by Rebecca Sanderson
Is the milk expiration date really a cut-off where you can no longer safely drink the milk? Is it guaranteed to stay good until that date? How can we tell if milk has gone bad or not?
You go to your kitchen one morning, just like any other. You pour yourself a bowl of cereal, set it on the counter, and then open the fridge for the milk. After dousing your cereal, you take a huge bite only to spit it out. The milk has gone sour! Looking at the milk carton, it is dated two days past. I am certain that I am not the only one who has played out this exact scenario at least once in their life. However, I have also experienced times when the milk stayed good for several days past the milk expiration date. What makes the difference?
Some milk processing plants will use an expiration date on the carton of milk while most will use the term “best by” before the printed date. While the two terms are often viewed the same, they aren’t quite. The milk expiration date is the estimated time frame in which that milk should stay good if handled and stored properly. This is based upon processing methods, how the milk is packaged, and time. A manufacturer guarantees the quality of a product up to the “best by” date even though the product may be perfectly safe to eat for a period of time after that date. After the “best by” date the taste, freshness, or nutrient quality may be diminished. When it comes to milk, as long as the carton has remained unopened, whole milk is good for five to seven days after the “best by” date, reduced fat and skim milk for seven days, and lactose-free milk for seven to 10 days. If you have already opened the carton of milk, you can expect it to still be safe to drink for five to seven days past the printed date (How Long Does Milk Last?¹). True expiration dates, when a product is deemed unsafe from that time on, aren’t used very often on food items.
There are a number of factors that will determine the shelf life of milk. First, the way that a processing plant processes the milk will affect the milk expiration date. Standard pasteurization methods quickly raise the temperature of the milk to 161 degrees for 15 seconds then rapidly cool it. This is called High Temperature short time pasteurization. Vat pasteurization brings milk to the temperature of 145 degrees for 30 minutes then before rapidly cooling (Pasteurization²). A recently tested method at Purdue University takes already pasteurized milk and sprays tiny droplets through a machine that brings the temperature up by 10⁰ Celsius (50 degrees) for less than a second before rapidly lowering the temperature thereby killing 99 percent of the bacteria that was left after standard pasteurization. Milk processed by standard pasteurization lasts for two to three weeks while milk that has gone through the new method can last up to seven weeks (Wallheimer, 2016³). The way milk is stored highly affects how long it lasts. Pasteurized milk is sensitive to light, so keeping it in a dark environment and toward the back of the fridge will help it last longer. It is also sensitive to temperature fluctuations, adding another reason to store milk in the back of the fridge rather than in the door which temporarily raises the temperature with every opening. Keeping your fridge set to 40 degrees or lower will keep your food fresh longest. This temperature should even be maintained in the door of the fridge and should be occasionally checked by thermometer. When not kept at safe storage temperature, your milk (and other foods) will not remain fresh or safe to eat even as long as the expiration dates. Milk can be safely frozen for up to three months, but the quality will be greatly affected. Previously frozen milk is often yellow in color and lumpy.
How can you tell if your milk has gone bad? First, is it near or past the milk expiration date? Second, open the carton and breathe deeply. Bad milk has a strong sour smell. It is also usually lumpy in texture. It is not likely that you will mistake milk that has gone bad. Milk turns sour due to the small number of bacteria that survived the pasteurization process having time to multiply and produce lactic acid. Sour milk is NOT safe to drink! I doubt that you would be tempted.
The milk expiration date, or “best when used by” date is mostly a guideline of how long the milk will taste optimally fresh when handled and stored properly. It can last a solid week longer when stored well; however, not storing milk properly will cause it to go bad early. Pasteurization methods have extended the shelf life of milk to several weeks from the time of processing when otherwise it would go bad after only a single week if not used. By following the guidelines outlined in this article, you may rest assured that you may enjoy your milk for the longest period of time possible.
¹ How Long Does Milk Last? (n.d.). Retrieved May 25, 2018, from EatByDate: http://www.eatbydate.com/dairy/milk/milk-shelf-life-expiration-date/
² Pasteurization. (n.d.). Retrieved May 25, 2018, from International Dairy Foods Association: https://www.idfa.org/news-views/media-kits/milk/pasteurization
³ Wallheimer, B. (2016, July 19). Rapid, low-temperature process adds weeks to milk’s shelf life. Retrieved May 25, 2018, from Purdue University: https://www.purdue.edu/newsroom/releases/2016/Q3/rapid,-low-temperature-process-adds-weeks-to-milks-shelf-life.html