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Figure 4. Schematic diagrams showing: (a) structure of surfactant molecule, (b) surfactant improving the wettability of water, and (c) surfactant removing fat from a surface and dispersing it in suspension.

other nonpolar compounds such as fats and oils. This aids cleaning in two ways: it reduces water surface tension and removes and suspends fats.

Surface tension is reduced as the polar head enters the water and breaks down hydrogen bonding. If a surfactant is added to a drop of water on a surface, the surface tension of the water is reduced and the drop collapses to wet the surface (Fig. 4). Increased wettability allows greater water penetration into soils and hence better cleaning action. Fats and oils are removed from surfaces as the polar head of the surfactant molecule dissolves in the water, whereas the hydrophobic end dissolves in the fat. Due to the forces acting on the fat-water interface, the fat particle tends to form a sphere, as this has the lowest surface area for a given volume. In so doing, the fat particle will roll up, become detached from the surface, and remain in suspension (Fig. 4).

Inorganic Alkalies. Alkalies are useful cleaning agents because they are cheap, break down proteins, saponify fats, and may be bactericidal. A typical saponification reaction is shown below in which a nonpolar R group is changed to a soluble form that has good surfactant properties.

CH2COO—R CH2OH

CH2COO—R CH2OH

Triglyceride Sodium Glycerol Soap fat hydroxide

Generally, the stronger the alkali the greater the degree of saponification, although this is a compromise, as corro-siveness also increases with alkali strength. Alkaline compounds can also precipitate scum, a reaction of the soap with water hardness ions (Ca2 + and Mg2+), and may have poor rinsability. For some applications, alkaline detergents are chlorinated, as at high pH chlorine increases peptizing of proteins and may reduce mineral deposition. Chlorine at high pH, however, is also very corrosive.

Inorganic and Organic Acids. Acids are not generally used in the food industry as they have little dissolving power for fats, oils, and proteins. They are useful, however, in making soluble mineral scales such as hard-water deposits, beer stone, and milk stone. A typical reaction would be as follows:

CaC03 + 2HC1 CaCl2 + H20 + C02 Insoluble Soluble

As with alkalies, the stronger the acid the more corrosive it is. Strong acids (mineral acids) include hydrochloric, nitric, and phosphoric acid, whereas weaker organic acids typically used include citric, lactic, and acetic acid.

Sequestering Agents. Sequestering agents are employed to sequester or chelate mineral salts by forming soluble complexes with them. Their primary use is in the control of water hardness ions although they are also useful in maintenance of alkaline conditions by buffering, emulsifi-cation of oils, and fats and increasing rinsability. Sequestrants can be both organic and inorganic, organic sequestrants being usually based on polyphosphates and inorganic chelating agents commonly being the potassium or sodium salts of ethylene diamine tetraacetic acid (EDTA).

A general-purpose food detergent may contain a strong alkali to saponify fats, weaker alkali builders to aid corrosion resistance, surfactants to improve wetting, dispersion, and rinsability and sequestrants to control hard water ions. Ideally, the detergent should also be safe, nontainting, stable, noncorrosive, biodegradable, and cheap. The detergent chosen for a particular application will depend on the soil to be removed; the solubility characteristics and cleaning procedure recommended for a range of food soils is shown in Table 1.

Table 1. Solubility Characteristics and Cleaning Procedures Recommended for a Range of Solid Types

Food (or soil)

Solubility characteristics

Cleaning procedure recommended

Sugars, organic acids, salt

Water soluble

Mildly alkaline detergent

High-protein foods (meat, poultry, fish)

Water soluble

Chlorinated alkaline detergent

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