The application of dehydration technologies on drying kinetics and physicochemical properties of orange-fleshed sweet potato

Abstract

Orange-fleshed sweet potato is a highly researched crop due to its nutritional content and more specifically, its high β-carotene content. Most developing countries around the world have adopted the use of orange-fleshed sweet potato as one of the staple food. The short growth period (3-6 months), low agronomical input, and dual-purpose use are properties to combat food security issues in most sub-Saharan African countries. The β-carotene compound is also a precursor for vitamin A, which is one of the most crucial nutrients, especially in children aged between 0-6 years, and pregnant women. African countries (Sub-Saharan region) are known to have a prevalence of vitamin A deficiency. There are challenges when it comes to the storage of fresh plant-based food such as orange-fleshed sweet potato, which is highly perishable and therefore has a short shelf life. Alternative processing methods such as drying and milling into flour can help to extend the shelf life of the orange-fleshed sweet potato. Traditional drying methods such as sun drying, solar drying and oven drying have been used for over decades. However, these methods generally destroy heat-sensitive compounds such as the critical β-carotene content, mainly due to exposure of the orange-fleshed sweet potato to oxygen, sunlight and high temperature for longer periods. The exposure of β-carotene to the mentioned factors can result in isomerization and auto-oxidation of β-carotene, which reduce the vitamin A content, thereby producing a product with poor nutritional content. The current study seeks to explore novel drying technology, such as freeze-drying, microwave and infrared drying methods. Orange-fleshed sweet potato (Bellevue and Orleans cultivar) was dried by application of oven (40°C, 4 hours, air velocity 5.2 m/s), microwave (80 W, 1 hour, air temperature of 40°C, air velocity 4.5 m /s), infrared (250 W, 2 hours, air temperature of 40°C, air velocity 4.5 m/s), microwave-infrared (80 W + 250 W, 45 minutes, air temperature 40°C, air velocity 4.5 m/s) and freeze-drying (-45°C, 100KPa, 5 days) technologies, and milled into flour. The drying kinetics were analysed by different models. The produced flour was analysed for physicochemical properties and nutritional composition. The analysed functional properties include water absorption capacity, swelling capacity, solubility index, bulking density, pasting properties and thermal properties. Proximate composition was also evaluated including total dietary fibre and β-carotene content of the flours. The oven-drying method was the slowest, due to a slow drying rate. The latter is due to the moisture transfer mechanism, as low temperatures are used during drying. The moisture transfer is by capillary diffusion which is a slow moisture transfer mechanism. Oven drying took about 4 hours to dehydrate the sweet potato slices to solid content of less than 13%. Infrared drying was the second slowest drying method, while microwave took only 1 hour to completely dry the sweet potato slices. The electromagnetic radiation by microwave and infrared cause a rapid structural collapse, releasing water from cytoplasm and vacuole, thus increasing cell membrane water permeability, which makes it easy for water to be transported out of the plant cell. The drying rate of the microwave-infrared drying method was the fastest (45 minutes). The coefficient of diffusion from the models also showed that the microwave-infrared combination had the highest diffusion rate, as compared to other drying methods which showed a lower coefficient of diffusion. The Page model was the most suitable for the oven drying method, Lewis model for infrared drying, while Henderson and Pabis for infrared and Logarithmic for microwave-infrared combined method. The pasting and thermal properties of the flours were not significantly affected by the different drying methods. However, infrared and microwave-infrared dried flours have indicated a higher final viscosity when compared to other drying methods. The freeze-dried flour showed a higher enthalpy value (4.29 J/g), as compared to other drying methods. Microwave-infrared drying methods, infrared, and microwave had a higher solubility index, while the oven and freeze-drying methods showed a lower solubility index. The freeze-dried flours exhibited the lowest bulk density as compared to other drying methods. Microwave-infrared combined drying methods revealed a higher retention of β-carotene (85.06-90.14%) and this seem to be mainly due to the fast drying rate of the combined drying methods. The microwave also had a higher retention of β-carotene, followed by infrared, while oven and freeze-drying method showed a lower retention of β-carotene as a result of longer drying periods, exposure to oxidative and destructive conditions, which can cause the degradation of β-carotene (high drying temperature, endogenous enzymes, and oxidative agents). The study suggest that a combination of microwave-infrared or microwave alone are energy efficient alternatives to produce dried orange fleshed sweet potato flour, with minimal reduction in β-carotene and change in functional properties