Poly ethylene glycol phase

To determine fatty acid composition, the method described in Modzelewska-Kapituła et al. [27 (link)] was used. Muscle lipids were cold extracted with chloroform/methanol (2:1 v/v) [28 (link)]. Then, fatty acids were methylated with a chloroform/methanol/sulfuric acid (100:100:1) mixture [29 (link)] and separated using an Agilent Technologies 7890A gas chromatograph (Agilent Technologies, Inc., Santa Clara, CA, USA) with a flame-ionization detector (FID) (30 m, 0.32 mm internal diameter fused silica capillary column (matrix active group: poly(ethylene glycol)phase, Supelco, Bellefonte, PA, USA)). The liquid phase was Supelcowax 10, and the film thickness was 0.25 μm. The conditions of separation were as follows: carrier gas, helium; flow rate, 1 mL min; detector temperature, 250 °C; injector temperature, 230 °C; column at temperature, 195 °C. The fatty acids were identified by comparing retention times with standards from Supelco (Bellefonte, PA, USA). The fatty acid content was presented as the relative percentage (% total fatty acids) and concentration (mg/100 g), which was calculated based on the fat content and the coefficient for lean fish (0.70) [30 ].

Fatty acid proportions and contents were determined using the method described in detail in Modzelewska-Kapituła et al. [14 (link)]. Briefly, muscle fat was extracted according to Folch et al.’s [18 (link)] method using a mixture of chloroform and methanol (2:1 v/v). Methylation of fatty acids was conducted using a chloroform–methanol–sulphuric acid (100:100:1) mixture [19 (link)]. Chromatographic separation was done using 7890A Agilent Technologies gas chromatograph equipped with a flame-ionization detector (FID) and a 30 m 0.32 mm internal diameter fused silica capillary column (matrix active group: poly(ethylene glycol)phase, Supelco, Bellefonte, PA, USA). Supelco (Bellefonte, PA, USA) standards were used to identify a particular fatty acid by comparing retention times. The results were presented as relative percentage (% total fatty acids) and content (mg/100 g wet tissue) of fatty acids in raw and sous-vide cooked fillets. The content of a particular fatty acid in fish muscle tissue was calculated separately for each sample based on fat content and coefficient for lean fish (0.70) [20 ].