Lactic Acid Fermentation of Carrageenan Hydrolysates from the Macroalga Kappaphycus alvarezii: Evaluating Different Bioreactor Operation Modes

Abstract

Lactic acid is a molecule used abundantly in the food, cosmetic, and pharmaceutical industries. It is also the building block for polylactic acid, a biodegradable polymer which has gained interest over the last decade. Seaweeds are fast growing, environmentally friendly, and economically beneficial. The Rhodophyta, Kappaphycus alvarezii, is a carrageenan-rich alga, which can be successfully fermented into lactic acid using lactic acid bacteria. Lactobacillus pentosus is a versatile and robust bacterium and an efficient producer of lactic acid from many different raw materials. Bioreactor strategies for lactic acid fermentation of K. alvarezii hydrolysate were tested in 2-L stirred-tank bioreactor fermentations, operating at 37 °C, pH 6, and 150 rpm. Productivity and yields were 1.37 g/(L.h) and 1.17 g/g for the pulse fed-batch, and 1.10 g/(L.h) and 1.04 g/g for extended fed-batch systems. A 3.57 g/(L.h) production rate and a 1.37 g/g yield for batch fermentation operating with an inoculum size of 0.6 g/L was recorded. When applying fed-batch strategies, fermentation products reached 91 g/L with pulse feed and 133 g/L with constant continuous feed. For control and comparison, a simple batch of synthetic galactose-rich Man-Sharpe-Rugosa (MRS) media was fermented at the same conditions. A short study of charcoal regenerability is shown. A scheme for a third-generation lactic acid biorefinery is proposed, envisioning a future sustainable large-scale production of this important organic acid.

The Macroalga Kappaphycus alvarezii as a Potential Raw Material for Fermentation Processes within the Biorefinery Concept: Challenges and Perspectives

“…Using similar conditions with an even higher biomass concentration of 50% (w/v) mix, Hargreaves et al [22] reached a galactose concentration of 82 g/L. The work carried out by Hargreaves et al [22] and Tabacof et al [24,73] showed that exposing K. alvarezii biomass to prolonged acid hydrolysis results in the formation of 1 g/L of HMF for every 4 g/L of galactose.…”

“…Hargreaves et al [22] determined that fine powdered charcoal is preferable to charcoal granules, and that approximately 1 g/L of charcoal should be used for each 1 g/L of HMF removed from the hydrolysate. Activated charcoal can also be regenerated via oxidation processes [79], desorption and decomposition methods [80], and thermal treatment [73,81]. Since HMF is a valuable chemical compound [29], its recuperation from the activated charcoal should be considered.…”

“…such as nanofiltration and reverse osmosis [90]. It is worth mentioning that activated charcoal can be regenerated by chemical and thermal methods [73], and the calcium sulphate (gypsum) produced after neutralization of the overlimed solution can be put to use for construction purposes.…”

“…In 2022, Sudhakar and Dharani [110] demonstrated that K. alvarezii biomass can be processed into lactic acid by L. plantarum. In the work of Tabacof et al [73] in 2023, it was shown that 115 kg of lactic acid can be produced by 1 ton of dry weight K. alvarezii using L. pentosus.…”