C# Extending Classes and Overriding Types

I have a scenario where I am working with a 3rd party library that I cannot change, and I'm trying to figure out a way to work with some of their types. Basically, they have the following classes:

public class Food
{
    ... properties that apply to all food ...
}
public class Pie : Food
public class Grapefruit : Food
{
   public int NumberOfSeeds = 50;
}
public class Cherry : Food
{
   public int NumberOfSeeds = 1;
}

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Now, "Grapefruit" and "Cherry" both have the "NumberOfSeeds" property, but that property is not part of the Food class.

I'm working on a "helper" class that removes seeds, so I'm trying to do something like this:

public class FoodHelper
{
  public Food item;

  public void RemoveSeeds()
  {
    item.NumberOfSeeds = 0;
  }
}

public class GrapefruitHelper : FoodHelper
{
  new public Grapefruit item;
}

public class CherryHelper : FoodHelper
{
  new public Cherry item;
}

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I'm trying to avoid having to redefine "RemoveSeeds" over and over again in the child classes (the real method is pretty monstrous), but when I define it in the parent class, I get a warning that "Food" doesn't contain the "NumberOfSeeds" property, which is correct.

Also, if I try to use any valid type like "Cherry" instead of "Food" in the parent class in order to make the RemoveSeeds work, then I'm forced to override / hide the "item" in the child classes and the parent methods don't even see the right "item".

I've experimented with trying to use dynamic <T> types in my class definitions, but with no luck. Can anyone point me in the right direction here?