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Powdered Milk Replacers -

Storage Considerations


When rehabilitators discuss milk replacers and formulas, the focus tends to be on products and recipes. Storage has gotten little to no attention, other than to follow instructions on the label (which are minimal). Storage has been assumed to be a minor consideration since it is a dehydrated, powdered product. Rehabilitators have thought that once they buy the powder, mix it, refrigerate (including allowing for ‘resting’/reconstitution time) or freeze for later use, and use it for feedings … then “that’s it.” However, growing scientific research has shown that storage, including both elapsed time and temperature conditions, can significantly impact the effectiveness of the formula – and ultimately the health, growth and survival of the wild animal. This page highlights some of the issues involved with powdered milk storage, discusses possible negative effects using/feeding an improperly stored product can have on wildlife, and offers some basic reminder tips in the form of an easy-to-use checklist.

Here are 10 questions on storing milk replacers that can help bring focus on the topic – and test your familiarity with basic storage issues. The answers to the questions are covered below in the discussion. 


1.) The lot number on the package contains the date of manufacture.   T   F


2.) It’s acceptable to store which of the following at room temperature:

            ___ An unopened, newly purchased package of powder for up to 12 months

            ___ An opened package of powder if very tightly resealed for up to 6 months

            ___ Mixed formula, kept in a tightly sealed jar and used that day  


3.) Freezing the powder extends the useful life beyond the expiration date by how much?

            ___ Doubles the time

            ___ Triples the time

            ___ Indefinitely

            ___ Has no effect 


4.) If immediately refrigerated, reconstituted formula is OK to use for ____ days after mixing.


5.) The expiration date on the package is governed by standards set by:

            ___ Association of American Feed Control Officials (AAFCO)

            ___ US Food & Drug Administration (USFDA)

            ___ The State Dept of Health where the product is made

            ___ The manufacturer can set the date


6.) Reconstituted formula can be frozen for 10-12 months and maintain full quality.  T   F


7.) Storing powder (any temperature) past the expiration date compromises reconstitution efficiency. T  F


8.) Rancidity is easily prevented by storing powder in dark, airtight containers.  T   F


9.) Some vitamins can lose (pick one - 35%    50%   75%) of their potency in just 3 months. 


10.) Storing in an air conditioned room is the same a ‘cool, dry place’.  T   F

Perspective on storage temperature and time 


Although storage temperature and time are the critical factors to consider and manage, they are very dependent upon the current state of the powdered milk replacer. For example, is the powder:


1.) In a new, unopened package as received from the manufacturer directly; a third-party vendor; or from another rehabilitator?


2.) In a previously opened package from prior use, either by another rehabilitator or yourself?


3.) In reconstituted liquid form ready for feeding?

What can cause powdered milk to go ‘bad’ if not stored properly?


There are likely a host of issues that can cause powdered milk to spoil, become unusable, or result in the powder becoming less effective in its performance and nutrition as when the manufacturer made it. Possible issues include the following:


Rancidity. Popular websites suggest that many of the human-grade powdered milk products can last indefinitely without any issues, including having little chance of the onset of oxidative rancidity. Considering that most of those products are made from non-fat milk, indefinite (or at least very long-term) storage may be possible.  However, since powdered milk replacers used by rehabilitators contain 25-50% fat, rancidity becomes a concern for even medium-term storage.


The availability or presence of oxygen, increased temperatures, and the impact of light can speed up autoxidation (oxidative rancidity) over a period of time. Autoxidation does not take place from one day to another, but the potential for the chain reaction to start increases over time and once triggered, takes place at a fast rate. The presence of oxygen and exposure to light, especially direct sunlight or even light from fluorescent tubes, can accelerate the onset of autoxidation. Interestingly, the oxidation of fat occurs at a faster rate at a reduced water content.  This is because water acts as a “barrier” against the reaction of fatty acids with oxygen. As a result, foods with smaller amounts of water are more prone to oxidation, like powdered milk replacers that are guaranteed at 5% maximum moisture.


While laboratory tests have not revealed rancidity to be a common problem with most of the milk replacers, it is still advisable to store the powders carefully and note if the product has an ‘off odor.’ Rancidity, however, HAS been a continuing issue with GME milk replacer even in newly purchased, unopened cans. The most recent 7 lots tested (made in 2019-2021) have all shown elevated Peroxide Value levels, a measure of rancidity. The specific cause for this chronic issue is unknown. As shown in the chart below, and as discussed above, the oxidation seems to be more pronounced the older the lot sample was from its manufacture date, beginning as soon as just 4 months after the manufacture date. 

Rancidity in GME.jpg

Reconstitution. A published study (Anema, 2006) examined the effects, if any, of storage temperature and time on the reconstitution of the proteins in powdered milk protein concentrate MPC85 (85% protein). The study used samples of MPC85 stored at 20°C; 30°C; 35°C; 40°C; and 50°C (shown as degrees °F in the image at right) for periods up to 60 days. 

The study concluded that the solubility of MPC85 decreased exponentially with storage temperature over a relatively short period of time. The sample stored at a moderately cool room temperature (20°C/68°F) showed no effect over 60 days but did fall to about 60% solubility at 210 days (7 months). The samples stored at even slightly higher temperatures showed marked degradation in even faster time periods. Each sample reached a lower plateau or floor at 20% solubility. This is explained by the fact that the whey proteins (about 20% of MPC85) were not affected by storage temp/time. It was only the casein proteins that were negatively affected.

Storage Temp Recon Time Eff.jpg

This study has several implications for rehabilitators using casein-rich substitute milk powders (which include most of them). First, attention should be given to purchasing the freshest products possible, as indicated by lot number. Second, the product (opened or unopened) can be stored at room temperature or below if used within 60 days of manufacture, without any noticeable effects on casein reconstitution performance. Finally, if longer storage is required, the product will perform best if stored at 40°F (refrigerator) or more preferably at 0°F (freezer) prior to use.


Vitamin potency. Much of the literature suggests that most vitamin supplements will retain some level of potency for several years if stored properly. “Proper” storage means cool, dry, airtight, and no exposure to light. Most people taking vitamins generally consume them before the expiration date, using them within a few months or less than a year from purchase, ensuring maximum potency.


Vitamins naturally degrade over time, with the rate of degradation influenced by improper storage methods. To illustrate this, consider the following example of another dry product used in rehabilitation – rodent chow. As a dry product, most rodent chow products have a generous shelf life, usually from 6 to 12 months. Some manufacturers indicate that storage of the product at higher temperatures (>70˚F) will tend to cause vitamin loss. For example, the two charts shown in the following table indicate the effect of storage temperature and time on LabDiet’s Certified Rodent Diet 5002 product (moisture 12% maximum). The manufacturer of this product recommends storage at temperatures of 72˚F or below and not beyond six months. 

Rodent Chow Storage Time Effects .jpg

Guidance on storage provided by the manufacturers


PetAg (label - outside). “Unopened powder should be stored in a cool, dry place. Reconstituted liquid must be kept refrigerated for up to 24 hours. Opened powder must be refrigerated for up to 3 months or can be frozen for up to 6 months to preserve freshness.” Use before date stamped on top or bottom. ”A ‘Best By’ date is stamped on the bottom of PetAg cans for the user to locate and follow.


PetAg (label – inside). Who knew you can cut off the label to find instructions inside? Anyway, you can and it provides freezing instructions as follows:


[Esbilac/GME/KMR] Powder may be frozen for up to six months after opening (in powder form). Seal well before freezing. Reconstituted [Esbilac/GME/KMR] may be frozen to preserve it past the 24 hours of refrigerated storage. It is best to freeze using the instructions given below.


1.) Pour individual or daily serving amounts into each section of an ice cube tray (small party size).

2.) Place the tray in the freezer and freeze until solid. NOTE: The product will turn brown when frozen, but will return to its normal creamy color when thawed.

3.) Remove the cubes from the tray and place in a heavy plastic bag or sealed plastic container.

4.) If kept frozen and sealed, the product is good for up to six weeks.


PetAg (website video). PetAg’s website includes a short video on storage for products sold in liquidform. It indicates this liquid form of formula can be refrigerated up to 72 hours if opened and not previously heated. It also suggests that it is possible to freeze unused formula in the freezer for up to six weeks if covered and airtight. There is no mention if these same suggestions apply to reconstituted powdered milk replacers on the video. [For perspective, PetAg also has a video on mixing powdered products that has some inconsistencies with label instructions. The video shows adding room temperature water (label specifies warm water) to the powder (label reverses the order of combining). Are the videos to be relied upon, or the label instructions?]


Fox Valley (label). “Use before date stamped on product. Store in a cool, dry place. Store leftover formula in refrigerator – feed within 24 hours.”


Fox Valley (website). No additional information provided on storage of products.



As shown, the two manufacturers provide guidance that is consistent in some respects - store reconstituted powder in liquid form in the refrigerator and use within 24 hours. PetAg provides more complete guidance about storing previously opened containers of powder and says to store no more than 3 months (refrigerator) to 6 months (freezer). Fox Valley provides no such similar guidance, other than to use the product before the expiration date, suggesting no difference in storage time or temperature recommendations between an unopened or opened package.


Is this guidance complete enough given the previously discussed factors that can cause powdered milk to go ‘bad’? For example, what specific temperature constitutes “cool?” Are the label recommendations accurate in all respects?  What is the reason for discarding reconstituted formula after just 24 hours? Fortunately, there are steps that a rehabilitator can pursue with proper storage methods that can help ensure optimal product freshness. The following discussion provides simple and easy steps that a rehabilitator can consider.



What does this mean? Here are some suggested steps and methods.


1.) Give more attention to the actual purchase of the product. Both manufacturers instruct YOU to store in a cool, dry place. What about all the others involved in storage and transit along the way from PetAg to you? Are they all keeping it in a cool, dry place? 


As for what constitutes ‘cool’ (and other designations), the U.S. Pharmacopeia (USP) in “Packaging and Storage Requirements” USP<659> provides these references:


Cold: Not exceeding 46° F.


Cool: Any temperature between 46° and 59° F.


Controlled room temperature: The temp maintained thermostatically for a usual and customary working [and living] environment between 68° and 77° F. 


Unless a package for delivery is marked ‘temperature-sensitive,’ requiring special handling (and costing more $$), the cargo areas of most delivery trucks (USPS and non-company owned UPS and FedEx trucks) are generally NOT climate controlled. This can be especially troublesome on cross-country ground shipping (3-5 days) in hot summer months. Additionally, metallic mailboxes located in direct sunlight for just a few hours can attain very high temps. All of this to say that while YOU as the end-user of the product may store the unopened container in a cool, dry place, its journey to you may have included exposures to warm or even hot temperatures and for unknown periods of time. 


A number of rehabilitators plan ahead and order enough product for the upcoming season during the winter months to reduce the powder becoming hot during transport and delivery. Keep in mind PetAg’s caution from their website:


"...While PetAg takes precise measures to ensure that product leaving the facility is good, there is always the possibility that product may become compromised in transit, delivery, storage, or on a retail store shelf. For this reason, the expiration or best-by date is not always an indicator as to whether the milk replacer should be used..." 


If circumstances require ordering during warmer or summer months, consider expedited shipping to minimize transit time and purchasing smaller quantities sufficient to cover only immediate needs until the next year-end buying opportunity. 


Another strategy to help ensure freshness when ordering is to contact the manufacturer, supplier, or vendor and request information on specific lot numbers for the products being ordered. It may be several months before the product will be used to feed animals. Purchasing a specific lot that is already 6 months past the manufacture date may push actual product use well past the mid-point of the expiration date and cause some of the product age-related problems mentioned previously. Seek those lot numbers that are no more than 2-4 months past manufacture and stipulate those specific lot numbers for the order. [For Fox Valley products, and if buying in sufficient quantity, consider calling them for a special order of just-made product.]


2.) Once received, properly store unopened packages. For medium (3-6 months) to longer-term (>6 months) storage, give strong preference to frozen storage (0° F) if space is available. At a minimum, store at refrigerated temperature (40° F). As discussed above, simply storing at room temperature will compromise the performance and nutrition of the product over time. If room temp is the only option, consider only buying enough product for short-term needs. Storing in an outside shed or garage (or other area without air conditioning) would present an even higher risk to the product – and the animals that consume it.


3.) Carefully store opened packages. A common strategy is to use a new, unopened package of powder from the freezer, open and remove enough powder for immediate use (next 1-2 weeks), and then tightly seal the package and return to the freezer. Store the immediate use portion in an airtight container in the refrigerator; remove as much as needed when preparing the next batch of formula; and then tightly reseal and return to the refrigerator. 


4.) Set a maximum time for storing reconstituted powdered milk. Both milk replacer manufacturers specify discarding any unused milk after just 24 hours in the refrigerator. Is this a hard rule or perhaps an overly cautious guideline? A recent study evaluated the shelf-life of raw and reconstituted whole milk (DANO full cream milk powder) kept at room temperature (≈82° F) and kept in refrigerated condition (40° F) during summer months. It was predicted that raw milk (containing bacteria) would spoil much faster than the powdered milk. At room temp, raw and reconstituted milk were acceptable up to 9.5 and 12.0 hours respectively. Under refrigerated conditions, raw and reconstituted milk were acceptable up to 12.7 and 23.7 days respectively. While each person needs to form their own storage threshold comfort level, based on this study, perhaps the <24-hour notion of reconstituted refrigerated storage is a bit restrictive and excessive. Some human-grade whole milk powder nutrition labels suggest 2-3 days (which concurs with PetAg’s recommendation of 72 hours for their own milk replacer products sold in liquid form).


Another way to store reconstituted milk is by freezing in ice cube trays or other suitable containers (PetAg suggests up to 6 weeks). If the ice cube tray does not have an airtight lid, transfer the cubes immediately after freezing to an airtight container for future use. Allow to fully thaw in the refrigerator (covered) prior to normal reheating.


5.) Decide which date is more important – lot# date or expiration date. Expiration dates on pet food containers are not required by the Association of American Feed Control Officials (AAFCO). Since no requirement or standard exists, it is up to the manufacturer to develop and include a date – or not. The expiration dates on PetAg and Fox Valley branded powdered milk replacers suggest a shelf life of 18-24 months, provided the product is unopened. ‘Shelf life’ can be defined as a period of time after processing and packaging during which the food product maintains a minimum level of quality acceptable for consumption. 


Dairy for Global Nutrition is a U.S. Dairy Export Council (USDEC) initiative designed to advance the science of dairy and document its benefits for the world's most vulnerable populations. The organization publishes extensive dairy research on its website. One set of information includes the table below that shows typical shelf life guidelines for dairy ingredients under ideal storage and shipping conditions (max temp of 77˚ F and max relative humidity of 65%.) Most of the ingredients shown in the table are included in the powdered milk replacers used to construct substitute wildlife milk formulas.

Shelf Life Guidelines USDEC.jpg

The USDA states that even if the expiration date on your [human grade] food item has passed, it should still be safe to eat (if handled properly) until the spoilage is evident. However, the food date labels and terminology used may not be as cut and dry as imagined: 

"Best if Used By/Before" indicates best flavor or quality. It is not a safety date.

"Sell-By" date tells the store how long to display the product for sale for inventory management. It is not a safety date.

"Use-By" date is the last date recommended for the use of the product while at peak quality. It is not a safety date except for when used on infant formula.

The USDA says foods are still safe for consumption after these expiration dates pass, “…but make sure to look out for an off odor, flavor, or texture that mean the food has spoiled and should not be eaten.” This last caution seems consistent with PetAg’s statement discussed above that warns, “…  the expiration or best-by date is not always an indicator as to whether the milk replacer should be used.” 

Given all these cautions about expiration date, and what it is and isn’t, perhaps a much more useful and informative date is the date the product was made – as included in the lot number coding on every package. [Click here to understand how to decode the manufacture date from the lot number. It is relatively straightforward and simple to do.] At least that date is known, firm, and not subject to interpretation. Knowing this date puts the user in control of purchase, storage management and use. 

For example, an easy calculation at time of purchase indicates the age of the product at that moment. 

Are you comfortable purchasing a package/lot that is already 3-6 months old? 

Once purchased, are you comfortable using a package/lot that is over 10-12 months old?

If uncertain about your answers to these two questions, perform a quick review of the material just presented:

1.) Re-read the section above discussing the rancidity issues for GME after just 4 months since manufacture. 

2.) Re-read the section above regarding loss of vitamin potency between 3 – 12 months. 

3.) Re-read the shelf life guidelines from US Dairy Export Council (USDEC) above.

4.) Then consider the following chart, which was discussed previously, but further annotated to explain in more detail the effect of storage time on the reconstitution and solubility of casein protein, the predominant protein contained in almost all of the milk replacers [whey protein solubility is not affected by storage time]. The right side of the chart focuses just on casein solubility. As shown, and using the right-hand scale for casein, solubility drops by 50% in just 50 days if stored at a warm temperature (86° F) and by 85% if stored at a hot temperature (95° F). Fortunately, there is no change in casein solubility at 50 days if stored at a cool room temperature (68° F), but notice that it does drop by 50% at 210 days (7 months) if stored at room temperature.

Storage effects on dried Casein .jpg

The information in this chart is consistent with PetAg’s caution on their website that reads in part:


 “…The nutrition, dissolvability, and chemical structure of milk replacers can be compromised by improper shipping and handling or storage…”

Why “Made in the USA” may be an additional consideration

First, what does that designation really mean? "Made in USA" is a label protected by the Federal Trade Commission, or FTC. In order for an item to be called such, the item must be made within the United States' borders from "all or virtually all" American parts — that is, with parts also made in the US. According to the FTC's website, "all or virtually all" means that "all significant parts and processing that go into the product must be of US origin. That is, the product should contain no — or negligible — foreign content."

Do Fox Valley and PetAg products meet this standard? Fox Valley labels contain the “Made in the USA”language, so presumably, they do. PetAg products apparently do not. A recent Esbilac label indicates ‘Crafted in the USA – with domestic and imported ingredients.’ A recent GME label provides no indication asto ingredient sourcing. PetAg’s website does provide this: 


[FAQ] “Why do some products say "Made in the USA" and some do not?

While we do our best to source everything from the USA, there are some ingredients that are simply too difficult to acquire locally. We obtain these ingredients from trusted sources internationally. We manufacture all of our products in the Midwest.”

Would not all ingredients be required to meet the same set of manufacturing and quality standards – domestic or imported? Probably, and for purposes of this discussion, let’s assume they do. The issue here is the storage time and temperature involved from the time an imported product is made overseas to when it is incorporated into a final product by a US-based manufacturer at its faclity. For certain additive ingredientsused in milk replacers that are in powdered form such as dicalcium phosphate, potassium chloride, or manganese sulfate, storage is likely far less an issue. 

But what about the more prominent ingredients such as dried milk or components of dried milk, especially if including dried casein proteins? Consider the map below showing where the world’s dried milk originates. And then consider if the product was subjected to rigorous storage protocols, during summer months, when on an ocean-bound container ship, or sitting at a major US shipping port such as Los Angeles or New Orleans, or a semi-trailer truck bound for the manufacturing facility in the Midwest US. 

World Dry Milk Production 2021.jpg

Conclusion, references, and suggested checklist


Some of these storage steps are similar to the approach people take in purchasing and storing the food they buy for themselves and their families.  Some quick takeaways and suggested storage methods are included in the following checklist. Each individual rehabilitator must obviously decide which of the points below to incorporate into the methods they use for the storage of powder and reconstituted formula.

References (not intended as an exhaustive list)


Anema, S. G., D. N. Pinder, R. J. Hunter, and Y. Hemar. Effects of storage temperature on the solubility of milk protein concentrate (MPC85). Food Hydrocolloids (2006) 20:386-393.

Kar, Sukanta, et. al. Shelf life of raw and reconstituted milk at room and refrigeration temperature. International Journal of Natural and Social Sciences, 2017, 4(4): 24-32.


U.S. Pharmacopeia (USP) <659> “Packaging and Storage Requirements”.

Storage checklist for powdered milk replacer

(Click here for printable version (PDF) of the checklist)


1.) Learn what a lot number is and how to decode the manufacture date (click here). This is the date that serves as the starting point for your storage timeline.


2.) Become an informed consumer as to the age of the package/lot you are considering purchasing. Engage with the merchant and stipulate the purchase must include only lot numbers that are quoted by the merchant and that you deem acceptable in terms of recent manufacture.


3.) Buy the freshest product available. Seriously consider avoiding any lot that is already over 3 months old.


4.) For Fox Valley products, consider placing an order directly from them. 


5.) If possible, schedule (all or most) purchases during the colder months of the year to avoid exposure to warmer temperatures during vendor storage, transit, and delivery. Some may even offer discounts that time of year.


6.) Upon arrival, immediately freeze all unopened packages if possible, and at a minimum, refrigerate.


7.) Upon first use, open the package, remove and thaw only enough powder to feed over the next 2-3 weeks. Return the package to the freezer in as airtight a manner as possible. Another strategy - once a new package has been opened, is to divide the powder between multiple airtight bags/containers to minimize exposing the entire larger package to air and light each time a withdrawal of powder is made. [Also, upon first opening, remember to quality check for any off-odors or unexpected characteristics – texture, color, etc.]


8.) If a rehabilitator offers you some unused powder (unopened or opened packages), check the lot number, calculate its age, and inquire about the storage history. If unknown or not provided, check for any signs of spoilage such as off-odor, off-color, clumps, unusual texture, etc. Determine suitability for use.


9.) After mixing into a liquid formula, label each container as to contents, date and time mixed. Always store in the refrigerator for ample reconstitution time (8 hours minimum) and use within the next 2-3 days. Reconsider the product label guidance of discarding prepared formula after only 24 hours. 


10.) Once mixed and reconstituted (after 8 hours in the refrigerator) and it is determined that the volume of formula just made exceeds the amount needed to be fed in the next 72 hours, immediately freeze as described above. Avoid waiting until 72 hours and then freezing any unused portion.


11.) Lastly, consider determining your own expiration or ‘discard’ date when receiving each new package and mark each container accordingly. Based on this discussion, your selected date may likely be earlier and well in advance of what is stamped on the container. Try to adhere to your own date, even if it means having to discard some of the remaining unused powder.

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