What causes egg-covered puffed rice cakes to become soft when pan fried (and how to control it)? While recently making fried bread slices dipped in egg (both white and yolk), I've run out of bread. Since I've already had everything else prepared, I've decided to go ahead and replace the bread with (Western-style) puffed rice cakes.

Anticipating the result would be not very palatable, I was instead surprised that the crispy and brittle (even after a long dip in the eggs) rice cakes became soft and mostly chewy during frying — turning out to be quite a viable substitute for this type of snack.

Interestingly, I've later tried to reproduce this process of softening up with just temperature and temperature and cooking oil, without success, implying that the egg dip has to be a factor in the transformation.

My question is: what in the combination of eggs and pan frying causes the softening of the puffed rice cakes, and what are the influencing factors in the context of controlling this process?

For completeness sake, some additional information:


the egg dip used was completely unseasoned, just egg white and yolk briefly stirred together with a fork,
the rice cakes were composed of puffed "pure grain brown rice" and 0.9% of "sea" salt (hence no seasoning in the eggs),
the cakes were submerged in the eggs for up to a minute (without losing rigidity),
they were pan fried on a Teflon pan in a small amount of rapeseed oil, near its smoking point, for about the same time you would fry bread.
They get softened up when the moisture from the egg gets hot enough to bathe the rice cake with steam, and rehydrate the starches with moisture and heat.

The rice cake was absorbing moisture while it was sitting in the egg - not too much if it still seemed light, and clearly not enough to start disintegrating, but some.  The texture of such dried products is open, but the strands (of gluten, or starch) are dried hard - the crumb can be heavy with moisture if soaked long enough, but it drives out the air not soaks into or softens the starch.  They will still be stiff (and dripping) after soaking, and they won't ever become soft and chewy from all the way dried out, with only added moisture.  

Add heat, though, and the combination lets the inner structures absorb the moisture from the surface, reconstituting the starches to a chewy consistency (the brittle stiff structures swelling, and becoming flexible and compressible like bread and not just brittle or mush like soaked crackers) - and it becomes flexible enough to let the steam wick upward through the rice cake to bring the combination of moisture and heat to transform the interior.  If the moisture form the soak hadn't reached the center, though, it can remain  dry and stiff - especially if the edges also hadn't soaked enough to get water for the steam, which helps mobility (turns it from soggy-edge and dry-center to moist all through).

This process isn't limited to eggs, or to rice cake.  If you have bread that's gone bone dry and hard, soaking will soften the edges to mush but it won't reconstitute it to a bread-like consistency without heat.  Soak it in water or milk, though, then pan fry or toast it - and the heat and moisture will make the gluten absorb the water, swell, and become flexible again.  I have rehydrated bread, rolls, or pastries this way (usually with water, it changes less) if they become totally dried out without otherwise spoiling - soaked in water for a while, then heated to transform from wet-and-brittle to soft and chewy.

You should see similar effects in your rice cakes (becoming soft/chewy) if you soaked them in water or milk or whatever, before frying again, to those you got when dipping them in egg first, and again similar effects from using a toasting or using an oven rather than frying.  The combination of heat and moisture makes a difference in re-hydrating starches, not just the specific combo of egg dip and ricecake (disclaimer - obviously, there will be different results from using egg, water, milk, or alcohol, and rehydrating rice-cakes, breads, and other dried starches - just, similar kinds of results from moisture, heat, and starches)