The Chemistry of Food Additives and Preservatives

2

Emulsifiers

Abstract: In food industries, forming a homogeneous mixture of food components which are totally immiscible (such as oil and water) is a challenge. Food emulsifiers are chemical molecules characterised by the presence of a hydrophilic and a hydrophobic part. The hydrophobic component is made up of fatty acid, while the hydrophilic portion consists of either glycerol or one of its ester derivatives generated from the reaction with organic acids such as lactic, citric, acetic or tartaric acid. As food additives, emulsifiers play a very important role in enabling hydrophilic components (e.g. water) and hydrophobic substances such as oils to mix together to form a stable continuous homogeneous product, or an emulsion. Examples of emulsion food products include mayonnaise, ice-cream and homogenised milk, which are composed of hydrophilic and hydrophobic substances.

Keywords: acacia gums; acetylated monoglycerides; cellulose alkyl esters; emulsifiers; glycol alginates; HLB; lactylated derivatives; sorbitan monoesters; stearoyl lactylates; succinylated derivatives

2.1 MECHANISMS OF FOOD EMULSIFIERS

By definition, emulsions are heterogeneous colloid mixtures of small molecule droplets of one component suspended in another component immiscible to it (Stampfli and Nerden 1995). This is possible because the surface tension that exists between the immiscible components is reduced by the action of emulsifiers which enable the immiscible phases to form one stable homogeneous phase known as an emulsion (Dziezak 1988). In the case of liquids, the term emulsion refers to the dispersion of two liquids that, under normal circumstances, are not miscible (e.g. oil and water). When mixed, an emulsion will form as tiny droplets of one phase dispersed into a continuous phase of another (Krog 1990; Stampfli and Nerden 1995). The availability of both the hydrophobic and hydrophilic affinities within one and the same material encourages the emulsifier molecules to position themselves accordingly at the junction between one phase and the continuous phase; the hydrophobic part of the molecule will be directed towards the oil phase while the water-loving hydrophilic part will orient itself towards the aqueous phase (Krog 1990; Stampfli and Nerden 1995). This mechanism prevents the droplets from lumping with other droplets which could result in the collapse of the emulsion (Figure 2.1a, b). This means that, in the real sense, emulsifiers do not create emulsions; instead, mechanical energy creates the emulsion and emulsifiers simply lock the emulsion that has been created.

Fig. 2.1 The emulsification process: (a) emulsifier at the water–oil interface; and (b) suspension and dispersion of droplets to the other phase (water or oil).

Among those emulsifiers used by the food industry are the monoglycerides, which are prepared using fats and oils as raw materials (Flack 1987). Fats are made up of a hydrophilic backbone of triglyceride with three fatty acid molecules attached to it, making the whole molecule lipophilic in nature (Lal et al. 2006). When the triglyceride molecule is cleaved, the lipophilic nature of the tail of the fatty acids balances the hydrophilic properties of the glyceride head to stabilise the emulsion (Lal et al. 2006).

Figure 2.1a shows how an emulsifier forms a film when it is adsorbed at the interface between the oil and water phases. The hydrophilic head end of the fatty acids in the emulsifiers points towards the aqueous phase while the lipophilic end points towards the oil phase in an air–water or oil–water interface, in order to lower the surface tension or interfacial tension (Jaynes 1985; Krog et al. 1985; Friberg and Larsson 1990). By forming a film at the interface, an emulsifier converts one of the two immiscible phases (water and oil in this case) into drops, which become suspended and dispersed over the other phase (Figure 2.1b). The emulsifier stabilises the droplets by encircling them to completely isolate them from each other; the immiscible phases will therefore not separate.

Emulsifiers are therefore liaisons between the two immiscible components and serve to stabilise such mixtures. In terms of their physical appearances and texture, emulsions are thick and normally find application in foods as well as in pharmacy, where they stabilise pharmaceutical products and formulation (Lal et al. 2006).

2.2 THE ROLE OF EMULSIFIERS IN FOODS

The primary role of emulsifiers as food additives is to improve food palatability, maximise the volume and aeration of food items, reduce the stickiness, enhance food flavour, improve the textural properties of foods and impart foam stability (Dieffenbacher and Martin 1987; Suman et al. 2009).

Apart from the role of assisting oil and water to remain in stable emulsions, a property useful in the preparation of many food products such as salad dressings, emulsifiers have many other roles that they play in the food industry. Their unique molecular structures enable them to perform a variety of other roles in the improvement of the quality of a wide variety of food products (Baker 2010). This may explain why many processed foods consist to some extent of emulsions and, in some cases, the whole food may be an emulsion (or the food may have been in an emulsified state at some stage of the processing).

Emulsifiers are also attractive for use as food additives due to their safety record; they can be present in a human body up to 125 mg kg−1 body weight without causing any health problems (FAO 1963; FAO/WHO 1963).

2.2.1 Emulsification

Emulsification (the process of maintaining the emulsions) is one of the primary roles of emulsifiers in foods. The choice of the emulsifier is largely dependent upon the type of food material which forms the dispersed phase and the continuous phase. For example, if oil is the continuous phase, the emulsifier must be more lipophilic; if it is an aqueous system such as water then a hydrophilic emulsifier will be the best choice for use.

2.2.2 Starch complexing

Another role of emulsifiers in food is that of starch complexing, which has a wide range of applications. Starch granules comprise a linear polymer polysaccharide of water-insoluble D-glucose sugar units known as amylose (Figure 2.2a) and another highly branched water-soluble glucose polymer molecule known as amylopectin (Figure 2.2b). When starch is dispersed in water and heated the granules tend to absorb water and swell. They become gelatinised in the sense that the starch molecules attain a viscous state, forming a gel structure. When the product is cooled the starch molecules will tend to be closer to one another, squeezing the absorbed water out. This causes the starch to recrystallise in a process known as retrogradation.

Fig. 2.2 Chemical structure of (a) amylase; (b) amylopectin (Baker 2010); and (c) phospholipase B.

In products where retrogradation takes place (e.g. bread), emulsifiers are incorporated to retard this process and maintain the softness of the product. During gelatinisation, the linear water-insoluble amylose molecule forms a helical structure, the inside of which tends to possess a mild lipophilic property. When the emulsifiers complex with the amylose through the attachment of their hydrophilic tail ends inside the helical structure, it automatically physically inhibits the amylose molecule from retrograding.

Since there is a diverse range of emulsifiers, there are differences in the extent at which different emulsifiers complex to starch molecules and the shape of the molecule has a great effect on whether it fits within the helical structure. For instance, the fatty acid has to be either fully saturated or trans-oleic fatty acid for it to fit inside the helix. If it exists in cis isomeric form or in a polyunsaturated configuration, the structure with an optical bend on the fatty acid will make it impossible to fit into the helix; the molecular structure and shape must be straight. Structures such as lecithin which contain two sets of fatty acids will not be effective starch complexing agents unless they undergo some chemical modifications by cleaving one of the fatty acids using a phospholipase enzyme (Figure 2.2c).

The ability of food emulsifiers to form complexes with starch components, especially amylose, is evaluated using the amylose complexing index (ACI). The ACI takes a value between 0 and 100; 100 represents no iodine affinity (IA), implying that complete amylose-emulsifier complex formation has been attained. Iodine affinity (IA) for iodine complexes is defined as the fraction or ratio of amylase to starch.

2.2.3 Foam stabilisation and aeration

Food emulsifiers also play the important role of foam stabilisation and provide aeration in baked and dairy products by improving the mechanism by which air is incorporated and retained. Air is important in the process of baking to give the required texture; air sacks in the baking batter expand due to the carbon dioxide generated from the leavening of the baked...