HYGIENIC MANUFACTURE AND PRESERVATION 791 that the correct conditions of steam saturation are rapidly achieved where such evacuation cannot be arranged, thorough venting is essential for the first few minutes during which steam is being admitted to the equipment. Some models are now fitted with an automatic air purge which allows entrapped air to be displaced during the run-up and subsequently. Com- ponents of glass or metal being steam-sterilized may be wrapped in Kraft paper, so that they remain sterile after removal from the autoclave, until they are needed for use. Heat-stable, water-insoluble powders may be sterilized effectively by autoclaving, preferably with subsequent evacuation of the sterilizer to avoid a high residual moisture-content in the powder. Contaminated glassware and discarded laboratory cultures should be sterilized by autoclaving at 138 kN m-2 (126 ø) for 20 min. 2.3 Ultraviolet irradiation The most effective bactericidal range of ultraviolet radiation lies between 280 nm and 240 nm. Whilst a radiation intensity of the order of 5 mW cm-2 may achieve a virtually 100% kill of micro-organisms in a few seconds, the exact requirements for a practical sterilization procedure always need to be established empirically. Specialist equipment suppliers should be consulted for apparatus of suitable design for the purpose envisaged they will take into account, for example, whether an air-gap has to be irradiated above a conveyor belt or whether the irradiation can be arranged in the form of a continuous pipe-line system for clear liquids. UV transmission is probably the most important variable factor involved for example, an arrangement capable of giving excellent results when newly-installed, may soon fail to achieve sterilization if the surfaces are not kept thoroughly clean. Even slight turbidity will markedly reduce sterilizing efficiency. UV irradiation is seldom used as the sole and final means of achieving sterilization, but it may be a useful adjunct to other procedures. However, monitoring by periodic microbiological tests is necessary to ensure that sterilizing efficiency is being maintained. 2.4 Ethylene oxide treatment Ethylene oxide is highly toxic by inhalation at levels undetectable by smell, and is explosive unless mixed with carbon dioxide or nitrogen. It is, therefore, only suitable as a sterilant under strictly controlled operating conditions. A further complication is that ethylene oxide is a skin irritant

792 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS and, since it is soluble in rubber, may produce eruptions even when rubber gloves are worn. Concentrations of ethylene oxide used in practice have been given as 500 mg 1-1 of air, or preferably nitrogen, for 4 h at room temper- ature and also 10-15% in air or nitrogen overnight. Special equipment is not essential, although chambers suitable for preliminary evacuation are sometimes employed. 2.5 Filtration Numerous methods of filtration are available for sterilizing various types of liquids and for air sterilization. For large-scale treatment of water and aqueous solutions, it is usual to employ either diatomite filter candles in a specially-designed unit or special grades of filtering mats in a con- ventional filter-press. Filters intended to remove all micro-organisms from a liquid or vapour might appear to need a maximum pore-size below about 0.2 gm. In practice, adsorptive filtering materials do not rely solely on mechanical trapping of micro-organisms and will usually operate satis- factorily with a maximum pore-size of 1-2 gm. It is important to ensure that the correct sterilizing grade of filtering agent is used (not the coarser clarifying grades) and to see that the filter is properly installed without accidental damage or channelling, e.g. around the edges of the mats. Excess pressure intended to improve flow rates during use will often damage a sterilizing filter unit and allow micro-organisms to pass careless cleaning efforts may have a similar effect. Filters for sterilizing applications must be sterilized and operated within sterile equipment in particular, pipe-lines and vessels taking the filtered material must obviously be presterilized with the aid of saturated steam or chemical sterilants, or the product will rapidly become contaminated again. Sterilization by filtration is one of the most suitable and economical ways of handling large volumes of mobile liquids, such as shampoos, but it calls for considerable skill and experience to ensure routinely good results. Frequent microbiological tests are necessary to give assurance that steril- ization is being achieved and that filters have not been damaged. 2.6 Chemical sterilization References are made in this monograph to sterilization by means of chlorine (e.g. solutions of hypochlorite yielding 200-250 ppm of available chlorine) and formalin (e.g. 0.5% equivalent to 0.2% of formaldehyde). Used in accordance with the operating conditions prescribed, especially with respect to preliminary cleansing, these chemical sterilants will prove suit-