Effect of Microwave Roasting and Extraction Solvents on the Bioactive Properties of Coffee Beans

Coffee is an intricate mixture of thousands of chemical compounds that are accountable for its flavor and aroma. Roasting is a key step in the processing of coffee beans. This study assessed the effect of microwave roasting (MW) and extraction solvents (ES) on the total polyphenol content, total flavonoid content, and antioxidant activity of coffee beans. The untreated and microwave-roasted (MR) coffee beans showed a total polyphenol content of 40.40 and 35.15 mg GAE/gm DW, respectively, when methanol was used as the solvent for extraction. Similarly, for the untreated coffee beans, the methanol extracted coffee had a significantly ( p < 0.05 ) higher total flavonoid content (39.34 mg CE/g DW) as compared to ethanol (34.82 mg CE/g DW). The obtained IC50 for the untreated and microwave-roasted samples as extracted by methanol were 4.13 and 5.68 mg/mL, respectively, while the IC50 values of untreated and microwave-roasted samples extracted by ethanol were 4.59 and 6.24 mg/mL, respectively. Untreated coffee beans exhibited a higher reducing power (1.237) than that of the microwave-roasted ones (0.839) when extracted with methanol. Chlorogenic acid was the major (2.31–2.68%) phenolic compound found in all the coffee samples whether it was untreated or microwave-roasted. Vanillin demonstrated the lowest (0.118–0.166%) phenolic compound found in the coffee bean samples. These results might be helpful for obtaining the maximum health benefits from coffee.

Microwave energy for gold ore processing

Quite a few studies have been carried out that look at the application of microwave energy for material processing in various spheres. Microwaves can selectively and quickly heat the target bulk material thanks to its dielectric properties. Gold ores have a complex mineral composition and the effect of microwaves is different on different minerals. This enables to heat selected areas of the rock causing a strong thermal stress. This leads to microcracks occurring at the mineral boundaries resulting in weaker grains and hence better grinding performance. Refractory gold concentrates contain a lot of sulphides, which get hot instantly when exposed to microwave radiation. Thus, microwave energy can be efficiently used to roast sulphide gold concentrates. During microwave roasting, heat conversion takes place directly in the sulphides and the reaction starts immediately. That’s why microwave roasting is a much faster process compared with convection roasting. This paper provides a brief overview of microwave energy used for gold ore and concentrate processing and describes the behaviour of minerals in the microwave field, as well as the results of a number of studies that looked at using microwave energy to destroy rocks, roast refractory sulphide gold concentrates and regenerate charcoals. The key advantages of microwave radiation include an easy process control, the possibility to quickly and selectively heat the product and low energy costs.

Study on the effect of microwave roasting pretreatment on nickel extraction from nickel-containing residue using sulfuric acid

Various means have been proposed to solve problems such as high ash content and complex composition in the recycling of nickel-containing residue produced by battery manufacturing enterprises. Microwave roasting pretreatment is proposed to improve the nickel leaching rate from the residue. The effect of different experimental conditions like microwave roasting temperatures, roasting times, and microwave powers on the nickel leaching rate was studied. It was found that the effect of roasting temperature on the nickel leaching rate was more significant than those of roasting time and microwave power. Meanwhile, after microwave roasting pretreatment, the rate of nickel leaching from the residue could be increased by 20.43%, and weight of the material could also be reduced by more than 21%. After microwave roasting at 450°C, there was no significant change in the main phases of the material, but the surface of the particles exhibited an apparent stratified dissociation phenomenon. Response surface methodology (RSM) was used to optimize the parameters of microwave roasting with nickel leaching rate as the response value. The results showed that the nickel leaching rate could reach 93.11% for roasting at the microwave power of 962 W for 6.2 min under the temperature of 452°C.