The beginner’s guide to shelf life stability and packaging

Over-packaging erodes profits

Why do shelf-life testing? Insufficient packaging allows water activity in food products to rise or fall over time—causing undesirable physical changes, moisture migration, chemical degradation, and susceptibility to microbial growth. Over-packaging, on the other hand, is expensive and can erode profits. How can you figure out the exact amount of packaging your product needs? All of these issues are controlled by water activity. If you understand how water activity works, you can develop and package products that stay safe and desirable for the entirety of their shelf life—without overspending.

Figure 1 is a stability diagram showing water activity and moisture content. Water activity is shown on the x-axis, and the y-axis illustrates reaction rates (think of moisture content as an increased rate of reaction). The dark blue trace is a generic moisture sorption isotherm. A moisture sorption isotherm illustrates the relationship between water activity and moisture content in a product. The other traces represent modes of failure. You can see that the growth rate of mold, yeast, and bacteria increase exponentially as water activity increases. The rate of enzyme activity begins to increase significantly at just under 0.9 and increases as water activity increases. However, lipid oxidation is different. At very low water activities it is high, but then as water activity increases to about 0.3 to 0.5, it becomes stable. Above 0.5, lipid oxidation again starts to increase. Browning reactions peak at approximately 0.6. The blue shaded section in Figure 1 illustrates that at a water activity range of 0.3 to 0.5, physical deterioration or texture changes can occur: loss of crispness, caking, or collapse of the food matrix.

Figure 2 is a graph showing several different moisture sorption isotherms generated by the AQUALAB VSA. It plots water activity along the x-axis and moisture content along the y-axis. The medium blue trace illustrates that milk powder has a critical water activity of about 0.42. How can you tell? At about 0.42, you see a large increase in moisture content with a small increase in water activity. This is where caking and clumping will start to occur. 

Interestingly, milk powder has a second critical water activity between 0.7 and 0.8 where crystallization starts to occur. For cereal (dark blue trace), the critical water activity is approximately 0.5. Grain-free kibble’s RH_c is closer to 0.7 because of microbial growth. Pet food companies have to stay below this level. Sucrose has a critical water activity of about 0.85 where there is a sudden increase in moisture content with just a small increase in water activity. 

Also note that a critical water activity is temperature-dependent. As you increase the temperature of your product, your critical water activity will decrease. Thus, it’s important to know the RH_c and your product’s storage conditions (temperature). You can then combine product formulation and packaging to stay below the critical water activity throughout production and shelf life.

If rancidity is your mode of failure, then oxidation levels must be measured. If a change in texture ends shelf life, you’ll need a moisture sorption isotherm (see Figure 2). For vitamin degradation, you’ll measure vitamin levels. Color changes can be assessed using color imagery (also called colorimetry). And enzymatic reactions can be assessed by looking at enzyme activity. After identifying what type of data you need to collect, you can perform accelerated shelf-life testing to identify the ideal water activity range for your product. The following chart illustrates a few scenarios of what an accelerated shelf-life testing process might look like after identifying your most likely mode(s) of failure.

Here are the details of how to set up a test: 

“Accelerated” testing means elevating the temperature and the water activity to see things happen faster. Because RH_C decreases as the temperature increases, you’ll select three different water activities and three different temperatures and hold your product at a combination of each of these three (nine subsamples) until the product becomes undesirable. During this time, you’ll track the progress of the chosen mode of failure. For example, if you’re tracking lipid oxidation, you would measure the oxidation level until it becomes unacceptable to the consumer. (You decide what “unacceptable” is since some changes occur in the product when it’s still acceptable.) Over time, collect time versus quantity of change data to determine the rate. Then model that collected data accounting for both time, temperature, and water activity. When you achieve this correlation you’ll be able to model the shelf life and the reaction rates for your mode of failure. 

The next step is to determine at what water activity the chips lose texture. To find out, the manufacturer needs a moisture sorption isotherm which will pinpoint exactly when he’ll see that change in texture (Figure 4). 

Figure 4 is an isotherm for a kale chip with water activity along the x-axis and moisture content along the y-axis. Visually, you might guess that loss of texture occurs at an inflection point in the trace where the sorption properties dramatically increase. However, it is tricky to exactly identify that point, so the simplest way is to perform a second derivative evaluation on that slope. A second derivative evaluation looks at the slope to determine when a change in the slope occurs, indicating a change in the moisture take-up (Figure 5). 

In the second derivative on the right of Figure 5, the first peak is going to be the RH_C. You can see when it is compared to the isotherm on the left, it correlates well. Thus, if the kale chip manufacturer can keep the chips below this critical water activity of 0.57, they will maintain their crisp texture and no longer be susceptible to microbial growth. Maintaining correct water activity levels is easy by using an AQUALAB 4TE water activity meter (watch the video to see how it works).

In any environment, there will be a certain amount of water in the air or relative humidity (RH). The packaging you select only allows a certain amount of that water to pass through and interact with your product. This is generally measured in grams per meter squared per day. Your packager tests the packaging under certain conditions (usually approximately 38 degrees Celsius and 90% relative humidity). Those conditions will come into play for your shelf life calculations. In addition, you need to know the surface area of your packaging in meters squared and the mass of your product within the package.

Other information required is the product storage conditions: temperature, humidity, and atmospheric pressure. The atmospheric pressure depends on your elevation and can also vary with differences in weather. 

Finally, you’ll need to know your product’s water activity. This includes the initial water activity and also the critical water activity. 

The software will ask you to enter your water vapor transmission rate, testing temperature and humidity (usually about 38 degrees Celsius in 90% humidity). Next you’ll enter the storage conditions of your product and some information about the product itself. In Figure 7, we placed the product somewhere with a 60% humidity and 100 kilopascals of atmospheric pressure. The product is 454 grams and it’s stored in an environment that’s 30 degrees Celsius. You calculate the surface area for your packaging and then input your initial and critical water activities. Using the software, you can quickly select your previously saved isotherm file (Your product isotherm is automatically calculated and stored in the software using the AQUALAB VSA instrument). 

After inputting the information, hit calculate, and the software provides you an estimated shelf life (in this case 30 days). To change or extend the shelf life, look for packaging with a smaller water vapor transmission rate. 

If you want to increase shelf life, you can use another calculator in the moisture analysis tool kit designed just for this scenario (Figure 8).

Figure 8 shows that to obtain a shelf life of 180 days, you need a packaging WVTR of 1.3. You can take this information to your packager and tell them that you need something with this WVTR to achieve your desired shelf life. 

Table 2 is a comparison of some common packaging materials.

It’s important to know that these water vapor transmission rates were obtained at 38 degrees C and 90 percent relative humidity. But that’s not always the case. Sometimes they are obtained at 30 degrees C and 75% relative humidity. Also note that this table is in metric units, and that’s how the software calculates it, but sometimes WVTR is listed as grams per meter squared per 24 hours. Or it could be in standard units such as inches squared. So it’s important that the units are correct when you’re inputting WVTR for packaging material. Note that the polypropylene has a WVTR of 8.2, but a polypropylene that is oriented and has a metalized layer, the WVTR reduces to 1.0. It’s good to understand what kind of packaging you need because you don’t want to under-package. Using the kale chips as an example, Figure 8 shows that with a packaging WVTR of 7.5 the food manufacturer is only able to maintain the product for 30 days. But if the manufacturer chooses packaging with a WVTR of 1.3, the product will last for six months. However, note that the lower the WVTR, the higher the cost will be, so don’t over-package, or you’ll pay for packaging you don’t need.

To the upper left, you add different ingredients. At the bottom left are the results. For one pound of kale chips, we entered the initial water activity and mass to find out what happens to the water activity when adding five grams of garlic powder. After inputting the information, hit calculate, and the software provides a new final water activity for the blend. In Figure 9, the water activity came down slightly. The software also tells you the final moisture content for the kale chips and garlic. The graph on the right shows how the isotherms are being combined (the kale chips are the blue trace, and the garlic is the green trace). The red trace is a combined isotherm, and the program also provides the equilibrium water activity (0.449) which will be the final water activity of the mixture.