Enhanced bacterial inactivation in apple juice by synergistic interactions between phenolic acids and mild food processing technologies

Abstract

This study demonstrated an innovative processing approach based on synergistic antimicrobial activity of two phenolic acids with mild thermal and non-thermal processing technologies. The two selected model phenolic acids were gallic acid (GA; 10 mM) and ferulic acid (FA; 1 mM). The processing technologies evaluated for processing of a model clarified apple juice were UV-A light, mild heat (55 °C) and moderate pressure (250 MPa), with processing times ranging from 1 to 30 min. The results demonstrated that combinations of selected phenolic acids and a mild physical processing were able to lower E. coli O157:H7 and Listeria innocua counts from 6-log CFU mL −1 to below the detection limit of 1-log CFU mL −1 . Bacterial inactivation was significantly enhanced by the combination of UV-A light processing and FA, where 10 min of treatment enhanced bacterial inactivation by 5-log as compared to light processing alone, which presented no bacterial inactivation. In contrast, the combination of GA and mild-temperature thermal processing (55 °C) or mild-levels of high-pressure processing (250 MPa), enhanced bacterial inactivation by 4-log as compared to the physical treatments alone, which presented only 1-log of inactivation. The influence of these synergistic combinations on bacterial membrane damage was assessed by selective plating technique under osmotic pressure. Furthermore, the total intracellular thiol content was also measured to assess for thiol oxidation. Overall, the results demonstrated enhanced bacterial inactivation based on synergistic interactions of selected phenolic acids with both mild-thermal and non-thermal technologies in a model food system and illustrate potential to create diversity of novel antimicrobial strategies for food processing. This study showed that the presence of naturally-based compounds can significantly reduce intensity levels of physical processing required to inactivate model gram-positive and gram-negative bacteria in apple juice. In this study two phenolic acids, ferulic acid and gallic acids were selected as these compounds are naturally present in many fruits and other food products including grains. Furthermore, the levels of these natural phenolic acids used in this study are comparable to the levels naturally present in some of the food materials. • Ferulic acid plus UV-A and gallic acid plus 55 °C or pressure showed synergistic inactivation in apple juice. • Ferulic acid at 1 mM resulted in 5-log CFU/mL bacterial inactivation with UV-A exposure. • Gallic acid at 10 mM resulted in 4-log CFU/mL bacterial inactivation with 55 °C or 250 MPa exposure. • Combination of gallic acid with heat or pressure increased intracellular thiol oxidation. • All observed synergistic combinations showed damage to bacterial membranes.