All I speak from is my experience. My demi-glace was made over the course of 3 days - strong stock, day 1, remouillage, and final reduction/clarification; a total of 14 hours or so of simmering. If the gelatin was that heat labile, I would expect that on day 3 - when bones are no longer present, and collagen extraction was no longer taking place - the stock would thin, rather than thicken; I am not extracting any collagen any longer, but merely reducing. Even though we are evaporating water, if the gelatin was broken down, all we should end up with is less, thin stock, not a viscous demi or glace. Additionally, since collagen is preferentially labile when compared to gelatin, its breakdown product, I also wouldn't think it would remain in favor of that breakdown product.
I did find some interesting info in this regard:
Gelatine Use in the Dairy Industry
Among other things, what they write squares with my understanding:
Gelatine, with its unique molecular structure has some very interesting properties which are virtually unaffected by "denaturants" like modification of the thermal or charge environments, because the protein is already denatured.
(denatured from collagen).
From what I have read, it is true that gelatin undergoes thermo-labile destruction at temps higher than boiling (actually, from what I've read, high temps induce reformation to helical, collagen-like structure - not dissolution to simpler forms), such as are obtained under pressure; but at normal boiling, no such destruction occurs. See
Stanford Study - Properties of Gelatin
-A very interesting study on the chemical properties of gelatin, from the point of view of many stressors and conditions. Among other things, I think a concluding comment is relevant:
Thirdly, gelatin features a rather wide temperature range of the glassy state with the upper limit of 205°C-210°C. This characterizes gelatin as a material with a fairly wide range of heat resistance.
In the range of normal simmering, in particular, gelatin is shown to exhibit reversible properties:
In the second region (20° to 120°C) the behaviour of gelatin is mainly determined by its moisture content. The thermal contraction of cold and hot gelatin films observed in this temperature region (Figure 7) is not associated with relaxation processes since it is independent of the conformational state of gelatin macromolecules. Rather, it is in essence a purely physical contraction of samples caused by water desorption. This contraction is completely reversible on repeated moisturing of the gelatin and correlates well with the contraction caused by decreasing air humidity at room temperature1.
(In other words, at least as I read it, changes are not structural in nature - the coil structure is maintained - but rather due to water desorption; and even this change is completely reversible).
The article is a bit to go through, but it does square with my experience in the kitchen (again - I can only speak from my experience). Intuitively, the idea that gelatin is destroyed by prolonged simmering just doesn't intuitively register with me - the gummy "lip smack" of a glace, for instance, obtained after only truly prolonged simmering, tells me otherwise.
Edited by paul o' vendange, 11 January 2007 - 07:24 AM.