Taylor, Geoffrey; (1990) The membrane filtration and the vacuum drying of proteins and enzymes. Doctoral thesis (Ph.D.), University College London.

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Abstract

This study examines the process integration of membrane filtration and vacuum drying to produce a dried protein or enzyme with the protein in an undenatured or active form. The concentration and drying of proteins and enzymes forms a main constituent part of a series of biochemical engineering unit operations to produce protein products from feed materials such as fermentation broths. Pilot-scale membrane filtration cartridges of short length - large diameter hollow-fibre membranes were used to produce dewatered, protein precipitate suspensions suitable for feeding a pilot-scale vacuum drier. The flow properties of these suspensions in the hoilow-fibres displayed non-Newtonian characteristics which were accurately described by independent rheological measurements provided suitable viscometric geometries were employed. The membrane filtration of protein precipitates proceeded by two different mechanisms depending on the degree of membrane polarization. The extent of polarization was controlled by the suspension shear rate at the membrane surface and the permeate flux rate. A critical operating point existed whereby polarization of the membrane alternated between pre-gel- and gel-polarized states. An experimental procedure for examining the temperature, moisture content and product quality during vacuum drying is presented. For the enzyme glucose oxidase in the presence of precipitated soya protein no product damage was observed at low, 4.25 kPa, and moderate, 10 to 25 kpa, absolute pressure. At higher pressures, above 25 kPa, the enzyme was irreversibly damaged at a rate dependent on the highest product temperature during the apparent constant-rate drying period. An economic appraisal indicated that the protein precipitate suspensions should be dewatered to a high degree before feeding to the vacuum drier to enable fast drying rates at low absolute pressures without the product expanding significantly. To minimize membrane filtration costs the hollow-fibre membrane channels should be long and narrow provided the protein concentrate can be recirculated through the channels without exceeding the bursting pressure of the membrane.

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