FUNDAMENTALS OF MICROBIOLOGY IN RELATION TO CLEANSING 55 (e.g. in a steam chest) or under pressure (e.g. in an autoclave) usually at 15 lb/sq in. Hot air requires 160øC for 3 h or 170øC for 2 h to ensure sterility of equipment or powders in thin layers. Although heat,- especially. autoctavi-ng, is still the. preferred method in medical work, it is often inconvenient or impractical in industry for one or more of the following reasons: (i) it is expensive (ii) it causes deterioration of materials (e.g. plastics and delicate fabrics) (iii) it causes distortion in equipment (e.g. pipelines and gaskets) (iv) considerable time is taken to heat and to cool (v) residues may be baked on unless the equipment is thoroughly cleaned (vi) inefficient heating may result in the incubation of micro-organisms in inaccessible parts of the equipment. tn general, it may be asserted that heating a liquid for a few seconds at 75øC will destroy non-thermoduric vegetative cells, at 90øC all vegetative cells, and at 130-140 øC all cells including spores. When considering surfaces, even of such easily cleansed materials as glass and polished stainless steel, a more drastic treatment is necessary, mainly because of the possible presence of very thin invisible films of soiling matter which are often present although the utensil or equipment appears to be clean. Rubber and similar materials are extremely difficult to' clean and sterilize. Thus Ander- son, Sage and Spaulding (3) showed that it is necessary to hold contamin- ated rubber nipples in boiling water for 5 min to destroy C. albicans, although a few seconds at 100 øC is sufficient to destroy it in milk (4). Sterilization by chemicals Broadly speaking-there are four ways of chemically sterilizing equip- ment: (i) cleaning by a detergent (e.g. alkali) and then sterilization by a sterilant (e.g. hypochlorite) or a quaternary ammonium compound (QAC), (ii) cleaning by a stronger concentration of a detergent-sterilant followed by sterilization by a weaker concentration (e.g. iodophors) (iii) cleaning and sterilizing by a detergent-sterilant (e.g. QAC+alkali) followed by a 'sterile rinse' (e.g. QAC or hypochlorite).

56 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (iv) using a single substance which has powerful cleaning and sterihzing properties (e.g. sodium hydroxide or nitric acid) followed by a sterile rinse. Inter-relationship between detergents and sterilants In practice it is impossible to dissociate the action of a detergent from that of a sterilant. All detergents have some killing power in addition to their ability to remove most of the micro-organisms from a surface by cleaning which effects mechanical removal. Some sterilants exert a deter- gent action by chemically attacking a constituent of the soil, e.g. hypo- chlorites accelerate the degradation of proteins, and dilute acids can dissolve the calcium salts in heated milk deposits and hard water scale and so disintegrate a strongly adhering film. The action of a detergent is complex. The main aspects are chemical hydrolysis of fat and protein, wetting of the equipment surface, solution of certain constituents, and, when an oxidizing agent such as chlorine, iodine, nitric acid, percarbonate or hydrogen peroxide is present, destruction of substances by oxidation. The combined effect of these activities is to dis- integrate and loosen the soil so that it can be washed away. Bactericidal action of detergents Many detergents have marked germicidal properties although they are used primarily as detergents, and this may be the only claim made by manufacturers of proprietary products. Hot water at 60-80 øC will kill most or all vegetative cells but few spores. A detergent will always enhance the killing effect of heat. Probably the best example is sodium hydroxide. A treatment of 63øC for 30 min in water will kill all bacteria except thermo- duric ones and spores, but a 1-3% NaOH solution under these condi- tions will kill all thermoduric cells and a considerable proportion of spores. Detergents are almost invariably used hot and so act by enhancing the bactericidal effect of heat. This effect is especially valuable against spores in those industrial applications, e.g. bottle-washing followed by cold filling, where excessive temperatures have to be avoided. Detergent-sterilants are particularly useful where high temperatures cannot be used, as in manual dishwashing or because of delicacy of the material.