## 1 Introduction  
For the first time, the authors propose a scalable method represented by a single-cascade flowchart for extracting proteins from unsorted tuna side streams made up of a dehydrated mix of canned tuna leftovers obtained through an innovative industrially patented procedure [1, 2]. The advantages of managing this kind of dehydrated raw material lie in the reduction in the volume of extremely perishable biomass and its increase in microbiological stability. In addition, the treatment of mixed non-separated side streams allows one to eliminate the laborious sorting step of tuna leftovers, which usually involves isolating skin and bones for collagen/gelatin extraction, representing a severe step for fish stakeholders. The exploitation of this unsorted biomass could also open up the recovery of other unsorted fish materials such as by-catches or unwanted catches [3].  
The handling and valorization of unsorted biomass are considered the two main goals of the Horizon 2020--EcoeFISHent project [4] addressed to the extraction of gelatin (G) and gelatin/collagen peptides (HGPs) for the nutraceutical, cosmetic, and packaging industries.  
In recent years, G and HGPs have received increasing attention for their numerous technological and functional properties, respectively. These include their gelling points, viscosity [5] and antioxidant activity [6], anti-photoaging properties [7], and anti-inflammatory and inhibitory effects on dipeptidyl peptidase-IV (DPP-IV) and angiotensin-converting enzyme (ACE) [8]. HGPs derive from enzymatic hydrolysis, which allows one to obtain low-molecular-weight peptides that show a significant increase in digestion and absorption when compared to collagen and gelatin [9].  
Fish waste and fish side streams like skin, muscles, viscera, and blood have been highlighted in many research studies as sustainable sources of proteins and peptides [10]; however, little or almost no research has paid attention to the exploitation of unsorted fishery biomasses.  
This work suggests a single-cascade flowchart for the extraction of all the protein fractions, namely gelatin (G), hydrolyzed gelatin/collagen peptides (HGPs), and non-collagenous proteins (NCs and ALKs) as reported in Figure 1.  
Proposing a single-cascade extraction process minimizes, as much as possible, the process waste according to one of the principles of green extraction [11].  
For G, NCs and ALKs, a preliminary characterization in terms of yield, proximate analysis, Fourier Transform Infrared Spectroscopy (FTIR) spectra, the amino acid composition, and CIELab color analysis were reported in the first paper (Part I), with a focus on oven-dried gelatin and its rheological properties (viscosity and gelling point).  
Hydrolyzed gelatin peptides (HGPs) and (spray-dried) gelatin (G) are at the core of this second paper (Part II), in which the following results are discussed: proximate compositions, pH, CIELab color profiles, amino acid compositions, FTIR spectra/molecular structures, dynamic viscosity, dynamic viscous-elastic behavior (DVB), thermo-gravimetric analysis (TGA), molecular weight distributions, and antioxidant activity.  
This work aims to provide a complete overview of the extracted collagenous proteins to open new perspectives of investigation for their further applications in both high-value food supplements and skin care products as well as in biodegradable and compostable packaging materials.  
Figure 1: Protein fractions obtained from the proposed single-cascade flow chart extraction.