Conventional drying of spices, as hot air treatment, often needs an additional downstream inactivation step to decrease the microbial load of the dried product and improve its microbial safety and microbial quality. In this regard, the present work explored the possibility to dry and decontaminate food in a single step by the use of supercritical carbon dioxide (scCO 2) as a drying agent. A case study was focused on the drying of herbs and the antimicrobial effects were evaluated on the naturally present microbiota. For this purpose, experiments were carried out on coriander leaves using a high pressure vessel at 10 MPa, at two different temperatures (40 and 50 °C) with drying time of 0 and 150 min 2 to establish the influence of each parameter on the microbial inactivation. Yeasts and molds appeared to be the least resistant to scCO 2 as they could never be detected after the treatment (< 2 log CFU/g). Mesophilic bacteria were also significantly reduced, up to 4 log CFU/g, but remained above the limit of quantification. The quality of the dried product was comparable with the quality of air dried samples in terms of phenolic constituents. Overall, the results indicated that scCO 2 drying was a promising green drying technique combining both drying and microbial inactivation in a single step with a relevant impact on safety and costs.
This work explores the use of supercritical carbon dioxide (scCO 2) drying in combination with High Power Ultrasound (HPU) to enhance both the dehydration kinetics and the microbial inactivation on coriander leaves. Indeed we compared the scCO 2 drying process alone and in combination with HPU at different powers (10, 40 and 80W), different drying times (up to 90 min) and two process temperatures (40 and 50°C). In the most effective condition tested (40W; 10 MPa; 40 °C), mesophilic bacteria were reduced up to 4 Log, mesophilic spores up to 1 Log, while yeast and molds resulted under the detection limit. We identified 40W as the threshold HPU value to achieve a beneficial effect on mesophilic spores reduction. Besides, the use of HPU enhanced the water loss and lowered the water activity of the sample, compared to the one processed with scCO 2 alone. The appearance of the dried sample did not show significant differences after the two processes. Overall, scCO 2-HPU combined process resulted a promising technology to enhance both the dehydration and microbial inactivation kinetics compared to the use of scCO 2 alone.
This work explores the feasibility of applying in situ Raman spectroscopy for the online monitoring of the supercritical carbon dioxide (SC-CO2) drying of fruits. Specifically, we investigate two types of fruits: mango and persimmon. The drying experiments were carried out inside an optical accessible vessel at 10 MPa and 313 K. The Raman spectra reveal: (i) the reduction of the water from the fruit slice and (ii) the change of the fruit matrix structure during the drying process. Two different Raman excitation wavelengths were compared: 532 nm and 785 nm. With respect to the quality of the obtained spectra, the 532 nm excitation wavelength was superior due to a higher signal-to-noise ratio and due to a resonant excitation scheme of the carotenoid molecules. It was found that the absorption of CO2 into the fruit matrix enhances the extraction of water, which was expressed by the obtained drying kinetic curve.
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