ISO 2173:2003

INTERNATIONAL ISO
STANDARD 2173
Second edition
2003-12-01


Fruit and vegetable products —
Determination of soluble solids —
Refractometric method
Produits dérivés des fruits et légumes — Détermination du résidu sec
soluble — Méthode réfractométrique




Reference number
ISO 2173:2003(E)
©
ISO 2003

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ISO 2173:2003(E)
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ISO 2173:2003(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 2173 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 3, Fruit and
vegetable products.
This second edition cancels and replaces the first edition (ISO 2173:1978), which has been technically revised.

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INTERNATIONAL STANDARD ISO 2173:2003(E)

Fruit and vegetable products — Determination of soluble
solids — Refractometric method
1 Scope
This International Standard specifies a refractometric method for the determination of the soluble solids in fruit
and vegetable products.
This method is particularly applicable to thick products, to products containing suspended matter, and to
products rich in sugar. If the products contain other dissolved substances, the results will be only approximate;
nevertheless, for convenience the result obtained by this method can be considered conventionally as the
soluble solids content.
NOTE For the determination of the soluble solids in fruit juices (not containing suspended matter) and in
concentrated juices (clarified), the pyknometric method specified in ISO 2172 is applicable.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
soluble solids determined by the refractometric method
concentration of sucrose in an aqueous solution which has the same refractive index as the product analysed,
under specified conditions of preparation and temperature
NOTE This concentration is expressed as a mass fraction in percent.
3 Principle
The refractive index of a test solution is measured at 20 °C ± 0,5 °C using a refractometer. The refractive
index is correlated with the amount of soluble solids (expressed as sucrose concentration) using tables, or by
direct reading on the refractometer of the mass fraction of soluble solids.
4 Reagents
Use only reagents of recognized analytical grade.
4.1 Water
The water used shall have been distilled twice in borosilicate glass apparatus, or shall be water of at least
equivalent purity.
5 Apparatus
Usual laboratory apparatus and, in particular, the following.
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ISO 2173:2003(E)
5.1 Refractometer
Use one of the following.
5.1.1 Refractometer indicating the refractive index, by means of a scale graduated in 0,001, in order to
allow readings to be estimated to 0,000 2.
This refractometer shall be adjusted so that at 20 °C ± 0,5 °C it registers a refractive index of 1,333 0 for
distilled water.
5.1.2 Refractometer indicating the mass fraction of sucrose, by means of scale graduated in 0,10 %.
This refractometer shall be adjusted so that at 20 °C ± 0,5 °C it registers a mass fraction of soluble solids
(sucrose) of zero for distilled water.
5.2 Means for circulating water, to maintain the temperature of the prisms of the refractometer (5.1.1 or
5.1.2) constant to within ± 0,5 °C, in the neighbourhood of 20 °C, which is the reference temperature (see 8.1).
5.3 Beaker, of capacity 250 ml.
6 Sampling
It is important that the laboratory receive a sample which is truly representative and has not been damaged or
changed during transport or storage.
7 Procedure
7.1 Preparation of test solution
7.1.1 Clear liquid products
Thoroughly mix the laboratory sample and use it directly for the determination.
7.1.2 Semi-thick products (purees, etc.)
Thoroughly mix the laboratory sample. Press a part of the sample through a gauze folded in four, rejecting the
first drops of the liquid and reserving the remainder of the liquid for the determination.
7.1.3 Thick products (jams, jellies, etc.)
Weigh into the tared beaker (5.3), to the nearest 0,01 g, a suitable quantity (up to 40 g) of the laboratory
sample and add 100 ml to 150 ml of water. Heat the contents of the beaker to boiling and allow to boil gently
for 2 min to 3 min, stirring with a glass rod. Cool the contents and mix thoroughly.
After 20 min, weigh to the nearest 0,01 g, then filter through a fluted filter or a Büchner funnel into a dry vessel.
Reserve the filtrate for the determination.
7.1.4 Frozen products
After thawing the sample and removing, if necessary, stones, pips and hard seed-cavity walls, mix the product
with the liquid formed during the thawing process and proceed as described in 7.1.2 or 7.1.3 as appropriate.
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ISO 2173:2003(E)
7.1.5 Dried products
Cut a part of the laboratory sample into small pieces. Remove, if necessary, stones, pips and hard seed-cavity
walls, and mix carefully. Then weigh into a tared beaker, to the nearest 0,01 g, 10 g to 20 g of the sample.
Add 5 to 10 times this mass of water and place on a boiling water bath for 30 min, stirring from time to time
with a glass rod. If necessary, prolong the heating time until a homogeneous mixture is obtained. Cool the
contents of the beaker and mix well.
After 20 min, weigh to the nearest 0,01 g, then filter into a dry vessel. Reserve the filtrate for the determination.
If the test solution is too dark to be read in the refractometer, dilute the test solution with concentrated sugar
solution; never use water for this purpose. Mix weighed amounts of the solution under examination and a

solution of pure sugar of about the same strength (see reference [1]).
7.2 Determination
Adjust the water circulation (5.2) in order to operate at the required temperature (between 15 °C and 25 °C)
and allow it to flow to bring the prisms of the refractometer (5.1.1 or 5.1.2) to the same temperature, which
shall remain constant to within ± 0,5 °C during the determination.
Bring the test solution (7.1) to the measuring temperature. Put a small quantity of test solution (2 or 3 drops
are sufficient) onto the fixed prism of the refractometer (5.1.1 or 5.1.2) and immediately adjust the movable
prism. Suitably illuminate the field of view. The use of a sodium vapour
...