Effect of freeze-thaw and PEF pretreatments on the kinetics and microstructure of convective and ultrasound-assisted drying of orange peel

The main waste generated by juice industry comprises orange peels, which have a great upcycling potential once stabilized. Drying is the most used method for this purpose, but the high energy consumption prompts interest in its intensification. This study assessed the influence of freeze-thaw and pulsed electric field (PEF) pretreatments in conventional and airborne ultrasound-assisted drying (50 °C) of orange peels. None of these pretreatments alone got to reduce processing times significantly, but combined with ultrasound-assisted drying produced a significant shortening of the process. This was particularly important in the lower intensity PEF pretreatment tested (0.33 kJ/kg), indicating the existence of optimum conditions to carry out the pretreatments. Microstructure analysis revealed that the application of ultrasound during drying led to better preservation of the sample structure. Thus, the integration of pretreatment techniques to ultrasound-assisted drying may not only shorten the process but also help to preserve the original structure.

Acerola processing waste: Convective drying with ethanol as pretreatment

The acerola seed is an agro-industrial waste. It is a high moisture content product, rich in bioactive compounds. Drying is an alternative to make this waste available in a safe condition. The use of ethanol as a pretreatment could improve the drying process besides reducing the operation time. This study aimed to investigate the influence of ethanol pretreatment (ET) on the content of bioactive compounds, cell wall thickness, and color. The drying kinetics was studied, and the influence of external and internal resistance was discussed. The samples were immersed in ethanol for 2 min with subsequent convective drying (40 °C and 60 °C; 1 m s-1) until they reached the equilibrium condition. The ET reduced the drying time up to 36.36 %. The external and mixed control of mass transfer were identified as the governing regimes for drying this material, depending on the use of ethanol. ET led to an increase in effective diffusivity, a reduction in cell wall thickness, and preservation of the color of the dried waste. The ET positively impacted the conservation of ascorbic acid compared to untreated dried samples but was not relevant to phenolic compounds, carotenoids, and antioxidant activity. The drying process increased the bioactivity of the anthocyanins. The best condition was drying at 60 °C, pretreated with ethanol.

Impact of pretreatments with ethanol and freezing on drying slice papaya: drying performance and kinetic of ultrasound-assisted extraction of phenolics compounds

BACKGROUND

Pretreatments of drying can represent an alternative to minimize the negative aspect process on the dry samples. Thus, the influence of ethanol (ET) and freezing (FG) as drying pretreatments was analyzed for slices of papaya. The slices (5 mm) were submitted for drying kinetics (60 °C and 1.5 m s^-1 ). Drying kinetics experimental data were fitted using Page's model and a diffusive model with boundary condition of types I and III. Also the thermophysical properties (thermal conductivity and specific heat) were determined and finally, the kinetics of ultrasound-assisted (40 kHz and 132 W) extraction of total phenolic compounds (TPCs) were realized.

RESULTS

The combined method (ET + FG) was more efficient in reducing the drying time of papaya slices (48%) and the model 2 simulating the boundary condition of the third type (type III) showed the best fit to the experimental data. Effective diffusivity and convective coefficient were higher for ET + FG, where the maximum reduction in water content was 91% compared to fresh slices. The pretreatments did not influence the thermal conductivity, however, they were significant in the specific heat and in the extraction of TPCs, being higher in the time of 180 min.

CONCLUSION

It was confirmed in the results presented that the combined pretreatment ET + FG is the most viable for drying papaya slices. Furthermore, it was found to be the most efficient in minimizing the loss of TPCs. Therefore, this pretreatment has great potential for application in the development of high value-added foods. © 2022 Society of Chemical Industry.