ISO/DIS 9877:2025(en)
IDF 258:2025(en)

ISO/TC 34/SC 5

Secretariat: NEN

Date: 2025-05-02

Milk and milk products — Determination of ash

© ISO and IDF 2025

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Contents

Foreword iv

Foreword v

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Principle 1

5 Apparatus 1

6 Sampling 2

7 Preparation of the test sample 2

8 Determination 3

9 Expression of results 3

10 Precision 4

Annex A (informative) Ashing time 6

Annex B (informative) Precision data 7

Bibliography 8

Foreword

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This document was prepared by Technical Committee ISO/TC 34 Food Products, Subcommittee SC 5, Milk and milk products, and the International Dairy Federation (IDF). It is being published jointly by ISO and IDF.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

IDF (the International Dairy Federation) is a non-profit private sector organization representing the interests of various stakeholders in dairying at the global level. IDF members are organized in National Committees, which are national associations composed of representatives of dairy-related national interest groups including dairy farmers, dairy processing industry, dairy suppliers, academics and governments/food control authorities.

ISO and IDF collaborate closely on all matters of standardization relating to methods of analysis and sampling for milk and milk products. Since 2001, ISO and IDF jointly publish their International Standards using the logos and reference numbers of both organizations.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. IDF shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not constitute and endorsement.

This document was prepared by the IDF Standing Committee on Analytical Methods for Composition and ISO Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products. It is being published jointly by IDF and ISO.

The work was carried out by the IDF/ISO Action Team on Determination of Ash in milk products, including dairy permeates powders (C43), of the Standing Committee on Analytical Methods for Composition, under the aegis of its project leader Mr. Richard Johnson (NZ).

Milk and milk products — Determination of ash

This document specifies a method for the determination of the ash content in milk, milk powder, whey powder, whey protein concentrate, infant formula, milk permeate powder and milk protein concentrate.

Note: For the determination of ash (“fixed ash”) of caseins and caseinates, see ISO 5544|IDF 89.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 3310‑1, Test sieves — Technical requirements and testing — Part 1: Test sieves of metal wire cloth

ISO 707, Milk and milk products — Guidance on sampling

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

ash

substances determined by the procedure described in this document

Note 1 to entry: Ash of milk products is conventionally expressed as a percentage by mass, numerically equivalent to grams per 100 g of product.

The ash residue is determined, by weight difference, by applying an incineration procedure, at 550 °C ± 25 °C, to the test portion.

Usual laboratory apparatus, and in particular the following.

5.1 Analytical balance, capable of weighing to the nearest 0,000 1 g.

5.2 Silica, porcelain or quartz crucibles or platinum dishes, of diameter about 70 mm and depth 25 mm to 50 mm.

5.3 Electrical furnace, with air circulation, capable of being controlled at 550 °C ± 25 °C.

5.4 Desiccator, suitable for high temperature application, containing an effective desiccant.

5.5 Thermogravimetric analyser (TGA)^[1], incorporating analytical balance (5.1), electrical furnace (5.3) and suitable dishes. The dishes may not conform to the dimensions specified in 5.2.

5.6 Water bath, capable of maintaining a water temperature between 35 °C and 40 °C.

5.7 Fibreglass pads^[2], in paper like form.

5.8 Boiling water bath, provided with openings of adjustable size.

5.9 Hotplate, controlled at 300 °C to 400 °C.

5.10 Bunsen burner.

Sampling is not part of the method specified in this International Standard. A recommended sampling method is given in ISO 707|IDF 50.

It is important that the laboratory receive a sample which is representative and has not been damaged or changed during transport or storage.

7.1 Milk powder, whey powder, whey protein concentrate, infant formula, milk permeate powder and milk protein concentrate.

Transfer the sample to a container of internal volume approximately twice the volume of the sample. Close the container immediately to avoid changing the moisture content of the sample. Thoroughly mix the sample by repeatedly rotating and inverting the container.

After the test sample has been prepared, the determination should proceed as soon as possible.

7.2 Whole, partially skimmed or skimmed liquid milk

Place the test sample in the water bath (5.6) set at 35 °C to 40 °C. Mix gently by inversion without causing frothing or churning. Once the sample is mixed thoroughly, cool to room temperature.

After the test sample has been prepared, the determination should proceed as soon as possible.

Heat the dish (5.2) in the electrical furnace (5.3), maintained at 550 °C, for a minimum of 30 min. Allow the dish to cool in the desiccator (5.4) to the temperature of the balance room and weigh to the nearest 0,1 mg.

Where a thermogravimetric analyser (5.5) is used the dishes should be heated to 550 °C and maintained at that temperature for a minimum of 30 min. Allow to cool in the analyser furnace and weigh to the nearest 0,1 mg.

For liquid milk samples fiberglass sample pads (5.7) may be used to aid evaporation. Prior to use they shall be preheated for a minimum of 30 min at 550 °C cooled and stored in a desiccator (5.4).

For liquid milk, weigh to the nearest 0,1 mg, directly or by difference, approximately 10,0 g of the test sample into the prepared dish or approximately 5,0 g when using thermogravimetric analyser (TGA).

For all other sample types, weigh to the nearest 0,1 mg, directly or by difference, approximately 1,0 g of the test sample into the prepared dish.

Evaporate liquid milks to dryness on a vigorously boiling water bath (5.8) before charring. Alternatively, liquid samples can be evaporated to dryness via hotplate (5.9), flame (5.10) or electrical furnace (5.3) operated at a lower temperature prior to charring. Ensure that only water is lost during this phase.

Heat the dish containing the test portion until it is completely charred, taking care that it does not burst into flame. This stage of the incineration can be performed manually via hotplate (5.9) or flame (5.10) prior to transferring the dish to the electrical furnace (5.3). Alternatively charring can be incorporated into a programme which gradually increases temperature of the furnace or thermogravimetric analyser until the 550 °C is achieved although care shall be taken to ensure ash loss does not occur.

Maintain the sample in the furnace or thermogravimetric analyser for at least 1 h at 550 °C until all the carbon has disappeared from the dish. For electrical furnace (5.3), allow the dish to cool in the desiccator (5.4) to the temperature of the balance room and weigh to the nearest 0,1 mg.

Repeat the operations of heating in the electrical furnace (5.3), cooling and weighing, until the mass remains constant to within 1,0 mg or begins to increase. Record the minimum mass.

Guidance on typical ashing times is provided in Table A.1.

TGAs continually weigh samples throughout the incineration of the sample. The constant weight criteria may be via a fixed or relative mass change within a defined time period. The criteria selected should achieve a similar outcome to the constant weight criteria for the electrical furnace (5.3).

The ash of the sample, w_a, as a percentage by mass, is given by Formula (1):

(1)

where

m₀ is the mass, in grams, of the test portion (8.2);

m₁ is the mass, in grams, of the dish and residue (8.4);

m₂ is the mass, in grams, of the prepared dish (8.1).

Calculate the ash to the nearest 0,01 g/100 g by mass and report the final result to the nearest 0,1 g/100 g for dried milk products and to the nearest 0,01 g/100 g for liquid milk.

Details of the interlaboratory test of the precision of the method are summarized in Annex B. The values derived from the interlaboratory test may not be applicable to analyte concentration ranges and/or matrices other than those given in Annex B.

The absolute difference between two 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 not in more than 5 % of cases be greater than:

a) a mass fraction of 0,23 g/100 g for dried milk products;

b) a mass fraction of 0,04 g/100 g for liquid milk.

The values are:

The absolute difference between two single test results, obtained using the same method on identical test material in different laboratories with different operators using different equipment, will not in more than 5 % of cases be greater than:

a) a mass fraction of 0,34 g/100 g for dried milk products;

b) a mass fraction of 0, 11 g/100 g for liquid milk.

The values are:

1. 
   (informative)
   
   Ashing time
   1. Typical ashing times required to achieve constant weight

Table A.1 — Ashing time

NOTE these times are indicative, laboratories should determine appropriate conditions for their equipment and furnace loading.

• 1. Example programme for charring and ashing with an automated furnace

Table A.2 — Ashing programme

• 1. Example programme for charring and ashing with an automated furnace TGA

Table A.3 — TGA programme

Step 1 can be omitted if liquid samples are not being tested.

Step 3 is concluded when the constant weight criteria is achieved.

1. 
   (informative)
   
   Precision data

The data in Table B.1 were obtained in an international interlaboratory study organized in 2025 by Fonterra (International Dairy Federation, IDF, 2025)^[1] in accordance with ISO 5725-2^[2] and the AOAC-IUPAC Harmonized Protocol for collaborative study procedures to validate the characteristics of this method of analysis^[3].

Table B.1 — Precision data

Bibliography

[1] International Dairy Federation (2025) Report title (Bulletin of the IDF no xxx), in press

[2] ISO 5725‑2, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method

[3] AOAC INTERNATIONAL. AOAC Official Methods Program, Associate Referee’s Manual on development, Study, Review, and Approval Process. Part IV AOAC Guidelines for Collaborative Studies, 1995, pp. 23–51

1. The Leco TGA710 is an example of a suitable commercially available thermogravimetric analyser. ↑

2. CEM Corp. Glass fibre sample pads (part# 200150/300150) are commercially available examples. ↑