Fresh or Frozen: The Facts about Freezing Milk

Written by: Katie Rodger, Ph.D. | Issue # |

The first thing that comes to mind when many people think about freezing milk is ice cream. Ice cream, frozen yogurt, and custards are all sweet treats that are notable for their creamy consistency. Yet milk that has been frozen sometimes seems less appealing once it’s brought back to a liquid state. Though some might not prefer milk that has been previously frozen, the ability to freeze and store milk and dairy products can be a safe and economical way to provide beneficial nutrition that people need. Likewise, the ability to keep human milk frozen for one’s own child helps families around the world every day. A common concern about freezing milk—whether produced by human or cow—is whether nutrients are lost in the process. So what, exactly, happens when we freeze milk?

From Farms to Freezers

Though most people simply associate “milk” with a commercially produced beverage consumed from cartons and containers, we know that the milk of different animals can vary in interesting ways. The freezing points of milks likewise can vary. Goat milk, for example, freezes at −0.509°C (31.084°F) and buffalo milk at −0.558°C (30.996°F). Cow milk freezes in a range of −0.564 to −0.516°C (30.985 to 31.071°F) with a mean value of −0.540°C (31.028°F) [1]. Variations in environment, management, and breed can all influence the freezing point of animal milk. Even the time of day and season in which the milk is collected, as well as the type of feed and the amount of water the animal consumes, can affect milk’s freezing point, which is also referred to as its melting point [2].

Though we often consider the effects of fat on milk’s consistency, it might be surprising that fat content doesn’t actually affect milk’s freezing point. Osmotic pressure, or salt balance, of a cow’s blood must balance with that of her milk. The greater the amount of salts in the blood, and therefore the milk, the higher the pH. Thus the pH—as well as lactose concentration—of milk may alter its freezing point. Potassium, chloride, sodium, citrates, urea, and other components also have an impact on the freezing point of milk, depending on their molar concentration. During pasteurization, the calcium phosphate complex and the pressure of carbon dioxide both can change, which can alter the freezing point of milk. When milk is cooled to freezing, soluble salts aggregate and transfer to casein micelles [2].

For dairy farmers and producers, the freezing point of milk can be used as an indicator of milk quality, especially adulteration with water. Typically, milk is approximately 87% water and 13% solids, including fat. When this balance is altered, usually by adding or removing water, the freezing point changes. According to the International Dairy Federation, many countries use the freezing point of milk as one of the criteria for ensuring high-quality milk, as it is considered one of the most effective ways to test the “natural” quality of milk and to detect contaminants. Any additives to milk—whether accidentally introduced to cows or deliberately added into collected milk—will alter the freezing point and thus be more easily identified [2]. Most dairy producers will conclude that milk has been watered down if the freezing point is anywhere above –0.250°C (31.55°F).

From Moms to Freezers

Breast milk is often frozen for easy storage and transport while a mother is nursing her baby, and NICUs keep the mothers’ and donors’ milk frozen for babies in hospitals around the world. Thus, there is ample research on the effects of freezing on human milk at different temperatures for various amounts of time. Interestingly, it is harder to pinpoint the freezing point or range for human milk, as the variation of components in the milk expressed by a woman can be significant, and can even differ between her own breasts. One of the most comprehensive studies was published in 1953 and looked specifically at the phenomenon of adulterating human milk “destined for milk banks” with water and/or cow milk. That paper, by R.A. Miller and R.W.B. Ellis, reported the range of freezing temperatures as −0.531 to −0.586°C (31.044 to 30.945°F), with a mean of −0.564°C (30.985°F), meaning it tends to freeze at a temperature slightly lower than cow milk does [3].

Though human milk is stable at room temperature for hours, freezing it promptly ensures its nutritional benefits. When frozen and stored for more than 90 days, however, fat, protein, and calories may decrease, and acidity may increase due to ongoing lipase activity [5]. A small but notable decline in pH and a rise in free fatty acid concentrations may also be attributed to persistent activity of lipolytic enzymes [6].

From Freezers to Families

Frozen milk—both animal and human—holds its nutritional properties well; when returned to a liquid state, its nutritional profile remains mostly unchanged. Enzymes and fat-soluble vitamins are usually unharmed, though sometimes depleted, making the decision to freeze milk a smart move for many families.

According to both the American Academy of Pediatrics and the Human Milk Banking Association of North America, storing human milk at −20°C (−4°F) is ideal and using it within 3–12 months is optimal [4]. A. English and L. Simon, in collaboration with the Academy of Breastfeeding Medicine, note that “frozen human milk should be stored in the back of the freezer to prevent intermittent re-warming due to freezer door opening, and should be kept away from the walls of self-defrosting freezers” [5].

Upon defrosting, the odor of once-frozen human milk may be different from fresh milk due to lipase-mediated triglyceride breakdown, which releases fatty acids that may become oxidized. This lipolysis process has antimicrobial effects preventing the growth of microorganisms in thawed refrigerated milk. According to researchers at the Academy of Breastfeeding Medicine, there is no evidence that infants reject defrosted breast milk due to changes in odor [5].

That said, searching the Internet for information about freezing human or cow milk for future consumption reveals many seemingly conflicting opinions on whether physical changes like odor and texture indicate spoilage. This might result in consumer uncertainty and the loss of a beneficial source of nutrition.

One common issue that arises when cow milk is frozen is its tendency to change consistency or texture. In particular, when frozen milk is brought back to liquid form, fat can separate, resulting in a graininess that some find worrisome or even off-putting. “Milk is a very complex liquid,” notes Bristol University physicist P. Barham [7]. “It consists of many small droplets of fat, each surrounded by a membrane that helps to keep them suspended in a solution of proteins and sugars in water. The delicate balance that keeps the fat droplets suspended can be upset in many ways.” Freezing milk creates ice crystals that can puncture the membranes around fat molecules. If, when defrosting milk, the temperature becomes high enough to melt the fat, “it can escape through the punctured membrane, making small ‘puddles’ of oil.” This can result in the grainy, separated consistency that some consumers don’t like. Shaking or blending defrosted milk can easily remedy this issue.

Most research advises that frozen cow milk is safe to store for 1–3 months. To reduce the likelihood of consistency issues, milk should be frozen quickly and in small batches. Containers of approximately one cup of liquid, with ample room for expansion, should be placed near the wall or back of a freezer. Many consumers prefer to freeze whole containers of one quart or more, and this is safe as well. Defrosting milk in a refrigerator for 1–2 days is ideal.

For many consumers, access to refrigerated milk in a store on a daily or even weekly basis is difficult. Likewise, for parents who want to feed breast milk to their infants, the option to provide freshly expressed human milk is not always available. Thus, the option to freeze milk for future use provides a safe and healthy solution for families around the world.

References

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