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If the temperature of boiling water is about 100 ºC (depending on air pressure), why are boiled potatoes on a high stove flame cooked faster?
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From the English Wikipedia:
Acrylamide was discovered in foods in April 2002 by Eritrean scientist
Eden Tareke in Sweden when she found the chemical in starchy foods,
such as potato chips (potato crisps), French fries (chips), and bread
that had been heated higher than 120 °C (248 °F) (production of
acrylamide in the heating process was shown to be
temperature-dependent). It was not found in food that had been boiled
or in foods that were not heated.
If I boil potatoes on the highest stove flame, will the potatoes not reach the temperature of 120 ºC?
Best Answer
Whether your stove is on 'high' or 'low' doesn't change the temperature of the water, i.e. the boiling point remains $100\ \mathrm{C}$. So the potatoes can NEVER reach $120\ \mathrm{C}$.
But objects being heated by hot water follow Newton's Law of Cooling (and Heating), which describes the heat transfer per unit of time, $\frac{dQ}{dt}$:
$$\frac{dQ}{dt}=hA[T_{BP}-T_{pot}(t)]$$
We can derive from this, for potatoes plunged into boiling hot water at $t=0$, with $T_0$ their ambient (and initial) temperature and $T_e$ their end temperature, their temperature evolution in time $t$:
$$T_e=T_{BP}-(T_{BP}-T_0)e^{-\frac{hA}{mc_p}t}$$
And the heating time:
$$t=\frac{mc_p}{hA}\times \ln\Big({\frac{T_{BP}-T_0}{T_{BP}-T_e}}\Big)$$
($m$ is mass of potatoes, $c_p$ their specific heat capacity and $A$ their surface area)
The factor $h$ is the heat transfer coefficient (in $\mathrm{Wm^{-2}K}$). It is obvious that when $h$ goes up, $t$ goes down.
In fluids, like water, $h$ is known to be somewhat dependent on the Reynolds Number, which is higher in turbulent conditions. More turbulent conditions favour higher heat transfer rates.
For that reason there may be a slight increase in heat transfer (and thus a slight reduction in cooking time) when comparing a vigorous boil (high heat) to a gentle simmer (low heat).