Why Food Manufacturers Need to Understand Shelf Life Testing
Dictionaries define "shelf life" as the length of time a product may be stored, as on a supermarket shelf or in a home pantry, without deteriorating.1 Unfortunately, such definitions create misconceptions because the process of deterioration for most agricultural crops begins the moment they're harvested, and for manufactured foods, prior to packaging.
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Fortunately, the rate of deterioration in many foods is slow, which means they can be stored for some time before they become unacceptable or significantly deteriorated. Because deterioration is an ongoing process, it's important to keep food distribution time as short as possible and avoid unnecessary stockpiling.
Food stored in a warehouse in Phoenix, Arizona, for example, will not deteriorate at the same rate or in the same manner as the same food stored in International Falls, Minnesota. Hence, shelf life can never be defined as a fixed amount of time but rather expressed as a range of estimated acceptable shelf time.
When a food distributor chooses to use an open code date (i.e., “best if used by" date) or set a maximum length of storage requirements, good judgment must be used to select the most appropriate time within the shelf life range.
Food stored in a warehouse in Phoenix, Arizona, for example, will not deteriorate at the same rate or in the same manner as the same food stored in International Falls, Minnesota.
It's important to remember, the judgment call you make can directly impact your bottom line.
Selecting a shorter shelf life of your food product could mean paying more than necessary to destroy overly aged products, special handling, packaging materials and, perhaps, controlled temperature storage. But, erring on the other side could mean losing repeat sales due to disappointed customers and possibly even undermining your brand's reputation.
How to Determine Shelf Life: 10 Factors Food Testers Need To Achieve Accurate Results
1. Testing Factors
Some food and beverage producers use a "test and estimate" approach to determine shelf life. They conduct tests in conditions that might be encountered in a boxcar or a warehouse in Florida in July and then extrapolate these results apply to more moderate conditions—the assumption being, one week of the abusive conditions might equal one month of shelf life.
Unfortunately, such extrapolations are often invalid. High temperature and high humidities are used to accelerate shelf life tests; however, the correlation between a given set of abusive and more moderate conditions will vary greatly for different types of foods. Thus, establishing the correlation requires either testing at several conditions or significant prior experience with a given type of food.
2. Establishing Limits
Understanding the innate stability of a food product is the classical goal of shelf life testing. It's crucial you know whether storage quality of a certain product changes significantly in a matter of days, weeks, or months. There are four basic factors to be considered when establishing reliable shelf limits and expiration dates:
- What is the innate stability of the product?
- How does distribution impact storage quality?
- How do ingredient quality, processing, and packaging impact storage quality?
- At what point does quality decrease enough for products to be unsuitable for consumption?
But, regardless of how extensive a shelf life study is, often the test is being run on only one lot of product. This inherently limits the test's effectiveness because it's unclear if that particular lot reflects typical, best, or worst product quality in terms of ingredient quality, food supply chain logistics, processing conditions, or package fabrication. Make no mistake, understanding the storage quality of any product you sell your consumers is absolutely critical.
This also raises an important question about how we understand distribution. In some cases, “accelerated” storage conditions such as our weather room (90°F day, 70°F night at 60-70% relative humidity) may be the reality for an unconditioned warehouse in the summer. But, many shelf life issues occur when products of marginal quality are stored under abusive storage conditions. Knowing what percent of your product will fall into this category will help you understand your risk of shelf life issues.
Shelf life testing is essentially a measure of quality versus storage time. For degradation reactions like lipid oxidation and microbial spoilage, there will be a point at which the product quality will degrade markedly. The data provided in a shelf life study can help you understand the end of shelf life in a given storage condition. Often, however, the quality of a sample will decline more gradually with changes impacting sensory attributes rather than actual food safety.
Here is where we need to determine the sensory quality level at which we no longer want a product available to consumers. For example, should the storage quality endpoint be when the product is less than "fresh?" Or, perhaps, when the food product is clearly objectionable to the consumer? In cases like these, a shelf life study provides the food manufacturer with the storage quality versus storage time data.
The bottom line is: Where shelf life ends for a product is a business decision that needs to be made by the manufacturer of the food product.
3. Testing Requirements
Before setting up a shelf life test, it's important to collect a few pieces of data to obtain a better understanding of a specific food. For example, moisture and water activity (aw) should always be known. Other desirable pieces of information may include the amount and type of fat, the protein content, the type of sugars and starches, ash content, and pH. For high moisture foods, microbiological tests may be needed.
The objective of most shelf life testing is to determine how rapidly microbiological, chemical and physical changes occur in the food during distribution and storage. Since different storage conditions accelerate some changes but not others, it helps to know what changes are likely to occur prior to testing.
Knowing the water activity, for example, will provide some indication of what the major mode of deterioration will be. Water activities above 0.8 are more susceptible to microbiological growth. At an aw greater than 0.55, enzymes can be active. In the intermediate aw (0.25 to 0.55) range, nonenzymatic browning is often the major mode of deterioration. At an aw below 0.25, rancidity often contributes to the loss of quality. Loss of vitamins, flavors, leavening, and changes in color are all dependent on aw, too.
Should the storage quality endpoint be when the product is less than "fresh?" Or, perhaps, when the food product is clearly objectionable to the consumer?
Remember, water activity is only a guide. Some yeast and molds can grow in foods with an aw as low as 0.65. Enzymatic hydrolysis has been reported to occur at aw as low as 0.2. And, lipid oxidation can occur in high moisture systems.
Most foods, when stored, will either gain or lose moisture unless they are contained in moisture impermeable materials. Moisture changes may directly affect the food quality—i.e., its texture—or indirectly by changing the deterioration rate due to change in aw. Some of the product changes (texture, caking, crystallization, etc.) which occur with changes in moisture levels can be determined by placing the food in a variety of saturated salt solutions and allowing the moisture in the food to reach equilibrium. Weighing the samples allows you to calculate the moisture change at the different RH.
By plotting the moisture against RH levels, the moisture sorption isotherm—also called moisture equilibrium or equilibrium relative humidity curve—can be generated.
4. Test Package Concerns
A product's shelf life testing requires packaging designed to deliver accurate and reliable results—it should come as no surprise that the packages in which the product is sold may not be the best choice. As previously mentioned, water activity is critical. If one tests under abusive conditions, the commercial package may not provide the protection required to maintain the normal water activity and reactions may occur which normally would not.
On the other hand, sometimes it's important to understand what would happen if the water activity changes greatly. In such a case, one might want to test the standard package against packages with intentional defects. Either way, just know packaging can make a big difference in the quality of your results and it should be chosen intentionally.
5. Rate of Deterioration
Measuring the rate of deterioration of a product can be more challenging that one might expect. If major changes in the quality of the food occur rapidly, the task of measuring the rate of deterioration is fairly simple. But, if the changes are very gradual, it is difficult to differentiate between deterioration and normal quality variations after storage for a reasonable period of time, even with a control sample stored at 0°F or -40°F.
6. Olfactory Changes
It's important for those conducting shelf life tests to smell all test samples and taste most of them. This allows testers to differentiate between stages of deterioration with clarity. But, because of differences in taste acuity between individuals, sensory evaluations are often unreliable unless large numbers of tasters are used. If evaluations must be done with only a few tasters, it is recommended that differences be accentuated using accelerated storage conditions, containers that will trap off-odors or packages with intentional defects.
Moisture analyses are fast and inexpensive, but moisture changes for many foods are most reliably monitored by package weight changes using empty containers as tare weights. This method is non-destructive, so it's possible to determine variations between multiple individual packages and still be able to return the product back to storage for future testing.
8. Color Changes
Color changes are some of the most obvious components in shelf life testing. Color changes due to non-enzymatic browning can be detected visually somewhat reliably. Color comparison measurements can supplement visual detection if necessary.
9. Lipid Deterioration
For low moisture, high-fat foods—i.e., 25% or more fat—lipid oxidation can be tracked by measuring the changes using peroxide values. For high moisture foods, thiobarbituric acid (TBA) values are often used to detect lipid oxidation. The best rancidity measure for low moisture, low-fat foods is the hexanal2 method, provided the fat contains linoleic acid.
10. Microbiological Tests
For high moisture foods, microbiological tests are needed to track spoilage. To determine the effect of shelf life from a microbiological standpoint, there are several aspects that have to be considered. First, one must determine whether the product will support microbiological activity—if the product does support growth, it is important to determine if this growth would result in product spoilage or present potential consumer safety issues.
To answer these questions, testers must consider pH, water activity, how the product will be distributed and retailed, length of shelf life, preservatives used, and type of packaging. Once the key storage factors of the product are determined, then an inoculated storage test can be designed using the appropriate combination of microorganisms, times, and temperatures.
The bottom line
Shelf life depends on factors like storage temperature, humidity, packaging, water activity, and ingredient quality to determine the rate of deterioration of food. If left unchecked or unknown, the deterioration of your products can negatively impact product quality, brand reputation, and even your bottom line. Submitting your products to consistent shelf life testing will protect your consumers and provide peace of mind for your stakeholders.
If you want to read more about the science behind shelf life testing, check out our in-depth look at the factors that drive food deterioration.
- The American Heritage Dictionary, 1987, Houghton Mifflin, s.v. “shelf life.”
- Fritsch, C.W. and Gale , J.A.; J. Am. Oil Chemists Soc. 54:225