Product shelf-life testing: All you need to know
By Linda Everis, shelf-life expert at Campden BRI
How do we define shelf-life? It’s typically the time after production during which a food or drink product remains acceptable for consumption. A straightforward definition, right? But the food and drink industry faces many hurdles when setting a shelf-life protocol. It’s a complex task. From manufacture to consumer storage, products can encounter a range of fluctuating temperatures as they are passed from one stage of the cold chain to the next – and this is just the start of many factors to consider when assigning a shelf-life. This article describes some of these factors in detail to help you get a better understanding when considering the shelf-life of your food or drink products.
Understanding your product
Understanding your product is one of the first places to start on your shelf-life determination journey. Just some of the factors to consider include:
Raw materials - The type and level of organism(s) present and their ability to grow will influence shelf-life. Higher initial loading will mean higher levels of organisms could be reached more quickly and therefore limit life.
Product formulation - The pH and water activity (aw) values will influence what organisms are likely to grow and how quickly they will grow. Each organism has a minimum pH and aw necessary for growth and below these values, growth will not occur. The presence of preservatives may increase shelf-life by preventing growth. Preservatives tend to be effective against specific organisms, e.g. sorbate is effective against yeasts and moulds. However, the efficacy of preservatives can also be influenced by other factors, such as pH. The presence of natural antimicrobials may act as a natural preservative.
Food structure – many food products can contain emulsions of two immiscible liquid phases: typically, oil-in-water (e.g. milk, mayonnaise) or water-in-oil (e.g. butter, low fat spread), or “Double emulsions” – containing both at the same time. Variable amounts of each phase can restrict microbial growth by physical space or availability of nutrients.
Ingredient shelf-life - The microbial and sensory quality of ingredients will change over shelf-life. The point at which the ingredients are used will influence final product shelf-life.
Seasonality - Raw materials and ingredient quality is likely to be subject to variation throughout the seasons when they are sourced from different suppliers and/or countries.
Competitive microorganisms – The presence of some organisms can prevent growth of other organisms - for example, lactic acid bacteria can produce bacteriocins that can prevent Listeria growth. Organisms such as lactic acid bacteria often reduce pH during growth which can result in the prevention of growth of some organisms.
Hurdles technology - This is where factors such as pH or aw on their own are not sufficiently low enough to prevent growth on their own, but a combination of a lower than optimal aw and lower than optimal pH is sufficient to prevent growth. These two factors work in combination.
Drift in pH/ aw - If there is an increase in pH or aw then shelf-life may be shortened due to an increase in microbial growth rate. The pH can drift due to microbial growth or issues with the buffering capacity of the food. The aw can increase in packaged products subjected to storage temperature fluctuations which can lead to a build-up of condensation and an increased aw on the product’s surface. Unpacked products subjected to an increase in humidity can lead to water uptake even if there is no change in temperature. Water can also be redistributed if a product separates and starts to thin.
Heat processing - Heat can be used to destroy organisms, toxins or enzymes. This can influence shelf-life limited by microbial growth or chemical spoilage. If a product is fully heat processed i.e., sterilised in-pack and not subject to cross contamination, then shelf-life will not be influenced by microbial growth and this will extend the product’s shelf-life. If a pasteurisation or a hot fill process is applied, then other factors need to be considered such as pH or temperature to control the growth of surviving organisms.
Non-thermal processing - A product may be subjected to a non-thermal process, e.g. UV treatment or high pressure. The shelf-life will be influenced by which organisms are able to survive and their ability and rate of growth and by the inactivation of key enzymes.
Packaging - The packaging material used can influence shelf-life by allowing migration of gases and water through the pack. Careful consideration needs to be given to this especially if the packaging is recyclable/biodegradable. The gas atmosphere used in packaging will also influence shelf-life as reduced oxygen will impact on chemical deterioration by supressing oxidation and rancidity and will also influence microbial growth by reducing growth rates of certain microbes.
Gas atmosphere - A reduced oxygen atmosphere, by way of modifying the atmosphere or reducing oxygen, could allow the growth of C. botulinum. Reduced oxygen presence could also be due to limited headspace or anaerobic spots on the product e.g., large joints of meat. Carbon dioxide also has an antimicrobial affect.
Hygiene - Factory hygiene and ensuring good manufacturing practices are in place will influence shelf-life. If the product is highly contaminated during the manufacturing process, then this will decrease shelf-life.
Filling technology – e.g., cold-fill, hot-fill, aseptic. This links to factory hygiene and choice of heat processing. Use of these technologies can offer advantages such as allowing the product to be preservative-free and reduction of the microbial load in packaging which can influence shelf-life.
Distribution - The product should be distributed according to the optimal storage conditions, i.e. chilled /ambient/dark. This will limit enzymatic changes and will limit any increase in condensation which could influence the product formulation and increase the rate of microbial growth which may limit shelf-life.
Storage - Product should be stored as recommended as any temperature abuse or prolonged exposure to light may affect shelf-life. Dried products should be packaged and stored so that they remain dry and frozen products should remain frozen. Microbial growth is not usually associated with dried and frozen foods if they are stored correctly. Organisms will survive, but not grow so it is important to ensure the products meet any required microbial specifications at the start of life. Products can often be deep chilled or super chilled prior to release to chill storage. This is storage of product at -1 to -2°C so a portion is frozen, but some remains liquid. This extends shelf-life by reducing the amount of time at temperatures that allow microbial growth.
Consumer handling - How the consumer handles the product will affect shelf-life. Any prolonged temperature abuse is likely to decrease the shelf-life. Consumer use such as suitability for freezing and instructions given as to how a product should be handled once opened should be given. The packaging format often has a significant influence on the shelf-life of foods. Consideration should be given particularly to products designed as multi-portion or bulk commodity packs, the effect on shelf-life of opening the pack should be assessed.
Fermented foods - Fermented foods rely on either naturally present organisms to reduce pH or specific starter cultures to reduce pH. The final pH and rate of pH drop will influence shelf-life.
Dried foods -The rate of aw drop and final aw is critical for foods that are raw and dried with no heat processing.
The factors described above exert their effects on microbiological, chemical and physical parameters within the food, which often result in a loss of sensory quality. The point at which these effects influence the product such that the change becomes noticeable or the product unacceptable is the endpoint and therefore denotes the end of life. It is the time taken to reach the endpoint that must be determined when assessing product shelf-life.
When to use a laboratory and what to expect from them
Answering ‘when to use a laboratory’ is tricky as it depends heavily on an SME’s specific situation and product. For example, if a food business wants to develop and sell a chewing gum product then this is one that does not require a shelf-life and therefore doesn’t require assistance from an independent expert. In a similar vein, products like biscuits are generally considered quite safe from a microbiological perspective (due to their low water activity) meaning this product’s shelf-life could be determined in-house with the use of a guidance document. However, for other products there may be a greater need for expert advice or independent testing.
What to expect from a laboratory can differ depending on who you go to and what you need testing. For example, not every laboratory can test for Clostridium botulinum. This organism can only be worked on in a biosafety level 2 laboratory. This is something that we have at Campden BRI and allows us to challenge test products with this organism to see if it grows under certain conditions.
Other considerations before receiving shelf-life testing include whether the laboratory you contract work with can also produce specific advice based on your results or whether they just provide results without an interpretation. Identifying a laboratory that can do this is crucial if your team does not have the knowledge to understand how to apply the results. On top of this, analytical measurements done as part of shelf-life evaluation must be performed by suitably qualified and trained personnel using appropriate methods and laboratory facilities such as those accredited to ISO 17025. Laboratories should be accredited to a recognised standard (e.g., UKAS or INAB) or in accordance with customers’ requirements. Also, the microbiological test methods used should be the recognised standard test method, e.g. ISO, or should have been fully validated in accordance with standards such as ISO 16140 or other recognised standards.
Other things to be mindful of when considering who to perform your shelf-life testing with include the experimental design. Are they doing enough samples? What will the timepoints be and will they be sufficient for your needs? If the food business operator does not know this informationit’s important to commission testing with experts who can explain why the test is being conducted in a certain way. Experts in this area will also be able to consider all the factors listed above to ensure the test is designed correctly.
With all these factors in mind, it’s easy to see that setting a shelf-life isn’t quite as simple as it sounds.This article is a good starting point for your shelf-life setting journey, but further guidance is really needed to develop a good understanding. Our team at Campden BRI updated shelf-life determination guidance that is backed by expert advice and practical shelf-life assessment and has been aligned with EU regulation and recommendations for setting a shelf-life.