Brine concentration influences physicochemical composition stability indicators and microbiological quality of Nile tilapia Oreochromis niloticus in Beni Democratic Republic of the Congo

Nile tilapia ( Oreochromis niloticus ) is one of the most important fish species for food security and household income in eastern Democratic Republic of the Congo. However, its high perishability, coupled with limited cold-chain infrastructure, necessitates the use of traditional preservation techniques such as brining. Despite its widespread application, information regarding the effects of brining on the physicochemical characteristics, stability indicators and microbiological quality of Nile tilapia remains limited under local conditions. This study aimed to evaluate the influence of three brine concentration levels (0%, 7.5% and 15% NaCl) on the physicochemical composition, water activity, Water Phase Salt (WPS) and microbiological quality of Nile tilapia. An experimental design comprising three treatments was conducted. Physicochemical and microbiological analyses were performed using standard AOAC and ISO methods. Correlation analysis, hierarchical clustering and Principal Component Analysis (PCA) were applied to investigate relationships among quality parameters. The results revealed significant effects of brining on all analyzed parameters (p < 0.05). Moisture content decreased from 75.42% to 56.37%, while pH declined from 6.82 to 6.41. Conversely, NaCl content increased from 0.52% to 10.89%, ash content from 1.24% to 8.47%, and Water Phase Salt from 0.69% to 16.19%. Water activity decreased from 0.984 to 0.881, indicating improved product stability. Protein and lipid contents increased from 18.72% to 24.13% and from 2.85% to 4.18%, respectively, mainly due to concentration effects associated with moisture loss. Microbiological analyses showed substantial reductions in total aerobic mesophilic flora (6.84 to 3.15 log CFU/g) and total coliforms (4.80 to 1.85 log CFU/g). Salmonella spp. and Shigella spp. were eliminated in brined samples, whereas moderate increases in halotolerant microorganisms, including Staphylococcus aureus and yeasts-molds, were observed. Correlation analysis and PCA confirmed that salt concentration was the principal factor driving changes in physicochemical and microbiological characteristics. Overall, the 15% NaCl treatment provided the greatest improvement in product stability and microbial safety. These findings demonstrate the effectiveness of brining as a preservation method and support the development of standardized brining protocols for artisanal fish processing in eastern Democratic Republic of the Congo.