Microbial polysaccharides (xanthan being the most successful example) are important commercial products having a wide range of applications in a variety of industries. Production processes of xanthan typically involve aseptic fermentation and a number of downstream operations. A key issue in the economics of the production of biopolymers such as xanthan is its recovery. This operation is commonly performed by precipitation with low molecular weight aliphatic alcohols. In this work, three particular aspects were evaluated: the effect of the electrolyte concentration (KCl), the amount of alcohol, and the heat treatment of the fermentation broths on the precipitation yield and on the quality of the final product. A Rushton turbine and a marine propeller were used as mixing devices. Dehydrated-reconstituted fermentation broths to which the ionic strength was adjusted (with NaCl) to a value close to that exhibited by an actual broth, behaved-from the precipitation point of view-as fresh broths. The Rushton turbine gave slightly higher yields as stirring speed was higher. On the other hand, the marine propeller showed a pronounced drop in yield as stirring speed was increased. In both cases, composition analysis (xanthan as equivalent sugars, nitrogen, and ash) revealed that lower yields were associated with higher precipitate purities. This means that mixing conditions importantly determine the quality of the product and can be fixed to meet a given specification. However, it is clear that a compromise has to be established between quantity and quality. The heat treatment applied to the broths substantially improved the precipitation yields, compared with nontreated broths. On the basis of precipitable solids, using 2% (w/w(gum)) KCl as a electrolyte and two volumes of alcohol per volume of broth, a recovery of 80% was achieved in the heat-treated broths (as compared with 65% in the nontreated broths). When 2 volumes of alcohol per volume of broth were used, the increase of electrolyte (KCl) concentration from 2% (w/w(gum)) to 3% increased precipitation yield (on the basis of precipitable solids) from 70 to 80%. Increasing further the electrolyte concentration (up to 5%, w/w(gum)) improved the yield only slightly. In general, the effect of KCl concentration on the precipitation yield was very similar in heat-treated broths than in nontreated broths. When 3% (w/w(gum)) KCl was used as electrolyte and 1:1.4-volumes of alcohol per volume of broth were employed in order to precipitate xanthan from heat-treated, reconstituted broths (containing 25 kg m(-3) of xanthan), yields up to 99% (based on xanthan) were achieved.
