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Wikipedia says that the shelf life of UHT milk is "typically" between 6 and 9 months.
Does it mean that UHT milk always falls in this range? Is there UHT milk which has a longer or shorter shelf life? (I am not talking about ESL milk here, just UHT). What determines which milk has 6 months, which has 9 months, and which has more (if more is possible)? Is there a variation in the process used?
I'd never heard of ESL until your question, so I had to look that up. A local company did a lot of work developing UHT milk; I remember samples when they were first starting up production, and they were trying to find flavors additives which would mask the flavor change from the UHT processing. Since then a lot of companies have adopted the processing methods for other products, or just adopted the packaging. So, before I relied on any random box-packed product to be shelf stable, I'd check into how it was produced.
Proper UHT milk doesn't go bad until it's opened. It's sterile, and in a pretty sturdy container. Once opened, bacteria will get back into the container, and the milk will go bad as normal. When the local company started production of UHT milk, they were putting 2 year code dates on packages, and that was just the longest date the government would allow; I've had boxes 3-4 years old, which were fine.
McGee's on Food and Cooking states that once opened
Pasteurized milk stored below 40°F/5°C should remain drinkable for 10 to 18 days.
It continues on the different types of pasteurization (bold mine)
There are three basic methods for pasteurizing milk. The simplest is batch pasteurization, in which [...] a few hundred gallons [of milk is heated at] 145°F/62°C for 30 to 35 minutes.
Industrial-scale operations use the high temperature, short-time (HTST) method, in which milk is pumped continuously through a heat exchanger and held at a minimum of 162°F/72°C for 15 seconds. [...]
The third method of pasteurizing milk is the ultra-high temperature (UHT) method, which involves heating milk at 265-300°F/130-150°C either instantaneously or for 1 to 3 seconds, and produces milk that, if packaged under strictly sterile conditions, can be stored for months without refrigeration. [...]
Sterilized milk has been heated at 230-250°F/110-121°C for 8 to 30 minutes; it [...] keeps indefinitely at room temperature.
I would assume that longer pasteurization (e.g. 3 seconds) and the higher temperatures (e.g. 150°C rather than 130°C) could add a few months before the milk starts to spoil.
