SOP: Determination of the Degree of Concentration of Juice Concentrates

GIFSQ — Global Institute for Food Safety & Quality

Principle

Concentrates are described as 4-, 5- or 6-fold, meaning one volume of concentrate dilutes to 4, 5 or 6 volumes of juice. The fold of concentration is assessed from the specific gravity or the refractive index. The specific gravity of the concentrate itself cannot be measured directly (it tends to occlude air bubbles), so a weighed portion is diluted with a known weight of water, the specific gravity of the diluted juice is measured, and the Brix of the original concentrate is back-calculated using Domke’s table of apparent specific gravity of sucrose solutions at 20 °C.

Apparatus required

• 50-ml specific gravity bottle, standardized at 20 °C
• 150-ml conical flask with tight-fitting rubber cork
• Analytical balance
• Water bath at 20 °C

Procedure

Weigh 20–25 g of concentrate into a tared 150-ml conical flask fitted with a tight-fitting rubber cork

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Add a quantity of water equal to not less than 3 times the weight of the concentrate, and reweigh

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Mix thoroughly by swirling to minimize the formation of air bubbles, and let stand for 1 hour

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Fill the 50-ml specific gravity bottle (standardized at 20 °C) with the diluted concentrate, swirling the conical flask at intervals to ensure a proper proportion of cellular matter enters the bottle

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Adjust the temperature to 20 °C and determine the specific gravity as in the specific-gravity-bottle method

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From Domke’s table (the book’s Table 28-16), read the °Brix corresponding to the specific gravity of the diluted juice; interpolate to express the Brix value to two decimal places — call this value b

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Calculate the Brix of the original concentrate, B = b × (x + y) / x

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From Domke’s table find the specific gravity of the original concentrate corresponding to Brix B

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Calculate the fold of concentration: Fold = (Specific gravity of concentrate × Brix of concentrate) / (Specific gravity of juice × Brix of juice)

Calculation

B = b(x + y) / x

Where,

• B = Brix value of the original concentrate
• b = Brix value of the diluted concentrate from Domke’s table
• x = weight of concentrate in grams originally taken
• y = weight of water in grams added to the concentrate

Fold of concentration = (Specific gravity of concentrate × Brix of concentrate) / (Specific gravity of juice × Brix of juice)

For the juice used in preparing the concentrate the USDA assumes a specific gravity of 1.0743 at 20/20 °C, i.e., 11.75 °Brix.

Precautions

• Mix gently and let the diluted concentrate stand for the full hour so that occluded air escapes.
• Swirl the flask while filling the specific gravity bottle so the cellular matter is representatively transferred.
• The degree of concentration may also be obtained from the specific gravity or refractive index using the book’s Table 28-17.

References

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

Official Methods of Analysis, Assoc. of Official Agricultural Chemists, 11th edn., 1970 — as cited in the book.

Basker, H. B., Analyst, 94, 410 (1969) — as cited in the book.