SOP: Examination of Sink Waters
GIFSQ — Global Institute for Food Safety & Quality
Principle
Washing and rinse waters pick up contamination as work goes on. Samples taken when washing starts and at intervals while it continues show when the water turns unhygienic and must be changed. Where hypochlorites are used for washing, the sample bottle is charged with sodium thiosulphate.
Apparatus required
- Water sample bottle containing sodium thiosulphate (for use where hypochlorites are used in washing)
- Sterile pipettes
- Petri dishes
- Test tubes with Durham’s tubes
Reagents
a. MacConkey’s single strength broth — To 1,000 ml of water add 5 g of sodium tauroglycocholate, 20 g of peptone and 5 g of NaCl, and dissolve by steaming. Dissolve 20 g of lactose in the mixture, bring the pH to 7.0–7.5 and filter if needed. Add about 10 ml of 1% aqueous neutral red to give a satisfactory red colour. Dispense 5 ml into each test tube containing a Durham’s tube and autoclave at 5 psig for 10 min. Do not overheat after the lactose goes in, or the sugar hydrolyses. (In place of neutral red, some workers use bromocresol purple at 1.5% — add enough of the dye solution to reach 0.04%.)
b. Yeast extract agar (routine work) — The chapter’s ingredient table is partly illegible in the available copy: the legible routine-work figures are peptone 5 g, agar 10 g and water 1,000 ml. Heat and dissolve the ingredients, adjust the pH to 7.3 and filter if necessary. Bottle or tube the medium and autoclave at 10 psig for 10 min. Check the complete recipe against the printed book.
Sample preparation
Take 100 ml of washing or rinse water by immersing a water sample bottle (containing sodium thiosulphate in case hypochlorites are used in washing) into the sink and allowing it to fill. Stopper and cool under the tap.
Procedure — Examination of sink waters
Immerse the sample bottle in the sink and allow it to fill with 100 ml of washing or rinse water
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Stopper the bottle and cool it under the tap
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Carry out plate counts and coliform counts on the sample
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Test at the beginning of washing and at intervals during the washing
Calculation
No calculation is required — report the plate and coliform counts obtained at the beginning of washing and at each interval, to judge when the water becomes unhygienic.
Precautions
- Use a sample bottle containing sodium thiosulphate where hypochlorites are used in washing, so that residual chlorine does not continue to act in the sample.
- Sample both at the beginning of washing and at intervals during it.
References
- De Man, J.C., Rogosa & Sharpe, J. Appl. Bact. 23, 130 (1960).
- Knox, R. & J. Walker, J. Hyg. Camb., 45, 187 (1947).
- (Author name illegible in the available copy — printed as “Gos) C.H.”), Microbiological Methods, 2nd edn., Butterworth & Co. Ltd., London, p. 347 (year printed as 1997).