The firmness and flavor of kimchi is depends on the type of salt you use. For the best results, it’s worth buying Korean coarse sea salt (cheonilyeom). It is minimally processed using one of the older methods of salt production, which entails long and slow drying in the sun to remove the bitterness sometimes found in other salts. Korean coarse sea salt has a bit of a sweet aftertaste. The grain size is much larger, too. Different salts also have different sodium levels and all of that can affect the finished kimchi if you substitute another salt.