STABILITY or FREEZE—DRIED FOODS
197
2. ”1f 9hSa6t )
water lowers the effectiveness of metal catalysts such as copper and iron (Uri,
3. That water is attached to sites on the surface thereby excluding oxygen from these
sites (Salwin, 1959).
4. That water promotes non—enzymatic browning, and browning can result in the
formation of antioxidant compounds (Lea, 1958).
5. That water forms hydrogen bonds With hydroperoxides and retards hydroperoxide
decomposition.
The effects of water observed in many studies do not allow any one of these hypotheses
to be singled out as correct, since any one or several of them could account for the observed
results.
A food is a complicated system containing both pro-oxidant and antioxidant substances
which complicate interpretation. A model system was devised, therefore, to study the
effect of water under more controlled conditions.
_
A model system consisting of microcrystalline cellulose, highly puried methyl linoleate,
and controlled levels of water and other additives was developed and evaluated in detail
with respect to its physical characteristics and chemical composition, including trace
.
components. In connection with this evaluation, new methods were developed for analysis
of trace concentrations of heavy metals. The need for highly puried linoleate resulted in
the development of techniques for purication and analysis of esters of unsatùrated fatty
acids.
Oxidation of methyl linoleate was studied in the model system, in the absence of metal
catalysts. Oxidation was studied manometrically in samples adjusted to water activities
ranging from approximately zero to approximately 0.6. Water (as determined from kinetic
data) had an inhibitory effect on the autoxidation‘ reaction. The magnitude of the inhibitory
effect was dependent on water content up to activities of approximately 0.5.
Evaluation of the kinetic data indicated that this inhibitory effect was most pronounced
in early stages of oxidation including the period during which the hydroperoxides decompose
by a monomolecular reaction.
In a subsequent study aimed primarily at the careful comparison of kinetics of oxidation
in early stages of oxidation in absence and in presence of water, it was established that,
in the presence of water, monomolecular kinetics were followed up to signicantly higher
levels of total oxidation than was the case in the absence of water. These studies seem to
support a hypothesis of the mechanism of the inhibitory effect of water based on the
assumption that water binds hydroperoxides by hydrogen bonding and thus interferes
With the normal bimolecular decomposition of hydroperoxides in which two molecules
of hydroperoxides hydrogen bond to one another and then decompose into two free
radicals (Maloney, 1965).
The oxidation of methyl linoleate catalyzed by various salts of cobalt was studied in
identical model systems containing added catalyst salt. Since information on hydration
of these salts as a function of water activity was not available in the literature, studies were
conducted in which this information was obtained. The catalyst containing model systems
were then adjusted to selected water activities, and the effect of water on
the course of the
autoxidation was determined in experiments
in which oxidation was studied by manometrrc
ro
hotometric
rocedures.
"
anÎt î£îîforînd that wateÈ had an inhibitory effect on the catalyzed oxidation of the fatty
ester, as well as on the oxidation in absence of metals. The effect of water on the metal
197
2. ”1f 9hSa6t )
water lowers the effectiveness of metal catalysts such as copper and iron (Uri,
3. That water is attached to sites on the surface thereby excluding oxygen from these
sites (Salwin, 1959).
4. That water promotes non—enzymatic browning, and browning can result in the
formation of antioxidant compounds (Lea, 1958).
5. That water forms hydrogen bonds With hydroperoxides and retards hydroperoxide
decomposition.
The effects of water observed in many studies do not allow any one of these hypotheses
to be singled out as correct, since any one or several of them could account for the observed
results.
A food is a complicated system containing both pro-oxidant and antioxidant substances
which complicate interpretation. A model system was devised, therefore, to study the
effect of water under more controlled conditions.
_
A model system consisting of microcrystalline cellulose, highly puried methyl linoleate,
and controlled levels of water and other additives was developed and evaluated in detail
with respect to its physical characteristics and chemical composition, including trace
.
components. In connection with this evaluation, new methods were developed for analysis
of trace concentrations of heavy metals. The need for highly puried linoleate resulted in
the development of techniques for purication and analysis of esters of unsatùrated fatty
acids.
Oxidation of methyl linoleate was studied in the model system, in the absence of metal
catalysts. Oxidation was studied manometrically in samples adjusted to water activities
ranging from approximately zero to approximately 0.6. Water (as determined from kinetic
data) had an inhibitory effect on the autoxidation‘ reaction. The magnitude of the inhibitory
effect was dependent on water content up to activities of approximately 0.5.
Evaluation of the kinetic data indicated that this inhibitory effect was most pronounced
in early stages of oxidation including the period during which the hydroperoxides decompose
by a monomolecular reaction.
In a subsequent study aimed primarily at the careful comparison of kinetics of oxidation
in early stages of oxidation in absence and in presence of water, it was established that,
in the presence of water, monomolecular kinetics were followed up to signicantly higher
levels of total oxidation than was the case in the absence of water. These studies seem to
support a hypothesis of the mechanism of the inhibitory effect of water based on the
assumption that water binds hydroperoxides by hydrogen bonding and thus interferes
With the normal bimolecular decomposition of hydroperoxides in which two molecules
of hydroperoxides hydrogen bond to one another and then decompose into two free
radicals (Maloney, 1965).
The oxidation of methyl linoleate catalyzed by various salts of cobalt was studied in
identical model systems containing added catalyst salt. Since information on hydration
of these salts as a function of water activity was not available in the literature, studies were
conducted in which this information was obtained. The catalyst containing model systems
were then adjusted to selected water activities, and the effect of water on
the course of the
autoxidation was determined in experiments
in which oxidation was studied by manometrrc
ro
hotometric
rocedures.
"
anÎt î£îîforînd that wateÈ had an inhibitory effect on the catalyzed oxidation of the fatty
ester, as well as on the oxidation in absence of metals. The effect of water on the metal