Why can this milk be stored for months?
Generally speaking, milk has a short shelf life. Even milk that has undergone pasteurization to kill most bacteria can only be stored in the refrigerator for 10-15 days. This period seems long enough, but for the past 50-60 years, milk, without refrigeration, has been stored on convenience store shelves for months, not days or weeks. The ubiquitous wax-coated milk cartons undergo sterilization, some of the effects of which are well-known, while others you may never have heard of.
Pasteurization, named after its inventor Louis Pasteur, kills tuberculosis bacteria and several other pathogens. Pasteurization involves heating milk to 72°C and holding it at that temperature for about 15 seconds, then allowing it to cool. This temperature does not kill any microorganisms; many harmless bacteria will still survive when the milk is refrigerated and consumed. Hilton Deeth, a dairy expert who recently retired from the University of Queensland, says that most milk sold in countries like the US, Australia, and New Zealand is pasteurized.
However, many European countries favor ultra-high temperature (UHT) milk. This type of milk is sterilized at 140°C, twice the temperature of pasteurization, for only 3 seconds. This high temperature instantly kills all bacteria, including most heat-sensitive spores. This milk is virtually sterile and can be stored for a very long time as long as it's kept in sterilized containers. Without bacteria, it won't spoil as long as the packaging remains intact.
However, Deeth warns that even UHT milk cannot guarantee it won't spoil. Its designed storage temperature is 20-30°C. Therefore, if UHT milk is transported by cargo ship through tropical regions near the equator, or if the ship itself has poor ventilation, or if the milk remains at a tropical port for too long, the few surviving spores can become activated and cause the milk to spoil. At the same time, some peculiar changes occur in this long-term stored milk: gel may form inside the packaging, and if you open the package to pour out the milk, "it will be like thick yogurt or lumpy milk sauce," says Dis.
The cause of these problems can be traced back to the chemical reactions in the high-temperature sterilization process. The instantaneous ultra-high temperature causes the milk proteins to denature. The original whey proteins are broken down, forming poorly reactive chain structures. The Maillard reaction, famous for creating the delicious caramel flavor, is a chemical reaction between milk proteins and sugars, and is also the source of the brown color in toast, bacon, and many other foods. Furthermore, without pretreatment to remove a substance called "fibrinolytic enzyme," high temperatures can cause this enzyme to break down to a certain extent, producing a sulfur-containing complex that gives milk a rotten egg smell that takes up to a week to completely dissipate.
If fibrinolytic enzyme remains active, it will break down various proteins, releasing them from their original locations and causing them to recombine into new structures. This may be the source of the gel-like substance in milk. The Maillard reaction may be the reason why UHT milk is sweeter than pasteurized milk. Dis emphasizes that UHT milk is also whiter than pasteurized milk. This may be because whey proteins and other substances can reflect light. In addition, molecules also contribute to the milk's off-flavor.
Because different groups of people have varying tolerances to these off-flavors, the market share of UHT milk varies greatly around the world. For example, in China, where milk consumption is growing rapidly, UHT milk is sold everywhere. "The Chinese dairy market is expanding rapidly at a rate of 10% annually, and this trend has continued for several years," Dis says. "China's sales of UHT milk are enormous." One of the driving forces behind the development of the dairy industries in countries like Australia, New Zealand, and Germany is the export of long-shelf-life milk to China.
However, UHT milk, with its long history, has a significant drawback: it cannot be used to make cheese. Cheese making is a two-step process: first, rennet is used to break down the proteins, then they clump together and solidify into a gel-like consistency. Dees says that the spoiled whey proteins throughout the mixture prevent gel formation (only 5-10% of the whey in pasteurized milk spoils, so this problem doesn't occur). Dees has done arduous research on this. He and a postdoctoral researcher tested all the possible scenarios but still couldn't solve the problem.
"One morning I came to the lab," Dees recalls, "and the postdoctoral researcher said, 'I prepared a piece of gel to make cheese... but I didn't leave until 1 a.m. Cheese usually solidifies in two hours, but this so-called "cheese" took a full 11 hours to solidify.'" "Making cheese from UHT milk still requires further research," he said. "Even if it succeeds in the future, the result will mostly be a product with a high water content, similar to soft cheese. Furthermore, due to insufficient effective enzymes, the taste of cheese made from UHT milk is unlikely to be very good."