Water activity and processed meat

Water activity and processed meat

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. 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.”

But why?

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” (Marianski et al., 2009). 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 molecules from high energy boiling water steaming 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 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 FDA, and HACCP. It’s a powerful way to measure and understand water in your product.

The most accurate way to test water activity

This primary method of measuring water activity gives you the best speed and accuracy

Moisture super-spec for quality control

There’s a guy in our office who likes his jerky to be just a little tougher than old shoe leather. Most other people in our taste tests preferred a moister, more easily chewed product.  Only you know your market and the texture you’re aiming for. But water activity specs should be part of your recipe for consistency. Because once you’ve identified the water activity sweet spot–the point at which your jerky is safe and savored–you want to turn that point into a moisture super-spec.

The beauty of water activity is that you actually know when it’s done–done right–and you can dry to that exact point every time.

  • You can measure during drying to make sure that each batch reaches that point
  • You know you have satisfied safety requirements
  • You know that you are not over- or under-drying your product

Quick, easy, at the line

Traditional moisture content measurements are nearly impossible to make during the drying process. Water activity can be measured at the line in 5 minutes or less.

Make anyone an expert

If you can microwave a burrito, you can measure water activity to 0.003 aw. Really. No science degree required.

Consistency batch to batch, supplier to buyer

Unlike other moisture measurements, water activity is standards based.  This makes it easy to comply with HACCP procedures that require verification, and it also makes it easy to compare values from batch to batch, between processing locations, and even between suppliers and buyers. Water activity makes your life simple because it’s a moisture number tied to safety on one side and standards on the other.

 

Water activity is a powerful formulation tool

Find the best packaging, determine shelf life, model what happens when you vary drying time and temperature

The bottom line: 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. Use water activity to determine when your product is dried correctly, and water activity could start adding to your bottom line.

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.

BrandType% Moisture ContentWater Activity
Refrigerated brandCrumbled Italian sausage51.180.9714
Refrigerated brandTurkey sausage slices55.020.9404
Local store brandBeef jerky40.660.8655
Generic brandSmoked and cured beef steak in sweet BBQ sauce36.070.8524
Leading brandItalian recipe pepperoni (slices)25.890.8344
Chain store brandBeef jerky32.370.8366
Vegan brandMeatless vegan mushroom jerky–hot and spicy26.040.8309
Bulk sellerNatural buffalo jerky (slab style)–no preservatives24.900.8286
National brandNatural beef jerky–no preservatives27.700.8198
National brandNatural BBQ flavored pork jerky–no preservatives23.550.8151
National brandSmoked meat sticks (beef, pork, and chicken)17.230.7903
Bulk sellerOrganic teriyaki beef jerky25.590.7899
Leading brandPremium smoked beef jerky25.050.7836
National brandExtra tender beef jerky27.190.7764
National brandBacon jerky18.580.7542
National brandLong pepperoni snack sticks11.060.7094
Variety brandNatural wild caught salmon jerky–no preservatives16.430.7094
National brandOld fashioned beef jerky20.130.6701
Variety brandNatural solid ahi tuna jerky–no preservatives14.740.6069
National brandOld fashioned beef jerky (slab style)13.950.5798
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

References

Marianski, Stanley, and Adam Mariański. The art of making fermented sausages. Bookmagic LLC, 2009. Book link.

Predict shelf life scientifically–no degree required

Water activity simplifies accelerated shelf life testing. Find out how you can do these tests in-house.

Step by step guide to adding humectants

You can control water activity by adding salt, sugar, soy sauce, and other humectants.