Medial letters (tussenletters: s or e(n) the medial -e or -en
Main rule:
You use the medial -en whenever the first word of the contraction is a noun that has only one plural form, ending with -en.
It is kippenei, because ‘kip’ has only got one plural: kippen. (chickens)
But it is aspergesoep because the plural of asperge is asperges,
Exceptions
Some words keep their medial -e, even though according to the main rule, they should be written with -en:
a) words that refer to a unique person or object: zonnestraal and maneschijn for there is only one of those.
b) In adjectives of which the first part is only there to accentuate the second part: apetrots, reuzeleuk and beregoed.
c) The first part of the word is a noun without a plural: rijstepap and roggebrood. ‘Rijst’ does not have a pluralform, ‘rijsten’ is a non-existing word.