Hi there
Here is a copy of training material used in New Zealand to train people in
the local industry, it repeats some of the concepts already expressed and
add some interesting points on the effects of thawing on yield. Apologies
for the lost of format.
Francisco Blaha
----------------------------
Thawing (Unit 6203)
In NZ thawing was not common until the early 80's when freezing on board
(especially from Joint ventures boats) started to be a common practice for
some industries.
Each thawing tank (or batch) should not deliver more than 1/2 hr to 1 hr of
production
The distribution and flow of water inside the tanks, needs to be as
uniformed as is possible, in order to produce an even thaw on the load.
This process can be made more efficient by the use of compressed air,
breaking not only the blocks, but also homogenising the water temperature by
avoiding stratification.
In general terms a water flow coming from the bottom of the tank is
preferred as more efficient.
Most of the bibliography coincides that maintaining a constant water
temperature is crucial for the effectiveness of the process and the
consequences in the quality and yields of the product.
In general terms an inlet water temperature of 16 to 20°C is considered as
appropriate
Excessive thawing times should be avoided, not only because quality and
yield issues, but also because of resources efficiency.
The flow rate influence affects the process in both ways, by increasing the
thawing time or by wasting water.
The bigger or thicker the block of fish to be thawed, the longer would be
the thawing period.
Uniformity on the blocks and if possible in the size of the fish per tank is
recommended in order to standardise the thawing process.
Care must be taken to avoid the blocks to stick together and act as one
"big" block. This problem can be overcome by keeping the plastic bags on the
block in the early stages of thawing, and staking the blocks with the bag
opening to the bottom of the tank.
Plastic bags should be removed after the outside layers of fish have been
thawed.
Manually braking up the blocks, reduce the times of thawing, but care must
be taken by the personnel, in order to not use to much force at separating
the fish.
Thawing time and final product temperature
Exposing the product to temperatures above 7°C should be definitively
avoided.
Long thawing times, is also long exposition of fish to a fresh water
environment, this would affect palatability, increase gapping and affects
texture in the cooked product.
The temperature of the product can be controlled by different methods, but
the thawing time has to be carefully managed in order to fit production
requirements, therefore sequential thawing is advised.
Yields
The ability of thawed fish to stand up to the knife at filleting (affecting
yields either at the unfrozen or refrozen pack) depends to a great extent on
the temperature to which the fish is raised during thawing. Best results are
obtained in fish barely thawed near the backbone; on the other hand, to much
frost in this area makes it difficult for the filleter to remove the fillet
completely.
The thawing process should be ended while some ice still remains in the
centre of the fish.
These fish was left equilibrate to the point where traces of "ice bone"
remained in some fish.
Fish thawed following these practices showed an 8-9% yield gain over fish
that was completely thawed.
Storage between thawing and reprocessing
· Storage should be avoided as much as possible, immediate
processing should be the aim.
· A time of 4 hr. in the thawer (in a mild ice-water slurry)
appears to be suitable if there are production delays.
· Holding thawed fish on ice overnight result in an additional 1.5%
weight loss.
· Holding thawed fish in the chiller (with no ice) increases the
loss up to 2-3%.
Conclusions
The suggested best practices can be summarised as follows:
1. Control inlet water temp. around 16-18°C.
2. Adjust the flow rate for the tanks as close as possible to the 3.3
lt./kg of fish/hour.
3. Fish should never get above 7°C
4. Stop thawing process when core temperature is close to 3°C and let
equilibrate for 30 minutes.
5. Process immediately
6. If delays occur keep the fish to a maximum of 4 hs. on the thawing
tanks controlling the temperature and adding ice if necessary.
7. Overnight storage should be avoided, but if necessary ice
abundantly before storage.
-----Original Message-----
From: owner-seafood@ucdavis.edu [mailto:owner-seafood@ucdavis.edu]On Behalf
Of Aa & Madsen
Sent: Tuesday, 4 March 2003 9:25 a.m.
To:
Subject: Re: Thawing Techniques
Hello Greg and Peter
As former developer of thawing systems for an equipment manufacturer, and
now consultant, I have worked intensively with this process, and designed
complete systems.
You can thaw with the described method.
The needed water circulation are made by adding air to the bin, through bars
placed in the bottom of the tank.
Do NOT use standard compressed air, since the air has to be oil free.
The air should only agitate the water, so no high pressure is needed.
Further the equalizing temperature should be + 4 °C, which is achieved in
4-5 hours.
(going for +15 °C will ruin the flesh)
Then add some ice, and leave for another hour or so.
This ice will raise the yield with app 2-3 %, in the filleting process.
Gaining this yield takes very good management, but 2-3 % is a lot of money.
Remember: this method are only working for H&G whitefish. H&G salmon, all
types of fillets etc needs other thawing methods.
Do not hesitate to contact me for further information’s. Process, equipment,
yield management etc.
Yours sincerely
Hans Morten Henriksen
M.Sc. Seafood Processing
Alcedo - Seafood Business Consultants
Kirke Vaerloesevej 109
DK-3500 Vaerloese
Denmark
hmh@alcedo.dk www.alcedo.dk
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