Establishment of an Ensiling System for the Production of Lactic Acid from Sorghum bicolor (Sweet Sorghum) Biomass

Abstract

Lactic acid is an economically important organic acid that has a wide range of applications in industry, including in the manufacture of sustainable and biodegradable plastic alternative poly-lactic acid (PLA). Transition towards a bio-based economy requires increased production of lactic acid and other industrially important chemicals from biomass. Lignocellulosic biomass is a preferred feedstock for production, as it is inexpensive and abundant, however the recalcitrant nature of lignocellulose makes its utilisation challenging. Therefore, development of a consolidated bioprocessing (CBP) system for efficient conversion of lignocellulosic biomass to value-added products is required. Lactic acid is currently produced via fermentation of expensive carbohydrate-based feedstocks by lactic acid bacteria (LAB). Application of an ensiling system may overcome problems associated with current fermentation methods and enable conversion of lignocellulosic biomass to lactic acid via CBP. Ensiling is commonly used for the preservation of freshly harvested crops for animal feed, including sweet sorghum, which contains a high concentration of soluble sugar and lignocellulose. In this study, a series of lab-scale ensiling experiments were performed, in which the effects of various conditions and additives on ensiling product yields from sweet sorghum were evaluated. Addition of CaCO3 was identified as an effective method of pH control, resulting in a 2.8 x increase in lactic acid production by endogenous LAB. Inoculation with two different species of LAB (Lactobacillus plantarum and Pediococcus acidilactici) and addition of a commercial cellulose enzyme package were demonstrated to have minimal influence on lactic acid yield compared to ensiling temperature and time. An ensiling temperature of 45 °C resulted in high lactic acid selectivity and potential degradation of some lignocellulose. However, due to variability between ensiling experiments, further investigation is required before optimal ensiling temperature and time can be confirmed for maximum production of lactic acid from sweet sorghum.