Method of Analysis Library
Determination of Dispersibility for Instant Nonfat Dry Milk
v 2.0 | Effective 10/30/2023
1.0 Purpose
This Analytical Method defines the ADPI standard operating procedures for determining the dispersibility of instant nonfat dry milk according to the modified Moats-Dabbah method.
2.0 Scope
This SOP is applicable to the determination of solubility index of instant nonfat dry milk. The hydrometer readings corresponding to percent of the milk dispersed at a given temperature are specific to nonfat dry milk and are not suitable for the quantitative determination of dispersibility in other dairy powders.
3.0 Definitions
| 3.1 | Instant nonfat dry milk is nonfat dry milk which has been produced in such a manner as to substantially improve its dispersing and reconstitution characteristics over that produced by the conventional processes. Instant nonfat dry milk shall not contain dry buttermilk, dry whey, or products other than nonfat dry milk, except that lactose may be added as a processing aid during instantizing. Instant nonfat dry milk shall not contain any added preservatives, neutralizing agents, or other chemicals. |
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| 3.2 | Dispersibility is the degree to which the powder will readily and evenly distribute into another substance such as water, under a given set of conditions. Dispersibility is distinctly different than solubility and also wettability and should not be used interchangeably with these terms. |
| 3.3 | Specific gravity is the ratio of the density of a liquid to the density of water, at the same specified temperature. |
4.0 Principle
Instant nonfat dry milk is mixed in a predetermined ratio with water at controlled temperature, and the resulting dispersion is subjected to screening using specific standard testing sieves. Specific gravity of the filtrate is measured using a hydrometer, and the specific gravity reading is directly proportional to the percentage of the milk powder which is dispersed. Hydrometer readings are corrected for temperature and also for any observed deviation from a control preparation of completely dissolved nonfat dry milk.
5.0 Reagents and Materials
Adhere to the following requirements carefully for consistent and accurate results.
| 5.1 | Laboratory balance, with capacity of approximately 100 grams and with sensitivity of ± 0.05 gram or better; |
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| 5.2 | Water bath, or equivalent, capable of holding a consistent temperature of 25°C, for heating the sediment-free water as described below; |
| 5.3 | Water, sediment-free, distilled or efficiently filtered, at a temperature of 25°C ± 1°C; |
| 5.4 | Graduated cylinder, 250 mL; |
| 5.5 | Teaspoon, or equivalent; |
| 5.6 | Sieves, Tyler, 8” diameter, 60 mesh, 100 mesh, and pan; |
| 5.7 | Towels, or equivalent, suitable for the preparation of the sieves and pan as described below; |
| 5.8 | Pipette, or equivalent, suitable for adding the defoaming agent in a dropwise manner; |
| 5.9 | Defoaming agent, “Antifoam B” emulsion, item #A 5757 (Nelson-Jameson; Ecolab, Inc.; or equivalent); |
| 5.10 | Funnel, 150 mm diameter, or equivalent; |
| 5.11 | Hydrometer, precision, specific gravity scale, range 1.0000 – 1.0070, in 0.0005 subdivisions, Fisher #11-555 or equivalent; |
| 5.12 | Thermometer, centigrade scale, suitable for measuring temperature at and around 25°C, accuracy ± 0.5°C; |
| 5.13 | Laboratory timer, or equivalent, suitable for measuring the 3-minute standing interval as described below; |
| 5.14 | Pipette, aspirator, or equivalent, suitable for drawing off any foam that may be present in the graduated cylinder during reading of the hydrometer, as necessary. |
6.0 Personal Safety Precautions
In all cases, the practitioner company’s internal policies and procedures regarding personal safety supersede the following ADPI recommendations:
| 6.1 | Milk (dairy) is globally classified as an allergen and should be properly handled with personal safety needs in mind. |
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| 6.2 | Read and understand all precautions for safe handling and disposal shown in the various reagents’ Safety Data Sheets (SDSs), including use of any prescribed Personal Protective Equipment (PPE). |
| 6.3 | Dairy ingredients are foods and as such are exempt from U.S. requirements regarding Safety Data Sheets (SDSs), where ingredient-specific safe handling instructions would be provided. Despite this exemption, many dairy ingredients are manufactured and marketed in powder form, and powders should be recognized as potential physical irritants, such as to the eyes, nose, and if inhaled. |
| 6.4 | Some testing apparatus described above may be susceptible to breakage, therefore be aware of associated personal risks. Inspect apparatus before use and replace any items which are compromised. |
7.0 General Procedural Considerations
Because visual interpretation of the hydrometer reading is the means by which the specific gravity reading is ultimately made, good results depend on adequate lighting.
Avoid common mistakes such as parallax when making the hydrometer reading. Use good technique for volumetric glassware; hold up the graduated cylinder with the interface between the milk layer and the hydrometer scale at eye level.
8.0 Sampling Procedure
As often as necessary to ensure the continued accuracy of the dispersibility determination, check the hydrometer reading with fully dissolved nonfat dry milk as described below:
| 8.1 | Accurately weigh 26 g ± 0.05 g of nonfat dry milk in a suitable container. |
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| 8.2 | Completely dissolve the nonfat dry milk in 235 mL ± 0.1 mL of water at 25°C ± 1°C. |
| 8.3 | Transfer the reconstituted milk to a graduated cylinder. |
| 8.4 | Place the hydrometer and the thermometer into the graduated cylinder. |
| 8.5 | Record the temperature, confirming that it remains at 25°C ± 1°C. |
| 8.6 | Remove the thermometer and read the specific gravity via the hydrometer. |
| 8.7 | Consult the specific gravity table to confirm that the reading corresponds to 100% dispersion. |
| 8.8 | If the reading agrees with 100% dispersion, no correction factor is required. Proceed to the dispersibility determination for the sample. |
| 8.9 | If the reading does not correspond to 100% dispersion, calculate the necessary correction factor as follows: |
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Example:
9.0 Sieves and Pan Preparation
| 9.1 | Wash the sieves and pan and rinse them thoroughly with water. |
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| 9.2 | Lightly wipe the sieves with a towel to remove all excess water. |
| 9.3 | Dry the pan completely with a towel. |
| 9.4 | The sieves are now ‘wet’ and are ready for immediate use, stacked in the order of the 60-mesh sieve, followed by the 100-mesh sieve, followed by the pan. |
10.0 Dispersibility Determination
| 10.1 | Accurately measure 235 mL of water at 25°C ± 1°C into a 600 mL beaker. |
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| 10.2 | Accurately weigh 26 g ± 0.05 g of instant nonfat dry milk. |
| 10.3 | Add the milk powder to the beaker and immediately begin stirring with a teaspoon, moving the spoon briskly back and forth across the diameter of the beaker 25 times in 15 seconds, counting each time the spoon returns to its starting position in the beaker (i.e., across and back equals one “time”). |
| 10.4 | Pour the contents of the beaker uniformly over the surface of the ‘wet’ stack of sieves and pan, allowing for a total drainage time of 1 minute 30 seconds, as follows: |
| a. Drain for 15 seconds. | |
| b. Lightly rotate the stack and drain for another 15 seconds. | |
| c. Lightly rotate the stack and drain for another 15 seconds. | |
| d. Lightly rotate the stack and drain for another 15 seconds. | |
| e. Separate the stack of 2 sieves from the pan, tilting the sieves over the pan to encourage additional drainage into the pan, collecting any additional drainage for another 30 seconds. | |
| 10.5 | Add 3 drops of defoaming agent to the contents of the pan. |
| 10.6 | Using the funnel, quantitatively transfer the contents of the pan to a 250 mL graduated cylinder. |
| 10.7 | Add additional water at 25°C ± 1°C to bring the total volume of the graduated cylinder to the 250 mL mark. |
| 10.8 | Pour the contents of the graduated cylinder back into the beaker, and then back into the graduated cylinder. |
| 10.9 | Again pour the contents of the graduated cylinder back into the beaker, and back into the graduated cylinder. |
| 10.10 | Place the thermometer and the hydrometer into the graduated cylinder. |
| 10.11 | Allow the graduated cylinder to stand for 3 minutes. |
| 10.12 | If foam is present on the surface of the milk, draw it off using a pipette or similar. |
| 10.13 | Confirm that the temperature of the milk is 25°C ± 1°C and record the temperature to the nearest degree Centigrade. |
| 10.14 | Remove the thermometer from the graduated cylinder and take the specific gravity reading of the milk as the first mark visible above the milk’s surface. |
| 10.15 | Proceed to 11.0 to interpret the results. |
11.0 Interpretation and Reporting of Results
| 11.1 | If a correction factor was determined necessary in section 8.0 above, subtract that correction factor from the specific gravity of the sample preparation; if no correction factor was required, use the raw specific gravity reading for the sample preparation. |
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| 11.2 | Consult the dispersibility table (see the Appendix) for the percentage dispersed that corresponds to the recorded sample temperature and to the specific gravity reading for the sample preparation at that temperature, corrected (if necessary) as described above. |
| 11.3 | Report the result to the nearest whole percent, rounding up any values from the dispersibility table that are 0.5% or higher. |
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Examples: Report a table value of 74.2% as 74%. Report a table value of 79.5% as 80%. Report a table value of 93.7% as 94%. |
12.0 External References
No external references are cited.
13.0 ADPI Document Linkages
Appendix
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