SOP: Coliform Count and Escherichia coli Test in Fresh Vegetables

GIFSQ — Global Institute for Food Safety & Quality

Principle

Vegetables, particularly leafy vegetables grown in polluted streams or sewage farms, can carry faecal contamination. Washing the vegetable in Ringer solution recovers the surface organisms, a presumptive coliform count is done as in the examination of water, and positive tubes are tested for Escherichia coli. Good quality vegetables show less than 500 E. coli per 100 g.

Apparatus required

• Sterile container for shaking
• Sterile pipettes
• Culture tubes

Reagents

a. Ringer solution — for washing the vegetable and recovering the organisms. (Preparation is given in SOP: Preparation of Culture Media for Spoilage Examination.)

Sample preparation

Take 100 g of the vegetable

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Shake the 100 g of vegetable with 200 ml of Ringer solution

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Allow it to stand for 20 min and decant — use the decanted liquid for the counts

Procedure

Do the presumptive coliform count as in the examination of water

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Test the positive tubes for the presence of E. coli

Calculation

Report the results as the number of organisms per 100 g of vegetable.

Interpretation

• Good quality vegetables should show less than 500 E. coli per 100 g.

Precautions

• Leafy vegetables that may have been grown in polluted streams or sewage farms should always be tested for E. coli.
• Keep the standing time at 20 min — longer standing changes the recovery.

References

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