2009_Book_FoodChemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 131
Table 2.12. Thermal inactivation of enzymes to prevent deterioration of food quality
Food product Enzyme Quality loss
Potato products, Monophenol Enzymatic browning
apple products oxidase
Semi-ripe peas Lipoxygenase, Flavor defects;
peroxidase bleaching
Fish products Proteinase, Texture (liquefaction),
thiaminase loss of vitamine B 1
Tomato purée Polygalacturonase Texture (liquefaction)
Apricot products β-Glucosidase Color defects
Oat flakes Lipase, Flavor defects
lipoxygenase (bitter taste)
Broccoli Cystathionine Off-flavor
Cauliflower β-Lyase
(cystine-lyase)
rioration caused by enzymes which can be eliminated
The co-called “D-value” represents the time
t = 2,3
k = D (2.83) (2.85)
e. g., by thermal inactivation.
needed to reduce the initial concentration
Temperature and time are two parameters responsible
for the effects of a thermal treatment. They
(activity, germ count) by one power of ten.
It refers to a certain temperature which has
should be selected carefully to make sure that all to be stated in each case. For example:
necessary changes, e. g., killing of pathogens, are Bacillus cereus D ◦ 121 C = 2.3s, Clostridium
guaranteed, but still all undesired changes such as
degradation of vitamins are kept as low as possible.
botulinum D ◦ 121 C = 12.25 s. For a heat treatment
process, the D-value allows the easy determination
of the holding time required to reduce
the germ count to a certain level. If the germ
count of B. cereus or Cl. botulinum in a certain
2.5.4.1 Time Dependence of Effects
food should be reduced by seven powers of ten,
the required holding times are 2.3 × 7 = 16.1s
The reaction rates for different types of enzymatic and 12.25 × 7 = 85.8s.
reactions have been discussed in section 2.5.1.
The inactivation of enzymes and the killing of microorganisms
can be depicted as a reaction of 1st 2.5.4.2 Temperature Dependence of Effects
order:
c t = c 0 e −kt (2.80)
A relationship exists for the dependence of reactionrateontemperature.Itisexpressedbyan
with c 0 and c t = concentrations (activities, germ
counts) at times 0 and t, and k = rate constant
for the reaction. For c t and t follows from equation
equation of Arrhenius:
k = A · e −E a/RT
(2.84)
2.80:
with k = rate constant for the reaction rate,
logc t = − k
E a = activation energy, R = general gas constant
2,3 · t + logc 0 (2.81) and A = Arrhenius factor. For the relationship
between k and T, the Arrhenius equation is only
t = 2,3
k log c 0
(2.82)
an approximation. According to the theory of
c t the transition state (cf. 2.2.1), A is transferred
via the active state A # into P. A and A # are in
c 0 /c t = 10 gives:
equilibrium.