Shelf Life with(out) Water Activity
Water activity is a geeky concept known mostly to specialists in the field of food safety/quality. Little wonder, then, that a Time article entitled “Food that Lasts Forever” (subscription) came tantalizingly close to mentioning water activity, but failed to do so. Not that we’re bitter. It actually makes sense to avoid the topic in a light piece meant to draw the reader in, since explaining the concept of water activity could take the article down a rabbit hole.
Still, many of the items that show up as long-lasting owe their staying power to water activity. Exhibit A: the “indestructible” sandwich – a bread envelope stuffed with barbecue chicken or pepperoni. These show up in MRE’s used by the US Military, but aren’t really that new. They first garnered attention in 2002 in a news piece that actually does mention water activity. The concept is simple: the bread and the filling must both be formulated at the same water activity, or moisture will move between the two, harming the product. These sandwiches are said to last for three years.
The Time article does focus on a technology that is rather new: using high pressure instead of temperature to kill bacteria. This method tends to result in food that tastes, looks, and feels fresher. Most of these types of foods are high moisture, though, so water activity isn’t much of a factor.
Water activity does have a role to play in shelf life determination for intermediate moisture foods. Most of the reactions that degrade food products like potato chips, crackers, beef jerky, and protein bars are affected by water activity. Sometimes even a small change in water activity can cause rates of degradation to double or triple. This effect is one we are researching at METER, so we’ll share more information as it becomes available.