Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for the re-
covery of fish meal and fish oil from raw fish. Particularly, the
invention concerns the production of fish meal of high protein
quality for animal feeding.
From various tests for the purpose of producing fish
meal products of high nutritional value, it is known that the
various heat treatments at raised temperature, to separate oil
and evaporate the relatively large amount of water contained in
the raw material, have a detrimental influence on the quality or
the nutritional value of the fish proteins. The time factor is
certainly also important, and therefore at least one of the con-
ditions of short process time and low process temperature is im-
portant for maintaining the functional characteristics of the
fish proteins.
With the knowledge that a process temperature not ex-
ceeding 50-60C has no substantial detrimental influence on the
nutritional value of the fish proteins, a conventional fish meal
plant could be directly modified by arranging to carry out all
the heating steps under vacuum. Such a process, however, would
probably be unsatisfactory due to unreasonably long residence
time in the different process steps and due to separation diffi-
culties and clogging problems that would arise.
The principal object of the present invention is to pro-
vide a process for the recovery of fish meal with high protein
quality, which process makes possible the use of sufficiently
mild process conditions as to time and temperature to avoid said
separation and clogging problems and which process does not re-
quire unacceptable increases in capital costs and operating~costs.
A process according to the present invention comprises
heating a fish mass obtained by disintegrating raw fish, separat-
ing and removing oil from the heated fish mass by means of a cen-
trifuge provided with a separate outlet for a separated oil phase,
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and evaporating and drying sludge and stick water discharged from
the centrifuge to a fish meal with desired low water content, the
process is characterized mainly in that the stick water and sludge
discharged from the centrifuge, possibly after having been sub-
jected to a disintegration treatment, are fed together to an eva-
poration unit where the mixture of sludge and stick water are fed
esssentially in plug-flow through at least one indirectly heated
tube, the inner walls of which are continuously cleaned by means
of a scraping device, whereafter the evaporated product is separa-
ted from the steam formed in said tube and dried in a drying step.
The basic idea of the present invention is that a largerwater amount than in a conventional fish meal process is removed
in a step where the stick water and sludge from the oil separation
step are evaporated together in an evaporator through which the
fish mass is passed in a so-called plug-flow during very short
residence times, that is, in the range of 2-6 minutes. According
to one embodiment of the invention, a great part of the evapora-
tion is carried out at atmospheric pressure and at comparatively
high temperature, that is, a product temperature of 80-90C.
This provides an advantageous heat economy, while the fish mass
stays in the evaporator such a short time that the detrimental
influence on the fish proteins caused by said high temperature is
very limited.
The evaporation step according to the present invention
also provides other differences and advantages compared with con-
ventional processes. For example, the stick water evaporation
heretofore carried out separately was a critical step. If the
stick water fed to the evaporator contained a large quantity of
fine particles ("fines") due to deficient separation in the pre-
ceding centrifugation step, or the raw fish was excessively dis-
integrated, clogging of the evaporation plant and costly stoppages
easily occured. This problem is practically eliminated in the
~present process, in which the evaporation is carried out by means
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of tubular heat excha~gers provided with rotating scraping means.-
Evaporators of this kind not only allow large amounts of solid
material to pass through the tubes without risk of clogging but
also they can even evaporate the fish mass up to a high dry con-
tent in the range of 60-80%. Further, the disintegration of the
raw fish can be carried out to a considerably greater degree than
before, since there are no demands for separation of solid par-
ticles from stick water in the oil separation centrifuge because
the conventional stick water evaporator and thus the incrusting
problem in the same are eliminated.
The initial heating step can be carried out in any way.
The temperature should, however, not exceed 55-60C, particularly
if a boiling apparatus is used that requires comparatively long
residence time for the fish rnass. For the process of the present
invention, preferably a h~at exchanger is used which is of the
same type as the one used in the evaporation plant, that is, a
heat exchanger in which the fish mass is fed through an indirectly
heated tube provided with rotating scrapers which keep the inner
walls of the tube clean from incrustations and cause turbulence.
In this way, the heating time can be kept very short, for example,
in the range of 0.5-3 minutes. Condensed steam from the evapora-
tion can be used as heating medium in the initial heating step.
If the heat exchanger with rotating scraping means and
said short residence time are used for heating the raw fish mass,
a higher temperature (for example, in the range of 70-90C) can
be used also in this step without any severe detrimental influence
on the fish proteins. Such a heating of the fish mass for a
short time provides pasteurization. This is a further advantage
of the process according to the invention, since this pasteuriza-
tion is obtained for nothing compared with a vacuum process of aconventional type, in which the pasteurization step would require
additional equipment. If in the process of the invention heating
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of the fish mass to a temperature in the range of 70-90C is de-
sired only in order to obtain said pasteurization effect, said
heating can be effected either in the raw fish heater or in the
evaporator. The first alternative is advantageous with respect
to the short residence time. On the other hand, the advantageous
heat economy by using higher temperature in the evaporator has
already been mentioned.
The final step of drying can be carried out in any way.
By evaporating the fish mass according to the invention to a very
high dry content in the range of 50-80%, a considerably abbrevi-
ated drying procedure is obtained. This means an improvement of
the protein quality even if a conventional drying method at a
temperature in the range of 80-100C is used. In the preferred
process according to the invention, however, a milder drying
method is used such as vacuum drying or spray drying. If the
drying is carried out under vacuum and the evaporation occurs at
atmospheric pressure, the steam produced in the evaporator can be
used as heating medium in the drying step.
Due to the fact that the amount of water to be removed
in the final drying step is less than heretofore, even such a
method as freeze-drying can be economically possible in this con-
text, particularly if a product of very high quality is desired.
Even if the evaporation in the new process is carried
out for economic reasons at atmosphere pressure and at "normal"
process temperature, the process can be carried out entirely as
a low temperature process. In such a case also the evaporation
step is carried out under vacuum, which makes the process more
expensive. This additional cost, however, can be more than com-
pensated by a high product quality in a process taking full ad-
vantage of low temperature steps as well as short residence times.
For the oil separation a centrifuge is used in whichthe separated oil phase is directly removed through a separate
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outlet for storing of further finishing. For this purpose, a so-
called three-phase decanter is suitable, that is, a centrifuge
provided with a horizontally journalled conveyor screw for
separated sludge and three separate outlets for separated oil
phase, stick water phase and sludge phase. According to the in-
vention, the separated stick water phase and sludge phase are
directly remixed. Before the suspension is fed to the evaporation
plant, a further fine division of the particles by means of a
disintegrating device can be advantageous. Since in this case
the separation of sludge phase from stick water phase is not re-
quired, it is also possible to use a modified centrifuge in which
the oil phase is taken out separately while the stick water and
the sludge are discharged through a common outlet.
The invention will be further described below by means
of an embodiment shown as an example in the attached drawing, in
which the single illustration is a flow diagram of a continuous
fish process according to the invention.
Raw fish is finely divided in a disintegrator 1 and
fed to a buffer tank 2. From the tank 2 the fish mass is fed by
means of a pump 3 through a heat exchanger 4 in which the fish
mass is fed in plug-flow through an indirectly heated vertical
tube provided with a rotating scraping device. Lines 5 and 6 are
a supply line and a discharge line, respectively, for the heating
medium. The broken line 7 designates an optional supply line in
which the heating medium is condensate from the subsequent evapo-
ration step. The fish mass leaving the heat exchanger 4 is fed
to a three-phase decanter 8, from which there is continuously dis-
charged separated oil phase through line 9, the stick water phase
through line 10 and a sludge phase through 11. The sludge phase
and stick water phase are then remixed in a collecting space 12,
from which the remixed fish suspension is fed by means of a pump
13 through a further disintegrator 14 and introduced into the
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bottom of the evaporating plant 15.
The evaporating plant 15 comprises a vertical heat ex-
changer tube 16. The latter is provided internally with rotating
scrapers and a separating space 17 for the separation of evapora-
ted product which is discharged at the bottom through line 19
and which is discharged at the top through line 18. The heating
steam is supplied to the evaporator through line 20 and the cor-
responding condensate is discharged at the bottom of the plant
through line 21. The three-way-valve 22 and the line 7 designate,
as mentioned above, the option of using the condensate as heating
medium in the heat exchanger 4.
From the evaporation plant, the fish mass is fed through
line 19 and a conveyor 23 to a dryer 24, which is connected to a
vacuum pump through line 27. Fish meal dried to a water content
in the range of 8 to 10% is discharged through a lock 28. Heating
medium for the dryer is supplied through line 32. Heating gas or
steam is supplied through line 33 and the valve 34. The dotted
line 36 and the three-way-valve 35 designate the option of using
the steam produced in the evaporator 15 also as the heating medium
for the dryer.
When the drying step is carried out under vacuum, the
temperature of the fish mass during the drying step should not
be substantially greater-than 60C.
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