REVIEW ARTICLE

Kinetics of Cholesterol Oxidation in Model Systems and Foods:Current Status

Ilce Gabriela Medina-Meza • Carlo Barnaba

Received: 20 February 2013 / Accepted: 8 June 2013 / Published online: 19 June 2013

� Springer Science+Business Media New York 2013

Abstract In the last decades, cholesterol oxidation

products (COPs) have been one of the most intense topics

in food science because of their biological and pathological

effects on human health. The formation of COPs during

food processing is a thermodynamically governed phe-

nomenon and, as for all chemical and biochemical reac-

tions, this formation can be controlled through a kinetic

monitoring of oxidative trends, in a manner similar to lipid

oxidation. The use of kinetics modeling as a powerful

predictive tool has increased recently, although its appli-

cation in lipid oxidation and cholesterol is poor. Despite

the abundance of data on the subject, as evidenced by the

presence of numerous reviews about the content of COPs

in different foods, contributions to the kinetic modeling are

isolated. Moreover, the application of this mathematical

approach often demonstrates numerous errors in method-

ology. This paper summarizes the scientific advances in

kinetic modeling of thermo- and photo-induced oxidation

of cholesterol. We briefly describe the reaction mecha-

nisms of both degradative pathways, with particular

attention to involucrate variables. Then, state of the art of

mechanistic models that are proposed is discussed in detail.

This analysis shows that it is necessary to broaden and

deepen the kinetic study of cholesterol oxidative phenom-

enon from a mechanistic perspective with a more specific

application of kinetic principles. The development of

effective predictive models may help to monitor COPs

during processing of the food and thus prevent their

accumulation in the final products.

Keywords Cholesterol oxidation � Kinetic models �Oxysterols � Lipid oxidation � Model system

Introduction

Cholesterol (5a-cholesten-3b-ol, Fig. 1) is an essential

biochemical compound in mammalian cells, representing

the most prominent lipid in eukaryotic cells. It is involved

in numerous body function tasks, ranging from the syn-

thesis of bile acids and hormones, to its different roles in

cellular membrane function [29]. Cholesterol is insoluble

in blood, and its effect in biological systems is determined

by the type and nature of lipoprotein complexes that carry

it through the bloodstream [60]. This molecule can be

synthesized de novo or obtained by diet, but it is also

widely distributed in foods of animal origin [31].

As with other lipid molecules, cholesterol is susceptible

to oxidation to form cholesterol oxidation products (COPs)

or oxysterols by heating [32, 45, 66], illumination [6, 46]

and enzymatic activity [3]. COPs are the derivatives of

cholesterol that contain a second oxygen atom as part of a

carbonyl, hydroxyl, ketone or epoxide group on the sterol

nucleus and/or a hydroxyl group on the side chain of the

molecule. Oxysterols usually occur at low levels, accom-

panied by a high concentration of the parent cholesterol.

Accumulation of oxysterols in the body can occur in sev-

eral different ways, the major ones being dietary intake and

internal chemical and enzymatic oxidation [54]. Oxysterols

are present in various foodstuffs, notably cholesterol-rich

I. G. Medina-Meza (&)

Center for Nonthermal Processing of Food, Washington State

University, Pullman, WA 99164-6120, USA

e-mail: ilce.medinameza@wsu.edu

C. Barnaba

Departamento de Procesos Tecnologicos e Industriales, Instituto

Tecnologico y de Estudios Superiores de Occidente (ITESO),

Periferico Sur Manuel Gomez Morın 8585, 45604 Tlaquepaque,

Jalisco, Mexico

123

Food Eng Rev (2013) 5:171–184

DOI 10.1007/s12393-013-9069-0

foods such as dairy products, milk, eggs, dried egg powder,

meat products, and dried or stored fish [31, 55].

These compounds have been shown to exert several

in vitro and in vivo biochemical activities of both physio-

logic and pathologic relevance [52, 54], which are medi-

ated by their biophysical effects on membranes and/or

stereospecific interactions with proteins [32]. COPs are

implicated in several pathophysiological mechanisms,

including atherosclerosis, lung disease, liver disease, car-

cinogenesis and neurodegeneration [3].

Due to the high pathological impact of these molecules

and the necessity to control their content and formation in

food processing, the mechanisms and kinetics of the for-

mation of COPs are of crucial importance. One of the

major objectives of food engineering is to develop pre-

dictive models that allow knowledge and control of the

variables that affect the production of certain compounds

harmful to the consumer [62]. Mathematical modeling is a

tool that allows the synthesis of data from one or many

experiments into an integrated system from which quanti-

tative changes in many components may be calculated. In

any mathematical model, the endpoint selected for shelf

life is critical. Most of the time, this endpoint is arbitrarily

selected, whereas it should be based on consumer accept-

ability. Moreover, it is a necessary tool for preventing its

effects on food. Generally, the equations used to model the

mechanisms of reactions allow us to obtain information

about kinetic factors such as constant rates and activation

energy necessary to degradation [18]. Many parameters

need to be considered during modeling, such as loss of the

inducer, use of antioxidants, auto-oxidation and hydroper-

oxides reduction. Consequently, the use of simulation

programs permits evaluation if the proposal model is in

agreement with the experimental data.

In the last few years, thorough reviews have covered

selected aspects of the formation of oxysterols and their

presence in foods [8, 31, 32, 59]; however, these reviews

do not provide a comprehensive analysis of the kinetic

models proposed in the formation of these compounds. In

this work, we have reviewed the effect different mecha-

nistic and empirical mathematical models used to describe

thermo-oxidation and photo-oxidation of cholesterol in

food and model systems. Particular attention has been paid

to the mechanisms involved and the effect of process

variables (presence/absence of antioxidant, temperature,

light source) in the formation of COPs, and how these

variables were included in the different predictive models

proposed.

Mechanisms of Cholesterol Oxidation

Auto-oxidation

The formation mechanism of COPs during auto-oxidation

and heat-induced auto-oxidation (also known as thermo-

oxidation or thermal degradation) has been well docu-

mented and illustrated [15, 37, 45, 56]. Auto-oxidation is

an endothermic phenomenon; degradation by auto-oxida-

tion occurs under a variety of circumstances, leading to a

galaxy of products [56]. Cholesterol auto-oxidation occurs

in various model systems, including aqueous emulsion,

micelles, liposomes and monomolecular films spread on

water, and can also occur in the crystalline state [32, 37].

Initially, a series of reactions lead to the formation of

reactive oxygen species (ROS), a group of molecules that

includes oxygen radicals and nonradical derivatives of

oxygen. This auto-catalytic process generates primary

oxidation products followed by further reactions leading to

a high variety of nonvolatile and volatile secondary oxi-

dation products [37]. The ROS are formed enzymatically,

chemically, photochemically and by irradiation of food and

decomposition of hydroperoxides. ROS are also formed by

decomposition and interreaction of ROS.

The oxygen radicals are alkoxy (RO�), hydroxyl (HO�),

hydroperoxy (HOO�), peroxy (ROO�) and superoxide anion

(O��2 ) radicals. Heat accelerates homolysis of hydroperox-

ides and produces peroxy radicals [16]. Nonradical deriv-

atives are hydrogen peroxide (H2O2), ozone (O3) and

singlet oxygen (1O2) [53]. The most reactive ROS is

hydroxyl radical followed by singlet oxygen. Lipid and

1

2

3

4

5

10

19

67

8

9

11 13

12

15

16

17

1820

2122

23

24

25

2627

14

Fig. 1 Chemical structure of cholesterol molecule

172 Food Eng Rev (2013) 5:171–184

123

cholesterol oxidation by ROS produces low-molecular-

weight volatile alcohols, aldehydes and hydrocarbons.

It has been shown that in the absence of other promoters

of oxidation, cholesterol is relatively stable up to temper-

atures below the melting point (148.5 �C) [15, 38, 45], so

the kinetic studies are generally carried out in the presence

of heat sources. In the next paragraphs, we will refer only

to heat-induced auto-oxidation (thermo-oxidation).

Oxidation usually takes place through a radical initiated

chain mechanism [34], involving the following steps:

Initiation RH! R�

R� þ O2 ! RO�2Propagation RO�2 þ RH! RO2Hþ R�

R� þ O2 ! RO�2Branching RO2H! RO� þ �OH

RO� þ OHþ O2 ! ROHþ RO�2�OHþ RHþ O2 ! H2Oþ RO�2

Inhibition InHþ RO�2 ! In� þ RO2H

In a cholesterol molecule, the reaction can be divided into

two major routes of C-7 oxidation and epoxidation (Fig. 2).

The first free radicals that are formed seem to be located in

positions C-7 and C-25, giving 7ab-hydroperoxycholesterol

(7ab-OOH) and 25-hydroperoxycholesterol (25-OOH). The

7-OOH epimers are the only ones to remain at room

temperature, while 25-OOH usually occurs in dry solid

cholesterol, and to a lesser extent in dispersion or solutions

[45]. During heating, 7-OOH epimers can be reduced and

give rise to the corresponding hydroxyl derivatives:

cholesterol-5-en-3b,7a-diol (7a-OH) and cholesterol-5-en-

3b,7b-diol (7b-OH). The formation of epoxy derivatives

occurs by bimolecular reaction mechanism. In fact, 5,6a-

epoxy-5a-cholestan-3b-ol (5a-epoxy) and 5,6a-epoxy-5b-

cholestan-3b-ol (5b-epoxy) are formed by the interaction of

a hydroperoxy radical and cholesterol [26].

Depending on the intensity and time of exposition, choles-

terol and its oxidation products can generate other oxygenated

compounds. Dehydration of 7-OOH and dehydrogenation of

7-OH lead to the formation of 3b-hydroxycholesten-5-en-7-

one (7-keto), as reported by Chien et al. [15].

The formation of COPs in positions other than C-7

during thermo-oxidation has been poorly addressed, espe-

cially the cholest-5-en-3b,4b-diol (4b-OH) and 3b-hydro-

xycholest-4-en-6-one (6-keto) [45]. According to Maerker

and Jones [43], the 3b-hydroxy function of cholesterol

could be oxidized to a 3-keto function, and the 5,6 double

bond could move to the 4,5 position, in conjugation with

the carbonyl group.

The hydration of epoxides in acid is initiated by pro-

tonation of the ring oxygen and is followed by an approach

of a nucleophile, in this case water, at the more highly

branched carbon from the opposite to that of the protonated

oxygen. In the a-epoxide, the rear side of C-5 is sterically

hindered by the angular methyl C-19, and nucleophilic

attack occurs not at C-5, but, reluctantly, at C-6, resulting

in the 5a-cholesta-3b,5,6b-triol (triol).

Oxidative attack at tertiary C-20 and C-25 positions

generates monohydroperoxides 20-OOH and 25-OOH and

their corresponding decomposition products [45], which

are mainly hydroxyl derivatives in position 20 (cholest-5-

en-3b,20a-diol, 20-OH) and 25 (cholest-5-en-3b,25-diol,

25-OH). These last are more stable and can remain for

6 months during continuous heating at 100 �C [58]. The

C-25 radical may form directly from cholesterol by a pri-

mary hydrogen abstraction reaction similar to that reported

at C-7, but it could also be formed by a radical transfer

reaction involving a radical already formed by reaction

with cholesterol [57]. Furthermore, the authors of this

paper found that the formation of 25-OH is strongly

dependent on the physical state of the sample. In anhydrous

cholesterol, the exposition of the aliphatic chain is favored,

resulting in a higher production of 25-OH [45].

Photo-oxidation

Cholesterol is known to be as susceptible to peroxidative

modification as other unsaturated lipids when exposed to

exciting light of suitable wavelength and molecular oxygen

[25]; however, the kinetics of cholesterol photo-oxidation

has not been sufficiently studied [46]. Chemical studies on

model systems have led to rapid advances in the under-

standing of a number of different photo-oxidation pathways

that can occur.

As discussed above for auto-oxidation (see ‘‘Auto-oxi-

dation’’ section), photo-oxidation processes can be initiated

by ROS. Among physical agents, near ultraviolet (UVA,

340–400 nm) and visible (400–700 nm) radiations are of

special interest in photo-induced oxidation. These, together

with appropriate photoexcitable compounds (sensitizers)

and ground state molecular oxygen (3O2) produce oxidative

injury through photodynamic action [21, 24].

Oxygen has two metastable molecular state excitements;

in the presence of light, 1O2 is typically generated by

energy transfer from a sensitizer in the relatively long-lived

triplet excited state (3S) to ground molecular state oxygen

(3O2). 1O2, wherein having high energy of 93.6 kJ above

the ground state triplet oxygen [24], reacts with lipids at a

higher rate than triplet oxygen. Since 1O2 is electrophilic

due to a completely vacant p2p orbital [25], it directly

reacts with high-electron-density double bonds via

6-membered ring without lipid radical formation [16]. The

lifetime of 1O2 is from 50 to 700 ls, depending on the

solvent systems of foods. The reaction temperature has

little effect on the oxidation rate of this nonradical

Food Eng Rev (2013) 5:171–184 173

123

derivative with foods due to the low activation of 0–6 kcal/

mol [20].

The major pathway for 1O2 formation in food is pho-

tosensitization [16], which can be initiated in the presence

of sensitizers. These last are pigments such as porphyrins,

carotenoids and chlorophyll, and dyes such as methylene

blue, eosin and curcumin [17], which can absorb energy

such as radiation and transfer it to the triplet oxygen, which

is less active than singlet oxygen. Riboflavin also acts as a

sensitizer [16]. Excited singlet state sensitizers return to

ground state via emission of light, internal conversion or

intersystem crossing. Fluorescence or heat is produced

from the excited singlet sensitizer by light emission or

internal conversion, respectively.

There are two different mechanisms of photoreaction

(Type I and Type II), one in which the primary interaction

of the triplet sensitizer is with oxygen, and the other in

which the interaction is with the substrate. The efficiency

of each path depends on the relative concentrations of

oxygen and substrate, the rates of reaction of sensitizer

triplet with substrate and with oxygen and the rate of triplet

decay [21].

Type I photoperoxidation is similar to ordinary lipid

auto-oxidation, with the exception of the mechanism of

primary oxidant generation. The excited triplet sensitizer

can interact directly with a substrate such as phenol or lipid

compounds by donating an electron or accepting hydrogen,

resulting in the production of free radicals. After initiation

of the free radicals (R�), the radical compound may react

with triplet oxygen via the free radical oxidation mecha-

nism [22]. The initiation of radical formation in the food

molecule will be at the site most liable for the loss of a

hydrogen atom. Each of these reactions pathways may give

rise to a large number of primary and secondary oxidation

Fig. 2 The formation pathways of some COPs during thermo-oxidation. The boxes indicate the kinetic routes studied by Chien et al. [15] and

Medina-Meza et al. [45]

174 Food Eng Rev (2013) 5:171–184

123

products. The excitation energy of the triplet sensitizer can

be transferred to triplet oxygen to produce singlet oxygen,

and the excited triplet sensitizer then returns to its ground

state (Type II) [16]. The rate of these two types of pro-

cesses depends on kind of sensitizers and substrates, con-

centrations of substrate and oxygen in the reaction

environments [10]. Type II photoperoxidation is consider-

ably less complex mechanistically than Type I; in general,

there are fewer products [25]. Both mechanisms are sum-

marized in Fig. 3.

Light promotes oxidation in lipids and steroids, such

cholesterol, especially in the presence of photosensitizers.

Medina-Meza et al. [46] reported that the mechanism of

cholesterol photo-oxidation presents both processes: Type

II photo-reaction may be divided into two major routes, the

C-6 oxidation and epoxidation, while in Type I it can be

observed two major pathways, the C-7 and the side-chain

oxidation (Fig. 4). The most reactive group of the choles-

terol side chain is the ternary carbon C-25. Thus, the for-

mation of alcohols in position C-20, C-24, C-25 and C-26

can occur via a direct free radicals reaction without any

intermediate forms being generated [56].

Cholesterol photo-oxidation begins with a sensitizer

absorbing energy from light and transferring it to triplet

oxygen to form singlet oxygen. Then, singlet oxygen reacts

with C-5,6 double bond, producing hydroperoxides

(OOH�), resulting the formation of 5-OOH and 6-OOH

[23]. It is well known that an allylic rearrangement of

5-OOH to epimeric 7ab-OOH [56] is more stable. Next,

hydroperoxides are reduced to diols, continuing the chain

reaction of free radical resulting in the 7-keto and other

species, such as epimeric 5,6ab-epoxides. However, other

mechanisms may also be operating in relation to further

oxidations of epimeric 7ab-OH in order to account for the

rather specific accumulation of 7-keto. The reaction rate of

cholesterol with singlet oxygen is 2.5 9 108 M-1 s-1 [33].

The hydration of epoxides in acid is initiated by pro-

tonation of the ring oxygen and followed by the approach

of a nucleophile. In the 5,6a-epoxide, the rear of C-5 is

sterically hindered by the angular methyl C-19, and

nucleophilic attack occurs not at C-5, but at C-6, resulting

in the formation of triol, which results in one hydroxyl

group being sterically hindered [45]. Isomers 6ab-OOH

give 5a-cholestan-3,6-dione (3,6 dione) and 4-cholestan-

3,6-dione, as results of dehydration via free radicals and/or

pyrolysis, having as intermediate product 3b-hydro-

xycholest-4-en-6-one.

It is known that in the A-ring, 3b-hydroxy function is

oxidized to a 3-keto function, and the 5,6-double bond may

move to the 4,5 position, in conjugation with the carbonyl

group; this process is promoted by the crystalline state of

cholesterol, probably due to the rigidity conferred by the

physical state [43, 46].

It is important to point out that the light source is of

great importance in the dynamics of photosensitization.

Most studies reported in the literature consider sources of

fluorescent light, with emission characteristics in the visi-

ble spectrum [5, 6, 14, 30, 40, 46]. The use of this type of

lamp simulates the situation present in retail shops [6].

COPs can also be formed in aqueous systems when cho-

lesterol is exposed to ionizing radiation. The products are

similar to those formed by auto-oxidation, but in different

relative amounts [35]. Maerker and Jones [42] reported that

cholesterol in aqueous environments exposed to gamma

radiation leads to generation of COPs in ratios different

than those produced by auto-oxidation. They stated that

7-keto and the isomeric 5,6-epoxides are the principal

products.

Kinetic Modeling of Cholesterol Oxidation in Model

and Food Systems

Despite the scientific impact of chemical kinetics in food

technology and engineering as a powerful predictive tool

against certain compounds (whether harmful or healthy),

its application in the oxidative processes is still poor.

Kinetic studies are important, since empirical and semi-

empirical rate expressions based on those studies are

obtained, which can then be used for predictive purposes to

Fig. 3 Scheme showing photo-

induced oxidation by Type I or

Type II chemistry (adapted from

[23])

Food Eng Rev (2013) 5:171–184 175

123

Fig. 4 Cholesterol oxidation products (COPs) generated via Type I and Type II photochemistry (adapted from [46])

176 Food Eng Rev (2013) 5:171–184

123

analyze a system’s dynamic behavior or to design chemical

reactors. As a result of the kinetic study, the authors have

composed a scheme of the mechanism of the process, as

well as analyzing it and comparing it with experimental

data in the literature, finding that there is a need for new

testing experiments, and if necessary supplementation of

the scheme and repeated checking [18]. The rate constants

can be calculated in different ways, such as empirically,

mechanistically and by performing molecular and quantum

level calculations.

There is a great deal of literature on the kinetics of

oxidation in oils and fats [7, 10, 49] as well as pigments

[20, 28], although the primary source of information refers

to works published a few decades ago [34]. The difficulty

lies in understanding the cascade of reactions and in the

limitations associated with the different methods of

assessing the degree of oxidation [7]. The study of complex

chemical reactions, such as lipid oxidation has shown that

most of those reactions are a composite of elementary steps

that involve very reactive intermediate species, known as

radicals. Generally, chemical kinetic studies in food sci-

ence do not use a macroscopic deterministic approach,

coupling differential equations (rate equations) and solving

them, sometimes analytically, but often by numerical

methods because of their complicated forms. This method

is based on the identification and quantification of each

species, which usually is not viable. Furthermore, in the

case of lipid oxidation, many intermediate products have a

limited half-life and are difficult to isolate. So, an alter-

native heuristic approach is usually used, defining a system

of few equations that represent the main reactions involved

during oxidation, or more frequently considering each

oxidative pattern as independent [34]. In this last case, the

food engineer’s goal is to find an empirical nth order rate

expression since such a form can often match the data

reasonably well within the experimental accuracy and is

easier to use than the complex rate form. Table 1 sum-

marizes the rate laws for zero, first and second order in

their differential and integrated forms.

In the next section, we will discuss the kinetic models

(empirical and mechanistic) proposed for the oxidation of

cholesterol in model systems, namely those in which the

cholesterol is alone, or combined with inhibitors/activators

of known nature. Then, we discuss the application of these

models in real systems, with a focus on their predictive

efficacy.

Thermo-oxidation

Kinetic Modeling in Thermo-oxidation of Cholesterol

The first proposals of a rigorous mathematical model of

cholesterol thermo-oxidation were conducted by Chien’s

group [11–13, 15]. Previously, Yan and White [65] studied

the kinetics of cholesterol oxidation during the heating of

lard and found that the reactions for the formation of 7-OH,

7-keto and 5,6-epoxy fit the first order. In contrast, in

another study, Park and Addis [51] reported that the reac-

tion for the formation of 7-keto during heating fits the zero

order. These two works will be discussed in full in

‘‘Applications in food systems’’ section. In 1996, Park et al.

[50] predicted chemical stability of 7-keto during hot

saponification at 45, 55, 65 and 75 �C using the Arrhenius

equation. Chemical reaction of 7-keto was described by a

pseudo-first-order reaction, because base reagent was in

excess. The rate equation for a first-order reaction is:

log½7� keto�½7� keto�0

¼ � k

2:303t

where [7-keto]0 is the initial concentration of 7-keto. The

study confirmed that higher temperature and prolonged

exposure promoted higher degradation of 7-keto. The cal-

culated activation energy for the reaction was about

113 kJ/mol.

Chien et al. [15] developed a mathematical model in

order to describe kinetic degradation of thermo-oxidized

cholesterol. Their model system consists of a microfilm

deposited on the inner surface of a flask; the authors did not

investigate the crystalline characteristics of sample

obtained, which could markedly influence oxidative pat-

terns, as speculated by Smith one decade before [56]. Due

to the complexity of COPs formation, they selected only

four major COPs (7-OOH, 7-OH, 7-keto and 5,6-epoxy),

dividing the reaction into two principal routes, C-7 oxi-

dation and epoxidation, considering these last as irrevers-

ible reactions. Another assumption made by the authors is

that oxygen concentration could be considered constant

and so can be despise in numerical analysis.

The results presented by Chien et al. [15] show that

cholesterol oxidation can be initiated by a second-order

reaction, and the first-order reactions follow afterward

(propagation). It is understood that the authors consider the

reactions as elementary, though they were not. For ele-

mentary reactions, the one-to-one correspondence exists

between order and molecularity of reaction. Unimolecular

reactions are first order; bimolecular are second order, etc.

Table 1 Summary of basic rate laws

Order Differential rate law Integrated rate law

0 dCi

dt¼ k Ci ¼ �kt þ C0

1 dCi

dt¼ kCi ln Ci ¼ �kt þ ln C0

1 dCi

dt¼ kCi Ci ¼ C0e�kt

2 dCi

dt¼ kC2

i1Ci¼ 1

C0þ kt

Food Eng Rev (2013) 5:171–184 177

123

[18, 34]. The system of differential equations that describe

the overall oxidative system considered is as follows:

d½Chol�dt

¼ �k1½Chol�½7� OOH� � k5½Chol� ð1Þ

d½7� OOH�dt

¼ k1½Chol�½7� OOH� ð2Þ

d½7� OH�dt

¼ k2½7� OOH� � k3½7� OH� ð3Þ

d½5; 6� epoxy�dt

¼ �k4½Chol� ð4Þ

d½7� keto�dt

¼ k3½7� OOH� þ k0

3½7� OH� ð5Þ

Equation 2 could be reduced to the logistic equation

proposed by Ozilgen and Ozilgen [49] to describe lipid

oxidation:

½7� OOH� ¼ ½Chol�ek1t

½Chol�½7�OOH�max

ð1� ek1tÞð6Þ

where [7-OOH]max is the maximum attainable value of

concentration at the end of the cholesterol oxidation pro-

cess for 7-OOH, k1 is the reaction rate constant (h-1) and t

is the time. Equations 1, 3–5 were integrated and resolved

by numerical analysis. If [7-OOH] � [7-OOH]max. The

equation can be rewritten as ½7� OOH� ¼ ½Chol�ek1t,

which is a first-order rate equation. Chien and coworkers

reported good correlation between experimental and theo-

retical data (R2 [ 0.80), especially for cholesterol degra-

dation and C-7 route.

Nonetheless, Chien’s approach seems to be incomplete,

considering that at least eight/ten different compounds are

found in considerable amounts during cholesterol degra-

dation in experimental conditions [45, 56], including both

7ab-OH and 5,6ab-epoxy isomers, as well as side-chain

oxysterols (20-OH, 25-OH). It is well known that during

auto-oxidation, the quasi-axial 7b-OH is produced more

abundantly than the quasi-equatorial a-isomer, due to steric

effect [41]. Both are transformed in 7-keto at different rates

[47]. Furthermore, after the C-7 position, ternary carbon

C-25 is the most susceptible to radical attack, followed by

C-20, giving their respective hydroperoxides 25-OOH and

20-OOH. These may be subject to a further degradation to

give 25-OH and 20-OH, respectively, which are currently

found in a great amount of animal products [31], as well as

dairy foods [59].

Medina-Meza et al. [45] studied thermo-oxidation of

cholesterol at 150 �C using a model system that consisted

of a thin crystalline microfilm deposited on a flask. The

hypothesis underlying the study was that the physical state

of cholesterol would markedly affect the oxidative pattern.

The cholesterol was re-crystallized in order to obtain

anhydrous crystals, as described by Loomis et al. [38]. The

authors observed that the solid state limited the mass

transference, with a lower diffusion of radical species and

availability of substrate. As a result, the kinetic profile was

deceleratory in the case of primary oxidation products

(hydroperoxides) during the initiation phase (0–60 min). In

the propagation phase, a rapid decrease in hydroperoxides

content was followed by the formation of different COPs.

Nine COPs were present at the end of the heat treatment:

7a-OH, 7b-OH, 5,6a-epoxy, 5,6b-epoxy, triol, 7-keto,

25-OH, as well as 4b-OH and 6-keto; these last two scar-

cely found in large amount in previous studies.

Barriuso et al. [1] recently realized a heat-induced oxi-

dative kinetic study of different steroids, among other

cholesterol. Similar to Chien’s work, they focused on 7ab-

OH and 5,6ab-epoxy epimers, 7-keto and triol. Although

expressed in the text, they did not propose a kinetic model,

but used nonlinear regression to fit experimental data,

considering each molecular formation as independent. The

rate constants thus obtained were not very indicative and

cannot be related to the reactions present during the oxi-

dative phenomenon. They used inverse, logarithmic and

exponential equation, obtaining good correlation between

data; even so the fitting parameters have no physical or

chemical significance.

As mentioned above, those models consider oxygen as

reactant in excess, thereby simplifying the kinetic system.

It is essential to mention that an effective model should

consider the concentration of oxygen as a key variable, as

well as the setup proposed by Bimberg [7]. This author

develops an oxidative model for fats, in which the oxida-

tion is characterized by the classic three stages (see ‘‘Auto-

oxidation’’ section), whereas the intake of oxygen is not

constant over time. Incidentally, the approach developed

by Bimberg, despite considerable interest, had no sub-

sequent applications.

Effects of Antioxidants in the Kinetics

One of the most effective methods to control lipid oxidation

is the incorporation of antioxidants [36, 44]. Antioxidants

work by a variety of different methods, including control of

oxidation substrates (for example, oxygen and lipids), con-

trol of pro-oxidants (for example, reactive oxygen species

and pro-oxidant metals) and inactivation of free radicals.

It is well established that c-tocopherol has a good

inhibitory effect in thermo-induced cholesterol oxidation,

retarding induction period, whereas a and d-tocopherol do

not [61]. Chien et al. [13] extended the proposed kinetic

model by including the antioxidant effect of a secondary

amine, stearylamine on cholesterol heating. Stearylamine

has a protective effect against oxidation of cholesterol,

since it reacts with the epoxy ring formed by the radical

178 Food Eng Rev (2013) 5:171–184

123

attack in a double bond, resulting in a stable amine product.

In this study, the cholesterol was solubilized in oil, and

thermo-oxidative study was carried out at 140 �C. The

results lend themselves to doubts about the effectiveness of

the proposed model, since the residual cholesterol found at

the end of kinetics is quite similar in both tests, although

the protective effect seems to be strong during the first 2 h

of treatment. This consideration is confirmed by the

unsatisfactory coefficient of determination (\0.80) for

most of the oxidative routes. In a subsequent work, Chien’s

group widened the kinetic model to triol (derived from

acidic hydration of isomeric 5,6ab-epoxy), studying the

antioxidant effect of quercetin (0.002 % w/w) on choles-

terol thermo-oxidation [12]. Quercetin has a marked ability

to scavenge highly reactive species such as peroxynitrite

and the hydroxyl radical, which are responsible for the

process of radical oxidation [4, 61]. On this occasion, a

good correlation is shown between theoretical and experi-

mental data. The principal outcomes of quercetin are in the

reduction in rate constants of the free radical chain reaction

of 7-OOH (from 453.5 h-1 to 117.7 h-1) and subsequent

dehydration of the last to form 7-keto (from 155.5 h-1 to

31.0 h-1). However, as in the case of stearylamine, the

effects are significant only for short periods of heat

treatment.

Finally, Yen et al. [66] developed a GC–MS analytical

method to determine COPs and conjugated linoleic acids

(CLAs) in a model system. CLAs are usually found in high

amounts in various meat products, whereas their occurrence

is scarce in vegetable oils and butter [2]. The role played by

CLAs as antioxidants is not clear, and there is still much

uncertainty about their actual scavenger properties [9, 68]. In

their work, they fitted kinetic curves with Chien’s model

(corresponding to a first-order integrated law, as shown in

Table 1), obtaining good correlation values. However, their

results are mathematically inconsistent, because the equa-

tion used by the authors in the kinetic modeling of COPs

refers to an exponential degradation.

Applications in Food Systems

Kinetic modeling is gaining increasing interest in food

science because it provides the possibility of controlling

changes in foods, i.e., to control food quality during pro-

cessing and shelf life [62]. If the development of kinetic

modeling in the thermo-oxidation of pure cholesterol is

limited, it will be even more so in more complex systems,

like foods. A high number of papers have been published

on the quantification of COPs in different food products,

and there are many reviews that regularly update the state

of the art [31, 59, 63]. Nevertheless, the kinetic approach is

rarely used, and even less so the application of mathe-

matical models.

The first attempt was made by Yan and White [65], who

studied the kinetics of cholesterol oxidation during the

heating of lard and found that the reactions for the formation

of 7-OH, 7-keto and 5,6-epoxy fit the first order. An analog

experiment with lard was carried out by Derewiaka et al.

[19], considering the geometry of the product, an essential

factor during any phenomenon of transference of energy, as

this can be a thermo-induced oxidation. Surprisingly, the

authors do not use the valuable information of Yan and White

about the reaction orders, but describe the change in the

dynamics of COPs using a second-degree polynomial.

In contrast, in another study, Park and Addis [49],

studying the thermo-oxidation of tallow at 155 and 190 �C

for 250 and 400 h, respectively, reported that the reaction

for the formation of 7-keto during heating fits the zero

order between certain temperature ranges, while choles-

terol presented an exponential decay behavior. The absence

of COPs at 190 �C leads the authors to hypothesize that at

an extremely elevated temperature, there may either be less

chance for cholesterol oxidation to occur, or more probably

for COPs to break down quickly after formation. The rate

of formations can be inferred from the graphics presented

in the article: for cholesterol degradation 3.2 9 10-3 h-1

and 2.4 9 10-3 h-1, at 155 and 190 �C, respectively,

whereas for 7-keto it is approximately 0.03 h-1 at 155 �C.

It is important to point out that in these early attempts the

authors used mechanistic models that consider each oxi-

dative step separately.

Ohshima et al. [48] studied the decomposition of cho-

lesterol in fish oils, approaching a real food system. This

study is interesting because the authors created a microfilm,

mixing crystalline microcellulose with cholesterol and fish

oil, which was subjected to auto-oxidation for a few weeks.

Despite the fact that the authors did not propose any kinetic

model, an accurate interpretation of their data suggests that

COPs formation (expressly, 7-keto, 7-OH and 5,6-epox-

ides) follows a hyperbolic behavior, similar to that

observed by Medina et al. [45].

Although the kinetic model proposed by Chien et al.

[15] (see ‘‘Kinetic modeling in thermo-oxidation of cho-

lesterol’’ section) could be used to predict the concentration

changes of main COPs during heating, its use was limited

to model system. Still, the literature does not report

applications in foods. In a recent work [67], the authors

prefer to use a simple exponential equation to fit the

experimental data obtained from thermo-oxidation of lard

in the presence/absence of CLAs at 150 and 200 �C.

Cholesterol degradation and COPs shows a first-order

kinetic at 150 �C, whereas at 200 �C the authors preferred

a pseudo-second-order equation:

dðCi=CmaxÞdt

¼ kf Cmax 1� Ci

Cmax

� �� �2

ð7Þ

Food Eng Rev (2013) 5:171–184 179

123

where Ci is the COPs concentration at time t, Cmax is the

COPs concentration at the equilibrium, kf is the formation

rate constant and t is the heating time. Equation (8) was

recently introduced for the description of adsorption kinetic

in liquid-phase adsorption systems [64]. The dimensionless

concentration, d(Ci/Cmax)/dt, is proportional to the square

of the residual amount of reactant, 1 - Ci/Cmax. Conse-

quently, the proportional constant kfCmax can be defined as

the second-order rate index [64].

The different kinetic approaches used for thermal cho-

lesterol degradation and single COPs formations are sum-

marized in Table 2.

Photo-oxidation

Kinetic Modeling in Photo-oxidation of Cholesterol

Cholesterol photosensitized oxidation has been studied

both alone and in the presence of pro- or antioxidants.

Considering the scarcity of works about oxidation kinetics,

in this section, we will deal with both situations. In two

similar works, Hu et al. [30] and Chien et al. [14] studied

the protective effect of riboflavin and fatty acid methyl

esters (stearic, linoleic and docosahexaenoic acids) on

cholesterol during illumination; a model system consisting

of an organic solution of cholesterol subjected to exposure

of three 40-W fluorescent tubes was used. The distance

between sample vials and the light source was about

50 cm, and the illumination intensity was 2,000–3,000 lux

at 25 �C for 28 days.

The authors determine the degradation rate constant of

cholesterol using the following equation, representing first-

order degradation:

k ¼ln½Chol�½Chol�0t

ð8Þ

where [Chol] is the concentration of cholesterol after light

exposure, [Chol]0 is the initial concentration of cholesterol

and t is the illumination time (days).

In the first study, Hu et al. determined the rate of deg-

radation of cholesterol (expressed in d-1) in coexistence

Table 2 Kinetic models proposed for thermal oxidation of cholesterol in food and model systems

Oxidation route Kinetic type Model Reference

Cholesterol degradation First order Tallow (155 and 190 �C) Park and Addis [51]

First order Lard (180 �C) Yan and White [65]

First order Lard (100, 150 and 200 �C, CLAs as antioxidant) Yen et al. [67]

First order Solid state, anhydrous (150 �C) Medina-Meza et al. [45]

C-7

7-OOH Second order Solid state (150 �C) Chien et al. [15]

Second order Solution (140 �C, stearylamine as antioxidant) Chien et al. [13]

Second order Solution (150 �C, quercetin as antioxidant) Chien et al. [12]

First order Lard (100, 150 and 200 �C, CLAs as antioxidant) Yen et al. [67]

Deceleratory Solid state, anhydrous (150 �C) Medina-Meza et al. [45]

7a,b-OH First order Lard (180 �C) Yan and White [65]

First order Solid state (150 �C) Chien et al. [15]

First order Lard (150 �C, CLAs as antioxidant) Yen et al. [67]

First order Solid state, anhydrous (150 �C) Medina-Meza et al. [45]

Pseudo-second order Lard (200 �C, CLAs as antioxidant) Yen et al. [67]

7-keto Zero order Tallow (155 �C) Park and Addis [51]

First order Lard (180 �C) Yan and White [65]

First order Solid state (150 �C) Chien et al. [15]

First order Lard (150 �C, CLAs as antioxidant) Yen et al. [67]

Pseudo-second order Lard (200 �C, CLAs as antioxidant) Yen et al. [67]

C-5

5,6ab-epoxy First Lard (180 �C) Yan and White [65]

Second Solid state (150 �C) Chien et al. [15]

Second Solution (140 �C, stearylamine as antioxidant) Chien et al. [13]

Side-chain

25-OH First order Solid state, anhydrous (150 �C) Medina-Meza et al. [45]

180 Food Eng Rev (2013) 5:171–184

123

with stearic acid methyl ester, linoleic acid methyl ester

and docosahexaenoic acid methyl ester, and obtained the

values 0.0322, 0.0152 and 0.0198, respectively, with high

correlation coefficients ([0.9). Regarding the rate of deg-

radation of cholesterol, in the presence of 50 ppm and

100 ppm of riboflavin, the constant values were 0.0655 and

0.0747 d-1, respectively. No degradation rate of riboflavin

was calculated because it was not detected after 1 day of

light exposure.

In the second paper [14], the authors also analyzed the

hydroperoxides formed under the same conditions of

treatment. Cholesterol hydroperoxides were measured in

the first 7 days; after that, both linoleic acid methyl ester

and docosahexaenoic acid methyl ester were completely

degraded. The degradation rate constants for cholesterol in

the presence of stearic methyl acid ester, linoleic acid

methyl ester and docosahexaenoic acid methyl ester were

0.0549, 0.0567 and 0.0512 d-1, respectively, about 3 times

more rapid than cholesterol exposed to treatment in the

absence of the sensitizer (0.0175 d-1). The results are

higher than those found in previous research, showing that

the addition of riboflavin facilitates cholesterol degradation

during light exposure.

Medina-Meza et al. [46] studied photo-oxidation of

crystalline cholesterol, reporting the formation of side-

chain COPs. They formed a thin crystalline film of anhy-

drous cholesterol by slow evaporation of a cholesterol/

chloroform solution, adding a certain amount of hemato-

porphyrin as a sensitizer. The system was illuminated in an

incubator at 25 ± 2 �C for 21 days. The light source was a

lamp with a fluorescent tube of 18 W and 6,500 K, lumi-

nous flux 1,300 Lm and light intensity of 4,300 ± 100 lx.

Among the results achieved, a hyperbolic trend was

observed in hydroperoxides formation, which could be

justified considering the inhibition due to end-to-end

crystalline arrangement that slows peroxidation rate for the

reduced availability of sites susceptible to radical attack.

Cholesterol degradation was fitted with a first-order expo-

nential decay equation [28, 66], obtaining a good correla-

tion value (R2 [ 0.98) and a kchol = 0.0395 d-1, about half

of the value obtained by Hu et al. [30]. This result could be

attributed to increased activity of hematoporphyrin as a

sensitizer, and to the different conditions of the experiment.

The rates of formation of Type II photoreaction products

(namely 5,6a-epoxy, 5,6b-epoxy, 6-ketostanol and 6-OH)

were lower than Type I ones (7a-OH, 7b-OH and 7-keto).

This result is probably due to the reaction mechanism and

radiolytic stability of these COPs, since susceptibility of

7-keto and 5,6b-epoxi has been found to be dependent on

its microenvironment [43]. To emphasize the importance of

the physical state in the oxidative pattern, the formation

rate and the final concentration of 25-OH (a side-chain

oxysterol) are higher than 7-keto; in particular, the value of

the constant rate is 0.8 9 10-2 d-1 for 25-OH and

0.2 9 10-2 for 7-keto.

Applications in Food Systems

There is some negligence in modeling the kinetics of photo-

oxidative phenomena in foods, and information regarding

cholesterol and COPs is therefore poor. Butter and meat

products are the most studied foods, and a comprehensive

review of the content of COPs in these products can be found

in Cardenia et al. [8]. Luby et al. [39] kinetically analyzed

butter photo-oxidation, since these oxidation products are

detectable only after prolonged exposure to light. They

further investigated the effect of packaging and light source

on oxidative stability [40]. It is interesting that the authors

consider oxidation as a light-induced surface phenomenon,

realizing their food model on this basis. A wide range of

wavelength (276–700 nm) was chosen to prove the effect of

different packaging material (HD and LD polyethylene

films, wax paper and aluminum foil, among others), moni-

toring peroxide value at 1 and 2 weeks of light exposition,

and obtaining a first-order trend in hydroperoxides accu-

mulation. It should be pointed out that an analysis carried out

with a reduced number of progressive sampling points is

insufficient for a correct determination of the kinetic

dynamic.

Storage under artificial light is a strong pro-oxidant agent,

especially for meat products. Boselli and coworkers studied

the formation of COPs in turkey [5] and beef [6] meat, using a

kinetic approach. In their first work, the samples were

exposed to warm-tone and daylight lamps. The first one

emanated red light with low emission in the blue radiation

(3,000 K, 36 W), while the second one was a fluorescent

light (6,000 K, 36 W). The oxysterols concentration in

samples irradiated with the warm-tone lamp was comparable

to that of meat kept in darkness with a similar storage time,

thus being much lower than that of irradiated samples with

the daylight lamp. The different degrees of oxidation

induced by the two types of lamps can be attributed in part to

their energy of radiation, which is higher in the daylight lamp

than in the red fluorescent lamp, because it contains a greater

amount of blue light. However, as in most of the works cited

above, the energy aspects relating to the source of light were

not examined in depth. For instance, Hansen and Skibsted

[27], studying light-induced oxidative changes in a dairy

spread, observed that lipid peroxidation increased with

decreasing wavelength.

Similar experiments were carried out by Boselli et al. [6]

in a subsequent work in cow meat; this time the authors

analyzed kinetic data of cholesterol degradation and total

COPs content by fitting it with a simple linear model:

Ci ¼ kit þ C0 ð9Þ

Food Eng Rev (2013) 5:171–184 181

123

where Ci is the concentration of the considered compound

and C0 is the initial concentration. ki assumes positive or

negative values, depending on whether it is a formation or

degradation of compounds. Equation (9) represents the

integrated law of a zero-order reaction, in which the rate is

independent of the concentration of reactant [18], as shown

in Table 1. The numerical value of the order does not need

to correspond to molecularity [63]; furthermore, in this

case the fit gives no statistically significant results, since

the coefficients of determination (R2) are 0.64 for choles-

terol degradation and 0.40 for total COPs formation. This

result demonstrates the lack of familiarity of many authors

with kinetic modeling and the fact that researchers in food

science have limited themselves to heuristic approaches

[62].

Table 3 reassumes the different kinetic models used for

cholesterol degradation and single COPs formations in

photo-oxidation.

Conclusions

In this paper, we have reviewed the state of the application

of kinetic models in the study of cholesterol oxidation

products. Despite the significance of kinetic modeling in

food engineering as a powerful tool to control/prevent the

formation and accumulation of these important molecules,

there has been little interest in researching this area.

Among the major points that make it difficult to design a

comprehensive model of the phenomenon of cholesterol

oxidation in food systems, we can highlight the following:

• The large number of molecules derived from oxidation

([80), many of which are volatile compounds difficult

to quantify.

• The presence/absence of antioxidants and pro-oxidants

and the interference of other macromolecules (proteins,

carbohydrates, lipids), which can exert a pro- or

antioxidant effect.

• Difficulties inherent in the analytical or numerical

resolution of systems of kinetic equations and the

correct interpretation of the obtained data.

Furthermore, analysis of the current status of the liter-

ature leads us to a future perspective view in which the

kinetic study of oxidative phenomena, and in particular of

cholesterol, will play a greater importance in Food Science

and Engineering. Among the different challenges that we

can mention are:

1. Correct application of the principles of chemical

kinetics, with the use of multiresponse approaches,

facilitated by the availability of more and more

powerful computational tools.

2. In the case of thermo-oxidation, evaluation of response

kinetics at different temperatures, better understanding

of the mechanisms of inhibition of antioxidants, ther-

modynamic considerations and transport phenomena.

3. In the case of photo-oxidation, there is an urgent need

to explore further aspects of transmission quantum

energy in food systems, considering not only the nature

of the source of induction, but also the effect of

inhibitors or activators of other molecules.

References

1. Barriuso B, Otaegui-Arrazola A, Menendez-Carreno M, Asti-

asaran I, Ansorena D (2012) Sterol heating: degradation and

formation of their ring-structure polar oxidation products. Food

Chem 135:706–712

Table 3 Kinetic models proposed for photo-oxidation of cholesterol in food and model systems

Oxidation route Kinetic type Model Reference

Cholesterol degradation First order Solution (fluorescent light, with fatty acid methyl

esters, riboflavin as sensitizer)

Hu et al. [30]

First order Solution (fluorescent light, with fatty acid methyl

esters, riboflavin as sensitizer)

Chien et al. [14]

Zero order Cow meat (warm-tone and fluorescent light) Boselli et al. [6]

First order Solid state, anhydrous (fluorescent light,

hematoporphyrin as sensitizer)

Medina-Meza et al. [46]

C-7

7-OOH First order Butter (fluorescent light, scanning at 276-700 nm) Luby et al. [39]

Deceleratory Solid state, anhydrous (fluorescent light,

hematoporphyrin as sensitizer)

Medina-Meza et al. [46]

Side-chain

25-OH Pseudo-zero order Solid state, anhydrous (fluorescent light,

hematoporphyrin as sensitizer)

Medina-Meza et al. [46]

182 Food Eng Rev (2013) 5:171–184

123

2. Benjamin S, Spener F (2009) Conjugated linoleic acids as func-

tional food: an insight into their health benefits. Nutr Metab. doi:

10.1186/1743-7075-6-36

3. Bjorkhem I (2009) Are side-chain oxidized oxysterols regulators

also in vivo? J Lipid Res 50:S213–S218

4. Boots AW, Haenen GRMM, Bast A (2008) Health effects of

quercetin: from antioxidant to nutraceutical. Eur J Pharmacol

585:325–337

5. Boselli E, Caboni MF, Rodriguez-Estrada MT, Gallina Toschi T,

Daniel M, Lercker G (2005) Photooxidation of cholesterol and

lipids of turkey meat during storage under commercial retail

conditions. Food Chem 91:705–713

6. Boselli E, Rodriguez-Estrada MT, Fedrizzi G, Caboni MF (2009)

Cholesterol photosensitized oxidation of beef meat under standard

and modified atmosphere at retail condition. Meat Sci 81:224–229

7. Brimberg UI, Kamal-Eldin A (2003) On the kinetics of the

autoxidation of fats: influence of pro-oxidants, antioxidants and

synergists. Eur J Lipid Sci Technol 105:83–91

8. Cardenia V, Rodriguez-Estrada MT, Boselli E, Lercker L (2012)

Cholesterol photosensitized oxidation in food and biological sys-

tems. Biochimie http://dx.doi.org/10.1016/j.biochi.2012.07.012

9. Chen ZY, Chan PT, Kwan KY, Zhang A (1997) Reassessment of

the antioxidant activity of conjugated linoleic acids. J Am Oil

Chem Soc 74:749–753

10. Chen JF, Tai CY, Chen YC, Chen BH (2001) Effects of conju-

gated linoleic acid on the degradation and oxidation of model

lipids during heating and illumination. Food Chem 72:199–206

11. Chien JT, Hsieh HC, Kao TH, Chen BH (2005) Kinetic model-

ling for studying the conversion and degradation of isoflavones

during heating. Food Chem 91:425–434

12. Chien JT, Hsu DJ, Chen BH (2006) Kinetic model for studying

the effect of quercetin on cholesterol oxidation during heating.

J Agr Food Chem 54:1486–1492

13. Chien JT, Huang DY, Chen BH (2004) Kinetic studies of cho-

lesterol oxidation as inhibited by stearylamine during heating.

J Agr Food Chem 52:7132–7138

14. Chien JT, Lu YF, Hu PC, Chen BH (2003) Cholesterol photo-

oxidation as affected by combination of riboflavin and fatty acid

methyl esters. Food Chem 81:421–431

15. Chien JT, Wang HC, Chen BH (1998) Kinetic model of the

cholesterol oxidation during heating. J Agr Food Chem

46:2572–2577

16. Choe E, Min DB (2006) Chemistry and reactions of reactive

oxygen species in foods. Crit Rev Food Sci 46:1–22

17. Das KC, Das CK (2002) Curcumin (diferuloylmethane), a singlet

oxygen (1O2) quencher. Biochem Biophys Res Commun 295:62–66

18. Denisov ET, Sarkisov OM, Likhtenshtein GI (2003) Theory of

elementary reactions. In: Chemical kinetics. Fundamentals and

new developments. Elsevier Science B.V., Amsterdam (Chapter 1)

19. Derewiaka D, Obiedzinski M (2010) Influence of lard heat

treatment on changes in the content of cholesterol and formation

of cholesterol oxidation products. Pol J Food Nutr Sci 60:19–23

20. Faustman C, Specht SM, Malkus LA, Kinsman DM (1992) Pig-

ment oxidation in ground veal: influence of lipid oxidation, iron

and zinc. Meat Sci 31:351–362

21. Foote CS (1968) Mechanisms of photosensitized oxidation. Sci-

ence 162:963–970

22. Foote CS, Shook FC, Abakerli RB (1984) Characterization of

singlet oxygen. Method Enzymol 105:36–47

23. Foote CS (1991) Definition of type I and II photosensitized

oxidation. Photochem Photobio 54:659

24. Girotti AW (1998) Lipid hydroperoxide generation, turnover, and

effector action in biological systems. J Lipid Res 39:1529–1542

25. Girotti AW (2001) Photosensitized oxidation of membrane lipids:

reaction pathways, cytotoxic effects, and cytoprotective mecha-

nisms. J Photochem Photobio B 63:103–113

26. Guardiola F, Dutta PC, Codony R, Savage GP (2004) Cholesterol

and phytosterol oxidation products. AOCS Press, Champaign

(Chapter 1)

27. Hansen E, Skibsted LH (2000) Light-induced oxidative changes

in a model dairy spread. Wavelength dependence of quantum

yields. J Agric Food Chem 48:3090–3094

28. Heaton JW, Lencki RW, Marangoni AG (1996) Kinetic model for

chlorophyll degradation in green tissue. J Agric Food Chem

44:399–402

29. Heimburg T (2007) Thermal biophysics of membrane. Wiley-

VCH, Weinheim (Chapter 2)

30. Hu PC, Chen BH (2002) Effects of riboflavin and fatty acid

methyl esters on cholesterol oxidation during illumination.

J Agric Food Chem 50:3572–3578

31. Hur SJ, Park GB, Joo ST (2007) Formation of cholesterol oxidation

products (COPs) in animal products. Food Control 18:939–947

32. Iuliano L (2011) Pathways of cholesterol oxidation via non-

enzymatic mechanism. Chem Phys Lipids 164:457–468

33. Kruck J, Lichszted K, Michalska T, Paraskevas SM (1992) The

influence of some biological antioxidants on the light emission

from the oxytetracycline oxidation reaction. Toxicol Environ

Chem 35:167–173

34. Labuza TP (1971) Kinetics of lipid oxidation in foods. Crit Rev

Food Technol 2:355–399

35. Lakritz L, Maerker G (1989) Effect of ionizing radiation on

cholesterol in aqueous dispersion. J Food Sci 54:1569–1572

36. Lee HW, Chien JT, Chen BH (2008) Inhibition of cholesterol

oxidation in marinated foods as affected by antioxidants during

heating. J Agr Food Chem 108:234–244

37. Lercker G, Rodriguez-Estrada MT (2002) Cholesterol oxidation

mechanism. In: Guardiola F, Dutta PC, Codony R, Savage GP

(eds) Cholesterol and phytosterol oxidation products: analysis,

occurrence, and biological effects. AOAC Press, Champaign,

pp 1–25

38. Loomis CR, Shipley GG, Small DM (1979) The phase behavior

of hydrated cholesterol. J Lipid Res 20:525–535

39. Luby JM, Gray JI, Harte BR, Ryan TC (1986) Photooxidation of

cholesterol in butter. J Food Sci 51:904–907

40. Luby JM, Gray JI, Harte BR (1986) Effects of packaging and

light source on the oxidative stability of cholesterol in butter.

J Food Sci 51:908–911

41. Maerker G, Bunick FJ (1986) Cholesterol oxides II. Measurement

of the 5,6-epoxides during cholesterol oxidation in aqueous dis-

persion. J Am Oil Chem Soc 63:771–777

42. Maerker G, Jones KC (1992) Gamma-irradiation of individual

cholesterol oxidation products. J Am Oil Chem Soc 69:451–455

43. Maerker G, Jones KC (1993) A-ring oxidation products from c-

irradiation of cholesterol in liposomes. J Am Oil Chem Soc

70:255–259

44. McClements DJ, Decker EA (2000) Lipid oxidation in oil-in-water

emulsions: impact of molecular environment on chemical reac-

tions in heterogeneous food systems. J Food Sci 65:1270–1282

45. Medina-Meza IG, Rodriguez-Estrada MT, Lercker G, Soto-

Rodrıguez I, Garcıa HS (2011) Unusual oxidative pattern in

thermo-oxidation of dry films of cholesterol. Rev Mex Ing Quım

10:47–57

46. Medina-Meza IG, Rodrıguez-Estrada MT, Lercker G, Garcıa HS

(2012) Oxidative pattern from fluorescent light exposition of

crystalline cholesterol. Food Biophys 3:209–219

47. Nielsen JH, Olsen CE, Skibsted LH (1996) Cholesterol oxidation

in a heterogeneous system initiated by water-soluble radicals.

Food Chem 56:33–37

48. Ohshima T, Li N, Koizumi C (1993) Oxidative decomposition of

cholesterol in fish products. J Am Oil Chem Soc 70:595–600

49. Ozilgen S, Ozilgen M (1990) Kinetic model of lipid oxidation in

foods. J Food Sci 55:498

Food Eng Rev (2013) 5:171–184 183

123

50. Park PW, Guardiola F, Park SH, Addis PB (1996) Kinetic eval-

uation of 3b-hydroxycholest-5-en-7-one (7-ketocholesterol) sta-

bility during saponification. J Am Oil Chem Soc 73:623–629

51. Park SW, Addis PB (1986) Identification and quantitative esti-

mation of oxidized cholesterol derivatives in heated tallow.

J Agric Food Chem 34:653–659

52. Poli G, Sottero B, Gargiulo S, Leonarduzzi G (2009) Cholesterol

oxidation products in the vascular remodeling due to athero-

sclerosis. Mol Aspects Med 30:180–189

53. Rawls HR, VanSanten PJ (1970) A possible role for singlet

oxidation in the initiation of fatty acid autoxidation. J Am Oil

Chem Soc 47:121–125

54. Ryan L, O’Callaghan YC, O’Brien NM (2005) Oxidized products

of cholesterol: their role in apoptosis. Cur Nutr Food Sci 1:41–51

55. Saldanha T, Bragagnolo N (2010) Effects on grilling on choles-

terol oxide formation and fatty acids alteration in fish. Cienc

Tecnol Aliment 30:385–390

56. Smith LL (1987) Cholesterol autooxidation 1981–1986. Chem

Phys Lipids 44:87–125

57. Smith LL (1991) Another cholesterol hypothesis: cholesterol as

antioxidant. Free Radical Bio Med 11:47–61

58. Tai CY, Chen YC, Chen BH (1999) Analysis, formation and

inhibition of cholesterol oxidation products in foods: an overview

(part I). J Food Drug Anal 7:243–257

59. Tai CY, Chen YC, Chen BH (2000) Analysis, formation and

inhibition of cholesterol oxidation products in foods: an overview

(part II). J Food Drug Anal 8:1–15

60. Uskokovic V (2008) Surface charge effect involved in the control

of stability of sols comprising uniform cholesterol particles.

Mater Manuf Process 23:620–623

61. Valenzuela A, Sanhueza J, Pilar A, Corbari A, Nieto S (2004)

Inhibitory action of conventional food-grade natural antioxidants

and of natural antioxidants of new development on the thermal-

induced oxidation of cholesterol. Int J Food Sci Nutr 55:155–162

62. Van Boekel MAJS (1999) Testing of kinetic models: usefulness

of the multiresponse approach as applied to chlorophyll degra-

dation in foods. Food Res Int 32:261–269

63. Vicente SJV, Sampaio GR, Ferrari CKB, Torres EAFS (2012)

Oxidation of cholesterol in foods and its importance for human

health. Food Rev Int 28:47–70

64. Wu FC, Tseng RL, Huang SC, Juang RS (2009) Characteristics of

pseudo-second-order kinetic model for liquid-phase adsorption: a

mini-review. Chem Eng J 151:1–9

65. Yan PS, White PJ (1990) Cholesterol oxidation in heated lard

enriched with two levels of cholesterol. J Am Oil Chem Soc

67:927–931

66. Yen TY, Inbaraj BS, Chien JT, Chen BH (2010) Gas chroma-

tography-mass spectrometry determination of conjugated linoleic

acids and cholesterol oxides and their stability in a model system.

Anal Biochem 400:130–138

67. Yen TY, Lu YF, Inbaraj BS, Chen BH (2011) Cholesterol oxi-

dation in lard as affected by CLA during heating—a kinetic

approach. Eur J Lipid Sci Technol 113:214–223

68. Yu L (2001) Free radical scavenging properties of conjugated

linoleic acids. J Agric Food Chem 49:3452–3456

184 Food Eng Rev (2013) 5:171–184

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