WildAgain Wildlife Rehabilitation, Inc.
Solubility tests were conducted on several milk replacer powders that wildlife rehabilitators use to make formula for wild mammals (see box to right). While some of the products more quickly and more completely reconstituted than others, none proved to be a true instant mix as is indicated on the label of all of the products tested.
A quick summary of the test results
All products were mixed at a ratio of 1 part powder to 2 parts water. Two different temperatures of water were used, one at 125°F (hot tap water) and 175°F (from an instant hot water dispenser). Additionally, two different hydration times were used in the test. The first involved mixing the formula and then observing any unwetted powder after 30 minutes. The second involved mixing the formula, allowing it to rest in the refrigerator for 4 hours, then reheating and observing any unwetted powder. (Click here for the specific test methodology).
The numbers on the chart at right indicate the level in a 100ml graduated cylinder that flotation of unwetted powder was noticed after 30 minutes, for the various water temperatures and hydration times. For example, a number of 76 indicates that unwetted powder filled the cylinder from line marker 76 to the top, or occupying 24% of the cylinder. As such, larger numbers on the chart are better, as they indicate less unwetted powder. A number of 100 indicates that no unwetted powder was observed in the cylinder, indicating that the powder fully dissolved upon mixing with water. The legend at the bottom of the chart explains the color coding associated with each number.
As shown, in almost all cases, the milk replacer products showed a higher degree of more complete reconstitution when hotter water was used to mix with the powder and when the mixed formula was allowed to rest in the refrigerator for at least 4 hours.
Background of why solubility tests were conducted
Some rehabilitators prepare milk replacer formula once a day, keeping it in the refrigerator until a scheduled time to feed the pre-weaned wild mammals. Other rehabilitators preferred to prepare the milk replacer just before each scheduled feeding – and immediately feed the ‘fresh’ formula to the young wild mammals. Many rehabilitators believed that the prepared timing for preparing formula seemed to be a matter of rehabilitator preference or scheduling convenience. Some rehabilitators believed that making the formula ‘fresh’ was better for the animals; others felt that there was no difference.
In the summer 2009, some rehabilitators reported that juvenile wild mammals that were fed prepared with a base of Esbilac® powder developed gastrointestinal and growth problems. During research to identify the cause of the problem, PetAg, the manufacturer of Esbilac®, acknowledged that it had changed from a multi-step dry process to a single-step spray-dry manufacturing process in December 2008. PetAg was confident in the method since it had used that manufacturing method with other milk replacers since the early 1990’s. PetAg also was planning to change KMR® to that same method in 2009.
During the research into the potential causes of the problems reported by some rehabilitators using the Esbilac® powder, WildAgain Wildlife Rehabilitation started studying various milk drying methods in food science publications. The literature revealed that milk powders produced with single-step spray-dry powders took ‘more’ to get into solution. This prompted WildAgain to ask Karen Smith, Phd, at the Center for Dairy Research at the University of Wisconsin what this meant and how it might affect milk replacers fed to juvenile wild mammals.
Dr. Smith confirmed that initial solubility of single-step spray-dry powders is different from those made with multi-step drying and an agglomeration or instantizing step. She described inexpensive and relatively easy tests that could demonstrate the difference in the solubility between the products and encouraged WildAgain to conduct them.
The solubility tests revealed noticeable differences in the solubility of the previous Esbilac® powder that was considered a ‘quick and easy mix’ and the Esbilac® powder produced with the single-step spray-dry method (see Figure 2 at right). WildAgain then conducted the same solubility tests on many of the common milk replacers used by wildlife rehabilitators. The solubility tests showed that how rehabilitators mixed milk replacer powders made a significant difference in how the product was rehydrated and dissolved.
Follow-up communications confirmed that rehabilitators who had fed ‘fresh’ formula made with the Esbilac® powder to juvenile wild mammals were more likely to have seen the health problems than those allowed the formula to ‘rest’ for at least 4 hours. While other factors that also contributed to the reported health difficulties, solubility was certainly a major consideration (see Casey, 2010).
These discoveries prompted questions about how milk powders fed to juvenile wild mammals are manufactured as well as the solubility of several milk replacer products.
Single-step spray-dry processes
Single-step spray-dry processes are an extremely popular method of drying milk products, as well as other foods. The single-step spray dry process tends to be less expensive – which can benefit consumers and manufacturers. This method also allows companies to prepare a milk replacer powder that tends to be more stable and have a longer shelf life, as well as reduce the use or amount of preservatives needed. So it’s no surprise that many milk replacer powders are produced with the single-step spray dry process.
For some milk replacers, the dried ingredients, such as whey, casein, fats, vitamins and minerals are combined with water and heated, and then that liquid is spray-dried – resulting in the powdered formula. PetAg has stated that their Zoologic® Milk Matrix products, such as 33/40, 30/55, 23/30, GME®, KMR®, and Esbilac® are made with the single-step spray-dry process.
Milk replacer powders made with dry blend manufacturing
Other milk replacer manufacturers combine the dry ingredients – and package the blended ingredients. Some of the ingredients used for dry blending may be produced with single-step spray-dry processes. While mixing dry ingredients can be more cost effective for manufacturers mixing smaller lots, there are many challenges to ensuring that the dry ingredients are evenly mixed and distributed throughout the batch. Fox Valley Nutrition, which offers a variety of milk replacers for wild mammals, including Day One (32/40, 20/25, 20/50, 25/30) and UltraBoost uses the dry blend process.
Multi-step dry with instantizing step
PetAg formerly made their Esbilac® powder and KMR® powder with the multi-step dry with the instantizing step. This allowed the formula to mix more quickly and easily with warm water and a quick stir or shake.
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