Evaluation of Escherichia coli inactivation and quality attributes in apple juice subjected to Pulsed electric fields (PEF) in combination with high intensity light pulses (HILP)

The combination of novel, non-thermal technologies for preservation purposes is a recent trend in food processing research. Pulsed electric fields (PEF) and high intensity light pulses (HILP) represent emerging technologies for the preservation of beverages and when combined under mild processing conditions they could serve to ensure microbial stability and safety with little or no loss of product quality characteristics. In the present study treatments involving monopolar square wave PEF in combination with HILP were applied to reconstituted apple juice in a continuous system using a 2 × 4 factorial design with sequence and energy levels as main factors. Two PEF field strengths (24 kV/cm or 34 kV/cm) were selected with a treatment time of 92 s in each case to provide a "high" (H) and a "low" (L) energy input. The juice was subsequently cooled, then pumped through a HILP system (pulse length 360 s, frequency 3 Hz) at different flow rates in order to receive H or L energy dosages of 5.1 J/cm² or 4.0 J/cm², respectively. Microbiological analysis was performed by inoculating the juice with Escherichia coli K12 and counting microbial numbers pre- and post-processing. Selected product quality attributes (pH, Brix, colour, non enzymatic browning index (NEBI), total phenols content and total antioxidant activity) were also measured to compare juice processed by the combination of non thermal technologies with unprocessed juice. A sensory evaluation of selected combinations was conducted using 31 untrained panellists and the products compared to thermally processed juice (94°C for 26 s). The PEF/HILP combination reduced the E. coli population by approximately 6 log cfu/ml without a significant effect of either the PEF or HILP energy levels. When the reverse sequence was applied, energy levels of the selected hurdles had a significant impact (p<0.001) on viable counts. The higher reduction of 6.42 log/cycles was obtained for the HILP(H)/PEF(H) combination in comparison to an untreated control. In terms of product quality, the juice pH (3.56), Brix (12), NEBI (0.142), total phenols content (341 mg GAE/L) and antioxidant activity (1.87 mM TEAC) were not affected by any processing combination, while slightly noticeable (0.5<E<1.5) colour changes were found in the product, compared to the untreated control. Sensory evaluation revealed significant adverse effects for odour (p<0.05) and flavour (p<0.001) in the non-thermally processed apple juice compared to the pasteurised sample. The HILP(L)/PEF(L) processing proved to be the most acceptable among the selected non-thermal treatments. These results indicate that PEF in combination with HILP could successfully reduce levels of E. coli in apple juice at appropriate treatment conditions, though further optimisation studies are required to improve product sensory quality.