The polyglycerol part of PGEs consists of branched or linear oligomer ethers of glycerol. Polyglycerol is usually prepared by alkaline polymerization of glycerol at elevated temperatures. This reaction will, depending on the reaction conditions, give rise to more or less branched oligomers of varying length. The hydrophilicity, and HLB value, of the final polyglycerol ester will be positively correlated with the length of the polyglycerol moiety.
The final product can be produced by direct esterification between polyglycerol and the chosen fatty acid at alkaline conditions and with temperatures above 200 °C. The sources of the fatty acids are often plant oils, such as sunflower oil, palm oil, or rapeseed oil. The fatty acids can also be derived from animal fats such as lard or tallow. Information on the exact origin of the fatty acids can only be obtained by consulting the emulsifier producer, as chemically, the fatty acids are identical from the two types of sources.
During the reaction, mono- and diesters can be favored by using a larger surplus of polyglycerol relative to the fatty acid. The remaining free polyglycerol is subsequently removed by extracting with water combined with salts.
The functionality of the final product can be tailored by varying the degree of polymerization of the polyglycerol, by choosing short- or long-chain free fatty acid, or by fine-tuning the final esterification step to favor either a high proportion of monoesters or a more complete esterification of all the available alcohol groups.
Legislation and safety
In 1978, the Scientific Committee on Food (SCF) endorsed an acceptable daily intake (ADI) of 25 mg/kg body weight per day previously established by the joint FAO/WHO Expert Committee on Food Additives (JECFA). Absorption of intact PGE in the gastrointestinal tract is extremely low. Instead, PGEs are rapidly and almost fully hydrolyzed to polyglycerol and free fatty acids. These compounds have recently been assessed and no detrimental effects have been identified in available studies. No adverse effects were seen in nonchronic studies with PGE doses up to 9000 mg/kg or up to 2500 mg/kg in chronic studies, the highest doses tested. Based on these and a range of other recent studies, the ESFA panel concluded in 2017, that PGE (E 475) was not of safety concerns and that there was no need for a numerical ADI value. Within EU the amount of higher oligomers is restricted in food usage.
PGE find uses in many areas due to their versatile and highly customizable chemical structures. They are used mainly due to their excellent interfacial properties in combination with their ability to affect crystallization in food lipids and other emulsifiers.
Margarines are a special type of water-in-oil emulsions containing salts, triglycerides in the form of edible refined oils and fats, milk components, pH regulators and other ingredients. The most widely used emulsifiers in margarines are lecithins, monoglycerides and CITREM. PGEs are mainly used in specialized types of margarines to improve the functional properties such as mouthfeel of spreads, or stabilizing or aerating the food containing the margarine.
The organoleptic properties of low-fat spreads and margarines can be improved by PGEs by reducing the graininess of the lipid phase and give a plasticity and elasticity of the margarine mimicking that of natural butter. PGEs also play an important role in stabilizing the emulsion in low-fat spreads and margarines such as pourable frying margarine.
An important application of PGEs is in aerated cake batters containing very little fat and oil. By employing PGEs, the production of these cakes can be simplified to a one-step ‘all-in whipping’ of the cake batter. Due to PGE’s excellent interfacial stabilizing abilities, the resulting cake batter will be more uniform with a much higher number of small air cells. Furthermore, the final cake batter is more robust and stable against losing air for an extended period. This enables the production of very light, aerated cakes with an easy handling and stable production.
The PGEs used for this purpose are mainly based on saturated fatty acid mono- and diesters as these give the best air-incorporation in cake batters. As the pure waxy solid materials are next to impossible to use directly, the PGEs are normally sold in a powder or a paste formulation. The powder formulation is often preferred as this can be used directly in both flour and cake dry mixes. Powders containing PGEs can be produced by spray-drying an oil-in-water emulsion containing other components such as carbohydrates and proteins. Alternatively, the PGE can be added to starch to give a fine and stable emulsifier powder using extrusion technology.
PGEs are utilized in the cosmetics industry as excellent stabilizers of oil-in-water emulsions such as creams and lotions. Within the plastics industry PGEs are extensively used as safe food-grade additives conferring anti-static properties to plastic containers. Their surface-active characters can also be exploited for the prevention of condensation of water on the inside of food wrapping.
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