The Release of Compounds from Microbial Cells

Abstract

Compounds present in the microbial cells are of great commercial value. Their separation requires disintegration of cell walls and cytoplasmic membranes. The disintegration of microorganisms on an industrial scale is carried out, among the others, in bead mills (Chisti & Moo-Young, 1986; Geciowa et al., 2002; Hatti-Kaul & Mattiasson, 2003). A bead mill is a container filled with beads that are set in circular motion by a rotating stirrer. Microorganisms dispersed in the liquid are disrupted due to the impact of beads. The mechanism of cell destruction is a result of combined action of normal and tangential forces. There are three basic types of this mechanism: collisions of beads, grinding and rolling performed by the beads. Disintegration is a very complex process. In this process, cell walls of microorganisms are disrupted, intracellular compounds are released and dissolved in the continuous phase, cell walls are subjected to microgrinding, the released macromolecular compounds are cut and the released enzymes interact. Rheological properties of the suspension and its continuous phase are changed. Currie et al. (1974) described disintegration kinetics of microorganisms in the bead mill. The authors developed a linear model by comparing experimental data with results obtained during the disintegration process carried out in a high-pressure homogenizer (Hetherington et al., 1971). A logical model based on the analogy to the theory of gas kinetics was proposed by Melendres et al. (1998). They presumed that microorganisms could be destroyed due to collisions of dispersed beads of the packing. A phenomenological model based on the flow of suspension between two volumes was developed by Heim & Solecki (1998). The authors assumed that cells were disrupted while moving from a safe volume to the one in which no living microorganisms could exist. Basing on the sequence of events: cell disruption – the release of intracellular compounds, Melendres et al. (1993) developed a nonlinear model of the release of selected intracellular enzymes. Heim et al. (2007) described nonlinearity of the kinetics caused by changes in the disintegration conditions which was a result of the process run. Nonlinearity of the disintegration process resulting from subsequent decline of the biggest fractions of yeast cells was observed by Solecki (2009). Earlier, Whitworth (1974) described nonlinearity of cell disintegration kinetics in a suspension containing microorganisms (Candida lipolytica) which belonged to the same species and occurred in two morphological forms. The recently developed theory of random transformations of dispersed matter makes it possible to include in the description the fundamental phenomena observed during the process (Solecki, 2011). The aim of the study was to