Nano Girl Michelle Dickinson: Understanding structure of matter can improve your cooking

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Cake will last longer if you keep it in the freezer. Photo / Getty Images
Cake will last longer if you keep it in the freezer. Photo / Getty Images

As a materials engineer, my job is to understand how the structure and composition of materials change how hard or strong things are. Mostly I deal with traditional engineering materials like metals and polymers, but sometimes I get to study my favourite type of material - food.

Food is actually an engineered structure, consisting of water, proteins, carbohydrates and fats that each undergo a series of changes during mixing, whipping and cooking to create a product with the desired properties and hopefully taste.

Those engineering skills came in useful this week when faced with the important question: Why, when you leave them out do biscuits go soft, but cakes go hard?

Many people think that because stale cakes feels dry, the staling must be caused by a loss of moisture.

However, a simple science experiment using bread carried out by Jean Baptiste Boussingault back in the 19th century showed that even when hermetically sealed so no water was lost through evaporation, the bread still went stale over time.

This is because the secret to staling is not due to the lost moisture content but due to the organisation of the starch molecules in the flour. In its natural state, starch is a water insoluble crystalline granule with a rigid and repeating structure. However, when heated up past the gelatinisation temperature, which occurs during baking, the crystalline granules change their structure so they can absorb nearby liquid molecules and form amorphous swollen starch molecules. These swollen granules are soft and hydrated, which is why your freshly baked cake feels squishy and moist. When cooled, the starch starts a process of recrystallisation, where the molecules begin to transform back to their more ordered crystalline structure through a process called retrogradation. The more starch that is transformed, the harder and drier the cake will feel.

Biscuits also contain starchy flour and the same staling reaction occurs in biscuits, making them more brittle. However, because biscuits have a higher sugar content and a lower moisture content than cakes, the crystallised sugar granules absorb moisture from the surrounding atmosphere and become partially dissolved. The moist sugar swells losing its rigidity and leads to a softening of the biscuit which is much more noticeable than the brittleness from staling.

Knowing this, you can now rescue your slightly crusty muffin by reheating enough that the starch can re-gelatinise, resulting in a temporary soft, fresh texture. However, reheating also accelerates the staling process, meaning that once it cools down it will quickly harden back up, so eating the whole thing straight out of the oven will avoid any further disappointment.

Counterintuitively, science shows the worst thing you can do with your favourite baked treat is store it in the fridge. A recent Argentinian study found that the staling reaction occurs six times faster at 4C - the same temperature fridges are set to - compared to storing at room temperature.

Keeping items below -18C produced the slowest staling reaction, meaning those spare slices of cake would be much better in the freezer than in the fridge.

Even though staling may be bad for your cake, it's the secret ingredient for great fried rice. My mother always told me to refrigerate rice overnight before making her secret Chinese-style fried rice recipe, and when I ignored her my dish was sticky, gummy and mushy. What I now know is that when cooked rice is chilled, the outer starch shell hardens six times faster through the same staling retrogradation process and in this case it keeps the grains light and separate when you fry them up for a great engineered dinner.

- NZ Herald

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