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An investigation into the relationship between freezing rate and mean ice crystal size for coffee extracts

Coffee extracts—10, 20 and 40% solids content—were uni-directionally frozen. The parameters characterizing freezing kinetics, such as the rate of fall of temperature and the velocity of ice front, were determined experimentally and compared with the predictions of Neumann's model. Freeze dried samples were subsequently observed under a fluorescence microscope and the mean size of ice crystals were determined from the mean hydraulic radius of the pores. A model linking the mean hydraulic radius with freezing kinetics was developed, and the following equation was obtained: r H = n exp(– mX so )[2δ 2 α 1 / s i ] [( T m — T p )s i −1 ] −0.5 . Here, X s is solid content, δ is Neumann's dimensionless constant, t is time, T m is the initial freezing point of the extract, T p is the freezing temperature, α 1 is the thermal diffusivity of the frozen region, s i is the position of the ice front and finally n and m are material related constants. Experimental data and model predictions were found to be in satisfactory agreement.