What's the Science Behind Fruitcakes?
- Anjali Carl
- 3 days ago
- 3 min read
Every December, the fruitcake returns, dark, dense and studded with bright bits of dried fruit. It’s the dessert people seem to joke about, but it never seems to spoil. Properly stored, fruitcakes can last a month in the pantry and six months in the refrigerator.
What is it about fruitcake that makes it practically immortal in the baking world? It’s a mix of chemistry, preservation techniques and some physics.
Sugar. That’s the first secret to creating a long-lasting fruitcake. It isn’t there just for sweetness; sugar also protects against spoilage. High concentrations of sugars bind to water via hydrogen bonding. This lowers the water activity in a cake, which is a measure of how much free water is available for microbes to grow. Most mold and bacteria need a water level of 0.9 or higher to survive. Fruitcake’s water activity is usually between 0.6-0.7 due to the amount of sugar. The lower water activity doesn’t mean fruitcake is dry
The second secret to fruitcake’s longevity is the addition of dried fruit. The candied cherries, raisins and orange peels are dried and soaked in syrup. Getting rid of fruit’s water through dehydration and replacing the moisture with sugary syrup decreases fruit’s water activity. The candying process also removes moisture through osmosis- as fruit sits in syrup, water moves out while sugar moves into the fruit. With the decreased water activity and the increased sugar, the dried fruit helps contribute to reducing the overall water activity of the fruitcake.
Fruitcake also incorporates alcohol, usually brandy, rum or whisky. Alcohol is a pretty strong anti-microbial agent that works in two different ways against bacteria, yeast, and mold.
Alcohol in high concentrations can denature proteins, including important microbial proteins and enzymes and making them useless.
Alcohol is a desiccant and will draw out moisture. With microbial cells, it will pull the water out of the cell, thereby dehydrating it. Also, alcohol can disrupt the microbial phospholipid bilayer making it unstable and letting the cell content spill out which kills the cell.
Over time, the alcohol diffuses throughout the cake, which is called liquid-phase diffusion. This allows flavor molecules to move from higher to lower concentrations and explains why fruitcake develops a deep flavor over time. If fruitcake is wrapped tightly and fed alcohol periodically, over time it will essentially “self-sterilize”.
One of fruitcake’s classic characteristics is how dense it is but even the texture contributes to its longevity. Fruitcake’s tight crumb, combined with fat from butter and eggs, forms an environment where it’s hard for oxygen to diffuse. Oxygen is needed for microbes like bacteria and mold to grow, so without the essential element, fruitcake isn’t as quick to spoil.
Even the spices in fruitcake serve a purpose. Classic spices include clove, cinnamon, allspice, and nutmeg. They all have antimicrobial properties, and it was the way food was preserved before refrigeration.
All of these forces put together, low water activity, dried fruit, limited oxygen, dense structure and spices, fruitcake becomes one of the most stable foods ever. With proper storage and wrapping, it could last months at room temperature and even longer when kept cool and periodically soaked with alcohol.
While it’s easy to make fun of it, fruitcake is full of food science.
Anjali Carl is a junior at Cedar Park High School in Cedar Park, Texas. They have been excited about food science since making ice cream in a third-grade science class, and now enjoys baking and recipe development. Through their Girl Scout Gold Award project, they created two cookbooks focused on reducing food waste for food pantries and college students. They plan to study food science and eventually become an ice cream chemist.
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