# Valorization of fish processing by-products through combined enzymatic and microbial hydrolysis: nitrogen recovery and fertilizer efficiency in wheat

## TL;DR

This study shows that fish waste can be turned into effective fertilizers using enzymes and microbes, boosting wheat growth and sustainability.

## Contribution

The novel integration of enzymatic and microbial hydrolysis for nitrogen recovery from fish by-products is proposed as an efficient biostimulant.

## Key findings

- Combined enzymatic and microbial hydrolysis achieved 226 mM α-amino groups and 33% protein recovery.
- Microbial and combined hydrolysates increased wheat plant lengths to 52 cm and 54 cm, compared to 44 cm in the control.
- Plant biomass reached 1.7 g and 2.3 g with microbial and combined hydrolysates, versus 0.7 g in the control.

## Abstract

The bioconversion of fish by-products has been evidenced as a sustainable process to convert food waste into high-value products. In the present study, protein hydrolysates were produced from fish by-products by different bioprocesses and evaluated as fertilizers in wheat (Triticum aestivum L.) on a nitrogen-equivalent basis. Fish by-products were processed through grinding prior to bioconversion. Enzymatic hydrolysis was performed using Alcalase at 55 °C, pH 6.5, and a 3-h reaction, while microbial conversion was assessed using a lactic culture at 40 °C, pH 6.5, and a 10-day culture. Hydrolysates obtained by enzymatic and microbial bioconversion were evaluated as fertilizers by adding 30 mg after 7 and 14 days to wheat seeds sown under controlled conditions. Protease and microbial hydrolysis generated high concentrations of α-amino groups, yielding 100 mM and 170 mM, respectively. The combined process exhibited a synergistic effect, yielding 226 mM of α-amino groups and 33% of protein recovery. Plant growth assays were conducted under controlled conditions using nitrogen-equivalent doses of each hydrolysate. Microbial and combined enzymatic-microbial hydrolysates generated average plant lengths of 52 cm and 54 cm compared to 44 cm in the control, while plant biomass reached 1.7 g and 2.3 g with microbial and combined enzymatic-microbial hydrolysates compared to 0.7 g in the control. Photosynthetic parameters remained within normal physiological ranges from 2.5 to 3.3 for performance index (PI) and from 0.78 to 0.80 for maximum quantum efficiency (Fv/Fm). The integration of enzymatic and microbial catalysis produced the most effective biostimulant activity, highlighting the value of combining enzymatic specificity with microbial metabolic versatility. These findings support fish-derived protein hydrolysates as efficient and eco-friendly fertilizers that are capable of improving plant growth while contributing to sustainable and integral utilization of natural resources.