Stinky, Chunky, Very Cool

NDSU Microbiology
by NDSU Microbiology

Guest Blogger, Rhiannon Dockter, VMS Undergrad

Oh my dear Lord up above, what is that smell?! Oh, that’s right; that’s my science experiment: watching milk rot…for about a month. While spoiled milk is a nuisance for most people, I thought it was the best science experiment, like, ever. This was prior to my public school education, which began in middle school, so being normal wasn’t yet a concern. Now that I’ve been in college for a few years, that desire to be normal has gone out the window, yet again, so again, I want to talk about my rotting milk experiment.

This was run of the mill, acidophilus 2% milk. Lactobacillus acidophilus is a pretty dandy probiotic that strengthens digestive health. Milk contains other lactic acid bacteria and coliforms. The lactic acid bacteria are usually the good guys, while the coliforms come from poop and make your milk look like…well…the same. At least they do after a month.

Milk is pasteurized to reduce microbes (specifically pathogens); it also carries its own antibacterial mechanisms, like lactoferrin and lactoperoxidase, which work to keep bacterial growth to a minimum. These factors help to prevent milk spoilage and keep it safe. However, these factors can only hold out for so long against a weirdo with time on her hands. In my experiment, I was able to bear witness to waves of different colors, textures, and smells as the environment changed to allow for optimum growth of different microbes. I didn’t know about all of the intrinsic and extrinsic properties of food at this time, so I can’t blame this on anything other than morbid curiosity.

I witnessed a few things which would pop up again later in my microbiology labs, such as gas production, color changes, and colony growth on a mass of solid, jiggly, nasty smelling stuff. Dr. Teresa Bergholz’s Food Microbiology course has really brought my weird experiment full circle, giving proper explanations for my observations:

  • The enzyme lipase was responsible for the horrendous smell, which infiltrated my garage. This water soluble enzyme hydrolyzes lipids into fatty acids, resulting in rancidity if left to fester.
  • Protease is another enzyme, which was likely at work in my milk experiment. This enzyme performs proteolysis, or hydrolysis of the peptide bonds in a protein.
  • Pseudomonas aeruginosa probably contaminated my milk and proceeded to release proteases into the environment. Milk contains many proteins, including casein, lactalbumin, and lactoglobulin. These proteases then degraded proteins and created free amino acid chunks for P. aeruginosa to use. This proteolysis explains why the milk became chunky.
  • The black growth in the milk could have been any number of molds that had been introduced after pasteurization. As the lactic acid bacteria began to run amok, the pH became low and selective for fungi.
  • These lactic acid bacteria may have included lactococci, lactobacilli, and Leuconostoc.The lactococci would account for the high viscosity of the portion that eventually floated to the top. Lactococci and Leuconostoc are heterofermentative bacteria, which also may have broken down sugars in the milk and produced the gas (CO2) that I observed. These bacteria flourish at 18°C, which is about the temperature that my garage is held at.

Though I’m an imperfect, amateur researcher, who has no authority to suggest a particular research MO, I’m still going to: Do the weird thing. Do the thing that no normal person would do. Life doesn’t have to be lived with a target in the sights. When in doubt, consider Alexander Fleming. He was messing around with mold and ended up saving innumerable lives.

Image of lactococci: CDC/Dr. Richard Facklam

This entry is part of the MICR 354 (Scientific Writing) student-blog series.