Actually, Kefir making produces carbon dioxide (the stuff that gives soda pop it's 'fizz'. So you can have 'fizzy' Kefir, especially if you are brewing with a tight fitting lid on.
However, the longer it brews, the stronger the flavor. It should smell kinda like a cross between buttermilk and yogurt, depending on the medium it's brewed in. (Goat milk, cow milk, or other liquids). It might just have brewed too long and developed really 'strong' flavor/smell.
Store bought Kefir is closer in nature to 'strong tasting yogurt', and really isn't the same thing as Kefir produced from living 'grains'. I'd be surprised if the stuff you purchased is all that similar to 'Real Kefir' as the 'Dom of Kefir' would call it.
I would think that stuff 'could go bad' in comparison to brewed Kefir, which just gets a stronger flavor and odor.
You can substitute Kefir for any recipe calling for buttermilk or sour milk. It makes the dough really tender and soft.
The Kefir 'grains' which look like slighty soft, (like sticky rice), 'masses' are the symbiotic organisms that produce the polysaccharides and 'convert' your milk to the cultured product. They are composed of healthy yeasts and bacteria. You need someone to send you about a teaspoon to Tablespoon worth of this 'starter'; they will grow and reproduce, eventuallly you can then send out a 'starter' to another or make back up (dried) grains or experiment with your extras in different liquids (water/juice kefir, etc).
Here is the link to the 'King of Kefir'. It's a bit much to read, but worth taking the time!
I made Kefir 'sourdough' by using Kefir soaked whole wheat flour. If there wasn't 'fizzy' carbon dioxide production, then it would not have been able to rise...and...it rose
All brewed products with yeast will have CO2 as a by product. (Beer/mead/wine making, sourdough, 'real' Kefir, etc)
Hth and here is 'Dom's' website on Kefir:http://users.chariot.net.au/~dna/kefirpage.html