Changes in tissue structure and physico-chemical quality characteristics of bamboo shoot slices during microwave drying process

Effect of drying methods on physico-chemical and bioactive compounds of mandarin (citrus reticulata) peel

Mandarin peel, an agro waste has an immense potential for bio utilization. The present study highlights the effect of drying on the physicochemical and biochemical properties of dried mandarin peel. Microwave drying and forced air drying accomplished at three different power levels (180, 360 and 540 W) and temperatures (30, 50 and 70 °C), respectively, while freeze drying was carried out at shelf temperature of –35 °C. The results revealed highest recovery of bioactive compounds from microwave drying at 540 W viz total phenolic (43.61 mg GAE/g), flavonoid (8.08 mg QE/g), tannin (8.73 mg GAE/g), saponin (159.91 mg EE/g−1), as well as, gallic acid antioxidant activity (3.58 mg GAEAC/g) and ferric reducing antioxidant power (50.61 mg TE/g−1). Furthermore, results from HPLC and UV–vis spectroscopy revealed presence of major polyphenols in dried peel. Microwave drying can be concluded as an industrial method for the bio utilization of mandarin peel.

Effect of drying methods on drying kinetics, energy features, thermophysical and microstructural properties of Stevia rebaudiana leaves

BACKGROUND

Stevia leaves were subjected to convective hot-air, infrared and vacuum drying at 40, 60 and 80 °C, followed by an assessment of thermophysical properties and microstructure, along with drying kinetics modelling and evaluation of energy features for all drying operations.

RESULTS

Effective moisture diffusivity (D_eff ) showed dependency on temperature with values ranging from 1.08 × 10^-12 to 7.43 × 10^-12  m²  s^-1 for convective drying, from 0.71 × 10^-12 to 6.60 × 10^-12  m²  s^-1 for infrared drying, and from 1.29 × 10^-12 to 5.39 × 10^-12  m²  s^-1 for vacuum drying. The thermal properties of the dried Stevia leaves under different drying conditions showed values of density, specific heat, thermal diffusivity, thermal conductivity and thermal effusivity ranging from 95.6 to 116.2 kg m^-3 , 3050 to 3900 J kg^-1  K^-1 , 4.28 × 10^-7 to 5.60 × 10^-7  m²  s^-1 , 0.16 to 0.23 W m^-1  K^-1 and 244 to 305 W s^0.5  m^-2  K^-1 , respectively. As for microstructure, convective hot-air drying showed better preserved leaf characteristics, compared to infrared- and vacuum-drying, whereby scanning electron microscopy (SEM) image analysis also revealed noticeable differences at higher temperatures. Statistical analysis showed that the Midilli-Kuçuk model fitted best the experimental data of drying curves (0.961 < r²  < 0.999, 0.000064 < SSE < 0.005359, and 0.000074 < χ²  < 0.006278). Comparison of the drying methods with respect to energy features showed that convective drying at 80 °C led to lowest specific energy consumption (61.86 kW h kg^-1 ) with highest efficiency (8.5%).

CONCLUSION

The results of this study contribute to a better understanding of the drying behaviour and showed that thermophysical properties of dried Stevia leaves and energy features are affected by drying methods. © 2021 Society of Chemical Industry.

Investigation on Spontaneous Shape Change of 4D Printed Starch-Based Purees from Purple Sweet Potatoes As Induced by Microwave Dehydration

The time evolution of three-dimensional (3D) printed food structures as affected by their composition and postprinting stimulus is an area of research that has recently received increasing attention. In this study, the spontaneous shape change of 3D printed purple sweet potato purees of different formulations as triggered by microwave dehydration was investigated. The rheological properties, water distribution behavior, and dielectric properties of the purees were first studied. Addition of salt reduced the viscosity, storage modulus, loss modulus, and yield stress but increased the relaxation time of the purees. Addition of fructose syrup resulted in opposite results. Addition of both salt and syrup decreased the dielectric constant but increased the dielectric loss of the purees. Increased microwave power and salt content increased the rates of dehydration and deformation but decreased the maximum deformation degree of the printed samples. The syrup also decreased the maximum deformation degree. A desirable deformation pattern could also be achieved by manipulating the infill parameters. Transformation of two-dimensional planar flowers and butterflies into 3D configurations as a result of varying the aforementioned parameters is illustrated. The proposed technique to induce spontaneous shape change of a 3D printed starch-based product should lay a foundation for further application of four-dimensional food printing.