Water activity for inspectors
USDA inspectors expect to see water activity as a critical control point (CCP) in HACCP plans, typically during drying and packaging. A water activity below 0.85 is an FSIS regulatory requirement for shelf stability HACCP model. Other moisture measurements, including MPR and moisture content, are “not safety considerations” and aren’t included in HACCP plans. The USDA explains, “Product water activity is best correlated to inhibition of each pathogen’s growth.”
Why does lowering water activity inhibit the growth of pathogens?
As the Marianskis so vividly put it in The Art of Making Fermented Sausages, controlling water activity “is like stealing food from the bacteria.” Lowering water activity “locks up” water, ultimately making it impossible for bacteria to reproduce. Let’s look at this in more detail.
Listeria, E. Coli, Staph, and Salmonella are tiny organisms, and like any organism they need water to grow and reproduce. They get that water by sucking it in through the cell membrane that surrounds them. This suction power comes from energy differences between the water outside and the water inside the cell.
It’s easy to see that water moves from high to low energy: picture a pot of boiling water.
Water molecules move (as steam) from the high energy boiling water into the lower energy atmosphere.
Energy differences between a cracker and a piece of cheese are less extreme, but as the cheese sits on the cracker, water molecules will move from the cheese, in which the water has a higher energy, to the cracker, in which the energy is lower.
This principle holds true at the molecular level too, and pathogenic bacteria use it to pull water from the higher water energy environment outside the cell to a lower water energy environment within the cell.
If, however, you lower the water energy outside the cell enough, it causes “osmotic stress”: the cell can’t take up water and becomes dormant. Osmotic stress doesn’t kill pathogenic bacteria, it just makes them unable to reproduce.
Water activity is simply a measure of the energy status of water in a material. You can use it to see if the water in a piece of jerky has enough energy to support a particular strain of bacteria. Different pathogens cope with osmotic stress in different ways. That’s why Staph is able to survive at lower water activities than Listeria.
Lowering water activity doesn’t kill bacteria, but after a kill step like heat treatment, water activity will control bacterial growth, and that’s why it’s the FSIS’s moisture measurement of choice. It can be much more, however. Water activity has significance beyond FSIS, the USDA, and HACCP. It’s a powerful way to measure and understand water in your product.
Process control with water activity
When you’re drying, water activity is the best way to control your process.
That’s because it directly gets at the three questions you have to answer:
-Is my product safe?
-Will consumers like it?
-Can I make money producing it?
Customer satisfaction: Guaranteeing consistency batch to batch
To satisfy customers, you need to figure out how to consistently deliver the product your customers want.
When you’re drying your product, you have to hit the same drying target time after time. If you’re measuring moisture content, MPR, or some other attribute, you’ll end up chasing your tail.
As you adjust time or temperature, the water activity values respond. And when you taste test your product, you’ll discover that the water activity measurements are highly correlated with jerky texture.
Unlike other moisture measurements, water activity is standards based. Meaning that the measurement you make today can be compared to the one you made last year, or to the one a different operator made. You can compare values from batch to batch, between different lines, and between processing locations. And you can use real-time water activity data to adjust your oven temperature or your baking time.
It also lets you verify ingredient quality coming from suppliers and prove the consistency of your product quality to buyers.
Profitability: Prevent losses from over-drying
In product testing, we found that over-drying jerky is easy to do if you don’t monitor water activity. Even small variations have an effect on quality and profits.
In fact, we found that a small difference in water activity spec was like tossing a quarter into each bag of jerky before shipping. Using water activity to determine when your product is dried correctly can prevent hundreds of thousands of dollars in waste.
By measuring water activity, you can hit the sweet spot-–the point at which your jerky’s water activity is low enough to be safe and high enough to satisfy your customers. Then you can hit that spec every day to guarantee profitability.
Process control with real time data
There are two keys to using water activity as a process control. First, the water activity measurement has to be fast. And second, the data need to be delivered in real or near-real time.
Quick, easy, at the line
Traditional AOAC-approved moisture content measurements are nearly impossible to make during the drying process. And rapid loss on drying isn’t accurate enough. In contrast, chilled mirror dew point and tunable diode laser water activity measurements can both be made at the line in 5 minutes or less.
And if you can microwave a burrito, you can use these instruments to measure water activity to 0.003 aw. No science degree required.
Real time data
Automated data collection is essential for using any measurement as a process control. New technologymakes it possible to collect data directly and automatically from water activity meters and other existing instruments. Learn how one manufacturing facility overcame the “nightmare” of manual data collection so they can get the near real-time data they need to monitor and control their oven temperature.
No one right answer
We tested a wide variety of shelf stable meat products in our lab. These products were controlling water activity using a number of different humectants, including salt, soy sauce, and sugar. Only one contained glycerine. As these products show, there are many strategies you can use to get to a safe water activity.
Table 1. Water activities of a variety of commercially available shelf-stable meat products
More sophisticated formulation
This article has principally explained the basics of water activity and shelf-stable meats. If you’re interested, you can go much deeper in understanding and controlling moisture in your product. For example, you can:
-learn how to control water activity by adding humectants (salt, sugar, soy sauce)
-model what happens when you vary drying time and temperature
-determine the shelf life of your product
-understand what happens if your product is stored at high temperatures or humidities
–evaluate packaging materials and understand the interaction between packaging and shelf life
-discover how wetting up or drying out your product affects its water activity
Marianski, Stanley, and Adam Mariański. The art of making fermented sausages. Bookmagic LLC, 2009.