Dairy Chemistry And Biochemistry

342 DAIRY CHEMISTRY AND BIOCHEMISTRY

8.3.9 Exogenous enzymes in food analysis

Exogenous enzymes have several applications in food analysis (Whitaker,
1991). One of the principal attractions of enzymes as analytical reagents is
their specificity, which eliminates the need for extensive clean-up of the
sample and makes it possible to quantify separately closely related mol-
ecules, e.g. D- and L-glucose, D-and L-lactic acid, which are difficult to
quantify by chemical or physical methods. Enzymatic assays can be very
sensitive; some can detect concentrations at the picomole level. Enzymes can
be immobilized as enzyme electrodes and as such can be used continuously
to monitor changes in the concentration of a substrate in a product stream.
Disadvantages of enzymes as analytical reagents are their relatively high
cost, especially when few samples are to be analysed, relatively poor stability
(due to denaturation or inhibition) and the need to use highly purified
enzymes.
Enzymes are rarely used by industrial food laboratories but find regular
application in more specialized analytical or research laboratories. Impor-
tant applications are summarized in Table 8.5 (see Boehringer Mannheim
(1986) for methods). There are alternative chemical and/or physical
methods, especially some form of chromatography, for all these applications,
but extensive clean-up and perhaps concentration may be required.
The use of luciferase to quantify ATP (Blum and Coulet, 1994) in milk is
the principle of modern rapid methods for assessing the bacteriological
quality of milk based on the production of ATP by bacteria. Such methods
have been automated and mechanized.

Table 8.5 Some examples of compounds in milk that can be analysed by enzymatic assays

Substrate Enzyme

D-Glucose Glucose oxidase; glucokinase; hexokinase
Galactose Galactose dehydrogenase
Fructose Fructose dehydrogenase
Lactose
Lactulose
D- and L-Lactic acid
Citric acid Citrate dehydrogenase
Acetic acid
Ethanol Alcohol dehydrogenase
Glycerol Glycerol kinase
Fatty acids
Amino acids Decarboxylases; dearninases
Metal ions

ATP Luciferase
Pesticides (inhibitors) Hexokinase; choline esterase
Inorganic phosphate Phosphorylase a
Nitrate Nitrate reductase

b-Galactosidase, then analyse for glucose or galactose

b-Galactosidase, then analyse for fructose or galactose

D- and L-Lactate dehydrogenase

Acetate kinase + pyruvate kinase + lactate dehydrogenase

Acyl-CoA synthetase + Acyl-CoA oxidase

Choline esterase; luciferase; invertase

(inhibitors or activators)