SOP: Direct Microscopic Examination of Spoiled Can Contents

GIFSQ — Global Institute for Food Safety & Quality

Principle

A stained smear of the can contents shows the shape and staining behaviour of the organisms at once. The kinds of cells seen — Gram-positive rods, cocci, yeasts — point straight to underprocessing or to leakage before any culture result comes back. But organisms seen may be dead from the heat process, so microscopy is always read together with the cultures.

Apparatus required

• Sterile inoculating loop
• Microscope

Reagents

a. Methylene blue stain — for the general morphology of the organism(s) present.

b. Carbol fuchsin stain — alternative general stain for morphology.

c. Gram stain — for Gram reaction.

Sample preparation

Transfer the contents of the spoiled can into a sterile container for subculturing, then proceed with the smears.

Procedure

Make smears with a sterile inoculating loop

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Stain with methylene blue or carbol fuchsin — this gives the general morphology of the organism(s) present

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Also stain with the Gram stain

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Read the smear against the following guide

Interpretation

• The presence of Gram-positive rods suggests underprocessing, while cocci, yeasts, etc., suggest leaker spoilage.
• If spoilage is due to bacteria surviving the heat process, not more than one or two types would be present, usually only one — except in gross underprocessing.
• In container leakage in products of pH above 4, the microflora would usually be a mixed flora.
• The presence of micrococci and/or yeasts is sure evidence of leakage.

Precautions

• The organisms seen may be dead or killed during processing: normal unspoiled cans may contain nonviable organisms in large numbers, due to poor raw materials or growth during the lag between preparation and processing. The food may show large numbers microscopically while cultures stay sterile because of auto-sterilization of the organisms. Such organisms stain poorly or unevenly and may show other signs of degeneration.
• Never report spoilage cause on microscopy alone — confirm by culturing.

References

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