The Effect of Pulsed Electric Fields (PEF) Combined with Temperature and Natural Preservatives on the Quality and Microbiological Shelf-Life of Cantaloupe Juice

Abstract

Pulsed electric field (PEF) is an innovative, non-thermal technology for food preservation with many superiorities. However, the sub-lethally injured microorganisms caused by PEF and their recovery provide serious food safety problems. Our study examined the effects of pH, temperature and natural preservatives (tea polyphenols and natamycin) on the recovery of PEF-induced, sub-lethally injured Saccharomyces cerevisiae cells, and further explored the bactericidal effects of the combined treatments of PEF with the pivotal factors in cantaloupe juice. We first found that low pH (pH 4.0), low temperature (4 °C), tea polyphenols and natamycin inhibited the recovery of injured S. cerevisiae cells. Then, the synergistic effects of PEF, combined with cold-temperature storage (4 °C), a mild treatment temperature (50 and 55 °C), tea polyphenols or natamycin, on the inactivation of S. cerevisiae in cantaloupe juice were evaluated. Our results showed that the combination of PEF and heat treatment, tea polyphenols or natamycin enhanced the inactivation of S. cerevisiae and reduced the level of sub-lethally injured cells. Moreover, PEF combined with 55 °C heat treatment or tea polyphenols was applied for cantaloupe juice. In the practical application, the two combined PEF methods displayed a comparable inactivation heat pasteurization ability, prolonged the shelf life of juice compared with PEF treatment alone, and better preserved the physicochemical properties and vitamin C levels of cantaloupe juice. These results provide valuable information to inhibit the recovery of PEF-injured microbial cells and shed light on the combination of PEF with other factors to inactivate microorganisms for better food preservation.

Keywords: Saccharomyces cerevisiae; cantaloupe juice; natural preservatives; physicochemical characteristics; pulsed electric fields; shelf life; temperature.

Figure 1

Effects of different electric field intensity (A) and treatment time (B) of PEF on inactivation and sublethal injury of S. cerevisiae. YPD, non-selective medium; YPD-SC, selective medium. Data are expressed as mean ± standard deviation. Significant differences are indicated by different letters (p < 0.05).

Figure 2

The effect of pH and temperature on the recovery of PEF-injured S. cerevisiae. Survival fraction of S. cerevisiae after the PEF treatment (20 kV/cm for 200 μs) in citrate-phosphate buffer at pH 4.0 (A,C) and pH 7.0 (B,D), and in the subsequent recovery process incubated at 30 °C (A,B) or 4 °C (C,D) for 4 h. YPD, non-selective medium; YPD-SC, selective medium. Data are expressed as mean ± standard deviation. Significant differences are indicated by different letters (p < 0.05).

Figure 3

The effect of natural preservatives on the recovery of PEF-injured S. cerevisiae. Survival fraction of S. cerevisiae after the PEF treatment (20 kV/cm for 200 μs) in citrate–phosphate buffer with pH 4.0 (A,B) and pH 7.0 (C,D), and in the subsequent recovery process incubated at 30 °C for 4 h in mediums with tea polyphenols (A,C) and natamycin (B,D). YPD, non-selective medium; YPD-SC, selective medium. Data are expressed as mean ± standard deviation. Significant differences are indicated by different letters (p < 0.05).

Figure 4

Inactivation of S. cerevisiae in cantaloupe juice by combining PEF with temperature or natural preservatives. The synergistic effects of PEF combined with low temperature (4 °C) (A), mild temperature (50 and 55 °C) (B), tea polyphenols (C) and natamycin (D) on the inactivation of S. cerevisiae cells in cantaloupe juice. Data are expressed as mean ± standard deviation. Significant differences are indicated by different letters (p < 0.05).

Figure 5

Microbial changes in PEF-treated cantaloupe juice stored at 4 °C. (A) Changes of total aerobic bacteria in PEF-treated cantaloupe juice stored at 4 °C. (B) Changes in yeast and mold in PEF-treated cantaloupe juice stored at 4 °C. Data are expressed as mean ± standard deviation. Significant differences are indicated by different letters (p < 0.05). Control, untreated cantaloupe juice; 90 °C, treated at 90 °C for 3 min; 30 kV/cm, treated at 30 kV/cm for 400 μs; TP + 30 kV/cm, treated with 400 mg/kg tea polyphenols and then treated at 30 kV/cm for 400 μs; 20 kV/cm + 55 °C, treated at 20 kV/cm for 400 μs and then treated at 55 °C for 5 min.