Over the last few decades, many researchers have investigated in detail the characteristics of bioactive compounds such as polyphenols, vitamins, flavonoids, and glycosides, and volatile compounds in fruits, vegetables and medicinal and aromatic plants that possess beneficial properties, as well as consumer acceptance and preference. The main aim of this article is to provide an updated overview of recent research endeavors related to the effects of the drying process on the major bioactive/effective compounds in agricultural products. Particular emphasis was placed on details related to the changes occurring in vitamin C, polyphenols, flavonoids, glycosides and volatile compounds, as well as the antioxidant activity. An analysis of the degradation mechanisms of these compounds showed that vitamin C, phenols, flavonoids and glycosides react with oxygen during the convective drying process under high drying temperatures, and the reaction rate results in degradation in such bioactive compounds due to high reducibility. On the other hand, high temperature results in a short drying time, thus minimizing the degradation of bioactive compounds. The reviewed research works addressing this trend revealed that the ideal drying temperatures for retaining vitamin C, polyphenols, flavonoids, glycosides, volatile compounds and their antioxidant activity were 50–60 °C, 55–60 °C, 60–70 °C, 45–50 °C, 40–50 °C and 50–70 °C, respectively. In conclusion, to maintain plant bioactive components, convective drying at relatively low drying temperatures is strongly recommended.

The gas-liquid two-phase flow is widely encountered in many industrial applications, and its on-line and non-separation flow rate measurement for each phase has plagued the industry for many years. Based on the one-to-one correspondence between differential pressure fluctuations and the movement of Taylor bubbles and liquid slugs, a novel method is proposed to measure the flow rate of gas-liquid slug flow in this paper. Theoretical correlations between flow characteristics (velocity, length, and void fraction) and differential pressure fluctuations were quantitatively analyzed by experiments, and the gas-liquid ratio was found to be linearly related to the length ratio of Taylor bubble and liquid slug. The gas flow rate and liquid flow rate were predicted based on the measured flow characteristics. Experimental tests on air–water two-phase flow show that the proposed method and correlations were effective, and the average relative errors in the gas flow rate and liquid flow rate are 5.50% and 5.62% with maximum values of 11.46% and 12.06%, respectively, under the conditions of 0.16–0.56 m/s of air superficial velocity and 0.23–0.48 m/s of water velocity. This method is based only on the differential pressure fluctuations and is on-line, non-separation, non-invasive, and radiation-free, which provides a new cost-effective solution for the flow rate measurement of industrial two-phase flows.