Bioconversion of waste animal fat (WAF) to polyhydroxyalkanoates (PHAs) is an approach to lower the production costs of these plastic alternatives. However, the solid nature of WAF requires a tailor‐made process development. In this study, a double‐jacket feeding system was built to thermally liquefy the WAF to employ a continuous feeding strategy. During laboratory‐scale cultivations with Ralstonia eutropha Re2058/pCB113, 70% more PHA (45 g PHA L −1 ) and a 75% higher space–time yield (0.63 g PHA L −1 h −1 ) were achieved compared to previously reported fermentations with solid WAF. During the development process, growth and PHA formation were monitored in real‐time by in‐line photon density wave spectroscopy. The process robustness was further evaluated during scale‐down fermentations employing an oscillating aeration, which did not alter the PHA yield although cells encountered periods of oxygen limitation. Flow cytometry with propidium iodide staining showed that more than two‐thirds of the cells were viable at the end of the cultivation and viability was even little higher in the scale‐down cultivations. Application of this feeding system at 150‐L pilot‐scale cultivation yielded in 31.5 g PHA L −1 , which is a promising result for the further scale‐up to industrial scale.

Fat‐containing animal by‐product streams are locally available in large quantities. Depending on their quality, they can be inexpensive substrates for biotechnological processes. To accelerate industrial polyhydroxyalkanoate (PHA) bioplastic production, the development of efficient bioprocesses that are based on animal by‐product streams is a promising approach to reduce overall production costs. However, the solid nature of animal by‐product streams requires a tailor‐made process development. In this study, a fat/protein‐emulsion (FPE), which is a by‐product stream from industrial‐scale pharmaceutical heparin production and of which several hundred tons are available annually, was evaluated for PHA production with Ralstonia eutropha . The FPE was used as the sole source of carbon and nitrogen in shake flask and bioreactor cultivations. A tailored pneumatic feeding system was built for laboratory bioreactors to facilitate fed‐batch cultivations with the solid FPE. The process yielded up to 51 g L −1 cell dry weight containing 71 wt% PHA with a space–time yield of 0.6 g PHA L −1 h −1 without using any carbon or nitrogen sources other than FPE. The presented approach highlights the potential of animal by‐product stream valorization into PHA and contributes to a transition towards a circular bioeconomy.