SOP: Fermentation Test for Concentrates

GIFSQ — Global Institute for Food Safety & Quality

Principle

The concentrate is inoculated into a sterile sugar medium and incubated; fermentative yeasts reveal themselves by gas formation accompanied by a white deposit of yeast cells. The test is empirical but gives the needed information on fermentative spoilage organisms in the concentrate.

Apparatus required

• 150-ml or 250-ml conical flasks
• Cotton-wool plugs
• Incubator (26.5 °C)

Procedure

To 9 g of sugar add 89 ml of water in a 150- or 250-ml conical flask; close with a cotton-wool plug, sterilize at 120 °C for 30 minutes, and cool

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Add 10 ml of the concentrate diluted to 12 °Brix with sterile water under aseptic conditions, and mix

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Incubate the sample at 26.5 °C for 14 days

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Watch for gas formation accompanied by a white deposit of yeast cells — this is a positive result

Interpretation

• In clarified juices it may be difficult to see gas evolution in the early stages because the gas dissolves in the juice as soon as it forms; on reaching saturation the carbon dioxide effervesces when shaken.
• If a positive result is observed within 6 days of incubation, reject the sample.
• If a positive result is observed during the last 8 days of incubation and the concentrate is preserved using sulphite, raise the sulphite concentration to 1500 ppm and the acidity to 3.5% w/v; mix thoroughly, let remain for 14 days, and retest.
• If a positive result is observed in the retest during 14 days of incubation, reject the sample; if the test is negative, the sample may be considered to have passed.

Precautions

• Sterilize the sugar medium fully (120 °C for 30 minutes) before inoculation, or background organisms give a false positive.
• Inoculate under aseptic conditions.

References

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