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Light Protection by Glass and Plastics

The light protection of glass and plastic depends on mate-

rial thickness and type and concentration of pigment.

Higher pigmentation will drastically reduce the light trans-

mittance and increase the protection of the material. As

shown by Figures 2 and 3, clear glass and plastic (injection

molded polypropylene) transmit about 90% of the visible

light and, respectively, 35% and 56% of the UV light, and

they are therefore poor UV and visible light barriers. Blue

glass and plastics transmit light in the UV, blue, and the

red regions of the spectrum. As many photosensitizers

absorb light in the UV and blue regions of the spectrum,

this light is particularly harmful to many foods. Green

glass and plastics transmit light in the UV, violet/blue, and

green regions of the spectrum. In this area the porphyrins

have some small absorption maxima, and prolonged ex-

posure in this region may cause food deterioration.

Brown glass transmits light in the yellow, orange, and

red region of the spectrum, an area where chlorophylls

also have absorption maxima. However, brown glass is

usually densely pigmented, offering good light protection

for a variety of food products including milk and beer.

Figure 3 shows the light transmittance through an

SiO

-pigmented (white) injection-molded polypropylene

plastic. Addition of this pigment results in a relative sharp

cut off around 400 nm offering good protection against UV

radiation. However, because normal artiﬁcial light sources

used in retail normally emits only small amounts of light

in the UV region (many have only a small emission peak in

the 370-nm area), the transmittance of light by packaging

materials in this region is not a serious threat during

indoor distribution, storage and display. A limited effect is

therefore achieved by adding a UV block to a transparent

packaging material.

Light Protection by Liquid Paperboard

Different qualities of liquid paperboard coated with low-

density polyethylene are used worldwide for packaging

of fresh milk products. Figure 4 shows that despite

their modest thickness (0.5–0.6 mm) commercial liquid

% Light transmission

200 300 400 500

Wavelength (nm)

600 700 800 900

Bleached, unprinted

Bleached, pink printed

Unbleached, unprinted

Unbleached, pink printed

Figure 4. Light transmission in three different

qualities of unprinted and pink printed liquid paper-

board: bleached, unbleached, and bleached with

oxygen and light barrier as measured by a Perkin

Elmer Lambda 800 UV–vis spectrometer with an

integrating sphere (Perkin Elmer Ltd., Buckin-

ghamshire, UK).

200

% Light transmission

(Transparent film)

% Light transmission

(Metallized films)

100

300 400 500

Wavelength (nm)

600 700 800 900

Transparent film

Metallized film, blue print

Metallized film, unprinted

Metallized film, red print

Figure 5. Light transmission in unmetallized, me-

tallized, and printed, metallized polypropylene ﬁlms

as measured by a Perkin Elmer Lambda 800 UV–vis

spectrometer with an integrating sphere (Perkin El-

mer Ltd., Buckinghamshire, UK).

658 LIGHT PROTECTION FROM PACKAGING

paperboard qualities offer a relatively good protection

against light. Only 4.3% of the light in the visible (400–

700 nm) region of the spectrum and 0.03% in the UV

region are transmitted through the bleached paperboard.

The unbleached board offers an even better light barrier.

Only 0.8% of the visible light and 0% of the UV light is

transmitted. Additional protection is achieved by printing.

Only 2.6% of the light is transmitted through pink printed

bleached paperboard, and most of the reduction appears in

the region of the spectrum that is most detrimental to milk

products—that is, up to 550 nm. Pink printed unbleached

paperboard offers very good light protection. Less than

0.1% of the visible light is transmitted through this

material.

If liquid paperboard is foiled with aluminum, it be-

comes virtually impermeable to light. Also the SiO

/car-

bon black/EVOH-based light and oxygen barrier used by

one supplier for certain products in some markets also

offers very good light protection. Only 0.3% of the visible

light is transmitted through this material (for unprinted,

bleached board).

Light Protection by Metallization of Plastic Films

Metallization may be used to improve both light barrier

and oxygen barrier of plastic ﬁlms. As shown by Figure 5,

the transmission in the visible region of the spectrum is

signiﬁcantly reduced from 92% for unmetallized ﬁlm to

0.5% when the same ﬁlm is metallized. The reduction in

light transmission caused by the metallization is rather

uniform in the visible area of the spectrum, similar to the

effect of a heavily pigmented gray (neutral) ﬁlter. The

effect of metallization is still very good in the UV region of

the spectrum, but not as good as in the visible region. As

for liquid paperboard, further reduction in light transmis-

sion is achieved by additional printing of the metallized

ﬁlm. The effect of blue print results in neglectable effect on

light transmission in the blue region because blue print

transmits light in the blue region of the spectrum. Simi-

larly, the effect of red print is neglectable in the red region

of the spectrum.

BIBLIOGRAPHY

1. A. Ryer, International light, Inc. web information. Available

at http://www.cbpf.br/pdf/Light%20Measurement%20Hand-

book.pdf. Accessed June 8, 2007.

2. Wikipedia Foundation Inc, web information. Available at

http://en.wikipedia.org/wiki/Candela. Accessed June 8, 2007.

3. D. G. Bradley and D. B. Min, Crit. Rev. Food Sci. Nutr. 31(3),

211–236 (1992).

4. S. K. Pushpan, S. Venkatraman, V. G. Anand, J. Sankar, D.

Parmeswaran, S. Ganesan, and T. K. Chandrashekar, Curr.

Med. Chem. 2, 187–207 (2002).

5. D. B. Min and J. M. Boff, Compr. Rev. Food Sci. Food Safety 1,

58–72 (2002).

6. C. S. Foote, Photochem. Photobiol. 54(5), 659 (1991).

7. J. Lindsay, Oregon Medical Laser Company, web information.

Available at http://omlc.ogi.edu/spectra/PhotochemCAD/

html/index.html. Accessed June 8, 2007.

8. M. Bekbo

let, J. Food Prot. 53(5), 430–440 (1990).

9. J. P. Wold, A. Veberg, A. Nilsen, V. Iani, P. Juzenas, and J.

Moan, Int. Dairy J. 15, 343–353 (2005).

10. J. P. Wold, Spectrosc. Eur. 18(5), 8–13 (2006).

11. M. B. Korycka-Dahl and T. Richardson, Crit. Rev. Food Sci.

Nutr. December, 209–241 (1978).

12. E. Choe, R. M. Huang, and D. B. Min, J. Food Sci. 70(1), R28–

R36 (2005).

13. H. Larsen, P. Lea, and M. Rødbotten, Food Qual. Pref. 16(7),

573–584 (2005).

14. L. Piergiovanni, and S. Limbo, Packag. Technol. Sci. 17(3),

155–164 (2004).

15. J. P. Wold, A. Veberg, F. Lundby, A. N. Nilsen, and J. Moan,

Int. Dairy J. 16, 1218–1226 (2006).

LIPID OXIDATION: CHEMICAL

STABILIZATION

KAREN M. SCHAICH

Department of Food Science,

Rutgers University, New

Brunswick, New Jersey

INTRODUCTION

Packaging must provide physical, microbiological, and che-

mical protection for foods . F or the most part, providing

physical protection is straightforward, as is preventing

contamination of foods after processing and packaging.

More challenging is meeting the requirement for inert

storage environments that inhibit respiration in fresh fruits

and vegetables, microbial growth in prepared foods and raw

meats, and chemical degradation in stored foods . Lipid

oxidation, or oxidative rancidity, is the chemical reaction

that most limits long-term shelf life of foods , producing

characteristic off-odors and ﬂavors, browning, texture

changes, co-oxidation of proteins and vitamins, and even

toxic products in foods. Lipid oxidation is also a major cause

of deterioration in cosmetics and many personal care pro-

ducts, changing surface tensions, breaking emulsions, oxi-

dizing active components, and producing off-odors. Because

of these extensive effects on product quality, lipid oxidation

also forces the greatest demands on food packaging .

This chapter will describe the reactions of lipid oxida-

tion in general terms and show how packaging can be used

to inhibit and control lipid oxidation. Additional details of

lipid oxidation reactions, factors affecting lipid oxidation,

and strategies for prevention may be found in comprehen-

sive reviews listed at the end of the chapter.

CHARACTERISTICS OF LIPID OXIDATION

Lipid oxidation is a chemical process in which unsaturated

fatty acids in phospholipids and triacylglycerides react

with oxygen and degrade to a variety of volatile and

nonvolatile products. Lipids oxidize by a free radical chain

reaction process involving three stages: initiation, propa-

gation and branching, and termination. The classical

LIPID OXIDATION: CHEMICAL STABILIZATION 659

sequence describing lipid oxidation is shown in Figure 1,

along with catalysts and protectants at each stage.

The radical chain reaction has several critical conse-

quences for lipid oxidation:

. Once started, the reaction is self-propagating and

self-accelerating; that is, it keeps itself going.

. Many more than one LOOH is formed and more than

one lipid molecule is oxidized per initiating event.

. Very small amounts of pro- or antioxidants cause

large rate changes.

. Multiple intermediates and products are formed, and

they change with reaction conditions and time.

Unlike most chemical reactions, lipid oxidation is a

dynamic process with no ﬁxed intermediates or endpoints.

That the reactions of lipid oxidation change with time and

conditions presents distinct challenges in measuring and

controlling lipid oxidation, and it explains in part why

lipid oxidation is a major problem in storage stability of

foods and other lipid-containing products.

STAGES OF LIPID OXIDATION

Initiation

LH ! L



ð1Þ

Oxygen cannot add directly to the double bonds in

unsaturated fatty acids, but it adds almost instantaneously

to L



free radicals to form peroxyl radicals that drive

radical chain reactions. Thus, lipid oxidation requires

special catalysts, or initiators, to form L



,oncarbonatoms

of lipid (L) molecules. Common initiators include ultraviolet

light, photosensitizers and visible light, radicals from other

sources, ionizing radiation, metals, heme compounds, and

heat.

Propagation



þ O

! L



ð2aÞ



þ L

H ! L

OOH þL



! L



ð2bÞ

•

+ L

OOH + L

•

etc. L

OOH (2c)

Propagation reactions establish the free radical chain

and keep it going. In this stage, lipid alkyl radicals, L



react with oxygen almost instantaneously to form peroxyl

radicals, LOO



, that are major players in lipid oxidation.

Peroxyl radicals very selectively abstract hydrogens from

adjacent lipid molecules, forming hydroperoxides and

generating new L



radicals in the process. Each new L



radical in turn repeats the process to provide the driving

force in the chain reaction. Chain propagation continues

indeﬁnitely until no hydrogen source is available or the

chain is intercepted.

Initiation (formation of ab initio lipid free radical)

Propagation

Free radical chain reaction established

etc.

Free radical chain branching (initiation of new chains)

HOH

OOH + L

•

+ O

•

+ L

•

+ L

OOH

+ L

•

OOH

+ L

•

OOH

•

+ L

OOH

•

+ L

OOH

•

LOO

•

OOH

•

+ H

(oxidizing metals)

•

+ OH

–

(reducing metals)

•

OH (heat and UV)

OH + L

•

Termination (formation of nonradical products)

Radical recombinations

polymers, nonradical monomer products

(ketones, ethers, alkanes, aldehydes, etc.)

Radical scissions

nonradical products

(aldehydes, ketones, alcohols, alkanes, etc.)

CLASSICAL FREE RADICAL CHAIN REACTION MECHANISM OF LIPID OXIDATION

Catalysts Inhibitors

Dark, filters

Refrigeration

Metal chelators

quenchers

Dark, filters

Refrigeration

Metal chelator

Water

Light

Heat

Radicals

Metals

Oxygen

Metals

Light

Heat

>~0.3

Heat

High pO

Water

Protic

solvents

Vacuum

Low pO

Radical

scavengers

Radical

scavengers

Radical

scavengers

Low pO

Dry systems

Neat oils

OOH

Figure 1. The stages and individual reactions involved in lipid oxidation as described by the classical free radical chain reaction, and the

catalysts and inhibitors associated with each stage.

660 LIPID OXIDATION: CHEMICAL STABILIZATION

Chain branching

OOH

•

+ OH

–

(3a)

OOH L

•

+ H

more chain

reactions

(3b)

OOH L

•

OH (3c)

Branching reactions are a second stage of propagation

in which the radical chain reaction is expanded, establish-

ing new chains at faster rates. In basic propagation (Reac-

tion 2), peroxyl radicals are the only chain carriers; they

react rather slowly (k = 36–62 L mol

1

) and quite speci-

ﬁcally, and the chain continues one abstraction at a time

from the initiation point. Without forces that decompose

hydroperoxides, this process can continue indeﬁnitely

while hydroperoxides accumulate. However, UV light,

moderate heat, metals, or heme compounds decompose

hydroperoxides to give alkoxyl and peroxyl radicals plus

ions (metals) or alkoxyl radicals and hydroxyl radicals (UV

light and heat), all of which initiate new free radical chains

that branch off of and expand the original chain. Hence,

the net effect of LOOH breakdown is a transition in

mechanism and kinetics. Lipid oxidation gathers steam,

increasing in rate and extent as LO



becomes the domi-

nant, faster chain carrier (k =10

–10

Lmol

1

) and

secondary chains dramatically amplify and broadcast lipid

oxidation. A single initiating event can thus lead to

sequential oxidation of literally hundreds of molecules in

the primary chain and in secondary branching chains.

In early stages of lipid oxidation while hydroperoxides

are just beginning to accumulate, the hydroperoxide de-

compositions occur monomolecularly (Reaction 3). As oxi-

dation progresses and hydroperoxides accumulate,

decomposition shifts to bimolecular mechanisms that

convert stable or slowly reacting species to multiple highly

reactive radicals:

2 LOOH LOOH…HOOL LO

•

O+

•

OOL (4)

LOO



þ LOOH ! LO



þ epoxy  LO



OH ð5Þ

The initial effect of bimolecular decomposition is a dra-

matic acceleration of lipid oxidation, but eventually a

point of net LOOH decomposition is reached. At that

point, termination reactions such as radical recombina-

tion or alkoxyl radical scission take over, and oxidation

slows, stable secondary products form, and off-ﬂavors and

odors become detectable.

Termination

Termination implies that a process is coming to a close.

For lipid oxidation as a process, ‘‘termination’’ is a mis-

nomer because lipid oxidation never fully stops. A speciﬁc

radical may be terminated as it forms some product, but it

leaves behind another radical so the chain reaction con-

tinues. Net oxidation slows down when radical quenching

processes exceed the rate of new chain production, and the

momentum of oxidation shifts from radical propagation

and chain expansion to generation of stable products. To

reﬂect this shift, ‘‘termination’’ in the discussion below

refers to stopping an individual lipid radical, not the

overall reaction. The cumulative effect on a reaction

system such as food is determined by the number of

radical chains being terminated.

Lipid free radicals terminate to form nonradical pro-

ducts primarily by three mechanisms:

(a) Radical recombinations

(b) Scission reactions when proton sources (e.g., water)

are present to stabilize products

The mechanisms dominating in a given system are inﬂu-

enced by the nature and concentration of the radicals, the

temperature and oxygen pressure, and the solvent.

Radical Recombinations. The limitless number of varia-

tions possible for radical recombinations is one reason for

the broad range of oxidation products detected in lipid

oxidation. Despite the possibilities, recombinations are

not random, and distinct patterns of favored recombina-

tions have been identiﬁed. Some of the most important

recombinations and their products are

Alkyl radicals ðL



þ L



Þ!alkane polymers ð6aÞ

Peroxyl radicals ðL



þ L



! alcohols and ketones; alkyl peroxides

ð6bÞ

Alkoxyl radicals ðR



þ R



! ethers; ketones; alkanes

ð6cÞ

ðR



þ R



Þ!alkyl peroxides; ketones; alcohols ð6dÞ

Temperature and oxygen pressure are key determi-

nants of radical recombination pathways (Figure 2). L



reactions dominate under low-oxygen (pO

= 1 to about

80–100 mm Hg) and high-temperature (reduced O

solubi-

lity) conditions. High pO

and lower temperatures favor

LOO



reactions. LO



contributions to the product mix

dominate in secondary stages of oxidation and at moder-

ate temperatures and oxygen pressures when LOOH or

LOO



decompositions are faster than their formation.

Radical recombinations are diffusion- and concentration-

controlled, so they are favored in neat oils; they decrease

with lipid dilution.

Formation of dimers and polymers increases viscosity

in oxidized oils. Perhaps just as importantly, alcohols,

ketones, ethers, and so on, are volatile compounds and

ﬂavor components that augment those produced in scis-

sion reactions and provide the undertones and secondary

notes that round out ﬂavors. Ketones and dialkyl perox-

ides, in particular, are unique to recombination reactions.

Scission Reactions of LO



. Of all the pathways active in

lipid oxidation, scission of alkoxyl radicals probably has

the greatest practical consequences for any kind of con-

sumer product because the products generated are

LIPID OXIDATION: CHEMICAL STABILIZATION 661

(mm Hg)

ROO

•

rxs.

•

rxs

1 10 100

% Termination

•

rxs

(mm Hg)

ROO

•

/ R

•

1.0

20 40 60 80

45°C

35°C

25°C

Figure 2. Effects of oxygen and temperature

on termination processes in lipid oxidation.

[Redrawn from Labuza (1) and Schaich (2).

Used with permission.]

Table 1. Typical Scission Products Produced from Important Oxidized Polyunsaturated Fatty Acids Undergoing

Oxidation and Associated Off Flavors

Product Classes Carbon Chain Lengths Major Products Associated Flavors

Oleic Acid

Hydrocarbons 6–8 Nonanal Fresh, citrus

Alkanals 2–4, 5, 6, 7, 8, 9, 10, 11 Octanal Fresh, citrus

2-Alkenals 6–9, 10, 11 Undec-2-enal Sweet, fatty

Acids 1, 6, 7, 8, 9 Undecanal Fatty

Alkanols 5–8 2-decenal Tallowy

Alkylformates 2–8

Linoleic Acid

Hydrocarbons 3–5 Hexanal Green-fruity, bitter almond

Alkanals 3, 4, 5, 6, 7, 8 2,4-Decadienal Fried

Alkenals 7, 8, 9, 10 2-Octenal Woodbugs-fatty, nutty

Dienals 9, 10 2-Heptenal Fatty-putty, bitter almond

Oxo-alkanals 7–9 1-Octene-3-one Metallic moldy, mushroom

Ketones 7, 8 Pentanal Sharp, bitter almond

Alcohols 3, 4, 5, 6, 7, 8 Heptanal Oily putty, soapy, fruity

Acids 1, 5, 6, 7, 9

Esters 1, 6, 7, 8

Linolenic Acid

Hydrocarbons 1–3 2,4-Heptadienal Rotten apples, fried

Alkanals 1, 2, 3, 6 3-Hexenal Green leaves, green tomatoes

Alkenals 4, 5, 6, 7 Propanal Sharp, irritating

Dienals 7–9 2,4,7-Decatrienal Fishy

Trienals 10 2-Pentenal Sharp, painty, green

Ketones 5, 18 Octadienal

Alcohols 3, 4, 5, 6, 7, 8 Pentene-3-one Sharp, ﬁshy

Acids 1, 5, 6, 7, 9 Octadiene-2-one Fatty-fruity

Esters 1, 6, 7, 8

Oxo-alkanals

Arachidonic Acid

Alkanal 2, 6, 7 Hexanal Green-fruity, bitter almond

Alkenal 7, 8, 9, 11 2,4-Decadienal Fried

Dienal 9, 10, 11, 12 2,4,7-Tridecatrienal

Ketones 2-Heptenal Fatty-putty, bitter almond

Alkanes 5, 6 2-Octenal Woodbugs-fatty, nutty

Aldehyde esters 4, 5 Pentanal Sharp, bitter almond

1-Octen-3-one Metallic, moldy, mushroom

4-Decenal Tallowy

3,5-Undecadien-2-one

2,6-Dodecadienal

662 LIPID OXIDATION: CHEMICAL STABILIZATION

responsible for the distinctive volatile off-odors and off-

ﬂavors associated with rancidity. Lipid alkoxyl radicals

undergo scission of the C–C bond on either side of the LO



group to yield a mixture of carbonyl products (typically

aldehydes, alkanes, and oxo-esters from the initial alkoxyl

radicals) and free radicals:

–CH–R

-COOH R

•

+ CH–R

-COOH

CH +

•

-COOH

(7)

•

βα

Unsaturated radical fragments oxidize still further and

undergo secondary scissions in similar fashion to produce

carbonyls and alkanes of still shorter chain length. Con-

sequently, the mix of breakdown products that accumulate

in oxidized lipids can become quite complex, as shown in

Table 1 for the four major unsaturated fatty acids.

Scission requires a strong proton donor such as water

to stabilize the products and drive the reaction forward.

Scission is a minor process in neat lipids at room tem-

perature. However, in the presence of water (e.g., in

emulsions), acid, or other polar solvents, scission is rapid

and competitive with hydrogen abstraction, accounting for

about half of alkoxyl radical reactions, even in early

oxidation. Scission is also favored in dilute solution of

lipids where there is reduced competition from hydrogen

abstraction. However, because LO



scission has a large

activation energy and Arrhenius factor, the greatest con-

tribution of this termination (and propagation) reaction is

at elevated temperatures. Heat accelerates alkoxyl radical

scissions in all solvents, although the pattern of cleavage

may change as temperature increases.

The most notorious scission product of lipid oxidation,

malondialdehyde (MDA), arises from multiple scissions

of cyclic internal hydroperoxides formed in fatty acids

with three or more double bonds (linolenic and higher).

MDA formation is facilitated in aprotic solvents and by

low lipid concentrations, limited oxygen pressures, and

photosensitized oxidation; and it requires conditions for

endoperoxide cleavage, namely, mild heat and acid. In

autoxidizing lipids, yields of authentic MDA are usually

less than 0.1%; but in photosensitized fatty acids where

internal hydroperoxides are formed in high concentra-

tions, MDA concentrations can reach 5% or higher. Mal-

onaldehyde is probably the most common measure of lipid

oxidation in complex food systems, but its analysis is quite

often misused and misinterpreted.

Co-oxidation of Nonlipid Molecules. Hydrogen abstrac-

tions by lipid alkoxyl and peroxyl radicals are not selective

for fatty acids. Nonlipid molecules such as amino acids,

proteins, nucleic acids, antioxidants, carotenoids and

other pigments, carbohydrates, and even vitamins also

have abstractable H’s and thus can serve as substitutes for

fatty acids, intercepting the lipid free radical chain. This

process terminates lipid oxidation chains, but reroutes

radicals and oxidation to proteins and other biomolecules.

As in lipids, these radicals add oxygen to form peroxyl

radicals that may also abstract H’s from other molecules

or lead to oxidative degradation of the molecular target,

whatever class of compounds it may be. In this way, lipids

serve to ‘‘broadcast’’ oxidation damage to other molecules

which then provide footprints of lipid oxidation in foods

and biological systems.

Radical transfer to nonlipid molecules has several

critical consequences. Co-oxidation reactions alter lipid

oxidation kinetics, mechanisms, and overall effects in

foods and biological systems. Immediately, there is an

apparent antioxidant effect in terminating individual lipid

radical chains and slowing accumulation of measurable

oxidation products. However, if initiation is not concur-

rently blocked, what appears to be inhibition of lipid

oxidation is false: Detectable oxidation products are

merely shifted from lipids to other molecules, but ab initio

generation of new chains by initiators continues. If tradi-

tional lipid oxidation products such as peroxide values

and aldehydes are analyzed, oxidation appears to be

very low because radicals are transferred before lipid

products are formed. However, production of off-ﬂavors,

browning, texture changes, nutritional losses, etc., con-

tinues, now mediated by other molecules, particularly

proteins:

In some co-oxidations, lipids add to the target mole-

cules, particularly proteins. This limits extractability of

LH L

•

LOO

•

LOOH + L

•

etc (8a)

Protein

•

Protein-OO

•

Protein-OOH (8b)

Protein

Amino acid loss

Crosslinking and fragmentation

Inactivation of enzymes

Loss or alteration in functionality

Browning

LIPID OXIDATION: CHEMICAL STABILIZATION 663

lipids for analysis and removes lipids from product

streams normally analyzed, compounding problems

in assessing the true extent of lipid oxidation. Conse-

quently, in systems where co-oxidations are active, lipid

oxidation must be followed by loss of fatty acids in addition

to oxidation products such as peroxides, malonaldehyde,

and anisidine values. Footprints of lipid oxidation in

the form of speciﬁc co-oxidation products or molecular

property changes (e.g., protein crosslinking, amino acid

loss, degradation of functionality) must also be tracked

to avoid severely underestimating both the extent and

the effects of lipid oxidation in complex systems.

Time Course of Lipid Oxidation

Lipid oxidation is often divided into three rate periods

described in terms of hydroperoxide reactions:

1. Induction period—very low level oxidation, forma-

tion of LOOH undetectable.

2. Monomolecular rate period—initial stages of

oxidation up to B1% oxidation, LOOH accumulate

slowly and decompose as single, isolated mole-

cules when exposed to UV light, metals, or heat

(Reaction 3).

3. Bimolecular rate period—later stages of lipid oxida-

tion up to B7–15% oxidation, LOOH accumulate

rapidly, begin to decompose in pairs via LOOH

(Reaction 4) or LOO



+ LOOH (Reaction 5), and

secondary products form.

The integration and progression of individual reactions

of initiation, propagation, and termination into an overall

oxidation process with these three rate periods is shown

in Figure 3. During the induction and early monomolecu-

lar period, LOO



is the main chain carrier and propaga-

tion is relatively slow. As active oxidation develops

and LOOH decompose, both LOO



and LO



contribute to

propagation. However, LO



reactions are much faster

and more speciﬁc, so LO



becomes the dominant

chain carrier and controls directions of reactions in late

monomolecular and throughout the bimolecular rate

periods, partially accounting for increasing oxidation

rates.

FACTORS AFFECTING LIPID OXIDATION: PRO-

OXIDANTS

Many factors alter rates of lipid oxidation. Some of the

most common catalysts and inhibitors are:

(a) Nature of lipids: number of double bonds

i. Conﬁguration of double bonds (trans versus cis

and conjugation)

ii. Level and type of phospholipids present

iii. Free fatty acids versus esters versus

triacylglycerols.

(b) Surfaces: Bulk oil – exposed surface versus emul-

sions versus dispersal on solid surface

i. Pro-oxidants

. Preformed hydroperoxides

. Metals

. Porphyrins, chlorophylls, heme compounds

. Lipoxygenase and cyclooxygenase

. Amino acids

. Ascorbic acid (low concentrations)

ii. Antioxidants (endogenous and added)

. Polyphenols

. Amino acids

. Metal chelators and complexers

. Synergists

. Glutathione peroxidase

iii. Interceptors

. Proteins

. DNA

. Vitamins

. Pigments

(d) Environment and solvent system: temperature,

light, oxygen pressures, water, pH, packaging.

Of these, light, oxygen pressure, metals, water, chela-

tors, and antioxidants are amenable to control by

packaging.

Light

Light is a key catalyst of lipid oxidation in foods and

cosmetics, yet is often discounted during handling, proces-

sing, and even storage. At low wavelengths (oB254 nm),

Time of reaction

Extent of reaction

LH L

•

LOO

•

2 LOOH LO

•

+ LOO

•

LOOH

Secondary

oxidation processes

LOO

•

products

LOOH, epidioxide

•

+ OH

–

•

Figure 3. Changes in dominant lipid oxidation reactions

and products over the course of lipid oxidation. Induction

period (I), monomolecular rate period (M), and bimolecular

rate period (B). [Modiﬁed from Schaich (2). Used with

permission.]

664 LIPID OXIDATION: CHEMICAL STABILIZATION

ultraviolet light has enough energy to break bonds and

form L



radicals, although most uv damage to lipids occurs

at wavelengths o200 nm. Much more active is UV-in-

duced decomposition of hydroperoxides to generate two

reactive radicals that rapidly propagate the radical chain:

ROOH RO

•

OH ROH (9a)

HOOH HO

•

OH 2 L

•

+ H

O (9b)

LOOH LO

•

OH LOH (9c)

hν(uv)

(ROOH is any organic hydroperoxide, HOOH is hydro-

gen peroxide, LOOH is a lipid hydroperoxide).

Hydrogen peroxide is actively generated in foods by

metal autoxidation, but can also be produced by UV

degradation of aqueous solutions. Light decomposition of

hydroperoxides is a major contributor to destabilization of

stored foods, particularly under ﬂuorescent lights in la-

boratories, storage facilities, and stores.

Visible light (W400 nm) initiates lipid oxidation indir-

ectly through photosensitizers, molecules that absorb

light and transfer the excitation energy to molecular

bonds in lipids to form free radicals directly (Type 1) or

to oxygen to form singlet oxygen

, which then adds to

double bonds and forms hydroperoxides in unsaturated

fatty acids without intermediate radicals (Type 2). Photo-

sensitizers in foods and biological materials are usually

molecules with multiple carbonyls and/or an extended

conjugated double bond system. Most pigments and other

molecules listed below have this required structure.

Chlorophyll, in particular, is a very potent photosensitizer

that produces both free radicals and singlet oxygen, with

the dominant reactions depending on substrate and reac-

tion conditions. Food dyes are listed with a question mark

because many food colors have requisite structures that

make them good candidates for sensitization, but their

action has not yet been documented.

Sensitizers

Quenchers

Type 1: Free Radical

Chlorophyll

Pheophytin

Hemes

Protoporphyrin

Myoglobin

Hemoglobin

Flavins (especially

riboﬂavin)

Xanthenes

Anthracenes

Anthroquinones

Crystal violet

Food dyes (?)

Phenols, polyphenols

Tocopherols

(carotenes)

Type 2: Singlet

Oxygen

Chlorophyll

Hemes

Erythrosine

Rose bengal

Flavins

Methylene blue

Proﬂavine

Eosin

Food dyes (?)

Carotenes

Tocopherols

Quenchers are antoxidant compounds that can selec-

tively react with light-induced radicals and singlet

oxygen, thereby preventing their reaction with lipids

and other biomolecules. Carotenes are important

scavengers in foods. They are also radical scavengers,

but listed in parentheses because in high concentrations

they can become pro-oxidants.

Light catalysis is nearly always present, but can be

veriﬁed by comparison of samples incubated open versus

in the dark, by oxidation kinetics, and by presence of

speciﬁc photolysis products. Type 1 photosensitization is

qualitatively indistinguishable from autoxidation, but

much faster. It characterized by the following patterns:

Mechanism:

Kinetics:

Products:

Free radical

Induction period present

Dependent on pO

Rate not p number of double bonds

Relative reactivity:

18:1 B1

18:2 17

18:3 25

Conjugated dienes formed with LOOHs

in external C-9, C-13, and C-16

positions

Scission products all from external

LOOHs

Type 2 singlet oxygen photosensitized oxidation has dis-

tinctively different oxidation patterns:

Mechanism:

Kinetics:

Products:

‘‘ene’’ rx, concerted addition of O

double bonds to form LOOH without

free radicals; radicals are generated

subsequently by LOOH decomposition

No induction period

Independent of O

(when O

not limiting)

Dependent on sensitizer concentration

Rate directly p number of double bonds

Relative reactivity:

18:1 B1

18:2 2

18:3 3

Both nonconjugated and conjugated

LOOHs

LOOHs at all positions, high proportion

at internal positions (e.g., C-10 and

C-12)

High proportion of internal cyclics, en-

doperoxides, epoxides, di- and tri-hy-

droperoxides (all are precursors of

light-induced off-ﬂavors)

Scission products from internal LOOHs,

altered scission position preferences

When these kinetic characteristics and products are

observed in products, light-mediated initiation can be

presumed, and protection from light during processing

and with packaging will be critical for stabilization.

Indeed, blanking out light (e.g., by packaging) prevents

both photosensitized generation of LOOH and UV decom-

position of LOOH and thus has huge immediate effects on

LIPID OXIDATION: CHEMICAL STABILIZATION 665

kinetics and consequences of lipid oxidation. However,

caution is required when deducing causes of oxidation or

mechanisms of a given agent from product mixes and

kinetics because many photosensitizers change mechan-

isms with solvent and concentration. For example, ribo-

ﬂavin normally photosensitizes by free radicals but at low

concentrations it converts to the

mechanism. Thus,

sensitivity to light often changes with product formulation

and phase localization and distribution of photosensitizers

and quenchers. For example, cheese will behave differ-

ently than milk under light, even though the fat and

sensitizer composition is similar for each. This needs to

be kept in mind when tracking and comparing causes of

oxidation in different oils, foods, and cosmetic products

and planning protection strategies.

Oxygen Pressure

Lipid oxidation requires oxygen, so removal or limitation

of oxygen has been a traditional approach to inhibiting

lipid oxidation in packaged processed foods. Nevertheless,

effects of oxygen are evident mostly at low oxygen pres-

sures and in early stages of lipid oxidation, so use of low

oxygen to control lipid oxidation must focus on a rather

narrow reaction window.

Since oxygen does not react with lipids directly, lipid

oxidation depends critically on the concentration and

reactivity of initiators that produce reaction sites for

oxygen, either L



by conventional initiators or

photosensitization, particularly in the low pO

range

(1 to 80–100 mm Hg). Once reaction sites are available,

oxidation rates increase dramatically as pO

increases

(Figure 4), but the catalytic effect of oxygen levels off at

B 100 mm Hg or when [O

]W[LH] and [L



] (exact level

depends on reaction system). Similarly, once enough radi-

cals are present to keep chain reactions going, formation

of new LOO



becomes much less important than hydrogen

abstraction to propagate chains, and the effect of in-

creased oxygen on rate reaches a steady-state equilibrium

as oxidation progresses.

Kinetic rate equations show the complex relationship of

oxygen to lipid oxidation rate. At low pO

, the rate of lipid

oxidation may be expressed simplistically as

Rate of oxidation ðlow pO

¼ k

ðk

1=2

½LOOH½O

ð10Þ

where rate constants refer to L



oxygenation, initiation,

and termination by L



recombinations, respectively. Dur-

ing early oxidation (monomolecular LOOH decomposi-

tion), the rate-limiting step is formation of L



and

peroxyl radicals, which requires oxygen. As oxidation

progresses and LOOH increases, propagation reactions

(i.e., hydrogen abstractions by LOO



and by LO



from

LOOH decomposition) become the dominant rate determi-

nants and the relative effect of oxygen diminishes.

At high pO

, more than enough oxygen is present to

add to every L



formed, so the rate dependence shifts from

oxygen to the concentration of available oxidizable lipids

and propagation processes:

Rate of oxidation ðnonlimiting pO

¼ k

ðk

1=2

½LOOH½LH

ð11Þ

The overall rate then is determined by the balance be-

tween initiation (k

) and peroxyl radical recombinations

), and it is proportional to the total amount and degree

of unsaturation of available lipid plus propagating LOOH

concentrations. The rate-limiting step now is abstraction

of hydrogens to form LOOHs (k

Hence, low oxygen needs to be combined with mini-

mization of initiators and catalysts to maximize inhibition

of lipid oxidation.

Metals

Redox-active metals are probably the most important

initiators and catalysts of lipid oxidation in oils, foods,

and biological systems because they are ubiquitous and

active in many forms, they exert critical inﬂuence on both

initiation and propagation, and trace quantities (nanomo-

lar) are sufﬁcient for effective catalysis. Only metals

undergoing one-electron transfers (e.g., iron, copper,

nickel, cobalt, vanadium, titanium, cerium) are active

catalysts. Metals in their higher valence (M

n+1

) states

are primary initiators: They form L



both directly by

oxidizing double bonds in unsaturated fatty acids (Reac-

tion 12, RCHQCHR is in a lipid chain) and indirectly by

oxidizing other molecules to produce radicals that abstract

hydrogens from unsaturated lipids LH (Reaction 13, R =

lipid or any other oxidizable molecule).

RCH¼CHR þM

ðnþ1Þþ

! RC



HC

HR þ M

nþ

!



þ RH

ð12Þ

RH þM

ðnþ1Þþ

! R



þ H

þ M

nþ

!



þ RH ð13Þ

Metals in their reduced forms (M

) —for example,

and Cu

—are not good initiators themselves, but

they form reactive species by complexing with oxygen

0 200 400 600 80

Oxygen pressure (mm Hg)

Rate of oxidation (arbitrary units)

Target range for packaging

Figure 4. Effects of oxygen on the rate of lipid oxidation. Rate of

oxidation increases rapidly with increasing pO

when oxygen is

limiting but is not affected by additional oxygen when [O

]W[LH].

[Figure generalized from data of Bolland, (3).]

666 LIPID OXIDATION: CHEMICAL STABILIZATION

(Reactions 14a–c) and by reducing oxygen to superoxide

anion O





, which dismutates to hydrogen peroxide and

very reactive hydroxyl radicals (Reactions 14d–f).

nþ

þ O

! M

nþ1

þ O



ð14dÞ



or O



=HOO



! H

!

nþ



þ OH



ð14eÞ



þ LH ! H

O þL



ð14fÞ

Metals also decompose any traces of preformed hydro-

peroxides ROOH present in foods (in lipids or any other

molecule) to alkoxyl radicals (RO



) (Reaction 15a), and

higher valence state metals oxidize hydroperoxides to

peroxyl radicals (ROO



) (Reaction 15b); both of these

radicals initiate lipid oxidation chains. Note that unlike

UV reactions, metal decomposition of hydroperoxides re-

leases only a single reactive radical that reacts with lipids

to generate L



2þ

þ ROOH !

fast

3þ

þ RO



þ OH



!

ROH þL



ð15aÞ

3þ

þ ROOH !

extremely slow

2þ

þ ROO



þ H

!

ROOH þ L



ð15bÞ

When both reduced and oxidized metals are present,

catalysis of lipid oxidation is ampliﬁed tremendously by

redox cycling (Reaction 16). Redox cycles occur slowly with

metals and hydroperoxides alone, but rates can be accel-

erated several orders of magnitude in the presence of

appropriate reducing agents, particularly ascorbic acid,

that maintain the faster reacting lower valence state in

metals:

n+1

+ ROOH M

+ ROO

•

+ H

(16a)

ROOH

n+1

+ RO

•

+OH

–

(16b)

The mere presence of metals is not sufﬁcient to guar-

antee catalysis of lipid oxidation. In foods and other

products, metals are complexed to a wide variety of

molecules, including organic acids, carboxylic acids, vic-

inal diphenols, amino acids, and proteins. How the metals

are bound controls the rate of initiation by altering the

availability of orbitals for electron transfer, the redox

potential and oxidizing or reducing capacity, partitioning

between aqueous and lipid phases, and associations with

other molecules. In general, higher redox potentials make

metal complexes better primary initiators, and low redox

potential metal complexes markedly enhance propagation

branching reactions. Selection of appropriate chelators

can thus greatly modify metal reactivity in foods.

Heat

At frying temperatures, there is sufﬁcient thermal energy

to break covalent C–C or C–H bonds in the acyl backbone

and form a variety of lipid alkyl radicals which then start

the radical chains of oxidation. At moderate temperatures,

heat exerts its main effects on processes that have high

activation energies, so they can beneﬁt most from a

thermal boost. The rate constants in the list below refer

to propagation, termination, and LOOH dissociation in

sequence. As can be seen, most steps of lipid oxidation

require little or no activation; only monomolecular and

bimolecular decompositions of hydroperoxides have en-

ergy hurdles. Thus, in early stages of lipid oxidation, low

to moderate heat breaks O–O bonds in traces of ROOH or

LOOH preformed by other reactions, particularly metals,

lipoxygenase, or photosensitizers. The RO



,LO



, and



thus generated abstract hydrogens from neighboring li-

pids to form L



and initiate radical chains. As oxidation

progresses and LOOH accumulate, the major effect of heat

shifts to acceleration of propagation rather than initiation.

Reaction



)

(LOO



+ LH)

(2 ROO



)

(2 R



)



+ ROO



)

(monomolecular)

(bimolecular)

Activation Energies (E

)

0 kcal/mol

B5–15

50 uncatalyzed system

Accompanying faster oxidation kinetics is a heat-in-

duced shift in dominant reaction pathways and resulting

products that markedly affect food and cosmetic quality.

The most important changes include:

. Abstraction of hydrogens from LOH and LOOH in

preference to LH, shifting dominant products from

volatile a and b scission products to W 90% dimers

and polymers.

. Shifts in types of dimers formed: b-scission of peroxyl

oxygen increases - more C–O–C and C–C dimers

and fewer C–O–O–C crosslinks.

. Altered scission patterns; for example, hexanal is

major product of L under mild oxidation conditions,

but 2,4-decadienal becomes dominant when systems

were heated.

•

+ [M

(n+1)+

…

–

(n+1)+

…O

–

•

]

+ O

•

+ M

+ HO

•

(14b)

(14a)

•

+ [M

(n+1)+

…

–

OH] (14c)

LIPID OXIDATION: CHEMICAL STABILIZATION 667