One of the options proposed by Bunjes. Also supported by FWB Chemie und chemische Technik, and Babel/Hagemann/Höhne, FWB Biotechnologie.

Bunjes also lists initial reactant (not sure if that would be quite the right term for your context); Babel et al. also list
initial product
(and cross-reference Ausgangsprodukt to Ausgangsmaterial for which they list raw material, (feed)stock, basic [primary] material)

A couple of examples:
Leaving the skin smooth and clean its use prepares and enhances the action of other products with active ingredients it represents the starting product for every treatment of RAPHAEL'S SKIN REJUVENATION SYSTEMS
www.salsaforhair.com/Purchase_page.htm

In biotechnology, a living organism - either a whole organism, a cell, or a part of a cell, such as an enzyme - acts as an intermediary to transform a starting product into a desired end product. In fact, biotechnology is not a recent invention.
www.unesco.org/opi/science/content/press/anglo/13.htm

Maybe this will fit into your context. HTH.
And Happy New Year!