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Palm Oil Developments 70 (June 2019) p. 4-8

PALM-BASED SUGAR ESTERS

Palm-based sugar esters can be produced by esterifying sugar with palm fatty acid or transesterification between sugar and palm-based methyl ester or triglyceride. The fatty acids, methyl esters and triglycerides can be sourced from palm oil and palm kernel oil, depending on the desired fatty acid chain length. On the other hand, sugars that can be used as feedstock include monosaccharides such as glucose, fructose and

Recent Development in Enzymatic Production of Palm-based Sugar Esters as Bio-based Surfactants

Arniza Mohd Zan*; Hoong Seng Soi* and Nik Siti Mariam Nek Mat Din*

BIO-BASED SURFACTANTS

The demand for bio-based surfactants is increasing and is influenced by the growing consumer awareness of the benefits of bio-based products. Bio-based surfactants are surface-active biomolecules derived from renewable feedstocks, such as sugar and palm oil. One advantage of utilising renewable feedstocks in surfactant production is to reduce carbon dioxide emissions (Foley et al., 2012). Sugar ester, a type of bio-based non-ionic surfactant, is an amphiphilic compound composed of a sugar and fatty acid moiety (Figure 1). They are usually odourless, biodegradable, non-toxic and non-irritating, and are therefore commonly used in food, cosmetics and personal care products (Ye and Hayes, 2014).

* Malaysian Palm Oil Board (MPOB), 6 Persiaran Institusi, Bandar Baru Bangi, 43000 Kajang, Selangor, Malaysia. E-mail: arniza@mpob.gov.my

galactose as well as disaccharides such as sucrose, maltose and lactose. One interesting fact of sugar esters is that they can be designed to suit a specific application by modifying the sugar head and the fatty acid tail, which changes the surfactant properties of the sugar esters. Therefore, these bio-based surfactants cover a wide range of applications. Sugar esters have been reported to function as emulsifier, antimicrobial, drug delivery and insecticidal agents. Besides commonly used in food and personal care industries as bio-based surfactants, they have also been used in detergent, pharmaceutical, medical, agriculture and other areas such as enhanced oil recovery

and environmental detoxification (Figure 2) (Gumel et al., 2011).

PRODUCTION PROCESS

The esterification and trans-esterification reactions between sugar and fatty acid moiety can be conducted by conventional chemical or enzymatic route. In conventional chemical route, the process is conducted at high temperature, catalysed by acid or alkaline catalysts with the aid of organic solvents. Although this approach often gives high yield, the catalysts used are not selective, thus producing multi-esters and other by-products. The esters produced from conventional route are usually dark-colored products (Gumel et al., 2011). Enzyme-catalysed synthesis has been reported as a potential substitute to the traditional route. The benefits of enzymatic route are milder reaction conditions, high selectivity and high efficiency of the reaction (Lee and Kim, 2016; Enayati et al., 2018). Various types of sugar esters have been enzymatically synthesised by utilising palm-based material (Table 1). The important components involved in the production of sugar esters are shown in Figure 3.

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Recent Development in Enzymatic Production of Palm-based Sugar Esters as Bio-based Surfactants

DEVELOPMENT OF ENZYMATIC PRODUCTION PROCESS

The major challenge in the production of sugar esters is the solubility issue of the two substrates, sugar (polar) and fatty acid (non-polar). It is very critical to find the most suitable reaction medium to compromise the solubility of both substrates without affecting the enzyme stability. Polar organic solvents are popular in the early

Figure 2. Application of sugar esters as bio-based surfactant.

days of product development such as pyridine, dimethylsulfoxide and dimethylformamide because they can easily dissolve sugars. However, it comes with several disadvantages: easily deactivate enzymes and incompatible with requirements of many food and personal care applications (Walsh et al., 2009).

In the search to find greener solvents to replace the polar organic solvents, ionic liquids

emerge as an alternative due to their non-volatility, thermal stability, and widely tunable properties to dissolve both polar and non-polar compounds. Hydrophilic ionic liquids consist of strong coordinating anion are very good in dissolving sugars. For example, 1-butyl-3-methyl imidazol ium dicyanamide can dissolve more than 100 g litre-1 of sugars compared to hydrophobic ionic liquids that consist of weak coordinating anion such as 1-methoxyethyl-3 - m e t h y l i m i d a z o l i u m tetrafluoroborate which can only dissolve 5 mg ml-1 of sugar. However, hydrophilic ionic liquids have a tendency to denature enzymes because they can form strong hydrogen bonds with the enzymes, which will affect their three dimensional conformation, resulting in loss of activity (Zhao et al., 2008).

To overcome this problem, special ionic liquids capable of dissolving sugars but not deactivating enzymes are developed, a series of ionic liquids consist of glycol-substituted cations and acetate anions that can dissolve up to 80% sugar (Zhao et al., 2008). Utilising binary system of ionic liquid and organic solvent can also improve the product conversion. Incorporating co-solvent such as 2-methyl-2-butanol into the ionic liquid system can improve product conversion due to reduced viscosity of the reaction mixture and improved solubility of sugar in the initial reaction mixture (Li et al., 2015).

Nonetheless, ionic liquid has been questioned for its poor biodegradability, biocompatibility and high cost. Therefore, deep eutectic solvents (DES) have

Figure 1. Amphiphilic bio-based surfactant.

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TABLE 1. PALM-BASED SUGAR ESTERS SYNTHESISED THROUGH ENZYMATIC ROUTE

Palm-based material Types of sugar Solvent system Product Reference

Palm fatty acid distillate Fructose Tert-butanol Fructose esters Rakmi et al., 1997

Oleic acid Sucrose and fructose Tert-butanol Sucrose and fructose oleate

Dang et al., 2005

Capric acid Lactose Acetone Lactose caprate Zaidan et al., 2012

Caprylic, lauric and palmitic acid

Lactose Hexane Lactose monocaprylate, monolaurate and monopalmitate

Enayati et al., 2018

Methyl laurate Fructose Tert-butanol/ dimethylsulfoxide

Fructose monolaurate Lee and Kim, 2016

Lauric acid Fructose Ionic liquid/2-methyl-2-butanol

Fructose monolaurate Li et al., 2015

Oleic acid Glucose Solvent-free Glucose oleate Vitisant et al., 2012

Oleic acid Sucrose and fructose Solvent-free Sucrose and fructose oleate

Ye et al., 2016

Figure 3. Important components in production of sugar esters.

emerged as an alternative due to their similar properties with ionic liquids. In contrast, DES are more attractive because they are biodegradable, non-volatile, non-toxic, non-flammable and have much lower production cost (Paiva et al., 2014). Regardless of the great potential of ionic liquids and DES, there are several disadvantages associated with

their use: high viscosity, poor recyclability, and biocompatibility of ionic liquid and DES.

Alternatively, supercritical fluid such as supercritical carbon dioxide (SC-CO2) is also a promising green solvent. It is non-toxic, non-flammable and leave no residues in final product (Palocci et al., 2008). Moreover, the production

of sugar esters in SC-CO2 can be carried out without using molecular sieves, which lower the production cost and minimises mass transfer limitations. The SC-CO2

was employed in the synthesis of sucrose and fructose esters with a satisfactory yield of 61% even without the addition of molecular sieve (Habulin et al., 2008). In order to improve the solubility of sugar in SC-CO2, Tai et al., (2009) applied SC-CO2-expanded with acetone in the enzymatic synthesis of glucose palmitate. This action was reported to improve the viscosity of reaction medium, better product selectivity, milder production process and higher reaction rates and turnover frequencies (Tai et al., 2009).

Another interesting method is solvent-free enzymatic synthesis of sugar ester. Fructose oleate was successfully synthesised under solvent-free conditions with a conversion of 80% (Pyo and Hayes, 2009). A few other ways to increase efficiency of the synthesis are by utilising non-conventional energy sources such as microwave,

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Recent Development in Enzymatic Production of Palm-based Sugar Esters as Bio-based Surfactants

ultrasound and mechanochemical mixing. Ye et al., (2016) applied both high speed and high pressure homogenisation to form a metastable suspension of 2.0 to 3.3 micron sized sugar particles in the reaction medium, which was oleic acid. This method successfully increased the sugar ester content to 89%.

CONCLUSION

The enzymatic route of producing palm-based sugar esters offers milder reaction conditions, high selectivity and high efficiency of reaction as well as a more environmental friendly process. Numerous studies have been carried out to synthesise sugar esters from palm oil product, such as fatty acids, methyl esters and palm fatty acid distillates. The versatility of the sugar ester makes it an interesting bio-based surfactant because of its tuneable properties that can be designed to match certain application. These palm-based sugar esters can be an alternative to commercially available petroleum-based sugar esters as the demand for bio-based surfactants is increasing. This phenomenon is influenced by the growth of consumer awareness on the benefits of bio-based products. The production process of sugar esters has evolved from using hazardous solvents to greener solvents (ionic liquid, deep eutectic solvent and supercritical fluid) and solvent-free method. One interesting fact proving by the solvent-free approach is that the two substrates (sugar and fatty acid) do not need to be completely miscible for the reaction to occur. Despite that, continuous research activities are being conducted to improve the production process of

sugar esters, not only to meet the stringent requirement of applications such as food, cosmetic, personal care, pharmaceutical and medical. Also, to preserve the environment by minimising the energy usage, utilising less hazardous chemicals in the production process and minimising production of waste.

ACKNOWLEDGEMENT

The authors would like to thank the Director-General of MPOB for the permission to publish this article.

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