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

186 Part 2: Biotechnology and Enzymology

for desugaring eggs and removing head space O 2. Peroxidases

may also be used to remove phenolic pollutants from industrial

waste waters. However, some other effects of peroxidases in

foods are undesirable, such as browning in fruits and off flavors

in vegetables.

Xanthine Oxidase

XO is a molybdo-flavoprotein enzyme that is found largely in

the milk and liver of mammals, and it catalyzes the oxidation

of hypoxanthine and xanthine (breakdown products of nucleic

acids) to uric acid.

Hypoxanthine+H 2 O+^1 / 2 O 2 ⇔Xanthine+^1 / 2 H 2 O 2 (1)

Xanthine+H 2 O+^1 / 2 O 2 ⇔Uric acid+^1 / 2 H 2 O 2 (2)

It is variously suggested that XO is involved in the spontaneous

development of oxidized flavors in raw milks. This defect caused

by XO in raw milk may be prevented by treatment with the

digestive enzyme trypsin (Lim and Shipe 1972).

Ascorbic Acid Oxidase

Ascorbic acid oxidase (or more precisely,l-ascorbic acid oxi-

dase) catalyzes the oxidation ofl-ascorbic acid (vitamin C) to

dehydroascorbic acid.

2 L-ascorbate+O 2 ⇔2 Dehydroascorbate+2H 2 O

The above reaction is important as the presence of the enzyme

can lead to the destruction of vitamin C in fruits and vegetables.

The same reaction serves as the basis for its use for the enzymatic

determination of vitamin C (in fruit juices) and for eliminating

the interferences due to the vitamin in clinical analysis. Sources

of ascorbic acid oxidase include squash (plant source) and the

bacteriumAerobacter aerogenes.

Lactate Dehydrogenase

LDH occurs in plants, animals, and microorganisms (e.g., lactic

acid bacteria such asLactococcus,Enterococcus,Streptococcus,

Pediococcus). LDH catalyzes the interconversions between lac-

tate and pyruvate with the participation of nicotinamide adenine

dinucleotide (NAD+) as hydrogen acceptor (i.e., from lactate

to pyruvate) or NADH as hydrogen donor (i.e., in the reverse

reaction from pyruvate to lactate).

Lactate+NAD+⇔Pyruvate+NADH+H+

Lactate formed by the enzymes is an essential component of

dairy products such as sour milk products, yogurt, ititu, kefir,

and cottage cheese. The formation of lactate lowers the pH

and promotes curdling of casein in fermented milks; it also

contributes to the sour flavor of sourdough breads and is used in

beer brewing to lower the pH and add “body” to beer.

In general, the breakdown of body tissues is accompanied by

an increase in the levels of LDH, thus LDH levels are used as

part of the diagnosis for hemolysis and other health disorders,

including cancer, meningitis, acute pancreatitis, and HIV.

Sulfhydryl Oxidase

SO enzymes catalyze the oxidation of thiol groups into disulfides

as follows:

2RSH+O 2 →RS−SR+H 2 O 2

The enzymes have been described in animal milk (Burgess and

Shaw 1983) and egg white (Hoober et al. 1996, 1999, Thorpe

et al. 2002), and in microorganisms (de la Motte and Wagner

1987). SO enzymes from bovine milk, pancreas, and kidney

have been reported to be Fe dependent (Schmelzer et al. 1982,

Sliwkowski et al. 1984, Clare et al. 1988), while others from the

small intestine and the skin have been shown to contain Cu++

ions (Yamada 1989).

The enzyme has been used in the food industry for flavor con-

trol in ultra-high temperature (UHT)-processed milk (Kaufman

and Fennema 1987).

Transferases

The transferases catalyze the transfer of groups from one sub-

strate molecule (donor molecule) to another (acceptor molecule).

The reaction is typified by the equation:

A–X+B→A+B–X

where A is the donor and B is the acceptor. Examples in-

clude TGases, transglycosidases, transacetylases, transmethy-

lases, and transphosphorylases. Some of them like TGases,

and various glucanotransferases (e.g., fructosyl transferase, cy-

clodextrin glycosyl transferase (CGTase), and amylomaltase),

are used extensively in the food industry.

Fructosyl Transferase

Fructosyl transferases catalyze the synthesis of fructose

oligomers, also known as fructose oligosaccharides (FOS). The

enzymes have been found in plants, e.g., onions (Robert and

Darbyshire 1980) and asparagus (Shiomi et al. 1979), and in

microorganisms, such asStreptococcus mutans(Wenham et al.

1979),Fusarium oxysporum(Gupta and Bhatia 1980, 1982),B.

subtilis(Cheetham et al. 1989, Homann and Seibel 2009),As-

pergillusspp,Penicilliumspp, andAureobasidiumspp (Prapulla

et al. 2000).

Fructo-oligosaccharides account for about 30% of the sweet-

ness of sucrose, but are non-digestible in the gastrointestinal

tracts of humans (Oku et al. 1984, Yun et al. 1995), thus they

serve as dietary fiber and render the gut milieu more conducive

for the growth and proliferation of beneficial intestinal mi-

croflora. So, they are applied as low calorie sweeteners and

as functional food ingredients with health benefits (Lee et al.

1992). FOS are also used as sugar substitutes in foods such as

light jam products, ice cream, and confectionery. The use of FOS

in these foods helps to reduce the caloric intake and contributes

to a lowering of blood glucose levels and related health issues

with diabetes. The enzyme fromB. subtilishas been used to

synthesize fructose disaccharides such as xylsucrose, galactosu-

crose, and 6-deoxysucrose for use as antigens, in glycosylated