Handbook of Meat Processing

Drying 221

Fat - free raw meat has a content of water

of about 80%, with swelling protein content

of about 19% and soluble components at

about 1%. Figure 11.2 shows the domain of

a w water content connection calculated from

various experiments (Hamm 1972 ; Lewicki

2004 ; Ruiz - Ramirez et al. 2005a ). The fi gure

It needs to be stressed that the other compo-
nents of meat — for example, fat — are inef-
fective in terms of a w , and therefore, a w does
not depend on the material ’ s moisture content
but on the ratio of water to the effective com-
ponents. The raw fat has an a w close to 1
independent of its low moisture content.
The soluble components decrease a w value
by means of their “ diluting ” effect. The basic
relation for a w is:

a number of water particles number of

water particles numb

w= (

+ eer of particles in

solution Nernst law)( )

Number of particles mass of component in

the solution mass o

=

ff particles^

The mass of a particle is the molecular
mass in the case of molecules, and ion mass
in the case of ions. If the solubility is limited
in a saturated solution, no more soluble com-
ponent goes into solution, and consequently
the a w does not change either.
Some soluble particles, particularly ions,
further decrease the water activity because
they bind water particles.
During preservation, the task is to dimin-
ish the a w to the desired value. This value
depends on the storage temperature of the
product. Dried meat products need no chill-
ing; they can be stored at room temperature.
Traditional dried meat products have a pH of
about 6.0; they need an a w below 0.9. The
lactic fermented ones have a pH about 5.0,
and in this latter case, an a w below 0.95 is
needed for ensuring a safe product (Fig. 11.1 )
(Incze 2004 ).
The swelling components bind a little part
of water independent of the a w ; another part
of water is bound increasingly parallel with
a w - increase. The amount of bound water
depends on the state of swelling material
(denaturation).
The contents of water, soluble materials,
and swelling materials together determine
the a w ; the other components have no
infl uence.

4.50

5.00

5.50

6.00

6.50

7.00

0.85 0.90 0.95 1.00

aw

pH

stable range

spoiling

Figure 11.1. The stable a w - range as a function of

pH.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1020304050607080

moisture content of raw meat in %

aw

low limits high limits

Figure 11.2. Relationship between a w and moisture

content of raw meats.