SOP: Incubation Test for Spoilage of Canned Products

GIFSQ — Global Institute for Food Safety & Quality

Principle

Cans are incubated at 37 °C and 55 °C for 7 days to reveal living microorganisms: gas-forming mesophiles swell the cans at 37 °C, while flat-sour organisms (which do not grow at low temperatures) are detected at 55 °C; stained slide examination of the contents distinguishes leaker spoilage from under-processing.

Apparatus required

• Incubators set at 37 °C and 55 °C
• Microscope
• Gentian violet or methylene blue stain

Reagents

a. Sterile dextrose-tryptone broth containing bromo-cresol purple indicator.

b. Gentian violet or methylene blue — for the slide stain.

Procedure — Incubation

Hold the cans in the incubator at 37 °C and at 55 °C for 7 days

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Swelling at 37 °C means living mesophilic organisms are present, most often through leakage: these organisms typically spoil the product with gas (CO₂) formation, turning the cans into springers and then into hard swells — and cans left in the incubator too long can burst

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At 55 °C cans as a rule do not swell. Spoilage here is of the flat-sour type — organisms that do not normally grow at low temperatures, so this trouble shows up where storage is hot, and consignments bound for hot countries need special care. Flat-sour spoilage points to under-processing or an unusually heavy initial contamination

Procedure — Flat-sour test

To look for flat-sour organisms, check the pH of the contents first: a drop in pH points to their presence

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The more usual, better method: hold the can 4 days at 55 °C, then transfer some of the contents aseptically into sterile dextrose-tryptone broth with bromo-cresol purple indicator

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Incubate the inoculated tubes 3 or 4 days; flat-sour organisms turn the indicator from purple to yellow

Procedure — Examination of the cause of spoilage

For cans that swell at 37 °C, make a slide from a drop of the contents and stain it with gentian violet or methylene blue

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Cocci or short rods — bacteria of the non-sporing type — are positive evidence of leaker spoilage, since such organisms cannot withstand even mild processing

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Medium or long slender rods, by contrast — spore-formers — are definite evidence of spoilage from under-processing

Recording the results

Record the incubation temperature and duration, the condition of the cans (flat, flipper, springer, swell), the pH change, and the outcomes of the flat-sour test and the stained-slide examination.

Precautions

• Subject all swollen cans to bacteriological examination to determine the cause of spoilage.
• If only one can is available, sample first for bacteriological examination before opening the cans for cut-out examination.
• In the case of swells, check the internal condition of the can and the condition of the seams carefully before discarding the container.

References

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