Response to Chingling
1. In the following message, temperatures are in degrees Celsius.
2. Is there any evidence of a discernible difference in quality between tuna
stored at -50ºC and -65ºC?
3. Delta T can refer to any temperature difference. The difference between
external and internal temperature of a cold store does not directly affect
weight loss of products in a store. The relevant temperature difference is
between the cooling elements of the store and the product. In any cold store
water will migrate, (distil), from a warmer surface to a colder surface.
This is true even if both surfaces are well below 0ºC. In a cold store the
cooling element has to be at a temperature below that of the intended
operating temperature of the store to compensate for heat transmission
through the fabric of the store and to absorb heat introduced into the store
from operations in the store, for example opening of doors, operating of
loading trucks. This difference in temperature between product and cooling
elements can allow water to migrate from the product to the cooling
elements, (the cooling elements frost up), unless the products are protected
by a moisture impermeable barrier resulting in weight loss in the product.
Good store design and operation attempts to minimise the temperature
difference between the cooling elements and the product. Several factors
have to be considered. Adequate insulation is required in order to reduce
heat flow through the structure. For a given level of insulation heat flow
is dependent on the temperature difference between inside and outside of the
store, but this difference is taken into account when designing the store
and specifying the insulation. Good store operation is very important and
often overlooked in practice. Good operation means minimising the number of
times the door is opened, minimising the time the door left open, minimising
the use of powered fork lift trucks, and very important, ensuring the
temperature of the product is not above the operating temperature of the
store when it is put in. Also, as I have mentioned above packaging the
product to prevent dehydration is required.
4. The peanut gallery can write directly to you. The short answer from me
is: No! The topic of effect of freezing rate and quality of the product was
discussed in previous messages.
Peter Howgate
----- Original Message -----
From: "Chingling at Vaio-2" <crtanco@mida-group.com>
To: "P Howgate" <phowgate@rsc.co.uk>; <seafood@ucdavis.edu>
Sent: Tuesday, January 14, 2003 1:18 AM
Subject: general freezing issues
Hello everyone, here are some comments from a trader in the industry:
1. what are the temperature terms you are all using -
-18ºC and -20ÌSC
2. On the -50 vs -65degrees C freezing of fish, i know this terminology
refers to being able to maintain fish as sashimi grade. A distinction is
made commercially on frozen on board fish in commercial vessels and product
frozen at -50 degrees or less can be considered sashimi grade and product
frozen at -40 degrees or more is considered A-Grade. This is used for
Tuna, and for Swordfish. You can see the distincition in the bloodline of
the fish.
Further more, fish that may have originally been sashimi grade but not
stored in -50 degrees will lose its sashimi quality.
3. As to quality deterioration during storage - i once had an ineresting
conversation with a technical person from Stahl Astra - the makers of the
screw type compressor. He said that there is something called the delta T
factor which refers to the difference between the temperature of the cold
storage and the temperature outside the cold storage. if the Delta T is
high, there will be more loss of weight during storage than if the Delta T
is lower. Does anyone know anything more about this?
4. Question for the peanut gallery. Is it always true that the faster the
freeze, the closer to retaining the quality of it's prefrozen state is a
product? So something frozen by nitrogen will be better than if it were
frozen in 4 hours?
Chingling Tanco
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