Analytical Pyrolysis of fish (Oreochromis niloticus) muscle. Effect of different cooking method

Abstract

In this communication a detailed analytical pyrolysis of tilapia fish (Oreochromis niloticus) muscle is described. The fish, supplied by Valenciana de Acuicultura (fish hatchery of Valencia, Spain), were acclimatized in the laboratory and for 15 days in two aquariums (8 individuals/aquarium) with 96 L of tap-water at a constant temperature (21 ± 2°C). Fish were fed daily (0.3 g/day) with commercial fish food only (Dibaq S.L., Segovia, Spain). After acclimation, were dissected and each muscle sample was cut into approximately 4 g portions. Fish muscle samples were cooked for 2 min by boiling, steaming, microwaving and broiling. Briefly, for boiling and steaming, the fish muscle was introduced into the pot or onto the food steamer, respectively, with cool water, heated to boiling (100°C) and continued to boil for 2 min. A conventional household microwave oven (Samsung M17-13, 300W, 2450 MHz) was used for microwaving, and samples were broiled in Teflon pans for both sides of the fillet. A non-cooked fish muscle fillet was used as control group. The assays were always carried out by quintuplicate (n=5). All samples were kept at -80°C until freeze dry (Cryodos 80, Telstar, Tarrasa, Spain). The Py-GC/MS was performed using a double-shot pyrolyzer (Frontier Laboratories, model 2020i, Fukushima, Japan) attached to a GC system (Agilent Technologies, Palo Alto, CA,. USA, model 6890N). The muscle samples (2 mg freeze-dry tissue) were placed in crucible deactivated steel pyrolysis capsules and introduced into a preheated micro-furnace at (350 °C) for 1 min. The volatile pyrolysates were then directly injected into the GC/MS for analysis. The gas chromatograph was equipped with a low polar-fused silica (5%-phenyl-methylpolysiloxane) capillary column (Agilent J&W HP-5ms Ultra Inert, of 30 m × 250 μm × 0.25 μm film thickness. The oven temperature was held at 50 °C for 1 min and then increased to 100 °C at 30 °C min-1, from 100 °C to 300 °C at 10 °C min-1, and stabilized at 300 °C for 10 min, with a total analysis time of 32 min. The carrier gas was He at a controlled flow of 1 mL min-1. The detector consisted of a mass selective detector (Agilent Technologies, Palo Alto, CA. USA, model 5973N) and mass spectra were acquired at 70 eV ionizing energy. Compound assignment was achieved by single-ion monitoring (SIM) for the major homologous series and by comparison with published data reported in the literature or stored in digital NIST 14 (Maryland, USA) and Wiley 7 (Weinheim, Germany) libraries. In a first analytical step, a detailed pyrolysis fingerprint of raw fish muscle tissue is produced and the effect of the pyrolysis temperature from 150 to 550 ºC in 100 ºC increments is studied in both, i) applying each temperature to a different sample or ii) sequentially (multi-shot) applying each temperature to the same sample. In a second phase, after stablishing an optimum pyrolysis temperature of 350 ºC for 1 minute, the effect of the different cooking methods (boil, steam,microwave and broil cooking) in the fish muscle pyrolyzates was studied.