Immobilized Rhizopus oryzae lipase catalyzed synthesis of palm stearin and cetyl alcohol wax esters: optimization by response surface methodology

Abstract

Background: Waxes are esters of long-chain fatty acids and long-chain alcohols. Their principal natural sources are animals (sperm whale oil) and vegetables (jojoba) which are expensive and not easily available. Wax esters synthesized by enzymatic transesterification, using palm stearin as raw material, can be considered as an alternative to natural ones.

Results: Palm stearin is a solid fraction obtained by fractionation of palm oil. Palm stearin was esterified with cetyl alcohol to produce a mixture of wax esters. A non-commercial immobilized lipase from Rhizopus oryzae was used as biocatalyst. Response surface methodology was employed to determine the effects of the temperature (30-50 °C), the enzyme concentration (33.34-300 IU/mL), the alcohol/palm stearin molar ratio (3-7 mol/mol) and the substrate concentration (0.06-0.34 g/mL) on the conversion yield of palm stearin. Under optimal conditions (temperature, 30 °C; enzyme concentration, 300 IU/mL; molar ratio 3 and substrate concentration 0.21 g/mL) a high conversion yield of 98.52% was reached within a reaction time of 2 h.

Conclusions: Response surface methodology was successfully applied to determine the optimum operational conditions for synthesis of palm stearin based wax esters. This study may provide useful tools to develop economical and efficient processes for the synthesis of wax esters.

Figure 1

Contour and response surface plots showing the effect of enzyme concentration and temperature on the synthesis yield of palm stearin based wax esters. Substrate concentration and alcohol/palm stearin molar ratio were at their zero levels.

Figure 2

Contour and response surface plots showing the effect of enzyme and substrate concentrations on the synthesis yield of palm stearin based wax esters. Temperature and alcohol/palm stearin molar ratio were kept at their zero levels.

Figure 3

Contour and response surface plots showing the effect of enzyme concentration and alcohol/palm stearin molar ratio on the synthesis of palm stearin based wax ester. Temperature and substrate concentration were constant at their zero levels.

Figure 4

Contour and response surface plots showing the effect of alcohol/palm stearin molar ratio and substrate concentration on the synthesis yield of palm stearin based wax ester. The enzyme concentration and temperature were maintained at their zero levels.

Figure 5

Production of palm stearin based wax ester during alcoholysis reaction under optimal condition. Temperature, 30°C; enzyme concentration, 300 IU/mL; molar ratio, 3 and substrate concentration, 0.21 g/mL.

Figure 6

(a) Thin layer chromatogram of palm stearin based, wax ester synthesis. Mobile phase: hexane/diethyl-ether/acetic acid 90:10:0.5 (v/v/v). Lane 1: mixture before reaction showing palm stearin (A) and cetyl alcohol (B); lane 2: mixture after 2 h of reaction time showing wax esters (C) and residual cetyl alcohol (D). (b) HPLC profiles of palm stearin/cetyl alcohol mixture obtained before at 0 h (1) and 2 h (2) of reaction times. The column used was: Lichrospher 100 RP-18. 5 μm (4 × 250 mm) and the mobile phase used was acetonitrile/acetone (50:50. v/v) with a flow rate of 1.5 mL/min. Temperature was maintained at 45°C and UV detection was at 206 nm.