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2.6.1.1 Esterification

The reaction pathway of esterification of fatty acids with glycerol to produce monoglycerides is shown in Figure 2.3. The esterification of glycerol and fatty acid will yield a mixture of 1(3)-

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in the system to obtain high yield of monoglycerides. This can be achieved by removing the water through vacuum or molecular sieves (Yang et al., 2005b).

Glycerol Fatty acid 1(3)-Monoglyceride 2-Monoglyceride Water

Source: Bornscheuer (1995); Pouilloux et al. (1999)

Figure 2.3: Esterification of glycerol and fatty acid

2.6.1.2 Glycerolysis

Glycerolysis is another pathway in which monoglycerides can be synthetically derived. The glycerolysis reaction is depicted in Figure 2.4. This is often used to produce monoglycerides as the glycerolysis offers the advantage of a higher yield. This is due to the fact that 1 mole of triglyceride can subsequently produce 3 moles of monoglycerides (Yang et al., 2005b). However, in real reaction systems, mixtures of monoglycerides and diglycerides are often obtained with some residues of unreacted triglycerides (10 %), residual glycerol (3 - 4 %) and free fatty acids (1 – 3 %). Often the use of excess glycerol rather than the theoretical 2 moles are needed (Chetpattananondh et al., 2005). CH2OH CH2OH CHOH RCOOH CH2OH CH2OH CH2OCOR CH2OCOR CH2OH CHOH H2O + _{or +}

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The industrial setting of glycerolysis process usually involves high temperature between 200- 260°C and inorganic catalysts usually sodium hydroxide under a nitrogen gas atmosphere (Bradić et al., 2010; Garti, 2001; Nylander, 2004; Sonntag, 1982). The source of triglycerides used are often from hydrogenated vegetable oil such as soya bean, rapeseed and cottonseed oils, or animal fats such as lard and tallow (Krog, 2002; Mahungu & Artz, 2001). Often, apart from monoglycerides, derivatives of monoglycerides such as organic acid esters of monoglycerides are also used in food industries. These derivatives of monoglycerides can be produced by esterification of the monoglycerides with various organic acids, such as, lactic acid, citric acid, diacetyl tartaric acid, and or acetic acid. Due to the difference in type of organic acids, the resulting product often exhibit different attribute from those of the monoglycerides with regard to crystalline behaviour and surface activity (polarity) (Faergemand & Krog, 2006).

Triglyceride Glycerol 1(3)-Monoglyceride 2-Monoglyceride

Source: Bornscheuer (1995) Figure 2.4: Glycerolysis of triglycerides with glycerol

CH2OCOR CH2OCOR CH2OCOR + CH2OH CH2OH CHOH CH2OH CH2OH CH2OCOR CH2OCOR CH2OH CHOH + 2 2 C

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The catalysts require neutralisation when the reaction is finished to prevent the reaction to reverse, which can happen to an extent of about 30 % and also to prevent the undesirable soapy taste, unstable colour and foaming of the final mixture (Corma et al., 1998). Although the addition of inorganic catalysts can help achieving high reaction rate at higher temperature, it has a shortcoming due to the non-selective nature of the catalysts, resulting numerous by- products being produced. This requires product clean-up from the mixtures obtained (Bradić et al., 2010). The monoglycerides percentage in the equilibrium blend gained relies on the glycerol- fat ratio in the reaction mixture and may vary from 10 to 60 %. Majority of the commercial mono- and diglycerides usually contain 40 to 55 % monoglycerides, 30 to 45 % diglycerides, and 8 to 20 % triglycerides (Faergemand & Krog, 2006; Krog, 2002).

Due to the undesirable blends of by-product in the mixture obtained, purification of the monoglycerides are needed because they need to be highly pure for the use in the food industry since they have a better emulsifying properties rather than a mixture of acylglycerols (Bornscheuer, 1995). However, short-path distillation (molecular distillation) is not able to completely separate monoglycerides and free fatty acids (Shimada, 2005). Monoglycerides can be separated from the other components by further purification by repeated extraction or high- vacuum thin film molecular distillation process yielding relatively pure >92 alpha monoglyceride esters (Garcia et al., 1995; Krog, 2002; Mahungu & Artz, 2001; Nylander, 2004). The proportion of products after purification are typically 95 % monoglycerides, 3 - 4 % diglycerides, 0.5 - 1 % free glycerol, and 0.5 - 1 % free fatty acids (Krog, 2002).

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Monoglycerides can be categorised as saturated or unsaturated depending on the nature of fatty acids chain in the triglycerides used. Saturated monoglycerides are formed from triglycerides or fatty acids with chain lengths of C16 (palmitic), C18 (stearic) and to a lesser extent lauric (C12), whereas unsaturated monoglycerides are mainly comprised of C18:1 (oleic) fatty acid/ triglycerides (Golding & Sein, 2004). These different fatty acids composing the triglycerides can also influenced the physical form of the monoglycerides produced of either solid or liquid. Unsaturated fatty acids mainly oleic acids will produce liquid monoglycerides and are more susceptible to oxidation and degradation, whereas saturated fatty acids will yield solid powders of monoglycerides that resembles a more waxy appearance. The melting points of the monoglycerides produced depends on the composition, origin and structure of fatty acids (Garti, 2001).

2.6.1.3 Enzymatic hydrolysis of triglycerides

Apart from the glycerolysis and direct esterification route, monoglycerides can also be produced from the hydrolysis of triglycerides. The established industrial process is known as the Colgate- Emery method, which often employed high temperature to about 250 °C and at 50 atm. Similar with the industrial glycerolysis, these harsh conditions often results in side reactions and requires further purification of the final products (Noureddini & Harmeier, 1998). An alternative route for production of monoglycerides is the hydrolysis reaction catalysed by lipase enzymes, particularly by a 1,3-selective lipases as shown in Figure 2.6. One mole of triglycerides will produced 1 mole of 2-monoglycerides, which can turned into 1,3-monoglycerides due to acyl

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recognised as a more natural pathway by which this material can be produced (Kaewthong, 2004; Yang et al., 2005b).

Triglyceride Water 2-Monoglyceride Water

Source: Bornscheuer (1995); Kaewthong (2004); Rodrigues & Fernandez-Lafuente (2010a; 2010b) Figure 2.5: Hydrolysis of triglycerides by 1,3-selective lipase