Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates ~o new forms of processed
vegetables, and typically celery and peppers, which heretofore
exhibited a loss of crispness when frozen and more particularly
it concerns the process whereby such produce will now retain an
appreciable degree of desired crisp texture after freezing.
Certain produce such as celery manifest an undesired
loss of texture as a result of freezing, and/or canning.
To explain, the typical structure of fresh celery
; will consist of a thin, continuous boundary of epidermal cells
which protect the inner tissues from physical damage and water
loss. Underneath the epidermis will be found a layer of paren-
chymatype cells which may be involved in a number of functions.
Directly under the epidermis to a depth whereat light may pene-
trate, the parenchyma cells are pigmented and involved in
photosynthesis; this tissue will be referred to as the hypo-
dermis. As one progresses away from the hypodermis and peri-
meter of the stalk, these underlying cells are larger in size
and function as storage tissue for the stalk. The bulk of the
stalk tissue is the cortex made up of thin walled parenchyma
cells.
Upon conventional freezing of celery the tissue
systems are disrupted greatly. The epidermis will be generally
torn loose from the tissue beneath it with only a few random
pieces attached to the underlying collenchyma bundles and a
random attachment to the hypodermis. The hypodermis will be
dislocated as will the collenchyma bundles which as viewed will
appear to be torn out of place and surrounded by large voids
where thin parenchyma tissues were torn away from them. The
parenchyma tissue which makes up the bulk of the cortex also
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suffers much damage; the cell walls are torn apart and randomly
located throughout. Vascular bundles also are torn apart into
separate tissues. Overall the fresh-frozen celery will be con-
siderably damaged and almost all of the tissue systems will be
disrupted to a large extent resulting in a loss of crispness.
This damage is largely due to the physical stresses of ice
crystal formation. Size and type of ice crystal is partially a
function of freezing rate, but in all commercial equipment,
damage is significant with low solids turgid products such as
celery.
Prior art workers have suggested the practice of de-
hydrofreezing fruits and vegetables including such produce as
apples and peas which have been optimized in processing by cus-
tomary food technology routes such as variety selection, slice
sizing, control of blanching conditions and the like. The
primary incentive for dehydro-freezing is the economical
advantage of shipping in the order of 50% less weight compared
to fresh-frozen or canned produce. Weight reduction results in
substantial savings in refrigerated transportation, storage
charges and handling labor.
According to the invention there is provided a pro-
cess of produeing frozen preserved vegetables which eomprises
redueing the moisture eontent of the vegetable to aehieve a
weight reduetion of 35-55%, and then freezing the vegetable
under freezing eonditions having a freezing plateau of less
than 10 minutes.
The present invention is founded upon the diseovery
that dehydro-frozen eelery and like produee redueed in moisture
eontent to broadly 45-65% (i.e. a weight reduetion of 35-55%)
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and preferably uniformly to 50-60% (i.e. a weight reduction of
40-50%) of its original weight relieves the strain of freezing
such that there is generally less tissue damage provided also
that the vegetables are frozen rapidly, which rapidity can best
be expressed by the products having a brief freezing plateau or
that time during the total freezing cycle when the product
temperature remains essentially constant. (In this connection,
such fast freezing is to be contrasted with static freezing at,
say, 0F or even blast freezing at -40F whereat a relatively
slow growth of water ice crystals occasions ice crystal growth
sufficient to induce cell damage and loss of crispness parti-
cularly upon rehydration.)
Dehydro-frozen celery, reduced and frozen as describ-
ed above can be easily restored or reconstituted to its orig-
inal weight and solids content, exhibiting texture and flavor
superior to that of conventionally frozen celery.
The improvement in dehydro-freezing stemming from
this invention is not to be restricted to any particular
theory. However, it is believed that with vegetables which
derive their structural integrity from high osmotic pressures
within the cells per se and which suffer most upon freezing
because of cell rupture, such as celery and bell peppers, and
carrots to a lesser degree, by employing the aforestatéd mois-
ture reduction there is simply less water to freeze and with
the resultant faster freezing rates, particularly in individual
quick freezing, one promotes smaller crystals and less con-
sequent cell damage. In addition, by increasing the concentra-
tion of water soluble compounds by partial dehydration, one
increases the degree of super-cooling and enhances the
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spontaneity of crystallization of the water present. It is
believed that this occasions a delay in the change of state but
that once crystallization starts, it proceeds rapidly with only
small crystals being formed, the presence of water soluble com-
pounds in a concentrated state influencing not only crystal
growth but also alteration of the type of crystal.
In this connection, it is important to recognize
there is a criticality such that if one overdrys there will be
irreversible cell damage simply due to dehydration and other
adverse quality effects occurring during freezing which result
in an inferior and inadequately reconstituted product when com-
pared with conventional produce such as frozen fresh celery.
If, on the other hand, one does not partially dry enough,
concentration of solubles and the effect thereof on ice crystal
formation is not significant, enough to minimize substantial
cell damage. It is believed that by adopting a 35-55% weight
reduction, there is a sufficient increase of soluble concentra-
tion in the produce's aqueous phase to supplement the benefits
that stem from a rapid freezing.
Freshly harvested celery is washed, rinsed, deleafed,
trimmed and sliced in a slicer set up to produce 3/8" thick
transverse cuts. No chemical additives need be added to the
product. The sliced celery is deposited onto a shaker screen
which serves to remove undersized pieces and small broken
pieces with some leaf. The uniformly spread product is fed to
a water blancher wherein the product is blanched at 200F for
one minute. The product discharging from the blancher is then
hydro-cooled rapidly by a cooling water system which reduces
overall slice temperature to below 60~F by maintaining the
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cooling water at a temperature of 45F.
The hydro-cooled slices are then essentially uniform-
ly dehydrated in a two-zone through-circulation hot air dryer
whose circulating air temperature at no time exceeds 165F.
Hot drying air entering the first stage of the dryer has a
temperature not exceeding 165F and leaves the dryer at 150F
in the first zone thereof. Product entering the second zone is
exposed to air not exceeding 150F and in its terminal stages
of dehydration product will be exposed to no heat. Thus, the
product will be uniformly dried while in a bed depth of 1-2".
The sliced celery will thus be reduced from an initial solids
content of 6.25-7.25% to a final solids content of 12.5-14.5%.
The thus reduced sliced celery will then be
individually quick frozen so as to emerge from a freezer at
between 0F and -10F employing a freezing plateau of 2-5
minutes. Preferably a moving bed or fluidized bed freezer is
employed; an immersion freezer employing liquid nitrogen or
freon may also be employed providing a relatively instant
freezing plateau. Generally speaking, product quality and
particularly texture will improve as the freezing rate is
increased, faster freezing resulting in smaller ice crystals
and minimization of cell damage.
The products produced by the present invention
exhibit generally better texture and flavor relative to conven-
tionally frozen unreduced counterparts in the class of celery
and peppers. Cell integrity is relatively well preserved. ~
Whereas the epidermis is torn away from the hypodermis of a -
celery piece and takes a portion of the hypodermis with it, the
hypodermis is still present and recognizable even though it has
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been split. Collenchyma bundles are still intact with a few
voids present there-around. The parenchyma tissue in the cor-
tex does appear erupted and the cells have burst open, but cell
walls appear more numerous and the large voids present in a
frozen control which has not been reduced are not visible. The
effects on cell structure are most manifest in relation to a
control when the specimens are thawed and rehydrated, a re-
hydrated dehydro-frozen celery slice for instance produced in
accordance with this invention will return to at least 95% of
its original weight and when compared with regularly frozen
celery, will have an improved texture, as is, as well as in
both canned and refrozen applications.
Although the invention has been described by
particular reference to celery, it is of equal application to
other vegetables which are prone to undergo significant changes
in their morphology due to freezing such as peppers as explain-
ed above.
