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Combined pre-treatments effects on zucchini (Cucurbita pepo L.) squash microbial load reduction

Publication . Neves, Filipa I. G.; Silva, Cristina L. M.; Vieira, Margarida C.

Freezing vegetables requires pre-treatments to reduce microbial load and destroy enzymes that impair the frozen product quality. So far blanching has been the most effective pre-treatment, preferred by the food industry, despite its severity: heating up to temperatures close to 100 °C for 1–3 min causes sensory and texture changes in most horticultural products. Alternative blanching treatments, using UV-C radiation combined with milder thermal treatments or with thermosonication, may improve the quality of the final frozen vegetables. Zucchini (Cucurbita pepo L.), the vegetable under study, has an availability in fresh restricted to a season, needing therefore to be often frozen to be used throughout the year. In this study, its surface was first inoculated with two vegetable contaminants, Enterococcus faecalis and Deinococcus radiodurans cells, which are resistant, respectively, to high temperatures and to radiation and then submitted to several blanching treatments, single or combined, and the effect on these microorganisms reduction was evaluated. As single treatments, water blanching (the control treatment, as it is the blanching treatment traditionally used) was applied up to 180 s at temperatures ranging from 65 to 90 °C, and UV-irradiation applied in continuous. As combined pre-treatments, water blanching combined with UV-C (continuous or in pulses), and thermosonication (20 kHz at 50% of power) combined with UV-C pulses were also studied. The continuous UV-C radiation incident irradiance was 11 W/m2 up to 180 s, and the pulses at incident radiance of 67 W/m2, lasting 3.5 s each (35 pulses). Mathematical modeling of bacterial reduction data was carried out using the Bigelow, the Weibull and Weibull modified models, and estimation of their respective kinetic parameters proved that the latter models presented a better fit below 75 °C. The best results proved to be the combination of water blanching at temperatures as low as 85 °C during <2 min with 25 pulses of UV-C (incident irradiance of 67 W/m2) or thermosonication at 90 °C also combined with UV-C pulses, both resulting in 3 log reductions of both microorganisms under study. These results proved to overcome what industry is requiring so far (a 2 log microbial reduction in 3 min), hence minimizing quality changes of frozen zucchini.

2019Journal article

Open access

Inactivation kinetics of peroxidase in zucchini (Cucurbita pepo L.) by heat and UV-C radiation

Publication . Neves, Filipa I.G.; Vieira, Margarida C.; Silva, Cristina L.M.

The effect of traditional thermal water blanching, or its combination with a UV-C radiation pre-treatment (11 J/m2), on the inactivation kinetics of peroxidase in zucchini (Cucurbita pepo L.) was carried out in the temperature range of 80 to 98 °C, and up to 3 min of processing time. Peroxidase inactivation started being noticed only at temperatures around 85 °C. For both treatments, the inactivation kinetics followed a first order reaction with the Arrhenius model describing the temperature dependence of the reaction rate. The obtained kinetic parameters (kHeat 89.6 °C=7.37×10−7±3.47×10−7 min−1 and Ea Heat=925± 69 kJ mol−1; kHeat 89.6 °C+UV-C=2.42×10−5±6.58×10−6 min−1 and Ea Heat+UV-C=596±48.8 kJ mol−1) showed that UV-C radiation had a significant contribution to increase the peroxidase degradation rate. These results will help to design new pre-processing conditions for the production of frozen zucchini, using less severe thermal treatments and attempt to minimize quality losses. Industrial relevance: The use of UV-C radiation in processing creates novel methodologies, which can be complementary to classical techniques, providing reduced processing times and increased efficiency. Having been proved to be particularly useful in sterilization of liquids and surfaces, inhibition of microbial population and in decrease of protein synthesis, its application in combination with the traditional blanching may be in favor of less severe heating treatments. The evaluation of the kinetics of zucchini (Cucurbita pepo L.) peroxidase inactivation with the traditional blanching method and combined with ultraviolet radiation suggests that short UV-C treatments may be a useful non-chemical pre-freezing method, allowing the application of shorter blanching times, leading to a product with eventual higher quality retention which is highly relevant to industry.

2012Journal article

Restricted access