In this question I asked why lactose-free milk lasted such a long time. The answer was because it is ultra-pasteurized. This leads naturally to another question: does lactose-free milk have to be ultra-pasteurized?

My initial guess is that it does because:

  • Since the lactase added to make the product lactose free breaks down lactose into twice as many sugar molecules, any living bacteria have twice as much nutrients to thrive in.

  • Since the disaccharide lactose has been broken down into the simple monosaccharides glucose and galactose, the bacteria do not need to produce their own lactase to use the sugar; thus any non-lactase producing bacteria can thrive in the milk.

  • And finally, I have been adding lactase to regularly pasteurized milk to produce my own lactose-free milk. However, this milk almost always goes bad a couple of days before the Best-By date stamped on the container.

  • This all leads me to think that lactose-free milk must be ultra-pasteurized in order for it to have any reasonable shelf life.

    • $\begingroup$ Regular pasteurised milk always goes bad within a few days after opening the bottle, which contaminates it with a million of funny organisms from the air we breathe. $\endgroup$
      – Karl
      Jul 19, 2017 at 20:01
    • $\begingroup$ Hmm...good point. So your view would be that it would probably go bad before the best-by date with or without the added lactase? Although...the bacteria that live in the air wouldn't find much sugar to consume in regular milk, since they would probably not be able to use the lactose in the milk. $\endgroup$
      – BillDOe
      Jul 20, 2017 at 18:53
    • $\begingroup$ I'll say the answer is no, you don'T have to, but should probably drink the lactose-free stuff a bit faster. What you say sounds reasonable, but the sugar contents might not be the important factor. $\endgroup$
      – Karl
      Jul 20, 2017 at 19:08
    • $\begingroup$ Consider the question of how long you'd leave some other random meat product out in the refrigerator. $\endgroup$
      – Zhe
      Nov 16, 2018 at 13:58

    2 Answers 2


    You can now buy (at least in Switzerland where I live) lactose-free milk which is not ultra high-temperature pasteurized. They are kept chilled in the supermarket alongside "normal" milk. So I would say based on this, no lactose free milk does not have to be ultra pasteurized. Though I will note, they hardly tasted sweet at all and they are quite expensive, nearly double the price of ordinary milk.


    I can't answer why they ultra pasteurize it with absolute certainty but my guess would be because there is less demand for lactose free milk so the extended shelf life will prevent waste if it doesn't sell quickly.

    Sugar, like salt, retards bacterial growth, it doesn't increase it. That's why candy and sugary baked goods aren't refrigerated. The 2 types of milk contain the same amount of sugar.

    Regarding your milk going bad after you've added the enzyme, you could unwittingly be introducing more bacteria into your milk during that process.

    I know this is an old post, but I came across it because of having the same question about lactose free milk and this was the conclusion I've come to.


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