Examining Simplified Shelf Life Testing Methods

Shelf life is the practical time that a product remains desirable to a customer and includes the span in which a customer would want to purchase and consume the product. Shelf life also refers to how long a product can remain safe for consumption. The end of shelf life can be caused by microbial spoilage, mold, and hazardous pathogenic bacteria growth.

Factors that affect shelf life

Several factors affect shelf life. The type of product, how it is stored, and how it's handled during manufacturing all contribute to shelf life. Shelf life can refer to both the quality and stability of a product. 

Many manufacturers communicate a product's shelf life using best-by or use-by dates, though none of these terms is regulated. Despite the lack of regulation, many companies try to be mindful and conservative when determining a product's shelf life to protect both the consumer and the brand’s reputation.

Shelf life depends on intrinsic and extrinsic factors. Intrinsic properties include water activity, matrix type, chemical reactions within a product, the structure of a product, and ingredients. Extrinsic factors affecting shelf life include storage humidity, temperature, packaging, and oxygen content. Different ingredients will have different modes of failure when exposed to extrinsic factors. 

Within appropriate water activity limits, the shelf life of a product is related directly to quality. A product that goes beyond its shelf life could lose flavor, change flavor or oxidize. It could also experience moisture migration, loss of texture, nutrient loss, and/or changes in acidity that affect flavors and cause odor.

How to determine shelf life

Determining the shelf life of a product can be difficult because manufacturers must consider many variables. Some companies guess based on similar products and make conservative comparisons. 

Some also store the product and wait until its quality or stability is unacceptable, often using expensive sensory or instrumentation testing — though this method might not be very efficient, as you could wait a very long time for a product to become unacceptable.

An efficient method of accelerating shelf life testing involves subjecting the samples to increased temperature and water activity. These factors will speed up potential processes and shorten the shelf life for more efficient observation. Once you have your data, you can then extrapolate the findings to determine the shelf life under normal circumstances.

Shelf life simplified

However, shelf life simplified is a step-by-step approach for determining shelf life. This approach follows a predetermined process by eliminating the most unlikely modes of failure and focusing on the most likely modes of failure. 

Using the simplest method to track the mode of failure gives you an idea of when the product fails. In addition, this method allows for less expensive ways to track specific modes of failure and can factor in the challenges and pitfalls that accompany accelerated shelf-life testing.

The simplified shelf-life process is as follows:

1. What is the expected water activity of a product at a typical storage temperature? Use a water activity instrument to determine this value.
2. What is my most likely mode of failure? The most likely mode of failure can be determined using water activity limits and will isolate the most likely options. 
3. What is the ideal water activity range?
4. What packaging would be most appropriate?

A product with a water activity level of 0.85 will likely encounter microbial spoilage and will probably require refrigeration. Products with a water activity of 0.7 to 0.85 risk microbial spoilage and chemical instability. (Chemical instability is where the chemical reactions are at their highest, often affecting both flavor and smell.) A water activity limit of 0.4 to 0.7 is most likely to fail because of chemical instability and moisture migration. (Moisture migration is most common in multicomponent snack products.) Products with water activity between 0.2 and 0.4 risk texture change, chemical instability, and acidity.

Shortbread cookie experiment

A shortbread cookie has a water activity of 0.4 or less, meaning that the most likely modes of failure are chemical instability and texture. (We can eliminate moisture migration as a contender because a shortbread cookie is not a multicomponent product.) By taking several shortbread cookies and holding them at different water activities and temperatures, we can best identify the role of water activity and temperature in this product’s stability.

We can create a model that accounts for both temperature and water activity, forming a DDI isotherm curve, and look for the inflection points to identify when the water activity starts to affect the texture of the shortbread. (If there is no DDI curve, move on to an more in-depth texture study and sensory panel.)

Packaging for shelf life

Permeability must be considered when determining shelf life and packaging a product. You must consider how quickly water activity inside the container will change, what impact that change will have on the product and its texture, and the chemical stability of the product. In addition, packaging must consider the surface area of the packaging, the total mass of the sample inside the packaging, the humidity of the surrounding air, and worst-case scenarios for abuse conditions.

Identifying the ideal water activity range for a product will maximize shelf life beyond just determining what that shelf life is. Knowing the water activity limit will lead to good process control and appropriate packaging.