Cooking Energy Sources Radiation – visible light, infra-red, microwaves Convection – heated air, steam Conduction – contact with a heated surface or liquid
Steam Cooking Braising Pans (as a steamer) – boiling water provides atmospheric pressure steam (0 PSI) 212° F. Air acts as an insulator and slows cooking, compared to a “true” steamer. Kettles – jacket wall heats to the steam temperature, pressure inside jacket gives a temperature above 212° F for cooking. Steamers/Combis – steam at atmospheric pressure (0 PSI) 212° F condenses on food surface and transfers latent heat directly.
Heat versus Temperature Heat is energy – the energy of molecular motion in a material Temperature is a measure – the measure of kinetic energy of the molecules in a material
Boiling v. Pressure The temperature where water changes to steam varies depending upon the pressure At sea level (~14psi), water boils at 212 ° F At 6000’ water boils at 201 ° F At 50psi water boils at 298 ° F
Temperature is related to Heat… One BTU is the heat required to raise the temperature of one pound of water by 1 ° F
…but not always Sometimes heat changes the structure (phase) of a material instead of raising the temperature – Ice to Water – Water to Steam
Why Steam? Definition: Steam – invisible gas into which water is changed by boiling [Oxford American Dictionary] Advantages: – Flows easily through pipes and tubing – Carries more energy than air or radiation – “Wants” to become water – Has no odor or taste Disadvantages: – “Wet” environment – does not “brown” – Pressure and burn hazards – Lime scale
Enthalpy of Steam BTU (British Thermal Unit) = the amount of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at its maximum density, which occurs at a temperature of 39.1 degrees Fahrenheit. One BTU is equal to approximately 251.9 calories or 3.7 Watts. Enthalpy = heat content capable of doing work Sensible heat = portion of heat content introduced into water raising it from 32 to 212 degrees Fahrenheit (at 212° = 180 BTUs) Latent heat = amount of heat energy introduced into 212 degree water to convert it into 212 degree steam (= 970 BTUs at atmospheric pressure)
Sensible and Latent Heat Sensible heat is the heat that changes the temperature of a material – it can be sensed Latent heat is the heat associated with a phase change of a material – it is “within” the material but not sensed by a temperature change
Pressure and Temperature As pressure increases, so does the sensible heat and temperature of steam Latent heat decreases slightly PRESSURETEMPERATURESENSIBLE HEATLATENT HEAT 0 PSI212° F180 BTU970 BTU 5 PSI227° F195 BTU961 BTU 15 PSI250° F218 BTU946 BTU 50 PSI298° F267 BTU912 BTU 100 PSI338° F309 BTU881 BTU
Heat Transfer The heat transfer rate of condensing steam is much greater than the heat transfer of air convection Example: you can put your hand into a 212 ° F (or higher) oven momentarily and not get burned DO NOT put your hand into steam at 212 ° F. You WILL get burned.
Steam Cooking Steam is created when water absorbs heat (latent heat of 970 BTU/lb) and changes to steam…but the temperature stays at 212 ° F Food is cooked when steam condenses on the food and releases its latent heat AND THAT’S A LOT OF HEAT!!
Summary Steam cooks because it releases a large amount of heat to the food. This process occurs when the steam condenses to water, releasing the latent heat. The heat transfers more rapidly than normal convection air cooking. The product remains moist.