According to dharma SAstras, in the context of Bhojana niyama , "ApoSanam" is the correct usage and not "Abhojanam".
I
am giving a list of pramANam from dharma SAstras wherein the word "ApoSanam" is used.
ApOSanamakritvA tu yadyannam vistrutam bhavet !
tadannam MAmsa tulyam syAt bhuktvA cAndrAyaNam caret !!
---- Sangraham
ApoSanenoparishtA dadhastAccaiva bhunjatA !
----- YAjnavalkya
ApoSanam parIdhAnam pUrvamAcchAdanam param !
---- SAtAtapa
ApoSanam vAma bhAge surApAnasamam bhavet !
---- smrutyarthaSAra
ApoSanam tu ya: kuryAt vAmabhAge tathaiva ca !
---- Smrutiratnam