One of the main reasons for a kitchen sink to smell would be if there is food deposits and fat sat within the trap from when you have washed your dishes. If left to sit and deteriorate this waste would give off a smell.
If you can remove the trap from below the sink, do so and disregard of the waste. Safely secure the trap back into position and run clean water through it. If you are unable to remove the trap a great way to freshen it would be to place 2 large handfuls of bicarbonate of soda into the plug leave for approx. 15 minutes then add approx. 250-500ml white vinegar again leaving this to sit and fizz for 15 minutes.
When this has sat for this duration pour a kettle full of boiling water directly into the plughole. If you have 2 plugholes in your sink do the process to both plugs as they are sometimes installed on 2 traps rather than 1.
Nowadays many people have dish washes, therefore, do not use their kitchen sink too often. In this circumstance, the trap liquid may dry out and in able drainage smells to enter your property. If you think this may be the case then every 2-3 days run the tap to your sink to refill the sink trap with fresh water.
If you follow these simple steps you should have a fresh-smelling sink at all times!