Production of lactic acid on liquid distillery stillage

Abstract

Bioethanol produced by fermentation of biomass is one of the most important renewable and ecologically friendly energy resources today. After bioethanol distillation a considerable amount of liquid stillage remains (1 hl of the produced bioethanol, approximately 13 hl of liquid thin stillage is formed). Since the stillage contains high concentrations of organic and inorganic substances, it may cause serious environmental problems if it is disposed to water flows untreated. The aim of this work was to investigate potential of application of corn thin stillage as an inexpensive renewable feedstock for lactic acid production. In this way, it is possible to solve ecological problems and to improve the economy of bioethanol production taking into account growing demands for lactic acid for utilization in pharmaceutical, food, chemical, textile and leader industry. Lactic acid fermentation was conducted with nine different species from genera Lactobacillus and their growth, sugar utilization and lactic acid production on corn thin stillage were compared. The most productive strains were two facultatively heterofermentative species Lb. paracasei ssp. paracasei NRRL B-4564 and Lb. casei ssp. casei NRRL B-441, which had almost identical kinetics of lactic acid production during the first 48 hours, and after that the rate of lactic acid formation decreased more for strain Lb. casei ssp. casei NRRL B-441. For this reason the species Lb. paracasei ssp. paracasei NRRL B-4564 was selected for further study of effect of temperature, oxygen and shaking on lactic acid production. The lactic acid production was maximal at 41°C, without shaking, under anaerobic conditions. However, the biomass production was better at 30°C, also without shaking, under anaerobic conditions.

Bioetanol koji se proizvodi fermentacijom biomase predstavlja izuzetno značajno i ekološki pogodno biogorivo čija se proizvodnja danas konstantno povećava. Nakon proizvodnje bioetanola zaostaje značajna količina džibre kao otpadnog proizvoda (na 1 hl proizvedenog etanola nastaje oko 13 hl tečne džibre). Zbog prisustva značajnih količina organskih i neorganskih jedinjenja ispuštanje tečne džibre u vodene tokove može prouzrokovati ozbiljna ekološka zagađenja. Cilj ovog rada je da se ispita mogućnost korišćenja tečne džibre iz proizvodnje bioetanola na kukuruzu kao sirovine za proizvodnju mlečne kiseline. Na ovaj način je moguće rešiti problem otpada i značajno povećati ekonomičnost procesa proizvodnje bioetanola, imajući u vidu rastući trend u proizvodnji mlečne kiseline i njenu sve veću primenu u farmaceutskoj, hemijskoj, prehrambenoj, tekstilnoj i kožnoj industriji. U radu je ispitana mlečno kiselinska fermentacija devet vrsta bakterija iz roda Lactobacillus u toku koje je praćen njihov rast, iskorišćenje šećera iz supstrata (tečne džibre) i koncentracija mlečne kiseline. Najbolja aktivnost uočena je kod fakultativno heterofermentativnih vrsta Lb. paracasei ssp. paracasei NRRL B-4564 i Lb. casei ssp. casei NRRL B-441, kod kojih je zapažen gotovo identičan tok mlečne fermentacije u toku prvih 48 h, nakon čega brzina obrazovanja mlečne kiseline brže opada kod vrste Lb. casei ssp. casei NRRL B-441. Zbog toga je za dalji rad izabrana vrsta Lb. paracasei ssp. paracasei NRRL B-4564 sa kojom je ispitan uticaj temperature, kiseonika i mešanja na proizvodnju mlečne kiseline. Najveći sadržaj mlečne kiseline je postignut u fermentaciji na 41°C, u statičnim, anaerobnim uslovima. Proizvodnja biomase je bila bolja pri temperaturi od 30°C, takođe bez mešanja, pod anaerobnim uslovima.