How to track changes in linktable with System.Data.Entity.Infrastructure.DbChangeTracker

I'm using DbChangeTracker in my c#/asp.net project (Visual Studio 2017, Entity Framework 6.4.4), which works fine. However, I've problems with tracking all changes in tables in combination of a link table.

Example: two classes, bread and topping and a link table goodCombination

class bread {
  public string name;
  public int tasteScore;
}
class topping {
  public string name;
  public bool containsLactose;
}
class goodCombination {
  string breadName;   //foreign key to field name of class bread
  string toppingName; //foreign key to field name of class topping
}

In EF the class goodCombination isn't a seperate class but part of the two classes bread and topping:

class bread {
  public string name;
  public int tasteScore;
  public virtual ICollection<topping> toppings
}
class topping {
  public string name;
  public bool containsLactose;
  public virtual ICollection<bread> breads
}

My problem is: changes in bread.name and bread.tasteScore are trackable. Changes in bread.toppings are not trackable! That means:

var allChangedEntities = this.ChangeTracker.Entries().Where(c => c.Entity is IChangeTrackableObject && c.State != System.Data.Entity.EntityState.Unchanged).ToList();

results in an empty list, even when a goodCombination is added or changed.

My question is: how to track these changes?