Chapter 4 Flashcards
Heating and Cooling Foods
Prevents microbiological and enzymatic degradation
Systems for heating and cooling food products
Heat exchangers
Non-contact heat exchanger
Product and mechanism are kept physically separated usually by a thin wall
Contact heat exchanger
Direct physical contact between product and heating/cooling system
Plate heat exchanger
Stainless steel plates, better heat transfer from pressed patterns to increase turbulent, Simple maintenance, sanitary
Parallel Flow
Product stream vs. heating/cooling stream travel in the same direction
Counter-current flow
Product stream vs heating/cooling stream travel in opposite directions
Fouling
Deposition of solids, such as milk protein, onto the plate. It decreases heat transfer
Tubular heat exchanger
(Non-contact) check it out
Scraped surface heat exchanger
scraping action allows for rapid heat transfer to a relatively small product volume. scrapes film build up on tube wall
Steam infusion heat exchanger
(contact) direct addition of steam to product, amount of water added
Specific heat
(Cp) measures the total amount of thermal energy needed to raise the temperature of a
material
Sensible heat
Temperature change with no state change
Latent heat
Addition/removal of heat during state change
Relationship between specific heat and moisture content
Specific heat increases and moisture content increases
Apparent specific heat
When a change in state occurs, incorporates heat involved in the change of state in addition to the sensible heat.
Thermal conductivity
(k) measures the rate of heat transfer per unit time across a
given cross section of the material (W/mC)
Thermal diffusivity
(alpha) Ratio involving thermal conductivity, density, and specific heat. high thermal diffusivity means that heat travels through it quickly
Forced convective heat transfer
Use of some mechanical means such as a pump, fan, or stirring to induce movement of the fluid
Free convective heat transfer
Density differences caused by temperature gradients within the system
Conduction
Heat transfer by within solids by molecular movement
Convection
Heat transfer in liquids by movement of molecules due to density difference
Radiation
Transfer of heat by electromagnetic waves
Steady state heat transfer
Time has no influence on the temperature distribution within an object, although temperatures may be different at different locations within the object
Unsteady state heat transfer
Temperature changes with location and time
Two layers develop in pipe during convective heat transfer
hydrodynamic boundary (Near wall) thermal boundary (farther from wall)
Reynold’s Number
Indication of inertial forces and viscosity of the fluid
Nusselt number
Enhancement in rate of heat transfer caused by convection
Prandtl Number
Thickness of the hydrodynamic boundary layer compared with the thermal boundary layer
Log mean temperature difference
Determines area and overall resistance to heat transfer
