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:
- https://www.researchgate.net/post/Can_we_decrease_the_boiling_point_of_water_without_reducing_atmospheric_pressure
(These scientists seem to agree that what I accomplished wasn't
possible) - https://www.quora.com/What-substances-lower-the-boiling-point-of-water-when-in-solution (refers to Raoult's law, implying the addition of something like alcohol or acetone, rather than anything like my ingredients)
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.
- Braise all ingredients, covered, in an oven for 2.5 hours (150˚C)
- Cool overnight
- separate ribs from sauce. Heat sauce to liquefy
- Strain away solids in a 0.5mm mesh
- 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…?)
Best Answer
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