Porter, a dark style of beer that was the staple of London in the eighteenth and nineteenth centuries, is conventionally addressed as a discrete invention, suited to large-scale production, whose appearance led rapidly to enclosure of the trade by a few industrial-scale producers. This paper by contrast presents the capitalist industrialization of brewing as co-extensive with, and reinforced by, the long-term emergence of a consensus definition of porter; the invention story is a retrospective construct that telescopes a century or more of technical change. Balancing established economic accounts, I address the role of product identity as a rhetorical device. London's greatest brewers were in part assisted in capturing smaller competitors' trade by the enshrining of large-scale production as a 'secret ingredient' in its own right, essential to the nature of the 'true' product. © Taylor & Francis.