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Lebensm.-Wiss. u.-Technol., 22,192-195 (1989)

New ET Type Multilayer Sorption Isotherms.

Part

:

Modelling Water Sorption in Foods

Roberto

J.

Aguerre, Constantino Suarez and Pascual

E

Viollaz

Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Ciudad Universitaria, 1428 Buenos Aires

(Argentina)

Received December}2, 1988; accepted January 25, 1989)

Two equations derived from a modification of

BET

theory were evaluated for goodness of it over 74 experimentalfood isotherms

representing spices, fruits, vegetables, meats, proteins, starchy foods and mi/k products. About

77%

ofthe isotherms represented

principally by starchy foods, proteins

meatsand

spices obeyed to

Eqn

[1}.

Such isotherms when compared to

BET

equation showed

lower sorption capacity than BET at high water activity. On the contrary,

23%

of the products formed principally by high-sugar

foods, i.e. fruits and some vegetables, whose sorption capacity resulted much higher than BET isotherm for water activities above

0.45 were adequately correlated by Eqn [2j.

Introduction

Many mathematical equations have

been

proposed in litera

ture to model hygroscopic equilibrium data in food systems.

The different models proposed, empirical, semi-empirical

or

theoretical have had sorne success in reproducing equilibrium

moisture content data of a given type

of

food and in a given

range

of

water activity. A review

of

the literature

on

equations

for fitting water sorption of

foods was performed by Chirife

and Iglesias (1). The authors evaluated the capacity of eight

published two-parameter equations to describe experimental

isotherms of

39 different foods divided in six food groups.

More recently, Lomauro et al.

(2) conducted a similar analysis

and studied the fitting ability of three, two-parameter,

equations and

one

three-parameter (GAB isotherm) to fruit,

vegetables and meat isotherms.

The aim

of

the present work was to evaluate the ability

of

the

two parametric equations derived in part

one

as

Eqns

(15) and

(16), to describe hygroscopic equilibrium data of several

foods. Their simplicity (i.e. only two parameters) and the fact

that

both

equations were derived within

the

framework

of

BET theory make both equations particularly interesting in

the area of water sorption in foods.

Materials and methods

Hygroscopic equilibrium data of different foods and food

products were selected from literature. The foods were

grouped according to main constituents and divided in the

following groups: milk products, starchy foods, spices, nut and

oilseeds, fruits, vegetables, proteins and meats. The list of

the

selected foods, and sorne specifications are given in

Table 1.

Results and discussion

For practical reasons the two equations to be tested, Eqns (15)

and (16) of part one, will

be

rewritten he re in terms of the

variables more frequently used in the literature of water

sorption in foods, moisture content

m)

and water activity,

w

)

as:

Eqn [1]

and

Eqn [2]

where mm and

e

are the monolayer moisture content and the

BET

constant, respectively. The equilibrium data of74 experi

mental isotherms Table 1) were processed by Eqns [1] and [2]

using a non-linear regression method. As a consequence of

such analysis it was determined that 23% of the products

examined obeyed Eqn [2] to sorne extent while the rest of the

materials

(=77 )

only fitted Eqn [1]. These results are shown

in Table 2

together with the values of

e

and mm obtained from

the regression analysis. To evaluate the goodness of fit of each

equation, the mean relative percentage deviation modulus, E

will be used. This

parameter

is defined by Eqn

[3]:

N

E

=

100,\

Imi

-

mpil

Ni m

i l

Eqn

[3]

where mi and mpi are the experimental and predicted moisture

contents (dry basis), respectively and N is the number of

experimental data. This parameter was used in the literature to

evaluate the goodness of fit of different mathematical ex

pressions as applied to experimental equilibrium data (1,2). I t

is generally considered that E values below 10% give a reason

able good fit for practical purposes. As can be seen from

Table

2 a great amount of food isotherms were fitted with E values

lower than 10%, while a poor fitting was obtained with only

two products (banana and pineapple) with E values consider

ably greater than 10%.

The performance obta ined with Eqns

[1]

and

[2]

will be further

analyzed.

For this purpose, the equilibrium data of the ma

terials reported in

Table 1

were modelled

by

means of BET

equation in the range of water activities between approxi

mately 0.03 and 0.45. The parameters

of

BET equations were

determined by non-linear regression and used for

extrapol

ating the equilibrium curves up to water activities about 0.80-

0023-6438/89/040192 04 03.00/0

1989 Academic Press Limited

192


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Table 2

Continued

Protein

Salmin

Collagen

Gelatin

Eggalbumin

Lactoglobulin

Serum albumin

y-pseudoglobulin

Silk

Wool

05

0-4

. 0.3

:3

E

02

01

o

04

Fitting Eqn No.

1 )

(1)

1 )

(1)

(1)

(1)

(1)

1 )

(1)

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

08

I

I

I

I

I

I

I

Fig. 2 Comparison between predicted,

- Eqn

[2],

and

experimental

data, 0 for sultana raisins; BET

isotherm

equation usually leads to k-GAB constant values lower than

one (25) seerns to corroborate the present results. On the other

hand, although

Eqn [2]

fitted only 23% of the foods tested, it

is

interesting to observe that rnost of these products are high

sugar foods such as fruits and sorne vegetables like sugar beet

and carrots, this fact rnakes

Eqn [2]

particularly interesting

given the very well-known difficulty of correlating equilibriurn

data

of

high-sugar foods (3).

Conclusions

The following conclusions can be drawn: fruits, rnilk products

and sorne vegetables were rnodelled by

Eqn

[2], covering a

range

of a

w

frorn 0.03 to 0.85, approxirnately;

Eqn [1]

was very

appropriate for the correlation of

equilibriurn

data of

starchy

lwt/vol.

22 (1989) No.

4

mm

C ( ,d.b.)

E

( )

a

w

range

1.4

22.4 8.6 0.05--0.90

11.2 13.4 4.5 0.05--0.90

12.7

11.7 5.5 0.05--0.90

7.9 8.1 4.8 0.05--0.90

6.4 8.7 4.5 0.05-D.90

13.2

8.1

3.7 0.05-D.90

13.5 8.7

4.3 0.05-D.90

15.1 5.0

4.4 0.05--0.90

11.2 6.9 6.6 0.05-D.90

foods, proteins, rneats, spices, nuts and oilseeds and sorne

vegetables, i.e. grarns and beans, over a wide range of water

activities; the ability of Eqn [1] to correlate a great variety of

food rnaterials

is

related with the tendency of rnost foods to

present lower sorption capacity than that predicted frorn BET

theory, for high water activity.

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195

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