Reconstitution of Powdered Milk Replacer - Particle Dispersal Test Methodology

Objective of the dispersal test


The objective of the dispersal test conducted by WildAgain was to measure the efficiency of the various powdered milk replacers in the initial stages of the reconstitution process (wetting, sinking and dispersing). These types of tests performed by scientific labs and commercial powdered milk manufacturers employ extremely powerful and expensive equipment to conduct and measure such efficiencies. Lacking such equipment, WildAgain used a set of sieves to determine how much of the powdered milk had been dissolved under different time periods, and what percent of any undispersed milk particles would be retained (trapped) by the sieves when reconstituted milk was poured through them.



The testing procedure followed the following steps:﷯


1.) Two 15 gram samples of a dry milk replacer powder were weighed on a gram scale.


2.) Two pint glass jars were each filled with 60 grams of 110°F water.


3.) One powder sample was poured into each glass jar. Each jar was allowed to rest for 5 minutes to observe wetting and sinking of the powder, if any.


4.) The two jars were then stirred/mixed for 1 minute each with a small whisk until no solid particles were visible.


5.) One jar was capped and placed in a 40°F refrigerator for 8 hours to rest.


6.) The three sieves (shown above) were initially weighed when dry. The content of the other jar was immediately and very gently poured through the three sieves that were stacked in the order of 500µm sieve on top; 250µm in the middle; and the 125µm sieve on the bottom.


7.) The top sieve was sprayed with cold water to break the surface tension (see below) of the milk, such that only wet, solid particles were retained in the sieve. The top sieve was removed from the stack, and cold water sprayed through the middle sieve to break the surface tension of any liquid such that only wet solid particles would be retained in the middle sieve. Finally, the middle sieve was removed from the stack and the cold water spray was administered to the bottom sieve, allowing only wet particles <125µm to pass through the bottom sieve.


8.)The three sieves were then placed in a forced-air dehydrator at 135°F until the retained particles in each sieve were dry and had attained a constant weight. Each sieve was weighed on the gram scale (residue + dry sieve) and the weight was recorded. Photos were taken of the residue retained in each of the sieves (wide perspective and close-up).


9.) The sieves were thoroughly washed of any milk powder residue and placed in the dehydrator at 165°F until thoroughly dry. Each sieve was then weighed to insure the initial dry weight was the same (without any moisture or residue).


10.) After 8 hours from mixing, the previously refrigerated jar of reconstituted milk was removed and stirred/mixed with a small whisk until no solid particles were visible. It was then heated to approximately 110°F in a microwave. [Note: The use of the microwave was for test purposes only, and not recommended for use to heat or re-heat formula to be fed.]


11.) For this second jar, steps 6 - 9 were repeated.


This series of steps was then conducted for each of the individual powdered milk replacer products. Additionally, two samples were created and tested by blending two different dry milk powders together in equal parts, for purposes of step 1.) above (using equal amounts of 7.5 grams of each powder). An example of this was an equal part blend of 7.5 grams of Zoologic®33/40 and 7.5 grams of Fox Valley 32/40.



Sieve sizes


The sizes of the sieves used were chosen to be functional when testing the performance of a dense milk liquid while at the same time able to provide sufficient retention in order to dry and measure different sized solid milk particles. The gross dimensions of the sieves are 8" top opening diameter and bottom mesh diameter at 7", which allows for stacking of the sieves. The image below is a close-up perspective of the mesh sieves of the three sieves. The diagram following the is an accurate relative size comparison of the size particles retained by the sieves as compared to the target size of <2µm (white dot) - 10µm (dark green area) for a fully reconstituted liquid milk.

Surface tension﷯


As mentioned above, the testing procedure added a step of spraying cold water through the sieve to correct for the surface tension of the liquid milk. So what is 'surface tension'?


"Surface tension in water owes to the fact that water molecules attract one another, as each molecule forms a bond with the ones in its vicinity. At the surface, though, the outermost layer of molecules, has fewer molecules to cling to, therefore compensates by establishing stronger bonds with its neighbors, this leading to the formation of the surface tension." (USGS and Georgia State University) 

In other words, left to its own devices, a water droplet would form a perfect sphere in a zero gravity environment. Except for the element Mercury (Hg), water has the highest surface tension of any liquid. As shown in the image at right, a drop of water has little difficulty resting comfortably on the dry surface of the wire mesh of the 125µm sieve.


This also means that liquids with high water content will have high surface tension properties. The powdered milk replacers examined in this reconstitution test have a high water content of between 82-84%, so it stands to reason that they will also display a high surface tension. In fact, this is the case and why the cold water spray was necessary to break that liquid surface tension, to allow the milk to flow through the sieves, to then reveal only the retained solid particles at each sieve size. 


Confused? A little too much physics? Perhaps a couple of images will help demonstrate this physical characteristic visually. The two images below are an equal part blend of Zoologic®33/40 and Fox Valley 40/25 after resting for 8 hours. The image on the left is immediately after the formula had been gently poured onto the mesh of the top sieve (500µm). It appears very cream-like and easily rests on top of the mesh. No visible solid particles. The image on the right is the same sieve with the layer of milk after being sprayed with cold water. Though few solid particles are retained by the sieve that exceed 500µm in size, some are found to be present, once the surface tension of the liquid is broken, and the milk flows through to the next sieve.

References (not intended as an exhaustive list)


Anema, S. G., D. N. Pinder, R. J. Hunter, and Y. Hemar. Effects of storage temperature on the solubility of milk protein concentrate (MPC85). Food Hydrocolloids (2006) 20:386-393.


Baldwin, Alan J., Fonterra Research Centre, Palmerston, NZ. Insolubility of milk powder products- a minireview. Dairy Science Technology (2010) 90:169-179.


Boiarkina, I., N. Depree, W. Yu, D. I. Wilson, and B. R. Young. Rapid particle size measurements used as a proxy to control instant whole milk powder dispersibility. Dairy Science and Technology (2017) 96:777-786.  

Forny, Laurent and Stefan Palzer (Food Science and Technology Department, Nestlé Research Centre, Vers-Chez-Les-Blanc, CH-1000 Lausanne, Switzerland). Wetting, disintegration and dissolution of agglomerated water soluble powders. Conference Paper · June 2009

Ilari, Jean-Luc and Laila Mekkaoui. Physical properties of constitutive size classes of spray-dried skim milk powder and their mixtures. Le Lait, INRA Editions, 2005, 85 (4-5), pp.279-294. hal-00895603


International Diary Federation. In determination of insolubility index, Standard 129A. Brussels, IDF (1988)


Pugliese, Alessandro, Giovanni Cabassi, Emma Chiavaro, Maria Paciulli, Eleonora Carini, and Germano Mucchetti. Physical characterization of whole and skim dried milk powders. Journal of Food Science Technology (2017) 54(11):3433-3442.


Sharma, Anup, Atanu H. Jana, and Rupesh Shrikant Chavan. Functionality of milk powders and milk-based powders for end-use applications - a review. Comprehensive Reviews in Food Science and Food Safety (2012) 11:518-528.


USGS (source: Georgia State University). Surface tension and water. Water science school.

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