The Physics of Why Old Fruitcakes Don't Die

Now there's really no excuse for throwing out that old fruitcake — even if it's been sitting around for years.

According to physicist and food scientist Peter Barham, a little physics can revive a stale and crumbly relic into the moist, weighty item it once was.

"You can stick a fruitcake, or for that matter any cake in a bag, leave it on the shelf for three to four years, come back to it and make it better than ever," contends Barham, a physicist at the University of Bristol in England and author of the book The Science of Cooking.

Dismantling Crystals

The trick lies in the cake's starch crystals.

Cake flour is most often derived from wheat where starch molecules grow in rigidly arranged crystal forms. The alignment of the starch crystals in wheat is so regular that the human body's enzymes aren't capable of breaking them down for digestion. To ease that process, the starch (flour) is baked with wet ingredients.

At 140 degrees Fahrenheit, the crystals start to melt, lose their orderliness and mingle with water to form a jelly-like substance that's moist to the tongue. But, Barham explains, starch is not inclined to stay that way.

"The starch molecules would be happier in their crystal forms," he said. "But there's no way they can get back to their original forms, so they do the next-best thing."

For starch crystals trapped in a fruitcake, the next-best thing is to suck in surrounding water molecules and lock them into small water-starch crystals. Once moisture is absorbed by these order-craving molecules, the cake becomes tough and dry.

But, not to fear, Barham says there's a simple solution. He suggests wrapping the cake in aluminum foil to prevent moisture from escaping and slowly warming it in a 130-degree oven. The crystals will melt, release their water and — voila.

Barham concedes the cake might not be quite the same as its former self since some water could have evaporated into the environment and starch can also degrade over time.

"It might not be exactly the same," he said, "but it's very very similar."

Baked to Last

And even though it may be detested by many, the fruitcake holds many advantages over other baked varieties. All those pieces of dried fruit act like sentries — grabbing water molecules that might otherwise escape by evaporation.

Fruitcake also has high sugar content and since sugar is highly acidic, it blocks or weakens any invading bacteria that might try to form moldy layers. If the cake is spiked with brandy, all the better, since alcohol is also highly acidic and also an effective preservative.

Some even argue an old fruitcake ages and improves like a good wine over time. Barham explains the tannins inside the fruit (which are also part of the aging process of wine) eke out and mix chemically with the rest of the cake, creating new flavor compounds.

These flavor compounds, he says, become more intense and varied with time — and are all the more reason to pull that ancient fruitcake out of the freezer and reheat it.

Or ... give it to someone else.

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