Page 69 - Advances in Eco-Fuels for a Sustainable Environment

46 Advances in Eco-Fuels for a Sustainable Environment

complex and economically expensive, but they can be commercially viable if all the
byproducts are optimally utilized [52].

2.7 Future prospects

The starch-based bioethanol industry has been commercially viable for 30 years. In
that time, giant improvements have been made in developing high fermentable hybrid
strains, improved separation technology and fermentation processes, enzyme effi-
ciency, reduction of process costs and duration, and increasing bioethanol yields.
New tendencies in the corn-to-ethanol industry are currently aimed at improving
the dry-milling processes. Research is oriented to the development of corn hybrids
with higher extractable or fermentable starch content. Another area of industrial inter-
est is illustrated in the efforts of Syngenta Biotechnology (Syngenta Biotechnology,
Inc., SBI) to direct the accumulation of starch-hydrolyzing enzymes in the endosperm
of transgenic corn kernels. Stable accumulations of enzymes, without detriment to
grain viability and composition, allow processing capability to be built into the grain
itself. “Self-processing” grains could be designed to meet specific and novel process
constraints, due to engineered enzymes with specific biophysical and enzymatic
properties.
Modifications of dry-grind machinery have made recovery of corn germ possible in
dry milling. Dry fractionation technology can separate corn kernels into their compo-
nents without the soaking step, and advancements in fermentation led to improved
yeast quality and reactor configuration. SSF performed at a temperature above 34°C
with thermotolerant yeast enables the reduction of cooling requirements, improving
conversion process efficiency.
Microalgae can now be directly used as feedstock for biohydrogen fermentation,
whereby biomass concentration of 180g/L can produce butyric acid (5.8g/L) and
hydrogen (4478mL/L) as main products, with an energy yield for complete oxidation
of carbohydrates of 4kcal/g. Algal species such as Chloroccum sp., Spirulina
platensis, Chlamydomonas reinhardtii, Nitzchia closteriu, Phaeodactylum
tnicornutum, and Dunaliella tertiolecta have shown high photosynthetic efficiency,
and are able to accumulate high carbohydrate content, up to half their dry weight [53].
Although metabolic engineering is a key technology to enable transformation of
microorganisms into efficient cell factories [54–56], identifying suitable enzyme tar-
gets and metabolic pathways for modification or replacement is not easy. Diverse fac-
tors contribute to the performance of a new metabolic pathway, and problems linked to
mRNA abundance and enzyme activity may be difficult to identify. Bioproducts not
produced by natural microorganisms could be synthesized someday through emerging
developments in metabolic engineering [57, 58].
All such metabolic manipulation approaches could contribute to the development
of biofuel producer species and industrial process improvement. Several emerging
techniques, including high-throughput sequencing for genome deciphering [59, 60];
highly sensitive transcriptomics, proteomics [61, 62], and metabolomics methods
[63]; and advances in fluxomics [64] and computational tools are currently available,