He was comparing milk to blood plasma, which has long been collected from donors and made into valuable medical products like immune globulin, which helps fight infections, and clotting factors for haemophiliacs. Concentrated milk could be just the start. Researchers say that breast milk, which evolved over eons to provide optimal nutrition and protect babies from infection, is brimming with potential therapeutics, not only for babies but possibly for adults, to treat intestinal or infectious diseases such as Crohn's disease, for example. "We are at the tip of the iceberg for milk," said Bruce German, director of the Foods for Health Institute at the University of California, Davis, and chairman of Evolve Biosystems. Evolve and some other small companies like Glycosyn, Jennewein Biotechnologie and Glycom are trying to develop products based on complex sugars that are abundant in breast milk and that appear to nourish bacteria in the digestive tract that are important to health. But the commercialisation of breast milk makes many people uneasy. They worry that companies might capture most of the excess breast milk and make products that would be too costly for many babies, while leaving less milk available for non-profit milk banks.

"The competition comes in the form of how much surplus breast milk is there in the country and who's getting it," said Kim Updegrove, executive director of the non-profit Mothers' Milk Bank at Austin, Texas. "The non-profit milk banks have a long history of providing milk to the sickest babies, and provide it based on medical need and not on insurance reimbursement or financial resources." Debate is also intense over whether women should be paid for their milk or donate it altruistically. Opponents of payments, worried about breast milk "farming", say women might try to increase their milk output unsafely, hide health problems that could make the milk unsafe, mix in cow milk to increase volume or deprive their own babies so they can sell more. When Medolac Laboratories, a competitor of Prolacta, said last year that it wanted to buy milk from women in Detroit, it was accused of profiting at the expense of black women. "We are deeply concerned that women will be coerced into diverting milk that they would otherwise feed their own babies," the Black Mothers' Breastfeeding Association wrote in an open letter in January. Medolac, which said it was working with the Clinton Foundation and wanted to encourage breastfeeding by making it financially attractive, abandoned its plan.

Defenders of payments say that if companies are going to profit, it is only fair for them to pay the suppliers for their raw material, especially since pumping takes considerable time and effort. (Prolacta, which started paying only last year, was accused previously of not making it clear to women that their milk donations were going to a for-profit company). Moreover, they say, the commercialisation of breast milk could increase the supply available. The American Academy of Pediatrics says that because of the "potent benefits of human milk", all premature babies should receive breast milk, preferably from their mothers but if not, then from donors. But there is not enough donor milk for that, experts say, partly because many women do not know that they can donate or sell excess milk. Elster said Prolacta processed 2.4 million ounces, or 18,750 gallons, of milk last year and aimed to do 3.4 million this year. That compares to the 3.1 million ounces dispensed in 2013 by all 18 non-profit milk banks that belong to the Human Milk Banking Association of North America. Those milk banks do not pay women for milk but do charge hospitals a few dollars an ounce to cover the costs of screening donors and pasteurising the milk. Some women give milk directly to other mothers who need it, using milk-sharing sites like Eats on Feets. Some sell their milk to other mothers (or in some cases to male body builders who believe it builds muscle), through websites like Only the Breast, hoping to receive more than the $1 an ounce that Prolacta and Medolac pay. Some health authorities say direct sales or sharing pose risks because the milk is usually unpasteurised. Prolacta's products are intended for extremely premature infants who weigh less than 1,250 grams (2.76 pounds) at birth, babies who can fit in the palm of a hand. Those babies need more nutrition than they can receive from breast milk alone.

Prolacta makes a fortifier containing high levels of protein, fats and minerals, which is used to supplement breast milk. It costs about $180 an ounce, and a baby would typically consume $10,000 worth over several weeks. Generally, the cost is paid by the hospital or insurers, not the parents. However, not all doctors are persuaded, and others say that while they would like to use the product, it is too expensive. "I've been stymied by the cost equation," said Dr Jae Kim of the University of California, San Diego. He said his hospital spends no more than $25,000 a year for donor milk for all babies. Adding Prolacta's fortifier for the 50 to 70 extremely premature infants each year would cost more than $500,000. Each batch of milk arriving at the $18 million factory, situated outside Los Angeles, is tested for viral infections, nicotine, drugs of abuse, dilution and adulteration by cow's milk. The women who supply the milk take blood tests for infectious diseases, must provide notes from doctors saying they and their baby are healthy and must furnish DNA samples to verify that their milk came from them.

"Even the vitamins you take you have to report to them," Amaya said.