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Formula Mixing Guide (updated April 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 make the new process a ‘habit’ or ‘muscle memory.’ They also report improved results!!

Why it's important. Mixing methods used by rehabilitators impact the health of the wild mammal young in their care far more than many have realized. This  video describes impacts of mixing methods on the wild mammals and caregivers – and reasons to update mixing methods.
How it's done. This video shows the steps of updated mixing methods for milk replacers, regardless of milk replacers, formula recipes, species fed. Explains and provides a hands-on demo of the 2-page formula Mixing Guide.

First Time User

OK, I Need Some Explanations

Great - we have some for you! We have posted the entire 2-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 2-page Print Version

(plus the new 1-page '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 2-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 an accompanying 1-page 'Quick Guide'. This is a summary of the key steps for reference when preparing formula. Suggest copying the 2 page Mixing Guide front and back, and the Quick Guide separately.

Here's the 2-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.




NOTE: Prior to starting, if milk powders are stored in the freezer, remove enough for the next several days and store in the refrigerator to fully thaw prior to weighing and mixing.

  1. Wash hands. Declutter and clean the mixing area. Make sure all tools/utensils are clean.

  2. Assemble and organize all mixing tools/supplies on the counter that will be needed to prepare the formula (e.g., gram scale; measuring cups; mixing bowls/containers; hand sifter or strainer; whisk, fork, and/or tablespoon; instant read thermometer; marking tape and pen for labeling containers). Include printed copy of the recipe with ingredient weights (don’t rely on memory).


  1. Remove dry ingredient(s) from refrigerator. Always sniff powders for freshness or an off odor (i.e., rancidity, unexpected scents). Inspect for off-color, contaminants, etc. Discard if concerned. 

  2. Use the scale TARE feature for each measuring container to ‘zero’ it out before weighing each ingredient separately for the amount specified in recipe. Tightly seal and return unneeded dry ingredients to refrigerator. 

  3. For a single powder recipe, weigh the total amount of water needed in a tared mixing container. For multiple powder recipes, split the water in separate mixing containers to the water weights specified in the recipe to later mix with each powder. 


Step 3: PRE-MIX

  1. Allow each weighed milk powder to come to room temperature in its own measuring container. Use the instant read thermometer to verify ≈70°F (21°C). Do not rush this step.

  2. For clumpy, sticky and hard-to-mix powders, use a fork, strainer, or hand sifter to break up all large clumps to a finer, more granular consistency prior to mixing to enhance wetting and sinking.

  3. Heat water to >130°F (>54°C). Boiling OK but not required. Let cool to appropriate mixing temperature (≈110-130°F or 43-54°C). Check with instant read thermometer before mixing.


Step 4: MIXING  

  1. Single milk replacer powder recipe: Prepare the powder as described in steps 2-6 below. [Note: If the recipe calls for multiple powders, prepare each powder separately (steps 2-6; then combine all liquids later (step 7).]

  2. Pour the weighed warm water into a formula mixing container (clear glass/plastic; wider diameter better than narrow for more powder-to-water surface contact). Sprinkle the weighed milk powder on the warm water. Do not stir or disturb 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. Hand whisk powder into water until the powder is completely dispersed (≈5 minutes). Pour the liquid through a strainer and push any clumps back into the liquid using the back of a tablespoon. Do not use a countertop/immersion blender; electric mixer; or shaker bottle.

  4. Once fully mixed, cover container with air-tight lid to prevent evaporation and/or contamination. Label the container (formula recipe, date/time mixed). If making several formulas or dilutions, label each container individually (recipe/strength, date/time mixed).

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

  6. After the reconstitution time, remove formula(s) from the refrigerator. Stir lightly. For most powders, small clumps will have dissipated. At this point, discard any large clumps (still dry powder) that have not reconstituted rather than straining back into formula

  7. Blended recipes:  When mixing a recipe that incorporates two or more milk replacers (that were separately reconstituted into liquid form), pour those liquids into a single container. [If adding other liquids, such as extra fats (e.g., heavy whipping cream, MCT oil), weigh those cooled liquids and add them into formula.] Stir all liquids to fully mix (3-5 min.). It is then ready to warm enough for immediate use or return unheated excess to the refrigerator or freezer. Also return and store any extra fat ingredients to the refrigerator, such as heavy whipping cream, MCT oil, etc.



  1. Dairy scientists advise mixed/prepared milk formula can be properly stored in the refrigerator up to 3 days (≈72 hrs after the 8-hour rest period). 

  2. For longer-term storage, divide into small portions (e.g., silicone ice cube trays), label with product/recipe and date and freeze. Transfer individual frozen cubes to airtight container to minimize air exposure. Frozen cubes may be kept similar to time for ice cream (preferably used in less than a month, but possibly up to 2 months). Remove enough individual cubes for use, slowly 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 drop 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 and likely result in more clumps. 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 affect growth, GI function, and more if eaten. Rancidity has the potential to become a problem if such higher fat content milk replacers were not stored properly at any time in storage or transport. 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 manufacturers 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 questioned 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. Rehabilitators have found this is a good time to break up clumpy, sticky or hard-to-mix powders by using a fork, strainer or hand-sifter - thus reducing clumping during the reconstitution process.

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 previously combined the multiple ingredients in dry powder form to create a blend, and then added the prescribed amount of water. This 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. 

The total amount of water needs to be proportionately divided between the various powders when reconstituting them separately. The Measurement Guide in the WildAgain Formula Calculator has a feature to accurately divide the total amount of water (in grams) between multiple powders.  



...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) are combined by stirring together for 3-5 minutes. This includes the other liquid ingredients (e.g., heavy whipping cream, MCT oil, corn syrup) without any measurable negative impact on reconstitution.

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


Even though the labels on many milk replacer powders specify adding the powder to the water (in that order), many people have 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).


Use of a handheld immersion blender or countertop blender can work against effective and complete reconstitution.

While high speed mixing tools are faster, they can introducing bubbles to the formula that can make feeding difficult, cause bloat and GI distress, and other problems. Less commonly understood issues arise from those milk replacers 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 can 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 that may similarly damage the fat structure. Additionally, the high-protein milk replacers that contain casein will reconstitute less effectively if mixed at high speed (studies have shown that hand whisking speed is sufficient).

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; 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. Many 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. Label these containers with formula info (recipe, product names, date made, species, dilution, etc.) before freezing.


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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