Baby’s fussy palate inspires a breast milk storage start-up

The baby was rested, clean—and still crying. It appeared that she was hungry, but, maddeningly, she would not take her bottle. In fact, every time her parents offered her breast milk that had been stored in the freezer, 4-month-old Holly Silpe simply was not having it. What was a dad to do?

For Justin Silpe, then a Princeton University postdoctoral researcher, the answer was to try and understand what was behind his daughter’s rejection of the frozen and rethawed milk that his wife, Katie Silpe, had expressed.

Justin used high-throughput screening techniques to investigate how milk changes when frozen and to find food-derived compounds that can increase milk’s longevity (Foods 2025, DOI: 10.3390/foods14122018). He has founded a company to develop the results into a product that will help other parents.

When the milk tastes wrong

Freezing milk is an important tool for nursing parents, says milk glycobiologist Steven D. Townsend of Vanderbilt University, who praises the new study. “When mom pumps, she’ll typically be able to keep that milk at room temperature for about 3–5 hours. Toss it in the fridge and you get a little less than a week. Freezer gets you 6 months,” Townsend says.

But infants often refuse to eat frozen and thawed milk. A survey of about 1,000 nursing parents that Justin’s colleague Karla Damian-Medina conducted found that about 30% of babies reject thawed milk, forcing parents to waste their supply.

Researchers believe this happens because of rancidification—chemical changes to fats that wreck their flavor. Human milk is only about 3–5% fat, but as they do in other foods, these molecules can make an outsize contribution to the milk’s flavor.

When human milk goes rancid, its lipase enzymes cleave fatty triacylglycerols into glycerol and free fatty acids; the free fatty acids can then become oxidized.

Parents can sometimes detect rancid milk by sight or smell, but prevailing medical advice is that it’s safe to feed to the baby provided the milk was stored and thawed according to safe-handling guidelines. When she returned to her job in tech sales about 4 months after giving birth, Katie says she learned right away that Holly didn’t agree with that advice. “We tried, and she was like, ‘Heck no.’” Tricks that Silpe found online to mask the taste, like mixing thawed milk with formula or adding alcohol-free vanilla extract, didn’t make little Holly any more interested.

“When you look up this problem, the Google hits that you get are generally either to cow’s milk or to blogs where other parents complain about this problem,” Justin says. He was surprised to find no research in the scientific literature, he adds, especially since “this is a pretty basic biology question.”

When Justin pitched a study to his boss, Bonnie Bassler, she was immediately on board. Bassler, celebrated for her studies of chemical communication between bacteria, knew nothing about feeding babies. But the project appealed to her because the experiment Justin had designed was similar in reasoning to the rest of the lab’s work but much more finite in scope. "It just seemed so tractable compared to the kind of science Justin and I usually do,” she says.

Besides, Bassler says, she was “horrified and outraged” that the best society had to offer parents—most often mothers—trying to cajole their reluctant offspring into eating was the kind of hack Katie Silpe had found on nursing forums. Bassler adds that Justin’s personal motive dovetailed perfectly with his scientific background to add up to “an amazing little detective story.”

Protecting milk fat

To understand the chemistry of milk storage, Justin needed freshly pumped samples from nearby nursing parents. That turned out to be a challenging ask. “People who are pumping often have a lot of other demands on their schedules,” he says. Besides, as a man, he had to get past the awkwardness of asking about “a kind of personal topic.”

With help from Katie and from local breastfeeding support groups—and by driving all over New Jersey—Justin eventually collected about 30 mL of milk from each of 14 donors. Back in the lab, he mixed compounds from a commercially available library of food-derived molecules with the milk samples before freezing them. After thawing the samples up to a week later, he used fluorescence-based assays to screen for lipase activity and antioxidant capacity.

The screen uncovered 15 compounds that could alter these drivers of rancidity without blocking other enzymes found in milk, such as the bacteria-fighting lysozyme. The research team chose to move forward with two of the molecules—pectin and ascorbic acid—because they are so widely available and consumed. Adding ascorbic acid, or vitamin C, to the milk improved its antioxidant capacity, protecting free fatty acids from oxidation. Meanwhile, pectin blocked the lipolysis, or breakdown of fat.

While vitamin C is a well-known antioxidant, the way pectin blocks lipolysis is not yet entirely clear. To better understand why the compound is useful, the team struck up a collaboration with Itay Budin, a lipid biochemist at the University of California San Diego who is also affiliated with the university’s Human Milk Institute.

Budin says that most lipid droplets are made up of a core of neutral lipids, like triacylglycerols, packaged in a polar lipid monolayer. But in milk “the fat is packaged in a very interesting way,” with a phospholipid bilayer wrapped around the lipid droplets.

That bilayer, an extra layer of protection that the milk fats pick up during secretion from breast cells, shields the fats in the core of the globule from lipases and other enzymes found in milk. “This is kind of a maternal factor, this milk fat globule membrane,” Budin says. “It’s going to vary depending on the mother.” This, along with individual variations in milk lipase activity, could explain some of the variability in babies’ willingness to tolerate milk that has been frozen and thawed.

Budin’s lab is using mass spectrometry to analyze the fat composition of milk samples stored under different conditions to get a clearer idea of how pectin alters the chemistry of milk as it freezes.

‘A do-gooder company’

While Budin works to understand the underlying mechanisms at play, Justin Silpe has landed funding from several federal agencies that support entrepreneurial development and founded a public benefit corporation—what Bassler calls a “do-gooder company”—aimed at bringing his findings to market.

The company, PumpKin Baby, is developing a product in both powdered and liquid forms to add to milk to help maintain its palatability in the freezer, and working with regulators to determine a pathway to bring it to market. Though Holly—now almost 3, and thriving—drank only fresh or refrigerated milk for most of her infancy, they hope that future babies who share her refined palate may benefit.

Neither of the Silpes ever expected Justin’s career to take this direction. Thinking back, Katie says that without Bassler’s support, it never would have. “He comes to [Bassler] with a bag of breast milk and says, ‘I want to research this,’ and she’s like, ‘Let’s do it!’” Katie says. “You don’t get that everywhere.”