Scotto di Uccio, Antonella (2024) Conversion of Carbon- and Nitrogen-rich Dairy Residues into Valuable Microbial Protein: Process and Product Insights. [Tesi di dottorato]

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Abstract

This doctoral thesis is divided into seven chapters, including a general introduction on carbon and nitrogen rich effluents, five research chapters and a general discussion with future perspectives and conclusions. Chapter 1 provides an overview of carbon- (cheese whey) and nitrogen- (anaerobic digestate) rich effluents from the dairy sector, focusing on their characterization and potential possibilities of valorisation. Chapter 2 investigates use of cheese whey permeate (CWP) as a substrate for the production of MP through a mixed culture of yeasts and bacteria. The influence of different C/N ratios and dilution rates is evaluated with respect to key process performance indicators such as biomass yield and productivities, sCOD removal and nitrogen assimilation efficiencies, as well as biomass quality for the evaluation of the final MP product. Chapter 3 explores the zero-chemicals and low-temperature direct air stripping process applied to anaerobic digestate for reactive nitrogen recovery in both batch and continuous mode. Furthermore, an optimization in relation to operational parameters such as air to digestate (A:D) ratio, temperature and initial nitrogen concentration is proposed. Chapter 4 presents a study arising from the combination of the processes studied in chapters 2 and 3 aimed at MP production. The first purpose is to perform air stripping of ammonia from digestate and upcycle the recovered N into an aerobic fermentation reactor fed with CWP. The second purpose is to combine the N-rich anaerobic digestate with the synthetic C-rich CWP during aerobic MP production process. Furthermore, the study also aims to investigate the aforementioned MP production systems when using a digestate contaminated with heavy metals. Chapter 5 analyses the characteristics of a MP-based biofilm obtained from MP produced by valorising waste nitrogen from anaerobic digestate and carbon from cheese whey through a combined process as illustrated in Chapter 4. Particularly, physical and mechanical properties along with the degree of biodegradation of the obtained biopolymer are evaluated. Chapter 6 studies the production of MP through CWP under extremophilic conditions in both continuous and fed-batch mode. In particular, the impact of typical low pH values and high salt contents of CWP as well as processing temperatures used in the dairy sector is evaluated with respect to selected key performance indicators, such as biomass yield and productivities, sCOD removal and nitrogen assimilation efficiencies, and biomass quality (protein and carbohydrate content, amino acids profile) for the evaluation of the final MP product characteristics. Chapter 7 discusses the main findings of the research, highlights the main conclusions and offering recommendations for future works.

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