Lipase-catalyzed solvent-free synthesis of erythorbyl laurate in a gas-solid-liquid multiphase system

Lipase-catalyzed synthesis of erythorbyl oleate and its characterization as a multifunctional emulsifier

Erythorbyl oleate (EO), a novel emulsifier with multifunctional properties, was synthesized via lipase-catalyzed solvent-free esterification between erythorbic acid and oleic acid. The reaction produced EO of 30.75 ± 1.65 mM at 60 h, monitored by high-performance liquid chromatography (HPLC) analysis. Purified EO was chemically identified as 6-O-oleoyl-erythorbic acid by HPLC-electrospray ionization/mass spectrometry (ESI-MS), 1H nuclear magnetic resonance (NMR), and 13C NMR analyses, and further investigations on its multifunctionalities were conducted. EO displayed antioxidant activity equivalent to erythorbic acid in 2,2-diphenyl-1-picrylhydrazyl and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical scavenging activities. Investigation of the antibacterial activity of EO against foodborne pathogens showed MICs of 0.27 ± 0.09 and 0.40 mM for Bacillus cereus ATCC 10876 and Staphylococcus aureus ATCC 12692, respectively. EO also displayed interfacial properties, lowering interfacial tension between medium-chain triglyceride oil and water to 3.40 ± 0.04 mN/m with a critical micelle concentration of 2.54 ± 0.03 mM. Dynamic light scattering analyses of the emulsion stabilized by EO demonstrated a similar polydispersity index (0.15 ± 0.00) to Tween 80-stabilized emulsion (0.28 ± 0.01), with an average droplet size of 173.2 ± 1.8 nm. In addition, the EO-stabilized emulsion exhibited a negative surface charge (-56.83 ± 4.30 mV) in zeta potential measurements, indicating sufficient electrostatic repulsion between droplets. These results suggest that EO is a multifunctional emulsifier to simultaneously control oxidative rancidification and bacterial spoilage in emulsion-based foods. Practical Application: EO was synthesized via an enzymatic esterification reaction between erythorbic acid and oleic acid. The synthetic process excluded any use of organic solvent, diminishing potential health hazards or environmental pollution associated with chemical reactions. The interfacial properties of the resultant EO, along with excellent antioxidant and antibacterial activities, propose its potential application in emulsion-based foods. Simultaneous control of bacterial spoilage and oxidative rancidification by EO would fulfill the industrial needs for extending shelf life and enhancing consumer safety.

Recent advances on erythorbyl fatty acid esters as multi-functional food emulsifiers

Over the past few decades, food scientists have investigated a wide range of emulsifiers to manufacture stable and safe emulsion-based food products. More recently, the development of emulsifiers with multi-functionality, which is the ability to have more than two functions, has been considered as a promising strategy for resolving rancidification and microbial contamination in emulsions. Erythorbyl fatty acid esters (EFEs) synthesized by enzymatic esterification of hydrophilic erythorbic acid and hydrophobic fatty acid have been proposed as multi-functional emulsifiers since they simultaneously exhibit amphiphilic, antioxidative, and antibacterial properties in both aqueous and emulsion systems. This review provides current knowledge about EFEs in terms of enzymatic synthesis and multi-functionality. All processes for synthesizing and identifying EFEs are discussed. Each functionality of EFEs and the proposed mechanism are described with analytical methodologies and experimental details. It would provide valuable insights into the development and application of a multi-functional emulsifier in food emulsion chemistry.

Synergistic inactivation of Listeria and E. coli using a combination of erythorbyl laurate and mild heating and its application in decontamination of peas as a model fresh produce

We investigated the synergistic antimicrobial activity of erythorbyl laurate (EL) and mild heating co-treatment on the Gram-positive Listeria innocua and Gram-negative Escherichia coli O157:H7 bacteria. EL (2 mM) and mild heating (55 °C for 3 min) resulted in 3.1 and 0.5 log colony forming units (CFU)/mL reductions in the number of L. innocua, respectively, compared to a 6.4 log CFU/mL reduction induced by the combined treatment of EL and mild heating in saline. EL (10 mM) and mild heating (55 °C for 3 min) resulted in 1.3 and 0.7 log CFU/mL reductions in the number of E. coli O157:H7, respectively, compared to a 6.2 log CFU/mL reduction with the combined treatment in saline. EL, a membrane-active compound, showed a strong synergistic effect with mild heating, possibly due to enhanced disruption of the bacterial cell membrane. The synergistic antibacterial effect was evaluated using inoculated English peas (Pisum sativum) and this combined treatment (2 mM EL and mild heating against L. innocua and 10 mM EL and mild heating against E. coli O157:H7) resulted in more than 7 log reductions in the numbers of L. innocua and E. coli O157:H7, inoculated on the surface of fresh peas. The treatments did not show significant difference in the color or texture of treated peas compared to the non-treated controls. This is the first report illustrating synergistic activity of EL and mild heating for both the gram positive (L. innocua) and the gram negative (E. coli O157:H7) bacteria on food. Overall, this research will illustrate the development of more effective and rapid antibacterial surface disinfection method for application in the processing of minimally processed foods.

Rice straw enhancing catalysis of Pseudomonas fluorescens lipase for synthesis of citronellyl acetate

Citronellyl acetate as an important flavor, can be effectively synthesized by lipase catalysis in nonaqueous system. But lipases usually behave low catalytic activity due to aggregation and denaturation of them in organic phase. To enhance the nonaqueous catalysis, based on the mechanism of lipases activated at water/oil (organic phase) interface, the inexpensive race straw was processed into powder and filaments on which Pseudomonas fluorescens lipase was immobilized by physical adsorption, used for synthesis of citronellyl acetate via transesterification of citronellol and vinyl acetate. Results showed that the desired loading was 10 mg lipase immobilized on 30 mg rice straw filaments or 25 mg rice straw powder. When the two immobilized lipases were employed in the reaction system consisted of 1-mL citronellol and 2-mL vinyl acetate at 37 ℃ and 160 rpm, the conversions all reached 99.8% after 12 h. Under the reaction condition, the conversion catalyzed by 10 mg native lipase was 85.1%. Undergoing six times of 8-h reuses in the organic system, the filament and power immobilized lipases had weak activity attenuation rates 0.36 and 0.32% h^-1, lower than 1.52% h^-1 of native lipase. Even at the room temperature and the static state without shaking and stirring, the rice straw filaments immobilized lipase could brought conversion 62.9% after 10 h but the native lipase only gave 37.0%. Obviously, the rice straw, especially its filaments, is an inexpensive and available natural material to prepare immobilized lipase with desired catalysis in organic phase, meant significant potential in flavor industry.

Lipase-catalyzed two-step esterification for solvent-free production of mixed lauric acid esters with antibacterial and antioxidative activities

Mixed lauric acid esters (MLE) with antibacterial and antioxidative activities were produced through lipase-catalyzed two-step esterification in solvent-free system without purification. In the first reaction, erythorbyl laurate was synthesized for 72 h. Successive reaction for 6 h at molar ratio of 1.0 (lauric acid to glycerol) produced MLE containing erythorbyl laurate and glyceryl laurate with small amounts of residual substrates, by converting 99.52% of lauric acid. MLE addition (0.5-2.0%, w/w) to Tween 20-stabilized emulsions decreased droplet size, polydispersity index, and zeta-potential, possibly enhancing the emulsion stability. In the emulsions, MLE at 0.5 and 2.0% (w/w) caused 4.4-4.6 and 5.9-6.1 log reductions of Gram-positive (Staphylococcus aureus, Listeria monocytogenes) and Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa), respectively, within 12 h. Lipid hydroperoxide concentrations decreased to 50.8-98.3% in the presence of 0.5-2.0% (w/w) MLE. These findings support a novel approach without needing purification to produce multi-functional food additives for emulsion foods.

Synergistic Inactivation of Bacteria Using a Combination of Erythorbyl Laurate and UV Type-A Light Treatment

This study evaluated the synergistic antimicrobial activity of erythorbyl laurate (EL) and UV type-A (UVA). To investigate the mode of synergism, changes in gene expression and bacterial inactivation activity were examined. Individual treatments with EL (10 mM) or UVA caused a 1.9- or 0.5-log CFU/ml reduction respectively, whereas EL/UVA co-treatment resulted in a 5.5-log CFU/ml reduction in Escherichia coli viable cell numbers. Similarly, treatment with either EL (2 mM) or UVA for 30 min resulted in a 2.8- or 0.1-log CFU/ml reduction in Listeria innocua, respectively, whereas combined treatment with both EL and UVA resulted in a 5.4-log CFU/ml reduction. Measurements of gene expression levels showed that EL and UVA treatment synergistically altered the gene expression of genes related to bacterial membrane synthesis/stress response. However, addition of 10–50-fold excess concentration of exogenous antioxidant compared to EL reduced the synergistic effect of EL and UVA by approximately 1 log. In summary, the results illustrate that synergistic combination of EL and UVA enhanced membrane damage independent of the oxidative stress damage induced by UVA and thus illustrate a novel photo-activated synergistic antimicrobial approach for the inactivation of both the Gram-positive and Gram-negative bacteria. Overall, this study illustrates mechanistic evaluation of a novel photochemical approach for food and environmental applications.

Cold Active Lipases: Biocatalytic Tools for Greener Technology

Lipases are enzymes that catalyze the ester bond hydrolysis in triglycerides with the release of fatty acids, mono- and diglycerides, and glycerol. The microbial lipases account for $400 million market size in 2017 and it is expected to reach $590 million by 2023. Many biotechnological processes are expedited at high temperatures and hence much research is dealt with thermostable enzymes. Cold active lipases are now gaining importance in the detergent, synthesis of chiral intermediates and frail/fragile compounds, and food and pharmaceutical industries. In addition, they consume less energy since they are active at low temperatures. These cold active lipases have not been commercially exploited so far compared to mesophilic and thermophilc lipases. Cold active lipases are distributed in microbes found at low temperatures. Only a few microbes were studied for the production of these enzymes. These cold-adapted enzymes show increased flexibility of their structures in response to freezing effect of the cold habitats. This review presents an update on cold-active lipases from microbial sources along with some structural features justifying high enzyme activity at low temperature. In addition, recent achievements on their use in various industries will also be discussed.

Erythorbyl fatty acid ester as a multi-functional food emulsifier: Enzymatic synthesis, chemical identification, and functional characterization of erythorbyl myristate

Erythorbyl myristate (EM), a potential multi-functional food emulsifier, was newly synthesized by immobilized lipase-catalyzed esterification between antioxidative erythorbic acid and antibacterial myristic acid. The yield and productivity of EM were 56.13 ± 2.51 mg EM/g myristic acid and 1.76 ± 0.08 mM/h, respectively. The molecular structure of EM was identified as (R)-2-((R)-3,4-dihydroxy-5-oxo-2,5-dihydrofuran-2-yl)-2-hydroxyethyl tetradecanoate using HPLC-ESI/MS and 2D [¹H-¹H] NMR COSY. The hydrophilic-lipophilic balance of EM was 11.5, suggesting that EM could be proper to stabilize oil-in-water emulsions. Moreover, isothermal titration calorimetry demonstrated the micellar thermodynamic behavior of EM and determined its critical micelle concentration (0.36 mM). In terms of antioxidative property, EM exhibited the radical scavenging activity against DPPH (EC₅₀: 35.47 ± 0.13 μM) and ABTS (EC₅₀: 36.45 ± 1.98 μM) radicals. Finally, EM showed bacteriostatic and bactericidal activities against Gram-positive foodborne pathogens (minimum inhibitory concentration: 0.06-0.60 mM; minimum bactericidal concentration: 0.07-0.93 mM).

Antimicrobial Characterization of Erythorbyl Laurate for Practical Applications in Food and Cosmetics

In this study, antimicrobial spectrum of erythorbyl laurate (EL) against bacteria and fungi, leading to food-borne and infectious skin diseases, was evaluated for its practical applications in food and cosmetics. Furthermore, the influences of environmental factors including pH, oxidative stress, and dispersion medium on antimicrobial activity of EL were investigated. The three Gram-positive bacteria and the three molds were susceptible to 3.0 mM EL, while the yeast was susceptible to 6.0 mM EL. It was demonstrated EL retained antibacterial activity against Staphylococcus aureus after being oxidized while the antibacterial activity against the three Gram-positive bacteria including S. aureus, Bacillus cereus, and Listeria monocytogenes was significantly enhanced as decrease of pH from 7.0 to 5.0. Moreover, EL exhibited bactericidal effects against both Gram-positive and Gram-negative bacteria in an oil-in-water emulsion. Treatment of 5.0 mM EL for 4 h reduced 5.29 ± 0.24, 6.01 ± 0.18, 5.95 ± 0.13, and 6.24 ± 0.30 log CFU/mL against S. aureus, L. monocytogenes, Pseudomonas aeruginosa, and Escherichia coli, respectively. In a multipassage resistance selection study, it was observed minimum inhibitory concentrations of EL against S. aureus were not increased over 20 passages, indicating EL might not develop drug resistance of bacteria. This study suggests EL has a potential to be applied as the multifunctional additive in food and cosmetics.

Synthesis of flavor esters by a novel lipase from Aspergillus niger in a soybean-solvent system

To find a lipase for synthesis of flavor esters in food processing, a total of 35 putative lipases from Aspergillus niger F0215 were heterologously expressed and their esterification properties in crude preparations were examined. One of them, named An-lipase with the highest esterification rate (23.1%) was selected for further study. The purified An-lipase had the maximal activity at 20 °C and pH 6.5 and the specific activity of 1293 U/mg. Sixty percent of the activity was maintained in a range of temperatures of 0-30 °C and pHs of 3.0-8.5. The highest hydrolysis activity of An-lipase was towards pNPC (C8), followed by pNPB (C4) and pNPA (C2), then pNPL (C12). K _m, V _max, k _cat, and k _cat/K _m towards pNPC were 26.7 mmol/L, 129.9 mmol/(L h), 23.2 s^-1, and 0.8/mM/s, respectively. The ethyl lactate, butyl butyrate, and ethyl caprylate flavor esters were produced by esterification of the corresponding acids with conversion efficiencies of 15.8, 37.5, and 24.7%, respectively, in a soybean-oil-based solvent system. In conclusion, An lipase identified in this study significantly mediated synthesis of predominant flavor esters (ethyl lactate, butyl butyrate, and ethyl caprylate) in a soybean-oil-lacking other toxic organic solvents, which has potential application in food industries.