Winny Routray

Microwave-Assisted Extraction of Flavonoids: A Review

Flavonoids are major bioactive compounds known to be beneficial against many chronic diseases such as cancer, cardiovascular disorders, and inflammation. Food products obtained from plants are key sources of flavonoids for humans. However, during different food-processing steps, flavonoids are lost in remarkable amounts. Supplementation of flavonoids as a food additive will help in retaining the required health-beneficial amount of flavonoids in the diet, and extraction is an important step in the preparation of food additives. Microwave extraction is one of the most advanced extraction methods, which has the potential of playing a major role in flavonoid extraction and analytical quantification. The present paper reviews the potential application of microwave extraction for flavonoids and the advantage of microwave-assisted process over the other extraction processes generally employed for extraction.

Effect of Different Drying Methods on the Microwave Extraction of Phenolic Components and Antioxidant Activity of Highbush Blueberry Leaves

In the last few decades, researchers have found blueberry leaves to be an interesting source of different phenolic compounds, and drying is an important part of their postharvest and sample preparation processes. In the current study, blueberry leaves were dried using microwave-assisted hot air drying or hot air drying alone at 45, 60, and 75°C to 10–15% wet basis. Corresponding drying characteristics were determined by fitting the moisture ratio obtained in each case with eight theoretical and semitheoretical models for hot air and microwave drying stages. It was observed that best fitting drying models for blueberry leaves depended not only on the nature of the biomaterial but also on the method of drying and the temperature of drying. Phenolic content, monomeric anthocyanin content, and the corresponding antioxidant activity in terms of 2,2-diphenyl-1-picrylhydrazyl (DPPH) inhibition activity and ferric reducing ability of plasma (FRAP) activity were evaluated for the leaf extract obtained following microwave extraction of the dried samples obtained using the above-mentioned drying methods and were compared to the corresponding bioactive compositional characteristics of freeze-dried blueberry leaves. It was observed that the freeze-dried sample had the highest content of total phenolics and total monomeric anthocyanins along with high antioxidant activity. Furthermore, extracts obtained from the microwave-dried leaf samples obtained with a drying temperature of 60°C had highest total phenolic content and highest total monomeric anthocyanin content among all other drying methods and the extract had an antioxidant activity similar to the freeze-dried blueberry leaf samples, implying that microwave drying at 60°C is a potential alternative to freeze drying for preservation of the phenolic components and antioxidant activity of dried blueberry leaves.

Variation of dielectric properties of aqueous solutions of ethanol and acids at various temperatures with low acid concentration levels

Microwave-assisted extraction is gaining in popularity for the production of plant extracts and the dielectric properties determine the behaviour of different materials in a microwave environment; hence the measurement of the dielectric properties of different solvents used for analytical purposes and the interpretation of their heating rate can help in minimising the time and solvent consumption. In this study, the dielectric constant and dielectric loss of different combinations (generated using central composite design (CCD)) of ethanol concentrations and acid (HCl and citric acid) at different temperature levels were measured at frequencies varying from 0.5 GHz to 6 GHz, and the dissipation factor and depth of penetration were calculated for the respective cases, which were analysed for the interpretation of heating rates of the different solvent combinations. Overall, for all the responses (observed and calculated), ethanol concentration levels and temperature levels were found to be the most significant factors; it was interpreted and observed that the heating rate increases with an increase in ethanol concentration and decreases with increasing temperature. In general, the dissipation factor was interpreted to increase with increase in acid concentration (observed from screening analysis); however, citric acid concentration was found to have an insignificant effect on the dissipation factor (as interpreted from CCD with response surface methodology (CCD-RSM)), and hence was found to have insignificant contribution to the heating rate at lower concentrations. In case of CCD-RSM analysis of the values of dissipation factor for the combinations of ethanol with HCl, the effect of acid concentration on the dissipation factor was found to be significant; however, it was still found to be negligible when the solvent combinations were heated in the microwave reactor.