Thursday, October 28, 2010

Is this safe to eat?

Due to the massive storm that hit the Midwest earlier this week, we lost power at our house for 18 hours. After it came back on, we did the usual - reset the clocks, purge the air in the water lines (we have a private well - no electricity means no water means the system loses pressure and lets in air) and then the nasty task of triage in the fridge: what do we keep and what do we toss.

In a previous employment situation, I worked on developing a time-temperature indicator. Perishable foods are sensitive to both time and temperature - you can't keep food safe forever even if the storage temperature is ideal - the degradation kinetics still keep clicking, albeit slower at lower temperatures. As with any chemical reaction, the kinetics are governed by the Arrhenius equation:r = A exp (-Ea/RT), although it is usually plotted on a semilog plot where the slope is -Ea and the intercept is ln(A). The higher Ea is, the more sensitive it is to temperature variations.

So given this, we looked in the fridge. We had cups of yogurt (expiration date in about 1 week) and cups of pre-made pudding (expiration date sometime next year). You may be shocked, but I pitched the puddings.

Why? Keep in mind that these are all perishable food items that need to be refrigerated at all times - that means that at room temperature, they have a short shelf life. But since the puddings have such a long shelf life at proper storage temperatures, that means they have a very high activation energy, and that they suffered far more from the fridge warming up (it was 55 oF when the electricity came back on.)

Similarly, in the freezer we had some ground beef that is stored in a modified atmosphere package, a package which also elevates the activation energy for the degradation. We ate it last night, or else I would have pitched it.

Modern food preservation techniques are all prone to this problem: they increase the shelf life at proper temperatures but do little to help at abusive storage conditions. People see the long shelf life dates and feel reassured.

1 comment:

Anonymous said...

great insight. thank you!