Introduction

Palmitic acid ethyl ester is a colorless needle shaped crystal(Figure 1). Also known as , ethyl palmitate or ethyl hexadecanoate. The molecular formula is C₁₈H₃₆O₂, The relative molecular weight is 284.48, Melting point 24-26℃ (lit.), Boiling point 192-193℃ 10mmHg (lit.). At 25℃ (lit.), the density is 0.857g/mL and the refractive index is n20/D 1.440 (lit.). FEMA: 2451, flash point:> 230°F,?20℃ storage. Insoluble in water, soluble in organic solvents such as alcohols, ethers, acetone, benzene, chloroform, etc. Flammable when exposed to fire or heat. It has a faint wax, fruity, and creamy aroma. Palmitic acid ethyl ester is found in apricot, sour cherry, grapefruit juice, blackcurrant, pineapple, red wine, apple brandy, black bread, mutton, rice, etc.[1]. The United States federal regulations have confirmed that it is a natural product, which can have a coordinated natural flavor in the blending of Baijiu, and also partially exists in flue-cured tobacco and oriental tobacco.

The application of  Palmitic acid ethyl ester

Palmitic acid ethyl ester has the following main uses:

1. GB 2760-96 stipulates that palmitic acid ethyl ester is an allowed edible flavoring. It can be used for the preparation of essence for butter, tallow, milk, pork, fish, spices, and also for the preparation of rum essence. Sun F.[2] had found some ethyl palmitate in kiwifruit wine through GC-MS detection, which has not been reported in previous research on kiwifruit wine at home and abroad, and is the first report. Palmitic acid ethyl ester widely exists in Baijiu and fruit wine. It can increase the mellow feeling in the wine body, reduce the dry and astringent taste in the wine, and enhance the aftertaste of Baijiu. If its content is too high, it will produce greasy and slightly astringent sensory manifestations. Therefore, controlling the content of ethyl palmitate can effectively improve the taste and quality of Baijiu[3]. Use limited FEMA ((Flavor and Extract Manufactures' Association of the United States): for cold drinks and baked goods, both 20.0mg/kg; Candy 40.0 mg/kg. 

2. Palmitic acid ethyl ester can be used as a softener and lubricant.

3. Palmitic acid ethyl ester can be used for organic synthesis, etc. Using ethyl palmitate as raw material, certain sucrose esters and sucrose polyesters can be synthesized through transesterification reaction.

4. Palmitic acid ethyl ester is a type of biodiesel. In the process research of ester synthesis, biocatalysis has received increasing attention due to its mild reaction conditions and high regioselectivity.

The synthesis methods of Palmitic acid ethyl ester

Regarding the synthesis of palmitic acid ethyl ester, most of the reported methods currently use chemical methods. For example, using proton acids such as concentrated sulfuric acid to catalyze esterification reactions. Although inorganic acids such as concentrated sulfuric acid have the advantages of high activity and low cost as catalysts, they also have disadvantages such as poor selectivity, multiple by-products, difficult product separation, and serious environmental pollution[4]. With the deepening of research, solid superacids have been introduced as catalysts in the synthesis of palmitic acid ethyl ester. Although solid superacid catalyzed esterification reactions are pollution-free, there are still drawbacks such as complex preparation and cumbersome characterization[5].

In order to improve the shortcomings of the above synthesis methods, researchers attempted to use enzymatic catalysis to synthesize palmitic acid ethyl ester, meeting people's demands for production environment and high-quality natural products. This method has the characteristics of mild reaction conditions, low pollution, high rate, and high purity. It has been confirmed that palmitic acid ethyl ester can be efficiently synthesized using palmitic acid and ethanol as substrates under the catalysis of lipase. There is relatively little research on lipase catalyzed synthesis of palmitic acid ethyl ester both domestically and internationally. Li Y etal.[6]had successfully synthesized molecular sieve NH₂-SBA-15 which had a larger aperture.They used the molecular sieves as a carrier to immobilize the porcine pancreatic lipase which had good stability, so it was suitable for non-aqueous phase catalysis. In this experiment, they used the palmitic acid and ethanol as substrates, immobilized enzyme as a catalyst. The yield of palmitic acid ethyl ester was about 73.5% through process optimization. In addition,the synthesis of palmitic acid ethyl ester catalyzed by Elp2252 was investigated by Gao JX[7]. The optimal molar ratio of palmitic acid to ethanol was 1:2, the optimal reaction temperature and time were 40°C and 6h, and the optimal amount of water and enzyme were 4% and 0.2g respectively. Under the above condition, the yield of palmitic acid ethyl ester reached 50.3%.

In summary, the enzymatic synthesis of palmitic acid ethyl ester is reasonable and effective, breaking through the bottleneck of traditional chemical synthesis methods. Microbial enzymatic synthesis has the characteristics of mild reaction conditions, low energy consumption, minimal waste, high reaction rate and conversion rate. At the same time, the appropriate enzymatic reaction environment can be selected according to the required product, which has guiding significance for obtaining high content of palmitic acid ethyl ester.

References

[1] Guo J.Research on Aroma Components of Kiwi Fruit and Fruit Wine [D]. Master's thesis from Northwest A&F University 2007;30-83.

[2] Sun F.Research on the Selection of Excellent Kiwi Wine Yeast and Aroma Control Techniques for Kiwi Wine [D.] Northwest A&F University, 2006;1-233.

[3] Zhou H,Wang S,Yu J, etc. A rapid method for measuring ethyl palmitate in Baijiu [J]. Brewing Technology 2014;10(1):120-122.

[4]Ma H, He J,Evans DG, et al.Immobilization of lipase in a mesoporous reactor based on MCM-41 [J]. J.Mol.Catal.B:Enzym.2004;30:209-217.

[5]Yadav GD, Jadhav SR.Synthesis of reusable lipases by immobilization on hexagonal mesoporous silica and encapsulation in calcium alginate:transesterification in non-aqueous medium [J].Micropor.Mesopor.Mater. 2005;86:215-222.

[6]Li Y.The study on the synthesis of ethyl palmitate catalyzed byimmobilized lipase in mesoporous molecular sieves SBA-15[D].Jilin University,2014;1-95.

[7]Gao J.Study on the cloning and expression, characterization of lipase from Paenibacillus pasadenensis CS0611 and the synthesis of ethyl palmitate[D].South China University of Technology,2018;1-112.