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.