Sunday, April 29, 2007

Lactic acid production by lactobacillus rhamnosus using carob pulp

R. Yez1, S. Marques2, F.M. Grio2, and J.C. Roseiro2. (1) Chemical Engineering Department, Faculty of Experimental Science, University of Huelva, Avda. de las Fuerzas Armadas s/n, 21007 Huelva, Spain, (2) Biotechnology Department, INETI, Estrada do Pao do Lumiar, 22, 1649-038 Lisboa, Portugal

Lactic acid (LA) is an industrially important product with a large and increasing market as a precursor of biodegradable polylactide products. Carob pulp, an agro-industrial by-product largely available in Mediterranean countries, is an interesting source of sugars for low-cost culture media for LA fermentation.
In this work a biotechnological process for LA production was developed consisting of fermentation of extract obtained from carob pulp with L. rhamnosus DSM 20711, a producer providing high productivity and yield.
Our results demonstrated that carob pulp was a suitable substrate since it allowed good levels of LA production without inhibition. Supplementation of carob pulp extract with yeast extract and salts (MnSO4 and MgSO4) allowed production levels comparable to those obtained with the complete MRS medium.
In order to improve sugar utilization, a prior extract hydrolysis (by pH reduction and heating) was performed for sucrose conversion into glucose and fructose, readily fermentable by the microorganism. Since the conventional heat sterilization produced furfural at inhibiting levels, the fermentation performance was improved by filter-sterilization of the carob pulp hydrolysate. The highest production was obtained when hydrolysis was carried out at mild conditions: pH 2, 45ºC during 72 h. Fermentation of this hydrolysate supplemented with MRS components in the presence of calcium carbonate (for buffering effect) produced 146 g L-1 of lactic acid with a yield of 1.1 g lactic acid per g of consumed sugars, corresponding to a global volumetric productivity of 2.2 g L-1 h-1.
In conclusion, carob pulp can be employed as alternative nutrient for efficient lactic acid production.