Moisture diffusion by the fractional-time model in convective drying with ultrasound-ethanol pretreatment of banana slices

Effects of ultrasonic pretreatment on drying characteristics and water migration characteristics of freeze-dried strawberry

The effects of ultrasonic pretreatment on the drying characteristics and microstructure of strawberry slices were investigated. The rehydration characteristics of freeze-dried products, which were pre-frozen at -20 °C and - 80 °C were explored, with a focus on water mobility and distribution. The ultrasonic pretreatment significantly increased the water mobility of the strawberry slices, resulting in a reduction in their water content. However, the application of ultrasound significantly decreased the rehydration speed, indicating a lower moisture absorption capacity in the pretreated sample. The micrographs revealed that the structure of the tissue was more uniform after ultrasonic treatment, and water loss was accelerated. In addition, the contact angle measurements showed that the samples were more hydrophobic after ultrasonic treatment, and the eutectic temperature and fold point of the samples increased. Therefore, this study found that ultrasonic-assisted freeze vacuum drying technology effectively reduces hygroscopicity, improves product storage, and represents a potential method for dried production.

Drying strategies of spent coffee grounds using refractance window method

The objective of this study was to evaluate, during the drying of spent coffee grounds, the application of pretreatments with ethanol and the application of ultrasound assisting the refractive window (RW) drying, and to compare with convective drying by hot air (AC). The effect on the kinetics parameters of the Fick and Page models were evaluated, as well as on the content of total phenolics and antioxidant capacity. For AC drying, samples of spent ground coffee were prepared in the form of fixed rectangular beds 0.7 cm high, which were placed on polyester sheets and pretreated up to 10 times sprayed with ethanol, then dried by AC at 80 °C and 0.8 m/s. For RW drying, the samples prepared in the same way as for AC were used. For this case, the application of ethanol as pretreatment and the use of ultrasound during process were tested. As results, regardless of the conditions applied, drying by RW was up to 50 % faster, evidenced in the highest values of effective diffusivity (from the Fick model) and the kinetic parameter (from the Page model). Regarding the treatments applied and their effect on the drying kinetics, any treatment had a significant effect on AC drying. On the contrary, the strategies applied in RW drying had significant effects, both the application of pretreatment with ethanol as well as the application of ultrasound assisting the process accelerated the drying kinetics. However, the treatment with ethanol and RW drying was the one that best preserved the phenolic compounds and the antioxidant capacity in the samples. Therefore, these strategies could be a good option to improve RW drying by accelerating the process and preserving the bioactive compounds in the spent coffee grounds for subsequent utilization.

Ultrasound applications in drying of fruits from a sustainable development goals perspective

This review focuses on the many contributions of ultrasound technologies for fruit drying toward the United Nations Sustainable Development Goals (SDG). Along this review, several aspects attained from the application of ultrasound technologies are correlated with the SDGs. The main ultrasonic technologies applied for fruit drying, such as ultrasonic bath, probe ultrasound, air-borne ultrasound air-drying, and ultrasound-assisted contact air-drying, are presented. An in-depth discussion on ultrasound contributions, its advantages, disadvantages, and limitations are made. The effects of ultrasound on water diffusivity in several fruits are presented by correlating this effect with drying time and cost of energy. Ultrasound-assisted fruit drying, like other food processing technologies, directly impacts Zero Hunger, but ultrasound technologies contribute to much more than delivering long shelf-life food. This technology can be used to produce healthy foods and provide well-being, which will be discussed by correlating the effects of ultrasound-assisted air-drying with the concentration of nutritional compounds. Ultrasound-assisted fruit drying reduces wastewater toxicity and energy consumption and improves productivity, potentially improving workplaces and salaries. A walk through the technology is presented from Zero Hunger to No Poverty.

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.

Effect of gum Arabic and ethanol pretreatments on drying kinetics and quality attributes of dried carrot slices

Carrot is an important root vegetable to the food industry and consumers due to its nutritional and health benefits. Given the high moisture content and low shelf life of fresh carrots, preserving this highly demanded vegetable is vital. This current research modelled the drying kinetics and evaluated the quality of ultrasonic-assisted gum Arabic and ethanol pretreated and oven-dried carrot slices. Fresh carrots were processed into thin slices and immersed in 3% gum Arabic (GA), ethanol (99.9%), and distilled water (control), followed by ultrasonication (frequency: 50 Hz, power: 500 W, temperature: 25 °C) for 10 min and drying in a hot air oven at 50 °C. The loss of moisture from the carrots was periodically recorded, converted to moisture ratio before fitted to eleven semi-theoretical thin layer drying mathematical models. The effects of the pretreatments on the retention of bioactive compounds and carrots' physical and chemical properties were also evaluated. From the tested models, the Diffusion, Modified Henderson and Pabis, and Two-term models showed the best fitting (R2 = 0.9944-0.9985; RSME = 0.0103-0.0227) to the experimental data from 3% GA and ethanol pretreated carrots, while control samples followed the Aghbasho model (R2 = 0.9999; RMSE = 0.0033). Overall, the 3% GA pretreated carrot slices exhibited better colour (yellowness: 25.82-34.50; total colour differences: 8.12-13.06), water activity (0.37-0.44), total phenolic content (1.34-2.99 mg GAE/100 g DM), β-carotene (7.63-13.07 mg/100 g DM), and DPPH radical scavenging activity (5.67-8.02 mM AAE/100 g DM) than ethanol pretreated carrot slices and control samples. At the same time, 3 % GA pretreatment did not affect the drying rate of the carrot slices. The total soluble solids/titratable acidity ratio, rehydration capacity, and shrinkage ratio did not significantly (p > 0.05) vary among the treatments. The findings of this study can be used to develop an optimal drying protocol for pretreated carrot slices and to produce shelf-stable carrot products that can be used dried, rehydrated, or in combination with other products.