Introduction
What is freezing time?
How does fish freeze?
Why Measure Freezing Time?
What factors affect freezing time?
How is freezing time measured?
Can freezing times be calculated?
How accurate must a freezing time be?
Specifying freezing time
Freezing times for fish products
This note defines freezing time and explains why it is important, what it depends on and how it can be measured. Estimated freezing times are often inaccurate, and the note emphasizes the importance of measuring freezing time during the process. Advisory Note 20 gives advice on measuring fish temperature, and Note 94 outlines the principles of freezing fish; these should be read in conjunction with this note.
A list of measured freezing times for a number of fish products is given at the end of the note.
Freezing time is defined here as the time taken for the temperature of the warmest part of the fish, usually the centre, to be reduced to -20°C.
The recommended storage temperature for frozen fish in the UK is -30°C and, to ensure that the fish is frozen quickly, the temperature of the freezer must be lower than this. Thus when the surface of the fish is at freezer temperature and the warmest part is down to -20°C, the average temperature of the fish on removal from the freezer will be close to the required storage temperature of -30°C.
Fish muscle usually contains 60-80 per cent water, depending on species and season, but the tissue fluid contains salts and other compounds in solution and hence the muscle freezes in rather a different manner from water. The change in heat content of white fish muscle and of water when the temperature changes is shown in figure 1; it can be seen that fish has to be reduced to a much lower temperature than water before most of the heat is removed and the fish is completely frozen.
Fig 1
The size of a freezer and the capacity of its refrigeration plant depend on the quantity of fish to be frozen and the freezing time of the particular product; the plant designer must be given this information. The longer the freezing time, the bigger the freezer has to be for a given output. For example an output of 1 tonne an hour of a product that takes 1 hour to freeze will require a freezer to hold 1 tonne but if the freezing time is 2 hours the freezer has to hold 2 tonnes. Where several products are to be frozen, the freezer should be designed to accommodate the required load of the product with the longest freezing time; the freezing rate for that freezer is then fixed.
A knowledge of product freezing times will help especially to reduce errors in operating air blast and immersion freezers which, because of their design, are more versatile in the shape and size of product they can accommodate and can therefore be overloaded. A more detailed discussion of the problems of loading blast freezers is given in Advisory Note 35.
Freezing time depends mainly on
freezer type freezerFreezer type
operating temperature
air speed in a blast freezer
product temperature
product thickness
product shape
product contact area and density
product packaging
species of fish
The type of freezer will greatly influence the freezing time; for example a product will normally freeze faster in an immersion freezer than in an air blast freezer operating at the same temperature.
Operating temperature
The colder the freezer the faster the fish will freeze, but the cost of freezing increases when the freezer temperature is reduced, and in practice freezers are usually designed to operate a few degrees below the required storage temperature of the product. For example, plate freezers usually operate at about -40°C and blast freezers at about -35°C to freeze products for storage at -30°C.
Air speed in a blast freezer
The effect of air speed on freezing time is shown in figure 2.
Freezing time is reduced as air speed is increased; up to about 5 m/s the reduction is considerable, but at higher speeds there is little further improvement.
Product temperature before freezing
The warmer the product, the longer it will take to freeze; the initial temperature of the product should be given when quoting a freezing time. Fish should be kept chilled before freezing both to maintain quality and to reduce freezing time.
Fig. 2
Product thickness
The thicker the product the longer the freezing time. For products less than 50 mm thick, doubling the thickness may more than double the freezing time; doubling a thickness of 100 mm or more may increase the freezing time fourfold.
Product shape
In a freezer that is suitable for single fish, for example a blast freezer, a round fish will freeze in about two thirds of the time taken for a flat fish of the same thickness; thus product shape may be important.
Product contact area and density
In a plate freezer, poor contact between product and plate results in increased freezing time. Poor contact may be due to ice on the plates, packs of unequal thickness, partially filled packs or voids at the surface of the block. Surface voids are often accompanied by internal voids; these cause poor conduction of heat, thus increasing freezing time and also reducing the density of the block. The relationship between freezing time, block density and contact area for 100 mm blocks of white fish is shown below.
|
Block density |
contact area |
freezing time |
|
800 |
48 |
3.0 |
|
780 |
45 |
3.0 |
|
650 |
29 |
3.8 |
|
650 |
21 |
4.0 |
The type of wrapping material has some influence on freezing time, but often the method of wrapping, particularly when air is trapped between wrapper and product, has even more effect. The following example illustrates the point. Kippers in a wooden box with the lid on take 15 hours to freeze in an air blast freezer. Kippers in an aluminium box of the same shape and size and with the lid on take 12 hours, but if the lid is taken off the wooden box the freezing time is only 8 hours, because there is no trapped air.
Species of fish
The lower the water content of a fish, the smaller the amount of heat to be extracted. Since water content goes down as fat content goes up, fatty fish like herring for example may freeze more quickly than lean fish. But since fat content varies with season, it is usually safer to assume the heat content figure for white fish muscle, that is about 320 kJ/kg as shown in figure 1, in any calculations.
Freezing time is best measured by inserting a special type of thermometer known as a thermocouple into the product so that it measures the temperature of the last part to fall to -20°C. The correct use of thermocouples in fish is described in Advisory Note 20. Since a single thermocouple slightly out of position can give a misleading answer, as shown in figure 3, it is advisable to insert two or three thermocouples in each sample to ensure success. Samples should be measured at several positions in the freezer to check the uniformity of freezing. The leads from the thermocouple to the instrument should be led out of a sample along the centre line to reduce heat conduction, and should be anchored as shown in figure 4.
Fig. 3. POSITION OF THERMOMETER IN FISH
It is impossible to tell whether a product is completely frozen merely by examining the surface; probing the surface with a screwdriver or a knife is useless as an indication of whether the centre has reached the required temperature.
Fig. 4. CORRECT METHOD OF LOCATING THERMOCOUPLES FOR FREEZING
Freezing times can be calculated, but there is usually insufficient information to make an accurate estimate. Freezing times can be predicted fairly accurately for uniformly shaped products like laminated blocks under ideal conditions, but variations in freezer temperature, the presence of wrappers, and other factors can still make the calculation complicated and unreliable. Freezing time should always be measured during the process; once established in this way, it will alter only if the product or the operating conditions are changed.
A small error in a short freezing time may be much more significant than the same error in a long freezing time. For example an error of 5 minutes in measuring a freezing time of 8 hours is unlikely to have any effect on design or operation, but an error of 5 minutes in measuring a freezing time of 20 minutes could mean that for example the cost of an expensive stainless steel belt in a continuous shrimp freezer would be 25 per cent higher than necessary.
Processors often ask for the freezing time of a fish product without giving any other information; such a question is impossible to answer precisely. For example the answer for shrimp might be anything from 5 to 50 minutes; 5 minutes might be correct for individual meats frozen in liquid nitrogen spray at -80°C, while 50 minutes might apply to a 25 mm thick carton in a plate freezer at -35°C. Clearly it is necessary to specify both the product and the operating conditions when quoting a freezing time.
The following are observed times for a number of fish products; although the freezing time of a new product in a particular freezer should always be measured in the recommended manner, these typical freezing times will give designers and operators some idea of what to expect in practice.
|
Product |
freezing method |
product initial temp °C |
operating temperature °C |
freezing time |
|
|
h |
min |
||||
|
whole cod |
vertical plate |
5 |
-40 |
3 |
20 |
|
whole round fish |
air blast 5 m/s |
5 |
-35 |
5 |
00 |
|
whole herring |
vertical plate |
5 |
-35 |
3 |
20 |
|
whole herring |
air blast 4 m/s |
5 |
-35 |
1 |
40 |
|
cod fillets |
horizontal plate |
6 |
-40 |
1 |
20 |
|
haddock fillets |
air blast 4 m/s |
5 |
-35 |
2 |
05 |
|
haddock fillets |
horizontal plate |
5 |
-40 |
1 |
02 |
|
kippers in pairs |
horizontal plate |
5 |
-40 |
2 |
15 |
|
whole lobster |
horizontal plate |
8 |
-40 |
3 |
00 |
|
whole lobster |
liquid nitrogen spray |
8 |
-80 |
0 |
12 |
|
scampi meats |
air blast 3 m/s |
5 |
-35 |
0 |
26 |
|
shrimp meats |
liquid nitrogen spray |
6 |
-80 |
0 |
5 |
