Why does organic milk last so much longer than regular milk?
Craig Baumrucker, professor of animal nutrition and physiology at Pennsylvania State University, pours out an answer:
This longevity disparity actually has little to do with whether or not milk is organic. Rather organic milk frequently lasts longer—as long as a month, compared with about a week for regular milk—because producers use a different process to preserve it. According to the Northeast Organic Dairy Producers Alliance, organic milk needs to stay fresh longer because it is produced in fewer dairies and generally has to travel farther to reach store shelves.
The process that gives the milk a longer shelf life is called ultrahigh-temperature (UHT) processing or treatment. UHT-treated milk is heated to 280 degrees Fahrenheit (138 degrees Celsius) for two to four seconds.
Compare that with pasteurization, the standard preservation process. There are two types of pasteurization: “low temperature, long time,” in which milk is heated to 145 degrees F (63 degrees C) for at least 30 minutes, or the more common “high temperature, short time,” in which milk is heated to roughly 160 degrees F (70 degrees C) for at least 15 seconds.
The difference in temperatures hints at why UHT-treated milk lasts longer: pasteurization does not kill all the bacteria, just enough so that you do not get a stomachache. UHT, on the other hand, wipes out everything.
Retailers typically give pasteurized milk an expiration date of four to six days after delivery to the store. Before delivery, however, there was up to six days of processing and shipping, so the total shelf life after pasteurization is usually up to two weeks. Milk that undergoes UHT, if packaged properly, does not need to be refrigerated at all and can sit unopened at room temperature for up to six months.
Regular milk, like organic milk, can undergo UHT; much of the milk in Europe, for instance, is UHT-treated. So why isn't all milk produced this way?
One reason is that UHT destroys some of milk's vitamin content—not a significant amount—and affects some of its proteins, rendering milk unusable for cheese. More important, though, UHT-treated milk tastes different. UHT sweetens the flavor of milk by burning, or caramelizing, some of its sugars. Many Americans find this flavor offensive—just as they are leery of buying unrefrigerated milk.
How long does cellular metabolism persist after death?
Arpad Vass, a forensic anthropologist at Oak Ridge National Laboratory, examines this morbid mystery:
As best as anyone can gauge, cell metabolism continues for roughly four to 10 minutes after death, depending on the ambient temperature around the body.
During this interval, oxygenated blood, which normally exchanges carbon dioxide with oxygen, is not circulating. The buildup of carbon dioxide produced by cell respiration lowers the pH level of the cells, creating an acidic intracellular environment.
The acidic environment causes intracellular membranes to rupture—including those around the cells' lysosome, which contains enzymes for digesting everything from proteins to fats and nucleic acids. The burst membranes release the enzymes, which begin to digest the cells from the inside out—a process known as autolysis, or self-digestion.
The rate of autolytic spread depends on the local density of enzymes; the dispersion in liver tissue, which is rich in these proteins, is likely faster than in lung tissue, which has a smaller reserve. Autolysis also progresses more quickly in water-rich tissues such as those of the brain.
Environmental temperature is even more critical to regulating autolytic spread. Warm surroundings speed up the self-digestive process, whereas cold conditions retard it. For this reason, people who have drowned in very cold water can sometimes be revived even after relatively long periods. In such cases, the cold has slowed the autolytic process enough to prevent permanent tissue damage.