A writer from the Ming Pao Newspaper contacted the dietitian I volunteer for and asked,

I’ve heard that the expiration date on the milk carton is only good for milk that hasn’t been opened. Is that true? How long can milk last or is there any nutrition loss after the carton’s open?

So, the dietitian asked me if I wanted to do some research.  And of course, I said YES!

There are several categories of date labelling on pre-packaged foods (the first 3 are essentially the same):

  • Durable life
  • “Best before”
  • “Use by”
  • Expiration

I realised that all the milk cartons at my local supermarket had “best before” dates (not expiration).  According to the CFIA, the durable life is

the anticipated amount of time that that an unopened food product, when stored under appropriate conditions, will retain its freshness, taste, nutritional value, or any other qualities claimed by the manufacturer.

Something important to realise is that milk can be consumed even after the printed date because “best before” dates are not indicators of food safety.  Instead, they simply represent the time frame when the product is at its optimum.  Nonetheless, think twice before chugging that old milk since some invisible microorganisms can induce foodborne illnesses.  A rule of thumb:  “when in doubt, throw it out!”

An opened carton of milk can typically last around 5 days after the “best before” date, depending on:

  • Milk type
  • Processing method
  • Thermal cycling
  • Storage conditions

According to dairy researchers at Cornell University, there are certain bacteria that develop more slowly in skim milk.  So, milk with less fat tend to last longer.

In terms of milk processing methods, there are two main techniques:  pasteurisation and UHT.  Unlike UHT, pasteurisation doesn’t kill all microorganisms so pasteurised milk does not last as long as UHT-treated milk.  On the other hand, UHT-treated milk an remain shelf stable for 6 months without refrigeration.

As for thermal cycling, milk that is less frequently exposed to unrefrigerated temperatures will last longer.

Most importantly, milk must be stored at refrigerator temperatures 4°C or below.  Also, milk should be stored on refrigerator shelves (not the doors because they’re generally warmer).  And, milk cartons should be kept closed and placed away from strong smelling foods to avert any loss in flavour and quality.

Finally, after the milk carton is opened, there is no significant loss of nutrients (especially before the printed date).  However, when milk begins to go bad because of spoilage formers, those microorganisms will consume the nutrients.

PS.  If the food is expired, throw it out :)