Fermentation of Walnut Kernels: Process Optimization and Impact on Derived Products

Background: Fermentation can modify the characteristics of a product enhancing taste, chemical profile, or prolonging storage time. This study aimed to apply fermentation to walnuts to obtain an innovative derived paste with improved properties. Methods: Walnuts were characterized for pH, water activity (aw), moisture. Preliminary trials were carried out on a commercial walnut drink and on a diluted walnut paste (produced mixing walnut paste with water in 1:1 ratio) to verify the fermentation feasibility and select the most suitable bacteria (testing two Lacticaseibacillus paracasei strains and one Lactiplantibacillus plantarum). Grounded walnuts were fermented with log6 UFC/g of the more performant strains (L. paracasei and L. plantarum); two fermentation techniques (soaking and spraying using saline solution) were compared to understand their practicality and yield as growht. After incubation (48 h at 37 °C), the fermented kernels were roasted (15 min at 160 °C) and processed to obtain pastes that were characterized for physico-chemical properties (viscosity, color, moisture, pH, aw), oxidative stability (peroxide value, total acidity), and untargeted metabolomics profiling. Results: The strains used to ferment grounded walnuts through both techniques showed a good growth yield. The walnut pastes obtained from the fermented kernels really differed from the unfermented paste showing different colors and a greater oxidative instability. The same behaviour was confirmed by untargeted metabolomics that proved the presence of some positive compounds derived by the Lactobacillus metabolic path-ways (amino acids and antioxidant phytochemicals). Conclusions: The work showed that LAB fermentation can be applied on walnut kernels. Further studies are needed to implement sensory profile, nutritional value, and stability of fermented walnut pastes. The impact of reducing the concentration of the inoculated strains and/or incubation times and/or temperatures will be investigated in order to improve the fermentation process.