Curious Cook: Why bread goes stale and how you can unstale it

As an amateur baker, I have quite a bit of interest in bread - and if you're also a baker, you would have noticed that home-baked breads tend to get harder rather faster than the convenience breads you can buy in supermarkets.

This is, of course, no real surprise as almost all commercial bakeries need to find ways to keep bread soft and delectable-looking for a period of time on the shelves so that it maximises the chances of getting bought by people - and after that it needs to remain usable for as long as possible in the kitchen, perhaps even up to a week after purchase.

A modern loaf of bread is a somewhat curious evolution for an important food that originated some 30,000 years ago in Europe and morphed from flatbreads like khubz, lavash to modern versions like roti, chapatti and tortillas to the soft leavened sandwich loaves that you buy today.

In Germany, there are now over 1,300 varieties of breads and along the way, cakes, pastries and other wheat-based baked goods were also invented from basically the same ingredients as bread.

But I am digressing, as what I want to focus on is why my home-baked loaves get hard so quickly and how come supermarket loaves can stay soft up to a week in a plastic bag.

Firstly, let's start with why bread gets stale in the first place. You might think it is simply because the bread is losing moisture by just drying out - and that would not be a bad guess, except that it would also be quite wrong.

In 1852, a French scientist named Jean-Baptiste Boussingault hermetically sealed a loaf of bread in a special air-tight container - and the bread still went stale, despite losing no moisture at all.

What is rather more interesting is the fact that he could actually reverse the stale nature of the bread by heating it uniformly at 68°C. So that's your first tip on how you can un-stale a loaf of bread. And incidentally, it is also partly why we like toasted bread so much.

So of course, you are now curious why bread can become stale and then un-stale after re-heating at 68°C - well, you must be curious as you're still reading. The reason is quite complex but also rather interesting.

Consider what happens to a bowl of wheat flour if you just pour hot water on it and mix it around. It turns into a rather icky, gummy substance (which is also pretty inedible) because of a process called gelatinisation.

The bonds between the starch molecules are broken down in heated water, allowing the water to dissolve the soluble polysaccharides within the starch.

Basically, the crystalline structure of normal starch molecules become diffuse in heated water and separate into an amorphous form - it becomes gooey, in other words. This is the principal idea behind roux sauce, except that butter and other liquid fats are heated in combination with flour instead of water.

Why gelatinisation is mentioned is because something like it called gelation also happens in baked breads, albeit at a lesser density as the bread would have been leavened (or raised) by yeast or other raising agents.

So instead of becoming an amorphous gooey mess, gelation involves the broken down components of starch - gluten and the polysaccharides, amylose and amylopectin - to form cross-linked networked structures under heat.

The process of making home-made bread can be picked up from any baking book, so if you don't mind, we will instead investigate the chemical reactions that occur in breads during baking - which is something cookbooks rarely talk about.

In the first few minutes in a hot oven, the first product of baking bread is steam. This is very useful as a means to transport the heat of the oven evenly throughout the whole loaf and therefore help to cook the loaf uniformly.

The film of steam on the surface also stops the loaf drying out by gelating the surface starch, which will eventually become the lovely baked crust as a result of the Maillard reaction.

In the next stage, the dough becomes more fluid and expands, as gases such as steam from the water in the dough and carbon dioxide produced by the yeast start to form little pockets of hot gases.

This wobbly state is sometimes described as the "oven spring". Also, ethanol gas is involved as the fermentation process produces ethanol - and this is the compound which actually contributes the most to the aroma of baked bread.

At internal temperatures of between 68℃-80°C, the oven spring is arrested when the disulphide bonds between the gluten proteins in the flour hook up into stronger linkages between each other and provide more rigidity to the cooking dough - a bit like a very fine wire cage.

At the same time, the starch granules also absorb water, puffs up and gelates. By this, I mean that the polysaccharides in starch, amylose and amylopectin, start to form molecular chains which become little sponges in between the gluten linkages.

These tiny sponges trap the hot gases and steam in the bread. Mostly, it is amylose that is involved initially, but amylopectin gets into the picture later.

The sponge-like structure is important as it means that the interconnected bubbles have a certain structural strength - if they were all single large bubbles of heated gases, then when the bread cools, these gas bubbles will shrink, pull in the gluten structures and the bread would collapse.

Once cooked, the bread's outer surface would be around 185℃ to 200℃ with about 15 per cent water content.

The inside of the bread would be roughly as wet as the original dough at about 40 per cent water with an internal temperature of around 93℃.

The maximum loss of moisture occurs once the bread is removed from the oven - between 10-20 per cent of the weight of a loaf can be lost in the first couple of hours, especially from smaller loaves.

So what chemical processes causes this lovely fresh loaf to eventually become stale?

The main process is called starch retrogradation. Initially, starch retrogradation is an important process in firming up both the crust and the very soft structure of freshly-baked bread into something which can be sliced and identified as bread.

So, in fact, breads start to stale from the moment you remove it from the oven, and this process becomes more noticeable and a nuisance after about a day or so.

The reason is simply because the amylose chains are re-aligning themselves back into long crystalline strands - it's just what they do (as they are long repetitive chains of glucose molecules which have been disaggregated by the heat but would like to return back to their original structure).

This is cute initially because, as mentioned, this causes the oven-fresh soft loaf to develop a nice texture which is suitable for slicing.

However, after about 24 hours, the same process will start to line up more and more amylose chains into parallel chains which then start to expel moisture from the bread.

Then the matter is made worse in about 24 to 48 hours by the amylopectin, which is a branch-shaped molecule with fewer linear parts.

However, these linear parts will eventually start to align with the amylose chains to form hydrogen bridges which will squeeze even more water from the bread - and eventually the chains become longer and stronger. And then the bread goes hard as a cricket bat.

Now you might be able to guess why re-heating stale bread (but not too stale, please) to around 68°C will un-stale the bread.

This is the temperature at which the starch originally began to gelate - so reheating the amylose molecules to this temperature will cause them to disaggregate once more (temporarily stopping their realignment trick) and make the bread taste fresh again - this is provided there is still enough moisture in the stale loaf to permit gelation.

That is also the reason why stale bread tastes so much better after toasting - the heating elements in the toaster crisp the outer surface of the bread while gelating the inner bread bits.

So it becomes a delicious mix of crispy and soft, which the tongue loves. In fact, why the tongue loves certain textures is also an interesting subject, which may be reviewed later.

And now we understand why bread gets stale - it's just nature's amylose and amylopectin molecules trying to get back together like childhood sweethearts and become crystalline again, just as they were originally when they were molecules in starch granules.

However, they can never return back to their original powdery state because the baking process would have disengaged and altered the gluten proteins in starch (gliadin and glutenin) - and also significantly perturbed the structure of the amylose and amylopectin molecules themselves.

So this isn't going to be a reunion story with a happy ending, especially as it always ends with hard bread.

However, one would have noticed that supermarket breads don't get stale so easily - and of course there are good reasons for this.