SOP: Microbiological Examination of Pickles

GIFSQ — Global Institute for Food Safety & Quality

Principle

Pickles are high acid foods with generally low counts (1,000/g), and yeasts — gas-producing or film-forming — are the usual spoilage agents. Total counts and acid-producer counts on dextrose-tryptone agar at two pH values, lactic acid bacteria on Rogosa medium, and yeast counts by chamber or plate culture together reveal the pickle’s microbial state; selective salt media pick up film-yeasts and obligate halophiles.

Apparatus required

• Petri dishes
• Culture tubes
• Counting chamber
• Microscope
• Incubator (32°C)

Reagents and media

All media preparations are given in SOP: Preparation of Culture Media for Spoilage Examination.

a. Dextrose-tryptone agar at pH 6.8 and at pH 4.5 — for total counts and acid-producing colony counts; the acid-producing colonies show a yellow halo from the colour change of the indicator.

b. Rogosa medium or other suitable medium — for counting lactic acid bacteria.

c. Erythrosin stain (1:5000) — for the counting-chamber yeast estimate.

d. Sabouraud-type medium — alternative medium for estimating yeasts.

e. Dextrose-tryptone, Rogosa, MRS or V.8 medium — for culturing. (Note: the book gives no preparation for V.8 medium.)

f. Liquid mycological medium containing 5% and 10% NaCl — for growing suspected film-yeasts.

g. Broth medium containing 15% sodium chloride — for obligate halophiles.

Procedure — Counts

Use dextrose-tryptone agar at pH 6.8 and at pH 4.5

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Do total counts, and also counts of acid-producing colonies — these have a yellow halo due to the colour change of the indicator

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Count lactic acid bacteria on Rogosa or other suitable medium

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Estimate yeasts either by the counting chamber method, staining with 1:5000 erythrosin, or on Sabouraud-type medium

Procedure — Culturing

Use dextrose-tryptone, Rogosa, MRS or V.8 medium for culturing

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Grow suspected film-yeasts in a liquid mycological medium containing 5% and 10% NaCl for 3 days at 32°C

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For obligate halophiles, use a broth medium containing 15% sodium chloride

Interpretation

• Pickles are high acid foods with generally low counts (1,000/g).
• Yeasts generally cause spoilage and may be either gas-producing or film-forming type — in the former case enough gas may be generated to burst the container.
• Spoilage in pickles may be due to low salt content of the brine, incomplete curing, poor quality vinegar, underprocessing or use of poor closures.

Precautions

• Infected pickles are often soft and slimy — examine texture as well as the counts.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition; Rogosa, M., Mitchell, J. A. & Wiseman, R. F., J. Bact., 62, 132 (1951); Man, J. C., M. de Rugcsa & M. E. Sharpe, J. Appl. Bact., 23, 130 (1960) — the book’s reference list gives the second author’s name as “de Rugcsa”, carried as printed.