WildAgain Wildlife Rehabilitation, Inc.

(Excerpted from: Manufacturing Changes for Esbilac® Powder Affect Wildlife Rehabilitators. 2010.
 Allan M. Casey, III and Shirley J. Casey, WildAgain Wildlife Rehabilitation, Inc., Evergreen, CO)          


Download the Full Report (PDF 2.7 MB)

or the Short Report (PDF 1.5 MB).

Question 4 - Has the solubility of Esbilac® powder changed? Answer: Yes, significantly!


PetAg responded to inquiries regarding solubility of the ‘new’ Esbilac® powder by suggesting that the product be mixed with hot water (instead of warm) and stirred a little longer. To date, the Esbilac® label has not been updated with this new information and continues to read: “gently stir or shake into warm water.”

Rehabilitators accustomed to following those instructions were unlikely to have changed their established mixing practices, since they had not been told to do so. Several rehabilitators confirmed this. Some followed the mixing instructions on the label, added extra fats for a particular wildlife species as needed, such as Multi-Milk® or heavy whipping cream, and promptly fed the ‘fresh’ formula. They had used this method successfully for many years.

Others prepared the milk replacers by stirring very hot water into the powder, adding extra fats and, sometimes, yogurt - and then letting the formula rest overnight. A closer look showed that rehabilitators in this second group seemed to have rare - if any - unexplained gastrointestinal problems. This prompted a much closer look at this issue, and suggested that solubility might be a key problem with the new powder. More information on the potential problems that develop secondary to feeding formula that is not fully rehydrated and dissolved in solution will be discussed later.


Understanding how drying method can affect solubility

Commercial single-step spray-drying operations can use a hot gas, such as air, to dry a liquid into a powder. Spray drying is a common technique with highly perishable foods, such as milk products, proteins, and vegetables. Removing the water from the food product helps with preservation and storage, and prevents the growth of micro-organisms. The water on the surface of the particles evaporates during the drying process, creating a shell on the particles. The ‘shell’ is often protein, with other ingredients, such as fats and minerals, contained inside. The ‘hardness’ of the shell of the particles is determined by many factors: steam, temperatures, equipment, drying time, and more.

PetAg acknowledged that it had changed from the multi-step drying to the single-step spray-dry process. PetAg stated that it was confident in the manufacturing method since it had used the single-step spray-dry with its Zoologic® Milk Matrix products since the early 1990’s.

Food science publications and professionals who described the manufacturing process to us indicated that single-step spray-dried powders do not easily disperse or dissolve in water. The ‘hardness’ and permeability of the exterior shell on the powder particles influences the rehydration and dissolving processes, as described in the box to the right. Such powder particles may stick together and become lumpy during rehydration of that ‘shell’. Some companies have added another step in the drying process, called ‘instantizing’ or agglomeration, to make the powders more soluble and easier to mix. 

While the previous Esbilac® powder made with the multi-step drying resulted in a quick-mixing product, the ‘new’ Esbilac® powder made with a single-step spray-dry method has proven to not be quick mixing. This suggests that difficulty in dissolving the ‘new’ Esbilac® powder may be a factor in gastrointestinal problems in small wild mammals.


Solubility tests

WildAgain began a series of solubility tests as described in Appendix D below. The tests were designed to simulate how rehabilitators mix the Esbilac® powder into formula to be fed to infant and juvenile wild mammals. Since solubility had been brought into question, the primary objective of the tests was to determine if visible separation of the product was evident after a short period of time. The effects of different combinations of variables were assessed as follows:

Figure 6. Esbilac® powder - Amount of visible separation after 30 minutes.
Figure 7. Esbilac® powder - Amount of visible separation - 'New' versus 'Old'.


-   Temperature of product at time of mixing (70°F room temp; 40°F refrigerator; 0°F freezer)

-   Temperature of water used to mix (125°F hot tap; 175°F from instant hot dispenser)

-   Hydration time (immediate after mix; after 4 hours; after 8 hours)

-   Type of supplements (MultiMilk®; heavy whipping cream; yogurt)


Test #1 – Mix product according to labeling instructions. Result: Noticeable separation.

The purpose of this test was to determine how the product performed when mixed according to the label on the can, and to determine the extent of any noticeable separation. For the first 5 minutes, the formula appeared to be in solution, much like the prior quick mix Esbilac® powder had performed (Figure 6). But after about 10 minutes, there were three distinct layers of separation:

Above the 7.5 ml line, a cap developed on the top that appeared to be powder that never wetted and thus never began to dissolve. This cap had formed a seal that had to be broken up to allow the liquid to be emptied from test tube after 30 minutes.

Between 7.0 and 7.5, a semi-translucent layer developed, perhaps vegetable oil in the product that was beginning to liquefy.

Below 7.0 the liquid appeared to be a very dilute milk solution.

As a point of reference, the photos in Figure 7 show the relative visual difference in solubility comparing the new and old Esbilac® powder. As can be seen, the old, quick mix Esbilac® powder easily reconstituted and remained in a homogeneous solution for the time period indicated.


Test #2 – Determine if product temperature affects solubility. Result: Yes.

Figure 8. Esbilac® powder - Amount of visible separation and residue based on product temperature.

The purpose of this test was to determine if the temperature of the product at the time of mixing had a material effect on solubility as measured by visible separation of the product after a short period of time.

The initial tests were to be performed using product at three different temperatures, including room (70°F), refrigerator (40°F), and freezer  (0°F) temperatures. While the first comparison of room temperature and refrigerator temperature did show a slight difference as shown in Figure 8, it was considered insignificant. Since it is assumed that most rehabilitators store the product in the refrigerator immediately prior to use as is suggested on the product label, and product temperature does not seem to be a significant variable, all remaining tests were performed using the product at 40°F.

The photos in Figure 8 shows a predictable and very slight reduction of solubility (two tubes on the left) using the product at refrigerator temperature (40°F) which is 30ºF cooler than using the product at room temperature. The two tubes on the right show the residue left after emptying the tubes after 20 minutes. No discernable difference of the amount of residue was noted.


Test #3 – Determine if water temperature affects solubility. Results: Yes.

Figure 9. Esbilac® powder - Amount of visible separation based on water temperature.

The purpose of this test was to determine if the temperature of the water at the time of mixing had a material effect on solubility as measured by visible separation of the product after a short period of time.

Tests were performed using two different temperatures of water. The first was hot tap water measured at 125°F and the second was water from an instant hot water dispenser measured at 175°F.

The product was tested in this manner mixed 1:2 with the water. Additionally, the base formula also was tested with the addition of MultiMilk®, and separately with the addition of heavy whipping cream plus yogurt.

As shown in Figure 9, the water temperature did have a noticeable influence on visible separation when using the new Esbilac® powder as a quick mix. The hotter water temperature (175ºF) resulted in less visible separation as measured by the formation of the cap at the top of each tube. In WildAgain tests, the cap that formed at the top again acted as a seal that would not allow the liquid to be emptied from the tube after 20 minutes, even with the hotter water.

During testing, the temperature of the mixed formula was measured immediately after the 30 seconds of stirring, and prior to putting the formula into the graduated cylinders for observation. When using hot tap water (125ºF), the formula had cooled 30ºF on average to around 95ºF. When using the instant hot water (175ºF), the formula had cooled almost 60ºF to approximately 115ºF. A more complete discussion of this test result and the implications for mixing formula are included in Appendix D below.


Test #4 – Determine if hydration time affects solubility. Result: Yes, significantly.

Figure 10. Esbilac® powder - Amount of visible separation based on water temperature.
Figure 11. Esbilac® powder - Amount of visible separation based on water temperature.

The purpose of this test was to determine if the amount of time allowed for the product to reconstitute from powder form to a liquid solution had a material effect on solubility as measured by visible separation of the product after various periods of time.

Tests were performed at three different time intervals after mixing the product with water. The first was immediately after mixing. The second time period was 4 hours after mixing. The third condition allowed rehydration for 8 hours. For the longer two time periods, the formula was mixed and then placed in the refrigerator (40°F). Prior to filling the test tubes, the formula was removed from the refrigerator, stirred slightly, and then reheated in a microwave to approximately 100°F to simulate feeding temperature.

As shown in Figure 10, the two longer time periods produced no visible separation of the formula, suggesting that longer hydration time has a significant effect on the ability of the powder to reconstitute into a complete liquid solution with no observable unwetted or undissolved powder.

Figure 11 displays this same result of no unwetted cap at hydration times of 4 and 8 hours. It also shows the relative unreconstituted nature of the formula when used as an immediate mix, expressed in terms of the percent of test tube volume comprised of the unwetted powder. The darker shades of red indicate relatively higher degrees of powder that had not wetted and dispersed into a reconstituted formula.






Test #5 – Determine if microscopy confirms beneficial effect of longer hydration. Result: Yes.

Figure 12. Esbilac® powder - Microscopic view of reconstituted powder at 1:2 (400x).
Figure 13. Esbilac® powder - Microscopic view of reconstituted powder at 1:2 (1,000x).

The purpose of the next test was to determine if closer visual examination using differential interference contrast (DIC) microscopy would confirm the beneficial effects of allowing for longer hydration time, as observed in the test tube trials.

Product and formula samples were taken to the biochemistry lab of a local university to perform the tests. A Leica DM LB microscope that could produce jpg images was used. Images were taken at 400x and 1,000x magnification (Figures 12 and 13).

As shown, it was very apparent at a microscopic level that longer hydration times do, in fact, provide sufficient time for the powder to sufficiently wet and disperse into a homogeneous solution with minimal observable undissolved particles. The photo to the left also shows the relatively undissolved nature of the powder when attempting to use as a quick mix product. The right side images in the photos show the prior Esbilac® powder as a reference point.

In sum, WildAgain’s solubility tests show that the Esbilac® powder made with the single-step spray-dry method rehydrates and dissolves differently than the former Esbilac® powder.









Appendix D - Solubility Test Methodology

The equipment used for the solubility test is shown in the photo at right, which includes all of the following:


-          Esbilac® powder

-          Red scoop from Esbilac® can

-          Small glass jars*

-          Small wire whisk*

-          Digital kitchen thermometer*

-          10 ml graduated glass cylinders**

-          Small plastic funnel**

-          Test tube cleaning brush**


* Baby food jars work just fine and are easy to clean.

The whisk and thermometer are available at most kitchen stores.

** Available from Home Science Tools. Graduated 10 ml test tubes could be used, but these cylinders are easier to use since they have a free-standing base.


Test Methodology


Variables to assess

The tests were designed to visibly measure the affects of using different combinations of the following and as shown in the matrix at right:

- Product temperature at time of mixing

    (room temp; refrigerator; freezer)

- Water temperature at time of mixing

    (hot tap 125ºF; instant hot 175ºF)

- Hydration time (immediate; 4 hrs.; 8 hrs.)

- Supplemental additives

    (MultiMilk®; USDA heavy whipping cream

     >36% fat; fat-free yogurt)

Note: Only the tests shaded in yellow were performed. After conducting the first two series of tests comparing different product temperatures (at 70ºF and 40ºF), it became apparent that this variable had no visible affect on solubility. Further tests involving the product at 70ºF or at 0ºF (freezer) were not performed. For product used directly from storage in the freezer, tests of product at 0ºF may be of value.


For the immediate mix trials:

1.)     Esbilac® powder was scooped into the small end of the red scoop provided in the Esbilac® can as shown above.

2.)     The powder was placed in a small glass jar and mixed 1:2 with either hot tap water (125ºF) or with much hotter water (175ºF) from an instant hot water dispenser. The temperature of the water was measured with the digital thermometer prior to mixing.

3.)     The formula was stirred for 30 seconds using the small wire whisk shown above.

4.)     The temperature of the formula was measured using the thermometer and recorded.

5.)     The formula was poured into a clean and dry graduated cylinder using the small plastic funnel shown above, filling to the 10 ml mark.

6.)     The cylinder was then placed in front of a tripod mounted SLR camera with a close-up lens.

7.)     Photos were taken immediately and then at 5, 10, 15, and 20-minute intervals.

8.)     The cylinder was then emptied and a photo taken of any visible residue on the side of the cylinder.

9.)     The cylinder was then rinsed with hot water, cleaned with the test tube brush, dried thoroughly, polished with a clean, lint-free towel, and allowed to come to room temperature prior to the next trial.

 Note: Each trial (steps #1-9 above) takes about 30 minutes to complete.


For the trials involving supplements in the formula:

Same procedure as above, except for the following:

1.)     When using the addition of .3 part MultiMilk®, it was added in step #1 (above).

2.)     When adding whipping cream and/or yogurt, those items were added during step #3 (above).


For the trials involving longer hydration times (4 and 8 hours):

Same procedure as described above, except for the following:

1.)     Instead of proceeding immediately to step #5 (above), the glass jar with the formula was covered with plastic wrap, and placed in the refrigerator (40ºF) for either 4 or 8 hours.

2.)     After the designated hydration time, the jar of formula was removed from the refrigerator, stirred slightly, and warmed in a microwave to normal feeding temperature of approximately 100ºF.  Proceed with step #5 above.



Appendix E - Effects of water temperature on milk replacer powder

Figure E-1. Drop in water temperature when mixing formula.
Figure E-2. Undissolved MultiMilk® resulting from using warm water for mixing.

Use of hot water is essential to mix with  Esbilac® powder (or any powdered milk replacer) containing such a relatively high fat content (≈ 40%). Achieving a temperature of around 110ºF is preferable to begin the liquefaction of the various fats contained in the powder.

During the tests performed to determine the solubility of the 'new' Esbilac® powder, both hot tap water (125ºF) and water from an instant hot water dispenser (175ºF) were used. A significant issue with using either source of water is the rapid cooling effect when mixing with a much cooler product temperature (40ºF) over the 30 second mixing period.

As shown in Figure E-1, the temperature of the formula after mixing for 30 seconds drops 30ºF on average to approximately 95ºF, which is below the target temperature of 110ºF to achieve optimal liquefaction of the various fats, especially certain animal fats (such as contained in MultiMilk® where it is a major ingredient). The temperature dropped even farther (an average 58ºF drop) when the much hotter water was used, but resulted in a formula temperature, of around 115ºF after mixing.

This also serves to explain why the formula which included MultiMilk® mixed with only the hot tap water tended to have many more and larger undissolved particles when viewed under high magnification (Figure E-2). The temperature of the formula was likely not hot enough to liquefy the animal fat in the powder (bland lard has a melting point of around 109ºF).

It is important to mention that while the milk replacer powder is mixed with hot water (about 175ºF), exceeding those temperatures could affect the nutrients.

Click to continue to next section of the report - "Question 5 - Has the stability of the product changed and is rancidity an issue?"


Return to Main Page of the report.




Copyright 2010. © WildAgain Wildlife Rehabilitation, Inc. All Rights Reserved unless otherwise stated.