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

karwe@aesop.rutgers.edu

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