SOP: Determination of Total Soluble Solids by Refractometer

GIFSQ — Global Institute for Food Safety & Quality

Principle

Total soluble solids are determined from the refractive index of the juice. The Abbe or hand refractometer gives the reading as refractive index or directly as per cent dry substance (°Brix). Fruit acids, the main non-sugar soluble solids in fruit products, affect the refractometer and the Brix hydrometer differently, so a correction is applied to obtain true Brix and true soluble solids. The instrument is easier to use than a hydrometer, needs less sample, and is more accurate with viscous solutions.

Apparatus required

• Abbe refractometer or hand (sugar) refractometer, calibrated for 20 °C
• Dropping pipette
• Thermometer

Reagents

a. Distilled water, for checking the instrument accuracy.

Procedure

Checking the instrument

Allow water to circulate through the refractometer for a few minutes, then note the temperature and the refractive index of distilled water

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Compare the reading with the book’s Table 28-5 (refractive index of distilled water at various temperatures); if the variation is more than ± 0.002, adjust the refractometer to the correct reading

Determination

Place a drop of the well-mixed, filtered sample on the prism

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Read the refractive index, or read per cent dry substance directly if a sugar refractometer is used

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If the refractive index was read, note the equivalent Brix reading from the book’s Table 28-8

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Correct the reading to the standard temperature of 20 °C using the corrections in the book’s Table 28-4 (subtract below 20 °C, add above 20 °C)

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For samples containing invert sugar (which fruit products invariably do), add 0.022 to the per cent solids for each per cent of invert sugar in the sample

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Apply the citric-acid correction from the book’s Table 28-6 (corrections developed for citrus products containing citric acid, applicable to other products with known citric acid content) to obtain the true °Brix and the true soluble solids

Calculation

True °Brix = temperature-corrected refractometer reading + citric-acid correction (from Table 28-6)

True soluble solids = temperature-corrected reading + citric-acid correction for soluble solids (from Table 28-6)

Precautions

• The correction tables (28-4, 28-5, 28-6, 28-8) are printed reference tables in the book; use the printed values directly, do not interpolate new ones.
• Allow the instrument to reach a steady temperature before reading.
• Brix measured by refractometer or by Brix hydrometer is a true measure of soluble solids only for pure sucrose solutions; in fruit products the two instruments differ.

References

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

Stevens, J. W. and Baier, W. E., Ind. Eng. Chem., Anal. Edn., 11, 447 (1939) — as cited in the book.