Note: Descriptions are shown in the official language in which they were submitted.
--1--
METHOD OF PEELING THIN SKINNED FOOD PRODUCTS
The present invention relates to a method of
peeling thin skinned food products.
Most of man's foods in the natural state are
protected with a covering such as the peel tissues of fruits
and vegetables, hulls of legume seed pods, shuck covering of
corn ears, skins of onions, shells of nuts, scales of fish,
and the calcareous coverings of shellfish. In most cases,
these coverings are considered to be inedible and are
therefore removed when preparing the product for human
10 consumption. Current methods used in the food industry
include mechanical peeling with power operated knives,
abrasive peeling, flame peeling, scalding, steam peeling,
mechanical shelling of legume crop, mechanical cracking of
nuts, mechanical shelling of shrimp. Adequate washing and
15 varying amounts of scrubbing, sorting, and hand trimming are
required to complete the operations.
There are two common commercial types of steam
peeling methods. In the first of these methods, vegetables
are charged through a rotary-type sluice gate batcher into a
20 hermetically sealed chamber and are processed under a
pressure of 3.4-6.1 bar for 40-90 seconds. While being
processed by steam, the vegetables are displaced along the
chamber by a screw conveyor, from the point of charging to
the point of discharge. The vegetables are then discharged
25 through a sluice gate batcher into a washing and peeling
machine in which they are mechanically peeled (FMC
Continuous High Pressure Steamers Models 140 and 240 FMC
Catalog (1980) pp. 49-50). The second peeling method
utilises a high pressure batch-type steam vessel which is
30 filled from above, purged with steam for several seconds and
hermetically sealed prior to a gradual pressure build up.
Pressures up to 20.4 bar for 20-60 seconds are used. The
exhaust valve is then opened requiring several seconds for
the pressure to gradually fall, and before the discharge
35 hatch can be opened. The produce is then discharged on a
~JL
--2--
conveyor and is conveyed to a washer where the peeling is
mechanically completed (US Patent 4091722). A further
steam peeling ~ethod has been described, but is not utilised
by the industry, whereby the produce to be peeled is
conveyed by gravity through a system of chambers separated
by hydraulically actuated gate valves. In the process the
produce moves from an atmospheric chamber to a low pressure
steam environment to a vacuum chamber (US Patent 3759166).
The prior methods and apparatus for processing
10 vegetables by steam are disadvantageous because following
processing and due to slow venting of the pressure vessel
the surface layers of the produce are excessively heated.
The heat continues penetrating inside, thereby increasing
the depth of the layer being treated, which results in large
15 amounts of by-product and correspondingly higher peeling and
trimming losses. These methods are most applicable for
processing vegetables of a hard consistency (potatoes for
example). Fruits and vegetables of delicate consistency are
not well peeled in such methods because they quickly begin
20 to cook and become soft and/or are sensitive to high
pressures which disrupt their structural integrity. A
further "thermal blast" process is shown in US Patent
4524681 and US Patent 4569850, in which the product is held
for a brief period in a heated, closed vessel filled with
25 superheated steam under elevated pressure, and then
instantaneously releasing pressure. During the holding
period, thermal energy is transferred at a very fast rate to
the moisture just beneath the outer covering of the product,
by the simultaneous exposure of the product surface to two
30 sources of heat: namely direct contact with pressurised,
superheated steam and radiant heat from the hot vessel
walls. Becoming highly energised, the moisture immediately
beneath the surface flashes to the vapour state when the
pressure surrounding the product is suddenly released. The
35 rapidly expanding vapour utilises the thermodynamic
lZ91915
--3--
properties of water to cause an explosive process which
blasts the outer peel, pod, shuck, skin, scale, or shell
from the product under treatment. Loss of edible portion is
minimal and the exposed surface is smooth and attractive.
The thermal blast process has proven to be
effective in removing the covering from a wide variety of
food products including fruits, vegetables, nuts, fish and
shellfish. By varying the temperature and/or time of
heating, excellent results have been obtained in removing
10 coverings ranging from the very think peel of a ripe peach
to the relatively thick peel of an orange.
However, the process has the problem of requiring
a substantial amount of time and energy to carry it out.
According to the present invention there is
15 provided a method of peeling thin skinned food products with
minimal loss of or damage to edible portions thereof, said
method comprising the steps of feeding said food product
continuously into and out of a vessel, subjecting said food
products as they pass through said vessel, to radiant heat
20 at a temperature in the range of 340 C to 705 C and to
contact heat from the fluid in the vessel at a temperature
in the range of 340C to 705C, maintaining the pressure in
said vessel at a level not greater than two bars gauge and
arranging for the food product to spend between 8 and 75
25 seconds in said vessel and immediately thereafter subjecting
said food products to ambient temperature and pressure.
The thermal shock process as disclosed herein is
distinctively different from any of the current commercial
processes. It is related to conventional pressurised steam
30 peeling in that steam is utilised in both processes. The
manner in which the steam is utilised and the end results
are vastly different.
While the method of the present invention has
certain common features with the ~thermal blast~ process
35 there are a number of important differences. Their common
lZ91~1 ~
feature is that they both utilise radiant heat from the
walls of a vessel and contact heat from a fluid or gaseous
medium simultaneously for rapid surface heating of the
product. A major difference is that the present "thermal
shock" method is continuous and utilises relatively low
pressures compared with those of the "thermal blast"
process, and generally higher temperatures for the
superheated steam or other fluid or gaseous medium. The use
of lower pressures in the present invetnion limits somewhat
10 the range of products on which the invention is effective,
but has the advantage of allowing efficient and continuous
peeling to proceed, rather than having to accomplish the
peeling in discrete individual batches. Differences between
the present invention and the "thermal blast" patents are
15 set out in Table I below.
TABLE I
COMPARISON OF THERMAL BLAST AND
THERMAL SHOCK PROCESSES
Items of Thermal Blast Thermal Shock
20 Comparison Process Process
Product Effective in blasting Effective only in
Adaption the peel, pod, shuck, peeling certain
skins, scale or shell products with
from many vegetable relatively thin skins
and animal products. that separate easily
from the edible
portion, by thermal
shock treatment at
low pressure.
30 Properties 2 to 6.8 bar Pressure not greater
of Super- pre~sures and 204 to than 2 bar,
heated steam 427 C temperatures. preferably 0 to 0.68
barOpressures; 348 to
704 C temperatures,
preOerably 427 to
538 C.
1291~1~
TABLE I (cont'd)
Items of Thermal Blast Thermal Shock
Comparison Process Process
Flow of Batch process with Product flows
Product loading and discharge continuously through
from same end of vessel, entering and
vessel. This requires and leaving through
tilting of vessel seal means. Vessel
upward for loading and can remain in a fixed
downward for position. Continuous
discharge. Process flow results in
time is lost between higher production
batches. rate.
Thermal Rapid heating of Rapid thermal shock
Treatment product surface treatment at low
followed by blast pressure, followed by
discharge. scrub-wash, if
needed.
Steam Use Steam lost by exhaust Steam can be recycled
to atmosphere from and supplemented with
each batch. make-up steam
generated in heat
exhangers.
Product Special system Product discharged
Discharge required for catching gently to
product during blast conventional
discharge. conveyor.
25 Process Noise Muffling of blast No unusual process
noise is required. noise involved.
Controls and Multiple step process Continuous flow
Automation increases control and process minimises
automation control and auto-
requirements. mation requirements.
The thermal shock method utilises a combination
of radiant heat coupled with superheated steam or other
fluid or gaseous medium ranging in temperature into which
conditions relatively thin-skinned fruits, vegetables or
35 other food products, such as tomatoes, potatoes, peaches,
lZ9~91~
carrots, apples, plums, and kiwis are introduced. The
sudden exposure to intense heat weakens the bonds between
the peel and the subsurface matter, without significant
injury to the latter. Because the pressure of the
superheated steam or othr fluid or gaseous medium is
relatively low, the pressure beneath the peel resulting from
heated moisture immediately below the surface of the peel
cannot reach an equillibrium level sufficient to "blast" the
peel off upon rapid depressurisation as in the thermal blast
10 process. However, the pressure under the peel does aid in
further weakening or destroying of the bonds holding the
peeling to the subsurface matter, although less dramatically
and, in the case of relatively thicker skinned food
products, somewhat less thoroughly than in the case of the
15 thermal blast inventions. However, for relatively thin
skinned food products, the thermal shock method of the
present invention achieves excellent results in
accomplishing the desired peeling. Depending upon the
thickness of the peel and the degree of ripeness of the
20 fruits or vegetables being processed, some follow-up washing
or scrub-washing may be desirable to remove completely all
of the loosened peel to the extent not already removed by
the thermal shock method. However, such washing or
scrub-washing is minimal in comparison with other known
25 peeling methods, except the thermal blast method, and
generally leaves the subsurface matter of the food products
without significant damage. Table 11 below shows the
results obtain for given lengths of thermal shock processing
time for a variety of food products suitable for the method
30 of the present invention:
~Z~
TABLE II
PROCESS TESTS ON THERMAL SHOCK PEELING
OF FRUIT AND VEGETABLE PRODUCTS
Thermal Shock
Treatment
(Seconds Process
Product Time) Evaluation of Results
Tomato 15 Excellent peel, scrub-wash
required.
10 Peach 20 Excellent peel, needs water
wash only.
Carrot 20 Excellent peel separation,
water wash only.
Apple 30 Peel loosened, removed by
scrub-wash~
Apple 60 Complete peel with scrub-
wash, some caramelisation.
Potato 60 Complete peel with wash.
Plum 15 Excellent results, requiring
water wash only.
Kiwi 60 Peel washed off, some heating
of flesh causing outer layers
to break off in places.
The thermal shock treatment above was carried out
25 at a vessel temperature of approximately 482C, fluid
heating medium (superheated steam) at approximately 0.1 bar
and 344 to 372C temperature.
In order that the present invention may more
readily be understood, the following description is given,
30 merely by way of example reference being made to the
accompanying drawing, in which:-
Figure 1 is a very schematic view showing one way
of carrying out the thermal shock method according to the
invention; and
Figure 2 is an enlarged view showing the vessel
1291~i~
of Figure 1 with certain of its associated parts.
Referring first to Figure 1 there is illustrated
an insulating combustion chamber 10 having in its lower part
a burner 12 from which hot combustion gases 14 flow.
Mounted for rotation above the burner 12 is a cylindrical
vessel 16 which is rotatable about a horizontal axis on
support rolls 18. Further details of the vessel will be
described below.
Adjacent the upper end of the combustion chamber
10 10 is mounted a first heat exchanger 20, which is fed
ambient temperature water via an inlet 22. An outlet 24 for
this heat exchanger 20 is connected to the second heat
exchanger 26 which in turn is connected via an outlet 27 and
a fan 28 located in a fan housing 29 to a further heat
15 exchanger 30. Finally a fourth heat exchanger 32 receives
ambient air from an inlet 34 and this is fed via an outlet
36 to provide preheated combustion air to the burner 12.
It will be seen that the cylindrical vessel 16
has associated therewith two fixed, stationary heads 40 and
20 42 and annular vessel seals 44, 46 allow the vessel to
rotate while maintaining a super-atmospheric pressure within
the vessel.
Connected to the stationary head 40 is a product
inlet 48 and to the stationary head 42 a product outlet 50.
25 An inlet valve 52 in the form of a rotary cylindrical valve
in which discrete compartments 54 are provided in inlet 48
and a similar valve 56 is included in the outlet 50.
Extending along the inner surface of the wall of
the cylinder 16 is a helical conveyor auger 58 and a support
30 rod 60 is fixed to the two stationary heads 40, 42 and
carries a plurality of inclined baffles 62 at axially spaced
locations therealong. A superheated steam inlet 64 is
provided in the fixed head 42 and a superheated steam outlet
66 $n the fixed head 40, these being connected to the outlet
35 of the third heat exchanger 30 and to the inlet side of the
1~9191S
_9_
fan housing 29, respectively (see Figure 1). The outlet
valve 56 is positioned to feed products via a wash and scrub
washer 68 and then to a conveyor 70 for the next processing
or packaging operation.
Upstream of the inlet valve 52 is an inlet gate
72 which can be opened and closed to allow food products to
pass to the valve 52. Mounted coaxially adjacent the fixed
head 42 is a cylindric.al member 74 and between the last
flight of the auger 58 and the fixed head 42 is a product
10 elevator baffle 76. The upper end 78 of the outlet 50 is
formed as a hopper or chute, so that when the baffle 76
reaches the top, food products picked up by the baffle 76,
readily drop down and out through the outlet 50. To prevent
the product entering the cylindrical member 74 a grill 80 is
15 provided at the inner end, this preventing the product
entering, but allowing the superheated steam to flow from
the conduit 64 into the interior of the cylindrical vessel
16.
In operation a suitable fuel is fed to the burner
20 12 and ignited and the hot products of combustion flow
upwardly in the furnace combustion chamber 10 heating first
the cylindrical vessel 16 and then the heat exchangers 30,
26, 32 and 20. The heat exchanger 32 improves the
efficlency of the combustion, because the combustion air
25 arrives at the burner at an elevated temperature.
Ambient temperature water is fed in at 22 and is
preheated in heat exchanger 20 before it is fed to the
second heat exchanger 26 in which low pressure dry make-up
steam is produced. This is fed via the fan 28 to the heat
30 exchanger 30 in which superheated steam is created and this
is fed via conduit 64 to the vessel 16 in which it flows
from right to left, in Figure 1, and exits via conduit 66
for recycling by the fan 68.
A food product is fed from a supply (not shown)
35 to the inlet valve 52 which is cyclicly rotated so that the
--10--
product is deposited in the vessel 16 and is fed
therethrough by the auger 58 which rotates with the vessel,
the baffle 60 assisting the movement of the product which
then leaves via the outlet valve 56. The inlet and outlet
valves 52, 56 maintain an adequate pressure within the
vessel while allowing the product to be fed continuously
into and out of the vessel. The product then leaves via the
washer 68 and the conveyor 70.
The walls of the vessel are maintained at
10 tempera~ure levels of 340C to 705C and the superheated
steam is maintained also at temperature levels of 340C to
705C, with preferable temperature ranges for the vessel
walls and the superheated steam being in the range 427C to
538C. Pressures of the superheated steam can range up to 2
15 bar but are preferably in the range of 0 to 0.68 bar. The
combination of radiant heat from the walls of the vessle 1
and the contact heat from the superheated steam and the
pressure from the superheated steam synergisticially destroy
or substantially weaken the bond between the peel and the
20 underlying food substance of relatively thin peeled
products, i.e. generally less than 0.8 mm in thickness.
This is achieved in a matter of seconds, quickly enough to
prevent damage to the food product. The results of typical
tests are shown in Table II above. The relative low
25 pressure utilised in the present invention make the
continuous processing possible which would not be suitable
under very high pressure conditions. It is found that low
pressures can be maintained without sacrificing the
desirable feature of continuity in an assembly line type
30 processing operation, such as in a commercial food
processing plant. In general, with relatively thin skinned
fruit and vegetables, excellent results are obtained. The
degree of ripeness of the fruit or vegetables being
processed can affect the utility of the invention if the
35 peel has a thickness near the upper limit mentioned. In
general, the peel of riper fruit or vegetables is less
firmly bonded to the underlying fruit or vegetable matter
beneath the peel and can thus be removed more readily than
the peel of less ripe fruit and vegetables. Hence, in the
case of ripe fruit and vegetables, thicker peel can in
practice be removed than is the case with less ripe
products.
The recycling of the superheated steam exhausted
from the vessel 16 back into the heat exchanger 30 is not an
essential part of the invention, but improves the energy
10 efficiency. Similarily the air preheated 32 is not an
essential part of the invention but again improves the
thermal efficiency. Further efficiencies are achieved by
the baffles 62 which direct the superheated steam within the
vessel more precisely onto the product as it is processed
15 within the vessel. The insulated furnace housing 10 again
is not essential but improves the thermal efficiency.
The inlet and outlet valves 52, 56 form a seal
for the rotating vessel 16 but these could be replaced by
other known types of valve, such as rotating seals,
20 screwseals, water U-tube seals or any other convenient form.
While superheated steam is the preferred fluid
because it is inexpensive, plentiful and non-toxic and holds
its heat well, any other suitable fluid may be used.
The washing or scrub-washing in the washer 68 is
25 not an essential part of the invention but it can be useful
in removing residues of peel and can be an efficient and
effective way to improve the appearance of the fruit and
vegetables prior to further processing or packaging.
The superheated steam is shown circulated in the
30 vessel 16 countercurrent to the flow of the product. This
i8 preferred because it maximises the temperature to which
the product is exposed as it leaves the vessel and this
assists in breaking the bond between the peel and the edible
material. However, the invention will work, although less
35 effectively, if the fluid flows in co-current with the food
product.
