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User @rumtscho notes in another question's answer:

If you are trying to keep the milk for multiple days in the fridge

It will become unsafe after 3-5 days, there is no way around that. If you switch to UHT milk, it will still be unsafe 3-5 days after opening the container, but it won't taste bad, so ...

What does this mean for the milk and the decision of whether to discard it?

(Traditionally I would discard milk if it tastes off to me.)

Specifically, a trajectory explanation of which microorganisms are killed of in the milk via UHT, so that it might not go "bad" taste wise, would be helpful.

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    I'll leave @rumtscho to write a definitive answer but I think it will boil down to: in your life, 'safe' is a personal judgement; on this website, 'safe' means 'in accordance with government food safety regulations'. The two are not generally identical and in some cases will diverge dramatically, which is constantly confusing to new users, but it is the only reasonable approach from StackExchange's position.
    – dbmag9
    Commented Nov 11, 2022 at 11:22
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    In response to your update. All microorganisms are eliminated in the processing of UTH milk. That is the point of the process, to make it shelf stable.
    – moscafj
    Commented Nov 11, 2022 at 13:26
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    @moscafj Presumably the difference is that fresh milk generally has some (lactic acid?) bacteria, so it's those that will usually multiply first and cause spoilage. Heat-treated milk won't come with those bacteria, and so it'll get spoiled only after some ambient bacteria have colonised it and grown: not only does that take longer, but (depending what type(s) of bacteria grow) it's less likely to become noticeably spoiled when they do, and so lack of spoilage is no longer any guide to safety.
    – gidds
    Commented Nov 11, 2022 at 17:38
  • @gidds yes, the food safety risk for opened UHT occurs from the potential of pathogens introduced from the environment.
    – moscafj
    Commented Nov 11, 2022 at 17:43
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    @FuzzyChef Your comment seems like you think I'm advocating we use a personal opinion standard here. I'm not, I'm explaining that we don't ("it is the only reasonable approach from StackExchange's position").
    – dbmag9
    Commented Nov 11, 2022 at 20:21

1 Answer 1

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Most food borne pathogens are colorless, odorless, and flavorless, and will not always impact the taste of food. Smell and taste is not an indicator of food safety. There is a difference between spoilage (which would impact flavor and odor, but not always make something unsafe) and pathogen growth (which would make a food unsafe). For example, sour milk is generally unpleasant for most of us, but generally not unsafe. The response to your quoted answer is reasonable in terms of how food safety works. I would quibble with the 3 - 5 days. What I see on the internet is that UHT milk is shelf stable for months until opened. Then it should be refrigerated and used within 7 days-10 days if continually refrigerated.

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    I'd love to see some citations added to this answer. It's already good, but EG to support "Most food borne pathogens are colorless, odorless, and flavorless, and will not always impact the taste of food."
    – Matt Morgan
    Commented Nov 11, 2022 at 17:59
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    @MattMorgan many hits come up when you google this...here is the first one that came up for me: extension.wsu.edu/asotin/health-wellness/consumer-food-safety To quote, " Food that contains a foodborne pathogen will look, smell and taste normal for the most part. Generally speaking, most bacteria and viruses that cause foodborne illness are odorless, colorless and tasteless."
    – moscafj
    Commented Nov 11, 2022 at 18:09
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    There is also a difference between "most" (we probably don't know many of them at all) and "most common". I'd venture the guess that we have developed the ability to detect many common cases with our senses, specifically by smell. For example, rotten meat, a major and, without refrigeration, common health hazard, smells terrible. Of course, what risk was acceptable in the paleolithic and what's acceptable in an industrial nation differs.
    – Peter - Reinstate Monica
    Commented Nov 11, 2022 at 21:46

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