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Drying processes reduce the amount of available essential nutrients in dried plant-based foods to a large extent compared to fresh produce. This reduction is much larger than the differences in the final quality of products dried using various processing parameters and, in most cases, different drying methods. This aspect is, however, rarely highlighted. Here, the extent to which different convective drying methods reduce the nutritional content, namely vitamin C, carotenoids and phenolic content of dried fruits and vegetables, compared to fresh produce was quantified using literature data. The impact of different drying process parameters, such as air temperature, airspeed and relative humidity on the nutritional content of fruits and vegetables were compared. Results revealed that convective drying reduced the amount of vitamin C, carotenoids and the phenolic content of dried fruits and vegetables by up to 70%. The reduction in the residual vitamin C and carotenoid content of dried fruits and vegetables due to differences in air temperature (∼40%), airspeed (∼20%), and relative humidity (∼20%) is much less than the nutritional quality losses due to the drying process. The residual vitamin C, carotenoids and phenolic contents in convective-dried fruits and vegetables are ∼30% less than those in freeze-dried products. This study confirms that little absolute gains in nutritional quality can be achieved by opting for either an alternative drying method or optimizing processing parameters since the drying process already results in a low nutritional quality of dried products. As such, the remaining micronutrient concentration of dried products should not necessarily be a decisive criterion in selecting the most appropriate drying method or processing parameters for fruits and vegetables. Instead, other key performance indicators such as the drying time, energy consumption, or sensory properties such as color, texture, and rehydration capacity could eventually have a greater influence on the decision-making process., Food and Bioproducts Processing, 131, ISSN:0960-3085, ISSN:1744-3571

Summary In this study, the effects of convective, microwave and microwave–convective drying methods on the drying characteristics, colour, total phenolic content and antioxidant capacity of goldenberry fruits were investigated. To select the most appropriate thin-layer drying model for drying treatments, nine mathematical drying models were fitted to the experimental data. Based on the statistical tests used for evaluation, the Midilli et al. and Wang and Singh models were considered the best models to describe the drying behaviours of goldenberry fruits in all drying methods. The colour values (L*, a* and b*) of fresh fruit were decreased by drying. Compared with the fresh sample, the dried samples exhibited a 64–75% and 65–75% decrease in total phenolic content and antioxidant capacity, respectively. Among the different drying treatments, the values closest to those of fresh samples with respect to colour values, total phenolic content and antioxidant capacity were achieved with the 160 W microwave drying method.

In this study, the effects of microwave, convective and microwave-convective drying treatments on the drying parameters, colour and microstructure properties of mushroom samples were investigated. To select the best thin-layer drying models for the drying treatments, 9 mathematical models were fitted to the experimental data. Based on evaluation by statistical tests, the MIDILLI et al. model, the Diffusion Approach model, Logarithmic model, and WANG and SINGH model were found to be the best-fitting models to describe the drying behaviour of the mushroom samples. The shortest drying time was provided with microwave method at 500 W (35 min). However, the drying time was significantly reduced by combining microwave treatment with conventional drying. The drying time at microwave-convective power combination level of 90 W - 50 degrees C was by 33.8% shorter than drying by only microwave at a power of 90 W, and by 52.9% shorter than the drying by only convective treatment at a temperature of 50 degrees C. The colour values of the samples dried at 50 degrees C were closest to the fresh samples for all drying conditions. Scanning electron microscopy images revealed that a higher temperature or microwave power caused greater damage to the microstructure of the mushroom samples.