Manu RIshi talks about prohibited food items in Manu Smriti verse 5.5, the original verse is
लशुनं गृञ्जनं चैव पलाण्डुं कवकानि च । अभक्ष्याणि द्विजातीनाममेध्यप्रभवानि च ॥ ५ ॥
Garlic, leeks and onions, mushrooms and all that proceeds from impure things, are unfit to re eaten by twice-born men. Manu Smriti 5.5
The gloss by medhatithi does not specify as to what those impure substances/things are.
What are those impure substances/things discussed in this verse, please provide proper scriptural citations to support your answer.