Bioprocessing Approaches to Synthesize Bio-based Surfactants and Detergents - Food and Industrial Bioproducts and Bioprocessing - page 242

frequently form, requiring the need for an additional purification steps, such as molecular
distillation (Freitas et al ., 2010 ). Span ® is typically prepared by first conducting acid-
catalyzed dehydration of sorbitol to obtain sorbitan, followed by alkali (e.g., NaOCH 3 − )
catalyzed transesterification between FAME and sorbitan at 200-250 °C. Sucrose-fatty acid
esters are prepared via transesterification of FAME at elevated temperatures (>100 °C) and
reduced pressure for several hours, often in the presence of toxic solvents such as
dimethylformamide (DMF) or dimethylsulfoxide (DMSO). APGs, although readily
produced under solvent-free conditions via formation of an acetal linkage between fatty
alcohol and saccharide under mild reaction temperatures, require molecular distillation, an
energy-intensive method, to remove excess fatty alcohol reactant; the process employs
significant amounts of energy to operate (Hill, 2009).
Therefore, although chemical-based syntheses provide polyol surfactants at high yields
and reaction rates, they possess many disadvantages when examined from a sustainability or
life cycle assessment (LCA) based perspective. LCA refers to a quantitative analysis of the
costs involved with a product “from cradle to grave”, which includes cost factors for their
environmental “footprints” (e.g., production of greenhouse gases and toxicants which
persist in ecosystems) (Patel, 2004; Hatti-Kaul et al ., 2007 ; Cowan et al ., 2008 ). Firstly, the
harsh conditions can lead to degradation of double bonds present in the acyl donor, and to
other by-products, which can lead to discoloration at minimum, among other undesired
results. The latter may require additional purification steps to be employed. This issue is
very important for surfactants containing polyunsaturated acyl groups; for instance, omega-6
fatty acyl-enriched MAGs have been reported recently as effective anti-cancer agents
(Fortin, 2010). Moreover, the double bonds of polyunsaturated fatty acids are known to
undergo several undesired thermal and oxidative reactions at elevated temperatures
(>100°C), including the formation of polymers that can adversely affect human health,
alkehydes, and ketones (Chang, 1988). Secondly, the high temperatures require excessive
energy usage, leading to increased production of the greenhouse gas carbon dioxide. Thirdly,
the use of heterogeneous catalysts, acid/bases, and/or toxic solvents yields waste products
that can lead to environmental harm and also present safety problems for workers. Fourthly,
often these reactions produce broad product distributions of desired and undesired products,
and, moreover, have lower selectivity, which can impair product performance and impact
the products' biocompatibility and biodegradability.
The production of bio-based surfactants via bioprocessing - the use of enzymes in non-
aqueous media and fermentation - provides a significant improvement in process sustainability.
10.5 PREPARATION OF BIO-BASED SURFACTANTS VIA
ENZYMES IN NON-AQUEOUS MEDIA
Enzymes can, potentially, play an important role in the manufacture of many bio-based
surfactants (Hayes, 2004 ; Karmee, 2008 ) (Table 10.1 ). The use of enzymes provides many
advantages compared to chemical processing, particularly for the upgrading of process
sustainability: lower energy use (due to the use of lower temperatures), lower amounts of
waste products and by-products, and the absence of toxic metal catalysts or acids /bases, and
safer operating conditions (Cowan et al ., 2008). The major disadvantages are the prohibitive
costs for enzymes compared to chemical catalysts (although this effect is reduced when
enzymes are reused, which is enabled by immobilization, and when potential enzyme
inhibitors are absent) and the lower reaction rates that accompany many enzymatic reactions.