Citric Acid is derived from certain fruits like lemons. It can be used as a preservative in beverages and food. Also used as a flavour enhancer.
1 TBSP in 1L of warm water will also clean copper equipment and bring back that shiny look.

Also good for stainless steel passivation.
Use at a rate of 50g to 1L of water at 60 - 70 Deg C