Disinfection

The cleaning portion of the sanitation cycle has been shown to reduce bacterial numbers on surfaces by up to 99.9% or three log orders (25). Work in our own laboratories has shown that with detergent soaks and pressure washing, up to 4-5 log orders may be removed. However, given that bacterial numbers on surfaces could be between 107 and 1010 organisms/cm (2,25), viable bacteria are likely to be present on surfaces after cleaning. The aim of disinfection procedures is to remove or reduce the viability of these remaining microorganisms.

If possible, temperature is used as a disinfectant as it penetrates into surfaces, is noncorrosive, is nonselective to microbial types, and is easily measured and leaves no residue (20). Whereas high temperatures are often used as disinfectants in CIP systems, their use on open surfaces is usually uneconomic, hazardous, or impossible. In such cases, chemical biocides are employed and, as with cleaning chemicals, no single disinfectant satisfies all the performance requirements. Biocides are rarely mixed, however, so a choice has to be made from a limited number of disinfectant types. Universally used biocides include chlorine releasing components, quaternary ammonium compounds, iodine compounds, amphoterics, and peracetic acid.

Chlorine is the most widespread and cheapest disinfectant used in the food industry and is available in fast-acting (chlorine gas, hypochlorites) or slow-releasing forms (e.g., chloramines, dichlorodimethylhydantoin). Quaternary ammonium compounds (QUATS or QAC's) are based on ammonium salts with substituted hydrogen atoms and a chlorine or bromine anion, whereas iodophores are soluble complexes between elemental iodine (active ingredient) and nonionic surface active agents. Amphoterics are based on the amino acid glycine, often incorporating an imidazole group. Peracetic acid may be used by itself or formulated with hydrogen peroxide. A range of characteristics for examples of these disinfectant types is shown in

Table 2. Other disinfectants used to a limited extent include biguanides, formaldehyde, ozone, chlorine dioxide, and bromine; however, biocides successfully used in other industries, eg, phenolics or metal ion-based products, are not used for food applications due to safety or taint problems. Frank and Chmielewski (26) confirmed the effectiveness of quaternary ammonium compounds and chlorine in reducing S. aureus 1,000-fold on stainless steel and domestic food preparation surfaces.

Disinfectant concentration and contact time are important considerations in the reduction of microbial viability. The relationship between death and concentration is not linear but follows a sigmoidal curve dependent on the resistance of organisms within the population. Disinfectants do not, therefore, necessarily kill all microorganisms in a population, and increasing concentration may not enhance this effect. For disinfectants to be effective, they must find, bind to, and transverse microbial cell envelopes before they reach their target site (27). Chemical disinfectants are also effective in preventing cross-contamination from food contact surfaces (28). Sufficient contact time is therefore critical to give good results. Amphoterics and QUATS may be left on surfaces for extended periods between production runs without rinsing, as they are FDA approved as indirect food additives at low concentration.

The performance of the cleaning procedure may influence disinfection efficiency. Any soil or cleaning chemical residues remaining may protect microorganisms from disinfectant penetration or may react with the disinfectant and destroy its antimicrobial abilities. Biocides are best used within their specified pH range, although performance can generally be increased by increasing the temperature. A study by Best et al. (29) examined the efficacy of 14 disinfectants against Listeria spp. Their effectiveness varied depending on whether the test organism was dried on a steel surface, in suspension, or in the presence of organic material. Consequently, it is important to select the appropriate disinfectant for the particular contaminated surface, especially if organic material is present.

Table 2. Characteristics of Some Universal Disinfectants

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