First of all, there is a reason Louis Pasteur, the French chemist, became famous. Thanks to his 19th Century discovery of a technique known as pasteurization, the public health issues associated with food spoilage and the consumption of raw milk were significantly reduced or eliminated. Today, like two centuries ago, the process is essentially the same. The heating of food and beverages (including milk) to a certain temperature kills most pathogens and inactivates the enzymes responsible for food spoilage, resulting in safer food and longer shelf life of fresh products. However, the temperature to which milk is heated, and the length of time it remains heated determines the shelf life. So, if you’ve noticed “on farm” pasteurized milk expiration dates are different than milk purchased in grocery store, there is a reason. Here’s the scoop on the comparison of pasteurization processes and why expiration dates are different.
Low Temperature — Low temperature or vat pasteurization milk is heated in small batches to a lower temperature (145°F) for a longer time of 30 minutes and then rapidly cooled. This was the original method used in the milk industry. Today, it’s used only for artisanal butters, cheeses and for some “farm fresh” milk. Due to the “low and slow” process, flavor may be different, and the milk is not homogenized (fat dispersed within the milk) so a visual layer of cream coats the top. Some proponents cite the nutrient content is better than the high temp pasteurized milk, but studies show there are no significant differences. The shelf life of low-temp pasteurized milk is shorter so if you want to experience the “cream on top” and are willing to pay the difference, this is an option.
High Temperature — High Temperature Short Time (HTST) is the most common method of pasteurization in the United States today for conventional milks. This process uses metal plates and hot water to raise milk temperatures to at least 161° F for not less than 15 seconds, followed by rapid cooling. The shelf life is longer than the low temp due to the temperature used for eliminating the enzymes responsible for food spoilage.
Ultra Pasteurized — Ultra Pasteurized (UP) milk uses the same process as HTST pasteurization but must be heated to not less than 280° F for two seconds. UP pasteurization results in a product with a longer shelf life but refrigeration is needed. Typically, organic milk available in retail grocery stores is UP pasteurized.
All pasteurized milk is safe and nutritionally equal. The deciding factors come down to the desired “at home” shelf life, taste, price and preferred production method. The good news is that we have choices!
- Pasteurization Explained, International Dairy Foods Association.
- “Raw Milk Alternatives,” by Cookson Beecher, Food Safety News, August 12, 2010.
- Rachel Kopay, Dairy and Food Safety Consultant, Boston, Massachusetts.
- Ask the Experts: Why does organic milk last so much longer than regular milk? Scientific American, September 1, 2008.