Influence of cell concentration, temperature, and press performance on flow characteristics and disintegration in the freeze-pressing of Saccharomyces cerevisiae with the X-press.

The pressure required for initiation of flow when freeze-pressing with the X-press is related to the phase boundaries of water, particularly those between ice I and liquid even at temperatures around -25 degrees C and lower. Widening the orifice of the pressure chamber to diameters larger than 2.5 mm leads to lower pressures and less extensive cell disintegration. Pressing Saccharomyces cerevisiae slowly with the aid of a manual hydraulic jack at -25 degrees C produces a disintegration of 60-75% irrespective of cell concentration. Pressing at -35 degrees C shows no clear differences. Pressing more rapidly with the aid of a motor-driven hydraulic press produces a similar extent of disruption of diluted cell suspensions (5.4 mg/g) as slow pressing. However, freeze-pressing a paste of baker's yeast (270 mg/g) increases the degree of disintegration. Under these conditions the disintegration is further enhanced by a lower temperature, -35 degrees C, and by a high velocity of flow through the orifice, such that more than 95% of the S. cerevisiae is disrupted by one pressing at less than 2 X 10(8) Pa. Mechanisms for flow through the X-press are suggested and discussed in relation to the phase diagram of water.