In response to the increasing consumers demand for so-called minimal processing food with least amounts of chemical preservatives, alternative methods of food preservation have begun to be promoted, such as the use of natural antimicrobials. Grape seed extracts (GSE) have shown a broad range of bioactive properties, including antimicrobial activity (AA).

The AA is usually determined by the microtiter dilution method, evaluating the inhibition of microbial growth by measuring the optical density (OD). However, this method is not adequate in case of turbidity or interference of the extracts colour on the OD detection.

In this study, a method based on the indirect measurement of impedance was chosen for studying the antibacterial activity of GSE against five food pathogenic bacteria (Salmonella enteritidis, Escherichia coli, Staphylococcus aureus, Listeria monocytogenes and Pseudomonas aeruginosa). The extracts were obtained from Tempranillo grape seeds by accelerated solvent extraction using water as solvent. For the determination of microbial growth, indirect impedance measurements were carried out by the MicroTrac measuring system (SY-LAB, Austria). The impedance changes were followed by an indirect measure on KOH solution (2 mL). 3 mL of Mueller Hinton broth (MHB) with 0, 7.5, 15 and 30 g/L of GSE were inoculated using a suspension of the pathogenic bacteria from 18 to 24 hours incubation standardized to 2 x 10⁵ cfu.

The parameter chosen for defining and quantifying the AA of the GSE was the detection time (DT) at 37ºC, defined as the time required by the microbial population to reach a fixed impedance value. For L. monocytogenes, St. aureus and P. aeruginosa, the fixed impedance value (M= -15) was not reached for any GSE concentration assayed, showing an inhibitory effect on bacterial growth. Low effectiveness was obtained for GME against S. enteritidis and E. coli, although DT increased with increasing concentration of GSE compared with the control.