Freezing time

Jones G J 1984 On estimating freezing times during tissue rapid freezing J. Microsc. 136 349-60... 

Figure 16.2 Freezing time for 150 mm wrapped boxed beef (Courtesy of AFRC Institute of Food Research, Bristol Laboratory)...

A different way obtaining short cooling and freezing times is to evaporate a part of the water in the product under vacuum. The evaporation energy of water at 0 °C is approximately 2.5 103 kJ/kg. To cool 1 kg of beef from 0 °C to -20 °C, 240 kJ have to be re-... 

Fig. 1.74. Temperature in the product as a function of freezing time, measured by one RTD and one Th each in three vials. The vials had been distributed diagonally on one shelf. (The differences between the three vials are within the accuracy of the drawing).

In Eq. (4) w u - 1.7 1, showing, that the influence of the conductivity is almost double compared with influence of the transfer. Assuming that d is only 0.2 cm, the freezing time falls to... 

The heat transfer coefficient /fsu in air varies strongly with the gas velocity, surface conditions of the product, and the geometry of the installation. In practical operations it will be difficult to achieve Ksu values of 1.7-2.5 10"3 J/cm2 s °C, or= 75 kJ/m2 h °C, and in many applications only half of this value (or less) may be possible. However, even with this high /fsu the above discussed slice of beef (2 cm thick) has a freezing time... 

Fourier s equafion does nof fake into account convection, which in the case of food freezing governs heat transfer befween fhe food surface and fhe refrigerafing medium, for example cold air in the case of a fluidized bed. Second, and more importanfly, if cannof accounf for fhe removal of lafenf heaf and fhe resultant phase change. Of the models available to predict freezing time. Plank s equation (Plank, 1913, 1941) is one of fhe simplesf and most widely used and is derived in many standard texts (Singh and Heldman, 2001 Smith, 2003). The principal... 

The freezing time using Nagaoka s equation is then... 

The freezing time must now be equal to the residence time in the bed (see Mixing, dispersion and residence time, below) a mean residence time can be assumed to be equal to the mass hold-up in the bed divided by the mass flow rate. If the mass hold-up is the product of bed volume and the bulk density of the bed, and the bed depth is H, then... 

Vazquez and Calvelo (1983b) presented a model for the prediction of the minimum residence time in a fluidized bed freezer which can then be equated to the required freezing time. The model is defined in terms of a longitudinal dispersion coefficient D, which is a measure of the degree of solids mixing within the bed in the direction of flow (and has the dimensions of a diffusivity, and hence units of m s ), a dimensionless time T... 

Table 3.6 Reported freezing times in fluidized bed freezing.

Conroy, R. and Ellis R.E, Pre-cooling berries improves quality, shortens fluidized freezing time. Food Processing, 42 (13) (1981) 82-83. 

Pham, Q.T., Simplified equation for predicting the freezing time of foodstuffs, /. Food Tech., 21 (1986) 209-219. 

Hung. Yen-Con "Prediction of Cooling and Freezing Times/ Fond Technology. 137 tMay 1990). 

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