelf life of the food by reducing the numbers of spoilage microorganisms (that cause food to go off).

It is important to be aware that many processes applied to food, e.g. washing fresh produce or pasteurizing, will not eliminate all the microorganisms present and a few may survive processing. Processed foods other than those that are retorted or receive ultra-high heat treatment (UHT) are not sterile. So while any pathogenic vegetative bacteria should have been reduced to a safe level by a validated process (and may now be below the limit of detection) if the conditions during storage permit the bacteria to grow, the food could become unsafe to consume. Some pathogenic and spoilage bacteria produce spores that can be very resistant to heat 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.

• 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

Nutrients levels may decrease in a food with time. The rate of loss will depend on the stability of the particular nutrient. If the level of the nutrient could decrease to below the level expected by the consumer (i.e. as stated on the label), a ‘use-by’ date would be needed to indicate the point in time at which the nutrient will drop below the level stated on the label

Step by step determination of shelf life by the direct method

• Identify what may cause the food to spoil or become unsafe

Each product will have its own set of factors that may limit its shelf life. Use the following lists as a starting point to help identify all the possible ways that the product may deteriorate in quality and/or safety. At the same time, identify the factors that help prolong the shelf life. Do not forget to consider the entire process, from the purchase of ingredients and packaging materials right through to the end use by the consumer. You may also need to consider the time of year, as some products will deteriorate faster in summer than in winter due to higher temperatures.

• Which tests to use?

You need to select suitable tests for determining the safety and quality of the product. All tests are not appropriate for all products. For example, you may test raw meats for numbers of lactic acid bacteria but you wouldn’t test fermented raw meats (salami) for these organisms. It may also be important to test for food poisoning organisms such as Listeria in order to verify product safety. If laboratory tests are needed, check that the laboratory is accredited for those tests

In general, tests can be divided into the following four categories:

1. SENSORY EVALUATION: Sensory evaluation assesses a food’s smell, appearance, flavor, and texture. It can be used to monitor and record obvious changes that occur over time, and is therefore, useful when determining the shelf life of a food. The food should be assessed under the conditions at which it is designed to be stored and consumed.Always check the food is safe to eat before using a taste panel.

2. MICROBIOLOGICAL: These tests can be used to evaluate both food quality and safety. Tests may be done to estimate changes in the number and type of spoilage organism (yeasts, moulds or bacteria) occurring over time. Table 1 in Appendix 1 provides examples of standard plate count levels that apply to various ready-to-eat food groups. If a specific food is not included in the table, use your judgement to determine where it would fit.Identification of any food poisoning organisms present is important for food safety. Tests required will depend on the particular product. Microbiological standards and guidelines give guidance on the types of organisms and their number that can be considered acceptable, or unsafe, in a food.

3. CHEMICAL: Chemical tests can detect changes in the product’s quality throughout its shelf life. Examples of instrumental chemical tests include pH, headspace gas analysis, free fatty acids and total volatile nitrogen.

4. PHYSICAL: These include tests for measuring product texture, examination of packaging, ‘travel tests’ and determining the best, worst and average retail conditions.A ‘travel test’ helps to identify any hazards involved in transport and handling. It involves transporting the product through the expected distribution and storage chain. An examination of the product at various points, and at the end of the chain, is required. A data logger may be used to record the temperature at preset times for later analysis. Experimental design should attempt to mimic real life practices e.g. include predicted transport temperatures to retail outlets, during commercial control, consumer purchase and transport, and consumer storage.

• Plan the shelf life study

Consider the following points when preparing your detailed shelf life study plan:

1. What tests need to be carried out?
2. How long will the study run for, and how often will the tests be carried out? Include the actual sampling dates in your plan. It is suggested that sampling be carried out at the beginning, at the target end point and at about three occasions in between. Another sampling should be carried out beyond the target to confirm the end point selection
3. How many samples will be tested each time?At least triplicate packs of product should be tested at each sampling

4. How many samples will be needed for the whole study period?
5. When will the study be run? Ideally it should be carried out in the season most likely to cause problems, usually summer. The study should be carried out more than once to take account of variability of the product

The product, process and packaging should be the same as you intend to use for the final product. Keep written records of everything you use or do, as these can be helpful when interpreting results.Now the study is fully planned and timetabled.

• Run the shelf life study

During the study samples should be stored under the same conditions as your normal production samples, from manufacture through to consumption. If this is not possible the samples should be stored at a known temperature and humidity. These need to be checked and recorded regularly.

• Determine the shelf life

Eventually a point is reached when the product no longer meets the quality standard. Using all the information you have recorded and observed, decide how long the product can be kept and still be of an acceptable quality and safety. Maximum storage times for quality and safety may not be the same. The shelf life of a product should be which ever.