Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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transport fluid containing high levels of dissolved

solids, starch or sugar compounds.

0023 The ideal geometry of a fluid disinfection system

for maximum utilization of the UV energy is a single

lamp running along the axis of a cylindrical treatment

chamber.

Practical Applications of UV

0024 Microbiological contamination can occur at any

stage of a production process from the incoming

process water to the surfaces of packaging. In many

cases, UV disinfection can offer a safe and effective

method of providing microbe-free process water,

fluids and air. It is effective against food pathogens,

including viruses and fungal spores (see Table 2).

0025 Unlike chemical biocides, UV does not introduce

toxins or residues into process water and does not

alter the chemical composition, taste, odor, or pH of

the fluid being disinfected. This feature is especially

important in food and drink processing plants, where

the chemical dosing of incoming process water can

cause off-flavors and alter the chemical properties of

the product.

0026 Ultraviolet treatment systems can be used as the

primary disinfection system or as a back-up for

other water purification methods, such as activated

carbon filtration, reverse osmosis or pasteurization.

As UV has no residual effect, the best position for a

treatment system is immediately prior to the point of

use. This ensures that incoming microbiological

contaminants are deactivated and there is minimum

chance of posttreatment contamination. Many users

install UV systems after filter beds and storage tank

outlet valves, to reduce the likelihood of contamin-

ation from these sources.

0027 Ultraviolet systems can also be installed as an

alternative to, or in conjunction with, air filtration

systems. The UV source will deactivate any micro-

organisms present and prevent colonization of air-

filtration systems.

Clean-in-place (CIP) Rinse Systems

0028It is essential that CIP final rinse water, which is used

to flush out foreign matter and disinfecting solutions,

is microbiologically safe. Fully automated UV dis-

infection systems can be integrated with CIP rinse

cycles to insure that final rinse water does not

reintroduce microbiological contaminants. Although

most town supplies are free from coliforms, they are

rarely sterile. Resistant to the effects of acid, deter-

gents, steam and chemical sterilants, UV systems are

an effective method of providing disinfected water for

final rinse systems.

Sugar Syrups

0029Specially modified UV treatment chambers are rou-

tinely used in the brewing and soft drinks industry to

disinfect concentrated sugar solutions and syrup. Al-

though high-Brix syrups will not support microbial

growth, any dormant spores present may become

active after the syrup has been diluted. Treating the

syrup and dilution water with UV prior to use will

insure that any dormant microorganisms are deacti-

vated.

0030Systems for syrup treatment are designed with a

chamber of relatively small diameter. This helps to

insure that sufficient UV light penetrates the liquid

and deactivates spores, which generally require higher

doses of UV than other microorganisms for complete

deactivation. In food and dairy processing plants, the

same equipment can be used to disinfect injection

brine and cheese whey.

Air Disinfection

0031For machine blanketing, and in sanitary and food

preparation areas, the exclusion of airborne contam-

inants, such as bacterial spores, yeasts and viruses, can

be a major problem. Ultraviolet disinfection systems

are now available for treating airflows entering a

sterile environment. Air-disinfection systems are fitted

into ductwork, and any microorganisms present are

deactivated as they are exposed to the UV source. (See

Plant Design: Designing for Hygienic Operation.)

0032Ultraviolet systems can also be used to disinfect

displacement air for pressurizing tanks, or pipelines

holding perishable fluids or free-flowing solids. Stor-

age tanks are particularly susceptible to bacterial col-

onization and contamination by airborne spores. To

prevent this, immersible UV treatment systems have

been designed to fit in the tank head airspace and

disinfect the air present.

tbl0002 Table 2 Experimentally determined UV dose, at 254 nm,

required for a 90% kill rate (D

) of various organisms

Organism D

(mJ cm

2

)

Bacteria

Escherichia coli 5.4

Bacillus subtilis 7.1

Clostridium botulinum 12.0

Micrococcus candidus 6.05

Pseudomonas aeruginosa 5.5

Mold spores

Aspergillus niger 100

Cladosporium herbarum 30–70

Mucor mucedo 50–70

Penicillium roqueforti 13

Yeasts

Saccharomycescerevisiae 6.0

5888 ULTRAVIOLET LIGHT

Packaging Applications

0033 Another aspect of contamination control that can

benefit from UV technology is the disinfection of fin-

ished products and their packaging. A typical system

incorporates a UV light source, shielded on three sides

and mounted over a conveyor or production line. As

foodstuffs or packaging pass under the light, any

microbes present on the surface are deactivated.

0034 After treatment by UV, the risk of microbial con-

tamination in filling and packaging lines is reduced,

and the shelf-life of treated products is extended. This

surface sterilization equipment can also be used to

disinfect bottle crowns, foil lids and plastic films.

(See Packaging: Packaging of Solids.)

Recent Advances in UV

0035 Modern UV treatment systems are microprocessor-

controlled with automatic UV intensity and lamp

status monitors. This facilitates the interfacing of

UV systems with other process control systems to

coordinate their operation.

Integral Wiping Systems

0036 The development of integral wiping systems has in-

creased the applications of UV to include treatment of

turbid fluids containing processing residues or high

solids concentrations. The wiping system is fitted to

the outside of the quartz sleeve. At preset intervals

during operation, a mechanical wiper moves along

the lamp surface, displacing any processing residues.

These would quickly foul conventional UV systems,

inhibiting the transmission of UV light and reducing

germicidal effectiveness. Wiped systems can con-

stantly reduce contamination to acceptable levels in

poultry, egg and meat processing, in the transport and

cooling of vegetables, and in the disinfection of

effluent flows. (See Effluents from Food Processing:

Disposal of Waste Water.)

Lamp Output

0037 High-output, single-lamp chambers emit the high

energy levels required to penetrate the turbid waters

that can be found in processing plants. This includes

virtually opaque fluids, such as transport water for

flour- and pasta-based products, and industrial efflu-

ent or recirculation water. Currently, the minimum

single lamp power available is 8 W (low-pressure),

increasing to a maximum of 5 kW (medium-pressure).

Limitations of UV Technology

System Sizing

0038 In theory, UV systems can be built to treat any flow

rate, air volume, or surface area. As UV demand

increases in terms of size, degree of contamination

and quality of water, the number and power output

of the UV lamps are increased proportionally to

maintain UV dose above the required minimum.

0039Obviously, practical constraints, such as the size of

the UV system, pressure drop through the system,

and power consumption, vary with each installation.

Single-chamber UV systems treating up to 250 m

1

are often more than adequate for process-water ap-

plications. Duty stand-by units, programmed to come

on-line when flow exceeds preset levels, can also be

installed as a back-up measure.

0040Ultraviolet systems have been successfully installed

to treat water flows over 1 10

per day. At these

flow rates, medium-pressure systems have the advan-

tage of fewer lamps, compared with low-pressure

systems, and therefore have lower maintenance

requirements.

Residual Disinfection

0041The only potential drawback with UV water disinfec-

tion, compared with chemical treatment, is that UV

does not leave a residual disinfectant effect. In food

and drink manufacturing, this is a positive advantage

as the presence of chlorine could adversely affect

product quality. In industrial applications, the lack

of residual is rarely a problem, provided that down-

stream pipework is maintained in a hygienic condi-

tion, and no ‘dead legs’ or leaks are allowed where

recolonization could occur.

0042Ultraviolet systems are recommended for installa-

tion as close to the point-of-use as possible to reduce

the likelihood of recontamination. It is mainly in the

drinking-water treatment industry, where there may

be miles of distribution pipework following disinfec-

tion, that a small chlorine residual is often added to

the water after UV disinfection. A combination of UV

disinfection and electrochlorination is most common,

both systems requiring little maintenance and able to

operate almost unsupervised.

0043With air and surface disinfection, there is a remote

risk that an operator may be exposed to the UV light

source. Prolonged exposure to high-intensity UV

light an damage the eyes, but this kind of accident is

very rare and can be avoided by carefully designed

shielding around the UV source. In addition, access

to the UV source can be interlocked with the power

supply, ensuring that access can be obtained only

when the power supply has been turned off.

Conclusion

0044The unique ability of UV light to deactivate micro-

organisms in water and air, and on surfaces, without

creating by-products or residual effects, enables

ULTRAVIOLET LIGHT 5889

UV disinfection to be used for a wide variety of appli-

cations. Whether installed as the main disinfection

system, as a back-up to alternative methods, or in

specific clean-room environments where high stand-

ards of sterility must be maintained, UV treatment

systems can ensure that levels of microbial contamin-

ants are effectively controlled.

See also: Effluents from Food Processing: Disposal of

Waste Water; Packaging: Packaging of Solids; Plant

Design: Designing for Hygienic Operation

Further Reading

Anonymous (1988) High UV output disinfects high solids

process water. Prepared Foods 157: 114.

Mans J (1987) Disinfect water with ultraviolet light.

Prepared Foods 156: 72–74.

Maunder DT (1977) Possible use of ultraviolet sterilisation

of containers for aseptic packaging. Food Technology

31: 36–370.

Phillips R (1983) Sources and Applications of Ultraviolet

Radiation. London: Academic Press.

United States Food and Drug Administration See Food and Drug Administration

URONIC ACIDS

F G Huffman, Florida International University, Miami,

FL, USA

Chemistry

0001 Uronic acids are produced by the oxidation of the

alcohol group of monosaccharides. These compounds

are named by substituting -ose with uronic acid. The

structures of the most common uronic acids are

shown in Figure 1. d-Mannuronic acid is the 2-

epimer of d-glucuronic acid, and l-iduronic acid is

the 5-epimer of d-glucuronic acid.

0002 The uronic acids are important constituents of cer-

tain natural heteropolysaccharides. They play a sig-

nificant role in the detoxification of substances such

as drugs. Glucuronic acids are found in human urine

bound with glycosidic linkages to hydroxylated com-

pounds such as menthol, borneol, and estrogens.

Owing to the increased solubility of hydroxylated

compounds (when conjugated with glucuronic acid),

they are readily disposed by the body. Glucuronic

acid also forms a conjugate with bilirubin, a bile

pigment.

0003 The most important uronic acid in humans is the

d-glucuronic acid. It, or its epimer, is a constituent

of many glycosaminoglycans (GAG) as well as glu-

curonide derivatives of drugs and hormones. d-

Glucuronic acid is also the precursor of l-ascorbic

acid in animals. The schematic utilization of d-

glucuronate in microorganisms, animals, and plants

is shown in Figure 2. Glucuronic acid is synthesized

from glucose in the uronic pathway, an alternative

oxidative pathway for glucose without the produc-

tion of adenosine triphosphate (ATP). In the uronic

pathway, glucose 6-phosphate is converted to glu-

cose 1-phosphate which subsequently reacts with

uridine triphosphate to form uridine diphosphate glu-

cose (UDPGlc). This compound is then oxidized at

the six-carbon position in a two-step process by the

COOH

(a) (b)

fig0001Figure 1 Structures of uronic acids: (a) D-glucuronic acid

(Glu U);(b)

D-mannuronic acid (Man U); (c) D-galaturonic acid

(Gal U); (d)

L-iduronic acid (L-guluronic acid; Gul U).

5890 URONIC ACIDS

nicotinamide adenine dinucleotide (NAD)-dependent

enzyme (UDPGlc dehydrogenase) to form UDP-

glucuronate.

0004 UDP glucuronate is the form of glucuronic acid

which can be incorporated into proteoglycans or

conjugated with steroid hormones, certain drugs, or

bilirubin. In bilirubin, two molecules of glucuronic

acid are attached with ester linkages to the two pro-

pionic acid groups to form an acylglucuronide. This

conjugation process increases the water solubility

of bilirubin, thus allowing its secretion into the

bile, ultimately to be excreted via the gastrointestinal

tract.

0005 A second product of the uronic pathway is l-

ascorbic acid, which is produced in mammals, with

the exception of humans, primates, and the guinea-

pig. In a series of reactions glucuronate is reduced to

l-gulonate, which is subsequently converted to l-

ascorbic acid. The glucuronic acid pathway is most

active in the liver, kidneys, and intestines. The pri-

mary function of the pathway is to produce UDP-

glucuronic acid, which is needed for detoxification

of various compounds by elimination of glucuronides

in the urine or bile. Many carcinogens, and drugs,

including antipyretics, hypnotics, and antimalarial

drugs, may be eliminated by this mechanism. There

may be certain organs which may have an active

uronic pathway specific for a drug. Glucuronic acid

synthesis may be stimulated when the consumption

of substances that are excreted as glucuronides is

increased, or when steroids and barbiturates, which

induce the microsomal P-450 system, are consumed.

D-Glucuronate

microorganism

animals

plants

D-Fructuronate D-Glucaric acid

Pyruvate

Triose

phosphate

UDP-glucuronic

acid

Xylulose

L-Gulonate

L-Ascorbic

acid

Pectin,

hemicellulose,

etc.

Glucuronate

1-phosphate

UTP

fig0002 Figure 2 The fate of glucuronate in plants, animals and microorganisms. UTP, uridine triphosphate; UDP, uridine diphosphate.

URONIC ACIDS 5891

Uronic Acids in Animal Tissue

0006 Uronic acids are an integral part of GAGs, formerly

known as the mucopolysaccharides. They have

structural importance in vertebrate animals. Some

important examples of GAGs are hyaluronic acid,

the chondroitin sulfates of connective tissue, the der-

matan sulfates of skin, and heparin (Table 1 and

Figure 3).

Hyaluronic acid

0007 Hyaluronic acid functions in the body as an agent

which increases the viscosity of body fluids and acts

as a lubricant. It is present in the joints and in the

vitreous humor of the eye. Hyaluronic acid is

composed of equal molecules of d-glucuronic acid

and 2-acetamido-2-deoxy-d-glucose, which alternate

in the heteropolysaccharide. The linkages from the

amino sugar to the acid are b-1,4; those from

the acid to the amino sugar are b-1,3. Hyaluronic

acid is also present in skin, aorta, heart valve fibro-

blast, and the umbilical cord.

Chondroitin Sulfates

0008 Chondroitin sulfates are the major constituents of

hyaline cartilage. They are located in cartilage and

also at the calcification site of the bone. The disac-

charide unit of the chondroitin sulfates contains glu-

curonic acid and N-acetylgalactosamine. Glucuronic

acids are connected by b-1,3 linkages. The two major

chondroitin sulfates, A and C, differ from one

another in the position of the sulfates. Chondroitin

sulfate A is sulfated at position 4, and chon-

droitin sulfate C is sulfated at position 6. Thus the

names chondroitin 4-sulfate and chondroitin 6-sul-

fate are used for these two polysaccharides.

0009 The number of repeating uronic acids in the chon-

droitin chain varies depending on the source. The

number may even differ within the same tissue. The

ratio of chondroitin 6-sulfate to chondroitin 4-sulfate

units increases progressively with age. In humans, a

plateau is reached around the age of 50.

Dermatan Sulfate

0010 Dermatan sulfate contains l-iduronic acid as the

major uronic acid. Glucuronic acid is also present in

smaller amounts. Dermatan sulfate is found in the

cornea and the sclera of the eye, which helps to main-

tain corneal transparency and the shape of the eye.

Many other tissues in animals and humans also con-

tain dermatan, such as blood vessel walls, heart valve,

and the umbilical cord. Dermatan sulfates may occur

at the C4 or C6 positions of l-iduronic acid. They

are found to increase normally with age, except in

diseased conditions.

Heparin

0011Heparin is mainly stored intracellularly in the gran-

ules of the mast cells. It may be released in response to

a specific stimulus. The main uronic acid of heparin

is the l-iduronic acid, although d-glucuronic acid is

also present. Heparin is a well-known anticoagulant

that binds to factors IX and XI but, most importantly,

it interacts with plasma antithrombin III.

Heparan

0012Heparan sulfate differs from heparin in many ways.

The dominant uronic acid in heparan sulfate is d-

glucuronic acid. The degree of sulfation is reduced

in heparan sulfate because there are fewer l-iduronic

acids. Heparan sulfates are located in the extracellu-

lar medium of mast cells. They differ in their physio-

logical functions in that they may become receptors

and participate in cell growth and communication.

Alterations in Glycosaminoglycans Metabolism

0013The preceding discussion shows that GAG has spe-

cific functions in vertebrate animals. Recent research

in this field suggests that certain disease conditions

may alter the functions, synthesis, and composition of

GAGs. For example, increased chondroitin sulfate

and uronic acids and decreased dermatan were ob-

served in the breast biopsies of women with carcin-

oma, when compared with controls. Some tumor cells

have less heparan sulfate, which may reduce their

adhesiveness. Hyaluronic acid may facilitate tumor

cell migration through the extracellular matrix and

tumor cells are capable of synthesizing increased

amounts of GAG. Samples of degenerated cartilage

from human patellas had increased dermatan sulfate

and hyaluronate and decreased chondroitin 6-sulfate

and uronic acid content. Dermatan sulfate binds to

low-density lipoproteins (LDL) in plasma and is be-

lieved to play a significant role in plaque development

in arteriosclerosis. Significantly increased urinary ex-

cretion of glycosamines was reported in subjects with

hypothyroidism. The levels of heparan sulfate and

chondroitin sulfate were higher in subjects with

hypothyroidism than the controls, indicating alter-

ation in the metabolism of connective tissue in

tbl0001 Table 1 Composition of glycosaminoglycans

Glycosaminoglycan Uronic acid composition

Hyaluronic acid b-glucuronic acid

Chondroitin 4-sulfate b-glucuronic acid

Heparin Sulfated iduronic acid

Dermatan

L-iduronic acid

5892 URONIC ACIDS

hypothyroidism. Specific reactions responsible for

these changes need to be studied.

0014 In different types of arthritis proteins of GAG may

act as autoantigens, causing additional symptoms of

the disease. Recent research indicates that anti-DNA

antibodies cross-reacting with GAG are present in

patients with lupus erythematosus. Increased produc-

tion of hyaluronic acid was observed with increased

severity of the disease in patients with autoimmune

thyroid disease. It appears that GAG not only plays a

role in the pathogenesis of autoimmune diseases but

also may be used as an activity marker of the disease.

0015Increases in hyaluronic acid and keratan and

decreases in chondroitin sulfate may be contrib-

uting factors in the development of osteoarthritis. A

recent surge in the use of chondroitin sulfate and

COO

−

OH H

H HN CO CH

COO

−

OH H

HOH

COSO

COO

−

OH H

H NH SO

OSO

OH H

−

HOCH

H HN CO CH

β-Glucuronic acid N-Acetylgalactosamine sulfate

β-Glucuronic acid N-Acetylglucosamine

Sulfated

lucosamine Sulfated iduronic acid

Chondroitin 4-sulfate

Hyaluronic acid

Heparin

−

fig0003 Figure 3 Structures of glycosaminoglycans.

URONIC ACIDS 5893

glucosamine by sufferers of osteoarthritis is currently

being investigated.

0016 Heparan and dermatan sulfates are found to inhibit

calcium oxalate crystallization and thus prevent

kidney stones. Recently a uronic acid-rich protein

has been isolated from the urine of normal and

stone-forming individuals. This protein appears to be

a most efficient inhibitor of calcium oxalate nephro-

lithiasis.

Uronic Acids in Plant Tissue

0017 In plants, uronic acids are associated with the fiber

component of the plant cell. The fibers which have

significance in human and animal life are called diet-

ary fibers. Not all dietary fibers contain uronic acids.

0018 Dietary fibers have three major components:

0019 Structural polysaccharides and derivatives, such as

cellulose and cellulose derivatives (microcrystal-

line cellulose, semisynthetic gums, and noncellulo-

sic polysaccharides in land and sea plants).

0020 Structural nonpolysaccharides, such as lignin.

0021 Nonstructural polysaccharides, such as mucilages,

seed gums, plant exudates, and microbial gums.

The uronic acids are found in the noncellulosic, struc-

tural polysaccharides, and derivatives – hemicellu-

lose, pectic substances (land plants), and alginate

(sea plant) – and in the nonstructural polysaccharides

– mucilages, seed gums, plant exudes, and microbial

gums (xanthan). These substances are also referred to

as soluble dietary fiber. The most common uronic

acids in the plants are d-galacturonic and d-glucuro-

nic acids (Table 2). The uronic acid content and

physiochemical properties of dietary fibers are as

follows.

Hemicellulose

0022 Hemicellulose is a branched polymer of pentose and

hexose sugars, found in the plant cell wall. The uronic

acid composition is mainly d-glucuronic acid and 4-

O-methyl-d-glucuronic acid. There are two distinct

hemicelluloses in plants: the acidic and the neutral.

Acidic hemicelluloses contain a larger number of uro-

nic acids than neutral hemicelluloses. Hemicelluloses

are partially fermented by the microorganisms of the

colon, producing some volatile fatty acids. Hemicel-

luloses are insoluble in water but soluble in alkaline

solutions. They, along with other insoluble dietary

fibers, decrease the intestinal transit time; hemicellu-

loses also increase fecal weight and slow down starch

hydrolysis. Acidic hemicelluloses may bind to cations.

These characteristics of hemicellulose may be respon-

sible for its physiological effects.

Pectins

0023Pectins are a complex mixture of polysaccharides

containing d-galacturonic acid as the main constitu-

ent. They are found in the primary cell walls and

intercellular layers in land plants. Citrus fruits,

apples, and pears contain large amounts of pectin.

In the intestinal tract they are almost completely fer-

mented by the microflora. Pectins are water-soluble

and form gels under specific conditions. They are

hydrophilic and have ion-binding and water-holding

capacities. Pectins delay gastric emptying and in-

crease bile acid excretion.

Gums and Mucilages

0024Gums and mucilages are water-soluble, viscous, and

highly fermentable by the microorganisms of the

intestinal tract. The uronic acids that dominate gums

are d-glucuronic and d-galacturonic acids. The most

important gums used in the food industry are gum

arabic, gum karaya, tragacanth, carob, and guar,

which are obtained as exudates from trees or shrubs.

Xanthan gum, which is synthetically produced, con-

tains d-glucuronic acid. Because of their gel-forming,

water-holding ability, gums delay gastric emptying.

Alginate

0025Alginate is a noncell-wall component of seaweed

which contains d-mannuronic and iduronic acids. It

forms gels and is highly fermentable by intestinal

microorganisms. Alginates are used in food process-

ing and they enter into the human food chain as food

additives.

Physiological Effects of Dietary Fiber

0026The physical and chemical properties of dietary fiber

create physiological effects in animals. Because

dietary fiber cannot be hydrolyzed by the intestinal

enzymes it is unavailable for absorption and therefore

continues to exert effects in the gastrointestinal tract.

Soluble dietary fibers have been shown to reduce

plasma levels of glucose and cholesterol. These

physiological effects of soluble dietary fibers are at-

tributed to their ability to form gels and delay gastric

emptying. Viscous fibers bind to bile acids and

tbl0002 Table 2 Uronic acid composition of fiber

Fiber Uronic acid composition

Hemicellulose

D-glucuronic acid and/or

4-methyl

D-glucuronic acid

Pectin

D-galacturonic acid

Gums, mucilages

D-glucuronic acid and/or D-galacturonic acid

Alginate

D-mannuronic acid and/or iduronic acid

5894 URONIC ACIDS

prevent their reabsorption, causing body cholesterol

to be excreted and thus reducing plasma cholesterol.

Second, fiber interferes with digestive enzymes by

sequestering lipids, carbohydrates, and proteins, pre-

venting their absorption. Fiber may also interfere

with micelle formation, mixing of intestinal contents,

and inhibition of cholesterol synthesis.

0027 The viscous fibers are highly fermentable by the

intestinal microorganisms, producing volatile fatty

acids, which, by being absorbed into the portal

blood, suppress hepatic cholesterol synthesis by in-

hibiting human menopausal gonadotropin coenzyme

reductase activity. Some diabetics and hypercholester-

olemics have experienced alleviation of their condi-

tion by increasing intakes of dietary fiber.

0028 Isolated dietary fiber components have been used

to elicit a specific response. However, It has become

clear that fiber exerts its primary effect as a compon-

ent of whole foods rather than as an isolated entity.

0029 The low incidence of colon cancer in populations

consuming high levels of fibrous foods has prompted

scientists to study the effects of fiber on preventing

colon cancer. The roles of specific dietary fiber are

unclear, but diets high in fiber and low in fat are

increasingly recommended.

0030 Research indicates that the effect of fiber is not

always favorable. Reduced bioavailability of certain

vitamins and minerals has been reported. Among the

vitamins studied, availability of vitamins B

,A,

and E was reduced as a result of high fiber intake.

Among the minerals, the absorption of sodium, po-

tassium, magnesium, calcium, zinc, and iron was re-

duced by dietary fiber, especially by the fiber fraction

containing uronic acids. Research to determine the

effect of fiber on nutrient bioavailability continues

with vigor.

Methods of Analysis for Uronic Acids

0031 Since uronic acids are found as integral parts of

animal and plant tissues, they must be separated

from their native materials.

0032 GAGs which contain mammalian uronic acids may

be separated by density gradient centrifugation and

chromatography (ion exchange or gel). Hydrolysis by

acid or by specific enzymes is used. Uronic acids may

be measured either by colorimetry or by decarboxyla-

tion techniques.

0033 There are two primary methods for quantification

of dietary fiber. The gravimetric method measures

total fiber content and uses enzymes or detergents to

solubilize the nonfiber components, such as starch

and protein. Defatting using organic solvents is

usually performed prior to sample analysis. Acid

and base solutions are used to separate acid- or

base-soluble fractions.

0034The fractionation method, developed by Southgate

in 1969, has gone through many modifications over

the years. This method allows measurements of total

dietary fiber as well as the fiber fractions. It is neces-

sary to use the fractionation method for fiber analysis

to be able to free uronic acids for quantification.

Most fiber analyses have three steps:

0035Preparation of an extractive-free residue (alcohol-

insoluble residue)

0036Removal of starch and protein from the residue

(enzyme or detergent hydrolysis)

0037Analysis of destarched, deproteinated residue for

neutral sugars and uronic acids

In recent years highly specific enzyme preparations

have become available, improving recovery of various

fiber fractions. The detergent method of fiber analysis

is simple and fast, but the soluble fiber component,

which contains the uronic acids, is lost during extrac-

tion. The method of choice for the quantitative analy-

sis of uronic acids appears to be the enzymatic

method followed by either colorimetry or decarbox-

ylation.

Quantification of the Uronic Acids

0038Uronic acids may be determined in the hydrolysate

of food samples following enzyme and/or acid

hydrolysis. Dilutions of the hydrolysate to give

25–100 mgml

1

may be necessary and can be achieved

with a mixture of sodium chloride and boric acid.

Diluted samples are heat-treated in the presence of

concentrated sulfuric acid. After cooling to room

temperature, a 3,5-dimethylphenol solution is added

and 10–15 min later the absorbance is read at

400 and 459 nm. Appropriate glucuronic acid stand-

ards are used to develop a standard curve. Differences

in absorbance readings of the sample are plotted on

the standard curve and sample concentrations are

read.

0039The colorimetric methods using carbazole reagent

may also be used in determining uronic acids. The

sample hydrolysate is placed in a test-tube containing

cold acid borate. The tubes are placed in a boiling

water bath, followed by cooling to room tempera-

ture. Carbazole reagent is added and tubes are again

placed in the boiling water bath. After cooling to

room temperature, the intensity of the color is

measured by reading absorbance at 530 nm. Sample

concentrations may be calculated using values from

standard curve. Recently a colorimetric method using

1,9-dimethylmethylene blue was used to determine

GAG in partial urine from pediatric patients. Results

indicated the efficiency and sensitivity of this method

and it is recommended for widespread use. Decarbox-

ylation of uronic acids with hydroiodic acid seems

URONIC ACIDS 5895

to improve the accuracy of measurements over

colorimetry procedures.

0040 The gas–liquid chromatography (GLC) method has

improved the accuracy and specificity of uronic acid

determinations. Although free, low-molecular uronic

acids are readily analyzed by GLC, it is impossible to

measure the specific uronic acids by this method.

0041 Recently a microtiter plate assay for the determin-

ation of uronic acids in biological samples has been

validated. Modification of a commonly used proced-

ure promises to have less risk in handling strong hot

acid and increases accuracy of measurement.

See also: Ascorbic Acid: Properties and Determination;

Carbohydrates: Classification and Properties; Digestion,

Absorption, and Metabolism; Metabolism of Sugars;

Cellulose; Dietary Fiber: Properties and Sources;

Determination; Physiological Effects; Glucose: Function

and Metabolism; Gums: Food Uses; Pectin: Properties

and Determination; Single-cell Protein: Algae