OSMOTIC DEHYDRATION OF POTATO SLICES COUPLED WITH ULTRASOUND, OHMIC HEATING AND PULSED ELECTRIC FIELD SYSTEMS
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Date
2023-04-30
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ACHARYA NG RANGA AGRICULTURAL UNIVERSITY
Abstract
Potato (Solanum tuberosum L.) from the Solanaceae family is a healthy, commercial food with wide applications worldwide. Potato is one of the most important agricultural products besides wheat, corn and rice. Under tropical and sub-tropical conditions, the losses due to poor handling and storage are reported to be in between 40-50 per cent. The qualitative losses greatly reduce the price of potatoes. Preservation of potato slices by hot air drying is an ancient process to reduce moisture content and increase the shelf-life and stability of food products. A major drawback of hot air drying is the quality deterioration of the products due to exposure to a high temperature for extended periods of time. Blanching and osmotic dehydration are some of the conventional approaches to reduce the qualitative changes during drying. Osmotic dehydration is a slow process and takes a long time. The most popular pre-treatment methods for accelerating the mass transfer during osmotic dehydration without causing adverse changes in the product are ultrasound and pulsed electric field (PEF) under non-thermal and ohmic heating under thermal treatment. These technologies can be used to improve drying rates, reduce energy demand and minimize thermal degradation. To date, there is no literature available on the effects of combined application of ultrasound, ohmic heating and pulsed electric field on the osmotic dehydration of potato slices. Hence, the present research was undertaken to investigate the effect of ultrasound, pulsed electric field and ohmic heating on the osmotic dehydration of potato slices individually and in combinations and also study the effect of osmotic pretreatments along with ultrasound, ohmic and pulsed electric fields on convective air drying characteristics of potato slices. Ohmic heating (OH) system consisted of a rectangular chamber of 1 L capacity with the inside dimensions of 180 mm length × 70 mm width × 90 mm height. Two holes of diameter 5 mm were made at middle on length sides of the chamber to fix the two electrodes. Length and width of the electrodes were 170 and 80 mm, respectively. The distance between both the electrodes was kept as 55 mm. The thickness of the electrodes was 1.5 mm. The treatment chamber was covered with an acrylic sheet having a dimension of 220 mm length × 100 mm width and at middle a circular hole of diameter 5 mm was made to insert consealed T-type thermocouple
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which was connected to the digital temperature controller-cum-indicator. Two types of pulsed electric field (PEF) generators of 30 and 20 kV were developed. 30 kV PEF generator was developed by using fly back transformer and 20 kV generator was developed by using rectifier circuit, high voltage generator circuit and voltage multiplier circuit.
Experiments were carried out in Department of Processing and Food Engineering, Dr N.T.R. College of Agricultural Engineering, Bapatla. Blanched potato slices were subjected to different osmotic salt concentrations of 0%, 5%, 10%, 15% and 20% for 15, 30, 45, 60 and 90 min at different solution temperatures of 30, 40, 50, 60, 70 °C and optimum osmotic dehydration (OD) conditions of 10% salt solution, 50 °C solution temperature and 90 min immersion time were obtained based on the highest amount of water loss. Different pretreatments such as ultrasound (US) treatment with different power levels (54.26, 75.44, 95.84 and 130.87 W) and treatment times (1, 5, 10, 15 min), ohmic heating with different voltage gradients (12.73, 25.46, 38.18 and 50.91 V/cm) and holding times (2, 4, 6, 8 min) and PEF treatment with different voltages (20 and 30 kV) and treatment times (10, 20, 30, 40 s) were imposed on potato slices and then subjected to osmotic dehydration at optimum conditions of 10% salt solution, 50 °C solution temperature and 90 min immersion time. After osmotic dehydration the slices were subjected to convective air drying at 65 °C air temperature and air velocity of 0.04 m/s. The optimum level of the pretreatment conditions were determined based on the quality of the dried slices. After osmotic dehydration, the potato slices were subjected to air drying at different temperatures (35, 45, 55, and 65 °C) and air velocities (0.0199, 0.0266, 0.0333 and 0.04 m/s) and the quality during the storage period was determined. Combinations of pretreatments were done at the optimum levels and dried at optimum drying air temperature (65 °C) and air velocity (0.04 m/s) and quality during the storage period of 90 days was determined.
In ultrasonic pretreated potato slices after osmotic dehydration, lowest moisture content (298.60% (d.b.)) and solid gain (11.615%) and highest percentage of water loss (45.674%) and weight reduction (36.47%) were found for ultrasonic power level of 130.87 W and 15 min treatment time. In ohmic pretreated potato slices after osmotic dehydration, lowest moisture content (302.59% (d.b.)) and solid gain (12.833%) and highest percentage of water loss (44.897%) and weight reduction (32.065%) were found for voltage gradient of 50.91 V/cm and holding time of 8 min. In PEF pretreated potato slices after osmotic dehydration, lowest moisture content (273.80% d.b.) and solid gain (10.694%) and highest percentage of water loss (50.214%) and weight reduction (39.52%) were found for voltage of 30 kV and holding time of 40 s. Drying time of 420 min was observed in the US+OD dried samples treated at 130.87 W ultrasonic power for 15 min treatment time with lowest final moisture content of 5.66% (d.b.), 450 min in OH+OD dried samples treated at voltage gradient of 50.91 V/cm and 8 min with lowest moisture content of 6.039% (d.b.) and 450 min in PEF+OD dried treated samples for 30 kV and 40 s with final moisture content of 5.487% (d.b.). Highest drying rates of 12.88%/min, 12.89%/min and 12.62%/min were observed in US+OD (130.87 W for 15 min), OH+OD (50.91 V/cm for 8 min) and PEF+OD (40 kV for 40 s), respectively, compared to all other samples.
Quality parameters such as lowest moisture content (5.66% d.b.), shrinkage (59.9%), hardness (5.42 N) and browning (0.054) and highest rehydration ratio (3.2),
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bulk density (150.31 kg/m3), L* value (75.36), a* value (-0.197), b* value (13.35), carbohydrate content (58.33 g/100 g) and starch content (45.12 g/100 g) were observed in ultrasound pretreated osmo+air dried potato slices at ultrasonic power of 130.87 W and treatment time of 15 min. In ohmic pretreated osmo+air dried potato slices, lowest moisture content (6.04% d.b.), shrinkage (63.2%), hardness (5.42 N) and browning (0.052) and highest rehydration ratio (3.26), bulk density (150.41 kg/m3), L* value (69.85), a* value (-0.24), b* value (12.81), carbohydrate content (57.99 g/100 g) and starch content (44.20 g/100 g) were observed at voltage gradient of 50.91 V/cm and holding time of 8 min. In PEF pre-treated osmo+air dried potato slices, lowest moisture content (5.45%), shrinkage (61.49%), hardness (5.29 N) and browning (0.052) and highest rehydration ratio (3.27), bulk density (150.96 kg/m3), L* value (76.41), a* value (-0.17), b* value (13.41), carbohydrate content (60.376 g/100 g) and starch content (46.35 g/100 g) were observed for voltage 30 kV and treatment time of 40 s.
Highest percentage of water loss in US+OH+PEF (52.39%), solid gain in OH+PEF (16.83%), weight reduction in PEF+US (39.45) and lowest moisture content in PEF+US+OH (261.42% (d.b.)) were observed after osmotic dehydration. At the end of storage period of 90 days, in all the combinations treated osmo+air dried samples, the better quality parameters of lowest moisture content (5.4% d.b.), bulk density (151.6 kg/m3), browning (0.056), total bacterial count (100 cfu/mL) and total mould count (100 cfu/mL) were observed in US+OH+PEF and highest rehydration ratio (3.24), L* value (76.35), a* value (-0.297) and b* value (13.06), carbohydrate content (54.92 g/100 g) and starch content (44.2 g/100 g) were observed in US+OH+PEF. Finally, it was concluded that the quality of the dried potatoes treated with the combinations of treatments such as US+OH, OH+PEF, PEF+US and PEF+OH+US were observed better at the end of the storage period of 90 days. Keywords: Drying rate, drying time, quality parameters, solid gain, storage period, water loss, weight reduction
Description
OSMOTIC DEHYDRATION OF POTATO SLICES COUPLED WITH ULTRASOUND, OHMIC HEATING AND PULSED ELECTRIC FIELD SYSTEMS