Food stains on slate

We installed slate floors in the kitchen and dining areas of our newly constructed home. The floor was sealed, but food stains immediately began to appear. The installer scraped away at the surface stains, and reapplied the sealer twice . . . but the porous stone continues to stain - the sealer seems to be totally ineffective. How can we get the stains up . . . and what sealer should we use to prevent staining from food and drink?