Green Extraction of Palmitic Acid from Palm Oil using Betaine-Based Natural Deep Eutectic Solvents

In the palm oil refining processes, the free fatty acid content is reduced to an acceptable level by using steam-stripping that causes, also, the loss of nutraceuticals such as tocopherols and carotenoids. An alternative method, such as solvent extraction, to separate free fatty acids, especially palmitic acid as the major free fatty acid present in palm oil, will conserves not only the important nutraceuticals but, also, conserves energy because a steam requirement is eliminated. The objective of this research is to evaluate the performance of Natural Deep Eutectic Solvents (NADES), each consisting of betaine as the hydrogen bonding acceptor and a polyalcohol as the hydrogen bonding donor, to extract palmitic acid from palm oil. The goal is to obtain a NADES that is able to extract palmitic acid from palm oil with the highest extraction yield. The viscosity of various studied NADES was 10-236 cSt while the polarity parameter, determined by using Nile red as the solvatochromic dye, was 48.9-50.8 kcal/mol. The obtained data shows that, for NADES having similar polarity to that of betaine, the extraction yields reduce with increasing viscosity of the NADES. The highest extraction yield of 60% (w/w), corresponding to a distribution coefficient value of 0.75, was obtained by using NADES consisting of betaine and 1,2-butanediol. The extraction yield and distribution coefficient values indicated the potential of NADES, prepared by friendly compounds of betaine and polyalcohols, as alternative green solvents in the solvent extraction process when separating free fatty acids from palm oil.

Betaine; Deep-eutectic-solvent; Palmitic acid; Palm-oil; Polyalcohol

Palm oil is obtained from crude palm oil after going through the degumming, bleaching, and deacidification processes to get rid of impurities and unwanted components. Palm oil has a unique composition of saturated and unsaturated fatty acids (50:50) and contains desirable compounds such as triacylglycerols, tocopherols, tocotrienols, carotenoids and phytosterols (Edem et al., 2002). Generally, good quality oil contains more than 95% neutral triacylglycerols and 0.5% or less free fatty acids (Lin, 2002; Mba et al., 2015). The presence of free fatty acids in palm oil is undesirable because it is easily oxidized and can cause rancidity (Zahrina et al., 2018).

In some of the palm oil refining processes, valuable nutraceuticals, such as tocopherols and carotenoids, are lost or degraded in the deacidification process because of the chemicals and high temperature steam (240-260°C) used to evaporate the free fatty acids (Hamunen, 2010). It is preferable to carry out the deacidification process by using a solvent extraction method at ambient temperature and pressure because it reduces the nutraceutical compounds losses and reduces the consumption of energy (Gonçalves & Meirelles, 2004, Rodrigues et al., 2007). However, by using hydrated ethanol as a solvent for the extraction, it turns out that there is an unacceptable low amount of extracted free fatty acids (Gonçalves et al., 2007).

Rodrigues et al. (2006) reported that free fatty acids could be extracted from cottonseed oil without reducing the nutraceutical content by using ethanol as extracting solvent. The process was optimized by adjusting the ethanol/water ratio and extraction temperature. The addition of water to ethanol solvent can reduce the nutraceutical loss present in cottonseed oil. However, this process can reduce the palmitic acid content of the palm oil to only 0.3% which is still higher than the 0.1% representing the upper limit requirement for food products. Therefore, the search for an alternative solvent for the extraction of free fatty acids from palm oil continues.

Green solvents are widely used as substitutes of hazardous organic solvents in order to minimize the environmental problems and to improve safety and health and reduce the cost (Bi et al., 2013). The usage of green solvents in the extraction of natural products is known as green extraction (Chemat et al., 2012). Ionic Liquids (ILs) are used in a wide range of applications (Earle & Seddon, 2000; Earle et al., 2006; Han & Row, 2010). However, due to the high toxicity of some ILs and the high cost of their synthesis, ILs are not commonly used in the pharmaceutical and food industries (Choi et al., 2011; Angell et al., 2012; Dai et al., 2013a).

Abbott et al. (2004) reported firstly about deep eutectic solvents that had similar physical properties and phase behaviors to of ILs. Such solvents were regarded as alternatives to ILs due to their ease of synthesis, availability, biodegradability, negligible volatility, being environmentally friendly and of low cost (Zhang et al., 2012; Hayyan et al., 2013, Maugeri et al., 2012). Deep eutectic solvents can be formed by mixing a Hydrogen Bond Acceptor (HBA), such as a quaternary ammonium salt, as and one or more Hydrogen Bond Donors (HBD) such as amides, carboxylic acids and polyalcohols. The HBA and HBD form intermolecular hydrogen bonds with each other when mixed in a certain molar ratio and produce an eutectic mixture that has a lower melting point than its individual components. Dai et al. (2013b) reported the existence of Natural Deep Eutectic Solvents (NADES) by using mixtures of various cellular constituents (primary metabolites) from all kinds of organisms. NADES, which are still liquid at room temperature, have nontoxic and environmentally friendly characteristics that are used potentially as solvents for the extraction of bioactive compounds from plants (Bi et al, 2013; Mulia et al., 2015; Garcia et al., 2016).

The physicochemical properties of betaine-polyalcohol NADES and the molecular structure of polyalcohols affected the stability and performance of NADES extracting palmitic acid from palm oil. The viscosity and density of NADES from betaine and polyalcohols appeared to be influenced by the type of polyalcohols and the molar ratios of betaine-to-polyalcohols. The hydrogen bonds and free volume formation were expected to affect the value of viscosity and density. The polarity of the studied NADES was more likely to be affected by the type of polyalcohol used rather than by the molar ratio of betaine-to-polyalcohol. There is relationship between the extraction yield and viscosity, for NADES with closed polarity (E_NR) to the polarity of pure betaine, i.e. the extraction yield declines with increasing viscosity of the NADES. The highest single-stage extraction yield of palmitic acid from palm oil was 60% (w/w) with the distribution coefficient of 0.75; this was obtained by using NADES from betaine and 1,2-butanediol. This study’s extraction condition was mild and each NADES was prepared from environmentally friendly compounds that were less or non-toxic to human or living things. The results indicate that NADES, prepared by friendly compounds of betaine and polyalcohols, have the prospect of being alternative green solvents in separating free fatty acids from palm oils in the solvent extraction process.

The authors are grateful for the Universitas Indonesia’s financial support through the DRPM PITTA project, contract number 772/UN2.R3.1/HKP.05.00/2017.

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