Reconstitution - Transforming a Dry Powder to a Liquid - Plus WildAgain's Test Results
In addition to other testing done by WildAgain (in-house or through a nationally certified independent lab), tests were performed to assess a macro-level reconstitution efficiency of some of the powdered milk replacer products. The purpose of this type of test is to address the following primary questions:
1.) Can full and complete reconstitution be achieved by following the product labeling mixing instructions?
2.) Is reconstitution affected by different variables, such as water temperature and/or length of resting time after mixing?
3.) Do the various products all reconstitute at the same rate and with the same efficiency?
4.) Based on test results, are there procedural steps to maximize reconstitution outcomes?
Just add water - right?
Turns out, it is a bit more complicated than just adding water and mixing to achieve the optimal result.
The mixing instructions on the PetAg® and Fox Valley product labels say to simply:
Step 1 - Add the powder to water.
Step 2 - Mix.
Step 3 - Use.
Following these instructions will in fact produce a white, milky looking liquid.
However, at this point, following those simple instructions with just a simple stir, is the formula sufficient to be fed to very small nursing mammals? WildAgain's testing as well as the scientific literature suggests the answer is highly unlikely. Fortunately, improvements are easily achievable with a few extra preparation steps. Consider the image below that illustrates the magnitude of the challenge of transforming dry particles ranging from 75-600µm in size to liquid particles 2-3µm (and smaller) in size. It is a little more complicated than just adding water and mixing to achieve the optimal result.
400x magnification. Leica DMLB, DIC microscopy (Univ. of Colorado-Boulder)
Rehabilitators work hard to provide optimum diet and nutrition to the animals in their care. This is especially true for neonates whose digestive systems in the formative stages, as well as those who may have arrived for care in less than optimum health. The success of substitute milk formulas is critical in terms of their nutritional composition and digestive effectiveness, since they are serving, in most cases, as the sole diet and source of nutrition for neonates. Using the wildlife formula calculator, it is possible to construct a formula recipe that comes reasonably close to a proportional match to mother's milk (in terms of total solids, proteins, fat and kcals). However, the ultimate digestibility of the formula being fed is not only largely dependent on the quality of the products used, but also the level of reconstitution of the dry ingredients.
Mother's milk has evolved over many years to be highly nutritious and digestible by the nursing young - and is about as good as it gets for a given species. Without measuring the milk particle size of each individual species' milk, it is likely safe to assume that it is free of any extraneous aggregation or clustering of large milk particles beyond what what would be considered normal levels (<2-3µm in size).
Reconstitution versus 'dissolving'
The main purpose of making of dry milk powder is to convert a liquid, perishable raw material(s) into a product that can be stored without spoilage and substantial loss of quality when properly stored. The challenge for the rehabilitator is to reverse this process by transforming the powder back to a liquid state by means of a reconstitution process.
For purposes of this discussion, the term 'reconstitution' refers to the complete process of recreating, as complete as possible, the original liquid state of the natural or formulated milk product before it was dried into a powdered form. In that process, not everything will be 'dissolved', or go into a complete aqueous solution when mixed with water.
Whole fresh milk is actually a mixture of water and three major components which exist in various states in the liquid:
1.) Carbohydrates exist in true solution (such as a transparent and clear dissolved mix of Gatorade). Want proof? Take a spoonful of granulated, white table sugar and mix in a glass of water. The sugar completely dissolves into the water.
2.) Fat globules of different sizes are suspended in an aqueous medium as a natural emulsion since oil (fat) is not soluble in water (such as a vinaigrette dressing where the oil/fat and water are both liquified). People learn early on that "oil and water do not mix" - which really means that while they actually can combine as two liquids, they just cannot dissolve together.
3.) Proteins are in a suspended colloidal state similar to an emulsion, but in this case involving both a solid and a liquid. The proteins do not 'mix' but exist in a combined and suspended state, where the protein molecules are still solid and the water is a liquid. An example would be a slightly runny Jello.
The ease or efficiency and speed of reconstituting these three components follow the same order as listed above. The sugars (carbs) dissolve fairly quickly as they are totally water soluble. The fats/oils achieve the emulsified state next depending on the degree of mechanical mixing (stirring, whisking, etc.) and the water temperature. Lastly, the proteins are the most difficult, especially the casein molecules and casein clusters, which comprise about 80% of the milk protein concentrates used in making powdered milks.
The physicochemical steps of the reconstitution process
Milk powders that disperse in both hot and cold water with a minimum of stirring and without the formation of lumps or undissolved sediments are referred to as 'instant' powders. They are also required to disperse within about 15 seconds to be labeled as instant. An example would be a very fine, human grade instant powdered coffee creamer. While the PetAg® and Fox Valley products' mixing instructions infer or suggest that they are a form of 'instant mix' product, they really fall very short on that metric, requiring much more time.
The reconstitution of instant powders can be described through several steps. There are many studies and researchers describing these steps as to their order and degree of overlap, and using a variety of terminology. In general, in order to achieve the full and complete reconstitution of any powdered milk product, the person preparing the liquid must take the powdered product through these steps. For a simplified discussion of this process, click here or on the image above. Even if you are not that interested in the science behind this process, a quick review of it may be helpful. It may highlight some of the steps that may be omitted or alert of a shortcut that could compromise the quality of your substitute milk formulas. It is worth a quick read and review.
Milk particle size - why is that so important
The emulsified fats/oils and colloidal proteins described above that are suspended in liquid milk are very small particles in size. Almost 40% of the milk has particles ranging from 1 - 10µm in size (about the size of red blood cells) These are primarily the fat globules and fat globule clusters, as well as a much smaller number of large casein clusters. About 50% of milk has particles less than 1µm in size (about the size of cooking smoke). These are the the completely disaggregated and fully dispersed casein molecules.
Assuming that the mother's milk is a more complete 'liquid' product with particles in these size ranges, it is reasonable to presume that a very small, young animal could likely have issues eating and digesting a substitute milk formula that had much larger solid particles resulting from less than full and complete reconstitution. These issues could present as poor palatability and prompt more severe digestion and GI medical problems. To get a more complete appreciation of particle size and see a more thorough discussion of this subject, click here or on the image above.
WildAgain's reconstitution test results
In order to test the reconstitution efficiency of the various milk powders, WildAgain conducted two separate tests.
1.) Dispersal test (wetting, sinking, and disintegration of agglomerates). This series of tests examined individual powders and powders blended together using three successively smaller sieves (500µm, 250µm and 125µm) at a constant water temperature (≈120ºF), and involving a resting period of immediate mix and 8 hours.
2. Solubility test (final dissolution). Following a modified IDF-Standard 129A insolubility index test, this procedure was performed to assess the insolubility of the reconstituted powders as a proxy for dissolution efficiency.
How storage time and temperature can also affect protein reconstitution
A published study examined the affects, 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 methodology of the study is described in the paper referenced below (Anema, 2006).
The study concluded that the solubility of MPC85 decreased exponentially with storage temperature over a relatively short period of time. The sample stored at 20°C/68°F (a moderately cool room temperature) showed no affect 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, explained by the fact that the whey proteins (about 20% of MPC85) were not affected at all by storage temp/time. It was only the casein proteins that were negatively affected.
This study has several implications for rehabilitators using casein-rich substitute milk powders (which are most of them). First, attention should be given to purchasing the freshest products as indicated by lot number. Second, storage of the product (opened or unopened) should be at room temperature or below if used within 60 days without any noticeable effects on casein reconstitution performance. If longer storage is required (up to 14-16 months), the product will perform best if stored at 40°F (refrigerator) or more preferably at 0°F (freezer) prior to use.
Conclusions - and steps available to improve reconstitution
If the premise is valid that a small, young nursing mammal will thrive best if fed a more fully reconstituted milk powder, free of overly large milk particle solids, then the tests and studies discussed above suggest several take-home messages for rehabilitators:
1.) Powdered milk products showed a more complete dispersal (99+%) of larger solid particles (<125µm) when allowed to rest 8 hours or longer post-mixing, versus less dispersal (≈89%) using immediately after mixing. Longer resting times produced best results or about a 90% improvement. Additionally, it required to mix and use the formula immediately, longer (> 1 min.) and more aggressive mechanical stirring, whisking or mixing will improve the disintegration and dispersal of the 125-500µm sized agglomerated particles.
2.) WildAgain's tests confirmed a range from good to moderate dissolution (solubility) for the various products following the IDF standard insolubility index test procedure (rehydrating with 110°F water for only about 20 minutes). Scientific studies and papers indicate that the casein molecules require the longest time for complete dispersal and dissolution, and will measurably benefit from longer reconstitution time and hotter water temperatures. In addition to longer resting times just mentioned that improve dispersal, warm water temperatures in the upper portion of the range of 100 - 160°F will aid in the more complete dissolution of the casein-rich milk powders.
3.) Though not specifically tested by WildAgain, the study cited above on the affects of casein reconstitution as a function of storage time and temperature suggests buying the freshest product available (from date of production). Additionally, storing the product at room temperature or below for periods of less than 2 months if used within that time frame, and at much lower temperatures for longer periods (<16 months) prior to use. As a side note, storing the powders at 40ºF or lower for any period of time will help preserve vitamin potency, help to prevent the formation of rancidity in the high-fat powdered milk products, especially after opening, and help prevent the formation of any microbial growth or contamination.
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.
Baldwin, Alan J., Fonterra Research Centre, Palmerston, NZ. Insolubility of milk powder products- a minireview. Dairy Science Technology (2010) 90:169-179.
Boiarkina, I., N. Depree, W. Yu, D. I. Wilson, and B. R. Young. Rapid particle size measurements used as a proxy to control instant whole milk powder dispersibility. Dairy Science and Technology (2017) 96:777-786.
Forny, Laurent and Stefan Palzer (Food Science and Technology Department, Nestlé Research Centre, Vers-Chez-Les-Blanc, CH-1000 Lausanne, Switzerland). Wetting, disintegration and dissolution of agglomerated water soluble powders. Conference Paper · June 2009
International Diary Federation. In determination of insolubility index, Standard 129A. Brussels, IDF (1988)
Pugliese, Alessandro, Giovanni Cabassi, Emma Chiavaro, Maria Paciulli, Eleonora Carini, and Germano Mucchetti. Physical characterization of whole and skim dried milk powders. Journal of Food Science Technology (2017) 54(11):3433-3442.
Sharma, Anup, Atanu H. Jana, and Rupesh Shrikant Chavan. Functionality of milk powders and milk-based powders for end-use applications - a review. Comprehensive Reviews in Food Science and Food Safety (2012) 11:518-528.