Energy aspects in food extrusion-cooking.pdf

Int. Agrophysics, 2002, 16, 191–195

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Energy aspects in food extrusion-cooking

L.P.B.M. Janssen 1 , L. Moœcicki 2 *, and M. Mitrus 2

1 Chemistry and Chemical Engineering Institute, University of Groningen, Nijenborgh 4, 9747 Groningen, the Netherlands

2 Food Process Engineering Department, University of Agriculture, Doœwiadczalna 44, 20-236 Lublin, Poland

Received February 25, 2002; accepted May 20, 2002

A b s t r a c t. Theoretical and practical energy balance

considerations in food extrusion-cooking are presented in the

paper. Based on the literature review as well as on own measure-

ment results, the baro-thermal treatment of different vegetable raw

materials is discussed together with the engineering aspects of the

extruders’ performance as a whole.

Keywords:extrusion-cooking, energy balance, specific

mechanical energy

actions and to predict the influence of changes in parameters

on the performance of the extruder as a whole [1,2].

Many of the parameters needed for food extrusion mo-

dels are unknown, usually changed considerably during the

process and are related to a wide variety of other parameters.

Many of the numerical descriptions that can be of great be-

nefit for the description of the extrusion of synthetic poly-

mers may deviate considerably from the actual results in

extrusion – the cooking of food, thus limiting their value in

this field. Therefore, in evaluating the energy balance, it is of

great importance to be acquainted with material properties

like viscosity.

In plastics extrusion, viscosity is a unique function of

temperature and shearing, but in extrusion-cooking, che-

mical and physical changes occur during the process. This

immediately implies that viscosity is not only a function of

INTRODUCTION

Extrusion-cookers give the opportunity to combine

pumping, mixing, kneading and heating operations in one

machine (Fig. 1). As a consequence of this combination,

however, the different operations interact with each other

and can only be separated to a certain extent. An important

objective in designing extruders is to define these inter-

Fig. 1. Cross-section of a typical food extruder: 1 – drive, 2 – feed hopper, 3 – cooling water jacket, 4 – thermocouples, 5 – barrel steam

jacket, 6 – pressure transducer, 7 – die, 8 – discharge thermocouple, 9 – breaker plate, 10 – barrel with hardened liner, 11 – screw with

increasing root diameter, 12 – feed section, 13 – compression section, 14 – metering section [1].

*Corresponding author’s e-mail: moscicki@faunus.ar.lublin.pl

192

L.P.B.M. JANSSEN et al.

instant temperature and shearing but also to a large extent a

function of temperature history. Physical cross-linking and

gelatination may modify the viscosity and contribute to a

complex rheology that can hardly be interpreted as a mere

change of state (melting). This can result in process insta-

bilities and problems in control [2].

It is well known that within normal operating ranges,

starches and protein-rich materials are shear thinning. This

justifies the use of a power-law equation for viscosity. For

changing temperatures, the power-law equation may be

combined with temperature effects [3]:

ENERGY BALANCE

exp

(1)

The extrusion-cooker is a thermodynamic unit: for ste-

ady-state operation, it means all energy that is introduced

into the apparatus must also come out again. An energy ba-

lance consists of four terms [2,8,9]:

– mechanical energy added by the rotation of the screw,

– heat transferred through the barrel wall,

– mechanical energy partly used to increase the pressure of

the material,

– mechanical energy partly converted into heat by viscous

dissipation.

The thermal energy (generated by viscous dissipation or

transferred through the wall) results in an increase in the

temperature and phase changes (e.g., melting of solid mate-

rial or evaporation of moisture). The energy balance can be

described as [2]:

0 is the

Newtonian viscosity (N s m –2 ), n is the power-law index,

is the shear rate (s –1 ),

a is the apparent viscosity (N s m –2 ),

is a constant, T is the temperature

(K), T 0 is a reference temperature (K).

Both starch-rich and protein-rich materials show an

increase in viscosity when extrusion-cooked. This may be

attributed to network formation of the molecules, in protein-

rich material by cross-linking and in starch-rich materials by

entanglement of the amylose and amylopectin chains. One

may assume that the network formation may roughly be

described as a first-order reaction and that the viscosity

increase is roughly proportional to the thus – formed con-

centration of cross-links or entanglements. Therefore, the

viscosity of a fluid element with a residence time

EE Q P c T e

(3)

is the density (kg m –3 ), c p is the specific

heat of the food material (J kg –1 K –1 ), e is the phase change

enthalpy per unit weight includes the energy needed for

chain splitting (J kg –1 ), for cross-linking in protein rich ma-

terials and for the generation of new surfaces when the

material expands.

If the product temperature is measured after the material

has left the die instead of before, the pressure energy has also

been transferred into heat and the energy balance reduces to

[2]:

in the

extruder may be expressed as [2]:

exp

exp R

Tt Dt

(2)

E is the activation energy (J), R is gas constant

(8.3143 J mol –1 K –1 ), t is time (s). Dt denotes that the inte-

gration must be performed in a co-ordinate system travelling

with the fluid element; as a result T ( t ) is the temperature as a

function of the time that the fluid particle experiences while

travelling through the extruder.

Depending on the actual values of the constants

EE Q c T e

(4)

T is now the temperature change measured just after

the die, before any cooling by convective and radiative

losses.

To establish the ratio between energy added by the drive

unit and energy transferred through the wall, the Brinkman

number must be used defined as [2]:

and

E and the temperature profile, the viscosity of the material

may increase or decrease during extrusion [2,9]. A small de-

crease in throughput (e.g., by a small increase of die resis-

tance) may change the hold up, increase the residence time,

and therefore increase viscosity. If this viscosity change

strongly affects the pressure built up at the die, the pressure

flow increases and the throughput may decrease further,

especially in an extruder with soft material or insufficient

grooves. This gives rise to instabilities. If, on the other hand,

the increase of viscosity affects the back flow most strongly,

the process becomes more stable [2]. Further investigation

into the dependence of the stability on the actual value of the

parameters

(5)

is the viscosity (N s m –2 ), v is a representative velo-

city (m s –1 ),

is the thermal conductivity (W m –1 K –1 ),

is the temperature difference between the food material and

the barrel wall (K).

Assuming that the heat needed to melt solid fractions of

the material is much smaller than that needed for heating the

material and evaporation of the moisture, two extremely

simple and useful equations can be used. If no evaporation of

moisture at the die end occurs, the final material temperature

( T f ) is given by:

and

E in connection with the temperature

profile is needed.

where

where E is the effective motor power that is transferred to the

screw(s) (W), E h is the net heat added through the wall (W),

Q v is the volumetric throughput (m 3 s –1 ), P is pressure at the

die opening (Pa),

where

ENERGY ASPECTS IN FOOD EXTRUSION

193

(6)

GcT

p die

85 /

(8)

When a moisture content of fraction f evaporates at the

die, Eq. (6) can be modified as [2]:

where G s is steam formed (kg kg –1 ), c p is the heat capacity of

the wet extrudate (J kg –1 K –1 ), H vap is the heat of vaporisa-

tion of water (2.26 MJ kg –1 ), T die is the temperature behind

the die (K).

To estimate the c p of the extrudate requires a proximate

analysis for water, protein, carbohydrate, fat and the ash of

the extrudate, and the data summarised in the literature

[5,10]. The heat capacity of the extrudate is simply the

weighted average of the heat capacities of the individual

components. For example an extrudate which is 25% water,

10% protein, 59% starch, 5% fat and 1% ash, the heat

capacity c p = 2.61 kJ kg –1 K –1 . If the temperature behind the

die is 150°C, 75 g steam per kilogram of wet extrudate will

be liberated. A simple water balance reveals that the final

moisture of the product would be 18.9%.

In the absence of any heat inputs or losses, such as steam

injection or venting, barrel heating or cooling, or convective

or radiative losses, that is, in the adiabatic condition, the

temperature rise of the extrudate can be found from the

following relationship [6]:

TT EfeQ

(7)

Here e w denotes the phase change enthalpy of the vapo-

rising component (for water, e w = 2257.8 kJ kg –1 at 100°C).

Taking into account that the temperature rise of the material

during extrusion ( T f – T 0 ) is a unique function of motor

power, throughput, and material properties, the analysis

above is particularly useful since its application is not re-

stricted to one particular type of machine. With simple mo-

difications, various other effects (cross-linking, chain split-

ting, surface generation) can be taken into account [2].

Knowledge of moisture flash at the extrusion-cooker

outlet is needed to perform material and energy balances

around the extrusion system. The most common approach to

obtaining this information is to attempt a sample of the

product as it leaves the extrusion die, however that gives

very imprecise moisture analyses. The moisture of the extru-

date changes so rapidly that any variation in the collection of

the sample and the sealing of the sample container results in

a sample whose moisture varies appreciably [5].

The energy lost by the extrudate as it passes through the

die is equal to the energy available for the evaporation of

steam on the law-pressure side of the die. From a thermo-

dynamics perspective, the enthalpy of the steam entering the

die is equal to the enthalpy of the exiting steam.

As previously indicated, the extruder imparts energy

into the extrudate via the dissipation of mechanical energy

and/or the transfer of thermal energy. This energy results in

heating the extrudate. In fact, it is the storage of the viscous

and thermal energy inputs as thermal energy in the ex-

trudate. Since the temperature associated with the equili-

brium water vapour pressure on the law-pressure side of the

die, some of the water will be converted into steam until

equilibrium is attained. The energy ‘stored’ in the extrudate

must be conserved during this process. That is, the stored

energy that entered the die is equal to the total energy in the

two exit streams (puffed extrudate and steam). This can be

described by a very simple heat balance. One must know the

temperature on the high-pressure side of the die and the

equilibrium vapour pressure of water on the discharge side

of the die. This pressure defines the temperature at which the

flash occurs. There is always fresh air mixed with the steam

in the discharge area, so appreciable vaporisation occurs

even though the extrudate has cooled to below 100°C. The

observations of many authors suggest that the water flashes

at about 85°C. Using the assumption of an 85°C flash, the

heat balance to estimate the mass of steam G s released per

unit mass of wet extrudate is [5]:

cTmH

t t

SME ,

(9)

T is the temperature change of the extrudate (K), m t

is the mass of extrudate which can undergo phase transfor-

mation per unit mass of extrudate) (kg kg –1 ), H t is the

energy associated with the phase transformation (J kg –1 ),

SME is the specific mechanical energy input of the extru-

der’s motor (kWh kg –1 ).

During phase transformation (e.g., gelatinization of

starch and denaturation of protein) a reasonable estimation

of the energy required is calculated for 17 J g –1 [8]. The

SME is usually calculated from the percent torque of the

extruder motor and its speed or by direct measurement with a

watt meter [1,6]. If the raw materials enter the extruder at

30°C and the SME is 0.1 kWh kg –1 (a typical value), the 25%

moisture extrudate described above exhibits a temperature

rise of approximately 133°C, or a die temperature of appro-

ximately 163°C. Any value significantly different from this,

assuming that the die temperature measurement is correct,

indicates that an appreciable heat transfer from other sources

is taking place.

The heat transfer from all sources may be included in the

analysis with a simple modification of the following equa-

tion [6]:

cTmH

t t

SME STE,

(10)

where STE is the specific thermal energy (kWh kg –1 ) from

other heat sources or sinks.

A negative value of STE represents a heat loss, a positi-

ve value is a heat input. For large industrial extruders, in the

vap

where

194

L.P.B.M. JANSSEN et al.

absence of steam injection or venting, the magnitude of STE

is about 0.03 kWh kg –1 , or less, because large extruders

have little surface area per unit volume. Small laboratory

extruders may exhibit much larger values of STE. The key

point to remember is that any analysis of extrusion beha-

viour must include an estimate of both SME and STE since

ultimately the quality of the product is controlled by both of

these parameters [6].

Taking into account the previous example, we can

easily estimate the STE from the die temperature measure-

ment. If the exit temperature was measured as 143°C, in-

stead of the predicted adiabatic value of 163°C, the STE

would be – 0.015 kWh kg –1 (a heat loss). Conversely, if the

exit temperature was measured as 183°C, the STE would be

0.015 kWh kg –1 .

The direct measurement of STE is not simple, due to

convective and radiative losses to the environment and the

heat inputs or losses from electrical coils or jackets. More-

over to estimate the heat transferred by direct steam injec-

tion or venting is not so easy.

where Q is heat (J), m s is mass of steam (kg), c v is heat of

vaporisation (J kg –1 ).

The heat of vaporisation of water is obtained from steam

tables and is a function of the steam injection pressure or the

vent pressure. At 0.1 MPa pressure it has a value of 2.26

MJ kg –1 of steam. The water added can be measured with

flow meters or can be estimated by a mass balance.

The other heat sources and sinks of thermal energy are

through barrel heat transfer by using steam, water, or other

heat transfer fluids or by electrical heaters. Moreover, signi-

ficant heat losses occur via convection of heat from the bar-

rel surfaces to the environment (important when processing

at high temperatures).

The energy obtained from electrical heaters will be in

the form of units of watts (J s –1 ). If the jackets are heated

with steam, the energy input is the same as that given above

for steam injection, or venting, except where the mass of

steam is the quantity being condensed in the condensate

leaving the steam traps with a bucket.

If a thermal fluid, or water, is begin used, the thermal

energy being transferred is obtained by a simple balance [7]:

PRACTICAL REMARKS

EWcT

(14)

Estimation of the SME is usually accomplished by

electrical measurement. The mechanical energy input is

readily estimated for a direct current motor drive by [7]:

where E t is the thermal energy (W), W is flow of thermal

fluid (kg s –1 ).

The flow rate of thermal fluid, or water can be measured

by flow meters, c p of the fluid is given in heat tables (4.184

kJ kg –1 K –1 for water), temperature change can be measured

on thermal fluid.

Measuring the thermal losses that occur by convection

of heat from the jacket to the surrounding air can be difficult.

The losses mentioned can be measured directly with heat

flux sensors [4]. A number of sensor simultaneously, must

be used because the heat losses are different all over the

extruder’s surfaces. These sensors can correlate heat losses

as a function of the extruder barrels’ external surface tempe-

ratures, the environmental temperature, and location. The

heat losses can be calculated as [7]:

SME

PN t N m

(11)

is % of torque, N m is maximum motor speed

(rots –1 ), t is time (s), m is mass of the extrudate.

The actual value should be reduced by the power con-

sumption of the extruder when it is running empty. The input

power for an AC motor is given by [7]:

SME

Pt m

r / ,

(12)

is efficiency, P r is watt meter reading (kW).

The efficiency of an AC motor is the strong function of

the load. The value of the efficiency can be obtained only

from motor curves (easy to obtain from the producer). In

case of DC motors, the input power is a gross measurement

and needs to be reduced by the power consumption of the

empty extrusion-cooker.

Estimating thermal energy inputs can sometimes be

difficult. There are a number of thermal energy sources and

sinks. Steam injection (a source) and venting (a sink) are

calculated by measuring how much water is added as steam

or removed as water vapour. The heat added, or removed, by

these actions is given by [7]:

St T T

(15)

where h is the heat transfer coefficient (Wm –1 K –1 ), l h is the

heat loses (J), S is a barrel surface (m 2 ), T s is a surface

temperature (K), T a is the air temperature (K).

The barrel surface temperature can be measured by the

attachment of surface thermocouples at a number of places

on the barrel surface. The heat transfer coefficient is a func-

tion of the position (top, bottom or sides) and the tempera-

ture difference between the extruder barrel’s surface and the

environment. Karwe and Godavri [4] provide a number of

equations for heat transfer coefficients, h may be assumed to

be about 10–15 W m –2 K –1 .

Qm sv

(13)

where P is rated motor power (kW), N is motor speed

(rots –1 ),

where

ENERGY ASPECTS IN FOOD EXTRUSION

195

REFERENCES

5. Levin L., 1997. Estimating moisture flash upon discharge

from an extruder die. Cereal Foods World, 42, 3, 10–14.

6. Levin L., 1997. Further discussion of extrusion temperatures

and energy balances. Cereal Foods World, 42, 6, 485–486.

7. Levin L., 1997. More on extruder energy balance. Cereal

Foods World, 42, 9, 22–27.

8. Moœcicki L. and Mitrus M., 2001. Energy requirement in

extrusion-cooking process (in Polish). Commission Motori-

zation and Energetics in Agriculture. University of Agricul-

ture, Lublin, 186–194.

9. Moœcicki L. and Mitrus M., 2001. Heat transfer in twin

screw extruder (in Polish). Commission Motorization and

Energetics in Agriculture. University of Agriculture,

Lublin, 195–208.

10. Okos M.R., 1986. Physical and Chemical Properties of Food.

American Society of Agricultural Engineers. Michigan. St.

Joseph.

1. Harper M.J., 1981. Extrusion of Foods. CRC Press Inc. Boca

Raton, Florida.

2. Janssen L.P.B.M., 1998. Engineering aspects of food extru-

sion. In: Extrusion Cooking (Eds C. Mercier, P. Linko, M.

Harper Jamerican). Association of Cereal Chemists, Inc. St.

Paul, Minnesota.

3. Janssen L.P.B.M. and van Zuilichem D.J., 1980. Rheology

of reacting biopolymers during extrusion. In: Rheology (Eds

G. Astarita, G. Marrucci). Plenum Press, New York, 3.

4. Karwe J. and Godavari R., 1997. Accurate measurement of

extrudate temperature and heat loss on a twin-srew extruder. J.

Food Sci., 62, 2, 367–372.

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