Bread & Water
By: Ash Brown
Tags: Water, Food, Science, Bread, Moisture

If you leave the package open, why does bread get crunchy, but potato chips get chewy? Why is fresh for one stale for the other? The answer starts with something reasonably simple but gets a little more complicated.

Understanding why crispy foods lose their texture and soft foods become hard starts with the fairly straightforward concept of moisture exchange, but it also includes the theory of molecular crystallinity and the process of retrogradation.

To begin, the air around us has water vapor (or water in its gas form) in it. Water vapor is invisible.

The amount of water vapor in the air is referred to as the air’s humidity, which mostly is affected by the air’s temperature and weight (or atmospheric pressure). The average amount of water vapor in the air (or the relative humidity) is about 2% to 3%, and it can range from very little (about 0.2%) to around 4%.

As a side note, when water vapor in the air turns back into a liquid in the form of droplets attached to tiny particles, like dust, they form clouds (when higher off the ground), fog (when closer to the ground), or dew (when on the ground).

Alternatively, the amount of liquid water present in a system or in a material (like bread) is referred to as moisture. (Actually, moisture can refer to any liquid, but it’s usually water.)

The water contained in food products can be either bound, adsorbed or free. Bound (or organic) water is where the water molecules (H2O) are components of larger organic molecules. Adsorbed water is just attached to the surface of other molecules. Finally, free water is not attached to the material’s molecules and can be squeezed out of it, like juice from an orange.

The term water activity refers to the amount of free and adsorbed water in food. One way of determining the water activity is by measuring the relative humidity of the surrounding air in equilibrium with the food. In other words, water activity is a measure of the moisture in food that is available to be converted to vapor.

As Guy Crosby, the Cooking Science Guy, explains, the amount of moisture in a food influences the mobility of the food’s molecules. The more water, the more molecular mobility. The more molecular mobility, the more flexibility. Thus, foods with more water will be more flexible and softer, and foods with less water will be more rigid and hard.

Further, water acts as a softening agent (or plasticizer), which lowers the glass transition temperature of the molecules. The glass transition temperature is the temperature range where molecules change from having a rigid, crystalline structure to a more flexible, elastic structure.

The water activity and the glass transition temperature of food have a linear relationship — foods with lower moisture and water activity have a higher glass transition temperature and vice versa. Thus, foods with less water (say, crackers) will be crisp and brittle at room temperature. And, adding water lowers the glass transition temperature, so the food will become soft and moist at room temperature.

During baking, the starch molecules in bread begin to absorb water and to become more elastic. Consequently, when the loaf comes out of the oven, it is soft and fluffy. However, as it cools, the starch molecules begin to reform and harden, which is known as starch retrogradation or recrystallization. As that process continues over time, the moisture that was absorbed during baking evaporates, and the bread progressively becomes dryer and harder as the molecules realign. (The crust — the dryer part of bread — also steals moisture from the interior of the loaf.)

Conversely, chips lose most of their moisture by evaporation when they are fried or baked. Then, when exposed to air, the chips absorb moisture, which makes them softer and more rubbery.

So, generally speaking, foods containing more water are softer, because their molecules are more flexible and elastic. On the other hand, those with less moisture are harder, because those molecules are more crystalline and rigid. Further, if there is more water in the food than in the surrounding air, the water in the food will evaporate until there is an equilibrium. Likewise, foods with less water than the air will absorb the available moisture. [more]

Therefore, keeping the package or container closed keeps the moisture either in or out, which slows the process of moisture exchange and prolongs the food’s freshness. This holds true for most starchy snacks. However, while it’s probably the case with freshly baked chocolate chip cookies, few that I’m around last long enough for me to find out.