Evaluation of nonthermal effects of electricity on inactivation kinetics of Staphylococcus aureus and Escherichia coli during ohmic heating of infant formula

Abstract

This study investigated the nonthermal effects of electricity on microbial inactivation kinetics of Staphylococcus aureus and Escherichia coli during ohmic heating (OH) of infant formula. For this, temperature profiles during OH and conventional heating (CV) processes were matched, and the kinetic parameters were compared at temperatures ranging from 57.5 to 65.0 8C. The inactivation kinetics were analyzed using an inactivation model, which allowed the definition of the thermal tolerance from k max and D values. For S. aureus, significant differences (p < .05) between D and k max values were observed when OH and CV treatment were compared in all temperatures. The results demonstrated a higher efficiency of OH over CV, indicating that microbial inactivation may have been caused by electroporation in addition to thermal effects. For E. coli, a significant difference (p < .05) between D values was observed only at 57.5 8C. Moreover, D values for both heating technologies were lower at 65 8C, which suggests a faster inactivation and, therefore, a lower degradation of nutritional compounds. Taking into consideration both microbial species, the best treatment for human milk pasteurization is OH at 65 8C. Practical applicationsThe low-temperature long-time pasteurization (62.5 8C for 30 min) is the currently recommended method for heat treatment of human milk in human milk banks (HMB) because it is simple to be applied and guarantees microbiological safety. However, it can promote significant losses of nutritional and organoleptic quality due to the slow conduction of heat transfer. Therefore, alternative technologies for human milk pasteurization need to be explored. OH is a faster heating technology, appropriate for heat-sensitive products. Although already established in the food industry, the OH advantages have not been considered for human milk pasteurization. The objective is to provide an alternative technology to the HMB that ensure safety and lower-impact on nutritional compounds. This study contributes for the correct understanding of the nonthermal effects associated with passage of electricity through foodstuff on microorganism's inactivation. Moreover, it has significant economic consequences to industry, supporting modeling and optimization to produce safe products with higher nutritional and sensory characteristics.

Microbial inactivation by ohmic heating: Literature review and influence of different process variables

“…Himoto et al (2008) found lower D values for OH at all analysed temperatures for the total viable aerobes (D values varied from 0.44 to 11.25 min in CH and from 0.38 to 8.64 min in OH) and for Streptococcus thermophilus inactivation (variation from 0.2 to 7.54 min in CH and 0.16-6.59 min in OH). Rodrigues et al (2018) found higher inactivation rates for Staphylococcus aureus for all temperatures investigated: the D values varied from 3.41 to 15.06 min at 57.5°C and from 0.53 to 1.42 min at 65°C in OH and CH, respectively. In the case of E. coli, significantly different values were found only for the lowest temperature analysed.…”

“…It is important to emphasize that these two works did not statistically analyse the z value. On the other hand, Rodrigues et al (2018) obtained considerably different values for both microorganisms used: the z values were 9.49°C and 7.91°C for Staphylococcus aureus and 10.13°C and 4.88°C for Escherichia coli using ohmic and conventional heating, respectively. In addition, Pereira et al (2007) also found different values for this parameter studying Escherichia coli cells in goat milk (23.1 and 8.4°C for CH and OH, respectively).…”

“…In addition, Pereira et al (2007) also found different values for this parameter studying Escherichia coli cells in goat milk (23.1 and 8.4°C for CH and OH, respectively). A possible explanation for the difference among the studies is associated with the fact that Baysal and Icier ( 2010 with spores, while Rodrigues et al (2018) worked with vegetative species. Pereira et al (2007) worked with vegetative and spore species and demonstrated different z values just for the vegetative species.…”

“…One of the most relevant parameters in inactivation kinetics is the z value, which represents the temperature difference needed to increase or decrease the D value by 90%. This parameter was evaluated in both thermal processes in the works by Pereira et al (2007), Baysal and Icier (2010), Somavat et al (2012), Somavat et al (2013) and Rodrigues et al (2018). ANOVA was conducted to evaluate the significant differences between z values in OH and CH by Somavat et al (2013) and Baysal and Icier (2010), and no differences were observed.…”

Effect of pasteurization on Aspergillus fumigatus in apple juice: Analysis of the thermal and electric effects

“…Kim et al, 2017;S.S. Kim et al, 2017;Rodrigues et al, 2018;Schottroff et al, 2019;Tian et al, 2018). The high potential of OH for microbial death is related to two aspects: (i) compared to CH, in general the heating ramp is faster and the temperature profile is more homogeneous, reducing processing time, energy costs and losses of desirable compounds; (ii) the presence of nonthermal effects may provide additional inactivation rates.…”