Water in Food.
Before it is anything else, most food is water. That one fact sets a ceiling on how hot a wet surface can get and a price on drying it.
A cucumber is about 95 % water, a raw steak about two-thirds, cooked rice about two-thirds, bread a little over a third, flour about 12 %. Water dominates the thermal properties of food (II.1), and it does two more things that shape every method. While a surface is wet, it cannot get hotter than water's boiling point, because any extra heat goes into turning water to steam. And turning water to steam is expensive: it takes over five times as much energy to boil a gram of water away as to heat it from room temperature to the boil.
So browning (Vol. VIII), crisping, frying and roasting are all, in part, fights against water: the surface must first be dried before it can climb past 100 °C. The workshop lists the water in common foods from the USDA composition tables and prices the energy of drying them.
Below: water per 100 g of common foods (USDA FoodData Central), and for the food you choose, the energy to heat 100 g of it from 20 to 100 °C against the energy to boil all its water away.
- Know roughly how much water common foods hold.
- Explain why a wet surface stays at or below 100 °C.
- Compare the energy to heat food with the energy to dry it.
- Connect water to browning and crisping.
The ceiling and the price
Raw potato is 79 % water. Heating 100 g of it from 20 to 100 °C takes about 29 kJ; evaporating all its water would take 177 kJ, 6.1 times as much. That is why a pan of chips or a tray of roast vegetables seems to stall: most of the oven's heat is going into steam, not into raising the temperature. Raw beef is 66 % water and bread 36 %; the drier the food, the sooner its surface can brown.
While a surface is wet, its temperature is capped at the boiling point — 100 °C at sea level, less at altitude (III.3). Above that, water turns to steam and carries the heat away. So a wet steak in a hot pan sizzles at 100 °C instead of browning at 150, a crowded pan of mushrooms stews in its juices (XV.3), and a roast's surface stays pale until it dries. Patting food dry, salting it ahead to draw water out and let it be reabsorbed (XVIII.4), and leaving it uncovered in the fridge are all ways of paying part of that price before the cooking starts.
Weigh 200 g of sliced mushrooms, then sauté them until they stop releasing liquid and begin to brown. Weigh them again. Most mushrooms lose about half their weight — water you watched boil off before any browning could start.
Terms
- Water content
- Grams of water per 100 g of food.
- Latent heat of vaporisation
- The energy to turn water into steam at its boiling point: about 2 257 kJ per kilogram.
- Boiling-point cap
- A wet surface cannot rise above the boiling point while water remains to evaporate.
- Drying
- Removing surface water before cooking so the surface can heat past 100 °C.
Sources
- USDA FoodData Central, SR Legacy (April 2018), public domain.
- Y. Choi and M. R. Okos, "Effects of temperature and composition on the thermal properties of foods", in Food Engineering and Process Applications, vol. 1, Elsevier, 1986, pp. 93–101.