Stale bread

Photo by Evi Radauscher on Unsplash

Bread is only apparently simple and the processes that occur at the molecular level as dough is baked into bread and as bread then ages are still not well understood. In particular, bread staling has been the subject of considerable scientific study because of the economic implications at manufacturing, distribution and domestic levels. Staling is a complex process and much remains uncertain, however one thing that is known is that bread does not go stale because it dries out.

If a loaf is hermitically sealed to prevent water-loss and left a few days it will go stale, and in fact stale at the same rate as a loaf left open to the air.

This experiment was carried out and reported as long ago as 1852, so there’s been something of a communication problem. Certainly, it tastes dry, but that doesn’t mean that it is dry. The same study showed that reheating the loaf to 60C restored freshness, the exact opposite of what you might expect if the loaf had indeed dried out.

So what underlies staling if it is not drying? The prime offender turns out to be the starch.

Wheat flour contains two main starches, amylose (small, linear shape) and amylopectin (large, complex shape). These assemble in hard, dry, starch granules. When water is added and heat applied (during baking) these granules absorb water, swell, soften and ‘gelatinise’ (an unfortunate term as no gelatine is involved and besides, gelatine is a protein). The swollen, gelatinised granules make bread soft when chewed.

When bread is first removed from the oven it is soft, ‘doughy’ and difficult to slice. Bread has a more agreeable texture if it is left to cool for a while. This change in texture is the first stage of staling and takes about an hour. The culprit is amylose. The second stage of staling takes a day or two, and the culprit is amylopectin. Between these two stages of staling, the bread is considered most palatable.

The mechanism of staling is the same for both starches, the difference in time course is a result of their different structures; it happens quicker with the small straight amylose molecules than it does with the complex amylopectins.

What is happening is that as the starches cool they recrystallise back into hard granules. Water can be expelled in this process, but has little effect on texture other than that some migrates to the surface of the loaf where it softens the crust. The water that doesn’t escape is locked into the newly crystallised structure of the granules. This process of recrystallisation with cooling is known as retrogradation. Anti-staling agents added to bread work by interfering with retrogradation.

The hardened starch granules give us the impression of a hard, dry mouthfeel, even though water content is essentially unaltered. There is no change in nutritional content either, however, the crystallised starches are harder for our digestive system to break down into glucose (starch is made up almost entirely of glucose), so stale bread has a somewhat lower GI.

Retrogradation also explains why stale bread can be refreshed by warming. Raising the temperature to about 60C agitates and breaks up the crystalline structure of the hardened starch granules, and they re-hydrate and soften. This is why toast made with stale bread can be soft in the centre.

Retrogradation occurs most rapidly at slightly above refrigerator temperatures (5C), so bread will stale fastest in the fridge. Freezing halts the process (and locks up the water in ice), so frozen bread will remain freshest. Staling occurs slowly at temperatures greater than about 30C.

Retrogradation applies to other starches too, for example it is relevant to mashed potatoes. It is also why fried rice is best made with day-old refrigerated rice — retrogradation is sped up in the fridge and hardens the rice so that it does not break up when fried and tossed during the initial stages of cooking. As it heats it softens and becomes palatable again on serving.

Physically, bread dough is a soft foam that becomes a set foam during the first phase of baking, and then changes to a sponge structure. A foam is a gas trapped in a liquid or semi-solid medium. Kneading entraps air in the dough, and the CO2 released by the action of yeast expands these air pockets.

At the onset of baking, the dough becomes more liquid-like, the gas expands and the dough rises further (oven spring). The expansion is due to water and alcohol vaporising under heat (alcohol is a byproduct of yeast fermentation) and also because hot gasses occupy a greater volume.

The next stage of baking sets the gluten (protein) network and the bread can no longer expand. As heating continues, internal pressure builds up in the air pockets and they rupture and join up in a network of interconnected channels that equilibrate pressure and connect to the surface of the bread where gasses can escape. The bread is now a sponge.

This last step is crucial, without it the gases would remain trapped and as the bread cooled they would condense and the air pockets deflate — the bread would collapse. This is precisely what happens to a soufflé, which never gets beyond the foam stage because there is nothing to restrict expansion as it cooks.

As an aside, mass-produced sandwich breads use complex mechanical means to drive fine air pockets into the dough. Loaves are formed, and placed in a vacuum oven. A light ‘vacuum’ is applied and the trapped air expands, causing the loaf to rise. The oven then bakes the loaves while they are held in this condition, setting the loaf. The oven is slowly returned to atmospheric pressure, which slightly deflates the loaves but they retain lightness and form. Yeast is added for flavour, and presumably so that the manufacturer can call it ‘bread’.

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Science of cooking, eating and health. Retired neuroscientist.

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Science of cooking, eating and health. Retired neuroscientist.

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