K Nn mK - Amecamex.org.mx

product quality needs to be taken into
account during food processing.
Sorption isotherms. Figure 3 shows
the sorption isotherms at 15, 25 and
35C. The figure indicates that there is
a great influence of temperature on
the isotherm sorption. Data are
exponentially distributed as shown in
other high starch products (Chen,
2002).
Water content (g H2O/g d.m)
0.8
0.6
0.4
0.2
15°C
25°C
35°C
0
0.2 0.4 0.6
Aw
0.8 1
Figure 3 Sorption isotherm of Tejate at 15,
25 and 35 C
The Chung-Pfost equation was
linerized to calculate the constant
values
ln
⎛
( − ln Aw ) = ⎜ln
+ ( − b)
M ⎟
⎝ T ⎠
a
where: a=C1/R and b=C2
and also Henderson equation
[ − ln(
1−
A ) ] = ln aT b ln M
ln w +
a=C1 and b=C2.
Table 1 shows the constant values
and the correlation coefficient (r 2 ) for
Chung-Pfost and Henderson models.
The accuracy of models at the
studied temperatures is the same for
both equations. They represent
⎞
isotherm behavior but they were more
accurate at low temperatures than at
high temperatures. Chung-Pfost and
Henderson models have been used
to predict sorption isotherms for
cereals or starchy products (Sun
D.W., 1998, Silakul and Jindal, 2002,
Chen, 2002). Both equations have
been modified to increase their
accuracy. Chung-Pfost equation has
been recommended to use in rice
products and mushroom (Durakova
and Mendkov, 2004, Shivhare et al.,
2004). Figure 4 fits both equations
and experimental results at 15 C.
There is a region between 0.7 and
0.95 of Aw where the isotherm was
not well described by the models.
Table 1. Values of constants and r 2 of
Chung-Pfost and Henderson equations for
Tejate.
Chung-Pfost
a= 668.84
b= 8.5224
Henderson
a= 0.0243
b= 1.1473
r 2
15C 25C 35C
0.9841
0.9851
Water content (g water/g d.m.)
0.6
0.5
0.4
0.3
0.2
0.1
Experimental
Chung-Pfost
Henderson
0.9650
0.9718
0
0 0.2 0.4 0.6 0.8 1
0.9343
0.9469
Aw
Figure 4. Experimental and predicted
absorption isotherms by Chung-Pfost and
Henderson models at 15C