Cell wall and flocculation

4.4.1 The cell wall - an introduction

The brewing yeast cell wall is a hugely important and frequently underestimated organelle. Primarily it is made up of an array of carbohydrates (80-90% of the wall) with proteins embedded within it. As memorably described by Stratford (1994) it is not an 'inorganic egg shell' but a living organelle whose properties and functions change during the cell's lifetime. As if to emphasise its importance the wall accounts for 15-25% of cell dry weight. The yeast cell wall has been subject to a number of general reviews over the years (MacWilliam, 1970; Ballou, 1982; Fleet, 1991; Kreutzfeldt & Witt, 1991; Stratford, 1994; Cid et ai, 1995; Lipke & Ovalle, 1998; Smits et ai, 1999).

Structurally the yeast cell wall has been likened to reinforced concrete (Stratford, 1994). Where

'steel reinforcing rods are represented by enmeshed alkali-insoluble p-(l->3)-glucan fibrils, comprising some 35% of the wall. The reinforcing is surrounded by concrete, pebbles in a sand/cement matrix: secreted mannoproteins represent pebbles, some 25-50% of the wall, encased and bonded to the reinforcing fibrils by a matrix of amorphous (3-glucan and chitin'.

An idealised representation of the cell wall and its various strata is presented in Fig. 4.32 (Kreutzfeldt & Witt, 1991). In reality, the micro-architecture of the cell wall is ill defined. As noted by Cabib el al. (1982) the layers are amorphous as the polymers interweave and overlap.

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