Agriculture Reference
In-Depth Information
In those equations,
T
is the temperature at any given location (
x
,
y
,
z
) at any
given time (
t
) after the heat treatment is initiated.
T
0
and
T
a
are the initial and ambient
temperatures. The transfer of heat to the interior of food is affected by its physical
properties,
k
(thermal conductivity),
(density), and
C
p
(specifi c heat). All three
parameters determine the thermal diffusivity,
K
/(
ρ
C
p
), which describes how fast heat
is transferred. The transfer of heat to fruits and vegetables is affected by the initial
temperature (Eq. 13.1) and the physical condition of the heating medium (Eq. 13.2).
dT dn
is the temperature derivative normal to the boundary. The parameter
h
is
the surface heat transfer coeffi cient, governing the effectiveness of convective heat
transfer from the heating medium to the surface of fruits and vegetables. The heat
transfer equation (Eq. 13.3), together with its initial and boundary conditions, is
usually solved by numerical methods (see case study below). Models may be devel-
oped to design a thermal process for a specifi c type of fresh produce to achieve effec-
tive surface decontamination with minimal quality damage (Scheerlinck and others
2004 ).
ρ
Thermal Inactivation Kinetics
The inactivation of microorganisms on fruits and vegetables by heat usually follows
fi rst-order kinetics, which means that the log counts, log(C), of bacteria decrease
linearly at constant temperature conditions:
t
D
()
=
()
−
log
C
log
C
Eq. 13.4
0
In Equation 13.4 ,
D
is a measure of time needed to achieve a 1-log reduction of
the bacterial counts under a constant temperature condition. The D value is dependent
on temperature, and the relation between the two can be expressed as the following
(Eq. 13.5 ):
lo
DD
T
z
()
=
(
)
−
log
Eq. 13.5
0
The parameter
z
depicts the effect of temperature on the D values, and is a measure
of the increase in temperature needed to cause a 90% reduction in the D values.
Together with the temperature history, the D and z values are used to estimate the
extent of bacterial destruction during a thermal process:
TT
z
−
ref
10
t
LRD
∫
Eq. 13.6
=
dt
D
0
ref
In Equation 13.6, LRD represents the total log reduction in bacterial counts during
a thermal process, where
T
is temperature, and
D
ref
is the D value at a reference tem-
perature
T
ref
. It is apparent that the total bacterial reduction is affected by the D and z
values of the target microorganism and the temperature history at the location of inter-
est in the food.
One major disadvantage of thermal processing is that heat applied to kill foodborne
pathogens and spoilage microorganisms usually also causes quality damage to fresh
