SOP: Test for Sterility of Concentrated Orange Juice

GIFSQ — Global Institute for Food Safety & Quality

Principle

The method is based on adding diluted concentrate to a sterile dextrose medium and examining for the growth of yeasts. Yeast, if present, appears as small colonies; the count per plate approximates the number of yeasts per millilitre of the concentrate.

Apparatus required

• Sterile 20-ml pipettes
• Sterile Petri dishes
• Conical flask
• Incubator (25–30 °C)

Medium

Potato dextrose agar (preparation as given in the book’s microbiology section, page 721 — the preparation recipe is not repeated here; follow the printed book for the medium formulation).

Procedure

When sampling from a can, roll the can on its sides and invert it end over end several times to mix the contents

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Open the can under sterile conditions; if sampled from casks, take the precautions described under sampling (page 683), take several samples, mix them together, and use the mixed sample for the examination

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Withdraw a sample each from the top, middle and bottom of the can with a sterile 20-ml pipette and discharge into a conical flask containing 60 ml of sterile 20% dextrose solution

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Mix, withdraw 2 ml, and discharge into a sterile Petri dish

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Melt a tube of potato dextrose agar, cool to 45 °C, pour into the Petri dish, mix, and let it set

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Incubate the inoculated plates at 25–30 °C for 2–4 days

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Yeast, if present, appears as small colonies; the number present on each plate gives approximately the number present in 1 ml of the concentrate

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If no colonies appear after 4 days, the concentrate contains no yeast

Precautions

• Sample from the top, middle and bottom of the can — yeasts may not be evenly distributed.
• Prepare the potato dextrose agar exactly as in the book’s microbiology section (page 721) — the medium formulation is part of the method.

References

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

Dickinson, D. and Goose, P., Laboratory Inspection of Canned and Bottled Foods, Blackie & Son Limited, London, 1955, p. 78 — as cited in the book.