Microbial Milk

carton of milk next to Oreo cookies and a glass

Don't leave that milk out on the counter!

You can't see, smell, or taste them, but bacteria flourish in mild temperatures and can survive in the carton even after you remember to put it back in the fridge.

"There are 450 deaths, 100 stillbirths, and 1,700 serious illnesses in the U.S. every year due to bacteria growing in milk cartons," says Sajid Alavi, an agricultural and biological engineering graduate student researching microbial growth in milk. Microbial growth is the growth of pathogenic bacteria—bacteria that can cause harmful diseases—as opposed to the beneficial bacteria used to ferment milk for yogurt and cheese.

The pathogen Alavi is most concerned about is Listeria monocytogenes, the cause of listeriosis, a disease attacking children, pregnant women, and the elderly. Listeriosis is a form of meningitis, also described as "brain fever"; it has a 40 percent fatality rate. The bacteria come into milk through infected cows.

Milk can be sold up to 14 days after pasteurization, the standard method for preserving it. A carton may sit around in your refrigerator for a few days to over a week beyond that selling date, and the bacteria are given a fresh opportunity to multiply every time you have a bowl of cereal or a glass of milk.

The optimal growth temperature for Listeria monocytogenes is 35 degrees C (95 degrees F), Alavi says, but it can survive at as low as four degrees C (39.2 degrees F), which is on the cool side for a normal refrigerator. Says Alavi, "Because this bacteria can survive and grow even at very low temperatures—temperatures at which most other bacteria cannot grow—this poses a serious concern for dairy foods."

Alavi has created a mathematical model which predicts the growth of Listeria monocytogenes inside a milk carton. To get the necessary data, he took a colony of bacteria cells and added it to a broth of nutrients and water—the first step in creating an "all-night culture." The next morning, he added a milliliter of the broth to sterilized milk, then took regular samples at varying temperatures and examined them under a microscope for bacterial growth. By collecting data at regular intervals, Alavi was able to write mathematical equations describing the correspondence between growth rate and temperature.

Then, using data collected by a commercial software program called ANSYS, he traced heat transfer inside a cardboard milk carton. Combining the microbial growth equations with those for heat transfer, Alavi created a dynamic model of bacterial growth.

Once introduced to milk, he found, Listeria monocytogenes experiences a lag period, taking time to adjust to its new surroundings. Following the lag is exponential growth. Then, while the cells are competing for nutrients, growth levels off for what Alavi calls a stationary period. Finally, many cells die due to a lack of sustaining nutrients. "At 25 degrees C, the cycle takes about 20 hours. At four degrees, it takes about 30 to 40 days, so we can at least learn the importance of storing milk products at low temperatures."

Alavi believes his model will give the dairy industry an easier way to test samples before milk is sold to the consumer. "Plating"—the technical term for testing for bacteria—is time-consuming and expensive. If temperatures seem possibly harmful, we can quickly test how much growth to expect using this model," he says. The model could also aid in designing future refrigeration apparatuses for the dairy industry.

In the meantime, don t stop drinking milk for fear of listeriosis, Alavi says. The Food and Drug Administration regularly checks on the milk supply. "If even one cell is found in a product, the FDA will recall the entire batch," he notes. "Including recalls and loss of productivity due to this bacterium, $250 million is lost annually in the U.S."

His research on this deadly pathogen certainly hasn't scared off Alavi, who stores his milk at a low temperature and puts his faith in the FDA. "Knowing what I know," he says, "I still drink four or five glasses of milk a day!"

Sajid Alavi received his M.S. in agricultural and biological engineering in May; he is now a graduate student at Cornell University. His adviser was Virendra Mohan Puri, Ph.D., professor of agricultural and biological engineering, College of Agricultural Sciences, 229 Agricultural Engineering Bldg., University Park, PA 16802; 814-865-3559; v7p@psu.edu. Principal investigators on this project are professors Puri, S.J. Knabel of the department of food science, and Rabi H. Mohtar of the department of agricultural and biological engineering. Their work is funded by the USDA and the Pennsylvania Agricultural Experiment Station.

Last Updated January 01, 1998