FUNDAMENTALS OF MICROBIOLOGY IN RELATION TO CLEANSING 53 in a water tank. The water had been assumed to be pure because it came from the mains. Type of product and potential contamination Each type of product is liable to its own specific kind of contamination and microbiological growth according to (i) chemical nature (protein, fat, carbohydrate and mixtures of these) (ii) pH value (iii) oxygen tension (iv) surface tension (v) other biocidal factors operating (vi) presence of biocidal substances. In practice one organism (e.g. Pseudomonas, yeast, mould) may so establish itself that it 'gets away' and the material becomes virtually a pure culture. If this organism is dangerous or affects the life of the product, disaster is inevitable. PRACTICAL ASPECTS: PROCEDURES IN THE FACTORY Control of raw materials All materials used in cosmetics should be checked for quality visually, chemically and where appropriate microbiologically. Specifications to buyers should include microbiological standards where necessary. It may be possible to blend, improve or 'top up' the chemical or functional quality of a crude ingredient, but it is often not possible or practicable to improve the microbiological quality without damaging the product. A defect of this nature can persist right through the processing and packaging to retail sale and use. Formulation and preservation In general it is always better to prevent microbiological growth by formulation rather than by relying on preservatives. These, like anti- oxidants, are rarely completely satisfactory for a prolonged period, es- pecially with warm ambient temperatures, whereas control by formulation {i.e. by physical and chemical means) lasts indefinitely. No organism can grow without nutrients but almost anything organic can act as a nutrient for some organisms. Physical factors are highly specific for particular types of organism, e.g. acidity for most bacteria but not for yeasts or moulds,

54 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS absence of oxygen for obligate aerobes, aerobic conditions for anaerobes. A useful measure of control can be exercised by designing formulation, pro- cessing and cleansing methods with particular reference to the type of infection to which -the product is most vulnerable. Temperature control Temperature is in practice the most important factor controlling the growth of bacteria, and so the safety and keeping quality of the product, assuming satisfactory hygiene. Two of the biggest mistakes made in factory practice are to assume that dial thermometers and recorders are always accurate, and that calculations made in respect of heat transfer under ordinary conditions also apply in hot weather. Both these fallacious assumptions have led to major catastrophes in more than one industry. All working (dial) thermometers should be checked against a known accurate thermometer in the laboratory. Mercury in glass thermometers should never be used in the factory. All cooling systems should be calculated allowing for an atmospheric temperature of 27øC and a mains water temperature of gOøC, or alterna- tively provision made for additional cooling capacity in hot weather. Design of equipment In the food industries it took a whole generation to convince engineers that micro-organisms existed and could spoil a product. Early types of equipment were often a paradise for bacteria with their multiplicity of dead ends, crevices, unhygienic joints, indiscriminate use of absorbent materials, etc. and the impossibility of cleaning and sterilizing them. In early educational work we laid down as a basic principle that all equipment for materials of biological perishability had to be dismantled daily and each item individually cleaned before sterilization. There was even equipment on the market for this purpose which could not be dis- mantled. Fortunately engineers have now received the message and equipment in this field today is practically all well designed and hygienically con- structed. Sterilization of equipment by heat Heat is usually applied as hot water, steam at atmospheric pressure