How to clean stains from inside of vacuum flask? I use my vacuum flask to bring my lunch to work. I've been doing this almost every day for half an year or so. Now there are some sediments or stains in the bottom of the flask  although I wash the flask almost always immediately after it's emptied.

How can I get rid of the stain and smell inside the flask? I even used steel wool on a stick but could't make it clean 100% and I don't want to scratch it too much inside. Because I use it for food I am not sure if bleach is a good choice to go for.

Update

I have original Thermos made of stainless steel interior and exterior. Something like that one http://www.thermos.com/products/vacuum-insulated-10-oz-food-jar.aspx

Note that I am located in Austaralia so no cleaning products are available to me.
I have found that a tablespoon of Oxy Clean followed by boiling water will remove even the harshest stains inside a vacuum bottle. Fill it to the brim (it will bubble and overflow, so do it in a sink), let it sit for a bit, then rinse very very well, and you will be amazed, I promise. I do this periodically to remove the coffee stains inside our steel thermos flask. [Edited for typos]