A colleague of mine just got a new thermos (inox), this model, and actually read all instruction, and we were surprised to read that it is not recommended to pour hot milk in the thermos.

My opinion was that:

  1. Unlike tea of coffee, a lot of micro-organisms live in milk, will end up being a favorable place for those micro-organisms to generate a culture (which will be more and more difficult to get rid off, and will contaminate any other fluid poured in the thermos)
  2. If you don't wash it straight away, it will really stink.
  3. The milk being full of water-insoluble proteins/fat, it will "coat" the internal thermos walls (gross!) - I've already seeing that "coat" forming in plastic containers, and I guess it won't be any different in inox containers.

I've actually not found an answer online, so I thought that here I could find some experts. I realize this could be a cross-question for Biology SE, but I felt it more food-related.

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    @J.Doe please don’t post answers in the comments. It bypasses quality control measures such as downvoting. Thanks! – Tim Nov 21 at 16:12
up vote 16 down vote accepted

It depends on the design. Some can't be washed properly, and milky liquids are much harder to clean off by rinsing than water or most water-based drinks

My genuine Thermos brand flask wouldn't be a problem because all the surfaces that come into contact with the food are accessible.

My previous small cheap flask had a pouring system in the lid that meant the contents passed through a non-washable chamber (clipped together in a way that wasn't designed to be opened). I eventually forced it open to find it full of hot chocolate residue. My flask is mainly used for kayaking, it has to be made up beforehand as fiddling about with powders when on a riverbank in the pouring rain doesn't tend to get you a hot drink when you need it. At that point it would have gone in the bin even if I hadn't just broken it.

A point mentioned in the comments is that the instructions for one particular Thermos say This product must NOT be used for keeping milk products or baby food warm or cool, to avoid the possibility of bacterial growth. This is the only reference to warm contents in these instructions; the word used elsewhere is hot (excluding washing-up instructions). Warm implies attempting to maintain temperatures in the danger zone. (for completeness as comments don't always last)

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    makes sense! I added a link to the model of the thermos I am talking about. true that in out case the chamber seems to be fairly accessible, but the top is very hard to disassemble and clean, thus milk can be stuck in there if you pour it through that top (what usually people do) – aechchiki Nov 19 at 12:10
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    Yes, inside the inner lid is a hard-to-get-at incubator for all sorts of unpleasant life forms. – Chris H Nov 19 at 12:43
  • FWIW, "my genuine Thermos brand flask" would be a problem since it has one of those "pouring systems" in the lid that is not entirely washable. – MrWhite Nov 20 at 22:34
  • @MrWhite mine might be quite old. I don't know as I obtained it by accidentally swapping with another the same size and shape a few years ago. That had a pouring system but you could get into it. Next time I'm buying one I'll look for one that doesn't, or disable it. The pouring system has one advantage - it reduces cooling of the remaining contents while serving. – Chris H Nov 21 at 6:46

The cleaning is one thing (there are flasks without hidden crevices that can be easily cleaned), but there’s also another point to consider:

A thermos flask that is doing its job will keep food warm, or rather, slow down the cooling process. Even if you fill it with fairly hot liquid, it will slowly cool. If you do that with a perishable food like milk, you will easily create an environment in the upper range of the danger zone, where food will become unsafe after as little as two hours.

Yes, you can argue that you are filling hot milk in a clean container, but in the end, it’ll be not safe by food safety standards. (What you make of that is obviously your choice.) The manufacturer’s warning will protect the end users from food poisoning.

  • Comments are not for extended discussion; this conversation has been moved to chat. – rumtscho Nov 20 at 13:27
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    "...will keep food warm" - or cold. – MrWhite Nov 20 at 22:38
  • We often used make hot chocolate for the kids when we go out, as well as a flask of tea for ourselves. Twice, after quite a lengthy drive (>3 Hrs), we tipped out lumpy brown sludge. The first time, I put it down to contamination of the poorly-designed pop-up lid (hard to wash properly), so we got a new flask and then it happened again. I finally realised that what was really happening was this. Now we make milk-shake, and put ice-cubes in for good measure! – SiHa Nov 21 at 13:33

Another aspect might be the engineering behind thermoses.

A thermos has a layer of vacuum between two layers of glass(At least back in the day it used to be like this),

This meant that the glass will break in case of rapid expansion due to hot liquids poured immediately. Milk has a higher heat capacity compared to diluted beverages, so that means it can hurt the flask even more.

Maybe they have engineered this problem away, but might be one of the reasons! -BD.

  • This is the reason why my grandma used to put a metal spoon in any glass she would pour any hot liquid into (tea, coffee...), to avoid it to be broke by the hot liquid. However i see here that milk has lower specific heat than water ( engineeringtoolbox.com/specific-heat-fluids-d_151.html) - is this what you are referring to? – aechchiki Nov 20 at 20:30
  • Plus, nowadays the thermos (at least the model that I linked to in the question) are not made of glass anymore (safety issues I guess) – aechchiki Nov 20 at 20:32
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    The glass in an old thermos was very thin, and could easily take boiling water being poured in (thinner means less thermal stress). The specific heat capacity of milk is very close to that of water, given that milk is 88% water, and the other components are nothing remarkable. In fact it's a tiny bit less – Chris H Nov 21 at 7:00

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