Yes, I am serious. I thought it was interesting that I was reducing some fluid the other day, and I put in my alerting thermometer to warn me of a rising temperature which I expected as the water evaporated and the solution strengthened.

I anticipated to set the threshold to about 101˚C and then to monitor and adjust, taking note of the target temperature for a perfect consistency for future repetitions of this recipe.

To my surprise, the thin fluid was boiling already at 99˚C. And as the sauce thickened, the temperature continually transitioned downward until the sauce was ready - i.e. the consistency of barbecue sauce, because that's what it was now - still bubbling at 84˚C. When I agitated the probe, mixing it well into the steam bubbles at the bottom of the pot, the reading would tend to decrease rather than increase.

I live at sea level and my electronic thermometer is well calibrated within 1˚. Boiling tap water reads exactly 100 on the device. I've made both caramel and marmelade successfully, where with removal of water we observe the temperature rise above the boiling point of water as the solution or colloid thickens. I have never monitored a salt solution although I understand from general principles, that we would observe the temperature rise until crystals came out of solution.

A quick Google search shows this to be an unexpected phenomenon:

For clarity the recipe is: 900g pork ribs 500g tomato sauce 1.5l water 2T sugar 1T vinegar 1T paprika 1t salt, and smaller amounts of a few more flavouring items.

  1. Braise all ingredients, covered, in an oven for 2.5 hours (150˚C)
  2. Cool overnight
  3. separate ribs from sauce. Heat sauce to liquefy
  4. Strain away solids in a 0.5mm mesh
  5. Reduce fluid until thick (or now, reduce until temp. drops to 84˚C)

So comes the question. Is there an identified physical principle that describes the temperature reduction observed in the reducing of my barbecue sauce? (Or have I discovered a novel working fluid for heat engines...?)

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – David Z
    Dec 22, 2018 at 2:49

2 Answers 2


There is a well known physical chemistry effect when you boil two immiscible liquids, such as water and fatty acids (from pork). The vapor pressure of the mixture corresponds to the sum of the vapor pressures of the individual components. Assuming that the vapor pressure of the water in the barbecue sauce is 700 mm Hg at the same time that the vapor pressure of the fatty acids in the barbecue sauce is 60 mm Hg, the mixture will boil. Obviously, this boiling point is below 100 deg C. A bit more detail on this process can be found at http://www.separationprocesses.com/Distillation/DT_Chp01m.htm


If I had to guess at first glance it appears you maybe generating alcohol as a result of your process. Ethanol has a boiling point just below the 84C you are seeing, and it's likely you are generating some during your overnight settling process. Tomato wine is actually fairly common and you can find different recipes online. Although you do not list yeast as part of your ingredients, it is possible you are getting enough natural yeast reacting to the natural sugar in your tomatos and the extra sugar you are adding. Although I would normally expect all the alcohol to have evaporated early in the boiling process, it is possible that some is formed late as you boil down, or there is some entrained deep in the tomato sauce and is only released late in the boil.

Perhaps their is another explanation, but I am not sure what other compound beyond alcohol would be generated in enough quantity to have an observable effect.

  • 1
    $\begingroup$ Wow, what a fantastic line of reasoning. Actually, shortly after posting this my thermometer died completely - so I figure it was indeed poor performance from my previously stellar thermometer. $\endgroup$ May 19, 2018 at 22:08

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