Thermal Food Processing

452 Thermal Food Processing: New Technologies and Quality Issues
TABLE 14.8
Kinetic Constants and Thermal Inactivation Parameters for the Thermal
Degradation of β-GAL
Temperature
(°C)
D Value
(min)
z Value
(°C)
Pre-Exponential
Factor (k 0)
(s −1 )
intolerant and cannot eat milk and dairy products. When the intestine produces
little or no lactase, lactose (the milk sugar) is not digested and moves into the
colon, where bacteria ferment it, producing hydrogen, carbon dioxide, and organic
acids. The results of this fermentation are diarrhea, flatulence (gas), and abdominal
discomfort. However, conventional and ohmic inactivation mechanisms are
similar, and the results obtained (Table 14.8) do not show a clear additional effect
of the electric field on the enzyme inactivation. 60
In summary, no general behavior or trends are observed, and the effect of
ohmic heating is dependent on the enzyme and probably also on the food system.
The electric field reduces the D values for LOX and PPO, while z and E a values
are not greatly affected. For the other enzymes tested, the effect of the electric
field is unclear.
14.3.3 ASCORBIC ACID DEGRADATION KINETICS UNDER OH
Activation
Energy (E a)
(× 10 −3 J⋅mol −1 )
Conv. Ohmic Conv. Ohmic Conv. Ohmic Conv. Ohmic
65 182.00 —
70 33.90 12.24
72 12.89 9.70
75 2.99 2.77
78 0.52 0.64
80 0.50 0.28
5.12 5.13 0.997 0.997 0.370 0.350
Source: Adapted from Castro, I. et al., The Influence of the Presence of an Electric Field on Lipoxygenase
and b-Galactosidase Inactivation Kinetics, paper presented at Proceedings of NFIF 2003: New Functional
Ingredients and Foods: Safety, Health and Convenience, Copenhagen, Denmark, 2003.
Ascorbic acid (vitamin C) is frequently used as a food preservative or a vitamin
supplement. Industrially, its main function is to prevent browning and discoloration,
thus enhancing the shelf life of several products. Ascorbic acid is, for
example, an effective inhibitor of peroxidase in fruits such as kiwi.
It is known to be thermolabile, and its degradation mechanism is specific to
the particular system in which it is integrated. Degradation depends on whether
aerobic or anaerobic pathways are employed. 62 The aerobic degradation pathway
is related to the presence of oxygen (either in the headspace or dissolved). 63 The
anaerobic pathway is mainly driven by the storage temperature, with lower temperature
storage being the only way to minimize the degradation rate. 64 When oxygen