Heritage branding and heirloom cultures are twin strategies for building brands in global markets. In this Element, the authors analyze these strategies through skyr; a traditional, sour dairy from Iceland. They explore how live microbial cultures in skyr have been 'heritagized' as heirloom cultures to build a brand advantage. Live skyr cultures, they show, illustrate symbiotic relations over millennia between microbial cultures and human cultures. The industrialization of this species interaction in the twentieth and twenty-first centuries, they argue, ultimately converted a mutualistic relation into a parasitic one. Moreover, they demonstrate a parallel inversion of gender relations in the production and consumption of skyr as part of its industrialization and export. Ironically, these transformations undermine the industry's promotion of the cultures and heritage to which it has effectively put an end. They ask whether there is a more general lesson in this about the relationship between industrialization, capitalism, and heritage.