Waste frying oil (WFO) is a complex and heterogeneous waste collected mainly from restaurants and, to a lesser extent, from
food processors and households. In the last decades, interest for this waste has deeply grown and a strong competition in
grabbing this raw material has begun. About 90% of WFOs collected in Europe is recycled for biodiesel production, being the
high free fatty acids (FFAs) concentration in WFOs one of the main drawback limiting the yield of the conversion process. In this
study we proposed a pit stop in the WFO to biodiesel conversion race by introducing an upstream microbial fermentation step
aimed at reducing the FFAs content through its conversion into biopolymers, i.e. Polyhydroxyalkanoates (PHA). A properly
engineered Escherichia coli strain, able to produce PHA exclusively from FFAs, but not secreting lipases, was tested in this
process. Recombinant production of a near P(HHx) homopolymer was achieved in a process requiring an aqueous pretreatment
step aimed to reduce the content of non-lipid carbon sources, which compete with FFAs for microbial growth impairing the PHA
production. A WFO with a halved FFAs content was recovered after fermentation, thus rendering it more attractive for further
conversion in biodiesel. Despite the quite low production yield achieved at this stage, the idea of no-competitiveness behind the
process was verified, leaving space to further strain improvement strategies to boost PHA yield.