1. Introduction
Over the last few decades, the global warming that has resulted
from the increase in greenhouse gas emissions and the simultaneous
depletion of fossil fuels has necessitated the research of
alternative and clean energy sources. Among the available renewable
fuels, interest in bioethanol has been increasing. The increase
in global bioethanol production between 2000 and 2013 is shown
in Fig. 1 [1]. In 2011, bioethanol production reached 3.5 Mm3 [2].
The production of biofuels, such as bioethanol, usually involves
starches and simple sugars derived from sources such as sugar cane
and corn. The fermentation of these substrates is highly efficient,
but at the same time it is expensive and non-sustainable because of
the concurrent use of these substrates as essential components of
the foodefeed chain [3]
Agro-food residues are the main substrates used as an alternative
for bioethanol production. In the USA and in Brazil, over 87% of
bioethanol is obtained from maize and sugar cane feedstocks,
respectively [4], whereas in China, rice straw is the major substrate
used for ethanol production [5]. These substrates contain high
contents of lignin and cellulose, which show great potential for
ethanol production [6]. Cellulose is a biopolymer that consists of
thousands of glucose units, whereas lignin is a polymer that displays
high molecular weight, insolubility, and chains of complex
carbohydrates [7]. Many studies have focused on using cellulose
and lignin hydrolysates to obtain biofuels, and in particular bioethanol
[8,9]. The monomers of the glucose contained in the lignin
and cellulose structures can be converted into glycerol through
anaerobic fermentation [10]. Park et al. [11] studied the optimal
concentration of glucose and performed tests with Saccharomyces
cerevisiae batch fermenting glucose contents ranging from 30 to
120 g/L. The optimal glucose concentration for ethanol production
was found to be in the 40e70 g/L range, which corresponds to an
ethanol concentration of 6.9% v/v and an ethanol yield of 88.3%. An