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The survival of Escherichia coli ATCC 25922 in orange juice treated with microwave and/or ultrasound was evaluated; Weibull model was fitted to survival curves to describe inactivation kinetics; and the effect of combined microwave‐ultrasound treatments was assessed. Ultrasonic treatment (42 kHz) has no significant effect on the survival rate reduction (1.3 log for 60 min of sonication) of E. coli. However, sonication can increase the efficiency of microwave inactivation. The use of ultrasound as pre‐treatment was more effective than when used in post‐treatment, that is, a reduction of 8.0 log was achieved by a combination of ultrasound (20 min) followed by a microwave (900 W/30 s), while 4.0 log was obtained when these both processes were reversed. The scale parameter α estimated from microwave‐inactivation kinetics combined with an ultrasound pre‐treatment was found to be lower than those obtained with a post‐treatment, which, in turn, are lower than those estimated for microwave or ultrasound alone. These results demonstrate the effectiveness of ultrasound as a pre‐treatment to improve the inactivation of E. coli by microwave processing in orange juice beverages. Practical applications High‐temperature short‐time (HTST) type processes are preferred by the food industry to reduce the adverse thermal degradation in food quality while ensuring food safety for liquid food. The microwave heating is fast and can significantly reduce the come‐up time to the desired process temperature. The hydrodynamic action of ultrasound on microorganisms is a great potential to improve microbial decontamination efficiency by microwave processing. This study contributes to the design and control of an effective combination treatment of ultrasound and microwave to improve pasteurization processes for orange juice beverages.

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The aim of this study is to inactivate Enterococcus faecalis ATCC 29212 present in dairy wastewater effluent using microwave (MW) waves and/or ultrasound waves (US). The ultrasonic bath treatment (35 kHz) had no significant effect on the reduction of the survival rate (predominant declumping effect). At 650 W of microwave treatment, the total destruction was completed at 75 s, while at 350 W a 3 log reduction was achieved. The Weibull model was fitted to the survival curves to describe the inactivation kinetics, and the effect of the combined microwave-ultrasound treatments was evaluated. The scaling parameter α that was estimated from the inactivation kinetics for the microwaves combined with the ultrasound waves in pre-treatment was found to be lower than the scaling parameters obtained in post-treatment, which were in turn lower than those estimated for microwaves or ultrasound waves alone. The use of the ultrasound waves in pre-treatment was more effective than in post-treatment; a total reduction was achieved using a combination of US (30 min) followed by MW (650 W) with α = 28.3 s, while 4.0 log was obtained by reversing all processes with α = 34.5 s. The results from the protein assays indicate that the bacterial wall was damaged and that holes were formed from which protein leakage occurred.

The present study aims to optimize the extraction of phenolics by microwave-assisted extraction (MAE) using the response surface methodology (RSM), from Lemon verbena leaves. The optimized extract was tested for its antioxidant activity using two methods (DPPH and reducing power) and its antibacterial efficiency by using disk diffusion assay and broth microdilution, against two Gram-negative (Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853) and two Gram-positive (Staphylococcus aureus ATCC 25923, Bacillus subtilis ATCC 6633) strains. Under the optimized conditions (40% (v/v): of ethanol concentration, 188 s of irradiation time, 600 W of microwave power and 1:40 g/mL of solid-to-liquid ratio) the total phenolic content (TPC) was 67.87±1.61 mg GAE/g DW. The IC50 of the extract was 139.65±1.44 µg/mL and 56.60±2.79 µg/mL for DPPH inhibition and reducing power, respectively. The best antibacterial activity was shown by the extract obtained by MAE with lower MBC (1.56 to 18.75 mg/mL) and MBC/MIC ratio. Lemon verbena extract can be used as an ingredient in cosmetics, food supplements and herbal medicinal products due to its interesting biological properties.