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HYGIENE IN MANUFACTURING PLANT AND EFFECT ON EMULSIONS 399 piece of machinery is to be left idle for even a few hours, as thin films of product may trap and protect organisms beneath them, and diluted pools of product will allow infected foci to develop. Hot detergent solutions are satisfactory for most emulsion plants, but, particularly in plants used to make w/o emulsions, it is usually necessary to couple circulation of the solution with mechanical scrubbing and the dismantling of all inaccessible joints. This inevitably calls attention to plant design, and the necessity for smooth surfaces to working parts, the absence of "blind ends," and the need for easily opened joints cannot be stressed too strongly. If these conditions cannot be met in older machinery, it is often better to leave equipment unwashed before idle time, assuming the last made product contained an effective preservative, than to wash through and risk dilution of the product and the ensuing dangers of growth in stagnation areas. Figures 7 and 2 show examples of parts of plant which are difficult to dismantle and clean, while Fig. $ illustrates equipment of better design. PLANT STERILIZATION Cleaning and the removal of product residues should always be followed by steriliza.tion, especially if the plant is not to be used again immediately. The fact that a single bacterium can give rise in five hours to over 1,000,000 like itself shows the risks of delaying sterilization after washing, which in itself can never remove all micro-organisms. Sterilization by heat is the most effective means of ensuring destruction of all micro-organisms. Vegetative bacteria and fungi are destroyed by boiling for 5 to 10 minutes, but the only certain methods of destroying bacterial and fungal spores are by autoclaving at 120øC for 20 min or by holding at dry heat of 170øC for 2 hours. These procedures are, of course, not practicable for large manufacturing plants, and pasteurization which involves holding at 65øC for half-an-hour is not recommended since this, although effective against some pathogenic bacteria, does not affect heat resistant bacteria or spores. Chemical sterilization is an alternative to heat treatment, and is effective in most instances but, because spores are not always destroyed, must be preceded by thorough washing to remove mechanically as much con- tamination as possible. The use of strong concentrations of chemical disinfectants at very high temperatures is a safeguard if contamination from spores is suspected, but these measures are often difficult to carry out. Davis (6) has reviewed the application of chemical sterilization of