SOP: Cleaning of Glassware for Bacteriological Examination

GIFSQ — Global Institute for Food Safety & Quality

Principle

Residues of soap, alkali, disinfectant or culture material on glassware can inhibit or kill the bacteria being examined. Glassware used for bacteriological work must therefore be chemically clean and neutral: washing removes the dirt, and a phenol-red autoclave test checks that new glassware is not leaching free alkali into water.

Apparatus required

• Brushes (for scrubbing glassware)
• Tall glass cylinder (for soaking pipettes)
• Autoclave (for the alkali test)
• Bottles with caps or stoppers (for the alkali test)

Reagents

a. Hot water and soap — for the initial scrub; washing soda, sodium metasilicate, trisodium phosphate, or synthetic detergents may be used instead of soap.

b. 3% lysol, or other suitable disinfectant — for receiving containers with discarded cultures.

c. Ethyl alcohol containing 5% HCl, or cleaning solution — for the occasional extra wash.

d. 0.4% phenol red solution — for the alkali test.

e. Distilled water, pH 7.0 — for rinsing and for the alkali test.

Procedure — Washing

Scrub new glassware with hot water and soap, using a brush

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Rinse thoroughly, first in tap water and then in distilled water

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Place glass containers holding discarded cultures in 3% lysol or other suitable disinfectant

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Boil the containers in soap solution, brush them, and wash in running tap water

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From time to time, wash glassware in ethyl alcohol containing 5% HCl, or in cleaning solution

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Soak pipettes overnight in the cleaning solution in a tall glass cylinder; wash in tap water and give a final rinse in distilled water

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Keep the washed articles inverted and let them air dry

Procedure — Alkali test for new glassware

Fill the bottle nearly full with distilled water at pH 7.0

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Add enough 0.4% phenol red solution to give a yellow colour

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Autoclave for 30 min at 15 psi (120°C) and cool to room temperature

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Read the colour: yellow means the glassware is satisfactory; a pink or magenta colour means alkali is being liberated from the glass

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If alkali was liberated, repeat the test; if no alkali is liberated on the second autoclaving, the glassware may be used, otherwise discard it

Precautions

• Glassware for bacteriological examination must be neutral; new glassware, especially the cheaper varieties, gives off free alkali and must pass the phenol-red test before use.
• Keep washed articles inverted while air drying so dust does not settle inside.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.