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Why does bread become hard if left out in the open? Caused by humidity and starch retrogradation

Scientists have been studying the hardening mechanism of bread for centuries, but the truth is that this has not been entirely well defined. What we know for sure is that there are mainly two causes responsible for this alteration. There redistribution Of humidity between crust and crumb and the transformation of starch, more specifically the so-called starch retrogradationin which amylose tends to take on a crystalline structure and becomes less elastic. The opposite happens for example with biscuits, which tend to become softer and lose crunchiness over time.

Why stale bread is hard: the redistribution of moisture

Before understanding why bread becomes hard in the air, we must understand what it is made of. The main ingredient is flour and flour is made up of about 70-75%. starcha carbohydrate complex contained in cereals and potatoes. Starch granules present themselves with their shape spherical trapped in a sort of protein lattice that holds them in place, helping to give a little rigidity to the structure.

In nature, starch is present in two forms: amylose and amylopectin. THE’amylose it constitutes the internal part of the granule and is made up of long chains with an amorphous structure. THE’amylopectin instead it forms the external part of the granule and is highly branched, with an ordered and crystalline structure.

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Starch grains in a potato cell. Optical microscopy, Lugol staining (25% dilution). Magnification: 400x. Credit: Ganìmede

Now that we understand how bread is made, let’s find out what happens when it goes stale.

We all agree on what bread should be: crunchy on the outside and soft on the inside. Stale bread suffers a sort of opposite effect: the crust becomes softer, while the crumb hardens. This aspect is easy to attribute tohumidityor rather its redistribution in the leavened product. In fact, we witness the migration of moisture from the crumb to the crust. Typically the crust contains 10-20% moisture, while the crumb contains 45%. From the results of some experiments we can see that the crust, after a storage period of approximately 100 hours, goes from 15% to 28% humidity, exchanging it with the inside of the bread and with the outside.

The concept is intuitive: we can think of toasted bread. When we toast bread we irradiate it with heat until it becomes hard because the dry heat of the toaster eliminates the residual moisture of the starch. What influences the “freshness” of the bread is therefore the so-called hygroscopic balancei.e. the balance between the humidity of the bread and that of the surrounding environment. So basically the bread becomes harder because it loses water and dries out. But it’s not that simple!

because bread becomes hard retrogradation starches starch

Why the crumb hardens: the retrogradation of starch

More difficult to explain is why the crumb becomes hard. In fact, we are talking about a phenomenon called starch retrogradation.

Let’s think about the process of making bread. The flour is mixed with water – and other things – and the dough is baked.
Initially starch is not soluble in water, but at high temperatures it is able to absorb it. As the temperature rises and this causes the bonds between the molecules to break. The penetration of water causes the starch molecules to separate, increasing their random arrangement and decreasing the number and size of the crystalline regions. Start the gelatinization.

The starch begins to swell due to hydration and the amylose spreads outside its granule and forms a kind of gel, arranging itself around the amylopectin which in the meantime has become amorphous (has lost its crystallinity). Thanks to gelatinization, starch also becomes digestible for our body.
As soon as it comes out of the oven the bread is soft because the starch has gelatinised during cooking, i.e. it has absorbed the water and expanded. The result is the formation of a soft and porous structure.

When bread becomes stale, a sort of reverse process occurs, which is called retrogradation.
In retrogradation the starch does not return to how it was in its initial phase, but forms an intermediate structure in which the molecules rearrange themselves, excluding water and recreating a more crystalline structure. Specifically, it is amylose that tends to aggregate into ordered crystals. It is precisely this structure with the ordered molecules that makes the food harder.
This process begins immediately, as the bread begins its cooling phase, but we only notice it when the bread starts to become too unpleasant, usually after a few days.

And what can we do when bread goes stale? We heat the bread again, reactivating the gelatinization phase, which starts again once it reaches 60°C.

Factors that influence the shelf life of bread

To improve the aging aspect of the bread, various factors were taken into consideration, primarily the flour mix used for the dough. Numerous studies show that flours derived from waxy wheat, with a high amylopectin content and low amylose content, are better in this respect. Other studies show that barley flour helps reduce the rate of hardening of bread. For still others, soy flour can make the difference. The truth is that all these methods only work in relation to other precise quantities of ingredients in the dough in which they are inserted. In short, there is no universal formula but it is the mix of ingredients, hydration And cooking that makes the difference. Another important factor is the gluten; according to some studies, a greater quantity of this in bread allows you to preserve a fresher crumb consistency during storage.

In the meantime, the best way to preserve bread remains freezing. It has in fact been shown by various questionnaires that frozen bread is almost like fresh bread in terms of flavor and consistency.