Temperature‐dependent Henry's Law constants of 4‐alkyl branched‐chain fatty acids and 3‐methylindole in an oil‐air matrix and analysis of volatiles in lamb fat using selected ion flow tube mass spectrometry

Rationale 4‐Alkyl branched‐chain fatty acids and 3‐methylindole are characteristic flavor compounds associated with sheep meat. Determining their partitioning behavior between the gas and condensed phase and ultimately developing a correlation between the compound's headspace concentration and sensory descriptive grouping are important for high‐throughput characterization and grading classification. Methods The headspace concentrations of 3‐methylindole, 4‐methyloctanoic acid, 4‐ethyl‐octanoic acid, and 4‐methylnonanoic acid above corn‐oil‐based standard solutions and lamb fat samples were measured using selected ion flow tube‐mass spectrometry (SIFT‐MS). The standard solutions were equilibrated at 80, 100, 110 and 125°C while the fat samples were equilibrated at 125°C. Statistical evaluation, linear and polynomial regression analyses were performed to establish the compound‐specific and temperature‐dependent Henry's Law constants, enthalpy (ΔH) and entropy (ΔS) of phase changes. Results The Henry's Law constants ( k H cp ) were calculated from the regression analysis with a high degree of confidence (p < 0.05) and linearity ( r 2  > 0.99). The k H cp increased with increase in equilibrium temperature. The empirical calculation of ΔH and ΔS at different temperatures confirmed the temperature‐dependence of the Henry's Law constants. The headspace concentrations of the lamb‐flavor compounds were determined above actual lamb fat samples and the corresponding condensed‐phase concentrations were successfully derived. Conclusions The temperature‐dependent Henry's Law constants, Δ H , and Δ S of phase changes for 3‐methylindole, 4‐methyloctanoic acid, 4‐ethyloctanoic acid, and 4‐methylnonanoic acid in an air‐oil matrix were empirically derived. The effectiveness of SIFT‐MS for the direct, real‐time, and rapid determination of key flavor compounds in lamb fat samples was established.