Food Chemistry

2.6 Enzymatic Analysis 137

to 50%. However, very high pressures in-
creased the activity at 32–60◦C. It is possible
that high pressure denatures peroxidase to
a heme(in) catalyst (cf. 3.7.2.1.7).
−Lipoxygenase from soybeans (cf. 3.7.2.2).
This enzyme was inactivated in 5 min at
pH 8.3bypressuresupto750MPaandtem-
peratures in the range 0–75◦C. The pressure
stability was reduced by gassing with CO 2
and reducing the pH to 5.4.
−Polyphenol oxidases (cf. 0.3.3) in mushrooms
and potatoes require pressures of 800–
900 MPa for inactivation. The addition of
glutathione (5 mmol/l) increases the pressure
sensitivity of the mushroom enzyme. In this
case, the inactivation is obviously supported
by the reduction of disulfide bonds.

2.5.6 Influence of Water

Up to a certain extent, enzymes need to be hy-
drated in order to develop activity. Hydration of
e. g. lysozyme was determined by IR and NMR
spectroscopy. As can be seen in Table 2.15, first
the charged polar groups of the side chains hy-
drate, followed by the uncharged ones. Enzymatic
activity starts at a water content of 0.2g/gpro-
tein, which means even before a monomolec-
ular layer of the polar groups with water has
taken place. Increase in hydration resulting in

Fig. 2.41.Pressure–temperature diagram for the inac-
tivation kinetics ofα-amylase fromBacillus subtilisat
pH 8.6 (according toLudikhuyzeet al., 1997). Range of
the rate constants: k= 0 .01 min−^1 (lower line) to k=
0 .07 min−^1 (upper line)

Table 2.15.Hydration of Lysozyme

a monomolecular layer of the whole available en-

zyme surface at 0.4g/g protein raises the activ-

ity to a limiting value reached at a water content

of 0.9g/g protein. Here the diffusion of the sub-

strate to the enzyme’s active site seems to be com-

pletely guaranteed.

For preservation of food it is mandatory to inhibit

enzymatic activity completely if the storage tem-

perature is below the phase transition temperature

Tgor T′g(cf. 0.3.3). With help of a model sys-

tem containing glucose oxidase, glucose and wa-

ter as well as sucrose and maltodextrin (10 DE)

for adjustment of T′gvalues in the range of− 9. 5

to− 32 ◦C, it was found that glucose was enzy-

matically oxidized only in such samples that were

stored for two months above the T′gvalue and not

in those kept at storage temperatures below T′g.

2.6 EnzymaticAnalysis

Enzymatic food analysis involves the determinat-

ion of food constituents, which can be both sub-

strates or inhibitors of enzymes, and the determin-

ation of enzyme activity in food.