BLBS102-c09 BLBS102-Simpson March 21, 2012 11:15 Trim: 276mm X 219mm Printer Name: Yet to Come
182 Part 2: Biotechnology and Enzymologyfoods can be stopped relatively easily after the desired transforma-
tion is attained. The chapter is organized in six sections and covers
the major groups and sources of food enzymes, their properties and
modes of action, the rationale for their use in food, their use in
the manufacture of various foods, and the different strategies em-
ployed to control their undesirable effects in foods. The chapter also
provides information on the future prospects of enzymes.INTRODUCTION
Enzymes are biological molecules that enable biological reac-
tions proceed at perceptible rates in living organisms (plants, an-
imals, and microorganisms) and the products derived from these
sources. Living organisms produce basically the same functional
classes of enzymes to enable them carry out similar metabolic
processes in their cells and tissues. Thus, it is to be expected that
foodstuffs would also possess endogenous enzymes to catalyze
biological reactions that occur in them both pre- and posthar-
vest. The molecules that enzymes act upon are called substrates,
while the resulting compounds from the enzymatic conversions
are called products. Enzymes are able to speed up biological
reactions by lowering the free energy of activation (G*) of the
reaction (van Oort 2010). Unlike chemical catalysts, enzymes
are much more specific and active under mild reaction conditions
of pH, temperature, and ionic strength.
Humans have intentionally or unintentionally used enzymes
to modify foodstuffs to alter their functional properties, extend
their storage life, improve flavors, and provide variety and de-
light, and the use of enzymes in food processing has been in-
creasing for reasons such as consumer preferences for their use
in food modification instead of chemical treatments, their capac-
ity to retain high nutritive value of foods, and the advances in
biotechnology that are enabling the discoveries of new enzymes
that are much more efficient in transforming foods. The recent
advances in molecular biotechnology that are permitting the dis-
coveries of new and better food enzymes augur well both for the
need and capacity for food technologists and food manufacturers
to produce food products in highly nutritious, safe, and stable
forms as part of the overall strategy to achieve food security.
However, not all reactions catalyzed by enzymes in foods
are useful; naturally present enzymes in fresh foods and their
counterparts that survive food-processing operations can induce
undesirable changes in foods, some of which may even be toxic.
Thus, there is the need to also develop novel and more effective
strategies to control the deleterious effects of enzymes in foods
to reduce postharvest food losses and/or spoilage.Some Terms and DefinitionsSeveral useful terms are encountered in the study of enzymes.
The active enzyme molecule may comprise a protein part exclu-
sively, or the enzyme protein may require as essential nonpro-
tein part for its functional activity. The essential nonprotein part
that some enzymes require for functional activity is known as a
cofactor or prosthetic group. For those enzymes requiring cofac-
tors for activity, the enzyme–cofactor complex is known as theholoenzyme, and the protein part that has no functional activity
without the cofactor is known as apoenzyme. Thus, cofactors
or prosthetic groups may be regarded as “helper molecules”
for the apoenzymes. Cofactors may be inorganic (e.g., metal
ions like Zn, Mg, Mn, and Cu), or organic (e.g., riboflavin,
thiamine, or folic acid) materials. Examples of food enzymes
that do not require cofactors or prosthetic groups for activity
include lysozyme, pepsin, trypsin, and chymotrypsin; examples
of those enzymes that require cofactors or prosthetic groups
for functional activity include carboxypeptidase, carbonic an-
hydrase and alcohol dehydrogenase (all have Zn^2 +as cofactor),
cytochrome oxidase (has Cu^2 +as cofactor), glutathione peroxi-
dase (which has Se cofactor), catalase (with either Fe^2 +or Fe^3 +
as cofactor), and pyruvate carboxylase (that has thiamine in the
form of thiamin pyrophosphate or TPP as cofactor).
Enzymes have a catalytic region known as the active site where
substrate molecules (S) bind prior to their transformation into
products (P). The active site is a very small region of the very
large enzyme molecule, and the transformation of substrates to
products may be measured by either the rate of disappearance of
the substrate (−δS/δt) or the rate of appearance of the products
(δP/δt) in the reaction mixture. The activity of the enzyme is usu-
ally expressed in units, where a unit of activity may be defined as
the amount of enzyme required to convert 1μmol of the substrate
per unit time under specified conditions (e.g., temperature, pH,
ionic strength, and/or substrate concentration). Enzyme activity
may also be expressed in terms of specific activity, which may
be defined as the number of enzyme units per unit amount (e.g.,
milligram) of enzyme; or it may be denoted by the molecular ac-
tivity or turnover number, defined as the number of enzyme units
per mol of the enzyme at optimal substrate concentration. The ef-
ficiency of an enzyme (catalytic efficiency) may be derived from
knowledge of the binding capacity of the enzyme for the sub-
strate (Km′, which is the apparent Michaelis–Menten constant)
and the subsequent transformation of the substrate into products
(KcatorVmax). The catalytic efficiency of an enzyme is defined
as the ratio of the maximum velocity (Vmax) of the transforma-
tion of the substrate(s) into product(s) to the binding capacity of
the enzyme (or the apparent Michaelis–Menten constant,Km′,
i.e.,Vmax/K′morKcat/K′m). The catalytic efficiency is a useful
parameter in selecting more efficient enzymes from a group of
homologous enzymes for carrying out particular operations.Rationale for Interest in Food EnzymesBeneficial EffectsEnzymes have several advantages for food use compared to con-
ventional chemical catalysts. They are relatively more selective
and specific in their choice and action on substrates, thus obviat-
ing side reactions that could lead to the formation of undesirable
coproducts in the finished products. They have higher efficiency
and can conduct reactions several times faster than other cata-
lysts. They are active in low concentrations and perform well
under relatively mild reaction conditions (e.g., temperature and
pH); thus, their use in food processing helps to preserve the
integrity of heat-labile essential nutrients. They can also be