7 soybean Gandhi and Bourne 1991, Xu and Chang 2008
b
, tempe Handoyo and Morita 2006.
Thermal softening can be due to changes in cell wall matrix polysaccharides celluloses, hemicelluloses, pectins etc. which depend on many
factors such as pH, types and amounts of various salts present in the plant cell wall. Water uptake by polysaccharides results in reduction of cohesiveness of cell
wall matrix thereby resulting in lower adhesion. Other reason for softening is due to loss of turgor pressure which is pressure of cell components against the cell
wall and it is due to the water content inside the cell Lund 1982. The thermal treatments can result in plasmolysis which reduces the turgor pressure and it is
responsible for softening of food Rao and Lund 1986.
Thermal processing influences the nutritional value of some food products by changing the total of antioxidant capacity. Boiled and steamed eclipse black
beans exhibited significantly lower antioxidant activities than raw beans in total phenolic content TPC, DPPH free radical scavenging activity DPPH, and
oxygen radical absorbing capacity ORAC Xu and Chang 2008
a
. As compared to the raw soybeans, all processing methods caused significant decreases in TPC,
DPPH, ORAC, total flavonoid content TFC, condensed tannin content CTC, monomeric anthocyanin content MAC, and ferric reducing antioxidant power
FRAP, and in black soybeans Xu and Chang 2008
b
. In the other hand, thermal processing elevated total antioxidant activity
and bioaccessible lycopene content in tomatoes and produced no significant changes in TPC and TFC. The increase in total antioxidant activity of the heat-
processed tomatoes was due to the increased amount of lycopene as a major phytochemical in tomatoes and other bound phytochemicals released from the cell
matrix Dewanto et al. 2002. However, high pressure processed tomato and carrot purées had significantly higher antioxidant capacities when compared to
thermally treated samples Patras et al. 2009.
2.6 Kinetics of Quality Degradation
Some of quality degradation reaction in food product can be described well by first order kinetic reaction. This model was characterized by a straight line
when the logarithm of the texture property was plotted against heating time Rizvi and Tong 1997.
Generally, thermal softening in fruits and vegetables was suited to the first order rate with the firmness as the primary texture attribute Rao and Lund
1986, Bourne 1987,
Rizvi and Tong 1997
.
The heat-induced degradation of natural pigments and browning reaction also followed the first order reaction
Villota and Hawkes 2007. The equations for a degradation reaction for first order are Van Boekel
2008: -
dC dt
=kC
-ln C
C =kt
8 where C
t
is the concentration at time t, C is the initial concentration, t is time,
and k is rate constant.
a b
Figure 1 a First order reaction curve Villota and Hawkes 2007 and b Relationship between rate constant and temperature Berk 2009.
All chemical reactions are accelerated when the temperature is elevated Berk 2009.
The temperature dependence of a reaction rate constant can be expressed by the Arrhenius equation Villota and Hawkes 2007, Berk 2009:
k=k
o
exp - E
a
RT ⁄
ln k
o
k =
E
a
RT where k
o
is the frequency or collision factor, E
a
is the activation energy, R is the gas constant 8.314 JK mol or 1.987 calK.mol, and T is the absolute
temperature K. The design of optimal thermal processes relies on relevant kinetic data for
bacterial inactivation and quality changes Van Loey et al. 1995. Generally, every 10
o
C rise in temperature the rate of chemical reaction increases two-fold, while the rate of microbial destruction rises ten-fold or otherwise quality
degradation is less temperature sensitive than microbial destruction Holdsworth 1985. There are differences in thermal behaviour between safety and quality
factors during heating. Therefore, optimization is needed to ensure the product safety and maximize retention of quality attributes.
Greenwood et al. 1944 were the first to observe thermal behaviour between safety and quality attributes. They studied the destruction of thiamin in
cured pork luncheon meat at three levels, 50, 20, and 10, compared with microbial destruction. The rate of thiamine destruction is doubled with an 18
o
F increase in temperature as contrasted with a tenfold increase in the rate of
destruction of heat-resistant bacteria. They assumed that rapid heating method at the higher processing temperatures were more favorable to thiamine retention.
Since then many publications have referred to this technique to optimize processing conditions Holdsworth 1985.
