The Freezing Time of Fish (2023)

Introduction
What is freezing time?
How does fish freeze?
Why Measure FreezingTime?
What factors affect freezingtime?
How is freezing timemeasured?
Can freezing times becalculated?
How accurate must a freezing timebe?
Specifying freezingtime
Freezing times for fishproducts

Introduction

This note defines freezing time and explains why it isimportant, what it depends on and how it can be measured. Estimated freezingtimes are often inaccurate, and the note emphasizes the importance of measuringfreezing time during the process. Advisory Note 20 gives advice on measuringfish temperature, and Note 94 outlines the principles of freezing fish; theseshould be read in conjunction with this note.

A list of measured freezing times for a number of fishproducts is given at the end of the note.

What is freezing time?

Freezing time is defined here as the time taken for thetemperature of the warmest part of the fish, usually the centre, to be reducedto -20°C.

The recommended storage temperature for frozen fish in the UKis -30°C and, to ensure that the fish is frozen quickly, the temperature ofthe freezer must be lower than this. Thus when the surface of the fish is atfreezer temperature and the warmest part is down to -20°C, the averagetemperature of the fish on removal from the freezer will be close to therequired storage temperature of -30°C.

How does fish freeze?

Fish muscle usually contains 60-80 per cent water, dependingon species and season, but the tissue fluid contains salts and other compoundsin solution and hence the muscle freezes in rather a different manner fromwater. The change in heat content of white fish muscle and of water when thetemperature changes is shown in figure 1; it can be seen that fish has to bereduced to a much lower temperature than water before most of the heat isremoved and the fish is completely frozen.

Fig 1

Why Measure FreezingTime?

The size of a freezer and the capacity of its refrigerationplant depend on the quantity of fish to be frozen and the freezing time of theparticular product; the plant designer must be given this information. Thelonger 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 tofreeze will require a freezer to hold 1 tonne but if the freezing time is 2hours 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 productwith the longest freezing time; the freezing rate for that freezer is thenfixed.

A knowledge of product freezing times will help especially toreduce errors in operating air blast and immersion freezers which, because oftheir design, are more versatile in the shape and size of product they canaccommodate and can therefore be overloaded. A more detailed discussion of theproblems of loading blast freezers is given in Advisory Note 35.

What factors affect freezingtime?

Freezing time depends mainly on

freezer type freezer
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 thanin an air blast freezer operating at the same temperature.

Operating temperature

The colder the freezer the faster the fish will freeze, butthe cost of freezing increases when the freezer temperature is reduced, and inpractice freezers are usually designed to operate a few degrees below therequired storage temperature of the product. For example, plate freezers usuallyoperate at about -40°C and blast freezers at about -35°C to freezeproducts for storage at -30°C.

Air speed in a blast freezer

The effect of air speed on freezing time is shown in figure2.

Freezing time is reduced as air speed is increased; up toabout 5 m/s the reduction is considerable, but at higher speeds there is littlefurther improvement.

Product temperature before freezing

The warmer the product, the longer it will take to freeze; theinitial temperature of the product should be given when quoting a freezing time.Fish should be kept chilled before freezing both to maintain quality and toreduce freezing time.

Fig. 2

Product thickness

The thicker the product the longer the freezing time. Forproducts less than 50 mm thick, doubling the thickness may more than double thefreezing time; doubling a thickness of 100 mm or more may increase the freezingtime fourfold.

Product shape

In a freezer that is suitable for single fish, for example ablast freezer, a round fish will freeze in about two thirds of the time takenfor a flat fish of the same thickness; thus product shape may beimportant.

Product contact area and density

In a plate freezer, poor contact between product and plateresults in increased freezing time. Poor contact may be due to ice on theplates, packs of unequal thickness, partially filled packs or voids at thesurface of the block. Surface voids are often accompanied by internal voids;these cause poor conduction of heat, thus increasing freezing time and alsoreducing the density of the block. The relationship between freezing time, blockdensity and contact area for 100 mm blocks of white fish is shownbelow.

The type of wrapping material has some influence on freezingtime, but often the method of wrapping, particularly when air is trapped betweenwrapper and product, has even more effect. The following example illustrates thepoint. Kippers in a wooden box with the lid on take 15 hours to freeze in an airblast freezer. Kippers in an aluminium box of the same shape and size and withthe lid on take 12 hours, but if the lid is taken off the wooden box thefreezing 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 amountof 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. Butsince fat content varies with season, it is usually safer to assume the heatcontent figure for white fish muscle, that is about 320 kJ/kg as shown in figure1, in any calculations.

How is freezing timemeasured?

Freezing time is best measured by inserting a special type ofthermometer known as a thermocouple into the product so that it measures thetemperature of the last part to fall to -20°C. The correct use ofthermocouples in fish is described in Advisory Note 20. Since a singlethermocouple slightly out of position can give a misleading answer, as shown infigure 3, it is advisable to insert two or three thermocouples in each sample toensure success. Samples should be measured at several positions in the freezerto check the uniformity of freezing. The leads from the thermocouple to theinstrument should be led out of a sample along the centre line to reduce heatconduction, and should be anchored as shown in figure 4.

Fig. 3. POSITION OF THERMOMETER INFISH

It is impossible to tell whether a product is completelyfrozen merely by examining the surface; probing the surface with a screwdriveror a knife is useless as an indication of whether the centre has reached therequired temperature.

Fig. 4. CORRECT METHOD OF LOCATINGTHERMOCOUPLES FOR FREEZING

Can freezing times becalculated?

Freezing times can be calculated, but there is usuallyinsufficient information to make an accurate estimate. Freezing times can bepredicted fairly accurately for uniformly shaped products like laminated blocksunder ideal conditions, but variations in freezer temperature, the presence ofwrappers, and other factors can still make the calculation complicated andunreliable. Freezing time should always be measured during the process; onceestablished in this way, it will alter only if the product or the operatingconditions are changed.

How accurate must a freezing timebe?

A small error in a short freezing time may be much moresignificant than the same error in a long freezing time. For example an error of5 minutes in measuring a freezing time of 8 hours is unlikely to have any effecton design or operation, but an error of 5 minutes in measuring a freezing timeof 20 minutes could mean that for example the cost of an expensive stainlesssteel belt in a continuous shrimp freezer would be 25 per cent higher thannecessary.

Specifying freezingtime

Processors often ask for the freezing time of a fish productwithout giving any other information; such a question is impossible to answerprecisely. For example the answer for shrimp might be anything from 5 to 50minutes; 5 minutes might be correct for individual meats frozen in liquidnitrogen spray at -80°C, while 50 minutes might apply to a 25 mm thickcarton in a plate freezer at -35°C. Clearly it is necessary to specify boththe product and the operating conditions when quoting a freezing time.

Freezing times for fishproducts

The following are observed times for a number of fishproducts; although the freezing time of a new product in a particular freezershould always be measured in the recommended manner, these typical freezingtimes will give designers and operators some idea of what to expect inpractice.