A Comparative Study on the Effect of Chemical, Microwave, and Pulsed Electric Pretreatments on Convective Drying and Quality of Raisins

Abstract

In order to discourage the use of chemicals in raisin processing, the effect of microwave and pulsed electric field (PEF) pretreatments on the drying rate and other quality parameters like color, total soluble solids, bulk density, appearance, and market quality were compared with that of chemically pretreated raisins dried at 65 C. The untreated and pretreated samples had a statistically significant difference in drying rate (P < 0.05). The drying rate of chemically pretreated raisins was the highest when compared to others. The results showed that the PEF and microwave-treated samples had a significantly high Total Soluble Solids (TSS), along with good appearance and market quality. INTRODUCTIONRaisins are valued for their medicinal and nutraceutical properties. They are widely used and are popular as a good source of potassium, magnesium, and fiber. Fresh grapes in general contain about 85% moisture content on wet basis. In the process of raisin making it is reduced to 18% (wet basis). This is done traditionally by sun drying. The natural waxy coating on the grapes delays moisture removal. In order to reduce drying time, especially in mechanical drying and to improve quality, grapes are pretreated before the drying process.Pretreatments in raisin processing include dipping in a hot solution of sodium hydroxide and ethyl oleate and sulphur fumigation. With a view to find alternate methods that could reduce drying time using no chemicals, pretreatment with microwaves (MW) and pulsed electric field (PEF) has been explored in this study.

Optimization of pulsed electric field pretreatment parameters for preserving the quality ofRaphanus sativus

“…The optimization results for each of the 3 pulse parameters with respect to the 3 measured quality indicators were as follows: The optimal ascorbic acid oxidase activity Y was U g For this activity X V cm X s andX (11) In the samples without PEF treatment, the Vc content was 87.8 ± 2.87 mg•kg -1 , the ascorbic acid oxidase activity was 0.1240 ± 0.0048 U•g -1 , the drying rate was 0.72 ± 0.02 h -1 . These results of samples with PEF treatment demonstrate that drying rate and Vc content were optimized with a low pulse intensity, short treatment time, and appropriate pulse number.…”

“…Although PEF and microwave pretreatments can shorten drying times by only about 20%, the quality of the products processed using these pretreatment methods is better than that of chemically pretreated samples or untreated samples. [11] In addition, a study on carrot drying showed that the drying time of PEF-pretreated carrots was 20% less than that of untreated samples. [7] Another recent study reported that PEF pretreatment of red pepper reduced drying time by 34.7%.…”

Novel Concepts in Microwave Drying of Foods

“…[148] Similar results were found when MW and pulsedelectric field (PEF) pre-treatments were studied for their application to raisin production, where it was found that both had an effect on drying rate. [149] PEF has also been applied in order to disrupt the cellular structure of food products prior to air drying. Again, the applicability is in question, as in the case of potato slices, the drying rate was found to be unaffected during subsequent microwave-convective drying.…”

Drying of shiitake mushroom by combining freeze-drying and mid-infrared radiation

“…Samples dried by the MIRD (25 min)-FD combination showed the lowest rehydration ratio and the highest apparent density (p < 0.05). The pore size distribution in the internal structure of dried products greatly affects both the apparent density and rehydration (Karathanos, 1993;Dev et al, 2008). In the case of samples which were subjected to MIRD before FD, it is difficult for water within the mushroom matrix to diffuse out because the surface of mushroom forms a crust.…”