Determination of the Effect of Steeping and Blanching Pretreatments and Drying Temperatures on the Proximate Properties of White-Fleshed Sweet Potato Variety
Abstract
Agricultural products require suitable preservation methods for retention of their nutritional qualities and attributes. Sweet potatoes are underutilized due to cost implications of their preservation using modern storage techniques. Enzymatic discoloration of freshly cut potato flesh surfaces from exposure to air affects processed potato qualities adversely. While steeping/blanching pretreatment is deployed in curbing this challenge, varying the steeping/blanching and drying treatments can assist in arriving at the optimum process conditions for white-fleshed sweet potatoes. 4 kg of similarly sized and healthy white-fleshed sweet potato tubers obtained from Eke Awka market (Latitude 6.210528 °N and Longitude 7.072277 °E) in Anambra state, Nigeria, was washed with clean tap water, peeled and cut into 4 mm by 10 mm by 10 mm slices. The experiment was designed using the Response Surface Methodology (RSM) software. The chipped potatoes were divided into 24 samples of 100g each for the analysis. 19 samples were pretreated in water at temperature range of 11oC to 100oC and 5 samples were not pretreated (served as control). All the pretreated samples were held in the fluid for period of 2 mins to 18 mins as contained in the design matrix. Both the pretreated and control samples were grouped and dried in convective oven at set temperatures of 53 °C, 60 °C, 70 °C, 80 °C and 87 °C and weighed every 30 mins until there was no further weight loss at three consecutive readings. The dried sweet potato chips were grated into flour, for the determination of the proximate composition of the chips. Optimized drying and pretreatment conditions were analyzed from the results using RSM software. Result of the analysis showed significant increase in proximate composition of the samples with increase in both the steeping and drying temperatures and indicated significance of the model terms. The error analysis showed a high degree of correlation between the observed and the predicted values. From this work the optimal conditions predicted by the RSM model were blanching temperature of 70 oC, blanching time of 5 mins and drying temperature of 80 oC for 4 mm-thick 100g of sliced whitefleshed sweet potato chips to yield 2.40% Crude protein, 73.95% Carbohydrate, 2.54% Fiber, 4.31% Ash, 3.24% Fat and 9.86% Moisture content which served as a guide for high quality sweet potato chips production from freshly harvested whitefleshed sweet potato tubers. The dried potato chips can be grated into flour and used as the major ingredient for bread, noodles and cake making. The application of the outcome of the work will help in reduction of post-harvest losses of the crop.
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Introduction
Sweet Potatoes (Ipomea batatas) are among the world’s major food crops and most consumed herb belonging to the family of Solanaceace (Rahman et al., 2015). It is rich in active ingredients such as Vitamins B1, B3, B6 and C, minerals, antioxidants, fiber, carbohydrate (Kingsford, 2021; Saraiva and Rodrigues, 2011), with the leaves and tubers as the most vital parts of the plant. In some African countries such as Guinea, Liberia and Uganda, the young tips of the vine and young leaves are valuable as they are usually eaten as vegetable (Orhevba and Abimaje, 2019) and serve as good sources of Vitamins A, C, and B2 as well as an admirable source of lutein (Carotenoid Pigment). It has variety of sizes and color which include orange, white, and purple fleshed with its origin believed to be Central/South America before it spread to other countries such as Mexico, China, Japan and continent of Africa (Orhevba and Abimaje, 2019). Today, Sweet potato is widely grown and consumed throughout the world and can be used for various purposes in the food industry and households (Arum et al., 2022)
Conclusion
The study, targeted at reducing the rate of post-harvest losses and underutilization of Sweet potato tubers in Nigeria, seeks to ascertain the possible pretreatment and drying conditions suitable for sweet potato chips production from the local white fleshed variety. The results of the experiment showed that steeping temperature, steeping time and drying temperature are essential factors in the production of high quality sweet potato chips. It was also observed that sweet potato chips contain lots of nutrients such as protein, carbohydrate and other essential vitamins and minerals which were noticed to be enhanced in the pretreatment and drying processes. In conclusion, the quadratic regression model successfully revealed the effects of the independent variables on the pretreatment and drying process with coefficient of determination (R2 ) values as 0.9840, 0.9664, 0.9811, 0.9671, 0.9737, 0.9877.
The values of Protein, carbohydrate and crude fiber content of the samples were observed to be enhanced in pretreated samples than the control samples; especially samples blanched at temperatures above 32 oC as the values were observed to gradually increase with increase in both the blanching and drying temperatures across the samples. The moisture content was observed to be generally higher in samples steeped in water of temperatures lower than 64 oC. It increased with increasing blanching temperatures but gave peak values with blanching time variation in some temperatures. The dry samples moisture contents were within the recommended value for storage of sweet potato chips. The study has shown that steeping and drying temperatures of white-fleshed sweet potato for chips production should be above room temperature and less than 100oC for good quality and nutritious sweet potato chips production. The optimal process conditions were predicted using the RSM as 70 oC blanching temperature, 5 mins blanching time and drying temperature of 80 oC; which yielded 2.39% Crude protein, 73.95% Carbohydrate, 2.54% Fiber, 4.31% Ash, 3.24% Fat and 9.86% Moisture content. Application of study findings will help to reduce postharvest crop losses and enhance sweet potato chips production.