Bioprocessing Approaches to Synthesize Bio-based Surfactants and Detergents - Food and Industrial Bioproducts and Bioprocessing - page 244

In addition, due to the need to reduce any inhibitory agents, the starting materials must be
purified; for instance, fatty acyl-containing material must not contain phospholipids,
aldehydes/ketones, peroxides, and other contaminants. But, as energy costs increase (as
anticipated), the importance of sustainability increases (due to government regulation and/
or consumer demand), and the capabilities of enzymes and their production systems increase
(due to improved technology in screening, mutagenesis, protein engineering, recombinant
DNA technology, immobilization, bioreactor design, etc.), enzymatic bioprocessing is
anticipated to become more cost competitive.
For most of the applications listed in Table 10.1, the main role of the enzyme is the covalent
attachment of hydrophile and lipophile. Formation of the ester bond is the most readily
achieved using enzymes (particularly hydrolases, such as lipases). Bio-based surfactants with
ester bonds are particularly common for food and pharmaceutical applications due to their
high biocompatibility. They also have excellent biodegradability (Stjerndahl et al ., 2003 ).
However, ester bonds possess relatively low temperature stability and are hydrolyzed under
alkaline conditions (Stjerndahl and Holmberg, 2003). Carbonate linkages provide improved
stability under alkaline conditions compared to esters (but possess lower stability than
amides), and are also highly biodegradable (Stjerndahl and Holmberg, 2005a). Amide bonds
provide higher stability toward acidic and basic conditions, important for industrial
applications such as laundry detergents, yet retain excellent biodegradability (Stjerndahl and
Holmberg, 2005b). The ether linkage provides improved stability compared to ester bonds in
alkaline media, but undergoes degradation in acidic media; and, alkyl ethoxylates and APGs
possess excellent biodegradability (Steber and Wierich, 1985; Petrovic and Barcelo, 2000;
Bozetine et al ., 2008; Hill, 2009). Oxidation of fatty alcohols to long-chain aldehydes and
ketones via horse or yeast alcohol dehydrogenase may be useful for the chemo-enzymatic
synthesis of surfactants with acetal or ketal linkages (Orlich et al ., 2000 ).
Of the enzymes listed in Table 10.1, lipases are the “workhorses.” The employment of
lipases in non-aqueous media is an established art, with over 25 years of research serving as
a foundation. Lipases are abundant and relatively inexpensive enzymes that require no
co-factors and are easily immobilized. Lipases from several thermophilic organisms have
been isolated, cloned, and mass produced via recombinant DNA technology in common
vectors such as Escherichia coli . Some of the examples in Table 10.1 are surfactants formed
from enzymatic hydrolysis of oleochemical feedstocks, such as MAG formed from lipase-
catalyzed hydrolysis of TAG, and lysophospholipids via hydrolysis by lipases or
phospholipase A. In the following sections some specific examples from the literature are
given of enzyme-catalyzed synthesis of bio-based surfactants. Other examples not described,
such as the oxidation of fatty alcohols to aldehydes (Orlich et al ., 2000 ) and the covalent
attachment of fatty alcohols and bio-based diethyl carbonate (Banno et al ., 2007 , 2010 ;
Matsumura 2002 ; Lee et al ., 2010) are covered in the references provided.
10.5.1 Lipase-catalyzed synthesis of monoacylglycerols
(MAGs)
Preparation of MAGs using lipases is currently well known and can occur via several
different routes (Hayes, 2004; Watanabe and Shimada, 2009). [In addition to glycerol, other
glycols can serve as acyl acceptor for the preparation of bio-based surfactants (Hayes,
2004).] Key aspects for bioprocessing are the need to enable miscibility between acyl donor
and glycerol substrates and retain low water concentrations for reactions involving ester
bond formation, to increase the yield of ester upon the approach of thermodynamic