Thermal Transport in Selected Food Processing
Operations
Mukund V. Karwe, Ph.D. Professor, Department of Food Science
Dean of International Programs
School of Environmental and Biological Sciences
Rutgers University
New Brunswick, NJ 08901
Thermal
Mechanical
Heat Addition
Heat Removal
Molding, Extrusion
High Hydrostatic Pressure
Mixing, Emulsifying,…
Pasteurization, Retorting,
Drying, Baking, Frying, Ohmic
Freezing, Freeze drying, IQF
Field
(Non thermal)
PEF, PL, OMF, Irradiation,
Ultrasound
RF, MW
Non-thermal
Gas Ozone, CO2, Cold Plasma
Non-thermal (?) Processing
• High Hydrostatic Pressure (HHP)
• Pulsed Electric Field (PEF)
• Ultrasound
• Pulsed Light (PL)
• Irradiation
• Oscillating Magnetic Field (OMF)
• Cold plasma
Thermal Processing
• Thermal processing
• Aseptic packaging
• Baking
• Frying
• Ohmic heating
• Microwave • Radio frequency
• Infrared
• Impingement
• Drying
• Extrusion • Chilling
• Freezing • Freeze drying
Examples of Preservation Processes
Extruder Annular die
Transducer
Dowel
Pin
Extrusion of corn strips
I. Deo, Ph.D. Thesis, 2001. Rutgers University, New Brunswick, NJ 08901, USA.
Extruder-Die
Interface
Tinterface
Extruder
Screw-tip
Die Inlet
Pinlet, Tinlet
Die
Outlet
Steel Walls of Die
h, T
Axis of Symmetry
Conjugate Heat Transfer in a
Single Hole Die
I. Deo, Ph.D. Thesis, 2001. Rutgers University, New Brunswick, NJ 08901, USA.
20
Tinlet=394 K
Tinterface=344 K
Tinlet=394 K
Tinterface=444 K
Isotherms in flow of cornmeal (30%
moisture) in a single hole die
kelvin kelvin
k=20 k=20
I. Deo, Ph.D. Thesis, 2001. Rutgers University, New Brunswick, NJ 08901, USA.
Baking
Transport
process:
Heat & Mass
Transfer
Temperature rise,
moisture migration &
evaporation
Physical & chemical
changes:
Starch gelatinization
Protein denaturation
Crust formation
Color development
Flavor formation
Quality:
Texture, Flavor
Color, Shelf-life
Jet Impingement Oven
Forced convection oven
High velocity (10 to 50 m/s) jets of hot air
(100 – 250 C) impinge vertically on a food
product
A Commercial Scale Jet Impingement
Oven
A Commercial Scale Jet Impingement
Oven
Courtesy of Wolverine Corp.
Multiple Model (?) Cookies
0
50
100
150
200
250
0 20 40 60
DIAMETER OF COOKIE
(mm)
h (
W/m
2K
)
SINGLE COOKIE
MULTIPLE
COOKIES
Variation of h with Size of the Model Cookie
Experimental Results
Nitin and Karwe, Journal of Food Science, Vol. 69, Nr. 2, pp. FEP59-FEP65, 2004.
Why Microwave Oven?
accelerated heat transfer
accelerated moisture migration
shorter process time
used for dehydration, cooking, blanching,
thawing, pasteurization, sterilization
microwave condensation of the
vapor at the surface
soggy & rubbery texture
no crust formation & color development
cold air
rapid heating
of the inside
of the product
microwave
rapid heating
of the inside
of the product
evaporation at the
surface
crust formation
color development
jet Impingement
Model of Crust and Crumb
before crust is formed
convective
heat transfer
convective
mass transfer
convective
heat transfer
heat
conduction
mass
diffusion
crumb
Qgen
water vapor
transport
through crust
heat
conduction mass
diffusion
crumb
Qgen
crust
evaporation front T = 100 C
after crust is formed
M > Me, T 100 °C
M = Me, T > 100 °C
M > Me, T 100 °C
D. Kocer, Ph.D. Thesis, 2005. Rutgers University, New Brunswick, NJ 08901, USA.
Governing Equations
Heat transfer:
Mass transfer:
M).(Dt
M eff
Surface heat flux:
QT).(Kt
T)C(ρi
pii
Surface mass flux:
)PP(Hk)Th(Tn
Tk vavsvmsa
s
svavsmseff /)PP(kn
WD
Antoine’s law:
1346
4438163036183133
.T
..exp.P
s
vs
