2.2.1.Calculation of the ice requirement of cooling fish

2.2.1.Calculation of the ice requirement of cooling fish

            The mass of ice needed to cool fish from the initial temperature to the final holding temperature can be calculated from an expression, which equates the heat taken up by the ice with the heat lost by the fish.

 

           Heat taken up by Ice   =  Heat lost by fish  

                    (Mi) (Li)             =          (Mf) (Cpf) (ti-tf)                    (1)

Where

                Mi                    =    mass of ice which melts (kg)

      Li                     =     latent heat of fusion of ice (80 kcal/kg)

      Mf                   =      mass of fish (kg)

     Cpf                   =     specific heat of fish (kcal/kgoC)

     ti                       =    initial temperature of fish (oC)

     tf                      =     final temperature of fish (0oC) 

From equation (1) the ice requirement will therefore be:

    (Mf) (Cpf) (ti-tf)

Mi        = -----------------------                            (2)

(Li)

            The specific heat of lean fish is 0.8  kcal/kg oC and this value should be used if there is a species mix or if there is a possibility that all the fish are of a lean species.

            The specific heat value, however, may be refined to take account of variations in the oil content of the fish and this refined value may be used if the fish composition is reasonably consistent.

            Cpf                  =          0.5 Xl + 0.3 Xs + 1.0 Xw       (3)

Where Cpf                  =          specific heat of fish (kcal/kg)

            Xl                    =          mass fraction of lipids (oil)

            Xs                    =          mass fraction of solids

            Xw                  =          mass fraction of water

            To illustrate the effect of oil content on the quantity of ice required for chilling the following comparison is made between lean and fatty fish.

Example (1) – 100 kg lean fish with 1% lipids, 19% solids and 80% water at an initial temperature of 20oC:

            Cpf      =          (0.5 x 0.01) + (0.3 x 0.19) + (1.0 x 0.8) = 0.862 kcal/kg o 

            100 x 0.862 x (20-0)

            Mi                    = ----------------------------------- = 21.55 kg of ice

80

Example (2) – 100 kg of fatty fish with 21% lipids, 19% solids and 60% water at an initial temperature of 20oC.

            Cpf      =          (0.5 x 0.21) + (0.3 x 0.19) + (1.0 x 0.6) = 0.762 kcal/kgoC

            100 x 0.762 x (20-0)

            Mi                    = ----------------------------------- = 19.05 kg of ice

80

           Since the calculation for fatty fish shows only a small reduction in ice requirement and,  with most species the oil content is variable, it is advisable to treat all fish as lean fish.

Last modified: Saturday, 24 December 2011, 10:47 AM