Consumers expect to purchase high-quality, fresh food, but recently, they’ve also begun to look for foods with fewer or no food additives or preservatives, pressuring manufacturers to reformulate products to meet growing clean label demands and to ensure food safety and brand protection. Manufacturers are also challenged with determining and maximizing the shelf life for products that are exposed to varying conditions in the supply chain. Shelf life touches on all the issues mentioned, and shelf-life determination is an essential requirement in providing safe, quality food products to consumers.
What Is Shelf Life?
There are many definitions of shelf life provided by governments and organizations. 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.” Both food safety and quality are important aspects of acceptable shelf life. Although pathogens are usually monitored during shelf-life studies, a suitable food safety program 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.
Initial quality. For perishable food, the initial microbial load will influence the shelf life. Using ingredients that have already started to deteriorate (e.g. old oil) or overprocessing can result in loss of texture or nutrients (e.g. vitamin C).
Inherent nature of the product. Fresh or perishable foods have an inherently shorter shelf life than shelf-stable foods. The low water activity of a product such as rice makes it an inherently shelf-stable food, for example.
Product formulation. The addition of preservatives or antioxidants can extend the shelf life of the product. Formulation changes such as replacing the type of acid, removing nitrates from a processed meat, and reducing the amount of added salt can also change the shelf life of the product.
The following are extrinsic factors.
Packaging. For shelf-stable products, the barrier of the package can affect the shelf life. For example, moisture absorption for a cracker will affect the crispness of the product and a moisture barrier is required. If the product has a large fat component (e.g. potato chips), fat oxidation affects the shelf life and an oxygen barrier is required. Light protection may also be required. Without light protection, milk is susceptible to vitamin degradation and off-taste due to light-induced oxidation.
Transportation and storage conditions. Exposure of the product to variable temperatures and relative humidity in the supply chain (including the retail environment) can affect the shelf life of foods. For refrigerated products, higher-than-optimal temperature storage can accelerate microbial growth. Oxidation reactions are also accelerated by higher temperature exposure, thus shortening the shelf life of products.
Consumer handling. After purchase, transfer of food from the store to home can result in higher temperature exposure. Consumer refrigerators can also be at higher-than-optimal storage temperatures. Once the package is opened, the shelf-life date assigned by the food manufacturer is no longer applicable.
Understanding the End of Shelf Life
What constitutes the end of shelf life? The end point can be indicated from relevant food legislation, guidelines provided by government or professional organizations, or the use of 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 (e.g. paired comparison, triangle, duo-trio, etc.). Descriptive analysis with expert panelists describes the change in sensory attributes (e.g. odor, taste, appearance, and texture) and can indicate consumer rejection. Although acceptance testing or use of consumer panels for acceptability can be more accurate, it is seldom used since a large number of panelists are required, resulting in a more time-consuming and expensive process.
A commonly used approach is to establish key analytical and sensory attributes that 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 (e.g. peroxide results indicate fat oxidation and rancidity of baked goods).
How to Conduct a Shelf-Life Study
There is no universal protocol for direct determination of shelf life. Examples of guidance documents for determining the shelf life of food have been issued from the New Zealand Government and the Food Safety Authority of Ireland. The 10 steps below outline a methodical approach to setting up a shelf-life study.
- Define 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.
- Identify mode of deterioration. End of shelf life is specific to different food commodities. For chilled foods, the end of shelf life is attributed to elevated spoilage microbial levels. Other modes of deterioration may be oxidation of fats as in fried snack foods, vitamin degradation as in fruit juices and starch retrogradation or staling of breads.
- Define key attributes to monitor. Microbial examination, chemical analysis (e.g. lipid oxidation and vitamin degradation), physical testing (e.g. color and 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.
- 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 and descriptive or acceptance testing.
- 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.
- Set target end point and testing frequency. For product with a short shelf life (seven to 10 days), evaluation can be performed daily or every two days. For moderate shelf life (three weeks) and long shelf life (one year), testing can be done at the initial point, end point, two to three occasions in between, and one point beyond the end point.
- 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 minimize variation and enough samples should be stored for duplicate or triplicate testing. Select the appropriate sensory control; if the product deteriorates over time, use freshly manufactured product or chill or freeze samples to ensure minimal deterioration.
- Perform shelf-life study. Store the samples under conditions outlined in the study and test at the selected intervals.
- Analyze results. In the absence of standards (legal or voluntary), manufacturers must 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.
- 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 that accelerate shelf-life testing. One of the most common methods to accelerate oxidative reactions is to store product at elevated temperature. For simple systems, such as fat and oil, there is a direct relationship between oxidation rate and temperature. This mathematical equation can be used only if there is a correlation between the storage behavior under normal conditions and under accelerated conditions. In reality, foods are more complex and reactions may occur that would not proceed at normal temperature storage. Increasing storage temperature may lead to changes that affect the deterioration process such as melting of solid fats, crystallization 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.
It’s important to understand the mode of food deterioration to establish the product’s shelf life. Product formulation, process conditions, and storage conditions are important factors 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.
Zweep is senior manager of packaging, product development, and compliance for NSF International. Contact her at [email protected].