Application of a microwave-assisted extraction method for the extraction of organic acids from Greek cheeses and sheep milk yoghurt and subsequent analysis by ion-exclusion liquid chromatography

A method for the determination of 13 organic acids (OA) in Greek cheeses and sheep milk yoghurt was proposed and assessed. The method was based on microwave-assisted extraction (MAE) of OA by an aqueous solution (10 mM) of sulphuric acid; extracts were subsequently analysed by ion-exclusion liquid chromatography associated with PhotoDiode Array detection (IE-LC-PDA). Chromatographic conditions (column temperature and mobile-phase composition) were screened to optimise separation. MAE operational parameters (extraction temperature, duration, and extractant molarity) were also evaluated. The effect of extractant molarity was not significant; however, the extraction temperature and duration exerted a significant effect (P<0.05) on the overall recovery of the majority of the target compounds. Recoveries >92% were found for most target analytes in cheese but recoveries of tartaric, succinic, valeric, and propionic acids were lower (67-78%). In the case of yoghurt, recoveries ranged from 78% to 125% but that of succinic acid was quite low (53%). The overall performance of the proposed MAE-based method, compared to the commonly used extraction method by stirring, was found to be superior in terms of accuracy and repeatability.