Abstract:
Xylitol is a commercially important chemical with multiple applications in the food
and pharmaceutical industries. According to the US Department of Energy, xylitol
is one of the top twelve platform chemicals that can be produced from biomass.
The chemical method for xylitol synthesis is, however, expensive and energy-
intensive. In contrast, the biological route using microbial cell factories offers a
potentially cost-effective alternative process. The bioprocess occurs under
ambient conditions and makes use of biocatalysts and biomass which can be
sourced from renewable carbon originating from a variety of cheap waste
feedstocks. In this study, the biotransformation of xylose to xylitol was
investigated using Yarrowia lipolytica, an oleaginous yeast which, in this study
was firstly grown on a glycerol/glucose medium for the screening of a co-
substrate, followed by a media optimisation in shake flasks, scale-up studies in a
bioreactor and then downstream studies where done on the processing of xylitol.
A two-step medium optimization was employed using a central composite design
and an artificial neural network coupled with a genetic algorithm. The yeast
amassed a concentration of 53 g/L of xylitol whilst using pure glycerol (PG) and
xylose media, with a bioconversion yield of 0.97 g/g. Similar results were obtained
when PG was substituted with crude glycerol (CG) from the biodiesel industry
(titre: 51 g/L; yield: 0.92 g/g). Even when xylose from sugarcane bagasse
hydrolysate was used as opposed to pure xylose, a xylitol yield of 0.54 g/g was
achieved. The xylitol was successfully crystallized from the PG/xylose and
CG/xylose fermentation broths with a recovery yield of 40 and 35 %, respectively.
To the best of the author’s knowledge, this study demonstrates for the first time,
the potential of using Y. lipolytica as a microbial cell factory for xylitol synthesis
from inexpensive feedstocks. The results obtained are competitive with other
xylitol producing organisms.
