UGANDA STANDARD
 
        US ISO 2173
First Edition
2009-mm-dd
 
Reference numbe r
    US ISO 2173:2003
 © UNBS 2009
  
Fruit and vegetable products — Determination of soluble solids 
— Refractometric method 
 US ISO 2173: 2003 
 
© UNBS  2009 – All rights reserved ii Compliance with this standard does not, of itself confer immunity from legal obligations 
A Uganda Standard does not purport to include all necessary provisions of a contract. Users are 
responsible for its correct application  
© UNBS 2009 
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The Executive Director 
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P.O. Box 6329 
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Fax: 256 41 286 123 
E-mail: unbs@infocom.co.ug
Web: www.unbs.go.ug  
      US ISO 2173:2003  
© UNBS  2009 - All rights rese rved                iii National foreword 
Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry  of Tourism, Trade and 
Industry established under Cap 327, of the Laws of U ganda. UNBS is mandated to co-ordinate the elaboration 
of standards and is  
(a) a member of International Organisation for Standardisation (ISO) and  
(b) a contact point for the WHO/FAO Codex A limentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT/SPS Agr eements of the World Trade Organisation (WTO). 
The work of preparing Uganda Standards is carried out through Technical Committees. A Technical 
Committee is established to deliberate on standards in a gi ven field or area and consists of representatives of 
consumers, traders, academician s, manufacturers, governme nt and other stakeholders. 
Draft Uganda Standards adopted by the Technical Committ ee are widely circulated to stakeholders and the 
general public for comments. The committee reviews the comments before recommending the draft standards 
for approval and declaration as Uganda Standards by the National Standards Council. 
This Uganda Standard, US ISO 2173, Fruit and vegetable products — Determination of soluble solids —
Refractometric method,  is identical with and has been repr oduced from an International Standard, ISO 2173:
2003, Fruit and vegetable products — Determination of  soluble solids — Refractometric method, and 
adopted as a Uganda Standard.  
This standard was developed under the subcommittee on fruits and vegetables by the Food and agriculture 
Standards Technical Committee (UNBS/TC 2). 
Wherever the words, "International Standard" a ppear, they should be replaced by "Uganda Standard." 
  
 
 
  
 Reference numbe r
ISO 2173:2003(E)
© ISO 2003
  
 
 INTERNATIONAL 
STANDARD  ISO
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 
 ISO 2173:2003(E) 
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ii © ISO 2003 — All rights reserved
  ISO 2173:2003(E) 
© ISO 2003 — All rights reserved  iii
 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.  INTERNATIONAL STANDARD ISO 2173:2003(E)
 
© ISO 2003 — All rights reserved  1
 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. ISO 2173:2003(E) 
2 © ISO 2003 — All rights reserved
 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. ISO 2173:2003(E) 
© ISO 2003 — All rights reserved  3
 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 lamp allows more precise results to be 
obtained (especially in the case of coloured and dark products). 
Bring the line dividing the light and dark parts of the surface into the field of view to the crossing of the threads. 
Read the value of the refractive index or the mass fraction of sucrose, according to the instrument used (5.1.1 or 5.1.2). 
8 Expression of results 
8.1 Corrections 
8.1.1 If the determination has been carried out at a temperature other than 20 °C ± 0,5 °C, the following 
corrections are required. 
a) For the scale indicating the refractive index (5.1.1), apply the formula: 
()200,0013 20t
DDnn t =+ −  
where 
20
Dn is the refractive index at 20 °C; 
t
Dn is the refractive index at the temperature of measurement; 
t is the temperature of measurement, in degrees Celsius. 
b) For the scale indicating the mass fraction of sucrose (5.1.2), correct the result according to Table A.1.  
8.1.2 If the determination has been carried out for the products with added salt, correct the refractometer 
reading, expressed as a concentration of sucrose at 20 °C ± 0,5 °C, for added salt by the following formula 
(see reference [2]): 
S = (R − N) × 1,016  ISO 2173:2003(E) 
4 © ISO 2003 — All rights reserved
 where 
S is the mass fraction of soluble solids, in percent, as sucrose, corrected for added NaCl; 
R is the refractometer reading, as a mass fraction in percent, as sucrose;  
N is the total chloride content, as a mass fraction in percent, expressed as NaCl;  
1,016 is the correction factor for added salt.  
8.1.3 If the determination has been carried out for the highly acidic products, such as citrus juices and 
concentrated citrus juices, correct the refractometer reading, expressed as a masss fraction of sucrose at 
20 °C ± 0,5 °C, by making the following addition to the refractometric reading (see reference [3]): 
0,012 + 0,193 × M − 0,000 4 × M2  
where M is the total acidity expressed in grams per 100 g, at pH = 8,1, expressed as anhydrous citric acid 
(see reference [4]). The calculated values for this expression are given in Table A.2.  
8.2 Calculation method 
8.2.1 Refractometer with refractive index scale 
8.2.1.1 Read from Table A.3 the mass fraction of soluble solids corresponding to the value read in 
accordance with 7.2, corrected if necessary in accordance with 8.1.1 a). In the case of liquid or semi-thick 
products (7.1.1 or 7.1.2), the mass fraction of soluble solids is equal to the number found. If the determination has been carried out on a diluted solution (7.1.3 or 7.1.5), the mass fraction of soluble solids, in percent, is equal to 
( )10/ Pmm⋅  
where 
P is the mass fraction of soluble solids in the diluted solution, in percent; 
m0 is the mass, in grams, of the sample before dilution (7.1.3 or 7.1.5); 
m1 is the mass, in grams, of the sample after dilution (7.1.3 or 7.1.5). 
8.2.1.2 If the determination has been carried out on a dark solution (see 7.1.5) that has to be diluted with 
concentrated sugar solution, the mass fraction of soluble solids, in percent, is equal to (see reference [1]) 
()WB B
Wmm C m D
m +− 
where 
mW is the mass, in grams, of the sample diluted with sugar solution;  
mB is the mass, in grams, of sugar solution used in the dilution;  
C is the mass fraction of soluble solids, in percent, in the mixture ( mW + mB), obtained from the 
refractive index; 
D is the mass fraction of soluble solids, in percent, in the pure sugar solution, obtained from its 
refractive index. 
Express the result to one decimal place. ISO 2173:2003(E) 
© ISO 2003 — All rights reserved  5
 8.2.2 Refractometer with sucrose scale 
In the case of liquid or semi-thick products (7.1.1 or 7.1.2), the mass fraction of soluble solids, in percent, as 
sucrose, is equal to the value read in accordance with 8.1.1 b). If the determination has been carried out on a diluted solution (7.1.3 or 7.1.5), calculate the mass fraction of soluble solids by means of the formula given in 8.2.1.1, or the formula in 8.2.1.2 for a dark solution diluted with a sugar solution (see 7.1.5). 
Express the result to one decimal place. 
9 Repeatability 
The absolute difference between two independent single test results, obtained using the same method on 
identical test material in the same laboratory by the same operator using the same equipment within a short interval of time, will in not more than 5 % of cases be greater than 0,5 g of soluble solids per 100 g or per 100 ml of product. 
10 Test report 
The test report shall specify: 
 all information necessary for the complete identification of the sample; 
 the sampling method used, if known; 
 the test method used, with reference to this International Standard; 
 all operating details not specified in this International Standard, or regarded as optional, together with 
details of any incidents which may have influenced the test result(s); 
 the test result(s) obtained; 
 if the repeatability has been checked, the final quoted result obtained. 
 ISO 2173:2003(E) 
6 © ISO 2003 — All rights reserved
 Annex A  
(normative)  
 
Correction and convertion tables 
Table A.1 — Correction of readings of the refractometer with scale indicating sucrose for a 
temperature different from 20 °C ±±±± 0,5 °C 
Scale reading for soluble solids,  % (by mass) Temperature 
°C 5 10 15 20 25 30 40 50 60 70 
 Corrections to be subtracted 
15 
16 17 
18 
19 0,29 
0,24 0,18 
0,13 
0,06 0,31 
0,25 0,19 
0,13 
0,06 0,33 
0,26 0,20 
0,14 
0,07 0,34 
0,27 0,21 
0,14 
0,07 0,34 
0,28 0,21 
0,14 
0,07 0,35 
0,28 0,21 
0,14 
0,07 0,37 
0,30 0,22 
0,15 
0,08 0,38 
0,30 0,23 
0,15 
0,08 0,39 
0,31 0,23 
0,16 
0,08 0,40 
0,32 0,24 
0,16 
0,08 
 Corrections to be added 
21 22 23 24 25 0,07 
0,13 0,20 0,27 0,35 0,07 
0,14 0,21 0,28 0,36 0,07 
0,14 0,22 0,29 0,37 0,07 
0,15 0,22 0,30 0,38 0,08 
0,15 0,23 0,30 0,38 0,08 
0,15 0,23 0,31 0,39 0,08 
0,15 0,23 0,31 0,40 0,08 
0,16 0,24 0,31 0,40 0,08 
0,16 0,24 0,32 0,40 0,08 
0,16 0,24 0,32 0,40 
 
Table A.2 — Acid correction (for citrus juices and citrus juice concentrates) 
Total acid pH ==== 8,1 expressed as 
anhydrous citric acid (g acid/100 g) Correction 
value a Total acid pH ==== 8,1 expressed as 
anhydrous citric acid (g acid/100 g) Correction 
value a 
0,2 0,04 4,2 0,81 
0,4 0,08 4,4 0,85 
0,6 0,12 4,6 0,89 
0,8 0,16 4,8 0,93 
1,0 0,20 5,0 0,97 
    
1,2 0,24 5,2 1,01 
1,4 0,28 5,4 1,04 
1,6 0,32 5,6 1,07 
1,8 0,36 5,8 1,11 
2,0 0,39 6,0 1,15 
    
2,2 0,43 6,2 1,19 
2,4 0,47 6,4 1,23 
2,6 0,51 6,6 1,27 
2,8 0,55 6,8 1,30 
3,0 0,58 7,0 1,34 
    
3,2 0,62   
3,4 0,66   
3,6 0,70   
3,8 0,74   
4,0 0,78   
a The correction values given should be added to the refractometer readings with scale indicating sucrose obtained at 20 °C ± 0,5 °CISO 2173:2003(E) 
© ISO 2003 — All rights reserved  7
 Table A.3 — Refractive index and corresponding mass fraction of soluble solids (sucrose) 
Refractive 
index Soluble 
solids 
(sucrose) Refractive 
index Soluble 
solids 
(sucrose) Refractive 
index Soluble 
solids 
(sucrose) Refractive 
index Soluble 
solids 
(sucrose) 
20
Dn % (by mass) 20
Dn % (by mass) 20
Dn % (by mass) 20
Dn % (by mass) 
1,333 0 0 1,367 2 22 1,407 6 44 1,455 8 66 
1,334 4 1 1,368 9 23 1,409 6 45 1,458 2 67 
1,335 9 2 1,370 6 24   1,460 6 68 
1,337 3 3 1,372 3 25 1,411 7 46 1,463 0 69 
1,338 8 4   1,413 7 47 1,465 4 70 
1,340 3 5 1,374 0 26 1,415 8 48   
  1,375 8 27 1,417 9 49 1,467 9 71 
1,341 8 6 1,377 5 28 1,420 1 50 1,470 3 72 
1,343 3 7 1,379 3 29   1,472 8 73 
1,344 8 8 1,381 1 30 1,422 2 51 1,475 3 74 
1,346 3 9   1,424 3 52 1,477 8 75 
1,347 8 10 1,382 9 31 1,426 5 53   
  1,384 7 32 1,428 6 54 1,480 3 76 
1,349 4 11 1,386 5 33 1,430 8 55 1,482 9 77 
1,350 9 12 1,388 3 34   1,485 4 78 
1,352 5 13 1,390 2 35 1,433 0 56 1,488 0 79 
1,354 1 14   1,435 2 57 1,490 6 80 
1,355 7 15 1,392 0 36 1,437 4 58   
  1,393 9 37 1,439 7 59 1,493 3 81 
1,357 3 16 1,395 8 38 1,441 9 60 1,495 9 82 
1,358 9 17 1,397 8 39   1,498 5 83 
1,360 5 18 1,399 7 40 1,444 2 61 1,501 2 84 
1,362 2 19   1,446 5 62 1,503 9 85 
1,363 8 20 1,401 6 41 1,448 8 63   
  1,403 6 42 1,451 1 64   
1,365 5 21 1,405 6 43 1,453 5 65   
 ISO 2173:2003(E) 
8 © ISO 2003 — All rights reserved
 Bibliography 
[1] AOAC Official Method 932.14, Solids in sirups . AOAC Official Methods of Analysis, 1995, 44.1.04 
[2] AOAC Official Method 970.59, Solids (Soluble) in Tomato Products: Refractive index method . AOAC 
Official Methods of Analysis, 1995, 42.1.10 
[3] EN 12143:1996, Fruit and vegetable juices — Estimation of soluble solids content — Refractometric 
method  
[4] ISO 750:1998, Fruit and vegetable products — Determination of titratable acidity  
[5] ISO 2172:1983, Fruit juice — Determination of soluble solids content — Pycnometric method  
 ISO 2173:2003(E) 
ICS  67.080.01 
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