Direct fermentation of l(+)-lactic acid from cassava pulp by solid state culture of Rhizopus oryzae

Bioprocess Biosyst Eng (2012) 35:14291436 DOI 10.1007/s00449-012-0731-3

ORIGINAL PAPER

Direct fermentation of L(+)-lactic acid from cassava pulp by solid state culture of Rhizopus oryzae

Parichat Phrueksawan Songsri Kulpreecha

Sarintip Sooksai Nuttha Thongchul

Received: 6 January 2012 / Accepted: 21 March 2012 / Published online: 5 April 2012 Springer-Verlag 2012

Abstract This study shows that Rhizopus oryzae is capable of directly utilizing cassava pulp alone to L-lactic acid in solid state fermentation (SSF). pH control at 6.0 helped prevent end product inhibition. Increasing lactate titer was observed at the higher initial moistened water due to the higher degree of substrate swelling and hydrolysis. With shaking, limited ethanol production but no change in lactate titer was observed. Rigorous shaking gave better oxygen transfer but presumably caused cell damage leading to substrate utilization through the biosynthesis route. Supplementing cassava pulp with nitrogen enhanced growth but not lactate production. Under the optimal conditions, R. oryzae converted the sole cassava pulp into lactic acid at the titer of 206.20 mg per g initial dry pulp. With the help of commercial cellulase and glucoamylase, the dramatically increasing lactate titer of 463.18 mg per g initial dry pulp was achieved via SSF.

Keywords Solid state fermentation Lactic acid

Rhizopus oryzae Cassava pulp

Introduction

Lactic acid is a multifunctional organic acid that has been extensively used in food, chemical, and pharmaceutical industries. Lactic acid can also serve as a monomer backbone in the polymerization of an environmental friendly compostable plastic polylactic acid. If the commercial polylactic acid production plant is successfully developed, lactic acid demand will be increased rapidly. Lactic acid is commonly produced via bacterial fermentation for food and pharmaceutical applications. Bacterial fermentation usually yields lactate racemates at different percentages of the optical purity depending on the bacterial strain. It is unlikely that the optically pure isomer of lactic acid is required in polylactic acid production. Therefore, bacterial fermentation becomes somewhat problematic since sophisticated downstream processing is required for the purication of the stereoisomers. Alternatively, Rhizopus oryzae is capable of utilizing starchy materials and pentose sugars present in the agricultural commodities to produce the optically pure L-lactic acid because R. oryzae possesses only L-lactate dehydrogenase while bacteria can produce both D- and L-lactate dehydrogenases [1].

Nonetheless, fungal fermentation usually suffers from morphological...