Shelf life determination: The manufacturer’s challenge

What is shelf life?

There are many definitions of shelf life provided by governments and organisations. For example, the Institute of Food Science and Technology defines shelf life as “the period of time during which the food product will remain safe; be certain to retain its desired sensory, chemical, physical, microbiological and functional characteristics; where appropriate, comply with any label declaration of nutrition data, when stored under the recommended conditions.”¹ Both food safety and quality are important aspects of acceptable shelf life, as is reflected in the European Food Education Council’s definition: “the length of time a food can be kept under stated storage conditions while maintaining its optimum safety and quality.”² Although pathogens are usually monitored during shelf life studies, a suitable food safety programme is the best way to ensure the product’s safety.

Factors affecting shelf life

Both intrinsic and extrinsic factors influence the shelf life of food products. Intrinsic factors include the initial quality of the food (use of quality ingredients and low microbial load), the inherent nature of the product (fresh, perishable food will have a shorter shelf life compared to processed, shelf stable foods) and product formulation (presence of a preservative or anti-oxidant extends shelf life).

Extrinsic factors that affect shelf life are processing methods (heat treatment or high-pressure processing will reduce microbial load and extend shelf life), barrier properties of the packaging (materials with low oxygen transmission rate and low water vapour transmission rate protects the product from oxidation and moisture gain or loss, respectively), transportation and storage conditions (elevated temperatures and relative humidities can shorten shelf life), and consumer handling (temperature control is variable in a home environment).

Understanding the end of shelf life 

What constitutes the end of shelf life? The end point can be defined by relevant food legislation, guidelines provided by government or professional organisations, or acceptable industry practices. Often acceptability limits are chosen based on self-determined end points. For the most part, the food industry relies on sensory perception as an indicator of product failure. Product acceptability may be determined when there is a significant difference in the aging sample compared to a fresh sample by using discrimination testing (such as paired comparison, triangle, duo-trio, etc.). Descriptive analysis with expert panellists describes the change in sensory attributes (such as odour, taste, appearance and texture) and can indicate consumer rejection. Although acceptance testing or use of consumer panels for acceptability can be more accurate, these techniques are seldom used since a large number of panellists are required which results in a more time-consuming and expensive process.

A commonly used approach is to establish key analytical and sensory attributes which are correlated to consumer acceptability parameters. Once a good analytical indicator has been established, then further routine shelf life studies on the same product can use the analytical indicator to determine the end of the product’s shelf life (for example, peroxide results indicate fat oxidation and reflects development of undesirable odour and taste (rancidity) of baked goods).

How to conduct a shelf life study

Just as there are many definitions of shelf life, there is no universal protocol for direct determination of shelf life. However, several regulatory guidance documents for determining the shelf life of food have been issued, for example by the New Zealand Government[1] and the Food Safety Authority of Ireland[2]. The ten steps below, gleaned from industry experience and best practices, outline a methodical approach to setting up a shelf life study.

1. Define the objective – What is the reason for the shelf life study? The shelf life study can be initiated due to development of a new product, a formulation change or an alternate package evaluation.
2. Identify mode of deterioration – End of shelf life criteria vary for different food commodities. For chilled foods, the end of shelf life is attributed to elevated levels of spoilage microorganisms. Other modes of deterioration may be oxidation of fats for fried snack foods, vitamin degradation for fruit juices, and starch retrogradation or staling for breads.
3. Define key attributes to monitor – Microbial examination, chemical analysis (for example lipid oxidation and vitamin degradation), physical testing (for colour or viscosity) or sensory evaluation can be monitored throughout the shelf life study. Note that a key part of establishing the usefulness of any analytical measurement is the correlation with sensorial changes.
4. Select test methods – For chemical analysis, lipid oxidation could be monitored by measuring peroxide, free fatty acid or thiobarbituric acid reactive substances formation. Sensory evaluation could be determined by various methods such as discrimination, descriptive or acceptance testing.
5. Set storage conditions – Select the variables such as temperature, relative humidity and lighting conditions. Product storage conditions can be optimal, typical or average, or worst case scenario. The variables can also be fixed or fluctuating to simulate real-life product exposure during storage, distribution and the retail environment.
6. Set target end point and testing frequency – For products with a short shelf life (seven to 10 days), evaluation can be performed daily or every two days. For products with a moderate shelf life (three weeks) or long shelf life (one year), testing can be done at initial point and end point, plus at two to three occasions in between and at one point beyond the end point.
7. Determine appropriate test and control samples – Set the ingredients, process and packaging for the shelf life study. Test samples should be from the same batch to minimise variation and enough samples should be stored for duplicate or triplicate testing. Select the appropriate sensory control; freshly manufactured product if the product deteriorates over time or chill or freeze samples to ensure minimal deterioration.
8. Perform a shelf life study – Store the samples under conditions outlined in study and test at the selected intervals.
9. Analyse results – In the absence of standards (legal or voluntary), manufacturers have to set their own end point based on microbiological, chemical or sensory criteria. The shelf life date is usually assigned as the last day of an acceptable sensory score or analytical results. The preliminary shelf life date can be conservative and based on the worst case manufacturing and storage scenario.
10. Monitor and confirm shelf life – Once the product has been introduced into the market, sample at the distribution and retail levels and adjust the shelf life date accordingly.

Accelerated shelf life testing

Lengthy real-time studies have led food processors to seek methods to accelerate shelf life testing. One of the most common methods to accelerate oxidative reactions is to store the product at an elevated temperature. For simple chemical systems such as bulk fat and oil, there is a direct relationship between oxidation rate and temperature. This mathematical equation can only be used if there is a correlation between the storage behaviour under normal conditions and under accelerated conditions. In reality, foods are more complex and elevated temperature storage may initiate reactions that would not occur at normal temperature storage. Increasing storage temperature may lead to changes that affect the deterioration process such as melting of solid fats, crystallisation of amorphous carbohydrates, increased water activity, denaturation of proteins and decreased solubility of gases. Relative humidity may also affect reaction rate. Accelerated shelf life testing is not applicable for short shelf life chilled foods where microorganisms flourish at different temperatures.

In conclusion, it is important to understand the mode of food deterioration in order to establish any particular product’s shelf life. Product formulation, process conditions and storage conditions are important considerations for product shelf life. Careful consideration of experimental design and test parameters is essential for accurate shelf life evaluation. The shelf life of commercial products should be monitored and adjusted as required.

Following these considerations will help ensure a safe, quality food product that meets customers’ expectations. 

References

1. Shelf Life of Foods – Guidelines for its Determination and Prediction. London: Institute of Food Science & Technology; 1993.
2. Food shelf life and its importance for consumers [Internet]. European Food Information Council. 2013 Oct [cited 2015 Oct 27]; Available from: http://www.eufic.org/article/en/artid/Food_shelf_life_and_its_importance_for_consumers/.
3. How to Determine the Shelf Life of Food: A Guidance Document [Internet]. New Zealand Government. 2014 July 22 [cited 2015 Oct 27]; Available from: http://www.foodsafety.govt.nz/elibrary/industry/determine-shelf-life-of-food/how-to-determine-the-shelf-life-of-food-revision.pdf.
4. Guidance Note No. 18: Validation of Product Shelf-life (Revision 2) [Internet]. Food Safety Authority of Ireland. 2014 [cited 2015 Oct 27]; Available from: https://www.fsai.ie/faq/shelf_life/determination.html.

Works consulted

Calligaris S, Manzocco L, Anese M, Nicoli MC. Shelf life assessment of Food Undergoing Oxidation – A Review. Critical Reviews in Food Science and Nutrition. 2015 (accepted manuscript).

Galić K, Ćurić D, Gabrić D. Shelf Life of Packaged Bakery Goods – A Review. Critical Reviews in Food Science and Nutrition. 2009; 49: 405-26.

Kilcast D, Subramaniam P, editors. The Stability and Shelf-Life of Food. Cambridge: Woodhead Publishing Limited; 2000.

Man D. Food Industry Briefing Series: Shelf Life. Oxford: Blackwell Science Limited; 2002.

Steele R., editor. Understanding and Measuring the Shelf-life of Food. Cambridge: Woodhead Publishing Limited; 2004.

About the author

Carol has extensive experience in quality assurance, testing laboratories, and research and development in the packaging field. She has been at NSF-GFTC (formerly the Guelph Food and Technology Centre) since 2002, and manages a department that performs packaging and shelf life studies, and reviews and assists in food regulatory and labelling projects. Carol is actively involved in the industry, often delivering presentations and courses on the latest packaging technologies and trends, judging competitions, and contributing articles to trade publications. Carol has a B.S. in biochemistry from Queens University and an M.S. from McMaster University, and is a Lifetime Certified Packaging Professional (CPP).