Shelf life best practice
Isabel Campelos, senior technical advisor (food safety) at Leatherhead Food Research, explores the interplay of shelf life and packaging.
Extending, or more accurately predicting, the shelf life of food and drink is an enduring challenge for the industry. Clearly food safety and quality are critical factors. However, they can lead manufacturers to err on the side of caution. As pressure mounts to reduce food waste, the industry needs to adopt more intelligence-led processes to ascertain shelf life.
Packaging has an important part to play here. It serves three core functions: holding, preserving and conveying information about the packaged product. The part it plays in food and drink preservation is particularly complex. Most products undergo alterations during their shelf life, such as oxidation of lipids, degradation of colours, reactions to light, biochemical reactions, enzymatic reactions and microbial growth. These alterations can be directly influenced by packaging materials and processes.
It follows that manufacturers must pay particular attention to packaging during new product development. The entire dynamic interaction between food, packaging material and the trapped atmosphere should be considered. Design and manufacture of packaging materials needs to be a
multistep process involving numerous careful considerations to successfully engineer the final package. Properties to be considered in relation to food distribution may include gas or water vapour permeability, mechanical properties, sealing capability, thermoforming properties, resistance (to water, grease, acid, UV, light), machinability (on the packaging line), transparency, antifogging capacity, printability, material minimisation, availability and, of course, cost. The convenience of use by the consumer can also be a powerful characteristic influencing product success.
The mechanical function of food packaging – holding the product – has a symbiotic relationship with preservation. Material selection is a key consideration, especially for chilled foods. If the material is of poor mechanical strength, the stresses, humidity and low temperature during storage, transport and handling can damage the package, causing leakage or contamination. Adequate seal integrity is also vital to maintain the correct atmosphere in the package and the choice of package type – rigid or semi-rigid, lidded tray or flexible film pouch – has a significant bearing on durability and resistance.
The material needs to be appropriate to both the product and the manufacturing process. If a package is hot filled then chilled, the material must be a good heat conductor, not an insulator. Also, the properties of the material will change with temperature; plastic might become more brittle or less flexible. Food products may suffer chemical alterations (oxidation, enzymatic reactions, staling) or microbial changes (spoilage and foodborne diseases) during the process.
Processes and techniques
Awareness of these potential alterations and their impact on shelf life can drive the selection of appropriate packaging techniques, such as modified atmosphere packaging (MAP), aseptic or vacuum packaging.
MAP alters the internal atmosphere with an inert gas mixture to prevent or slow the growth of bacteria within a package. Some products, such as bread, are static; they do not undergo respiratory processes so the composition of MAP gases can be constant. This means the packaging should act as a barrier, maintaining the desired atmosphere composition inside the package. Other goods, like vegetables and fruits, undergo biochemical changes throughout their shelf life, releasing moisture, carbon dioxide and other gases. Replacing the natural ratio of nitrogen and oxygen found in ambient air with a carefully studied mixture of oxygen, carbon dioxide and nitrogen will delay some natural phenomena to keep these products fresh and good to consume for longer. In this situation, active packaging with some permeability is desirable in order to equilibrate the gas production inside.
The aseptic method, where both the package and the product are sterilised, has been developed in response to market demand for natural products. Tetra Pak is commonly used by juice and milk manufacturers opting for the aseptic technique, and PET (polyethylene terephthalate) bottles may be a better option as the use of glass bottles and jars loses momentum.
Vacuum packaging involves the removal of all gases from inside the package, thereby inhibiting the growth of most aerobic pathogenic and spoilage micro-organisms. However, a downside is that it promotes growth of anaerobic bacteria. Some of these, such as clostridium botulinum, are very dangerous, so extra precautionary measures should be adopted.
The absence of oxygen will avoid the oxidation of fats and can also induce undesirable colour changes during shelf life.
Assessing product stability
Once the packaging material and method have been determined, it’s important to assess product stability, including shelf life and pathogen growth within the packaging. If there is uncertainty over the best material or technique to use, head-to-head testing can aid the decision making process.
The simplest assessment mechanism is real-time microbiology, in which finished products are stored and appropriate microbiological screening is undertaken. This may involve temperature cycling and abuse periods designed to simulate factors such as mishandling or transit, providing a worst case scenario for the product and mitigating any risk of a food safety failure. Such abuse cycles typically include storage of chilled products at elevated temperatures of around 25°C for two hours, representing typical transit time from the retailer to a customer’s home.
An alternative method for non-microbiological effects is accelerated shelf life testing under controlled climatic conditions. This involves the use of environmental chambers that elevate temperature and humidity over a period of time. Levels can be adjusted to speed up degradation. Products are evaluated using a set of predefined sensory and physical data (such as texture and colour) then cross-referenced to previously mapped quality failures. This prediction of the rate of deterioration can then be used as a scientifically validated method to determine the shelf life of longer-life products for the purposes of food safety and quality certification. A number of time-points are evaluated to aid the extrapolation.
Evaluation methods can be tailored to account for the impact of packaging on sensory and physical characteristics during shelf life, in addition to food safety. For instance, organoleptic or visual measures may be used, as well as rheological evaluation, texture measurements or colorimetric analysis to quantify fading over time.
It is not uncommon for food and drink products to be deliberately ‘short dated’. This involves allocating a use by date set at around 75 per cent of a product’s actual failure point. The objective is to ensure optimal condition even if a product is purchased on its final day of life.
Organisations such as the Waste & Resources Action Programme (WRAP) are challenging this practice due to its potential contribution to unnecessary food waste. Yet, it is considered essential by many manufacturers; they naturally want to provide robust food safety assurance and maintain their reputation for product quality.
Ongoing packaging innovation is set to be a key enabler here. More intelligent and creative use of packaging – underpinned by rigorous assessment procedures – could have a vital role in addressing the issue of food waste without compromising either quality or safety.
A white paper that discusses shelf life best practice is available to download free at www.leatherheadfood.com/whitepapers.