The Laboratory Notebook

Answering important questions about quality.

By Robert Green

This month we tackle several questions about stability testing and “shelf-life” determination.

Q. I am ready to launch a new product and want to determine an expiration date. How can I do this efficiently and quickly?

A. In today’s modern world there can be long intervals of time between the manufacture, sale and consumption of a supplement or functional food. Products that degrade too quickly simply will not be successful. It is important therefore to know how long that interval can be, and you want to know it before you start to ship. The question you are asking then is how long your product will remain stable? In addition, when setting an expiration date for your product you should not just guess, but rather, compile data to support the period you select. The answer to both questions lies within a stability study, also referred to as a shelf-life study.

A stability study can be open-ended or closed-ended. An open-ended study answers the question: How long will my product last? A closed-ended study will determine if your product survives for a stated period of time. In either case, there are three ways to get your answer. The first method involves
manufacturing your product and keeping it on a shelf, testing it periodically over the selected time-period to determine if its original components are still intact. The shelf should be in a controlled environment. This is called a “real-time” study. It is accurate, but it is by no means quick. To determine
if your product has a two-year shelflife you need to keep it on the shelf for two years. Few have the patience and luxury of doing that.

The second method is an accelerated stability study, which will establish stability in a timelier manner. Here we duplicate the effects of a period of time in a much shorter timeframe, hence the name “accelerated.” This is accomplished using an environmental chamber. This is a specialized computer-controlled laboratory instrument that produces very specific atmospheric conditions inside the chamber. By exposing your product to specific temperature/humidity conditions we can simulate the effects of storage for a specified amount of time in a fraction of the “real-time.” Those conditions are static, that is, they are set at the start and remain constant throughout the study. An example would be 70% relative humidity, 30°C. The actual equivalency rate is determined by a very compli-
cated mathematical procedure based on the Arrhenius equation, which is something we are not going near in this column.

If you want your answer even faster, the third method you can choose from is called a “cyclic stress study.” Here, instead of conducting the entire study with static environmental conditions, we cycle the temperature/humidity from one extreme to another. An example would be gradually changing (we call it ramping) those conditions between two set points as follows: 12 hours to attain 75% relative humidity and 45°C and then 12 hours to attain 10% relative humidity and 6°C. Again, you need to refer to the Arrhenius equation to determine the time, but it is possible to duplicate one year of shelf-life in one month in the environmental chamber under cyclic stress conditions. Obviously, you need a sophisticated environmental chamber to do this.

If you are going for one of the accelerated methods you should also put some samples aside for a real-time study to further support the accelerated findings.

For some products like liquids, emulsions, and oils, the real-time study is the best method. Products with these consistencies tend to degrade much faster under accelerated conditions than they would on the shelf, which means an accelerated stability study may return an inaccurately short shelf-life. Tablets, capsules, and powders, however, return very accurate results when kept in the chamber.

So the next question is, “How do we know under what conditions we should conduct the accelerated or cyclic stress study? For that we go to the ICH or International Conference on Harmonization. It has established accelerated conditions representative of the four climactic regions where products are most likely to be stored. Although these conditions were established for the determination of shelf-life for pharmaceuticals, nutra-ceuticals are no less important, so we use theses standards too.

Q. I heard that water activity can greatly affect the stability of my nutrition bar over time. I do not add water to my product, so do I have to be concerned about this?

A. Yes you do. A stability chamber simulates the extrinsic conditions of storage like temperature and relative humidity, but there are multiple intrinsic factors inherent in all products that lead to its degradation over time. One is referred to as “water activity,” which is a factor that can have a significant effect on the shelf-life of a product.

Water exists in a product in two forms—bound and free. Bound water molecules are part of the molecular structures of the product’s components. You cannot remove bound water without fundamentally changing your product. Bound water generally does not affect the degradation of the product. On the other hand, there is also “free water,” and water activity value is based on this. Free water refers to water molecules that are “extra” and may actually work against the other components in the product. If a product has high water activity and contains components that are soluble or activated to it such as vitamins and enzymes, it is likely to have a much shorter shelf-life than a drier material. The free water will interact with these components and in time the product may no longer meet label claims. When these products react, changes in appearance, smell and taste may result.

Water activity can also lead to spoil-age. In cases where there are microbiological organisms initially present in a product, water activity provides the perfect environment for those microbes to multiply, spoiling the product faster than if the water activity was low. To prolong the shelf-life of your product, make sure it is as dry as possible.NW