Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell

href="#ulink_b0367966-85dd-5838-a753-045c4a84f527">Table 1.26). The basic components of many formulations are water and other solvents (such as alcohols); although many biocides and other excipients are soluble (ionic or hydrophilic) in water, many are insoluble (hydrophobic). Biocides are therefore often mixed with emulsifiers, including soaps and detergents, to increase their solubility or dispersion in a formulation. Emulsifiers form micelles, which are aggregated units of surface-active molecules (Fig. 1.26). Soaps are water-soluble salts or fatty acids, which are made by reactions of fats and/or oils with an alkali (e.g., sodium hydroxide). Detergents are mixtures of surfactants, which are defined as surface-active agents. Surfactant molecules act in low concentrations to change the properties of a liquid at its surface or interface with a surface. This increases the wettability of the liquid by breaking its surface tension, the force that holds the surface molecules together, and allowing it to spread over a surface better. Surfactants are further defined as nonionic (no charge), anionic (negatively charged), cationic (positively charged), and amphotheric (positively and negatively charged), and many possess antimicrobial activity (see section 3.16). Surfactants form micelles (Fig. 1.26), which are useful for the solubilization, dispersion, or emulsification of incompatible materials, as well as aiding in cleaning processes by the removal and dispersion of hydrophobic soils from a surface.

TABLE 1.26 Various constituents of formulated biocidal products

Ingredient	Purpose	Examples
Biocide	Antimicrobial or preservative activity	QACs, phenolics, biguanides
Solvent	A solvent is a substance (usually liquid) that is capable of dissolving other substances, with water being the most common. Solvents are used for dissolution and dilution of the biocide and other ingredients.	Deionized water, isopropanol, propylene glycol, urea
Emulsifiers, surfactants	Emulsifers are ingredients (including surfactants) that allow the formation of stable mixtures (emulsions) of water- and oil-soluble ingredients. Surfactants (“surface-acting agents”) can be used as emulsifiers. but also to reduce surface tension, improve wettability of a surface, disperse contaminants, and inhibit foam formation.	Sodium lauryl sulfate, potassium laurate, lecithin, nonionic and other surfactants (see section 3.16)
Thickeners	A substance used to increase the viscosity of a formulation	Polyethylene glycol; polysaccharides, like pectin, gums, and alginates
Chelating agents or sequestrants	Binding metals (like calcium and magnesium) and inhibiting their precipitation; water softening and prevention of mineral deposition	Ethylenediamine, EDTA, EGTA
Alkali or acid	pH stabilization. Alkalis are used as “builders” to optimize the activities of surfactants, including emulsification of soils. Acids are also used to prevent mineral deposition and for water softening.	Alkalis (NaOH, KOH, silicates); acids (acetic acid, citric acid, phosphoric acid)
Buffer	Maintaining pH over time and increasing alkalinity	Disodium phosphate
Corrosion inhibitors	Reducing the corrosion rates and protecting the surfaces of metals	Nitrates, phosphates, molybdates, ethanolamine
Others	Aesthetic qualities	Colors and fragrances

FIGURE 1.26 Basic structures of surfactants and soaps and micelles (a water-in-oil micelle is shown).

      Other formulation ingredients, including thickeners, buffers, chelating agents, and fragrances, allow further optimization of the stability, efficacy, and compatibility of the product. Due to the variety of ingredients that can be present in a given formulation, the preservative and/or antimicrobial activity can vary considerably. For this reason, users should pay close attention to the instructions (or labeling) provided with the product for its intended use, including the shelf life, application, dilution (if required), contact time for antimicrobial efficacy, spectrum of activity, safety, and compatibility.

      1.4.7 Process Effects

Just as the antimicrobial activity of a biocidal product can vary depending on various formulation effects, it is also affected by various process effects, including variables like temperature, humidity, pressure, time, and biocide concentration. This is particularly true in the development and optimization of processes. Important process variables in various disinfection and sterilization techniques are given in Table 1.27; they are discussed in further detail in chapters 2, 5, and 6.

The activity of a biocide is usually greater as the contact time, temperature, or concentration increases. The effect of contact time is often demonstrated by studying the loss of microbial viability over time, for example, D-value determinations (see section 1.4.2.1). Some biocidal applications are required to be rapid in action due to their practical use, as in the case of hand washing or surface disinfection. In contrast, preservative applications are only required to control microbial growth within a product over a longer exposure time or within a given shelf life. Temperature itself can be a reliable method of disinfection and sterilization, depending on the contact time and temperature for a given application (see section 2.2). In most cases, the activities of chemical biocides are also increased as the temperature increases; however, in the case of higher temperatures, increased degradation can also be observed, depending on the biocide type and the temperature conditions. Various materials or applications may also be restricted to low-temperature exposure conditions, for example, in the case of thermosensitive materials or due to safety concerns.

TABLE 1.27 Examples of process variables in various disinfection/sterilization techniques

Biocidal process	Variables
Steam (moist heat)	Temperature, time, pressure, quality of steam (including saturation), biocide penetration
Ethylene oxide	Formulation, temperature, humidity, biocide concentration, vacuum, time, biocide penetration
Liquid peracetic acid	Formulation, temperature, biocide concentration, time, biocide penetration