The effects of microwave blanching on the nutritional and textural quality of freeze-dried spinach

Abstract

The large leaf surface area of spinach makes this crop a logical choice for freeze drying. Microwave blanching has been suggested as an alternative to conventional steam and water blanching. Potential advantages include: conservation of vitamins, color retention, and decreased process time. Heating efficiency can be high as the product acts as its own heat source without heat loss to the surrounding environment. A possible disadvantage may lie in the high per unit cost of thermal energy. As a method of blanching, microwave energy was demonstrated to be convenient and effective. Thermal processing was found to result in a more intense yellow-green spinach product; though type of blanch was found to make relatively little difference in Gardner color values. Microwave blanching rated highest in ascorbic acid content, 309.87 mg/100g; intermediate in alpha-tocopherol values, 2.67 mg/100g; and lowest in beta-carotene, 24.86 mg/100g on a dry weight basis. High pressure liquid chromatography was found to be a rapid, reproducible method of separating alpha-tocopherol with quantitative accuracy. Rehydration was not found to correlate with Instron texture scores. The firm, chewy texture of the rehydrated microwave and steam blanched spinach was preferred. Water blanching was found to rupture the cells and destroy the cellular fine structure. Due to cell wall breakdown and the consequent leaching of cell contents, losses in ascorbic acid, flavor, and textural quality can easily occur. Steam blanching caused less tissue disruption allowing a greater retention of cellular structure which explains the high rehydration ratios obtained and the acceptable textural characteristics. Microwave blanching caused the most thermal damage as evidenced by the coagulated protoplasmic material surrounding the cell walls. After microwaving, the external tissue layers were compressed and acted as barriers to rehydration water. Photomicrographs of the surface revealed tiny, spherical holes believed to be caused by hot spots from microwave heating. Cell structure remained intact causing microwave treatment to be the preferred method for retaining ascorbic acid and textural quality attributes.