Using Moisture Levels to Predict Food Shelf Life

As food industry professionals and as consumers, we all know how a packaging moisture barrier is used to maintain the quality of a food product. As consumers, we’ve all gone to our pantries and discovered once crispy crackers have become soggy or once succulent dried fruits have become tough.

It's likely we deemed these products to be of unacceptable quality to consume ourselves or serve to our families. At times like these, we may blame ourselves as consumers for not doing an adequate job of closing the package after it's been opened. But as food industry professionals, are we doing all we can do to ensure our products are reaching our consumers’ pantries at optimal quality?

We're here to help you understand the quality implications of selecting a moisture barrier film for the food products your customers take home to their families. To do this, a few simple storage tests will help us understand: 1) the moisture tolerance of your product; 2) the moisture barrier our package provides; 3) how to apply this information to storage environments, using a classic packaging equation.¹

Finding the Moisture Tolerance of Food

To understand the moisture level of our product, we need to know the moisture level of our food at the moment of packaging, and the corresponding moisture level the product is no longer viable for consumers. These are typically referred to as the "production moisture range" and the "storage moisture range," respectively. Let's use a breakfast cereal with raisins as an example.

The production moisture range for the blended moisture of the flakes and cereal is likely between 8-9%. This is the moisture level of the product when it leaves the manufacturing plant. The flakes may become very tough at around 10.5% blended moisture, and the raisins may become tough at around 6.5-7% moisture. Thus, we must select a packaging film that provides adequate protection to ensure the food stays within the storage moisture range over the shelf life of our product.

But how do you determine the moisture range for a product? Shelf life testing can provide fundamental information that will help you make this critical business decision. Generally speaking, generating a moisture sorption isotherm at pertinent temperatures on the product will provide basic information on texture and moisture as a function of water activity and storage humidity.

Understanding product texture as a function of moisture can be further tested and refined in a simple storage study, in which packaged product is exposed to a humid storage environment and a dry (low humidity) storage environment. Degree of difference (DOD) sensory scores, initial analytical moisture values, and storage moisture values generated by package weight change will provide key insights into the moisture tolerance of your food.

Finding the Moisture Barrier of a Package

The first indication we have regarding the moisture barrier protection provided by any food's packaging is the water vapor transmission rate (WVTR) value stated in the packaging film specification. While this is a good and convenient place to start, it rarely provides the entire picture we need to understand the product quality consumers will be experiencing.

Knowing the moisture barrier protection provided by your finished package is critical to understanding the product quality your customers will see and ensuring you're getting the moisture barrier protection you paid for.

There are two potential caveats to applying the WVTR value to moisture limited shelf life questions.

First, the WVTR value is generally expressed as a maximum value. Film manufacturers typically produce their films at a lower (better barrier) WVTR to prevent any lots of film from being rejected. Thus, the WVTR specification is typically one end of a classic bell shape curve, with the actual film average being two or more standard deviations of production variability under (better) the stated specification value.

The second consideration is understanding how the process of converting film into packaging impacts the moisture barrier of our finished package.

An excellent packaging production process may result in finished packages with almost the same package WVTR as the film WVTR. A less-controlled process may result in finished packages that possess only a fraction of the film's moisture barrier. The same moisture by weight gain study used to calculate moisture tolerance can be used to calculate the average and the range of WVTR for the finished package.

This information may be applied to help understand the actual moisture barrier protection provided by your finished package. Knowing this information is critical to understanding the product quality our customers will see and ensuring you're getting the moisture barrier protection you're paying for from the packaging film.

Using the Data to Predict Shelf Life

Once we understand the moisture tolerance of our food and the barrier protection provided by the finished package, we can apply a modified version of the packaging equation developed by Labuza¹ to help us gain insight into how the product and package should perform in the field.

A caveat with the application of this equation is that we need to ensure the packaging is absolutely controlling the moisture transfer with the environment. For example, a 1-ounce bag of corn snack chips in a metalized packaging film is a good example of a package controlled product. The time it would take for the moisture to equilibrate throughout the bulk of the chips would be rapid compared to the rate at which the water vapor permeated the packaging film.

A five-pound bag of flour, on the other hand, is an example of a very poor product to which one would apply this equation. For the flour package, it would be the bulk of the food that would limit the rate of moisture transfer, not the paper-based packaging material.

The true value of the predictive tool is that it allows us to apply the data to any pertinent environment where we have temperature and humidity data. Temperature and humidity data from a recorder in the actual storage environment is preferred, but weather service information can generally be applied for storage in unconditioned warehouses in a given city or region. Combining this information with the data will help you answer several questions, including:

1. How will your product at the target moisture in an average package perform across a range of storage environments?
2. How will high-production moisture product perform in the most humid storage environment (location and seasonality) in the typical package versus the package with the poorest barrier?
3. How will low-production moisture product do in the driest pertinent storage environment?
4. How will the quality of the product change if you change the packaging barrier, production moisture range, or storage moisture range?

The final word on storage quality

We are making a number of assumptions when we answer these questions using the tools described. These tools can help us estimate the quality of market pull samples or long-term storage study samples, help us confirm or refine these assumptions, and help us better understand other mechanisms—e.g., rancidity, browning, flavor loss, etc.—that impact storage quality.

But basic moisture tolerance information can be invaluable in helping you make better business decisions regarding the moisture limited storage quality of the product your customers will be placing in their pantries. Check out our blog post "Shelf Life: Understanding What Drives Food Deterioration" for an in-depth analysis of other factors that can reduce shelf life.