SOP: Determination of Soluble Solids by Refractometer in Jam, Jelly and Marmalade
GIFSQ — Global Institute for Food Safety & Quality
Principle
The soluble solids content of the product is estimated from the refractive index of the sample at a standard temperature of 20°C. Where the product is too dark to be read directly, it is diluted with a weighed amount of concentrated sugar solution and the dry substance of the mixture is used to calculate the dry substance of the original product. Corrections are applied for invert sugar present and for water-insoluble solids.
Apparatus required
- Refractometer with water-jacketed prisms
- Constant temperature water bath (20°C)
- Analytical balance
Reagents
a. Concentrated sugar solution of known dry substance (refractive index reading) — for diluting samples too dark to be read directly.
Sample preparation
Use the prepared sample as received. Keep the refractometer prisms at 20°C by circulating water from a constant temperature water bath through the jackets long enough for the prisms and the product to reach equilibrium before taking the reading.
Procedure — Direct determination
Take the refractometer reading of the sample at 20°C
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If the reading is taken at room temperature, apply the temperature correction given in Table 28-4 to correct the reading to 20°C
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If condensation occurs on the exposed surfaces of the prism, take the reading at room temperature and apply the temperature correction to 20°C
Procedure — Dilution of samples too dark to read
If the jam is too dark to be read in the instrument, dilute it with concentrated sugar solution — never with water
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Mix weighed amounts of jam and the sugar solution, of about the same strength
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Determine the refractive index of the mixture and of the pure sugar solution separately, and obtain the corresponding % dry substance values from the refractive index
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Calculate the % dry substance of the product by the expression given under Calculation
Calculation
% Dry substance of the product, from the dilution method:
% Dry substance = [(W + B)C − BD] / W
Where,
- W = weight of sample in g mixed with B
- B = weight of sugar solution in g used in dilution
- C = % dry substance in the mixture (W + B) obtained from the refractive index
- D = % dry substance in the pure sugar solution obtained from its refractive index
Correction for invert sugar:
Jams, jellies and marmalades invariably contain invert sugar. Find the corresponding % sucrose content from the refractive index using Table 28-8 and add 0.022 for each % invert sugar present in the sample. (According to the US Federal and Codex Alimentarius Standards for fruit jellies, jams, marmalades and preserves, correction for water-insoluble solids, invert sugar or other substances is disregarded.)
Soluble solids in the presence of insoluble solids:
% Soluble solids = (% solids determined by refractometer / 100) × (100 − I)
Where,
- I = % water-insoluble solids
Note: the layout of this expression is garbled in the available print of the source; the form above is the reading consistent with the book’s own variable definitions — carried as printed with this note.
Precautions
- Allow the refractometer prisms and the product to reach temperature equilibrium at 20°C before reading.
- Dilute dark samples only with concentrated sugar solution, never with water.
- If condensation occurs on the exposed surfaces of the prism, take the reading at room temperature and apply the Table 28-4 temperature correction to 20°C.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
US Federal and Codex Alimentarius Standards for fruit jellies, jams, marmalades and preserves — cited in the book.