Food Biochemistry and Food Processing (2 edition)

(Steven Felgate) #1

BLBS102-c09 BLBS102-Simpson March 21, 2012 11:15 Trim: 276mm X 219mm Printer Name: Yet to Come


9 Enzymes in Food Processing 185

of large molecules (e.g., proteins, carbohydrates, lipids, and
nucleic acids) into smaller ones (e.g., amino acids and peptides;
mono-, di-, and oligosaccharides; free fatty acids, mono-, and di-
glycerides; nucleotides and purines). Examples of the hydrolases
include the proteases, carbohydrases, lipases, and nucleases. The
isomerases potentiate intramolecular rearrangements in their
substrate molecules. An example of isomerase used in food pro-
cessing is xylose isomerase (also known as glucose isomerase).

Oxidoreductases

Oxidoreductases are the group of enzymes that catalyze
oxidation–reduction reactions in their substrates. They occur
widely in plants, animals, and microorganisms. In the food in-
dustry, the oxidoreductases of economic importance include
GOX, PPO, peroxidase and catalase, lipoxygenase, xanthine
oxidase (XO), ascorbic acid oxidase, and sulfhydryl oxidase
(SO). Other well-known oxidoreductases are alcohol dehydro-
genase, aldehyde dehydrogenase, lactate dehydrogenase (LDH),
glutathione dehydrogenase, laccases, and lactoperoxidase.

Glucose Oxidase

GOX is produced commercially from the fungiAspergillus
niger,Penicillium chrysogenum(formerly known asP. notatum),
P. Amagaskinese,andP. vitale. GOX catalyzes the oxidation of
β-d-glucose initially tod-glucono-δ-lactone and hydrogen per-
oxide (H 2 O 2 ); the gluconolactone is subsequently hydrolyzed to
d-gluconic acid. In the food industry, GOX is used to improve
bread dough texture (in place of oxidants like ascorbate or bro-
mate), and the H 2 O 2 formed as coproduct in the oxidation of
d-glucose facilitates formation of a network of disulfide bonds
through the oxidation of thiol groups in proteins and cysteine
to result in stronger doughs. GOX is used in the food industry
to desugar egg white and prevent Maillard-type browning reac-
tions, and is also used in combination with catalase to eliminate
O 2 from the head spaces of beverages (e.g., wines) and dried
food packages to curtail enzymatic browning reactions and off-
flavor development.

Polyphenol Oxidase

The term PPO is used here to encompass all those enzymes
that have variously been referred to as catecholase, cresolase,
diphenolase, laccase, phenolase, polyphenolase, or tyrosinase.
PPO is widespread in nature and occurs in plants, animals, and
microorganisms, and is produced on a large scale from mush-
rooms and by fermentation from fungi such asAlternaria te-
nius,Chaetomium thermophile,andThermomyces lanuginosus,
and from bacteria, particularly from the genusBacillussuch as
B. licheniformis,B. Natto,andB. sphaericus.TheyareCu++-
containing enzymes and enhance enzymatic browning, whereby
phenolic compounds are hydroxylated and/or oxidized via in-
termediates such as quinones that polymerize into dark-colored
pigments known as melanins. These browning reactions are re-
sponsible for the undesirable darks discoloration in bruised or
freshly cut fruits (e.g., apples, avocadoes, and bananas) and veg-

etables (e.g., lettuce and potatoes), as well as in melanosis or
“blackspot” formation in raw crustacea (e.g., shrimps, lobsters,
crabs). Nonetheless, PPO-induced browning is desirable in other
products such as chocolate, cocoa, coffee, prunes, raisins, tea,
and tobacco.
In the food industry, PPO is used to enhance the color and
flavor in coffee, tea, and cocoa. PPO is also put to nonfood uses
in the pharmaceutical and biomedical industries: as components
of biosensors for distinguishing between codeine and morphine;
for the treatment of Parkinson’s disease, phenylketonuria, and
leukemia; and in wastewater treatment for the removal of phe-
nolic pollutants.

Lipoxygenase

LOX, also known as lipoxidases, are non-heme, iron-containing
enzymes that are found in plants, animals, and microorganisms.
LOX is obtained for commercial use from soy flour. The enzyme
causes oxidization of essential unsaturated fatty acids, e.g., oleic,
linoleic, linolenic, arachidonic, and other polyunsaturated fatty
acids such as eicosapentaenoic acid and docosahexaenoic acid,
as well as carotenoids, to form their breakdown products (e.g.,
hydrocarbons, alcohols, carbonyl compound, and epoxides). As
a result of the activities of LOX, off flavors develop in foodstuffs
(oxidative rancidity); as well, there is formation of free radical
intermediates that cause destruction of biomolecules and essen-
tial food components (e.g., proteins, astaxanthin,β-carotene,
and vitamin A).
In general, the action of LOX in foods is undesirable, partic-
ularly the generation of harmful free radicals, the destruction of
essential biomolecules like carotenoids and other essential food
components that manifest in effects such as gradual color loss
in salmonids during frozen storage, and off flavor development
in fatty foods. However, LOX is put to some beneficial uses in
bread making for the bleaching of flour and for improving the
viscoelastic properties of dough. In plants, LOX participate
in flavor formation through the formation of the aldehyde 2-
hexenal. Some of the aldehyde thus formed may be oxidized
to the corresponding carboxylic acid by the enzyme aldehyde
dehydrogenase or reduced by alcohol dehydrogenase to the cor-
responding alcohol. Esters form from the carboxylic acids and
alcohols to impart flavors to the plant material.

Peroxidase and Catalase

These are heme iron-containing enzymes that occur naturally
in plants, animals, and microorganisms. They use H 2 O 2 as co-
reactant to catalyze the oxidation of other peroxides to form
H 2 O and oxidized products, such as alcohols. Examples of per-
oxidases are horseradish peroxidase, glutathione peroxidase, cy-
tochrome c peroxidase, and catalase. Catalase is usually men-
tioned separately from the rest of the peroxidases as it displays
absolute specificity for H 2 O 2. In the food industry, thermal inac-
tivation of peroxidases is used as a measure of the efficiency of
blanching treatment. Catalase is used for the removal of H 2 O 2
from food products (e.g., to remove residual H 2 O 2 from cold
pasteurization of milk); it is also used in conjunction with GOX
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