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Formula Mixing Guide (updated Feb 2023)

When wildlife rehabilitators discuss formulas for a young wild mammal, the focus tends to be on the formula recipe and discussing what's in it. Such discussions rarely give much attention to preparation. While formula recipes are critically important, the preparation methods can significantly affect how the animal is able to utilize the formula to achieve overall health and well-being. Without realizing the preparation method is so important, rehabilitators have often mixed the formula according to personal habit, or conflicting instructions from labels or others. Recent research conducted on powdered milk replacer products and formulas have shown that adjusting the preparation method can make a significant difference for wild mammals fed formula, even though this may mean adjusting and changing our habits. Rehabilitators who have ‘tested’ these new methods mention that it takes following the mixing guidelines 3-5 times to get make the new process a ‘habit’ or ‘muscle memory.’ They also report improved results!!

First Time User

OK, I Need Some Explanations

Great - we have some for you! We have posted the entire 1-page version of the Guide below with certain steps underlined . These are clickable links to more full explanations and discussions of why steps are specified in a certain manner. Plus, many of those explanations have further links to provide even more explanation and science that governs how powdered milk products reconstitute back into liquid form most effectively. We encourage a complete understanding of some of the changes suggested in the Guide.

Returning User

I Just Need the 1-page Print Version

(plus the new 'Quick Guide') 


Having previously read through the explanations referenced at left and described below, many returning rehabilitators simply want a fresh copy or two of the summary 1-page print version. Just click on the icon below for a PDF version of the Guide, or even print several for your team.

     NEW! It now includes a 'Quick Guide' to be copied on the reverse side. This is a summary of the key steps for reference when preparing formula. Simply copy the 2 pages front and back.

Here's the 1-page Mixing Guide - With Explanations

(click the underlined and shaded links below for info):





Recent research on the milk powders, formula recipes and preparation methods identified ways to improve nutritional benefits of formula fed to young wild mammals.

READ ALL INSTRUCTIONS FIRST. Shortcuts/omissions compromise results.




  1. Wash hands. Generally declutter and clean the mixing area.

  2. Remove dry ingredients from refrigerator or freezer and allow to reach room temperature. Inspect for color, contaminants, etc. Sniff powders to check freshness or an off odor (i.e., rancidity, unexpected scents). Do not use if concerned with quality.

  3. Assemble and organize mixing supplies on the counter (e.g., gram scale, measuring cups, powders, whisk, instant read thermometer, recipe ingredients). Include printed copy of the recipe with ingredient weights (don’t rely on memory).

  4. Organize the recipe ingredients in mixing area: water (e.g., tap, filtered, distilled), milk powders, fats (i.e., liquid or powder), etc.      




  1. Use the scale TARE feature for each container to ‘zero’ it out before weighing each separate ingredient.

  2. When the milk powder is at room temperature, place the weighing container on the gram scale and then fill/weigh the needed amount as by the recipe.

  3. Weigh each powder and each other ingredient separately in their own individual containers.

  4. Heat a little more water than recipe requires to appropriate temperature (≈ 110-120°F or 43-49°C; not boiling).  Weigh the total amount of water specified by recipe in a separate container.  



Step 3: MIXING  

  1. For a single milk replacer powder in the recipe, prepare the powder as described in steps 2-6. If the recipe calls for multiple powders, prepare each powder separately (steps 2-6), and then combine all liquids later (step 7).

  2. Pour the weighed warm water into formula mixing container. Sprinkle the weighed milk powder on the warm/hot water. Do not stir at this point. Set timer for 5 minutes.

  3. After 5 minutes, the milk powder on the water in the container will start to wet and begin to sink. Whisk powder into water until the powder is completely dispersed (≈ 5 minutes). Don’t worry about small clumps. Follow the same steps when making a large batch. If considering using a handheld immersion blender during last dispersal step of large batch, limit use to a few short bursts (≈ 5 seconds) to reduce problems. Avoid any use of countertop blender.

  4. Cover container with lid to prevent evaporation and/or contamination. Label the container (formula, date/time mixed). If making several formulas or dilutions, label each container (formula, date/time mixed).

  5. When formula is slightly cooled, place in refrigerator (≈ 40°F or 4.5°C) to allow a rest time for the powder(s) to hydrate and fully reconstitute (8 hours minimum). 

  6. After the reconstitution time, remove the formula(s) from the refrigerator. Stir lightly; small clumps will have dissipated. Rehabilitators planning to gavage (e.g., opossums) may strain formula to prevent tube obstruction.

  7. If blending combining two or more milk replacers (now liquid form), pour those into a single container. If adding other liquids, such as extra fats (e.g., heavy whipping cream), weigh those cooled liquids and add those into cooled formula. Stir. Return to the refrigerator. Dairy scientists advise mixed formula can be stored in refrigerator up to 3 days.

  8. For longer storage: divide into small portions (e.g., ice cube tray), label with product/recipe and date and freeze. Remove frozen cubes to an airtight container to help minimize air exposure. Frozen cubes may be kept similar to time for ice cream (preferably used in less than a month, but up to 2 months). Thaw in refrigerator, warm water bath or at room temperature (not microwave).




  1. When ready to feed, briefly and lightly stir the formula liquid again to ensure formula is well mixed (but no bubbles). 

  2. Pour the estimated amount required for this feeding into a container(s) – and warm, such as in warm water bath or steam heat bottle warmer. Do not warm formula in microwave.

  3. Keep the formula containers separate depending on recipe, dilution (e.g., full vs. half strength).

  4. Follow standard practices to keep the formula containers separate for animals in quarantine.

  5. Do NOT reuse heated formula; discard leftovers.

Step 1: INITIAL PREPARATIONS - Selected notes and explanations

Remove dry ingredients from refrigerator or freezer...


The powdered milk replacers most commonly used to make formulas for wildlife are high in both protein (primarily casein) and fat (dairy, plant or animal based). The quality and reconstitution performance of these primary ingredients, as well as critical vitamins, can degrade significantly if not stored properly. Consider freezing (≈ 0°F or -18°C) these products if purchased and not used in the immediate future (< 1 month). Once opened and partially used, they can still be kept in the freezer, and at a minimum in the refrigerator (≈ 40°F or 4.5°C). Click here for the effects of storage time and temperature on powdered milk replacers.


...allow to reach room temperature.

Powdered milk replacers perform better when reconstituted with warm water (≈ 110-120°F or 43-49°C). When any powder at a lower room temperature (≈ 70°F or 21°C) is combined with the water, the higher temperature of the water will be affected to a lower temperature. If combining with powder taken directly from the freezer, the water will be lowered significantly more to the extent of negatively affecting the reconstitution process. Click here for the test results discussing the effects of water temperature on reconstitution. Simply plan ahead and allow the amount of powder the short time needed achieve room temperature.


Inspect for color, contaminants, etc.

Does the powder look different from last time? Foreign objects? Unfortunately, these things happen. Some are of little concern, others require attention. But unless you look, you may never know. A color of the same product that is different from the last can could simply be the result of a product formulation change from the manufacturer. Compare the labels of the two cans to see if ingredients have changed. Remember that labels may not not reflect changes for months or more than a year. Contact the manufacturer, and check with other rehabilitators using the same product to determine the reason and if a replacement is appropriate. A noticeably darker color could result from excessive heat in the production process of drying the milk to a powder. If a burnt odor is detected, contact the manufacturer to request a replacement.

Over the years, rehabilitators have reported foreign objects from bits of plastic, very small hard objects to other odd looking things that should not be present. Promptly report this to the manufacturer with the lot number of the container (photos are helpful). 


Sniff powders to check freshness or an off odor (i.e., rancidity, unexpected scents). 

Since most mammals in rehabilitation have much better olfactory senses than humans, animals are likely to not want to eat formula that has an off-odor, perhaps noticeable to the animal but not caregiver. Rancid products can cause GI upset and other problems if eaten. Rancidity has the potential to become a problem if such higher fat content milk replacers were not stored properly at any time. It is highly suggested to read the series of 3 articles on rancidity on this website.

Other off-odors can also be present, either intentionally or not. Unintentional odors could include the burnt odors mentioned previously. Some rehabilitators report that Fox Valley may be intentionally offering flavorings/scents in their products, such as bubble gum. Questions arise about the purpose of adding such scents (e.g., conceal product odors) as well as possible detrimental effects. Sniff the powder; if it smells off or bad, don't use it and contact the vendor or manufacturer for a replacement.

Initial Preperations

Step 2: WEIGHING - Selected notes and explanations the weighing container on the gram scale and then fill/weigh the needed amount as by the recipe.

Some have criticized weighing ingredients in a formula recipe as unnecessary and time-consuming. However, weighing the ingredients really is essential for accuracy considering the goal of meeting the animal's nutritional needs. Examples abound in bread or cake cookbooks that advise to weigh dry / powdered ingredients to achieve more consistent and positive results. Significant error can be introduced by simply scooping powder (by approximate volume) versus more accurate weighing. Many of the milk replacer powders are notoriously clumpy and sticky and do not scoop consistently. Each person will scoop differently - some fluff the powder up, others pack it down. Exact weights remove all of that error. Review the 3 articles on powder measurement on this website, starting with a quick summary, with links to the other two. You may surprised by how much a little extra investment in time increases accuracy significantly - and helps the wild mammals fed formula.


Weigh each powder and each other ingredient separately in their own individual containers.

The various powders can differ in weight by 20% or more. This means they need to be weighed separately. Each powder will need to reconstitute separately in its own container anyway, as discussed below.

Heat a little more water than recipe requires to appropriate temperature (≈ 110-120°F or 43-49°C; not boiling).  

The use of warmer water above room temperature produces superior reconstitution results in tests reported in the scientific literature as well as WildAgain lab tests. While some websites and other sources suggest room temperature or even cooler water is acceptable, they typically are not dealing with the relatively high protein and fat milk replacer powders used in wildlife formula recipes. Click for a summary the research and testing performed by WildlAgain.


Step 3: MIXING - Selected notes and explanations

...prepare each powder separately (steps 2-6)...

This may be a significant change for many rehabilitators who use recipes that call for using multiple milk powders. Said another way, if a recipe calls for an amount of a milk replacer product (Fox Valley, PetAg, etc.) and an amount of another milk replacer product (Fox Valley, PetAg, etc.), some have simply combined the multiple ingredients in dry powder form to create a blend, and then combined with the prescribed amount of water. This common approach can compromise the overall reconstitution process.

Each powdered milk replacer has its own individual characteristics based on ingredient content (percent of total solids, proteins, fats, carbs and minerals); manufacturing method (roller dried; single step spray dried; multi-step spray dried; etc.) and physical characteristics (porosity; texture; weight; etc.). As such, for optimum reconstitution back to liquid form, each powder should be reconstituted separately and independently from any other powder because each powder has its individual profile. This should allow each powder to reconstitute while not competing with or interacting with any other powder. 

It then becomes apparent that when reconstituting powders separately, the total amount of water needs to be proportionately divided between the various powders. Let's look at an example. A formula recipe calls for 1 part of powder A,1 part of powder B and 2 parts of water. Easy enough, since the powders are used at a 1:1 ratio (50%:50%), then the 2 parts of water would be divided using the same ratio. So half (50%) of the water would be combined with Powder A in a single container, and the other half (50%) of the water would be combined with Powder B, in a separate container.  

Take a slightly different recipe that calls for 1 part of powder J, 2 parts of powder K and 6 parts of water. Since these powders are used at a 1:2 ratio (33%:67%), then the 6 parts of water would be divided using the same ratio. So one-third (33%) of the water would be combined with Powder J in a single container, and the remaining two-thirds (67%) of the water would be combined with Powder K, in a separate container.  



...then combine all liquids later (step 7).

Once the powdered products have been allowed to reconstitute for at least 8 hours (explained below), then all products (now in liquid form) can be combined together, including with other liquid products (such as fresh cream) without any measurable negative impact on reconstitution.

Sprinkle the weighed milk powder on the warm/hot water.


Even though the labels on PetAg and Fox Valley products specify adding the powder to the water (in that order), many people do not. Most human milk products specify that same instruction. There is a reason and it makes total sense once you think about it.

For a powder to start the reconstitution process to a liquid form, it has to get 'wet.' This means it needs to go through a "wetting" and "sinking" process prior to dispersal in the water. This series of articles explains this process in more detail. Simply stated, the powder on top of the water begins to absorb the water (wetting), gets heavy, and breaks the surface tension of the water that has been supporting the weight of the powder (sinking).


If the powder is added to the container first, and the water is added on top of the powder, the wetting and sinking does not fully occur. This results in clumps of un-wetted powder remaining on the bottom of the container or rising and then floating on top of the water. Some have tried to reduce clumps by adding half the water into the powder, stirring into a paste, and then stirring in the remaining water. However, this often results in an incomplete wetting and sinking requirement.

Whisk powder into water until the powder is completely dispersed (≈ 5 minutes).

The previous paragraph discusses the reconstitution processes of Wetting and Sinking. Some powdered dairy products are designed to be an instant mix (instantized) such as coffee creamer. However, milk replacers used commonly by rehabilitators are not instant mix products - not by a long shot. The other way to accomplish the wetting and sinking (other than with heat, e.g. warm water) is through mechanical means, such as stirring or whisking for several minutes. This will accomplish most of the wetting and sinking required, then followed by an 8-hour resting period prior to use. Hand whisking and stirring is far preferred over high speed mixing tools (see below).


If considering using a handheld immersion blender during last dispersal step of large batch, limit use to a few short bursts (≈ 5 seconds) to reduce problems. Avoid any use of countertop blender.

There are mixed feelings in the rehab community about the use of hand stirring/whisking versus countertop mixers and even handheld immersion blenders. High speed tools are admittedly faster. However, the concerns with this approach are commonly cited as introducing bubbles to the formula that can make feeding difficult, cause bloat and GI distress, and other problems. Less commonly understood issues arise form those milk replacers that are manufactured to provide fat in an encapsulated form, where small fat particles are coated with proteins. This allows the the fat to go into solution and not separate out once the powder is dissolved. Use of a high speed blender could damage the encapsulated fat structure due to excessive agitation, and could compromise an effective reconstitution. Questions have arisen about similar damage to the fat particles by using shaker formula mixing bottles that contain balls with sharp edged ribs that may similarly damage the fat structure. 

If making several formulas or dilutions, label each container (formula, date/time mixed).

This is simply a tip to stay organized and minimize mistakes. During hectic spring/summer baby seasons when rehabilitators get overworked and tired, mistakes can happen. Or someone else helping with feedings may grab a formula that is not intended for a particular animal or litter. Labeling multiple containers of prepared formula and placing them in labeled areas reduces the chance of mistakes. Consider labels that can be easily removed (painters or masking tape) or washed off after the container is empty and ready for washing. Indicate the liquid contents (e.g. 'Squirrel Formula'; 1:2:1/4 Esbilac:Water:HWC; etc.); formula strength (full; half; quarter, etc); which animal or litter (for quarantine purposes); date/time prepared; and for those who really get overworked, indicate a discard date, if making enough for 1-3 days. The lot numbers of the milk replacers may be kept in other records, and need not be indicated on each mixing container label.


...fully reconstitute (8 hours minimum). 

This is another significant change for some people. Sure, the milk replacer labels provide simple instructions of 'add water, stir, use.' This might work for those products that are truly instantized. However, as discussed above, these milk replacer products have not performed as instant mixes - or even after several minutes of hand stirring/whisking. WildAgain's extensive testing shows a 90% improvement in reconstitution when the formula, once mixed, is allowed to rest in the refrigerator for at least 8 hours. Click here for an entire series of these tests for the most commonly used milk replacer products used for wildlife. All this requires is that the rehabilitator plan their work flow to prepare formula 8 hours in advance of first use (e.g., making the formula in the evening for use in the next couple days).


Remove frozen cubes to an airtight container to help minimize air exposure. 

Many people now have frost-free freezers (as part of a refrigerator or standalone). Many people have pulled out frozen items that have dried out, discolored or become freezer burnt. This is a common problem that is easily solved. A frost-free freezer works by periodically raising/lowering the temperature from 0°F (or -18°C) to 32°F (or 0°C); melting any accumulated ice buildup; circulating air to remove the water vapor; collecting the vapor in a drain; and then evaporating the collected water vapor. So if an item is either not tightly wrapped or stored in an air tight container, or stored for an excessive period, the item can develop one or more of the problem issues mentioned earlier. 


Step 4: FEEDING - Selected notes and explanations

Do not warm formula in microwave.

The scientific literature seems to coalesce in three areas when discussing human baby infant formula. First, potential microwave damage to proteins and fats appears very slight, if at all. Second, damage to certain vitamins is somewhat more likely. Lastly, the more significant risk is very uneven heating of the liquid that can result in scalding when fed. Since there are several very effective ways to heat formula, many people prefer other, more gentle techniques (e.g., placing a small container with formula, some pre-filled syringes, or feeding bottles in a warm water bath). 

Do NOT reuse heated formula


This is a universal recommendation when working with any human infant formula because of the rich medium for bacteria growth that is created when heating the formula initially. It seems logical that the same concern is present for substitute milk formulas fed to wildlife, once the formula has been heated. Estimating and then measuring the amount needed to feed the young animal(s) helps rehabilitator prepare the amount for the current feeding (e.g., determining the approximate amount for each animal to be fed in that feeding and adding them to reach a total amount).

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