Halacha recognizes that taste can be absorbed into surfaces and appliances through heat and use, and kashering is the process used to remove that absorbed taste.
Before Pesach
Any kitchen used during the year requires kashering before Pesach, since chametz is prohibited even in the smallest amount. This applies to sinks, countertops, stovetops, ovens, and other food-contact surfaces.
Before kashering, the kitchen must be thoroughly cleaned. Afterward, each surface is kashered using the halachic method appropriate to its material and how it was used. In some cases the counters need covering with material, like foil or contact paper.
Moving Into a New Home or Rental
When moving into a home, apartment, or rental where the previous occupants’ kashrut standards are unknown or different, the kitchen generally requires kashering. A kitchen that appears clean may still contain absorbed taste, which halacha takes into account.
Change in Kashrut Status
If non-kosher food was cooked, meat and dairy were mixed improperly, or hot non-kosher food came into contact with kitchen surfaces, kashering may be required. Even unintentional use can necessitate kashering, depending on the details of the case.
Long-Term Non-Use
In limited situations, if a kitchen or specific surface was unused for a very long period, certain leniencies may apply. These cases are not automatic and must be evaluated individually according to halacha.
Nachman Harris. Kosher Expert.
Available for any of your questions
