Why Milk?

By Pat Bailey

UC Davis "got milk" decades before legions of white-mustachioed celebrities proclaimed it fashionable. Incensed that young farmers had to go out of state to learn how to judge dairy products, State Agricultural Society President Peter J. Shields paved the way for the 1906 founding in Davisville of the University Farm, where the scientific and technological aspects of agriculture could be combined. Although the campus has now grown far beyond its dairy roots, animal and food scientists, veterinarians, economists, engineers and nutritionists continue to examine the many facets of milk in hopes of improving the ways we produce, process and consume it.

Forget for a moment the mystique of wine, the machismo of beer and the youthful effervescence of soda pop. Instead, contemplate milk.

From childhood we've glugged it down by the gallon and as adults admonished our kids to "Drink your milk; it'll make your bones strong and your teeth white."

But why? What is it about this murky white beverage that has positioned it over the ages as a staple food in the Western diet?

"Milk is such a hallowed commodity; it's the first thing a baby drinks from its mother's breast," says UC Davis dairy economist Bees Butler.

Others have a more technical view.

"I always say milk is a biological fluid selected by evolution to provide the best nutritional and immunological food source for the mammalian newborn," says UC Davis food scientist Moshe Rosenberg. "But we humans have decided over many thousands of years that we have other ideas."

Animal milk became part of the human diet sometime after people domesticated sheep around 9,000 B.C. and cattle and goats around 7,000 B.C. No one is quite sure exactly when the ancients started milking their animals, but pictures and written records from the Middle East indicate that dairying was part of daily life by 4,000 B.C.

Through the ages, people have obtained milk from a host of animals including goats, buffalo, sheep, yaks, horses, camels and, of course, the cow. The most productive dairy animals have one important thing in common: the rumen, a compartmentalized stomach that efficiently ferments grass, grains and other fibrous foods into the biochemical components of milk.

Behind its bland facade, scientists have discovered more than 100,000 different types of molecules in milk. In addition to water and fats, these include proteins, carbohydrates, minerals such as calcium, enzymes, gases and vitamins A, C and D, to mention just a few.

The proteins account for milk's creamy white color. Centrifuge the fat molecules out and milk will still be white, but continue centrifuging until the casein proteins drop out and you'll be left with a clear liquid.

A tale of two milks

And just how does cow's milk stack up against mother's milk?

No one knows that quite as well as UC Davis nutritionist Bo Lonnerdal, who studies breast milk to better understand its unique qualities and possibly also improve infant formula, which is made from nonfat cow's milk and vegetable oils.

Cow's milk and breast milk are ideal for their respective consumers--calves and infants, according to Lonnerdal.

Cow's milk--low in iron but rich in protein, calcium and vitamins--is perfect for the needs of rapidly growing calves. Remember, calves reach their adult size in just three years, compared to the roughly 18 years required by humans.

Breast milk, sweetened by high levels of the sugar lactose, seems more designed for fighting disease than for rapid growth. It contains only about one-third of the protein and one-fourth of the calcium found in cow's milk. And compared to cow's milk, human milk has much lower levels of casein, the proteins that make up the curds from which cheese is made. That makes sense, says Lonnerdal, because an infant's digestive system isn't equipped to handle so much curd.

"The digestive system of human infants is less efficient than that of cows or adult humans," Lonnerdal explains. "In adult humans, once a protein hits the small intestines our bodies jump on it and break it down into amino acids. It's a very quick process. If we have a meal, within an hour the proteins have broken down into amino acids, and they're already in the bloodstream."

But in an infant, some milk proteins are allowed to pass from the stomach intact, fighting off potential disease-causing microbes as they travel down the entire digestive tract. It's an ideal strategy that bolsters the baby's immature immune system. Studies have shown that breast-fed babies get fewer infectious diseases, particularly diarrhea, probably due to the antimicrobial components of the breast milk, Lonnerdal says.

These antimicrobials include antibodies from the mother plus lactoferrin and lysozyme, a powerful biochemical duo that gives a swift one-two punch to harmful bacteria. After lactoferrin pokes holes in bacterial cell walls, lysozyme penetrates and destroys the bacteria.

Lactoferrin is so potent that it is a likely candidate to be added to baby formula, Lonnerdal says. But finding sufficient supplies of this human milk compound, which is almost nonexistent in cow's milk, has been a challenge until now.

Lonnerdal turned to a seemingly unlikely colleague, UC Davis plant geneticist Raymond Rodriguez, to help develop a source of lactoferrin. Rodriguez, who for years has studied the genetics of rice and barley, was able to insert the gene for human lactoferrin into rice, developing rice plants that produce lactoferrin in the mature grain.

The lactoferrin-rich rice is similar to the much publicized "golden rice," genetically engineered to contain elevated levels of vitamin A. But rather than the yellowish color of golden rice, this rice is pink, due to the extra iron bound by the lactoferrin.

According to Rodriguez, new rice varieties like the golden rice and the lactoferrin rice mark a turning point for agricultural biotechnology. "In the next few years, consumers can expect to see new, genetically enhanced foods that will not only be more nutritious but capable of fighting disease as well."

But does every body need milk?

It's clear that milk is good for calves and as a basis for infant formula, but does it make sense for adult humans to be drinking cow's milk?

"Milk provides adults with protein and calcium, and has a lot of positive qualities," Lonnerdal says. "For women, milk is a very important source of calcium for preventing osteoporosis and is more convenient than taking vitamin supplements.

"It contains nutrients that are especially important for pregnant women and adolescent women whose bodies are still growing," he adds. "Many of the nutrients crucial for the body while it's growing are found in milk."

Of course children under the age of 1 year shouldn't drink cow's milk because it's too low in iron, vitamin E and some fatty acids, he cautions. And there is a significant segment of the population that literally can't stomach milk.

"The sugar lactose is the primary reason some people can't drink milk," Lonnerdal says. "Their bodies simply lack the enzyme necessary to digest lactose."

Interestingly, lactose intolerance is more common among Asian and African ethnic groups than among Northern Europeans, whose countries have a long tradition of dairying. The biochemical ability to digest milk may have evolved with the reliance of certain cultures on dairy foods.

Lonnerdal estimates that about 70 percent of the non-Caucasian population is lactose-intolerant. Some of those individuals absolutely can't digest milk, but most simply must limit their milk and dairy foods intake to avoid intestinal problems.

Another 3 percent of the population is actually allergic to milk and dairy foods, usually to a major protein called lactoglobulin, which occurs in cow's milk but not in human milk.

Making a good thing better

Convinced that milk truly does a body good, a number of UC Davis researchers are equally confident that they can improve on nature's recipe.

UC Davis animal scientists Gary Anderson and Jim Murray are using genetic engineering techniques to change the protein content of milk. They are trying to introduce the gene for human lysozyme--one of those powerful antimicrobial proteins--into the genetic machinery of cows so that they, too, will produce lysozyme in their milk.

Lysozyme-fortified cow's milk likely would reduce udder infections in dairy cows and intestinal ailments in humans who drink milk, the researchers believe. The gene transfer procedure has been successful in mice, and now Murray and Anderson hope to repeat it in goats and, eventually, cows. But the promising research moves slowly in cows, which have much longer gestation periods than mice and give birth to just one or two offspring at a time rather than a litter.

Down the hall from Anderson and Murray, other animal scientists are also tinkering with the composition of cow's milk.

Ed DePeters, an authority on dairy cow nutrition, is trying to make those changes in milk by altering the cow's diet.

To understand how a cow changes hay, grains and other feeds into milk, you first have to look at the rumen, that garbage-can-sized, four-compartment stomach.

"The rumen is a huge anaerobic fermentation vat filled with millions of bacteria," DePeters says. "We always say that we're feeding the cows, but in reality we're feeding the 'bugs'--the microbes.

"It's a symbiotic relationship between the microbes and the cow," he explains. "The animal maintains a level of acidity in the rumen that is good for digestion and provides a warm environment that's just right for the growth of the 'bugs.' In return, the microbes provide nutrients for the cow."

In the rumen, the fibrous feed stuffs are methodically fermented into fatty acids that are absorbed through the rumen wall, carried to the liver and eventually delivered to the udder where they are reconstituted into milk. The microbial population grows in the rumen as feeds are digested. Eventually the microbes pass out of the rumen to be digested in the small intestine. The protein in the microbes are a source of amino acids for the cow.

"Anything we do with the diet of the cow will affect its milk by altering the proportion of fatty acids produced in the rumen and available for metabolism by the cow," DePeters says.

Right now, he and his colleagues are particularly interested in a small group of fatty acids known as CLAs or conjugated linoleic acids, shown in laboratory rats to have anti-cancer properties.

In general, the chemical compounds called lipids that come from animal products are higher in CLA content than plant lipids, and lipids from ruminant foods are higher than non-ruminant foods. Even though CLA levels are quite low in ruminants, DePeters is certain they can be boosted significantly simply by feeding the cows more unsaturated fats, usually in the form of vegetable oils.

DePeters and animal scientist Juan Medrano are also working on selecting and breeding dairy cattle with higher levels of casein proteins in order to increase the amount of cheese that can be made from a given volume of milk. And they're trying to increase the levels of unsaturated fats in milk.

"The cow's rumen naturally tends to add hydrogen molecules onto unsaturated fatty acids and turn them into saturated fatty acids," explains DePeters. "Reducing the amount of saturated fatty acids in milk fat and increasing the monounsaturated fatty acid content can improve the nutritional properties of milk fat."

Medrano and DePeters have identified significant differences among various dairy breeds including Holstein, Jersey and Brown Swiss in levels of unsaturated fatty acids and CLAs, which are associated with the enzyme that converts saturated fats into unsaturated fats. In the process of identifying genetic markers for these traits, they have found that differences in the enzyme's level of activity are also genetically linked within breeds.

"We don't want to make dramatic changes," Medrano says. "We're simply interested in making a small increase in the proportion of unsaturated fatty acids because it would be healthier for people who consume dairy products and might allow us to produce butter that is soft and spreadable right out of the refrigerator."

More than a milk shake

As scientists develop a better understanding of what's in milk, they're also coming up with new ideas for utilizing those compounds.

"The more we study milk, the more we find that milk contains a complex array of components that, in addition to nutrition, bring along unique functional properties," says Cooperative Extension food scientist Moshe Rosenberg. "The question is, how can we fractionate milk and utilize these unique functional properties and introduce them into food and nonfood uses.

"For example, if you shake milk, you get a milk shake, which means there is something in milk that causes the formation and stabilization of air bubbles," he explains. "And for years we've been using milk in emulsions--mixtures of oil and water. So, what can we do with the emulsifying properties of wheys and caseins?"

Questions like these led Rosenberg several years ago to discover that whey proteins are ideal for use in microencapsulation, the process that makes tiny bead-like packages of various materials. He now holds the only patent in the world for using whey proteins for microencapsulation and studies how such proteins can be used to encapsulate and deliver not only food compounds but also pharmaceuticals and agrochemicals.

He's also busy developing analytical tools that will make it possible to identify and manipulate various attributes of cheese, such as texture, microstructure and flavor.

"We have developed new methods for sampling the flavor compounds that allow us to establish a chemical flavor fingerprint for cheese," Rosenberg explains. He's also studying the role that fat plays in maintaining the texture of cheese, hoping to establish a "filing cabinet" of different textures.

"The possibilities are endless for introducing new products," he says.

The bottom line

For researchers, the sky does seem to be the limit, but dairy producers and processors face very down-to-earth problems such as mastitis infections in cows, environmental issues related to waste management, food safety challenges and an ever-changing economic environment. If university research doesn't provide practical solutions to those problems, it won't go far beyond the pages of the scientific journals.

UC Davis agricultural engineer Michael Delwiche is doing just that as he designs "bio-sensors"--delicate instruments that can measure hormone levels that indicate where a cow is in the reproductive cycle. Some bio-sensors might detect unusually high levels of urea, suggesting an imbalance in the cow's feed and a potential waste-management problem. Eventually such sensors should also be able to pick up signs of elevated immunological activity in the cow, alerting dairy farmers to the onset of mastitis before the infection even produces symptoms.

In the environmental arena, Cooperative Extension animal scientist Deanne Meyer is spearheading the Environmental Stewardship Program, a statewide educational project aimed at helping dairy and other livestock producers develop plans for managing waste on their farms and ranches. Through a partnership with 14 other agencies, she's helping them understand the myriad of state and federal waste-management regulations and identify markets for their animal wastes, which can be sold as fertilizer.

Likewise, UC Davis agricultural economist Bees Butler provides educational outreach programs to help dairy farmers better understand the markets in which they are operating and to adjust to the gradual withdrawal of federal price supports for milk.

"The biggest issue facing the dairy industry is how it reacts to the risk environment created by the loss of price supports," says Butler, confident that the dairy industry will weather the changes and emerge stronger and more efficient as a result.

It's been 95 years since UC Davis was established with the intent of weaving science and technology into innovative solutions for California agriculture. Today the state is the nation's largest milk producer and despite the growing popularity of non-dairy beverages, the average Californian each year is still drinking more than 22 gallons of milk and gobbling down 28 to 30 pounds of cheese, plus massive quantities of ice cream, yogurt and butter.

UC Davis veterinarians and animal scientists, engineers and economists, nutritionists and food scientists would like to think that they and their predecessors have played some small part in that success story of milk.