Bend it Like Breakfast
Much like Al Gore, cereal bars burst onto the scene in the 90′s. I recall the novelty of the multi-component nutri-grain bars when they hit the shelves – kind of like a thinking man’s pop-tart. Now Kelloggs makes about 4 million of them a day. Although nutritional experts deride the category as a bunch of glorified cookies, sometimes a granola bar is all the breakfast a guy needs.
This brings me to my mom, a remarkable woman who raised nine kids (I’m number six) and lived to tell the tale. Feeding such a large group required lots of food, most of it kept below room temperature in a downstairs “fruit room” that remains well-stocked to this day. Over the past 25 years, all 9 kids have moved out. In short, the fruit room inventory turnover ain’t what it used to be. This fact was confirmed last summer when mom brought an unopened jar of mayonnaise from 2001 to a family picnic. It tasted like motor oil flavored aspic.
Another family gathering last week saw the appearance of some Nature Valley granola bars that were a year past the “best before” date. Let me first say that they really didn’t taste that bad – maybe a little stale. It was the texture I found interesting. Rather than their normal crispy, brittle texture, I could actually bend these bars (see picture). I thought that maybe the water activity had changed over time, so I took one of the old bars in to work on Monday, and tested the water activity. It came out to 0.386 @ 25C – relatively low. Still, I thought it probably wasn’t as low as a freshly made bar, which I bought from the store. Sure enough, the “fresh” granola bar (that snapped in half as soon as I tried to bend it) had a water activity of 0.247 @ 25.1C.
So what causes the change over time? The most likely suspect is moisture migration into the package. The “fruit room” humidity is in the 50% range. Although many consumers think of packaging as impermeable to moisture and air, this isn’t true. More expensive packaging can suppress conductance, but can’t stop it completely. Over the 18 months or so that the granola bars sat in the fruit room, they undoubtedly absorbed moisture from the environment. The laws of thermodynamics dictate that the difference in energy levels would cause water to flow from the ambient air (0.5 aw) into the bar (0.247 aw to start with). This probably caused the binder (in this case, brown sugar syrup) to change from a glassy to a rubbery state. Another possibility is a retrogradation of starch, a reaction which can release moisture. This would also have softened the binder over time.
A quick and shameless plug: the real interesting data to gather here would be isotherms. If we could run an isotherm on the bar, we could pinpoint the water activity at which it would lose its crispy texture. We could also calculate, in days, weeks, months, the time it will take for a fresh granola bar to turn into a stale, bendy one. My company makes an instrument that measures these things – it’s called the AquaLab VSA. End of shameless plug.
If you’re interested in a rather amusing Nature Valley granola bar cameo, please check out this old time baseball sketch from Conan O’Brien.