What is shelf life anyway?
The factors that affect shelf life.
What actually ends a product’s shelf life?
“Is it still good?” This is a phrase that is heard in many households when visiting the fridge or pantry for a meal or snack. The question is often answered by checking the expiration date on the label — the date stamp that indicates the product’s shelf life; but what is shelf life and how does a manufacturer determine it? Shelf life, simply put, defines the length of time that a product can be stored without unacceptable degradation in the quality of the product. It is straightforward on the surface, but there are a number of considerations to actually determine an accurate and useful shelf life.
Why Conduct Shelf Life Testing?
Shelf life testing may be conducted initially when a new product comes out of R&D. It may be tested periodically to confirm consistent performance, or with changes to formulation, packaging, or other factors impacting shelf life. Understanding the performance of a product and whether it’s meeting expectations is the baseline for establishing a customer base and ensuring the ongoing satisfaction of consumers of the product. Properly labeled shelf life is key to brand protection and it is often a customer requirement.
Quality vs. Safety
One important distinction in food manufacturing is that of quality versus safety. When speaking about shelf life, it is typically the quality of the product being discussed. A product could be past its prime, even have high levels of spoilage microorganisms, without being “unsafe” for consumption. On the other hand, products could contain foodborne pathogens with absolutely no indication of spoilage. While it’s certainly possible for a spoiled product to also contain pathogens (thereby posing a health risk), the two do not, ipso facto, go hand-in-hand.
Factors Affecting Shelf Life
The shelf life of every product is affected by various influences which can be broadly categorized as “intrinsic” or “extrinsic” factors. With intrinsic factors, think formulation, or elements which will not be changed during the storage life of the product. This includes characteristics such as pH, water activity (or the amount of available/unbound water in the product), preservatives, packaging, or competitive microflora in the case of cultured or fermented products. Extrinsic, or outside, factors such as temperature, humidity, and light, can also have a significant impact on shelf life. The manufacturer typically has very limited control over these conditions, but a robust shelf life study can incorporate ranges, such as storing the product at two or three temperatures over a range of expected conditions. Quality degradation is specific to the product and a number of these influences, so shelf life testing should be tailored to the product, packaging, and storage conditions.
End of Shelf Life Indicators
The end-of-shelf-life is differently defined for different products and a comprehensive shelf life study will be designed to incorporate quality indicators specific to the product under test. The end of a product’s quality shelf life may be defined by microbiological, chemical, physical, and/or sensory changes:
In perishable products, the end-of-shelf-life is often defined by microbiological spoilage, meaning that the growth of spoilage bacteria, yeasts and molds leads to a lower quality product. This is typically perceived by consumers as off-flavors, off-odors, sliminess or changes in texture such as mushiness.
Microbiological analyses are selected based on the product type and associated spoilage microflora, as well as packaging and storage conditions. For example, a standard aerobic plate count (APC) for a product stored at room temperature may be appropriate, while APC should incorporate psychrotrophic microorganisms (those which are capable of growth at lower temperature ranges) for refrigerated products. Vacuum packaged meat products might include a psychrotrophic count as well as lactic acid bacteria counts, but the quality of a fermented or cultured products would not benefit from these analyses since the product is prepared using lactic acid bacteria; in these products, quality would be better monitored by selecting for Gram-negative bacteria, yeasts and molds. The level of microbial spoilage required to cause sensory defects varies by product and packaging, but as a general guideline, microbial counts are typically considered unacceptable when a product contains 10,000,000 colony forming units per gram (CFU/g) of spoilage bacteria, 100,000 CFU/g of yeast, or when visible mold spoilage is observed.
Chemical indicators impacting shelf life are primarily rancidity, changes in moisture content or pH level, or the stability of nutrients. Rancidity is the chemical degradation of fats and oils in the product and can result in off-flavors and odors. Oxidative rancidity, the degradation of lipids by oxygen, often results in painty or cardboard-like odors and flavors, whereas hydrolytic rancidity results in textural changes when water degrades fats. Moisture changes in a product can result in freezer burn, staling or texture defects and microbial growth can result in pH changes that affect odor and flavor, such as when acid-producing microorganisms cause a product’s pH to drop and produce a sour or acidic odor and flavor.
Nutrient stability testing is conducted for products where a specific nutrient level, such as Vitamin C, is claimed. Products which contain natural sources of nutrients must contain at least 80% of the claimed value at the end of shelf life, while fortified products must contain 100% of claimed nutrients at the end of shelf life.
Microbiological and chemical changes will often lead to changes in the physical quality of a product. Analytical measures of viscosity, consistency, texture or color of a product can be incorporated into a study design to provide an analytical value correlating to sensory changes. This approach captures a number that can be associated with changes in sensory quality over time and help manufacturers better quantify their shelf life.
From a consumer perspective, sensory is arguably the primary driver of shelf life. While chemical, physical and microbiological changes all impact the quality of a product, the effect of this degradation is ultimately perceived as sensory defects by consumers – changes in appearance, aroma, texture or flavor. A complete shelf life will incorporate sensory evaluation as a component to give better definition to analytical values derived from the study. Chemical and physical changes typically do not have defined acceptance criteria unless specifications have been established by the manufacturer. Instead, correlating sensory data to these levels will provide a broader understanding of a product’s shelf life.
The sensory method used in a shelf life study will depend on the objectives for the interpretation of study results and the specific quality concerns for the product. Affective testing is used to understand consumer liking of a product, and whether the product maintains acceptable quality through the end of its shelf life. Descriptive testing is applied when the primary concern is to understand changes in specific attributes of the product over time, and difference testing can be applied to determine how similar or different a product is throughout shelf life when compared to the quality of a fresh or control product.
Putting it All Together
Designing a shelf life study tailored to the product requires an understanding of the product and any applicable end-of-shelf life indicators, storage conditions, packaging, shelf life expectations and homogeneity of the product. The length of the study will be defined by the desired shelf life, which may be driven by familiarity with the typical product performance, competitor matching, customer requirements, or sales and marketing goals. Optimally, the shelf life study can extend 25-50% beyond that expectation in order to understand how the quality holds up if it is held past its expiration date and several time points (typically a minimum of 5-7) over the course of the shelf life should be included in order to understand how the product changes over time.
If a product will have variation between lots or even between packages of the same lot (as in the case of heterogeneous products), multiple lots and/or multiple samples within the lot can be tested to better establish shelf life. Extrinsic factors, such as temperature ranges, humidity, and light can be incorporated into the study design to simulate real-world storage conditions.
By designing a study that incorporates these considerations, a comprehensive understanding of the true product shelf life can be gained. Shelf life can be revisited as appropriate, being mindful that any changes in formulation, packaging, processing — any of the intrinsic or extrinsic properties impacting shelf life — would warrant a re-evaluation. Incorporating a robust shelf life program into your research & development process will help ensure that customers continue to enjoy the high-quality product that you’ve always delivered!
What Can Mérieux NutriSciences Do For You?
Do you need assistance with a shelf life study? Mérieux NutriSciences’ Research Services division has extensive experience with designing and performing shelf life evaluations. Contact us today to learn more about how we can help.