Table Of ContentTHE INHIBITION OF
FAT OXIDATION PROCESSES
N.M.EMANUEL and
YU. N. LYASKOVSKA YA
Translated by
K.A. ALLEN
P E R G A M ON PRESS
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Copyright © 1967
Pergamon Press Ltd.
First English edition 1967
This is a translation of the original Russian
TOPMOMEHHE nPOIfECCOB OKHCJIEHHH MMPOB
published by Pishcheprom, Izdat., 1961
Library of Congress Catalog Card No. 66-25838
P R E F A CE
THIS book, The Inhibition of Fat Oxidation Processes, by Prof.
N.M.Emanuel, corresponding member of the Academy of Sciences
USSR, and Yu. N. Lyaskovskaya, candidate of technology, is a
complete survey intended for workers in the field of industrial
food science, and its pubUcation was received with great acclaim.
The book is outstanding owing to its world-wide collection of the
results of theoretical and experimental work.
As a result of the work of Academician N. N. Semenov and his
pupils, the USSR has become a principal source of the well-
known chain reaction theory. It is therefore natural that the col
lation and examination of fat oxidation processes, and of their
inhibitions, should be carried out by our scientists and specialists
in the hght of the chain reaction theory, and that this work is of
considerable scientific and practical value.
The book is concerned mainly with the special problem of re
tarding the oxidation of animal fats, but this in no way detracts
from its more general value. Animal fats in particular are the least
stable in the presence of oxygen, while fat-containing substances are
particularly unstable during processing and storage. Because of
these "defects" in animal fats (especially lard), and since it is
essential to use the whole output of high-quahty animal fats for
food purposes, there has been an intensive development of a new
branch of chemical industry which specialized in the production of
a wide range of synthetic, oxidation-inhibiting compounds, the
antioxidants, for protecting the fats and fat-containing products
from decay.
The authors of this book have not made it their task to deal
with the whole range of oxidation kinetics of all types of fats pro
cessed by various industries, and with the protection of diflferent
fat-containing food products. However, the fundamental principles
necessary for understanding these are presented, and some of the
details are also dealt with in subsequent chapters.
vil
vii i Preface
Consequently the theoretical principles given in this book throw
light on many oxidation phenomena encountered in industrial
production and storage of complex fat-containing food products.
What are the main aims in the development of the theory and
practice of the inhibition of oxidation processes? The authors out
line these as follows.
Firstly, all principles based on the modern concept of slow,
branched (degenerate) chain reactions can and must be used when
studying fat oxidation and the oxidative decay of fat-containing
food products, and also in the discovery of effective means to inhibit
these processes.
Secondly, the problems of the rational use of unstable fats for
human consumption, especially of animal fats such as lard, and of
the production of high quahty fat-containing, storable food pro
ducts can be partly solved at present in some cases by using natural
and synthetic antioxidants.
Thirdly, there is a distinct tendency to obtain greater resistance to
decay of fats and fat-containing food products by using not only the
different types of antioxidants with ''universal" activity, but also
mixtures of these and other suitable additives, to obtain the best
protective effect, i.e. synergism. The range of fat-containing pro
ducts is very wide and they represent a complex, multi-component,
colloidal, dispersed system. It is therefore very difficult (although
desirable) to select a single, universally active and specific anti
oxidant. For example, when comparing the technological anti
oxidant mixtures, one must consider the need for heat resistance in
cases where they are used in food products exposed to heat during
processing.
Fourthly, it is qxwte obvious that special attention must be paid
to the compounding of high efficiency antioxidants so as to obtain
a high degree of synergism. The latter is a very interesting and
promising phenomenon, and its use has already had a significant
effect on development in all fields of modern science and technology.
There is thus a whole series of composite mixtures of different
antioxidants, natural and synthetic (sometimes with other suitable
additives) which have a much greater effect together than each of
them individually, or a mixture prepared from only some of the
components. The preparation of a mixture thus ensures the use of a
product with more synergic and effective properties. On the basis
of these considerations cotton-seed oil containing a considerable
Preface ix
amount of an effective natural inhibitor, tocopherol, is often used
in such mixtures.
Fifthly, one must not forget the most frequently used synthetic
antioxidants such as butylhydroxyanisole, butylhydroxytoluene,
esters of gallic acid (higher and lower), and the other additives such
as phosphoric and citric acid in use today. It is thus obvious that
reports of analytical methods for the various types of antioxidants,
given by the authors in this book, will be most useful.
Specialists interested in problems of protecting edible fats and
fat-containing products from decay will find much valuable ma
terial in this book. One aspect of this problem has been fairly
thoroughly worked out and a single answer may be expected. In
other cases, however, some experiments have yet to be made, some
decisions taken and a hypothesis formed. All this is discussed in this
book and the characteristics of this rapidly developing branch of
modern science and technology are described.
Much still remains to be clarified in this new field and much is of
an empirical nature. Discussion of any problems presented in this
book is therefore desirable and would be useful to the authors as
well as to the discovery, development, and wider use of effective
methods for the protection of edible fats and fat-containing food
products from decay.
The rapid growth of our national fatstock production, the pro
cessing of vast quantities of meat and the consequent growth of
animal fat production, including lard, presents us with the need to
accelerate the development of the industrial production and apphca-
tion of all types of effective antioxidants. Our socialist code requires
that the variety, quality and property of synthetic antioxidants
produced and introduced into the food industry must be approved
by the appropriate medical authority, i.e. all theoretical work in this
field, as well as the practical use of antioxidants, should be carried
out under the supervision of the appropriate branch of the Ministry
of Health of the USSR
N. A. PETROV
Cand. techn. sei.
F ٢ R B W ٢ R ٢
THE PROBLEM of inhibiting the oxidative decay of fats is one of the
main problems of the food industry.
Fat oxidation is a degenerate branching reaction. The theory of
these processes was explained by Academician N.N. Semenov in 1931.
The solution of the problem of retarding the oxidative decay of
edible fats can be based on a valuable theoretical foundation.
Even before the discovery of chemical chain reactions a con
siderable quantity of material on the effect of very small amounts of
various compounds, the so-called inhibitors of chemical processes,
had been collected. Since the discovery of chain reactions, chemists
have correctly understood and interpreted the considerable sensitiv
ity of reactions to the addition of small quantities of chemical
compounds. This sensitivity (strong inhibition or acceleration) has
in some cases become characteristic of the chain reaction mechanism.
Additives accelerate the chain process if they cause an increase in
the rate of chain initiation by some means (formation of free radi
cals). Inhibitors retard chain reactions by eliminating the free
radicals propagating the chain, i.e. by terminating it.
When chain processes, such as oxidation, decomposition, halogen-
ation, and polymerization are undesirable, the reactions may be
terminated by the appropriate inhibitors. It is therefore quite clear
what path is to be followed so as to solve the problem of increasing
the resistance of edible fats to oxidation. One can also see the
inmiense practical importance of an economical selection and syn
thesis of inhibitors, their study, and the conditions of use under
which they will be most effective.
That the retardation of oxidation processes has interested a
large number of research workers has been very helpful in the
solution of this problem, and the needs of industry have also stim
ulated work on it.
There was a need to stabilize lubricants, and the products of
petroleum cracking, and to prevent the ageing of plastics and rub-
xii Foreword
ber, which is caused by the development of oxidative processes, and
also to avoid the oxidative decay of fats and other products.
There is, however, a major difference between the food industry
and ordinary technical usage in the properties required of an anti
oxidant or inhibitor. Inhibitors required by the food industry must
be non-toxic. One of the authors of this monograph has recently
established another field for the use of non-toxic inhibitors of free
radical reactions. It was found that this class of compounds is very
suitable for the efficient retardation of biological processes.
These inhibitors will find wider application, especially in the
fields of technology, chemistry, food industry, and biochemistry as
knowledge in these disciplines increases.
It is hoped that the publication of this monograph will be of
interest to the workers in various branches of science and technology.
The decision to write this book has been taken by the authors as a
result of their participation in work on problems of increasing the
resistance of edible fats to oxidation, which was undertaken on
behalf of the All-Union Scientific Research Institute of Meat
Production and the Institute of Chemical Physics of the USSR
Academy of Science. This work was started in 1953. It is only na
tural that there are a few gaps in this book. The authors know this
and would be grateful to readers for pointing out any short
comings. They will endeavour to cover these in their subsequent
work on non-toxic inhibitors of free radical reactions.
The authors wish to thank N. A. Petrov for his valuable advice
during the editing of this book, and also L. S. Vartanyan, G. N. Bog-
danov, L.G.Bulavin, and E.B. Burlakova for their cooperation
in the work, selection of the material, and preparation of the
manuscript.
CHAPTER I
KINETICS AND CHEMISTRY
OF FAT OXIDATION PROCESSES
AND MODEL SYSTEMS
A STUDY of the kinetics and chemistry of the autoxidation pro
cesses of fats and oils is of considerable practical significance.
The development of oxidative processes leads to the appearance
of compounds of a peroxide nature, of aldehydes, ketones, low
molecular weight acids, oxyacids, etc., in fats and fat-containing
products. This results in a loss of nutritional value because the
fats become toxic. The fats lose their biological value long before
the appearance of any distinct signs of decay; the fat-soluble vita
mins are destroyed and the unsaturated fatty acid content and pig
ment (in coloured fats), etc., are reduced. Oxidative decay results
in the majority of edible fats having to be transferred to the
category of "technical" fats or even in becoming completely un
usable.
A longer storage period for edible products in which quaUty is
preserved is one of the major needs of the food industry. It should
not only be solved by the use of refrigeration but also by more
positive methods such as the retardation and inhibition of oxidative
processes by adding various chemicals. This opens up a variety of
possibihties connected with the chain mechanism of the oxidation
processes of organic material, and of fats in particular.
It is well known that chain reactions are very sensitive to even the
smallest addition of compounds able to interact with the free radi
cals and to terminate the chain. A rational approach to the problem
of retarding oxidative decay of fats is possible only after thorough
study of the kinetics and chemistry of their reactions.
Fats, or triglycerides, are glycerine esters of predominantly higher
mono-basic, saturated and unsaturated fatty acids. The general
FOP 1
2 The Inhibition of Fat Oxidation Processes
formula of fats (triglycerides) can be written as follows
CH2—O—C—R
II
o
CH—o—C—Ri
II
o
CH2—o—C—R2
o
where R, R^, R2 are fatty acid radicals.
Those of animal origin contain mainly the radicals of saturated
stearic and palmitic acids and unsaturated oleic acid. One also
encounters the radicals of unsaturated linoleic, Hnolenic and arachi-
dic (eicosanoic) acids. Animal fats generally contain a fair number
of different fatty acids.
Natural fats are a mixture of the so-called simple (in which all
three acid radicals are identical) and mixed (different acid radicals)
triglycerides. They contain in addition a small quantity of phos
phatides, sterines, steroids, pigments, and vitamins.
Fat oxidation processes are identical with oxidation reactions of
paraffinic and olefinic series of hydrocarbons. This is quite natural
because radicals R, Ri, and R2 are long hydrocarbon chains with
an oxygen-containing group at one end only. A study of the kinetics
and chemistry of fat oxidation processes thus makes it possible to
apply generally the results of oxidation to other types of com
pounds, such as higher fatty acid esters and also paraffinic and
olefinic hydrocarbons.
l.THE MECHANISM OF CHAIN OXIDATION
PROCESSES
The basis of present-day theories on the mechanism of the oxi
dation of organic compounds is the peroxide theory postulated by
Bakh [1] and Engler [2], and the theory of initiation of chain
branching of Semenov [3, 4].
A direct proof of the chain character of the oxidation reaction is
given by the results of measuring quantum yields during photo
chemical oxidations. These are found to be much greater than one
Kinetics and Chemistry 3
(when considering the number of molecules of oxygen absorbed per
unit absorbed light). In the case of photo-chemical oxidation the
following quantum yields F and chain length ν were found for two
wave lengths [5]:
λλ F V
2537 900 45
3650 21 0 10
As regards chain length one can assess it only as a lower limit
from photochemical results. A calculation of chain length does not
take into account the reduction of quantum yields due to re-
emission of light by the excited molecule (fluorescence). Also, the
terminating collisions of excited particles, leading to the conversion
of excitation energy into vibrational energy, are not accounted for.
Another proof of the chain mechanism is the retardation of
oxidation processes by the addition of as little as 0Ό01-0·1% of
some compounds.
Compounds capable of retarding the chain reaction in trace
amounts are called inhibitors. When used in conjunction with oxi
dation reactions they are called antioxidants. Phenols and aromatic
amines are often effective antioxidants.
The inhibitory activity of these compounds can be understood only
in the light of the chain reaction theory. The inhibitor reacts with the
radicals initiating the oxidation chain. On the basis of the principle
of the non-destructibility of free valence, new radicals differing from
those initiating the oxidation chain are formed from the inhibitor.
The reaction between an inhibitor molecule, InH, and the free
radical initiating oxidation (e.g. the hydroperoxide radical RO2)
can be illustrated schematically as follows:
InH+ RÓ2-^R02H-Mn.
The inhibitor radical In is normally of low activity and in
capable of taking part in chain propagation. It therefore eventually
recombines with any of the radicals and is thus eliminated from
the reaction. It is thought that the In radical recombines with another
of the same type to form an Inj molecule.
If one assumes that chain termination in the presence of an
inhibitor (at very low concentrations) occurs due to the reaction
between the chain initiating radicals and the inhibitor molecule, one
