STABILITY OF FREEZE-DRŒD FOODS
193
tables, and of the proteins, in the case of foods derived from muscle tissue of animals.
It was our opinion at the time that, With regard to cellulose, the mechanism was increased
crystallinity due to increased hydrogen bonding but that in the case of proteins the situation
was extremely complex, that several mechanisms could be suggested, and that the nature
of the increased aggregation is not fully understood. As will be seen from the following
review of new work, our opinions have not changed in the intervening years.
-
The increase of crystallinity in cellulose during dehydration hasbeen demonstrated in
dehydrated carrots and in appropriate model systems by the elegant work of Shimazu
and Sterling (1961) and Sterling and Shimazu (1961). In this work it was clearly shown,
using X—ray diffraction, iodine absorption, density measurement, and enzymatic digestibility,
that both dehydration and subsequent storage in the dry state increase the crystalline content
of cellulose. It was also shown that blanching retarded this crystalzation to some extent.
The practical importance of this theoretical knowledge is beginning to become apparent.
For instance, different varieties of the same species would be expected to have dierent
crystalline contents, and varietal differences in suitabüity for dehydration might be expected.
Such differences have in fact been observed in practical studies (Spiess, 1964). Furthermore,
it would be expected that random scission of the cellulose chains by suitable agents would
counteract some of the eects of increased crystallinity. The susceptibility of cellulose to
scission by ionizing radiations is well known (Bovey, 1958), as is the softening effect of
radiation on plant and vegetable tissue (Maxie and Sommer, 1965). The application of
radiation to improve texture of dehydrated foods of plant origin, therefore, is a logical
extension of these ndings, and a patent for
been issued recently
to an American rm (Schroeder, l962)._» , ,
The other practical consequence is the use of freeze—_dried fruits in applications in which
complete reconstitution and softening are not required.
is notably true of cereal—fruit
combinations which are among popular breakfast foods in the United States. The combination of corn akes and freeze-dried peaches, strawberries, bananas, or blueberries has
resulted in commercial products which, to our knowledge, are enjoying a wide acceptance.
The loss of textural qualities in muscle tissue foods isconsiderably more diîcult to correct
and much less perfectly understoodthan similarevents in plant materials. Connell (1957)
stated that “it is probably true to say that.nodehydrated sh has as yet been prepared
whose texture is indistinguishable from,that of fresh sh”. This statement, applied to all
types of meats and sh, appears tous as true today as it was in 1957_
In reviewing theproblem in 1963,we concludedthat the actomyosin complex is probably
the site of changes in water—holdingand ofassociated îtextural changes and that the changes
may be due to one or,all of the
».
’
.
…
?
1. Aggregation or crosslinking of
‘
‘
.
2. Denaturation, or at least partial denaturation, of
proteins followed, by their
aggregation.
,
:…
w
':m
';
.
'
—
3. Interaction of the native,
With lipid—s or car-bohydrates.
The view that the changes causing 10ss of water—holding and—' texture occurin the actomyosin complex appears Well»substantiatedn0W.ln paicular,r1t 1'S known that
tenderness
of meats may be directly related
«sarcomeres
< of the—muscle brs (Locker and
Hagyard, 1963; Herring-et
'1î has
been demonstrated
recently
that the main features of
“?
freeze—‘dr1ed pork muscle
are
also produced by
is01&ttedmyobnls from
pork muscle (W1smerPedersen,
..
193
tables, and of the proteins, in the case of foods derived from muscle tissue of animals.
It was our opinion at the time that, With regard to cellulose, the mechanism was increased
crystallinity due to increased hydrogen bonding but that in the case of proteins the situation
was extremely complex, that several mechanisms could be suggested, and that the nature
of the increased aggregation is not fully understood. As will be seen from the following
review of new work, our opinions have not changed in the intervening years.
-
The increase of crystallinity in cellulose during dehydration hasbeen demonstrated in
dehydrated carrots and in appropriate model systems by the elegant work of Shimazu
and Sterling (1961) and Sterling and Shimazu (1961). In this work it was clearly shown,
using X—ray diffraction, iodine absorption, density measurement, and enzymatic digestibility,
that both dehydration and subsequent storage in the dry state increase the crystalline content
of cellulose. It was also shown that blanching retarded this crystalzation to some extent.
The practical importance of this theoretical knowledge is beginning to become apparent.
For instance, different varieties of the same species would be expected to have dierent
crystalline contents, and varietal differences in suitabüity for dehydration might be expected.
Such differences have in fact been observed in practical studies (Spiess, 1964). Furthermore,
it would be expected that random scission of the cellulose chains by suitable agents would
counteract some of the eects of increased crystallinity. The susceptibility of cellulose to
scission by ionizing radiations is well known (Bovey, 1958), as is the softening effect of
radiation on plant and vegetable tissue (Maxie and Sommer, 1965). The application of
radiation to improve texture of dehydrated foods of plant origin, therefore, is a logical
extension of these ndings, and a patent for
been issued recently
to an American rm (Schroeder, l962)._» , ,
The other practical consequence is the use of freeze—_dried fruits in applications in which
complete reconstitution and softening are not required.
is notably true of cereal—fruit
combinations which are among popular breakfast foods in the United States. The combination of corn akes and freeze-dried peaches, strawberries, bananas, or blueberries has
resulted in commercial products which, to our knowledge, are enjoying a wide acceptance.
The loss of textural qualities in muscle tissue foods isconsiderably more diîcult to correct
and much less perfectly understoodthan similarevents in plant materials. Connell (1957)
stated that “it is probably true to say that.nodehydrated sh has as yet been prepared
whose texture is indistinguishable from,that of fresh sh”. This statement, applied to all
types of meats and sh, appears tous as true today as it was in 1957_
In reviewing theproblem in 1963,we concludedthat the actomyosin complex is probably
the site of changes in water—holdingand ofassociated îtextural changes and that the changes
may be due to one or,all of the
».
’
.
…
?
1. Aggregation or crosslinking of
‘
‘
.
2. Denaturation, or at least partial denaturation, of
proteins followed, by their
aggregation.
,
:…
w
':m
';
.
'
—
3. Interaction of the native,
With lipid—s or car-bohydrates.
The view that the changes causing 10ss of water—holding and—' texture occurin the actomyosin complex appears Well»substantiatedn0W.ln paicular,r1t 1'S known that
tenderness
of meats may be directly related
«sarcomeres
< of the—muscle brs (Locker and
Hagyard, 1963; Herring-et
'1î has
been demonstrated
recently
that the main features of
“?
freeze—‘dr1ed pork muscle
are
also produced by
is01&ttedmyobnls from
pork muscle (W1smerPedersen,
..