Microwave hydrodiffusion and gravity as pretreatment for grape dehydration with simultaneous obtaining of high phenolic grape extract

The present study aimed to verify the efficiency of microwave hydrodiffusion and gravity (MHG) by partially removing water from grapes while simultaneously obtaining a phenolic-rich extract. In addition, the effect of heating on phenolic compound degradation of pre-dehydrated grapes, the extract, and economic viability to produce both products were evaluated. grape dehydration

Thus, a 50% reduction of moisture in the grapes was observed after 10 min while simultaneously obtaining a grape extract. Both products obtained by MHG presented high phenolic compound content and antioxidant capacity.

In addition, the use of MHG on an industrial scale was found to be economically viable based on the analysis time, labor cost, and financial return of the products obtained. Thus, the method is efficient, fast, economically viable, and sustainable while simultaneously producing pre-dehydrated grapes and a phenolic-rich extract.

Grapes are considered an attractive source of income due to their high consumption, whether in fresh or processed form . This fruit is characterized as a source of antioxidants, being potentially bioactive, and presenting high concentrations of phenolic compounds, especially anthocyanins that characterize the purple coloration .

However, grapes in their fresh form have high levels of moisture and sugar, which accelerates deterioration. As a result, its consumption must take place quickly, otherwise, one of the alternatives to preserve the fruit is to convert it into some derived product .

Dehydration by freeze drying

In this way, 100 g of the fruit were weighed on an analytical balance and stored at −18 °C for 24 h. After freezing, the grapes were subjected to dehydration by freeze drying (Terroni-Fauvel-LH2000/3) at −4 to −5° C until total loss of water in the fruit. This experiment lasted 149.9 h (Fig. 1c). To speed up the dehydration process and promote the release of water from the grape berries, holes were made in the skin with a needle.

Moisture content

Moisture content was determined according to the Lutz (2008) Institute in order to determine the amount of water remaining in the grapes after pre-dehydration.

Total phenolic compounds

Total phenolic compound estimation was based on the FolinCiocalteu spectrometric method according to Singleton, Orthofer, & Lamuela-Raventós (1998). A calibration curve with gallic acid (standard phenolic) was drawn with concentrations from 0.0109 to 0.0763 mg/L, being the values were expressed as mg gallic acid per liter of extract and mg gallic acid per 100 g of fruit on a dry basis. The reading wavelength was 760 nm for the curve and samples.

This study attested to the positive effect of grape pre-dehydration by MHG on the oven-drying and freeze drying methods, as it provided a 50% reduction in the moisture of the grape berries and significantly reduced processing time. In addition, pre-dehydration of grapes by MHG preserved the antioxidant activity and phenolic compounds present in the fruit.Moreover, MHG was effective in quickly extracting phenolic compounds and providing an extract rich in phenolic compounds and high antioxidant capacity. Thus, the extract obtained by MHG features great potential for the food or pharmaceutical industry and can be added in different products.