HYGIENIC MANUFACTURE AND PRESERVATION 733 aid of microbiological tests, to ensure that their hygienic objective is duly achieved. Although aseptic technique is seldom necessary for cosmetic manu- facture, one of the most important criteria is to avoid a cumulative increase of contamination in the plant from day to day. Detailed consideration is therefore given in the following sections to plant cleaning and sterilization. 3.7 Plant cleaning Continuous production rarely takes place in any piece of equipment and interruptions in the use of machinery may vary from hours to weeks the plant may, perhaps, be cleaned after production of the last batch of one product or just before the manufacture of the next. The idle time of the equipment is of great importance from the microbiological point of view, whether it occurs before or after the cleaning process. Undisturbed residues in damp, dark and warm regions of a plant are dangerous because the conditions are so admirably suited to the multiplication of bacteria and fungi. In pumps, joints, pipelines, pressure gauges and valves, growth is quickly initiated where small amounts of liquid are left undisturbed for a few hours. Complete and thorough removal of product residues is essential for the plant to remain in a hygienic condition. Films of product trap and protect micro-organisms and diluted material often allows the rapid development of massive numbers of contaminants. Hot detergent solutions should be used for cleaning, coupled with circulation of the solution, scrubbing and the dismantling of all valves, taps and joints. Plant should be washed, sterilized and rinsed immediately before use and if operations are to be interrupted at some stage during this process, the safest point is after chemical sterilization and before rinsing. Leaving the plant idle after detergent washing or final rinsing is potentially hazard- ous from the microbiological point of view unless the machinery is thorough- ly dried inside and out. 3.8 Plant sterilization Cleaning and removal of product residues should be followed by sterilization, especially if the plant is not to be used immediately. Steri- lization by heat is the most effective means of ensuring the destruction of all micro-organisms, but is not always convenient for manufacturing plants of moderate or large size unless special provision has been made in the

734 JOURNAL OF TIlE SOCIETY OF COSMETIC CHEMISTS original plant design. Pasteurization is not recommended since it is in- effective against heat-resistant bacteria or spores. Chemical sterilization is a possible alternative to heat treatment and is usually effective when it follows thorough washing to remove as much "soil" as possible this is important because spores are not always des- troyed by chemical disinfection and they may germinate in the residues. If contamination with spores is suspected, it is necessary to use strong con- centrations of chemical disinfectants maintained at elevated temperature. Chlorine is the best plant sterilizing agent in most circumstances. Cold solutions of hypochlorite yielding 200 to 250 ppm of available chlorine will sterilize glass and metal surfaces after $ min if all organic matter has been removed previously. Formaldehyde or formalin is a convenient sterilizing agent for use after plant washing but, like chlorine, its activity is greatly reduced by organic matter. Cold solutions should be used at a concentration of about 0.5% formalin (0.2% formaldehyde). This concentration will sterilize surfaces which are free from organic matter in 10 min. Cold solutions are recom- mended for both of these sterilants owing to the unpleasant or toxic effects that may result from inhalation of their fumes. Hot solutions may, however, be employed in totally-enclosed plant if proper precautions are taken. Quaternary ammonium compounds, although weight for weight less effective than chlorine and formaldehyde, have advantages such as being almost odourless, relatively non-toxic to man, and less corrosive than chlorine. They do not have a wide spectrum of antibacterial activity, being more active against gram-positive than against gram-negative bacteria they are also rendered less effective by organic matter and anionics. Since they are quite efficient surface-active agents, they can in some cases be used to clean the plant as well as to disinfect, although their detergent action would be inadequate for the removal of residues from w/o emulsions. A concentration of 0.5% of benzalkonium chloride at 60 ø will sterilize "soil"-free surfaces in less than 10 min. Residues of disinfectant solutions must be removed from the plant after sterilization in order to avoid re-contaminating the machinery, this should be done by irrigation with sterile water. The risk of recontaminating sterile machinery is high if cold water from a distilled or demineralized water storage tank is used. It is necessary, therefore, to use freshly boiled or autoclaved water, particularly if the plant is to be left idle before the next manufacturing schedule begins. The detailed advice given in the foregoing sections on plant cleaning and