Abstract
Glycolipids are considered an alternative to petrochemically based surfactants because they are non-toxic, biodegradable, and less harmful to the environment while having comparable surface-active properties. They can be produced chemically or enzymatically in organic solvents or in deep eutectic solvents (DES) from renewable resources. DES are non-flammable, non-volatile, biodegradable, and almost non-toxic. Unlike organic solvents, sugars are easily soluble in hydrophilic DES. However, DES are highly viscous systems and restricted mass transfer is likely to be a major limiting factor for their application. Limiting factors for glycolipid synthesis in DES are not generally well understood. Therefore, the influence of external mass transfer, fatty acid concentration, and distribution on initial reaction velocity in two hydrophilic DES (choline:urea and choline:glucose) was investigated. At agitation speeds of and higher than 60 rpm, the viscosity of both DES did not limit external mass transfer. Fatty acid concentration of 0.5 M resulted in highest initial reaction velocity while higher concentrations had negative effects. Fatty acid accessibility was identified as a limiting factor for glycolipid synthesis in hydrophilic DES. Mean droplet sizes of fatty acid-DES emulsions can be significantly decreased by ultrasonic pretreatment resulting in significantly increased initial reaction velocity and yield (from 0.15 ± 0.03 μmol glucose monodecanoate/g DES to 0.57 ± 0.03 μmol/g) in the choline: urea DES. The study clearly indicates that fatty acid accessibility is a limiting factor in enzymatic glycolipid synthesis in DES. Furthermore, it was shown that physical pretreatment of fatty acid-DES emulsions is mandatory to improve the availability of fatty acids.
Highlights
IntroductionGlycolipids are a class of biosurfactants that have been claimed to be non-toxic (Hirata et al, 2009), readily biodegradable (Baker et al, 2000; Hirata et al, 2009; Lima et al, 2011), and less harmful to the environment than the petrochemically produced ones (Dörjes, 1984; Poremba et al, 1991; Lima et al, 2011; Johann et al, 2016)
Glycolipids are of special interest to the pharmaceutical industry, e.g., as bioavailability enhancers (Perinelli et al, 2018), and for the food industry, since e.g., Glycolipid Synthesis in Hydrophilic deep eutectic solvents (DES)
The enzymatic synthesis of sugar surfactants is well established in volatile organic solvents (Castillo et al, 2003; Šabeder et al, 2006), but sugar solubility is limited in this system (Flores et al, 2002)
Summary
Glycolipids are a class of biosurfactants that have been claimed to be non-toxic (Hirata et al, 2009), readily biodegradable (Baker et al, 2000; Hirata et al, 2009; Lima et al, 2011), and less harmful to the environment than the petrochemically produced ones (Dörjes, 1984; Poremba et al, 1991; Lima et al, 2011; Johann et al, 2016). Glycolipids are of special interest to the pharmaceutical industry, e.g., as bioavailability enhancers (Perinelli et al, 2018), and for the food industry, since e.g., Glycolipid Synthesis in Hydrophilic DES sucrose fatty acid esters are approved as food additives (European Parliament, 2014; Younes et al, 2018). Hydrophilic DES consisting of choline as hydrogen bond acceptor and urea or glucose as hydrogen bond donor are proven to be readily biodegradable and have low cytotoxicity (Radoševicet al., 2015; Wen et al, 2015; Mbous et al, 2017). In that study, Zhao et al (2016), investigated various biphasic systems of an organic solvent with 10% of different choline-based DES, using urea, acetamide, glycerol or ethylene glycol as the hydrogen bond donor. Low or negligible glycolipid yields were reported (Zhao et al, 2016)
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