Abstract
Nearly seventy percent of fish become by-products during processing, highlighting a significant opportunity to reduce food waste and promote sustainable fish production. Effective utilization of these by-products is crucial for maximizing resource efficiency and minimizing environmental impact. This study explores the effects of pulsed electric fields (PEF) on endogenous lipases in fish by-products, specifically trevally (Pseudocaranx dentex) and snapper (Pagrus auratus). These species were chosen due to their economic and cultural importance in New Zealand fisheries and their potential for aquaculture applications. To address a critical gap in enhancing the value and sustainability of fish processing by-product, this study aims to investigate the effects of PEF on endogenous lipases in the fish frames of trevally and snapper and understand the impact on the oxidative status of treated fish by-products. Fish frames were chosen for their relative cleanliness (less surface area exposed to the environment, less susceptible to external contaminants) and suitability for processing, as well as being less studied than other fish by-products.
The study involved the characterization of fish frames from trevally and snapper through physical examination, proximate analysis, and profiling of free amino acids, fatty acids, and minerals. Endogenous lipase activity in these fish frames was assessed concerning optimal pH, thermal stability, and inactivation kinetics. The effects PEF on lipase activity were investigated under two conditions: (1) fish frames that were freeze-dried and ground prior to PEF treatment (ground freeze-dried/GFD samples) and (2) wet fish frames (WF samples) that were not subjected to drying or grinding. For the GFD samples, a square wave bipolar pulse with a constant pulse width of 20 µs and a frequency of 100 Hz was applied, with specific energy (SE) levels ranging from 0 kJ/kg (untreated group) to 600 kJ/kg. This treatment was conducted on 0.5 g of GFD samples and 10 ml of 0.01 M sodium phosphate buffer (pH 8) in a chamber with a 10 mm electrode gap, with electric field strength (EFS) varying from 1.13 kV/cm to 4.34 kV/cm. For the WF samples, an 80 mm electrode gap chamber was used, with EFS ranging from 1.1 kV/cm to 1.3 kV/cm and SE levels from 0 kJ/kg to 400 kJ/kg. A comparative study was conducted to evaluate the effectiveness of PEF for lipase inactivation compared to conventional heat treatment methods. The most effective PEF treatment scenario, which resulted in maximum lipase reduction, was subsequently used to study the oxidation status of the fish frames. This final stage involved comparing the impact of PEF and heat treatment on oxidative stability to assess their relative effectiveness in preserving the quality of the fish by-products.
Trevally and snapper frames exhibit notable differences in their physical characteristics, nutritional composition, free amino acids, fatty acids, and mineral content, highlighting species-specific traits that influence lipase activity and stability. The optimal pH for trevally lipases was 8, while snapper lipases exhibited optimal activity at pH 9. Both types of lipases remained stable at temperatures up to 40°C, with inactivation occurring beyond this threshold. Kinetic analysis revealed distinct differences in the properties of trevally and snapper lipases. Trevally lipases were more sensitive to temperature changes, as indicated by a lower z-value, but demonstrated greater overall stability, evidenced by a lower k value and higher Ea value. This thermal stability in trevally lipases is likely attributed to the species' larger size and higher levels of polyunsaturated fatty acids (PUFA) and tryptophan (TRP), which contribute to their thermal resilience.
PEF treatment influenced lipase activity in both fish species, though responses varied significantly under different treatment conditions and matrices. The EFS and SE on residual activity of GFD samples differed notably between species, particularly at an EFS of 1.13 kV/cm, where snapper showed inconsistent inactivation. Higher EFS and SE levels were associated with increased reductions in lipase activity. The greater reduction in lipase activity observed in WF samples (93.56% for trevally and 89.14% for snapper) compared to GFD samples (86.15% for trevally and 87.72% for snapper) suggests that preserving the material in its original form or minimizing tissue disruption enhances the effectiveness of PEF treatment. Larger cells, which are more susceptible to electrical fields, were less affected in WF samples, as they were only cut and not subjected to drying and grinding. In contrast, GFD samples experienced alterations in cell size due to these processes, making their cell structure less sensitive to PEF treatment. In GFD samples, trevally demonstrated faster lipase inactivation than snapper. However, in WF samples, although trevally exhibited slower inactivation compared to snapper, it ultimately achieved a higher reduction in lipase activity, possibly due to differences in the initial enzyme activity levels and species-specific factors. This discrepancy may be attributed to structural differences in the bone matrix and tissue composition between the two species, with trevally's less dense matrix potentially allowing for more effective energy distribution despite slower initial inactivation. Additionally, ohmic heating induced by PEF contributed to the reduction in lipase activity, with the highest reduction observed in samples experiencing the most significant temperature increases due to PEF treatment.
Evaluation of lipid and protein oxidation markers after PEF and heat treatments was conducted on WF samples. PEF proved more effective than HT in reducing lipid oxidation in trevally, while in snapper, PEF decreased lipid oxidation but increased protein oxidation. The observed differences between species are linked to their unique biochemical compositions. Trevally frames, with higher polyunsaturated fatty acids (PUFAs) and pro-oxidant metals, exhibited more oxidative damage but benefited more from PEF treatment, which effectively reduced lipid oxidation. Variability among fish frame samples reflects natural biological differences within species. Since the fishes were wild caught from the same season in New Zealand, variations in treatment responses may also be influenced by factors such as nutrient profiles, feeding habits, and stress levels. These results highlight the species-specific responses to the treatments and emphasize the need to consider both lipid and protein oxidation when assessing the effectiveness of PEF and HT.
This study highlights the impact of PEF on partially inhibiting endogenous lipase activity in trevally and snapper frames. Factors such as the matrix type (ground or wet) and PEF processing conditions (EFS and SE) influence the effectiveness of lipase inactivation. The findings underscore the potential of PEF as a pre-treatment for utilizing fish by-products, reducing lipase activity with minimal alterations to oxidative status and fatty acid composition.