Food Biochemistry and Food Processing (2 edition)

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