STABILITY OF FREEZE—DRIED rooms
195
withfree fatty acids, liberated from the phospho—lipid fraction of the sh by enzymatic
actrvrty of esterases (King et al., 1962). Experiments in model systems appear to conrm
this view, and, in any case, the possibility exists that changes in the lipoproteins, which are
components of structurally important membrane systems, will cause changes in permeability
to water, changes in ionic atmosphere surrounding sarcoplasmic and myobrillar proteins,
and redistribution of lipid components, all of which may have profound effects on waterholding capacity.
.
The role of lipoproteins and of phospholipids in changes occurring in dehydration is
almost completely unknown, but it is well established that their breakdown during
dehydration is at least possible, if not probable (Harrison and Trevelyan, 1963; Lea, 1958).
The crosslinking reactions initiated during the dehydration process are further intensied
in storage, even when the moisture content is maintained at a low level. This course of
events is exemplied by the study of Miller and May (1965) in which loss of tenderness
of chicken meat was studied as a function of freezing, dehydration, and storage. The mean
shear value for freeze—dried chicken meat one week after drying was 31.6 lbs ; after storage
for three months at refrigerated temperatures, the shear value was 38 lbs.
The nature of storage toughening may be similar to that occurring during freeze—drying
or may be due to non-enzymatic browning, usually by interaction with oxidizing lipids.
In the case of freeze-dehydrated jack mackerel, it was observed by Toyomizu et al. (1963)
that extractability of the myobrillar proteins decreased during drying, as well as during
storage. The changes in extractability during the period from the 5th to the 20th day of
storage at 37 °C were correlated with the simultaneous oxidation of lipids; the loss of
extractability during drying and in the rst few days of storage was caused by other factors.
Non—Enzymatic Browning
_
Non—enzymatic browning is an important mechanism of deterioration of dehydrated foods
and leads to loss of acceptable color, development of o-avor, and loss of solubility.
Low moisture contents usually retard browning, and the conditions whereby freeze-dried
foods which are dehydrated to water contents below 2 % and are paCkaged in impermeable
containers tend to reduce the likelihood of signicant deterioration due to this reaction.
In certain cases, however, non—enzymatic browning can occur at extremely low water
contents. Our work on orange juice crystals indicated that non—enzymatic browning could
occur at water contents as low as 0.8 % and that it was correlated with loss of ascorbic acid.
Interestingly enough, both the loss of ascorbic acid and the non-enzymatic browning
occurred at almost identical rates when stored in air or stored under vacuum (Karel, 1960;
Karel, 1963; Karel and Nickerson, 1964).
Our results have received some conrmation in recent years by the work of several
investigators who worked with other fruits. Clegg (1964) and Clegg
and Morton(l965),
working with single strength lemon juice (not dehydrated), established that_browmng
was
due to reactions involving ascorbic acid. Oxidation of ascorbic amd was an important step
in the development of browning. These results appear to substantiate the role of ascorbic
acid in browning of dehydrated orange juice.
_
_
The surprising observation made by us and referred to above, 1.e., that packagmg under
vacuum did not retard loss of ascorbio acid when water contents were suicreny high,
'
has also been conrmed under conditions similar to those in our own work. Krebes and
'
Behun (1964) studied freeze dehydrated fruit nectars, prepared from strawberry, rose h1ps,
195
withfree fatty acids, liberated from the phospho—lipid fraction of the sh by enzymatic
actrvrty of esterases (King et al., 1962). Experiments in model systems appear to conrm
this view, and, in any case, the possibility exists that changes in the lipoproteins, which are
components of structurally important membrane systems, will cause changes in permeability
to water, changes in ionic atmosphere surrounding sarcoplasmic and myobrillar proteins,
and redistribution of lipid components, all of which may have profound effects on waterholding capacity.
.
The role of lipoproteins and of phospholipids in changes occurring in dehydration is
almost completely unknown, but it is well established that their breakdown during
dehydration is at least possible, if not probable (Harrison and Trevelyan, 1963; Lea, 1958).
The crosslinking reactions initiated during the dehydration process are further intensied
in storage, even when the moisture content is maintained at a low level. This course of
events is exemplied by the study of Miller and May (1965) in which loss of tenderness
of chicken meat was studied as a function of freezing, dehydration, and storage. The mean
shear value for freeze—dried chicken meat one week after drying was 31.6 lbs ; after storage
for three months at refrigerated temperatures, the shear value was 38 lbs.
The nature of storage toughening may be similar to that occurring during freeze—drying
or may be due to non-enzymatic browning, usually by interaction with oxidizing lipids.
In the case of freeze-dehydrated jack mackerel, it was observed by Toyomizu et al. (1963)
that extractability of the myobrillar proteins decreased during drying, as well as during
storage. The changes in extractability during the period from the 5th to the 20th day of
storage at 37 °C were correlated with the simultaneous oxidation of lipids; the loss of
extractability during drying and in the rst few days of storage was caused by other factors.
Non—Enzymatic Browning
_
Non—enzymatic browning is an important mechanism of deterioration of dehydrated foods
and leads to loss of acceptable color, development of o-avor, and loss of solubility.
Low moisture contents usually retard browning, and the conditions whereby freeze-dried
foods which are dehydrated to water contents below 2 % and are paCkaged in impermeable
containers tend to reduce the likelihood of signicant deterioration due to this reaction.
In certain cases, however, non—enzymatic browning can occur at extremely low water
contents. Our work on orange juice crystals indicated that non—enzymatic browning could
occur at water contents as low as 0.8 % and that it was correlated with loss of ascorbic acid.
Interestingly enough, both the loss of ascorbic acid and the non-enzymatic browning
occurred at almost identical rates when stored in air or stored under vacuum (Karel, 1960;
Karel, 1963; Karel and Nickerson, 1964).
Our results have received some conrmation in recent years by the work of several
investigators who worked with other fruits. Clegg (1964) and Clegg
and Morton(l965),
working with single strength lemon juice (not dehydrated), established that_browmng
was
due to reactions involving ascorbic acid. Oxidation of ascorbic amd was an important step
in the development of browning. These results appear to substantiate the role of ascorbic
acid in browning of dehydrated orange juice.
_
_
The surprising observation made by us and referred to above, 1.e., that packagmg under
vacuum did not retard loss of ascorbio acid when water contents were suicreny high,
'
has also been conrmed under conditions similar to those in our own work. Krebes and
'
Behun (1964) studied freeze dehydrated fruit nectars, prepared from strawberry, rose h1ps,