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

Подождите немного. Документ загружается.

Effect on Bioavailability of Minerals

0015 High-oxalate foods have been known to inhibit cal-

cium and iron absorption. Even though vegetables

such as spinach, rhubarb, and Swiss chard are high

in calcium, the calcium cannot be absorbed due to the

presence of oxalates in these vegetables. When cal-

cium absorption from spinach, a high-oxalate and

high-calcium food, was compared with calcium ab-

sorption from milk, a high-calcium food, the results

showed that the calcium from spinach is not readily

available (only 5.1% absorbed), probably due to

the high content of oxalates. The adverse effect of

oxalates is greater if the oxalate-to-calcium ratio

exceeds 9:4 (or approximately 2). The oxalate-to-

calcium ratio in a food varies widely and can be

classified into three groups, as summarized in Table 2.

0016 Oxalate and calcium levels and the oxalate-to-

calcium ratio of specific foods are detailed in Table 3.

0017 Foods that have a ratio greater than two, as well as

containing no utilizable calcium, have excess oxalate,

which can bind calcium in other foods eaten at the

same time. Foodstuffs with a ratio of about one do

not encroach on the utilization of calcium provided

by other products and therefore do not exert any

demineralizing effects. However, these foods are not

good sources of calcium. Although parsley (Petro-

selinum sativum) contains average levels of oxalate

(140–200 mg 100 g

1

), its high calcium levels (180–

290 mg 100 g

1

) reduce the oxalate-to-calcium ratio

to a low level.

0018 Oxalate appears to interfere only slightly with zinc

absorption. A counteracting or protective mechanism

may prevent the precipitation of zinc by oxalates.

Increasing the proportion of magnesium ions in solu-

tion was reported to inhibit the binding of calcium

and zinc oxalates. This observation explains the

minor effect oxalates have on zinc absorption from

some leafy vegetables, such as spinach, which has

high levels of calcium and zinc, and relatively high

levels of magnesium.

0019 Oxalic acid may cause greater decreases in mineral

availability if consumed with a high-fiber diet but the

decrease may be only temporary. Negative calcium,

magnesium, zinc, and copper balances were detected

in males consuming a diet containing fiber and oxal-

ates. When spinach was replaced by cauliflower, a

low-oxalate vegetable, fiber had no effect on the

minerals studied, indicating that the apparent nega-

tive balances obtained were due to the presence of

oxalic acid.

Adverse Effects of Oxalates

Acute

0020A number of plants contain calcium oxalate crystals

(measured as insoluble oxalate). When ingested, they

are not absorbed into the blood stream and remain

largely undissolved within the digestive tract, so they

have no systemic toxicity, but the sharp raphide

crystals can penetrate the tissues of the mouth and

the tongue, causing considerable discomfort. Most of

the plants that contain calcium oxalate crystals are

members of the arum family. It has been suggested

that calcium oxalate crystals are responsible for the

irritating sensation in kiwi fruit (Actinidia sp.) and

soluble oxalates are thought to account for the bitter

taste present in some oca (O. tuberosa Mol.). Con-

ophor seeds (Tetracarpidium conophorum) are a

popular Nigerian snack, which have a bitter taste

when raw but are palatable when cooked. This obser-

vation was correlated with a 73% decrease in total

oxalate concentration after cooking.

Chronic

0021Oxalate is poorly absorbed under nonfasting condi-

tions. Once absorbed free oxalate binds to calcium

ions to form insoluble calcium oxalate, it remains in

the insoluble form.

0022Free oxalate and calcium can precipitate in the

urine and may form kidney stones. These stones

consist mainly of calcium oxalate (80%), which is

relatively insoluble in urine, and calcium phosphate

(5%). Oxalate crystallizes with calcium in the renal

vasculature and infiltrates vessel walls causing renal

tubular obstruction, vascular necrosis, and hemor-

rhage, which lead to anuria, uremia, electrolyte

disturbances, or even rupture. Kidney stones are

becoming more common in men between the ages

30 and 50 years in industrialized countries. The risk

factors involved in stone formation are a low volume

of urine, increased urinary excretion of oxalate, cal-

cium, or uric acid, a persistently low or high urinary

tbl0002 Table 2 Examples of plants with varying oxalate to calcium ratios

Oxalate-to-calciumratio Examples

Group 1: Plants with a ratio greater than 2 Spinach, rhubarb, beet, sorrel, cocoa

Group 2: Plants with a ratio of approx. 1 Potatoes, amaranth, gooseberries, currants

Group 3: Plants with a ratio less than 1 Lettuce, cabbage, cauliflower, green beans, peas

4284 OXALATES

pH, and a low concentration of urinary inhibitors,

such as magnesium, citrate, and high-molecular-

weight polyanions. Normal urine is usually super-

saturated with calcium oxalate. The normal urinary

excretion of oxalate is less than 40–50 mg day

1

with less than 10% coming from the diet. Intakes

of oxalate exceeding 180 mg day

1

lead to a

marked increase in the amount excreted. Small in-

creases in oxalate excretion have pronounced

effects on the production of calcium oxalate in the

urine, implying that foods high in oxalate can pro-

mote hyperoxaluria (high oxalate excretion) and

increase the risk of stone formation. Rhubarb, spin-

ach, beet, nuts, chocolate, tea, coffee, parsley,

celery, and wheat bran cause significant increases

in urinary oxalate excretion in healthy individuals

and have been identified as the main dietary

sources in the risk of kidney stone formation. It

has been reported that black tea increased oxalate

excretion by only 7.9%, compared with increases

of 300% and 400% for spinach and rhubarb, re-

spectively. Therefore 2–3 cups a day of black tea

would have little effect on the risk of urinary stone

formation when compared to spinach and rhubarb.

It appears that tea is a significant source of oxalate

intake in UK diets.

0023The main reason for the strong relationship be-

tween the risk of calcium stones and urinary oxalate

excretion appears to be the effect that the latter has

on the supersaturation of urine with calcium oxalate.

The amount of oxalate excreted in the urine was

higher in individuals with kidney stones than in

healthy individuals, suggesting that those with kidney

stones absorb more oxalate, consume more oxalate

or oxalate-producing substances such as ascorbate, or

metabolize more oxalate precursors. Excessive or

tbl0003 Table 3 Oxalate, calcium, and oxalate-to-calcium ratio (Ox:Ca) of some common foods

Foodstuff Oxalate (mg 100 g

1

FW) Calcium (mg 100 g

1

FW) Ox:Caratio

Range Mean Range Mean (mmoll

1

)

Group 1

Rhubarb (Rheum rhaponticum)

cv. Victoria, forced, stewed 260 12.4 9.32

raw 275–1336 805 40–50 45 7.95

Common sorrel (Rumex acetosa) 270–730 500 35–45 40 5.56

Red beetroot (Beta vulgaris) 121–450 275 121–450 275 5.09

Garden sorrel (Rumex patientia) 300–700 500 40–50 45 4.94

Pig spinach (Chenopodium spp.) 1100 99 4.94

Purslane (Portulaca oleracea) 910–1679 1294 13–236 125 4.60

Spinach (Spinacia oleracea) 320–1260 970 80–122 101 4.27

Garden orach (Atriplex hortensis) 300–1500 900 100 4.00

New Zealand spinach (Te tr a g o n i a ex p a n s a ) 890 100 3.96

Coffee (Coffea arabica) 50–150 100 10–15 12 3.70

Cashew (Anacardium occidentale) 231 41 2.50

Cocoa (Theobroma cacao) 500–900 700 100–150 125 2.49

Beet leaves (Beta vulgaris var. cicla) 300–920 610 100–120 110 2.46

Rhubarb (Rheum rhaponticum)

cv. Crimson, end of season, stewed 460 91.5 2.23

Group 2

Potato (Solanum tuberosum) 20–141 80 10–34 22 1.62

Amaranth (Amaranthus polygonoicles) 1586 595 1.18

Tea (Thea chinensis) 300–2000 1150 400–500 450 1.14

Amaranth (Amaranthus tricolor) 1087 453 1.07

Rhubarb (Rheum rhaponticum)

cv. Victoria, end of season, stewed 620 266 1.04

Group 3

Apple (Malus spp.) 0–30 15 5–15 10 0.67

Blackcurrant (Ribes nigrum) 2–90 50 19–50 35 0.63

Tomato (Lycopersicum esculentum) 5–35 20 10–20 15 0.58

Parsley (Petroselinum sativum) 140–200 170 180–290 235 0.32

Cabbage (Brassica oleracea) 0–125 60 200–300 250 0.11

Lettuce (Lactuca sativa) 5–20 12 73–90 81 0.07

FW, fresh weight.

Adapted from Zarembski PM and Hodgkinson A (1962) The oxalic acid content of English diets. British Journal of Nutrition 16: 627–634; Gontzea I and

Sutzescu P (1968) Natural Antinutritive Substances in Foodstuffs and Forages, pp. 84–108. Basel: S Karger; Meena BA, Umapathy KP, Pankaja N and Prakash

J (1987) Soluble and insoluble oxalates in selected foods. Journal of Food Science and Technology 24: 43–44; Noonan SC and Savage GP (1999) Oxalates

and its effects on humans. Asia Pacific Journal of Clinical Nutrition 8: 64–74 with permission.

OXALATES 4285

increased absorption of oxalate from normal diets is

the result of intestinal abnormalities or malfunction.

This is termed ‘enteric hyperoxaluria’ and is the com-

monest cause of increased renal oxalate excretion. It

has been indicated that people with abnormal gastro-

intestinal absorption are at greater risk for hyperox-

aluria and, as a result, kidney stone formation, than

healthy individuals and should reduce their intake of

oxalate and its precursors, such as ascorbate. A low-

oxalate diet has prevented stone formation in some

cases involving gastrointestinal disorders associated

with hyperoxaluria.

0024 An increase in calcium intake should be accompan-

ied by a lower oxalate consumption, because a low-

calcium and high-oxalate diet enhances oxalate

absorption and excretion, which carries an even

greater risk of stone formation than high calcium

excretion. An increase in calcium intake may reduce

urinary oxalate excretion by binding to more oxal-

ate in the gut, thus reducing the risk of stone forma-

tion. Varying the amounts of calcium does not

significantly alter levels of urinary calcium. From

experimental work, it has been concluded that hyper-

calciuria plays, at most, a secondary role in the

formation of calcium stones compared with mild

hyperoxaluria.

0025 Excessive ascorbic acid (vitamin C) intake may

increase urinary oxalate output with an increased

risk of forming kidney stones. An excess dose is con-

sidered to be 2000 mg of vitamin C per day. However,

ascorbic acid doses greater than 500 mg day

1

were

reported to induce a significant increase in urinary

oxalate, and doses of 1000 mg day

1

would increase

urinary oxalate excretion by 6–13 mg day

1

. The

recommended daily intake in many countries is in

the region of 80 mg.

Effects of Processing

0026 Oxalates may be removed from food by leaching in

water but this is not the most effective method as it

removes only the soluble oxalate. Although the

amount of oxalate in raw soybean (Glycine max)is

relatively low, soaking and germination of the seed

reduced the oxalate concentration. Cooking germin-

ated soybeans reduced oxalate concentration below

that in uncooked germinated soybeans. Soaking

followed by cooking also proved to be effective, al-

though not as effective as germination. Oxalate con-

tent in horsegram seeds (M. uniflorum) decreased by

38% when seeds were dehulled (508 and 315 mg

100 g

1

, for seed and dehulled seed, respectively).

Roasting was found to be the least effective method.

Roasting chicory roots was reported to increase

oxalate content. Roasting oca (New Zealand yam,

O. tuberosa Mol.) also increased oxalate levels by

10–26%. This may be caused by the decrease in mois-

ture content, a hypothesis supported by reports of dry

tropical leafy vegetables having higher oxalate con-

centrations than fresh vegetables.

0027A40–50% loss of total oxalates by leaching was

reported when yam tubers (D. alata and D. esculenta)

were boiled, compared to steaming (20–25%) and

baking (12–15%). Cooking proved most effective in

reducing total oxalates. However, it must be noted

that water-soluble minerals also leach out at the same

time. Mineral leaching appears to vary between plant

species. Blanching has been reported to decrease the

oxalic acid content in spinach. However blanching,

by conventional and microwave methods, reduced

the oxalic acid content of sweet potato, peanut, and

collard leaves only slightly whereas other antinutri-

tional factors such as tannic and phytic acid were

reduced significantly. Spinach, orach, and silverbeet

are generally eaten after being boiled. However,

rhubarb, cocoa, and common and garden sorrel may

be consumed in the raw state and therefore should be

eaten in smaller quantities.

0028Fermentation, frequently used in Asian countries,

has been reported to decrease the oxalate content of

foods. A marked decrease in oxalic acid content was

reported in Icacinia manni (a starch tuber) after

fermentation. Oxalic acid was observed to decrease

by 37% (86 to 54 mg 100 g

1

FW) during souring of

poi (a cooked taro paste) at 20



Recommendations

0029Foods high in oxalates should be consumed in mod-

eration to insure optimum intake of minerals from the

diet. Although some foods are reported to be high in

calcium and other essential minerals, the amount

available may be limited due to the presence of oxal-

ates. For instance, spinach is a high-calcium food,

yet because of its high oxalate content, the calcium

availability is almost negligible. The availability of

magnesium, iron, sodium, potassium, and phos-

phorus may also be restricted.

0030High-oxalate foods should be cooked to reduce the

oxalate content. Soaking raw foods will also reduce

the oxalate content but other useful nutrients such as

water-soluble vitamins and minerals may also be lost

at the same time. Oxalates tend to occur in higher

concentrations in the leafy parts of vegetables rather

than in roots or stalks.

0031For the general population, the occasional con-

sumption of high-oxalate foods as part of a balanced

diet does not pose any health problems. However,

there are some groups of people who may be at risk

from oxalate-induced side-effects.

4286 OXALATES

0032 Vegans and vegetarians should be aware that some

foods contain high levels of oxalates. The diets of

vegans and those persons with lactose intolerance

may be low in calcium as a result of the exclusion of

dairy products, unless their diet is supplemented by

some other high-calcium food products. It is recom-

mended that high-oxalate foods should be accompan-

ied by calcium-rich foods such as dairy products and

shellfish. If high-oxalate foods are consumed in con-

junction with a low-calcium diet, then the consumer

may be at risk of hyperoxaluria, which may lead to

kidney stone formation.

0033 Women tend to be more susceptible to calcium and

iron deficiencies than men. Osteoporosis is a concern

amongst females, especially after menopause. People

suffering from fractures should also be aware of the

potential effects of oxalates on mineral availability, as

high calcium levels are required for bone repair. Once

again, consumption of high-oxalate foods with an

adequate-to-high calcium intake should pose no

health problems. It must also be noted that calcium

is only absorbed and used when there are adequate

levels of vitamin D in the body, either obtained via the

diet or synthesized by the body when exposed to

sunlight. Women should eat red meats, which are

low in oxalate, to satisfy their iron intake. Adequate

levels of vitamin C are required for the absorption of

iron, but excess amounts are not advised because

ascorbic acid is converted into oxalate.

0034 The risk of stone formation is three times greater in

males and they should avoid eating excess amounts of

high-oxalate foods. Sufferers of hyperoxaluria and

kidney stones are advised to restrict their diet to

foods containing low or medium levels of oxalates,

as although urinary oxalate arises predominantly

from endogenous sources, it can be influenced by

dietary intake. Excess vitamin C intake is not recom-

mended in these patients.

0035 Inhabitants of tropical countries should be aware

that leafy tropical plants and tropical root crops tend

to contain higher levels of oxalates than plants from

temperate climates. People living in these areas are at

possible risk of stone formation due to hyperoxaluria,

and mineral deficiencies if sufficient minerals are not

consumed.

See also: Ascorbic Acid: Properties and Determination

Physiology; Calcium: Properties and Determination;

Physiology; Iron: Properties and Determination;

Physiology; Plant Antinutritional Factors:

Characteristics; Renal Function and Disorders:

Nutritional Management of Renal Disorders; Toxins in

Food – Naturally Occurring

Further Reading

Concon JM (1988) Food Toxicology – Principles and

Concepts, pp. 416–419. New York: Marcel Dekker.

Dobbins JW and Binder HJ (1977) Importance of the colon

in enteric hyperoxaluria. New England Journal of

Medicine 296: 298–301.

Gontzea I and Sutzescu P (1968) Natural Antinutritive

Substances in Foodstuffs and Forages, pp. 84–108.

Basel: S Karger.

Hagler L and Herman RH (1973) Oxalate metabolism I.

American Journal of Clinical Nutrition 26: 758–765.

Hanson CF, Frankos VH and Thompson WO (1989)

Bioavailability of oxalic acid from spinach, sugar beet

fibre and a solution of sodium oxalate consumed by

female volunteers. Food and Chemical Toxicology 27:

181–184.

Linder MC (1991) Nutritional Biochemistry and Metabol-

ism with Clinical Applications, 2nd edn. New York:

Elsevier.

Massey LK, Roman-Smith H and Sutton RAL (1993) Effect

of dietary oxalate and calcium on urinary oxalate and

risk of formation of calcium oxalate kidney stones. Jour-

nal of the American Dietetic Association 93: 901–906.

Meena BA, Umapathy KP, Pankaja N and Prakash J (1987)

Soluble and insoluble oxalates in selected foods. Journal

of Food Science and Technology 24: 43–44.

Noonan SC and Savage GP (1999) Oxalates and its effects

on humans. Asia Pacific Journal of Clinical Nutrition 8:

64–74.

Ross AB, Savage GP, Martin RJ and Vanhanen L (1999)

Oxalates in oca (New Zealand yam) (Oxalis tuberosa

Mol.). Journal of Agricultural and Food Chemistry 47:

5019–5022.

Sangketkit C, Savage GP, Martin RJ, Mason SL and Vanha-

nen L (1999) Oxalates in oca: a negative feature? In:

Jenson J and Savage GP (eds) Second South West Pacific

Nutrition and Dietetic Conference Proceedings, pp.

44–50. Auckland, New Zealand.

Strenge A, Hesse A, Bach D and Vahlensieck W (1981)

Excretion of oxalic acid following the ingestion of various

amounts of oxalic acid-rich foods. In Smith LH, Robert-

son WG and Finlayson B (eds) Urolithiasis: clinical and

basic research, pp. 789–794. New York: Plenum Press.

Wanasundera JPD and Ravindran G (1994) Nutritional

assessment of yam (Dioscorea alata) tubers. Plant

Foods for Human Nutrition 46: 33–39.

Zarembski PM and Hodgkinson A (1962) The oxalic acid

content of English diets. British Journal of Nutrition 16:

627–634.

OXALATES 4287

OXIDATION OF FOOD COMPONENTS

K L Parkin and S Damodaran,Universityof

Wisconsin-Madison,Madison,WI,USA

Scope

000 1 Antoine Lavoisier (1743–94) recognized oxidation

as a chemical process, concluding that oxygen was

the element responsible for the formation of acidic

residues, or oxides, upon combustion of certain sub-

stances. A contemporary definition of oxidation is the

process by which oxygen is added, or hydrogen or

electrons are withdrawn. For a component to be oxi-

dized, another has to be reduced, and reduction can

be defined as the withdrawal of oxygen, or the add-

ition of hydrogen or electrons. The component that is

oxidized and loses electrons is the reductant and the

component that is reduced and gains electrons is

the oxidant. Oxidation is distinct from oxygenation;

the latter is a noncovalent coordination of oxygen

with a component, as in the case where hemoglobin

and myoglobin bind oxygen to facilitate oxygen

transport in blood and muscle, respectively. In food

systems, oxygen is the most common oxidant, al-

though other endogenous and added chemicals can

also serve as oxidants. The principal negative effect of

oxidation in foods is that flavor quality is lost, giving

rise to the defect often referred to as oxidative rancid-

ity. In addition, functional, color, and nutritional

qualities of food components can be lost as a conse-

quence of oxidation in foods. However, there are also

some oxidative processes in foods that have beneficial

effects on quality.

The Basic Process of Oxidation

Oxidation–Reduction Potentials

000 2 The potential or thermodynamic favorability for

two components to be involved in an oxidation–

reduction (redox) reaction can be predicted from

the corresponding half-reactions of oxidation and

reduction. Table 1 provides a selective list of some

standard reduction half-reactions, using the hydro-

gen half-cell at pH 7.0 as a standard. As the reduc-

tion potential (voltage) becomes more positive, the

tendency for that half-reaction to take place in-

creases. Thus, the most powerful oxidants in Table 1

are hydrogen peroxide and oxygen. For each com-

ponent, oxidation half-reactions take place in the

reverse direction of what appears in Table 1 and

have voltages of the opposite sign of the same

magnitude.

0003The redox potential (E

) of food systems is depend-

ent on the concentration and redox states of the com-

ponents of that system. One of the most important

components is oxygen and, at limited dissolved

oxygen components, E

is strongly dependent on

the oxygen content. ‘Reducing’ conditions or a very

negative E

(e.g., 400 mV) exist when dissolved

oxygen is poised at near-anaerobic levels.

0004Transition metals such as copper and iron are be-

lieved to be involved in oxidation in foods via a redox

cycling mechanism. Since there is very little ‘free’ iron

and copper in biological systems, the types and con-

centrations of chelators present have a marked effect

on the redox behavior and thus oxidative activity of

these transition metals.

0005Oxidation in foods is often caused by free radical

reactions. There are three stages of free radical oxida-

tion, also referred to as autoxidation when the oxi-

dant is oxygen. The first step, or ‘initiation,’ involves

the formation of a free radical species (X

) from a

biological component (XH), usually by the abstrac-

tion of a hydrogen atom (H

) by active oxygen or

high-energy irradiation (eqn (1)). ‘Propagation’ of

free radical oxidation processes occurs by chain reac-

tions that consume oxygen and yield new free radical

species (peroxy radicals, XOO

) or peroxides

(XOOH), as in eqns (2) and (3). The products

and XOOH, see also eqn (6)) can further propa-

gate free radical reactions. ‘Termination’ of free rad-

ical oxidative reactions occurs when two radical

species react with each other to form a nonradical

adduct, as in eqn (4).

XH ! X

þ H

ð1Þ

! XOO

ð2Þ

tbl0001Table 1 Standard electrode potentials of selected reduction

half-reactions

Reaction Volts

þ 2H

þ 2e



! 2H

O 1.77

þ 4H

þ 4e



! 2H

O 1.23

2þ

þ e



! Cu

0.15

3þ

þ e



! Fe

0.11

Dehydroascorbate þ H

þ 2e



! Ascorbate 0.054

þ 2e



! H

0.00

RSSR þ 2H

þ 2e



! 2RSH 0.39

Estimated value for oxidized disulfide (RSSR) conversion to reduced thiol

(RSH) such as for oxidized/reduced glutathione and cystine/cysteine

couples.

Standard Conditions: pH 7.0, 1 mol l

1

for each component.

4288 OXIDATION OF FOOD COMPONENTS

XOO

þ XH ! XOOH þ X

ð3 Þ

þ X

! X  X ð4 Þ

Activation of Oxygen

0006 In food systems, molecular oxygen (dioxygen; O

)is

generally the source of oxidizing power. Other strong

oxidantsincludethefoodadditives hydrogen peroxide

),calcium andbenzoylperoxides, and bromates

(KBrO

). All of these compounds are conjugates

of oxygen. However, not all strong oxidants are

composed of oxygen, as fluorine and bromine are

also strong oxidants.

000 7 Although the reaction of ground state oxygen (

triplet oxygen) with organic compounds is thermo-

dynamically favorable, it is kinetically slow due to the

high energy of activation required for oxygen to react.

The electron configuration of

includes two un-

paired electrons in the outer shell, yielding a triplet

signal in a magnetic field. All known organic com-

pounds are in a singlet state, having all electrons

paired with another. Consequently, facile reaction

between

and organic molecules is forbidden due

to the incompatibility of their electron ‘spin’ states.

000 8 ‘Activation’ of

overcomes much of the energy

barrier to its reactivity as an oxidant. One means of

activation is the excitation of

to yield singlet

molecular oxygen (

); the latter has the two outer

electrons paired in a single orbital. Other forms of

‘activated’ or ‘reactive’ oxygen result from the first

three one-electron reductions to

in the process

of reducing

to water. These reactive species of

oxygen include the superoxide anion radical (O



) and

its conjugate acid (HO

), hydrogen peroxide (H

and the hydroxyl radical (

OH). The standard reduc-

tion potentials of each of these steps is provided in

Table 2. The electronic structure of these activated

forms of oxygen facilitate their reactivity with bio-

logical compounds. The strongest electrophiles (elec-

tron seekers),

OH and

are the most reactive

forms of ‘active’ oxygen, followed in reactivity by



and then H

0009Some of these active oxygen species can be inter-

converted, and these processes can be facilitated by

the presence of specific catalysts. Activated forms of

oxygen can also be formed by g irradiation and by

photosensitization of pigments in foods.

Catalysts of Oxidation Reactions

0010Catalysts of oxidation reactions can be enzymatic

(protein) or nonenzymatic. Transition metals (M

reduced form; M

n þ 1

, oxidized form) can participate

in redox reactions with

to yield O



=HO

,asin

eqn. (5). The resulting O



can initiate oxidation reac-

tions. Another manner by which transition metals can

cause oxidation reactions is by breaking down lipid

hydroperoxides (LOOH) (eqn (6)), and the alkoxy

radical (LO

) so formed can cause further oxidative

reactions. Since there are often small quantities of

LOOH in food systems this process is probably im-

portant. Transition metals can also take part in the

interconversion of active oxygen species, as in

the Haber–Weiss reaction (eqn (7)). This reaction

can be mediated by three partial reactions (eqns (8)–

(10)). In the first reaction (eqn (8)), O



acts as a

reductant and donates an electron to an oxidized

transition metal (e.g., iron). In the second step (eqn

(9)), O



, acting as both an oxidant and reductant,

undergoes dismutation to form H

and

. In the

third step (eqn (10)), also called the Fenton reaction,

the reduced transition metal donates an electron to

to form the extremely reactive

OH, and the

transition metal reverts back to its oxidized state

to allow another cycle. In food systems, several

endogenous components, such as ascorbic acid and

thiol compounds, can replace O



as a reductant.

This set of reactions also illustrates the partici-

pation in oxidative reactions of all activated oxygen

species generated by univalent electron reductions

! M

nþ1

þ O



ð5Þ

þ LOOH ! M

nþ1

þ LO

þ OH



ð6Þ



þ H

þ OH



OH ð7Þ



þ M

nþ1

þ M

ð8Þ



þ 2H

þ H

ð9Þ

þ M

! M

nþ1

OH þ OH



ð10Þ

0011Other nonenzymatic catalysts include photosensi-

tive pigments in foods. Photosensitive pigments

become elevated to an excited triplet state upon the

absorption of light energy, and can transfer that

energy to

or other biological components. Some

pigments favor transmission of energy to organic

tbl0002 Table 2 Standard electrode potentials for univalent reductions

of O

to H

Reaction Volts

þ e



! O



0.16 (0.33)



þ e



þ 2H

! H

0.89

þ e



þ H

!OH þ H

O 0.38

OH þ e



þ H

! H

O 2.32

Standard conditions: pH 7.0, 1 mol l

1

for each component. For O

electrode potential also provided, in parentheses, at 10

Pa (0.987 atm).

OXIDATION OF FOOD COMPONENTS 4289

compounds (type I process) which ultimately yield



and H

from

. Other pigments favor trans-

mission of energy directly to

to yield

(type II

process). Examples of photosensitizers of each of

these types in foods are riboflavin and chlorophyll,

respectively. (See Chlorophyl; Riboflavin: Properties

and Determination.)

0012 Enzymatic catalysts of oxidative reactions usually

cause oxidations of specific biological compounds.

For example, the enzymes lipoxygenase, polyphenol-

oxidase, sulfhydryl oxidase and xanthine oxidase are

common to foods and cause the specific oxidation of

unsaturated fatty acids, mono- and diphenolic acids,

protein thiol (cysteine) residues, and xanthine, re-

spectively. Glucose oxidase converts glucose to glu-

conic acid and also produces H

. Xanthine oxidase

and peroxidase can produce H

and O



and

respectively, and this is dependent on which sub-

strates are being utilized and the level of oxygen

present. These active oxygen species may cause the

oxidation of other biological compounds, leading to

losses in food quality.

Oxidation of Food Components

Lipids

001 3 Polyunsaturated fatty acids with 1,4-pentadiene func-

tional units are particularly sensitive to oxidative re-

actions. Using linoleic acid as an example, oxidation

can be initiated by two basic mechanisms, abstraction

(autoxidation) and ‘ene’ addition (Figure 1). Abstrac-

tion is when an electron (or hydrogen atom) is

removed from the fatty acid by reaction with an

electrophilic species such as

OH or X

, or by inter-

action with high-energy radiation. The initial abstrac-

tion step yields a fatty acid free radical (initiation step)

which can then undergo addition of

(propagation

step) and then abstract an electron from another

biological compound. The methylene or ‘allylic’

hydrogen atoms of the pentadiene structure are most

readily abstracted. The resulting free radical (L

) can

be stabilized by resonance along the original penta-

diene structure, and the fatty acid radical tends to

undergo addition of

when the unpaired electron

is most ‘delocalized’ or located at the terminal sites,

or C9 and C13, resulting in the formation of first

the linoleic acid 9- and 13-hydroperoxyl radicals,

and then the 9- and 13-OOH (hydroperoxides)

isomers. Further oxidative processes can be initiated

by interaction of these hydroperoxides with transi-

tion metals as previously described in eqn (6). (See

Fatty Acids: Properties.)

0014 The ‘ene’ addition reaction that can initiate lipid

oxidation is caused by the highly electrophilic

which will add directly to the double bond since this

is where the highest electron density can be found.

Thus, a mixture of 9-, 10-, 12-, and 13-OOH isomers

are produced by

reaction with linoleic acid.

0015Once the fatty acid hydroperoxides and hydroper-

oxyl radicals are formed, additional initiation reac-

tions can take place for these initial products, being

unstable, can be subject to secondary reactions, as

shown for linolenic acid oxidation (Figure 2). Oxida-

tion of any remaining double bonds can take place,

and in some cases the fatty acid radicals can attack

adjacent intramolecular double bonds, forming cyclic

structures. Alternatively, hydroperoxyl fatty acids can

react with adjacent fatty acids to yield polymerized

oxidation products. ‘Scission’ reactions lead to frac-

ture of the fatty acid chain and result in the emanation

of reduced molecular weight ketones and aldehydes.

These latter secondary products, being fairly volatile,

give rise to the off-flavors and odors that are associ-

ated with oxidized foods or oxidative rancidity. One

product that can be formed by secondary reactions of

oxidizing lipids is malondialdehyde (MDA). MDA is

often used by food scientists as an indicator of the

degree of oxidation of lipids in foods. In cases where

specific secondary products are formed by enzyme

reactions, such as by lipid hydroperoxide lyases in

freshly cut cucumber and tomato fruits, the resulting

volatile compounds are pleasant and contribute desir-

able aromatic qualities.

0016Thermally induced oxidation reactions can occur

in both saturated and unsaturated lipids at tempera-

tures encountered during processes such as deep-fat

frying. Oxidation generally proceeds via the initial

formation of hydroperoxides. The high temperatures

can cause many isomerization and scission reactions

to take place, producing a myriad of secondary or

breakdown products such as epoxides, dihydroper-

oxides, cyclized fatty acids, dimers, and aldehydes

and ketones resulting from scission reactions.

Proteins

0017Proteins, peptides, and amino acids in foods undergo

several oxidative changes during food processing.

The amino acids that are most susceptible to oxida-

tive degradation are methionine, cysteine (cystine),

histidine, and tryptophan. Under severe oxidizing

conditions tyrosine, serine, and threonine are also

oxidized to some extent. Oxidation of proteins and

amino acids is caused by several agents, such as light,

g irradiation, peroxidizing lipids, metal ions, the

products of enzymatic and nonenzymatic browning

reactions, and food additives such as hydrogen per-

oxide, benzoyl peroxide, bromates (KBrO

) and azo-

dicarbonamide. (See Amino Acids: Properties and

Occurrence; Protein: Chemistry.)

4290 OXIDATION OF FOOD COMPONENTS

001 8 Treatment of proteins with hydrogen peroxide or

calcium peroxide causes oxidation of methionine sulf-

oxide (reversible), which can be further oxidized to

methionine sulfones (irreversible) (eqn (11)). Cysteine

residues can be oxidized by peroxides or other forms of

activated oxygen to yield the sulfenic (Cy-SOH), sul-

finic (Cy—SO

H) and sulfonic (Cy—SO

H), acid de-

rivatives. Oxidation of cystine residues in proteins

results in the formation of mono-, di-, tri-, and tetra-

sulfoxides.

0019 Free thiol groups in proteins are readily oxidized by

atmospheric oxygen to form disulfide cross-links. Free

thiol groups also catalyze thiol-disulfide interchange

reactions, which often lead to polymerization of pro-

teins. Oxidizing agents such as KBrO

and azodicar-

bonamide are often used as additives in wheat flour in

order to improve dough formation. These additives

are believed to oxidize and block the free thiol groups

of protein and nonprotein constituents, and thus pre-

vent the occurrence of thiol–disulfide interchange

reactions in the dough. The modulation of oxida-

tion–reduction behavior in dough systems may also

be controlled by ascorbic acid, dehydroascorbic acid,

and glutathione.

0020When foods containing photosensitive substances,

such as riboflavin and chlorophyl, are exposed to

light, the amino acids histidine, cysteine, methionine,

tryptophan, and tyrosine are oxidized by the acti-

vated oxygen species O



and

. g Irradi-

ation of foods results in the formation of H

= −(CH

)

−COOH

= CH

(CH

)

−

Linoleic acid

13 12

10 9

Abstraction

'Ene' addition

OOH OOH

OOH

OOH OOH

fig0001 Figure 1 Initiation reactions for the oxidation of linoleic acid. Reproduced from Oxidation of Food Components, Encyclopaedia of

Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

OXIDATION OF FOOD COMPONENTS 4291

through radiolysis of water in the presence of oxygen,

which in turn causes oxidative changes in proteins.

Tryptophan residues can also be oxidized upon ex-

posure of proteins to acidic conditions.

0021 Substantial oxidation of free amino acids and

amino acid residues in proteins occurs in the presence

of peroxidizing lipids. Methionine, cysteine, histi-

dine, and lysine are the most susceptible amino

acids/residues. Two types of mechanisms, one involv-

ing the alkoxy (LO

) and peroxy (LOO

) free radicals,

and the other involving malondialdehyde and other

carbonyl compounds, are believed to be involved

in the oxidation of proteins by peroxidizing lipids.

In the first case, the lipid free radicals react with

proteins (P) and induce formation of protein free

radicals (P

), followed by polymerization of protein

molecules (eqns (12)–(16)). In addition to the free

radical-induced polymerization of protein molecules,

the lipid peroxides formed during the reactions oxi-

dize methionine, cysteine, histidine, and tryptophan

residues. The highly reactive malondialdehyde formed

in peroxidizing lipids reacts with amino groups

of lysyl residues, resulting in intermolecular cross-

links.

LOO

þ P !

LOOP

OOLOOP

POOLOOP

ð12 Þ

þ P ! LOH þ P

ð13Þ

LOO

þ P ! LOOH þ P

ð14Þ

þ P ! P  P

ð15Þ

P  P

þ P ! P  P  P

ð16Þ

0022Heat treatment of proteinaceous foods causes

several oxidative changes in proteins. While mild

heat treatment results in protein denaturation and

loss of functionality, severe heat treatment often

causes undesirable chemical changes in amino acid

residues and complex reactions of proteins with

= −(CH

)

−

COOH

= CH

−

Linolenic acid

16151312109

,XH

Endoperoxide

OOH

Scission

MDA

Scission

Dimerization

Transitional metal (eqn(6))

or high temperature

OOH

Plus 9−, 12−, and 16−peroxy

radical derivatives

Abstraction at C

or C

Addition of O

Aldehyde Aldehyde

fig0002 Figure 2 Initiation and secondary reactions for the oxidation of linolenic acid. Reproduced from Oxidation of Food Components,

Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

S CH

(11)

4292 OXIDATION OF FOOD COMPONENTS

other food components, such as carbohydrates and

lipids. (See Browning: Nonenzymatic.)

0023 When protein is heated at temperatures above

300



C, as commonly encountered during broiling

and grilling, several amino acid residues undergo ther-

mal decomposition and pyrolysis. Several of these

pyrolysis products have been isolated, identified, and

shown to be highly mutagenic. The most carcinogenic/

mutagenic products are formed from the decompos-

ition of tryptophan, glutamate and lysyl residues. (See

Carcinogens: Carcinogenic Substances in Food:

Mechanisms; Mutagens.)

Carbohydrates

0024 Carbohydrates are not as sensitive to oxidation reac-

tions as are lipids and proteins. In addition, since

many oxidation products are not volatile, the prac-

tical consequences of carbohydrate oxidations in

foods are limited. Oxidation of food carbohydrates

can take place, especially at high temperatures,

resulting in caramelization reactions. (See Caramel:

Methods of Manufacture; Carbohydrates: Inter-

actions with Other Food Components.)

0025 Some industrial processes employ oxidation reac-

tions to prepare functional derivatives of monosac-

charides for the chemical industries. However, the

carbohydrate oxidations of most relevance to foods

involve enzymatic reactions. Glucose oxidase oxi-

dizes glucose to gluconic acid while simultaneously

reducing O

to H

. The enzyme is added to foods

to reduce glucose levels (to prevent nonenzymatic

browning in eggs to be dried) or oxygen tension (to

stabilize beverages and salad dressings from oxidative

deterioration).

0026 Carbohydrates can be oxidized by the same free

radical mechanisms as described for lipids. Low-

molecular-weight carbohydrates such as glucose,

mannitol, and deoxyribose are known to react with

OH and produce oxidized derivatives. Again, these

derivatives, when present, have little impact on food

quality and are thus of little practical significance.

Minor Food Components

0027 Oxidation of minor food components can also

influence food quality. Oxidation of ascorbic acid by

enzymatic (ascorbic acid oxidase) or nonenzymatic

means can compromise nutritional quality. Ascorbic

acid, and other organic acids, can be degraded by

active oxygen species and by reactions initiated by

transition metals. Polyphenol oxidase and tyrosinase

are enzymes found in plant and crustacean foods that

oxidize phenolic acids and initiate secondary non-

enzymatic reactions that are responsible for

darkening and often loss of color quality. (See Ascor-

bic Acid: Properties and Determination.)

Environmental Factors

Temperature

0028Generally, as the temperature increases, the rate of

oxidation reactions also increases. Rate increases usu-

ally follow a Q

¼ 2 relationship, or a doubling in

rate for every rise of 10



C, provided no change in the

mechanism of reaction occurs with a corresponding

change in temperature and as long as competing reac-

tions have little impact on the reactants. For oxidative

reactions caused by enzymes, an optimum tempera-

ture exists. This is because enzymes, being proteins,

are denatured above a characteristic temperature and

will lose biological activity. Another factor is that, as

temperature increases, oxygen solubility in water de-

creases and this could attenuate the temperature acti-

vation of oxidative reactions if oxygen was a limiting

component in the process.

Moisture

0029Lipid oxidation reactions generally have rate minima

at intermediate water activities (a

) of about 0.3. As

increases above 0.3, rates of oxidation increase,

probably due to increased mobility and activity of

catalysts. At a

below 0.3, rates of oxidation in-

crease, perhaps due to solvation and removal of cata-

lysts and reaction intermediates from lipids and into

the aqueous phase. (See Water Activity: Effect on

Food Stability.)

Chemical Composition of Food

0030The relative concentrations of various prooxidants

and antioxidants in the food, and their identities and

relative reactivities, will greatly influence the rate of

oxidation reactions. Antioxidants, such as butylated

hydroxytoluene, butylated hydroxyanisole, propyl

gallate, ascorbic acid, and sodium bisulfite can be

added to impede oxidative reactions. On the other

hand, the inadvertent addition of transition metals,

such as iron and copper, from processing equipment

can hasten oxidative reactions in foods. In addition,

the nature of the substrate sensitive to oxidative reac-

tions is important. For example, foods rich in poly-

unsaturated fatty acids (vegetable and fish oils) are

more sensitive to oxidation than those rich in mono-

unsaturated fatty acids (animal depot fats). (See

Antioxidants: Natural Antioxidants; Synthetic Anti-

oxidants.)

Exogenous Factors

0031Packaging materials and strategies can have an influ-

ence on rates of oxidation. Foods containing photo-

sensitive pigments are often packed in opaque or

translucent containers to minimize photooxidative

OXIDATION OF FOOD COMPONENTS 4293