STABILITY or FREEZE—DRŒD FOODS
.
.:î$
The nature of the antioxidant activity of the amino compounds was found to be dieren 3
from that observed in the identical system with a phenolic antioxidant, since the
effect of the amino compound was to prolong the induction period, while the phenolic
compound prolonged the induction period and, in addition, reduced the rate of oxidation
throughout the course of oxidation (Karel and Tannenbaum, 1965; Karel et al., 1966).
Additional studies are in progress, including work on isolation and characterization of the
reaction products of the amino acids. One of the products isolated in the course of these
studies was methionine sulfoxide, formed from methionine during the course of oxidation
in the model system. Reaction products of histidine were also isolated but as yet not identied
.
'
Enzymatic Reactions
Freeze-dried foods are subject to deterioration by reactions catalyzed by a number of
enzymes, including phenolases, various esterases and some oxidases. These reactions may
occur in the frozen state prior to dehydration, during storage in dried state, or after rehydra—
tion. Reactions prior to dehydration may be minimized by freeze-drying the foods soon
.
after freezing, and reactions after rehydration arealmost certain to occur in unblanched
freeze—dried foods unless proper methods of enzyme inhibition are used.
Reactions during storage in dried state, however, occur only if the water content is above
a critical level. Considerable research has been devoted to dening this critical level, and
'
it has been suggested that it coincides with the onset of capillary condensation of water
‘
(Acker, 1963). Reactions have been reported, howeVer, at moisture contents denitely
below the level attributable to capillary condensation (Draudt et al., 1962). It
be
concluded therefore, that factors controlling enzymic reactions at low moisture contents
are as yet not fully elucidated.
_
.
Other Physico—Chemiœl Aspects of Deterioraon of
.
‘
In addition to the major reactions discussed abOVe, there are seVeral aspects of deterioration
of freeze-dried foods, which maybebf importance bUt are as
not understood. We Would
like to point out several areas Which seem to deserv‘e further study.
‘
_ '
'
l. Recent work (see f.i.
Butler, 1965; Grant and Album, 1965) shoWs that
various reactions, including hÿdfolysis and Oxidation, are accèler’atedin the froZen
state. Since freeze-dried“ foods are frozen priÔrΑto diyi‘ng, it is
that some
‘
precursors are formed in the
state. The work
systems had additional
signicance for dried materials;’siiiCe‘t‘he froZen ‘syStems,
dned'sÿStem‘S, have
a
low free—water Content.
'
_
2. Recent work (Mimdaÿ et al.,
indicates
that
free radicals may bë’formed‘in foodmaterialsduringÿthe‘
To What extent these
for initiation of déterior'ave reactions
is unknown? '
?
_
f
.
Ü
:
3. Additional Work is needed
the“ ox1dat1ve Changes m the'non4hp‘rd fraction
of freeze—dried feeds“,
1n prote1nsand amino
acrds
_
4. Our understanding of
role Of Changes in “morphological
tissues is imperfect. It appears to us thatabetter understandmg of changes in membrane
structures and their role in redistribution of food components
would ard
1n understanding the problem of changes;in
C&paCltY—.
1',
.
.:î$
The nature of the antioxidant activity of the amino compounds was found to be dieren 3
from that observed in the identical system with a phenolic antioxidant, since the
effect of the amino compound was to prolong the induction period, while the phenolic
compound prolonged the induction period and, in addition, reduced the rate of oxidation
throughout the course of oxidation (Karel and Tannenbaum, 1965; Karel et al., 1966).
Additional studies are in progress, including work on isolation and characterization of the
reaction products of the amino acids. One of the products isolated in the course of these
studies was methionine sulfoxide, formed from methionine during the course of oxidation
in the model system. Reaction products of histidine were also isolated but as yet not identied
.
'
Enzymatic Reactions
Freeze-dried foods are subject to deterioration by reactions catalyzed by a number of
enzymes, including phenolases, various esterases and some oxidases. These reactions may
occur in the frozen state prior to dehydration, during storage in dried state, or after rehydra—
tion. Reactions prior to dehydration may be minimized by freeze-drying the foods soon
.
after freezing, and reactions after rehydration arealmost certain to occur in unblanched
freeze—dried foods unless proper methods of enzyme inhibition are used.
Reactions during storage in dried state, however, occur only if the water content is above
a critical level. Considerable research has been devoted to dening this critical level, and
'
it has been suggested that it coincides with the onset of capillary condensation of water
‘
(Acker, 1963). Reactions have been reported, howeVer, at moisture contents denitely
below the level attributable to capillary condensation (Draudt et al., 1962). It
be
concluded therefore, that factors controlling enzymic reactions at low moisture contents
are as yet not fully elucidated.
_
.
Other Physico—Chemiœl Aspects of Deterioraon of
.
‘
In addition to the major reactions discussed abOVe, there are seVeral aspects of deterioration
of freeze-dried foods, which maybebf importance bUt are as
not understood. We Would
like to point out several areas Which seem to deserv‘e further study.
‘
_ '
'
l. Recent work (see f.i.
Butler, 1965; Grant and Album, 1965) shoWs that
various reactions, including hÿdfolysis and Oxidation, are accèler’atedin the froZen
state. Since freeze-dried“ foods are frozen priÔrΑto diyi‘ng, it is
that some
‘
precursors are formed in the
state. The work
systems had additional
signicance for dried materials;’siiiCe‘t‘he froZen ‘syStems,
dned'sÿStem‘S, have
a
low free—water Content.
'
_
2. Recent work (Mimdaÿ et al.,
indicates
that
free radicals may bë’formed‘in foodmaterialsduringÿthe‘
To What extent these
for initiation of déterior'ave reactions
is unknown? '
?
_
f
.
Ü
:
3. Additional Work is needed
the“ ox1dat1ve Changes m the'non4hp‘rd fraction
of freeze—dried feeds“,
1n prote1nsand amino
acrds
_
4. Our understanding of
role Of Changes in “morphological
tissues is imperfect. It appears to us thatabetter understandmg of changes in membrane
structures and their role in redistribution of food components
would ard
1n understanding the problem of changes;in
C&paCltY—.
1',