Improved Vacuum Frying Process for High Quality Sweet Potato Chips

摘要

Vacuum frying is a promising method for preserving the desired color, texture, and flavor of products with high sugar content. Since most vegetables and fruits degrade when processed with traditional frying, vacuum frying is an excellent alternative to high temperature processing. However, in vacuum frying the product should be pre-treated before frying to obtain a better texture.The kinetics of oil absorption and oil distribution in sweet potato chips (total, internal, and surface oil content) was studied so that effectiveness of the de-oiling system could be established. An analysis of product quality attributes (PQA) such as moisture content, oil content, microstructure, diameter shrinkage, and thickness expansion, as well as, color, texture, bulk density, true density, and porosity of chips fried at different temperatures (120, 130, and 140?C) was performed to evaluate the effect of process temperature on the product.The final oil content of the sweet potato chips was 0.178?0.007, 0.178?0.011, and 0.172?0.002 g/g solid for frying temperatures of 120, 130, and 140?C, respectively. These values were lower (~60% less) than those found in traditionally fried sweet potato chip, which indicates that the de-oiling mechanism is crucial in vacuum frying processing.It was found that the rate of change in PQAs is greatly affected by temperature; however, the final values of bulk density, true density, porosity, diameter shrinkage, and thickness expansion were not affected by temperature. The texture of the samples was affected by temperature, with the chips fried at 140 degrees C being crispier. In terms of color, the L* and color b* values decreased as temperature increased. While color a* was not affected by temperature.In this study, a two-stage frying process was also evaluated to improve the flavor and texture of sweet potato chips. First, a basket filled with the sweet potato slices was submerged into the oil under atmospheric conditions. As soon as the potato slices were partially cooked (1 min), the pressure was lowered to 1.33 kPa (vacuum frying stage) and the product fried for 2 more min. The products were fried at 130 degrees C for different interval of times. Starch gelatinization, texture, moisture content, and oil content were evaluated at each time interval. Sensory analysis was accomplished by using a consumer panel with 50 members. The samples were compared with the on-stage frying and atmospheric frying processes.The two-stage fried chips had better appearance and texture compared to the ones that were only fried under vacuum or atmospheric conditions. The samples were lighter and more yellow than the chips fried under the single-stage process. In vacuum frying, the temperature of the chips does not reach the gelatinization temperature until most of the water is evaporated. Therefore, there is not sufficient moisture content in the product for gelatinization to occur completely. As a result, the product has a glassy texture. In the two stage frying, the atmospheric frying prior to vacuum frying helps the starch to gelatinize thus producing a better product in terms of texture, oil content, and flavor. The atmospheric fried samples were darker in color and had a scorch taste. The degree of starch gelatinization was 21% higher for the two-stage fried samples than the single-stage fried ones.The application of the dual stage enhances the quality of sweet potato chips, improves consumer satisfaction, and reduces the need for space, cost, and any other needs of blanching pre-treatment to the sweet potato manufacturers.