SURFACTANTS: BASICS AND VERSATILITY IN FOOD INDUSTRIES
The nonionic surfactant can be of polyol esters, polyoxyethylene esters, and poloxamers or pluronics which are poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) tri-block copolymers. The polyol esters include glycol and glycerol esters and sorbitan derivatives. Polyoxyethylene esters majorly include polyethylene glycol (PEGs). The most commonly used nonionic surfactants are ethers of fatty Alcohol15. Amphoteric surfactants are very mild, making them particularly suited for use in personal care preparations over sensitive skins. They can be anionic, cationic or non-ionic in solution, depending on the acidity or pH of the water. Those surfactants may contain two charged groups of different sign. The frequently used compound is alkyl betaines15. The chemical structures of few commonly used surfactants are shown in Fig.3
2. Applicability of Surfactants
In recent years, surfactants have been widely applied, such as wetting agents, enhance oil recovery (EOR), emulsifiers and/or manufacturing textiles and leather finishing agents to reduce surface tension or speed the drying process7. Due to their interesting properties such as lower toxicity, higher degree of biodegradability, higher foaming capacity and optimal activity at extreme conditions of temperatures, pH levels and salinity, these have been increasingly attracting the attention of the scientific and industrial community. The world production of soaps, detergents and other surfactants was about 18 Mt (million tons) in 1970, 25 Mt in 1990 and 40 Mt in 2000 (not counting polymeric surfactants)7, 8. Fig.4 represents the advancement in utilization of surfactants in various industries like personal care, pharmaceutical, food, paint, agriculture, paper, mineral processing, electrical, etc. Versatile surfactants applied in industries in general as follows;
• Foods – emulsions, foams, dispersions, fouling etc.
• Pharmaceuticals – emulsions, dispersions, fouling, etc.
• Household products – dirt removal, foam control etc.
• Paints & coatings – cleanliness, wetting, adhesion, wicking, dispersion stability
• Mineral Processing – froth flotation
• Semi-conductors – cleanliness, adhesion of thin layers, characterization of surface treatments
• Heavy industry – lubrication/wear, degreasing
• Crude oil – oil recovery
• Paper – printing, adhesion in packaging etc.
• Biomaterials– fouling, tissue adhesion
Fig.4: Important surfactant-based products in the current market.
The widespread importance of surfactants in practical applications, and scientific interest in their nature and properties, have precipitated a wealth of published literature on the subject and many ways these materials are exploited by research community through quality papers in various journals(Fig.5).
Fig.5 : Reputed core research journals of Surfactant Science
3. Surfactant in Food Industries
Due to their unique chemical structure, surfactants strongly affect the stability of colloid systems and can interact with all the main components of flour (starch, gluten and lipids). Surfactants act as lubricants, emulsify oil or fat in butters, build structure, aerate, improve certain qualities of the final product, extend shelf life, modify crystallization, prevent sticking, and retain moisture16.
Naturally occurring surfactants such a lecithin from egg yolk and various proteins from milk are used for the preparation of many food products such as mayonnaise, salad creams, dressings, deserts, etc. Alanine, phenylalanine, leucine and isoleucine contain nonpolar aliphatic and aromatic side chains. Amino acids, such as arginine, lysine and tryptophane, contain amino groups, which promote cationic character to the protein. Aspartic and glutamic acids possess side chains with carboxyl groups, which contribute to anionic character. The nature, number and location of the polar amino acids determine the isoelectric point of a protein; e.g., the pH at which the protein is uncharged. In food systems where the pH is above the isoelectric point, the protein will behave as the anionic emulsifiers, while at pH values below their isoelectric point, it will become cationic. One complicating factor in using emulsifiers is that their charge makes them vulnerable to interactions with other charged species, such as calcium ions and some gums16. Later, polar lipids as monoglycerides were introduced as food emulsifiers. More recently, synthetic surfactants such sorbitan esters (Tweens) and their ethoxylates and sucrose esters have been widely applied in food emulsions.
In commercial food emulsifiers, in general, the hydrophilic part can consist of glycerol, sorbitol, sucrose, propylene glycol or polyglycerol. The lipophilic part is formed by fatty acids derived from fats and oils such as soybean oil, rapeseed oil, coconut oil and palm kernel oils.
3.1 Emulsions in Foods
The understanding of the formation, structures, and properties of emulsions is essential to the creation and stabilization of various structures in foods. Three main type of emulsions(shown in Fig.6) organized in foods are as follows;
i) Oil-in-Water (O/W) emulsions: Droplets of oil are suspended in an aqueous continuous phase. Such emulsions exist in many forms of food like creamers, cream liqueur, whippable toppings, ice creams mixes, mayonnaises. The properties of such emulsions are controlled through the surfactants utilized and the other components present in water phase.
ii) Water-in-Oil (W/O) emulsions : Droplets of water are suspended in an oily continuous phase. Mainly these emulsions exist in butter, margarines, and fat-based spreads. The stability of these emulsions depends more mainly on the properties of fat or oil, dispersed phase and also surfactant used in water phase.
iii) Water-in-Oil-in-Water (W/O/W) emulsions : In effect, an o/w emulsions whose droplets themselves contain water droplets. This type of emulsion often found in variety of baked products.
Fig.6 : Type of emulsions in Foods
A large number of surfactants traditionally used in foods which are water soluble as well as water insoluble. Surfactants, which have a Krafft point beneath room temperature, are classified as water insoluble as a contrast to ionic surfactants like SDS, which are classified as water soluble, because they form transparent aqueous solutions with large concentrations. Due to Micellization of Surfactant, aqueous solutions with high surfactant concentrations are transparent low viscosity liquids, which would indicate very significant solubility in water. However, in order to understand emulsion stability it is essential to realize that the surfactant molecules are not at all soluble to this extent. Generally surfactants/emulsifiers can be characterized by the Hydrophilic Lipophilic Balance. The balance is measured on molecular weight and is an indication of the solubility of the emulsifier. The HLB scale varies between 0 and 20. An emulsifier with a low HLB value is more soluble in oil and promotes water-in-oil emulsions. An emulsifier with a high HLB value is more soluble in water and promotes oil-in-water emulsions. The HLB value is a somewhat theoretical value, it only considers water and oil, and food systems are more complicated. But the HLB value of an emulsifier can be used as an indication about its possible use. The second group of surfactants, the ‘‘insoluble’’ ones, differs from the first group only by the structure of the association. This difference is in reality important to comprehend the stability of food emulsion17. The emulsion stabilization is in the phenomena at the critical surfactant concentration, when the self-aggregation in water is started (Fig.7). For the water soluble surfactant the association is limited to spherical aggregates, micelles, which form a thermodynamically stable dispersion in water, the system remains a one phase transparent liquid. In case of water insoluble surfactant the association structure is a lamellar liquid crystal (shown in Fig.7) which does not have size limitation like the micelle, the association continues infinitely and distinct a separate phase appears. For the water soluble surfactants the adsorption of the surfactant to the interface increases with concentration in the aqueous solution until the CMC is reached, at which the surfactant has formed a mono-layer at the interface. After that point the additional surfactant forms micelles, in the bulk.
Fig.7 : Surface tension of aqueous solutions of surfactants
The water insoluble surfactants behave in a completely different manner. It forms a separate phase and the adsorption to the oil/water interface is now not a question of individual molecules; the adsorption is mainly adjudged by the three interfacial free energies with four possible dispersed structures(Fig.8).
Fig.8 : Arrangements for an emulsion with LC as 3rd phase
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