2009_Book_FoodChemistry

2Enzymes
2.1 Foreword
Enzymes are proteins with powerful catalytic
activity. They are synthesized by biological
cells and in all organisms, they are involved in
chemical reactions related to metabolism. Therefore,
enzyme-catalyzed reactions also proceed
in many foods and thus enhance or deteriorate
food quality. Relevant to this phenomenon
are the ripening of fruits and vegetables, the
aging of meat and dairy products, and the
processing steps involved in the making of
dough from wheat or rye flours and the production
of alcoholic beverages by fermentation
technology.
Enzyme inactivation or changes in the distribution
patterns of enzymes in subcellular particles
of a tissue can occur during storage or thermal
treatment of food. Since such changes are readily
detected by analytical means, enzymes often
serve as suitable indicators for revealing such
treatment of food. Examples are the detection of
pasteurization of milk, beer or honey, and differentiation
between fresh and deep frozen meat or
fish.
Enzyme properties are of interest to the food
chemist since enzymes are available in increasing
numbers for enzymatic food analysis or
for utilization in industrial food processing.
Examples of both aspects of their use are provided
in this chapter in section 2.6.4 on food
analysis and in section 2.7, which covers food
processing.
Details of enzymes which play a role in food science
are restricted in this chapter to only those
enzyme properties which are able to provide an
insight into the build-up or functionality of enzymes
or can contribute to the understanding of
enzyme utilization in food analysis or food processing
and storage.
2.2 General Remarks, Isolation
and Nomenclature
2.2.1 Catalysis
Let us consider the catalysis of an exergonic reaction:
A k 1
⇋
k −1
P (2.1)
with a most frequently occurring case in which
the reaction does not proceed spontaneously. Reactant
A is metastable, since the activation energy,
E A , required to reach the activated transition
state in which chemical bonds are formed or
cleaved in order to yield product P, is exceptionally
high (Fig. 2.1).
The reaction is accelerated by the addition of
a suitable catalyst. It transforms reactant A into
intermediary products (EA and EP in Fig. 2.1),
the transition states of which are at a lower energy
level than the transition state of a noncatalyzed
reaction (A≠ in Fig. 2.1). The molecules of the
Fig. 2.1. Energy profile of an exergonic reaction
A → P; — without and ---withcatalystE
H.-D. Belitz · W. Grosch · P. Schieberle, Food Chemistry 93
© Springer 2009