Question: I live in Japan and milk is sold only in cartons here. I take milk to the fridge at work. I currently put it in a black plastic bottle with black cover but it stinks like it is off after only a few days. The milk is fine as far as I can tell. I doubt there is food safe plastic bottles here. I carry by bicycle and the bar fridge is small and at times very crowded as it shared so spillage is a great risk. What is the best solution? I use the milk for adding coffee and cocoa- I may have one drink or many drinks- depends on how sleepy I am that day. I buy 1 litre cartons as I am poor but I couldn't drink 1 litre at work in a week even. The main milk is always fine.

Answer: 
The key thing is that you **empty and wash the container regularly** (ideally daily, but at least every few days). If there is a bad smell that is a sign that something is wrong; most likely some milk remained somewhere like a screw top or drops on the lid, which has gone off in some way.

**Any bottle sold for drinking water should be food safe** in terms of its material, so I would be very surprised if you cannot find anything. However, I would try to find a bottle which:

* is **easy to clean** completely, so no complicated closing mechanism or built-in drinking straw.
* will provide reasonable **insulation** while the bottle is out of the fridge, for example a thermos bottle (which uses vacuum in the walls to insulate the contents).

Anything suitable for carrying hot coffee is likely to work well for your needs, in case that's a useful way to search.
Your question is not entirely clear, so I will answer for both possible options.

If you are trying to keep the milk for multiple days in the fridge
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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 some people prefer to do it that way.

So the solution here would be to reduce the amount you are bringing to your work.

If you are bringing new milk daily, but the bottle stinks
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You are probably cleaning the bottle improperly, as the dbmag9's answer and some comments mentioned. Here, you have several options, ordered by my personal preference:

1. Stop using a (typical) bottle.

* As ChrisH said, you can start using a jar, just test first if it leaks when you shake it vigorously. You can specifically search for buying a "leakproof jar" if you find that all random jars you have happen to be leaky.
* Other alternatives would be Weck bottles with a wide mouth and a flat lid held by metal brackets (I don't think they come in single-day-sizes though),
* or a gasketed clip-lid container, there are baby-food-sized ones that hold 200 ml (downside: they aren't convenient for pouring).
* Or get creative and use a baby milk bottle - nowadays, you can get bottles with a sealing cap to exchange the nipple during transport.
* Many people nowadays use an insulated travel mug with a leakproof lid.

Whichever container you choose, aim for a transparent one, so you can see if it has been washed properly or if it still has a milky layer sticking to it.

2. Start washing your bottle properly. Many people try to get away with filling the bottle and sloshing it a bit. That never works, especially for fatty contents like milk. You need a spongy brush on a long thin stem, it is sold especially for bottle cleaning (don't use the bristle ones for milk, they are for stuck-on pieces as in fruit juice bottles). You have to stick the brush in and make sure you scrub intensively the whole inner surface with soapy water, and then clean the threads of both bottle and cap. Afterwards, do 3-5 rinses by half-filling it with clean water and sloshing it thoroughly.
If you want to use a dishwasher, note that it only works with a wide-mouthed bottle (50 mm or more) and not too tall. Make sure that it stays vertically during the cycle, it is best to stick it on a spike and have other tall-ish items surrounding it.
3. As mentioned in comments, single-use bottles can also be used. My environmentalist hairs stand on end when I hear it :( but it would work. It would also be the most expensive solution.
4. If you only use tiny amounts of milk daily (e.g. to add to coffee or tea) consider buying single-serving milk or creamer instead, like the ones you get in coffee shops. In Europe, they are available in large supermarkets, and I assume that in Japan, you can at least find them online. Alternatively, there is also powdered creamer available.