UGANDA STANDARD
 
       US ISO 750
First Edition
2009-mm-dd
 
 Reference numbe r 
    US ISO 750:1998
© UNBS 2009
  
Fruit and vegetable products — Determination of titratable 
acidity 
 US ISO 750:1998 
 
© UNBS  2009 – All rights reserved 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  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilised in any 
form or by any means, electronic or mechanical, in cluding photocopying and microfilm, without prior written 
permission from UNBS. 
Requests for permission to reproduce this document should be addressed to 
The Executive Director 
Uganda National Bureau of Standards 
P.O. Box 6329 
Kampala  
Uganda 
Tel: 256 41 505 995 
Fax: 256 41 286 123 
E-mail: unbs@infocom.co.ug  
      US ISO 750:1998  
© UNBS  2009 - All rights reserved 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 Uganda.  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 750:1998, Fruit and vegetable products — Determination of titratable
acidity, is identical with and  has been reproduced from an International Standard ISO 750:1998, Fruit and
vegetable products — Determination of titratable acidity, and adopted as a Uganda Standard. 
This standard was developed by the Food and agricu lture Standards Technical Committee, UNBS/TC 2. 
Wherever the words, "International Standard" a ppear, they should be replaced by "Uganda Standard." A Reference numbe r
ISO 750:1998 (E)INTERNATIONAL
STANDARDISO
750
Second edition
1998-08-01
Fruit and vegetable products —
Determination of titratable acidity
Produits dérivés des fruits et légumes — Détermination de l’acidité titrableISO 750:1998(E)
©   ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by an y means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
iiForeword
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 technicalcommittees. Each member body interested in a subject for which a technical committee has been established hasthe right to be represented on that committee. International organizations, governmental and non-governmental, inliaison with ISO, also take part in the work. ISO collaborates closely with the International ElectrotechnicalCommission (IEC) on all matters of electrotechnical standardization.
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.
International Standard ISO 750 was prepared by Technical Commitee ISO/TC 34, 
Agricultural food products ,
Subcommittee SC 3, Fruit and vegetable products.
This second edition cancels and replaces the first edition (ISO 750:1981), which has been technically revised.INTERNATIONAL STANDARD    © ISO ISO 750:1998(E)
1Fruit and vegetable products — Determination of titratable acidity
1 Scope
This International Standard specifies two methods for the determination of the titratable acidity of fruit and vegetable
products:
 a potentiometric reference method;
 a routine method using a coloured indicator.
By convention, the latter method does not apply to wines.In the case of some coloured products, it may be difficult to determine the endpoint of the titration in the latter
method and the former method should preferably be used.
NOTE — The determination of titratable acidity is of no value in the case of products to which sulfur dioxide has been added.
2 Principle
2.1 Potentiometric method
Potentiometric titration with a standard volumetric solution of sodium hydroxide.
2.2 Routine method
Titration with a standard volumetric solution of sodium hydroxide in the presence of phenolphthalein as indicator.
3 Reagents
Use only reagents of recognized analytical grade, and distilled or demineralized water or water of equivalent purity.
3.1   Sodium hydroxide , standard volumetric solution, c(NaOH) = 0,1 mol/l.1)
3.2   Buffer solutions,  of known pH.
3.3   Phenolphthalein , 10 g/l solution in 95 % ( V/V) ethanol.
4 Apparatus
Usual laboratory apparatus and, in particular, the following.
4.1   Homogenizer  or mortar and pestle .
4.2   Pipettes, to deliver 25 ml, 50 ml or 100 ml.
                     
1) Previously expressed as "0,1 N standard volumetric solution."ISO 750:1998(E) © ISO
24.3   Conical flask , capable of being fitted with the reflux condenser (4.7).
4.4   Volumetric flask , of capacity 250 ml.
4.5   Beaker, of capacity 250 ml, together with a magnetic or mechanical stirrer.
4.6   Burette, of capacity 50 ml.
4.7   Reflux condenser .
4.8   Analytical balance , capable of weighing to the nearest 0,01 g.
4.9   pH-meter , accurate to at least 0,05 pH units.
4.10   Water bath .
5 Sampling
It is important the laboratory receive a sample which is truly representative and has not been damaged or changed
during transport or storage.
Sampling is not part of the method specified in this International Standard. As there is no specific International
Standard dealing with fruit and vegetable products, it is recommended that the parties concerned come to anagreement on the subject.
6 Preparation of test sample
6.1 Liquid products
Liquid products include products from which the liquid is easily separable (e.g. juices, canned fruit syrups, picklingliquids, brines, liquids from fermented products).
Take part of the previously mixed laboratory sample and filter it through cotton wool, filter paper or cloth. Transfer,
by means of the pipette (4.2), 25 ml of the filtrate (see note) into the volumetric flask (4.4). Dilute to the mark withwater and mix thoroughly.
It is necessary to remove carbon dioxide from carbonated liquid products by shaking under reduced pressure for
3 min to 4 min.
NOTE  It is also possible to take a sample by mass, weighing, to the nearest 0,01 g, at least 25 g of the laboratory sample.
6.2 Other products
Remove any stalks, stones, hard seed-cavity walls and, whenever possible, pips (after thawing in the case of frozenor deep-frozen products). Mix the sample thoroughly.
Allow frozen or deep-frozen products to thaw in a closed vessel and add the liquid formed during this process to the
products before mixing or blending.
In the case of dehydrated or dried products, cut a part of the laboratory sample into small pieces.Homogenize the product or grind it in the mortar (4.1).Weigh, to the nearest 0,01 g, at least 25 g of the laboratory sample and transfer it to the conical flask (4.3) with
50 ml of hot water. Mix well until homogeneity is obtained.
Fit the reflux condenser (4.7) to the conical flask and heat the contents on a boiling water bath for 30 min.© ISO ISO 750:1998(E)
3Cool, quantitatively transfer the contents of the conical flask to a volumetric flask (4.4) and dilute to the mark with
water. Mix well and filter.
7 Procedure
NOTE  If it is required to check whether the repeatability requirement (clause 9) is met, carry out two determinations in
accordance with 7.1.2 and 7.1.3, or 7.2.1 and 7.2.2.
7.1 Potentiometric method (Reference method)
7.1.1 Calibration of the pH meter
Check that the pH meter (4.9) is functioning correctly using the buffer solutions (3.2).
7.1.2 Test portion
Transfer, by means of the pipette (4.2), 25 ml, 50 ml or 100 ml of the diluted test sample (see clause 6), according
to the expected acidity, to the beaker with its stirrer (4.5).
7.1.3 Determination
Start the stirrer and add quickly, from the burette (4.6), the sodium hydroxide solution (3.1) until the pH is 7 ± 0,2.
Then, slowly add more until the pH is 8,1 ± 0,2.
7.2 Method using a coloured indicator (Routine method)
7.2.1 Test portion
Transfer, by means of the pipette (4.2), 25 ml, 50 ml or 100 ml of the diluted test sample (see clause 6), according
to the expected acidity, to a beaker with its stirrer (4.5).
7.2.2 Determination
Add 0,25 ml to 0,5 ml of the phenolphthalein solution (3.3) and, with shaking, titrate, using the burette (4.6), with the
sodium hydroxide solution (3.1) until a pink colour, persisting for 30 s, is obtained.
8 Expression of results
8.1 Method of calculation for laboratory samples taken by volume
The titratable acidity, expressed in millimoles of H+ per 100 ml of product, taking into account the dilution carried out
in clause 6, is given as follows:
250
V ¥ V1 ¥ c ¥ 1001000
00VVc
V=1
where
Vis the volume, in millilitres, of the test sample, i.e. 25 ml;
V0is the volume, in millilitres, of the test portion (7.1.2 or 7.2.1);
V1is the volume, in millilitres, of the sodium hydroxide solution (3.1) used for the determination (7.1.3 or
7.2.2);
cis the exact concentration, in moles per litre, of the sodium hydroxide solution (3.1).
Report the result to one decimal place.ISO 750:1998(E) © ISO
48.2 Method of calculation for laboratory samples taken by mass
The titratable acidity, expressed in millimoles of H+ per 100 g of product, taking into account the dilution carried out
in clause 6, is given as follows:
250
m ¥ V1 ¥ c ¥ 100
0V
where
V0, V1 and c have the same meanings as in 8.1;
mis the mass, in grams, of the test sample (see 6.1and its note, or 6.2).
Report the result to one decimal place.
8.3 Other methods of expression
It is also possible to express the titratable acidity conventionally in grams of acid per 100 g or per 100 ml of product,
as appropriate, by multiplying the formula (8.1 or 8.2) by a factor appropriate to the acid (see  table 1).
Table 1 
Acid Factor
Malic acid
Oxalic acidCitric acid monohydrateTartaric acidSulfuric acidAcetic acidLactic acidCitric acid0,067
0,0450,0700,0750,0490,0600,0900,064
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 2 % of the arithmetic mean of the two results.
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, together 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 750:1998(E) © ISO
ICS  67.080.01
Descriptors:  agricultural products,  food products,  plant products,  fruit and vegetable products,  tests,  determination,  acidity.
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