## 1 Introduction  
When the US Food and Drug Administration (FDA) announced their process analytical technology (PAT) directives, the investigation of PAT became a key research area in bioprocess development. The main objectives are designing, developing, and operating bioprocesses to guarantee a targeted final product quality [1; 2]. The focus of this initiative was predominantly on biopharmaceutical processes, while novel PAT tools could be integrated into any bioprocess. Especially, the implementation of PAT for polyhydroxyalkanoate (PHA) production can provide significant benefits to facilitate a consistent and highly efficient production. Techniques such as FTIR, Raman spectroscopy, fluorescence staining associated with flow cytometry, and enzymatic approaches were reported as novel methods for a rapid characterization of PHA production [3; 4; 5; 6; 7]. A comprehensive overview of qualitative and quantitative methods for PHA analysis was published by [PERSON] et al. (2016). However, the reported methods have not been applied for _in-line_ or _at-line_ measurements of the PHA production process so far. Photon density wave (PDW) spectroscopy is an _in-line_ technique, which has been used as an analytical tool for measurements of various highly turbid chemical processes ([PERSON] et al., 2012; [PERSON] et al., 2013; [PERSON] et al., 2014; [PERSON] et al., 2015). The method is based on the theory of photon migration in multiple light scattering material. If intensity-modulated light is introduced into a strongly light scattering but weakly light absorbing material, a PDW is generated. Absorption and scattering properties of the material influence the amplitude and phase of the PDW. By quantifying these shifts as a function of the emitter fiber and detector fiber distance and of the modulation frequency, the absorption coefficient \(\mu_{\mathrm{a}}\) and the reduced scattering coefficient \(\mu_{\mathrm{s}}\) can be determined independently ([PERSON] et al., 2012; [PERSON] et al., 2014; [PERSON] et al., 2015). The mentioned features make PDW spectroscopy very attractive for the monitoring of high-cell-density bioprocesses. Currently, PHA production costs are not compatible with the low-priced production of conventional plastics. The main cost driving factors are the feedstocks for PHA accumulation and the recovery process. Thus, alternative low-cost substrates, e.g., biogenic waste streams, are of high interest to reduce the final production price. Other attempts concentrate on finding more sustainable and price efficient purification strategies ([PERSON] et al., 2014; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015). _Ralstonia eutropha_ (also known as _Cupriavidus reactor_) is one of the main species studied for polyhydroxybutyrate (PHB) accumulation and the model organism for PHA accumulation ([PERSON] et al., 2015). Growth of _R. eutropha_ on oleaginous feedstocks is particularly attractive due to their high carbon contents, high conversion rates to PHA, and low culture dilution in fed-batch processes. Efficient growth on these feedstocks is facilitated by the expression of extracellular lipases, which emulsify the lipids ([PERSON] et al., 2014; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015). A large biomass accumulation prior to PHA accumulation is very important for a high final product titer. In this context, it has been shown that urea is an inexpensive nitrogen source, which allows excellent growth ([PERSON] et al., 2015; [PERSON] et al., 2015). Despite alternative substrates and downstream approaches, highly efficient bioprocesses are required for an economic feasible PHA production. Recently, high-cell-density cultivations with _R. eutropha_ on various renewable feedstocks have been published presenting the production of over 100 g\(\cdot\)L\({}^{-1}\) PHA and space time yields from 1 to 2.5 g\({}_{\mathrm{PHA}}\)\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\)[21; 24; 29; 30; 31].