SOP: Rinse Method for Bins and Large Utensils
GIFSQ — Global Institute for Food Safety & Quality
Principle
For bins and other large utensils that cannot be swabbed practically, the whole inner surface is washed with a known volume of sterile Ringer solution; the rinse then carries the surface flora, and plate counts made from it give the total count per container.
Apparatus required
- Screw-capped jar
- Sterile pipettes
- Petri dishes
- Incubator (37°C and 22°C)
Reagents
a. Sterile Ringer solution — To 1,000 ml of distilled water add 2.15 g of NaCl (AR), 0.075 g of potassium chloride (AR), 0.12 g of anhydrous calcium chloride (AR) and 0.5 g of sodium thiosulphate. Autoclave for 20 min at 15 psig. Use the solution as diluent at one-quarter of its original strength; it is ionically balanced.
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
None beyond sterilizing the Ringer solution as above.
Procedure — Rinse examination of bins and large utensils
Add 500 ml of sterile Ringer solution to the vessel
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Rotate the vessel to wash the whole of the inner surface
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Transfer the rinse into a screw-capped jar
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Make counts with 1 ml and 0.1 ml amounts of the rinse, in duplicate
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Incubate one pair of plates at 37°C and the other at 22°C for 24 hr
Calculation
Count per container = (Mean of the 37°C and 22°C counts per ml) × 500
Where,
- Mean of the 37°C and 22°C counts per ml = average of the colony counts per millilitre of rinse at the two incubation temperatures
- 500 = millilitres of Ringer solution used to rinse the vessel
Precautions
- The Ringer solution must be sterile before use.
- Rotate the vessel so that the whole of the inner surface is washed.
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).