Sometimes it's nice to have a guest writer and today I have the talented Mark S Whitehead BSc Hons who is not only a food chemist but an avid baker himself. This has been an age-old question and now it is finally answered!
Normal white bread contains a lot of starch – in the order of 45-50%. Whilst bread contains many other nutrients (e.g. protein) and fibre, it’s the starch content that is important in bread staling.
Starch
Starch is a very long chain of almost circular glucose molecules joined together. The number of glucose molecules (called “residues” in labs) in the chain varies but in bread wheat it’s in the thousands, 15,000 being on the low side.
In the grain, these long chains lie more or less parallel for long stretches and are attracted to each other. This attraction, although not always a full chemical bond, is still moderately strong.
When we add water to starch and then heat it, the chains tend to break away from their straight-line format and form curves and waves. They are still attached to each other, but not nearly as much, and molecules of water often lie between the chains for much of their length. This allows bread to have its “fluffy” consistency.
Firming Up
The bending and curving of the chains is brought about by heat of the oven and is kept that way, to an extent, by the water from the dough. Chains of protein, also in the wheat, behave in much the same way though not to the same extent. This is why a still-warm loaf is not very firm and tears easily.
As the bread cools and some more of the water evaporates, the chains pull a little closer together and the bread gets its normal firmness.
Staling
Whilst the starch can’t move much once the bread is cool, it can still move to a small extent. It is not held in rigid lines, so the bread remains flexible. However, the starch “wants” to get back into its original shape with the long chains parallel and attached to each other. It can do this only gradually and the water molecules, which are strongly attracted to the starch chains, get in the way.
However, over time the starch can get some of what it wants. Some parts of the chains can get parallel again firming up the bread a bit more. Eventually, given enough time, lots of the chains go back (“retrograde” if you listen to food scientists) to their original state. The bread becomes firmer, and as it is no longer closely attached to the starch, water will evaporate and the bread will become dry and firm.
This is what we call “staling”.
Preventing Staling
In most bread recipes, there is some fat – oil, lard, butter or something similar. This stops the water moving around so much, leaving it attached to the starch chains, and gets in the way of the starch chains when they try to come back together. This is what prevents immediate staling.
The fat also stops, or at least slows down, the movement of the water that is attracted to the starch chains. The water also helps stop the chains coming back together. However, as the bread dries out, there is less water to keep the chains apart. This makes it easier for the chains to meet up again. You don’t need to lose much water to allow your bread to become stale. It will manage to do this eventually, but the less water the easier it is for the starch to retrograde.
What To Do With Stale Bread
The usual thing to do is to chuck it out. However, if it’s your last cob and you’re starving, just heat it up a little – a microwave will do the job well. The heating causes the chains to flex out of their parallel state and the bread becomes edible again. However, you don’t get all that long, as you’ll lose some water when you re-heat the bread, thus allowing it to go stale that much quicker.
You can also use it for toast (and many people say that slightly stale bread makes better toast) or as a “trencher” – a warmed “slab” of bread that has some kind of moist, hot food on top, such as a casserole.
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