SOP: Cloud Stability Test by the Stevens Accelerated Procedure

GIFSQ — Global Institute for Food Safety & Quality

Principle

The Stevens accelerated cloud-stability test checks pasteurized citrus concentrates for leftover enzyme (pectinesterase) activity, as a measure of the cloud stability of the product. Added pectin is incubated with the reconstituted juice in the presence of barium chloride (which gels pectin whose methoxyl groups have been split by residual enzyme); separation, flocculation or clarification indicates residual activity, and a centrifuged sample that loses its cloud confirms enzyme action.

Apparatus required

• 250-ml beakers
• Clear screw-capped bottles
• Incubator (48.3 to 48.9 °C)
• Centrifuge tubes (15 ml) and centrifuge

Reagents

a. Pectin solution: weigh 3 g of rapid-set pectin, wet with a few drops of alcohol, add water with stirring (heat if necessary) to render it into solution, and make up to 100 ml. The rapid-set pectin should not form a gel when 1.3 ml of barium chloride solution is added to 100 ml of 0.1% pectin solution.

b. Sodium benzoate solution: dissolve 4.4 g in 10 ml of water.

c. Barium chloride solution: dissolve 30 g in 100 ml of water.

d. Citric acid solution: dissolve 50 g in 100 ml of water.

e. Sodium hydroxide solution (for adjusting lemon and lime juices).

Procedure

Reconstitute the concentrate to single-strength juice (11–12 °Brix)

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To 90 ml of the reconstituted juice in a 250-ml beaker add 3.6 ml of pectin solution, 1.3 ml of barium chloride solution and 0.5 ml of sodium benzoate solution

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Adjust the pH to between 3.1 and 3.2 with the citric acid solution — for reconstituted lemon and lime juice concentrates use NaOH solution instead of citric acid solution

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Mix well, transfer to a clear screw-capped bottle, and incubate at 48.3 to 48.9 °C for 5 days

Interpretation

Examine the sample during and after incubation for signs of separation, flocculation or clarification — the absence of these conditions indicates a negative test for residual activity

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If there are any indications of residual activity, dilute a portion of the test sample with an equal volume of water and centrifuge in 15-ml tubes at 2000 rpm (16-inch-diameter centrifuge) for 2 minutes — the supernatant should retain a good cloud; a clear serum after centrifugation is indicative of enzyme action

Precautions

• Use the pH adjustment prescribed for the product (citric acid for most, NaOH for lemon and lime) — the test pH controls the pectin behaviour.
• Check the rapid-set pectin against the barium-chloride gel criterion before use; pectin that gels spontaneously invalidates the test.

References

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

Stevens, J. W., Methods of Conserving Juices, US Patent 2,217,261 — as cited in the book.