Effect of Irradiation and Freezing Rate on Rehydration of Freeze Dried Beef

Abstract

Total water uptake, rate of water uptake, extract release volume and maximum shear force were measured to study the effect of irradiation dose and freezing rate on rehydration of freeze-dried beef. Lots cotaining six beef samples each were irradiated at 0, 1, 3 or 5 Mrad, either before freezing or after freeze drying. Samples were frozen in an air-blast freezer at —22° or —56°C. Freeze drying was carried out below 300 microns of Hg and at a maximum shelf temperature of 16°C.Rehydration of samples took place in three phases:1) A rapid uptake during first six seconds;2) An intermediate uptake in next 25 minutes; and3) A final uptake during the next hour of immersion.It is well established that the logarithm of the rate of liquids flow through a capillary tube is linearly related to the logarithm of the pressure drop along the tube. The rehydration of freeze dried beef follows the same relationship. For the latter phases the logarithm of water uptake was linearly related to the logarithm of immersion time (r > .98). In phase two (intermediate) the rate of water uptake of the control samples was significantly (P ≤ 0.05) lower than for the irradiated samples, and the shear force of the control samples was significantly (P ≤ .05) greater. In phase 3, the rate of water uptake was significantly (P ≤ 0.01) greater for the samples irradiated after freeze drying than for those irradiated fresh.Irradiation sequence and freezing rate had marked effects on total water uptake. The highest total water uptake was found for the meat irradiated when fresh and frozen slowly at —22°C.