Note: Descriptions are shown in the official language in which they were submitted.
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Method of drying portion pieces of fish and meat products
The present invention concerns a method of drying whole portion pieces of fish
or meat or portion
pieces of minced fish or forcemeat.
Background
An increasing time pressure in modern society has changed peoples life and
meal habits.
Contemporaiy consumers request food of a high quality and a short preparation
time. This change
in lifestyle has lead to an increasing demand for instant products.
Split cod is a 500 year old tradition. The present manufacture of split cod
involves a number of
steps. Fresh fish with a water content of 80 % are firstly salted. After the
salting process that takes
a few weeks the water content is reduced to about 55 %. The fish are
thereafter transported to a
drying plant and left for drying until a water content of about 40-45 % is
reached. Thereafter a
dilution process requiring 1-2 days takes place. All in all this is a time
consuming and expensive
process and therefore a production line that is simpler and less time
consuming has been developed
and the usefulness of the product has simultaneously increased.
Stockfish is produced according to the old natural method. Cleaned (gutted)
fish with heads
removed are suspended on fish flakes for two months until the water content is
about 15
Particularly stockfish has a varying quality due to variations in the weather.
Among the more modern methods could be mentioned that it is generally known to
freeze-dry
foodstuffs to obtain a better quality than with other drying methods and a
more rapid processing
than with traditional preparation of split cod and stockfish. As far as the
inventors know there has
never been developed a method to dry entire portion pieces of fish or meat
while preserving a
quality that is close to the quality of their fresh equivalents.
Objective
It is thus an objective of the present invention to provide a method of drying
products of fish, meat,
minced fish or forcemeat in the form of entire portion pieces, which yields
products of a high
quality that can rapidly be rehydrated.
It is furthermore an objective to provide a method as mentioned above which
inay be conducted
with inexpensive means within a comparatively short period of time.
It is still further an objective to provide a method as defined above to
prepare food products which,
when employing relevant fresh food, may be used instead of stockfish or split
cod in traditional
courses or as substitution for fresh food, said products having long shelf
life at room temperatures
when conveniently packed.
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The invention
The above mentioned objects are fulfilled by the method according to the
present invention as
defined by claim 1.
Preferred embodiments of the invention are disclosed by the dependent claims.
According to the method of the present invention portion pieces of fish or
meat or portion pieces of
minced fish or forcemeat are dried in two consecutive steps of which the first
takes place at a
temperature lower than the initial freezing point. The raw material is fresh
or frozen foodstuff which
is divided into pieces of a size suitable to be served. It is convenient to
use pieces of a mainly
uniform size since the process thus may be adapted with respect to retention
time to ensure an
optimal quality of the product.
The term "portion pieces" is not a clear scientific denotation, and as used
herein it is meant to cover
portion pieces that are typically put on a plate and served ready to eat. To
fit the definition of
portion piece the largest dimension should have a size of at least 40 mm and a
ratio between the
largest and the smallest dimension of at least 2:1. More typically portion
pieces have a length in the
range 70-90 mm, a width in the range 50-70 mm and a thickness in the range 5-
20 mm. The size
may, however, vary significantly and portion pieces of meat will often be
quite thin while portion
pieces of fish, minced fish and forcemeat will generally be thicker. It should
be emphasized that
though portion pieces, such as fish fillets, often have an accurately
rectangular shape the portion
pieces according to the invention may as well have an oval cross-sectional
shape or other cross-
sectional shapes.
Different types of equipment may be used for conducting the method of the
present invention but
the simplest end therefore preferred method comprises use of a tunnel dryer
with two or more
separate chambers or zones. Another preferred embodiment involves use of a so-
called belt dryer
that e.g. may be arranged in a spiral or helically. It is not required with a
complete isolation
between the zones, it is sufficient that different zones or chambers may be
operated separately with
individually controlled teinperature. It is thus applicable to use a tunnel
dryer with a number of
separate chambers separated from each adjacent chamber with whole or partial
partition walls that
allow "wagons" or a belt with portion pieces of raw material to pass through.
The temperature can
typically increase stepwise from chamber to chamber in the direction of the
movement of the
portion pieces through the dryer. If there as an example is 10 different
chambers, a number of e.g.
eight chambers may be operated at a temperature below the freezing point and
two chambers at a
temperature above the freezing point. An important aspect of the last chamber
is to obtain a
comparatively effective final frying of the product and the temperature should
therefore be
significantly higher than the freezing point in at least this particular
chamber. The relative humidity
in the dryer will also be able to affect the quality of the product as well
ass the economy of the
process. A convenient relative humidity for the entering air to the dryer is
30-50 %.
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According to the present invention there is no need for vacuum or reduced
pressure in any stage of
the process. To the contrary it is preferred that the entire process takes
place at ambient pressure.
In at least one step below the freezing point and preferably in the
temperature range from -2 to - 10
C, such as about -5 C, an initial drying is conducted. When portion pieces
enters this stage of the
dryer they have a water content that is typical of the fresh product. The
water content at this stage is
typically between 70 and 90 % and more typically between 75 and 85 %.
When the portion pieces have been subjected to drying in one or a plural of
such steps below the
freezing point, the portion pieces will have a water content in the range 20-
60 % and more preferred
in the range 25-40 %. The portion pieces are then immediately conveyed to the
next process stage
which involves diying in one or a number of steps above the freezing point.
The temperature in this
zone will be in the range 15-30 C and more preferred about 20 C. While some
products may well
tolerate a temperature close to 30 C without experiencing a quality loss,
other product need to be
kept around 20 C to maintain a good quality.
The product is dried to a final water content chosen with respect to a
combination of economy and
shelf life for the product. Generally, the lower the water content the better
the shelf life of the
product. The final water content will normally be lower than 20 %, more
preferred in the range 5-
15 % and most preferred in the range 8-12 %.
Below the process is described in further detail with reference to he
accompanying drawings,
where:
Figure 1 is a plot of rehydration data for a product dried in accordance with
the present invention
compared with a product dried above the freezing point,
Figure 2 is a bar chart of colour measurement of the product dried in
accordance with the invention,
compared with a product dried above the freezing point and with stockfish, and
Figure 3 is a plot of not denatured protein in a product dried in accordance
with the invention
compared with a product dried above the freezing point.
Portion pieces of fresh fish are placed on a grating or a tray on wagons and
conveyed into the drying
chamber. The drying plant comprises a drying tunnel where the drying air in
the first step is -5 C
and in the second step is +20 C. The chambers are separated by a lock in
order to maintain the
different temperatures in the zones. The wagons with the fish pieces will have
longer retention time
in the chamber with the lower temperature. With a size of 80x65x10 mm the
pieces will typically
have a retention time in the cold chamber of about 4 days and in the chamber
with the higher
temperature of about 12 hours. In the first of these chambers the products
typically are dried from
about 80 % to 25-40 % water content. The drying are conducted at atmospheric
pressure in both
chambers. The wagons are moving through the chamber b means of suitable
conveyance means
such as rails or a belt. The conveyance speed in the cold chamber is adjusted
so as to reach a desired
water content before being transferred to the warmer chamber, e.g. a water
content between 25 and
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40 %. This is determined empirically for the products in question and the size
of the portion pieces
in question.
After completed drying the products will be porous and easily rehydratable.
The products therefore
should be packed in airtight packaging to avoid undesired rehydration in air.
Conveniently packed
the product may be stored at room temperature for several months without
noticeable deterioration.
The end product is unique with respect to the ease with which it may be
rehydrated in water. It is
an instant fish product that needs very short time for rehydration, typically
less than 3 ininutes.
Examples
Experiments have been performed with fish (cod) as the raw material. In
addition to a subjective
assessment of the quality of the end product we have investigated the
(re)hydratability of the
product, its colour change and its protein content.
Rehydratability
The rehydratability of a product is a measure of how much water the product is
able to absorb and
thereby swell. The formula below shows the matliematical calculation of the
rehydratability:
ra='- =100%
mts
rQ = rehydratability [%]
rn,= water content in the product after relrydration [gJ
rnt,s= weight qf dry substance in the product [gJ
In order to draw a parallel to the original product water content, it should
be emphasized that a
rehydratability of 300 % corresponds to a water content of 75 % in the product
(3 parts of water per
1 part of dry substance).
The pieces were weighed prior to rehydration while the dry substance is known
from the drying
graph. A box was filled with cold water and the portion pieces were placed in
the water for
different periods of time. Pieces were removed after 30 s, I min., 2. mins., 3
mins., 10 mins., and 20
mins. Based on the weight change the rehydratability was calculated and the
rehydration graph was
established. The graph is shown in Figure 1.
As shown by Fig. 1 already in 30 seconds the product has taken up a lot of
water. The graph is very
steep and flattens off rapidly. Already after 1 minute the rehydration graph
is beyond 300%, which
means that the product contains more than 75 % water. The graph shows that the
product has
"instant" properties and is completely rehydrated in less than 3 minutes. The
properties of the
comparison product are not even close to this and is only rehydratable to an
extent of 50%, i.e. two
parts of dry substance per one part of water.
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Colour nteasurernents
The colour of the product is seen as an important quality paranieter. This is
a parameter that often
changes during storing and processing. The colour is important for the
acceptability among
consumers.
5 The measurement apparatus used is an X-Rite 948 colour measurement
instrument. This
instrument converts colour nuances to a numeric system. The system used is
Judd Hunters L*a*b-
system. This is a three dimensional system in which L represents a value for
lightness from 0 to
100 where 0 is darkest (black), the a value defines the position along a red-
green colour axis and the
b value defines the position along a yellow-blue colour axis. Four
measurements were made of each
piece of fish and the result is given as an average of these four. Figure 2
shows colour
- measurements for pieces of fish dried at 20 C and -5 C. The L value
increases with a drying
temperature of -5 C compared to 20 C. The values of b and a decreases.
As shown by Figure 2 the product dried by the method according to the present
invention
maintained its lightness much more than a piece dried at a temperature above
the freezing point
during the entire drying period. Figure 2 for comparison also shows values for
a traditional
stockfish.
Protein content
The amount of protein that is extractable from a protein-rich product is a
measurement of protein
denaturation. Denaturing of proteins means that the protein's secondary,
tertiary and quatemary
structure is modified in an undesired manner while the sequence of amino acids
is maintained.
Tests were made to determine how the amount of salt and water soluble proteins
were investigated.
The object was to see at which drying temperatures the fish is subjected most
to denaturing. Figure
3 shows the protein content in fish (cod) dried at -5 C and 20 C respectively.
Figure 3 shows that
there is a much higher content of not denatured protein in fish dried with the
method according to
the present invention compared to fish dried entirely at a temperature above 0
C. The amount of
salt and water soluble proteins seen in Figure 3 is a measurement of not
denatured protein in the
fish.
In general it was found that products dried in accordance with the present
invention have a veiy
high quality. For example when drying fresh fish it obtains a whiter colour,
higher rehydratability
and preserves the proteins better than fish dried above 0 C, cf. the above
discussed measurements.
Many consumers want food with high quality and short preparation times.
Products dried with the
method in accordance with the present invention is well suited for bachalao
type meals and meals
based on stockfish. They constitute an alternative to today's traditional
split cod and stockfish. The
process line is shorted down and the usefulness of the product is increased.
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With respect to the costs of the process, theses are related to energy
consumption, time consumed,
and not the least how labour demanding the process is. In all these areas the
method according to
the invention is good. Drying at low temperatures as an isolated element
certainly is a
comparatively costly step of the process and should therefore not be conducted
to a higher extent
than required to obtain the desired quality. The end drying at a higher
temperature is significantly
less expensive. By finding the optimal retention tiine in the low temperature
zone for each product
in question, the quality and the energy economical aspects can be optimally
balanced. Since the
entire process is rapid compared to competing methods and is well adapted to
automation, it is not a
labour demanding process.
Compared to processes with vacuum freeze-drying the present process is much
less expensive both
with regard to equipment and the process as such. With vacuum freeze-drying
the capacity is low
and the energy consuinption is high. A further advantage related to the
present method is the
possibility to use a temperature program. Combinations of drying temperatures
that yield an
optimal drying process with respect to quality and costs may then be found.
In general too short or limited drying at low temperature leads to a product
that is not of the finest
quality while a inconveniently prolonged drying at the same temperature leads
to a undesired high
process cost.
In an industrial process according to the invention it may be relevant to use
dryers with a
significantly higher number than two temperature steps.
Thus the entire drying process in a preferred embodiment of the invention may
be conducted in a
tunnel dryer with continuous or step by step increasing temperature, the
temperature being lower
than freezing point of the product for 65-90 % of the drying time and more
preferred for 75 to 88 %
of the drying time.
Alternatively the drying process may be conducted in a belt dryer with the
same conditions as
mentioned above, i.e. a continuous or step by step increasing temperature, the
temperature being
lower than freezing point of the product for 65-90 % of the drying time and
more preferred for 75 to
88 % of the drying time.
When the temperature is increased step by step as mentioned, the number of
temperature steps is
typically 2-12 and more preferred 2-8 steps.
Under any circumstances there will always be at least one drying step at a
temperature lower than
the initial freezing point of the product and at least one drying step at a
temperature above the
freezing point.
