Convective drying with ethanol pre-treatment and intermittent microwave drying of fresh and isomaltulose-enriched strawberry

Abstract

Strawberries are nutritionally rich, with appreciated sensory characteristics. However, as it is a highly perishable fruit, the processing of the strawberry is indicated. Osmotic dehydration (OD) is a process of mass transfers between the food and the hypertonic solution. One of these mass flows is solid incorporation, which makes OD an interesting strategy to promote the enrichment of fruits with solutes beneficial to health. Isomaltulose can be used as an osmotic agent. It is a carbohydrate with a low glycemic index and low cariogenic potential. Another mass flow of OD is water loss. However, the amount of water removed during OD is insufficient to guarantee the stability of the product for long periods. Convective drying is the most traditional method for reducing the moisture content of food. Ethanol pre-treatment is an efficient procedure in reducing drying time and minimizing the degradation caused during convective drying. In turn, the microwave method is an alternative for drying food, capable of promoting volumetric heating, which is associated with thermosensitive compounds preservation. Therefore, the aims of this work were (i) to enrich the strawberry with isomaltulose, by means of OD using osmotic solutions of 25 or 35%, at atmospheric pressure and with vacuum application during the first 10 or 20 min; (ii) to study the drying kinetics and quality of fresh and isomaltulose-enriched strawberries, pre-treated, or not, with ethanol; and (iii) to evaluate the influence of the use of intermittent microwaves in the drying of fresh and isomaltulose enriched strawberries. OD was shown to be efficient for the incorporation of isomaltulose in strawberries, especially using the osmotic solution with the highest concentration and applying a vacuum. However, some losses due to the leaching of anthocyanins, phenolic compounds and antioxidants were observed. In convective drying, the osmo-dehydrated strawberry demanded more time to complete the process and contributed to the reduction of shrinkage and hygroscopicity of the dried strawberry. The addition of ethanol contributed to the reduction of drying time and minimized the degradation of antioxidant compounds in strawberries. Drying by intermittent microwaves required less time and consumed less energy, in addition to contributing to the preservation of strawberry color and bioactive compounds and antioxidants. The osmotic process promoted the enrichment with isomaltulose and reduced the shrinkage of dried strawberries.