The effect of different temperatures on the drying kinetics and the phytochemical constituents of edible Irish brown seaweed, Himanthalia elongata were studied. This kinetic study involved the modelling of the terms of Fick’s diffusion equation, for estimation of the diffusion coefficients. The diffusivity coefficient increased from 5.6 × 10−07 to 12.2 × 10−07 m2/s as the drying temperatures increased with an estimated activation energy of 37.2 kJ/mol. The experimental data was also fitted to different empirical kinetic models, Newton, Logarithmic and Henderson–Pabis, and the goodness of fit for the different models was evaluated. The effect of drying temperatures on the phytochemical constituents in seaweed was also evaluated. Drying at 25 °C resulted in 49% and 51% reduction in the total phenol and total flavonoid content, respectively, as compared to fresh seaweed. However, the reduction declined as the drying temperatures were increased. The scavenging effect on DPPH radical was also greater for the fresh seaweed as compared to the dried form. An increase in the phytochemical content was seen for higher temperatures (35 °C and 40 °C) when the moisture content reduced by 50% indicating that this semi-dry state is even more nutritious than the fresh form and could be an interesting starting point for seaweed processing.

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Effect of different drying temperatures on the moisture and phytochemical constituents of edible Irish brown seaweed

Drying is one of the widely used methods of grain, fruit, and vegetable preservation. The important aim of drying is to reduce the moisture content and thereby increase the lifetime of products by limiting enzymatic and oxidative degradation. In addition, by reducing the amount of water, drying reduces the crop losses, improves the quality of dried products, and facilitates its transportation, handling, and storage requirements. Drying is a process comprising simultaneous heat and mass transfer within the material, and between the surface of the material and the surrounding media. Many models have been used to describe the drying process for different agricultural products. These models are used to estimate drying time of several products under different drying conditions, and how to increase the drying process efficiency and also to generalize drying curves, for the design and operation of dryers. Several investigators have proposed numerous mathematical models for thin-layer drying of many agricultural products. This study gives a comprehensive review of more than 100 different semitheoretical and empirical thin-layer drying models used in agricultural products and evaluates the statistical criteria for the determination of appropriate model.

A comprehensive review of thin-layer drying models used in agricultural products.

Mechanistic understanding of case-hardening and texture development during drying of food materials

Dehydration reduces water activity and extends shelf life of perishable agricultural products. The purpose of this research was to study the application of radio frequency (RF) energy in dehydration of in-shell Macadamia nuts and shorten the lengthy process times needed in conventional hot air drying operations. A pilot scale 27.12-MHz and 6- kW RF system was used to determine the operational parameters, the drying curve, and the quality attributes of the processed nuts. The results showed that an electrode gap of 15.5 cm and a hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures, and were used for further drying tests. The drying curves showed an exponential decay and required 750 and 360 min to achieve the final moisture content of 0.030 kg water/kg dry solid (3.0 % dry basis) in whole nuts in hot air drying and RF heating/hot air combined drying, respective- ly. The drying kinetics of the nuts were described well by the Page model for hot air drying, but a logarithmic model was more suited for RF/hot air drying. Peroxide value and free fatty acid increased with the drying time both for hot air and RF drying but remained within acceptable range re- quired by the nut industry. The RF process shows potential to provide rapid, uniform, and quality-acceptable drying technology for the nut industry.

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Developing Hot Air-Assisted Radio Frequency Drying for In-shell Macadamia Nuts

The objective of this study was to evaluate the application of: (1) freeze drying, (2) convective drying (50, 60, or 70 °C), (3) vacuum–microwave drying (240, 360, or 480 W), and (4) a combined method of convective pre-drying and vacuum–microwave finish drying in the processing of pomegranate arils and rind. The quality parameters under study included sugars and organic acids, punicalagins and ellagic acid, total polyphenols, total antioxidant activity, and sensory quality. In general, drying led to a reduction in all studied parameters; however, the behavior of arils and rind was different. Vacuum–microwave drying at 240 or 360 W was the best drying treatment for arils, while rind required freeze drying or soft conditions of convective drying (50 °C). Further research is needed to obtain proper results with convective pre-drying and vacuum–microwave finish drying of arils and rind. With proper selection of the drying protocol, high-quality dried arils will be available for consumers; these arils will be characterized by high contents of fructose (25 g 100 g−1), phytic acid (2.2 g 100 g−1), punicalagins (0.57 mg g−1), total polyphenols (1.6 mg eq gallic acid g−1), high antioxidant capacity (0.6 mg eq Trolox g−1), and high intensities of garnet color, sweetness, sourness, and fresh pomegranate aroma. Besides, dried rind with very high contents of active compounds (123 mg g−1 of punicalagins and 108 mg eq gallic acid g−1) and high antioxidant capacity (26 mg eq Trolox g−1) will be also available as functional material.

C Mota

Papers

Convective drying of onion: Kinetics and nutritional evaluation