Transcript steam water - Blodgett Oven

Why Steam?
What’s the Big Deal?
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
Latent Heat does the Cooking
STEAM
WATER
Pressure and Temperature
• As pressure increases, so does the sensible heat
and temperature of steam
• Latent heat decreases slightly
PRESSURE
TEMPERATURE
SENSIBLE HEAT
LATENT HEAT
0 PSI
212° F
180 BTU
970 BTU
5 PSI
227° F
195 BTU
961 BTU
15 PSI
250° F
218 BTU
946 BTU
50 PSI
298° F
267 BTU
912 BTU
100 PSI
338° F
309 BTU
881 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.