Deep Fat Frying

Dr. Sirichai Songsermpong

Department of Food Science and Technology

Kasetsart University

Introduction• Deep fat frying is a process of cooking foods by

immersing them in an edible oil which is at a temperature above boiling point of water (typically 150-200◦C)

• It is often selected as a method of choice for creating unique flavours and texture in process foods

Fig.1 Schematic cross-section of a piece of food during deep fat frying

(Right) Scanning Electron Microscope image of a cross-section of the

crust of a fried potato (Mellema, 2003)

Crust

Product/oil interface

(T<100°C)

Roughness

Steam bubblesOil

(T>150°C)

Porosity & capillary

Introduction• Deep fat frying is a process that simultaneously heats

and mass transfer (Fig. 1)- “Heat Transfer” have 2 steps1.Heat is transferred from the hot oil to product surface

by convection2. And from the surface to the center of the food

by conduction

Introduction

Mass transfer has 2 phenomena

water moves from the inside of the food to outside as vapor

The uptake of frying oil into the food

Introduction

Fig. 2 Heat and mass transfer during food-frying (Dincer and Yildiz, 1995)

Introduction• Why we need to know this transport phenomena ?

to control of the frying process

to obtain fried food of a greater quality

to consider the energy that want to use during the process of deep fat frying

Heat transfer

• Alvis et al. (2009) report the 3 methods to measure of the heat transfer coefficient during deep fat frying process 1. Steady-state measurement of surface temperature

2. Transient measurement of temperature

3. Heat flux measurement of surface temperature

Heat transfer

• based on the Newton’s law of cooling• requires constant conditions during the test and no mass

transfer• this method is used only for product similar to fresh

product and with high thermal conductivity

source : Alvis et al. (2009)

1. Steady-state measurement of surface temperature

Heat transfer

• based on transient temperature measurements

• can be determined by using the lumped capacity method

• essentially assumes that the Biot number of food will be lower than 0.1

source : Alvis et al. (2009)

2. Transient measurement of temperature

Heat transfer

• requires a sensor that can detect the heat flux at the surface (and change in temperature at the surface with time)

• this method would account for the mass transfer

source : Alvis et al. (2009)

3. Heat flux measurement of surface temperature

Heat transfer

Seruga and Budzaki (2005)

• To determine the convective heat transfer coefficient with varies oil temperature on Krostula dough

• And study influence of oil temperature on surface temperature of Krostula dough

Heat transfer

• The convective heat transfer coefficient was determined as Eqs.

(1))( 021

21

s

vap

TTA

H

tt

mmh

When ; m = mass of dough T0 = oil temperature

t = time of frying Ts = surface temperature

ΔHvap = Heat of vaporization of water (= 2257.104 kJ/kg)

A = surface area of dough (=0.000745 m2)

Fig. 3 Typical curves of heat transfer coefficient („) and temperature profile of Krostula dough in it centre and frying oil. (a) 160 ̊ C ; (b) 170 ̊ C ; (c) 180 ̊ C ; (d) 190 ̊ C

110˚C 118˚C

120˚C140˚C

579.12583.88

597.05 657.91

160 ˚ C170 ˚C

180 ˚ C 190 ˚C

Mass transfer

• Modeling to determine molecular diffusion in deep fat frying process has a difference levels of complexity to report such as

- Fick’s second law of diffusion

- First kinetic model

- And in more complex model has been treated as two regions separated (the crust and the core)

Mass transfer

Budzaki and Seruga (2005)

• To examine oil absorption and moisture loss phenomena during deep fat frying of Krostula dough

• Fick’s second law was used to describe moisture loss

and oil uptake during deep fat frying of Krostula dough

Mass transferAssuming the slice is an infinite slab

„ The average moisture concentration can be determined as in

(2)2

,2

0

2

48ln

h

tD

M

M wefft

Where ; Mt = moisture content at t=t Deff,w = moisture diffusivity

M0 = moisture content at t=0 t = frying time

h = half thickness of slab

Mass transfer

„ For the average oil concentration can be determine as in

(3)2

,2

0

2

48ln

h

tD

O

O oefft

Where ; Ot = oil content at t=t Deff,o = oil diffusivity

O0 = oil content at t=0 t = frying time

h = half thickness of slab

Mass transfer

• The effect of temperature on effective moisture and oil diffusitivity

is generally described using an Arrhenius-type relationship

RT

EDD a

ooeff exp,0,

RT

EDD a

wweff exp,0,

Where ; D0,w and D0,o = the moisture and the oil diffusivity constant

Ea = activation energy , R = gas constant , T = temperature

Fig. 4 Experimental and predicted data for moisture content of Krostula dough during frying at different temperatures as a function of time

190 ˚C

Fig. 5 Experimental and predicted data for absorption of oil of Krostula dough during frying at different temperatures as a function of time

0.45

0.509

0.547

0.6137

Mass transfer

Debnath et al. (2003)

• Study first-order kinetics model to describe moisture outflow and oil uptake during deep fat frying of ribbon snacks

• Study about effect of frying temperature 150,175 and 200 ˚C on moisture and oil mass transfer

Mass transfer• Moisture kinetics model:

(4) tKMmm

mmmr

e

et

lnln

0

Where ; mt = moisture content at t=tm0 = moisture content at t=0me = the pseudo-equilibrium moisture contentKm = the kinetics coefficient for moisturet = frying time

Mass transfer• Oil kinetics model :

(5) tKOOO

OOor

e

et

lnln

0

Where ; Ot = oil content at t=tO0 = oil content at t=0Oe = the pseudo-equilibrium oil contentKo = the kinetic coefficients for oilt = frying time

Fig. 6 variation of moisture content during deep fat frying at

difference temperature

● = 150 ˚ C

■ = 175 ˚ C▲= 200 ˚ C

Fig. 7 variation of oil content during deep fat frying at difference temperature

● = 150 ˚ C

■ = 175 ˚ C▲= 200 ˚ C

Conclusions

• Heat flux measurement of surface temperature can be used to determine convective heat transfer coefficient.

• Fick’s second law and first-order kinetics model can be used to describe moisture loss and oil uptake during deep fat frying.

Conclusions

• The oil frying temperature have effect on heat and mass transfer :

- the heat transfer coefficient will increase with increasing oil temperature.

- moisture loss and oil uptake become more intense at higher temperatures.