Fermentation optimization for the production of poly(beta-hydroxybutyric acid) microbial thermoplastic

Batch culture of Alcaligenes latus, American Type Culture Collection 29713, was investigated for producing the intracellular bioplastic poly(P-hydroxy butyric acid) (PHB). A central, composite experimental design was used to o ptimize the composition of the culture medium for maximizing the productivi ty of PHB. Investigated were the effects of temperature, the initial cultur e pH, the ionic strength of the medium, the concentration of trace elements , the type of nitrogen source and the carbon-to-nitrogen ratio. The optimal temperature for growth and PHB synthesis appeared to be 33 degrees C; howe ver, over the 25-37 degrees C range, the effect of temperature was negligib le. An initial pH value of 6.5 gave the best results; pH values that differ ed even slightly from the optimum reduced the culture performance. Typical culture characteristics were: 0.075/h maximum specific growth rate, 0.38 g/ l h maximum specific sucrose consumption rate, and 0.15 g/l h maximum speci fic PHB production rate. PHB was lost because of hydrolysis in the stationa ry phase, suggesting critical importance of timing the harvest. Under the b est conditions, PHB constituted up to 63% of dry cell mass after 93 h of cu lture. The average biomass yield coefficient on sucrose was about 0.4 kg/kg . Of the four nitrogen sources-ammonium chloride, ammonium sulfate, ammoniu m nitrate, and urea-used, only the first two supported the culture satisfac torily. The biomass and PHB showed clear yield maxima at 1.5 g/l ammonium c hloride (C:N ratio = 21.5) and 1.4 g/l ammonium sulfate (C:N ratio 28.3). T he yields were higher with ammonium sulfate and were relatively more sensit ive to changes in its concentration. Ionic strength had a strong negative e ffect on PHB productivity. The highest PHB yield occurred at 4 g/l phosphat e buffer concentration. Iron appeared to have the potential to enhance the proportion of PHB in the cells. (C) 1999 Elsevier Science Inc. All rights r eserved.