Food Chemistry

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 elimi-
nated e. g., by thermal inactivation.
Temperature and time are two parameters respon-
sible for the effects of a thermal treatment. They
should be selected carefully to make sure that all
necessary changes, e. g., killing of pathogens, are
guaranteed, but still allundesired changes such as
degradation of vitamins are kept as low as possi-
ble.

2.5.4.1 TimeDependenceofEffects

The reaction rates for different types of enzymatic
reactions have been discussed in section 2.5.1.
The inactivation of enzymes and the killing of mi-
croorganisms can be depicted as a reaction of 1st
order:

ct=c 0 e−kt (2.80)

with c 0 and ct=concentrations (activities, germ
counts) at times 0 and t, and k=rate constant
for the reaction. For ctand t follows from equa-
tion 2.80:

logct=−

k

2 , 3

·t+logc 0 (2.81)

t=

2 , 3

k

log

c 0

ct

(2.82)

c 0 /ct=10 gives:

t=

2 , 3

k

=D (2.83)

The co-called “D-value” represents the time

needed to reduce the initial concentration

(activity, germ count) by one power of ten.

It refers to a certain temperature which has

to be stated in each case. For example:

Bacillus cereus D 121 ◦C = 2 .3s, Clostridium

botulinumD 121 ◦C= 12 .25 s. For a heat treatment

process, the D-value allows the easy determin-

ation of the holding time required to reduce

the germ count to a certain level. If the germ

count ofB. cereusorCl. botulinumin a certain

food should be reduced by seven powers of ten,

the required holding times are 2. 3 × 7 = 16 .1s

and 12. 25 × 7 = 85 .8s.

2.5.4.2 TemperatureDependenceofEffects........................

A relationship exists for the dependence of re-

actionrateontemperature.Itisexpressedbyan

equation ofArrhenius:

k=A·e−Ea/RT (2.84)

with k= rate constant for the reaction rate,

Ea=activation energy, R=general gas constant

and A=Arrhenius factor. For the relationship

between k and T, theArrheniusequation is only

an approximation. According to the theory of

the transition state (cf. 2.2.1), A is transferred

via the active state A#into P. A and A#are in

equilibrium.

(2.85)