Reconstitution of powdered milk replacer products



In addition to other testing done by WildAgain (in-house or through an independent lab), tests were performed to assess a macro-level reconstitution efficiency of some of the powdered milk replacer products. The purpose of this type of test is to address the following primary questions:



1.) Can full and complete reconstitution be achieved by following the product labeling mixing instructions?


2.) Is reconstitution affected by different variables, such as water temperature and/or length of resting time after mixing?


3.) Do the various products all reconstitute at the same rate and with the same efficiency?


4.) Based on test results, are there procedural steps to maximize reconstitution outcomes?



Basic premise


Rehabilitators work hard to provide optimum diet and nutrition to the animals in their care. This is especially true for neonates whose digestive systems are still in the formative stages, and those who may have arrived for care in a less than fully healthy condition. The success of substitute milk formulas, serving as the sole diet and source of nutrition for neonates, is critical in terms of its nutritional composition and digestive effectiveness. While it is possible to construct a formula recipe that comes very close to a proportional match to mother's milk (in terms of total solids, proteins, fat and kcals) using the nutritional calculator, the ultimate digestibility of the formula being fed is largely dependent on the quality of the products used as well as a high level of reconstitution of the dry ingredients.


Mother's milk is about as good as it gets for a given species, as it has evolved over many years to be highly digestible by the nursing young. Without measuring the milk particle size of each individual species' milk, it is likely to assume that it is free of any extraneous aggregation or clustering of large milk particles beyond what what would be considered normal levels.

To put this into proper perspective, consider a quick look at the 'normal' size distribution of the particles that comprise milk. The image at left shows the relative distribution of particle sizes in milk with different levels of fat. As shown, since fat globules have a larger size than the casein micelles, milks with a higher fat content will have a higher percentage distribution of larger particles. This also suggests that most milk contains particles sized less than 1-2µm, with only a few fat globule clusters approaching 10µm in size. [For reference, to appreciate this small size, auto emission exhaust is in the range of 1-3µm. Particles of this size are commonly measured and estimated using forward lobe light scattering and laser diffraction techniques.]


Since WildAgain does not have access to such sophisticated equipment, or the resources to engage such testing, our tests at a more macro level were designed to simply quantify the amount of milk powder material that did not reconstitute enough to successfully pass through a 1,000µm mesh. Certainly with 1,000µm being one thousand times larger than an average milk particle, the expected result would be that all of the reconstituted liquid would easily and completely pass through the mesh. The tests were also designed to simulate typical mixing conditions and technique that would be used by a rehabilitator in following the instructions as specified on the package labeling.



Reconstitution versus 'dissolving'


For purposes of this discussion, the term 'reconstitution' refers to the process of recreating, as complete as possible, the original liquid state of the milk before it was dried into a powdered form. In that process, not everything will be 'dissolved', or go into a complete aqueous solution when mixed with water. Whole fresh milk is actually a mix of three components: 1.) fat globules of different sizes suspended in an aqueous medium as a natural emulsion (such as a vinaigrette dressing, since oil (fat) is not soluble in water); 2.) proteins in a suspended colloidal state (not unlike a slightly runny Jello); and 3.) carbohydrates in true solution (such as a transparent and clear dissolved mix of Gatorade). The main purpose of making of dry milk powder is to convert this liquid perishable raw material into a product that can be stored without spoilage and substantial loss of quality when properly stored. The challenge of the rehabilitator is to reverse this process by transforming the powder back to a liquid state, aka reconstitution.



Test design and methodology


Basic design


Based on the first three test purposes mentioned above, the following describes the assumptions and overall test design.


Products were chosen from different manufacturers and with different protein/fat formulations.


Each product would be mixed with water at a ratio of 1 part powder to 2 parts water, as suggested by the product labeling (the 'mix').


Two temperatures of water (100°F and 160°F) would be used to construct a separate mix for each product.


Each mix would be tested at different time intervals from initial mixing (at time of mixing ('instant'); after 8 hours; and after 12 hours).


Four samples would be prepared for each mix.


Each mix would be prepared following the normal order of rehydration steps (link).


A total of 12 products were selected resulting in 288 samples to be tested, requiring an elapsed time of 6 days.




Prior to testing, paper worksheets were prepared to record weights and observation notes during testing of all samples. Centrifuge tubes (Celltreat®, 15 ml, .5 ml graduation with conical bottom, flat surface (HDPE) leak-proof seal caps) were labeled to uniquely identify each sample. Each empty tube and cap were weighed (0.00gr) and recored on the worksheets.


Three sets of four products each would be tested over a 2-day period.


Each product was brought to room temperature (70°F) prior to mixing with water.


Based on the previously calculated weight for the product (Lab spreadsheet), two 2 Tbsp samples were weighed for each product (in grams).


A 60 gram amount of water was placed in a glass jar for each water temp (100°F and 160°F). Upon mixing, this would provide a volume of  approximately 130-140 ml for each product/water temp mix for testing (a total of 120 ml would be needed to provide 4 samples to be drawn at three different time intervals).


Two Tbsp of dry powder of each product was added to each of the two jars of 60 grams of water and allowed to 'wet" for 5 minutes prior to stirring.


Each jar was then stirred with a small wire whisk for 60 seconds, while minimizing incorporating any unwanted air. The sides of the jar were constantly scraped with the whisk to incorporate a maximum amount of dry powder into the liquid. Obvious clumps of powder were given added attention to visually achieve as complete a reconstitution as possible.


For the first time interval ('instant' mix), four 15 ml centrifuge tubes were each exactly filled with 10 ml of each mix and capped. Each filled tube was weighed (0.00gr) and recorded. The remaining volume of mix was refridgerated.


The four tubes were placed in low-speed centrifugation (1,000rpm, 450g RCF) for 10 minutes using a 800-1 bench-top centrifuge.


The tubes were removed from the centrifuge. The plastic caps were removed and recapped with 1,000µm cloth mesh with elastic fasteners. The tubes were inverted to allow the supernatant to drain and be discarded. The tubes were tapped several times to remove as much liquid as possible, with the mesh restraining any of the precipitant over 1,000µm in size.

The mesh was removed, scraping any solid material back into the tube, and each tube was placed upright in a Celltreat® plastic tray.


The tubes were placed in a forced-air dryer at 135°F and checked after 12 hours. Any material that had subsequently liquified was removed. The tubes were checked ever 6 hours until all remaining material was totally dry and moved freely when the tube was agitated


Each tube was recapped, weighed, and weights recorded.


For each of the two successive resting time intervals (8 hours and 12 hours), four tubes were again filled as described above 1 hour prior to centrifugation to allow the samples to attain room temperature. After another 60 seconds of stirring, the centrifugation and following steps as described above were then performed.


Efficiency calculation


The weights recorded above were transferred to a previously created spreadsheet that calculated the percent of the 10 ml sample that was attributable to the powder prior to centrifugation, draining and drying. The weight of the dried precipitant (taking into account the starting and ending weights net of the weight of the tube+cap) was then used to calculate the percentage amount of material that did not reconstitute.



Summary results of the tests

The table at right summarizes the best (green) and worst (red) reconstitution outcomes based on the tests described above and calculations performed. As shown, the results vary by product in that one single water temperature or one single resting period did not appear optimum across all products. Instead, some products reacted much more favorably to a given water temp or resting time period. This is a change from WildAgain's earlier round of testing in 2010 that suggested that the hotter water (160°F) and longer resting time (>4 hours) seemed to provide the best result. It should be noted the above table of results is only for the limited number of lots tested, and although provided mostly repeatable and consistent data values, additional test runs could provide a different set of results.


In addition to the table above and more in-depth discussion provided below (please take time to read!), a page on the website is also provided for each product. These pages include a more thorough discussion of the test results for each product and how they performed under each of the different test conditions. The page also includes a plot that shows visually how the products scored over the 4 samples tested for each condition.Click on the product name at right to access those pages.In addition to the table above and more in-depth discussion provided below (please take time to read!), a page on the website is also provided for each product. These pages include a more thorough discussion of the test results for each product and how they performed under each of the different test conditions. The page also includes a plot that shows visually how the products scored over the 4 samples tested for each condition. Click on the product name at right to access those pages.



More thorough discussion of findings (and suggested procedural steps!)


Rehydration process


Wetting - differences by product .  (powder to water)

Sinking/dispersal .    ""



Reconstitution differences between products


product specific

more sensitive to water temp/less time .  (Powder temp)


Incomplete reconstitution .  (≈90%)





Match water temp to product

If blending, recon separately. Then mix liquids together, including any other ingredients such as HWC

Adjust recipe powder amounts (+10%)

Strain formula prior to feeding



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