My response to a request for some science based insight into the toughening of bread on microwave reheating.
Wheat starch is the main culprit leading to the normal hardening in crumb texture as bread ages – and this occurs even in the absence of moisture loss but moisture redistribution has a big role to play under normal real-life circumstances.
The specific reason for the hardening of the crumb is the recrystallization [winding into a double helix] of the terminal chains of the branched amylopectin component of the starch. Amylopectin is about 75% of the dry starch weight. It is fairly clear that it is not gluten that is the culprit, as you can see similar firming rates in gluten free breads, in fact this was seen in breads made with only rice starch and some gums [which, by the way, are just atrocious to eat].
These terminal amylopectin chains – up to say 30 glucose units long can be unwound – remelted – by reheating in a conventional oven or a toaster – radiant heat. Therein lies the secret of the “refreshing” of bread when it’s reheated.
The other 25% of the starch is amylose – just chains of glucose but with effectively no branches, just a straight line. This component of the starch also recrystallizes when it is cooled but it is a much smaller molecule than amylopectin. As a result of its smaller size and linear nature amylose recrystallizes fast and strongly. It is one of the reasons that bread doesn’t collapse after it comes out of the oven. To “remelt” amylose takes temperatures above 100 deg C (212 deg F) so it does not participate in the softening of refreshed breads.
Now to the thorny question of hardening on microwave reheating.
There have been lots of hypotheses over the years, but the experience of mirowave reheated bread is both regretable and unforgettable. One clue was the fact that the toughening seems to occur as the bread cools, it is really really soft immediately out of the microwave in my experience. The latest idea that I can find is that it is related to amylose. This needs a slight diversion into how starch is packaged in the wheat grain and therefore the flour and dough. Starch comes in little granules in wheat up to about 50 micrometers in diameter. The granules have an internal structure in layers that in the simplest terms can be thought of as alternating mostly amylopectin and then mostly amylose.
When the starch is cooked some of the amylose leaks out of the granules [another phenomenon related to smaller size and linear nature] this is how it can form a 3D network that helps support bread structure. However, some is left behind and it is not recrystallized because the molecules are just too crowded to allow it.
Anyway back to microwaves;
the newest hypothesis I can find suggests that when bread is reheated by microwaves there is localized boiling — OK — I will let them tell you.
“Comparing breads reheated in conventional and microwave ovens revealed that the latter considerably toughens the crumb texture when internal boiling is induced. Moisture loss in itself has a relatively minor toughening effect… Substantially greater amounts of amylose are leached out of the granules in the case of sustained boiling during microwave heating, as compared to conventional oven heating. The free amylose solution is being ‘pushed’ by the generated steam pressure toward the air-cell wall interface. A rich amylose phase is accumulated at that interface and over the granules. Upon cooling, the amylose undergoes rapid phase changes; thus, toughening is apparent in a relatively short time after heating. Minimizing the textural deleterious effects in microwave reheating of bread-like products should entail: preventing or minimizing internal boiling; diluting of the starch concn. below the threshold level; and interfering with the amylose phase change by using comp lex forming agents”.
Mechanism of crumb toughening in bread-like products by microwave reheating.
Uzzan, M., Ramon, O., Kopelman, I. J., Kesselman, E., Mizrahi, S.
Journal of Agricultural & Food Chemistry. 55, (16): 6553-6560, 2007.
So the increase in amylose in the space between the granules means that when the bread cools the newly released amylose recrystallizes fast and strongly as we expect, but it is there in greater amounts [it was previously locked in the remnant granules] and when the bread cools it hardens as we experience it.
The use of shortening to stop this was once thought to stop the gluten from toughening, but fats can complex wiuth amylose and stop it recrystallizing – a reason for its effectiveness [from the paper above this relates to “interfering with the amylose phase change by using complex forming agents”.
Sorry it is so technical, but starch chemistry at is basis is, well, chemistry.
Cool videos of starch gelatinizing from Kansas State U
and some starch links