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Scalding flour is the process where the flour starch is gelatinised by the addition of very hot water (See https://en.wikipedia.org/wiki/Starch_gelatinization). I'm assuming here that a very similar process occurs when making hot water pastry (using lard or lard and butter plus hot water).

According to Wikipedia, this reaction depends on the starch but occurs between 55-85 °C. If my assumption here is correct, there is no need to boil the mixture for gelatinisation to occur. Chefs seems to be divided on this matter, some say the liquid must boiled, others not.

As bringing a fat and water mix to a boil is potentially very dangerous as the mixture could explode out of the pan if overheated, what is the minimum safe temperature that this reaction will occur at? Or is something different chemically occurring here and the mixture must be boiled for best hot water pastry?

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  • You've got a certain mass of flour and a certain mass of fat at room temp, and you've got a mass of hot water. The resulting temperature from mixing them together is simply a function of the specific heats of the three substances and the relative masses.
    – Scott Seidman
    Feb 7 at 21:42
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    In my experience, boiling water makes better pastry. This may be simply a matter of skill - somebody using colder water might have to work with much more precision, while the hot water gives you some leeway - or it may have other reasons. My point is, I doubt that there are many cooks interested in finding (or using) the lowest possible point, because the highest possible point (boiling) is culinary preferable.
    – rumtscho ♦
    Feb 8 at 11:06
  • More a safety point rather than anything else, I foolishly brought some up to boiling point far too quickly and the result was volcanic to say the least. Thankfully, I wasn't hurt, but the speed at which the solution turned from simmering to spraying boiling lava everywhere was close to instantaneous. Lesson learned, use a gentler heat and don't take your eye off the mixture for even a second. I'll know if it makes better pastry tonight, the formed dough has been resting for 24 hours before baking.
    – Greybeard
    Feb 8 at 17:26

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