Thermos food jars are only as good as they can serve their purpose; otherwise they can be such a disappointment. Pre-heating or cooling your thermos before filling it up could go a long way in ensuring best results, but never use the microwave for this. Always fill it up with hot or cold water before hand and leave it still for around five minutes, after which you can empty and use it as required.
The biggest gaffe one can ever pull is using the dishwasher and chlorine or bleach to clean a thermos food jar. Sturdy stain removers have an abrasive effect on the rubber washers in the covers and the lid. Before you know it, the thermos innards will begin to spill. The dishwasher makes work easier, but it denies you the opportunity to clean the flask to detail.
For best results, use trifling soaps or dishwashing cleansers like Dawn and Ivory in trivial soapy water, and hark back to rinse well before intended usage. Rough rasping cleansers will more often than not damage and smudge the surface and inner linings of the thermos food jars. When this goes on for some time, the thermos could even alter savors of your stored food and beverage.
To remove obstinate beverage stains, try leaving a few teaspoons of baking soda and warm water in the thermos for an hour then rinse meticulously. Soaking overnight for persistent stains and a bottle brush afterwards could produce exemplary upshot. Alternatively, you can mix white vinegar and water in 1:3 parts or more as necessary and let it set then rinse very well with water. Swab the exterior with a laxly loofa then dry the thermos, and it is ready for use.