Ohmic Pretreatment Research Articles

The shrinkage of orange slices during microwave drying and ohmic pretreatment

The present research was an attempt to investigate the effect of ohmic pretreatment on the shrinkage of orange slices during the microwave drying and check the effect of this heating process on the shrinkage percentage of the samples. Thomson oranges were chopped into 4-mm-thick slices and were then arranged in a way that they can fit into the ohmic heating system. The ohmic heating system consisted of a 10-mm-thick rectangular glass cell (11 × 8 × 15 cm3) with two 8 × 15 cm stainless steel electrodes and a voltage regulator. Samples were tested at three voltage gradients (30, 50, and 70 V) for 3, 5, and 7 min. After the heat treatment process, the orange slices were subjected to microwave drying at three power levels (90, 360, and 900 W). During the microwave drying process, samples were photographed and weighed at 1-min intervals to check the shrinkage percentages. The experiment was carried out with three replicates. The results showed that the use of ohmic heating system as pretreatment leads to a decline in the moisture content of orange slices. In other words, an increase in duration of heating (from 3 to 7 min) intensifies the moisture content reduction rate. During the microwave heating process, any rise in microwave power (from 90 to 900 W) led to intensification of orange slice shrinkage level. Practical applications The present research was carried out in an attempt to increase productivity of dehydration in the industrial sector and also in the fruit dehydration industry. This research was also an attempt to investigate and predict the shrinkage rate of orange slices subjected to microwave dehydration and investigate the effect of ohmic pretreatment on the shrinkage percentage of the specimens in an attempt to figure out if this method can be used as an alternative to other industrial methods.

Ohmic heating pretreatment of algal slurry for production of biodiesel

Suspensions of the model microalga Chlorella sp. TISTR 8990 were pretreated by ohmic heating to facilitate release of lipids from the cells in subsequent extraction and lipase-mediated transesterification to biodiesel. After ohmic pretreatment, the moist biomass was suspended in a system of water, hexane, methanol and immobilized lipase for extraction of lipids and simultaneous conversion to biodiesel. The ohmic pretreatment was optimized using an experimental design based on Taguchi method to provide treated biomass that maximized the biodiesel yield in subsequent extraction–transesterification operation. The experimental factors were the frequency of electric current (5–105 Hz), the processing temperature (50–70 °C), the algal biomass concentration in the slurry (algal fresh weight to water mass ratio of 1–3) and the incubation time (1–3 min). Extraction–transesterification of the pretreated biomass was carried out at 40 °C for 24 h using a reaction systems of a fixed composition (i.e. biomass, hexane, methanol, water and immobilized enzyme). Compared to control (i.e. untreated biomass), the ohmic pretreatment under optimal conditions (5 Hz current frequency, 70 °C, 1:2 mass ratio of biomass to water, incubation time of 2-min) increased the rate of subsequent transesterification by nearly 2-fold.

Effect of Extraction Methods on Antibacterial Activity and Chemical Composition of Chinese Chives (Allium tuberosum Rottl. ex Spreng) Extract

The Bang-Phae organic vegetable grower community enterprise group, Ratchaburi province is one of the largest Chinese chives (Allium tuberosum Rottl. ex Spreng) growers in Thailand. However, not all of its Chinese chives meet the standard of retail markets and were sorted out. Producing Chinese chives extract is an alternative way to add value to the rejected produces. The Chinese chives essential oil contains many new and known bioactive compounds. Therefore, this experiment was aimed to study the antibacterial activity of Chinese chives extracts using 17 difference extraction conditions (Steam distillation for 1.0, 2.0, 2.5, 3.0 h; Ohmic pretreatment followed by steam distillation for 1.0, 2.0, 2.5, 3.0 h; 95% ethanol, hexane, soy bean oil extraction using dried, fresh and frozen Chinese chives) against 6 different pathogenic bacterium (Escherichia coli ATCC25822, Salmonella enterica Typhimurium U302, S. enterica Enteritidis, S. enterica 4,5,12:i (human) US clone, Bacillus cereus and Listeria monocytogenes 10403S). The disc agar diffusion method with 3 different concentrations of extracts (25, 50, 75 mg/mL) was used to evaluate the antibacterial activity. The results showed that different extraction conditions significantly affected the antibacterial activity. The higher extract concentration resulted in the better antibacterial activity. This finding indicated that both extraction condition and extract concentration significantly influence antibacterial activity. The extracts obtained from thermal extraction seem to have higher antibacterial activity than the ones obtained from cold extraction. The extract obtained from steam distillation for 2.5 h resulted in the highest antibacterial activity; 75 mg/mL of the extract caused 0.688 ±0.023 cm clear zone against B. cereus and L. monocytogenes 10403S. The chemical composition profile using GC-MS showed high percentage of organosulfide volatile compounds in thermally extracted Chinese chives. Therefore, the organosulfide volatile compounds might be responsible for effective bacteria inhibition.

Effects of ohmic pretreatment on crude palm oil yield and key qualities

Pretreatment is an important step that can increase the oil yield and improve the extraction efficiency of palm oil production. However, the conventional pretreatment method widely employed in the palm oil industry requires a relatively long time and high energy. In this research, ohmic heating was employed to pretreat oil palm fruit and the effects were studied of heating conditions on the hardness of the oil palm fruit, yield and qualities of the extracted crude palm oil. Oil palm fruits were ohmic heated using various heating conditions (temperature: 60–100 °C; voltage: 0–150 V; heating time: 2–10 min). Crude palm oil was then extracted from the heated samples using a screw extruder and determined for its qualities (free fatty acid, moisture content and deterioration of bleachability index). The results revealed that the voltage, temperature and heating time significantly affected the hardness of the pretreated oil palm fruit. Increasing the voltage, temperature and heating time resulted in an increase in the yield (maximum at 46%w/w) and improved the oil qualities. It was also found that ohmic pretreatment can reduce the heating time to 8 min and decrease the energy consumption of the process to 0.12 kW-h/kg. The results suggested that ohmic heating can be used as an alternative method for oil palm fruit pretreatment.

Enhancement of Sugar Release from Taro Waste Using Ohmic Heating and Microwave Heating Techniques

The enhancement of sugar release from taro waste slurries pretreated by ohmic heating and microwave heating techniques was evaluated, and the potential mechanism of ohmic heating associated with the breakdown of cell structure was investigated. After ohmic and microwave pretreatments, images of the taro tissues were taken using a fluorescent microscope and a field emission scanning electron microscope (FESEM). The glucose yields from the samples pretreated by ohmic and microwave heating were 28.63 ±4.82 and 27.99 ±6.65 g per 100 g dry base weight of taro waste, respectively. Although there was no significant difference between the two glucose yields, ohmic pretreatment needed only 28% of the energy input of microwave pretreatment to produce the equivalent glucose yield. For validation of electrophoretic cell destruction by ohmic heating, slices of taro skin tissues were exposed to the electric field for 10, 20, and 30 s and then stained with fluorescent dye. In the dye diffusion experiment, microscopic images of taro tissues treated by ohmic heating indicated that the dye diffused well into most portions of the taro skin cells. FESEM images also showed that the cell walls of samples after ohmic treatment were completely disintegrated and damaged, and starch granules in the vacuolar interior became exposed to the surroundings for enhanced enzymatic reactions. Results showed that ohmic heating contributed to (1) the release of starch granules from taro waste, leading to higher sugar yields, and (2) enhancement of enzymatic activities for starch hydrolysis within a short incubation period. It can be concluded that the developed ohmic heating pretreatment technique prior to alcohol fermentation could be applicable to other feedstocks, and the oscillating electric field caused the breakdown of the recalcitrant taro cellular structure.

EFFECTS OF OHMIC PRETREATMENT ON OIL UPTAKE OF POTATO SLICES DURING FRYING AND SUBSEQUENT COOLING

ABSTRACT The effect of ohmic pretreatment on the oil uptake of potato slices during frying and subsequent cooling was investigated. The treated potato slices were heated by either directly sandwiching the slices between two electrodes or by heating them in a 0.11% salt solution. The treatment was carried out using an alternating current field of 35.5 V/cm. Results of this study indicate that oil uptake during frying and subsequent cooling of potato slices was decreased by the ohmic pretreatment without involving a liquid medium (direct sandwiching). However, when the ohmic pretreatment was carried out in a liquid medium, the effect was not evident. This might be due to water absorption during the treatment, which increased initial moisture content as well as porosity of the treated samples.

Effect of Ohmic Pretreatment on the Drying Rate of Grapes and Adsorption Isotherm of Raisins

Effects of ohmic pretreatment on the drying rates of seedless red grapes and adsorption isotherms of the raisins produced were investigated. The treated grape samples were heated in a medium containing 2% citric acid to a final medium temperature of 60oC using an alternating current at 14 V/cm field strength and 30 Hz, 60 Hz, and 7.5 kHz frequencies. Results of this study reveal that the drying rate of the grapes was significantly increased by the ohmic pretreatment, especially at low electrical frequencies. The effect of the ohmic pretreatment on equilibrium moisture content of the raisins produced was evident at 0.75 or higher water activities but there was no or limited effect at low to moderate water activities.

Ohmic heating of strawberry products: electrical conductivity measurements and ascorbic acid degradation kinetics

The effects of field strength and multiple thermal treatments on electrical conductivity of strawberry products were investigated. Electrical conductivity increased with temperature for all the products and conditions tested following linear relations. Electrical conductivity was found to depend on the strawberry-based product. An increase of electrical conductivity with field strength was obvious for two strawberry pulps and strawberry filling but not for strawberry topping or strawberry-apple sauce. Thermal treatments caused visible changes (a decrease) in electrical conductivity values of both strawberry pulps tested, but the use of a conventional or ohmic pre-treatment induces a different behavior of the pulps’ conductivity values. Ascorbic acid degradation followed first order kinetics for both conventional and ohmic heating treatments and the kinetic constants obtained were in the range of the values reported in the literature for other food systems. The presence of an electric field does not affect ascorbic acid degradation.

Effects of moderate electrothermal treatments on juice yield from cellular tissue

Ohmic and microwave heating were employed to investigate electro-thermal effects on apple juice yield by preheating apple samples to 40 or 50 °C. Cylindrical raw and pretreated samples from fresh apple were wrapped in a non-absorbent cloth and pressed to produce juice. Data on juice yield and applied force were recorded and compared with the raw material. The results showed that apple juice yield was strongly improved by both thermal pretreatments, and pretreated samples showed less resistance during extraction. The thermal effect of ohmic pretreatment on juice yield increased with the pretreatment temperature, while for microwave pretreatment, the low pretreatment temperature resulted in high juice yield. Ohmic heating as a pretreatment increased juice yield and required less input work, compared to microwave heating; thus frequency might play a major role in this effect. The thermal effect difference could be due to increased migration of cellular moisture during ohmic pretreatment. These observations indicate ohmic pretreatment has the potential to enhance juice yield and save energy.

EFFECTS OF THERMAL AND ELECTROTHERMAL PRETREATMENTS ON HOT AIR DRYING RATE OF VEGETABLE TISSUE

ABSTRACTCylindrical samples of carrot, potato and yam were dried in a hot‐air dehydrator after preheating to 50C or 80C by three different heating methods (conventional, microwave and ohmic). The results showed that enhancement of drying rate increased with pretreatment temperature. Ohmic pretreatment increased the drying rate more than conventional and microwave heating. Desorption isotherms showed that in the low aw range, desorption data of preheated and raw materials were similar. However, the isotherms of preheated samples shifted when aw was high, which indicated that thermal pretreatments altered the structure, and apparently, the water distribution within these materials. For all samples, ohmic pretreatment showed stronger influences on isotherms than microwave heating, while the pretreatment effect of conventional heating was only observed for potato tissue.

The effects of ohmic heating frequency on hot-air drying rate and juice yield

Ohmic pretreatment of fruit and vegetable samples has been shown to increase hot-air drying rate, shift desorption isotherms, and increase juice yields over raw samples or those pretreated with conventional or microwave heating. The frequency of alternating current has been found to alter heat and mass transfer properties. In this study, the hot-air drying rate of yam and the juice yields of apples were compared using a 60 Hz sine wave and a 4 Hz sawtooth wave to determine if lowering the frequency would result in additional improvements to these processes. The 4 Hz sawtooth wave resulted in a faster hot-air drying rate of yam cylinders than the 60 Hz sine wave. The drying rates of the 4 Hz pretreated samples were significantly greater during most of the drying process, with the most pronounced differences occurring during intermediate stages of drying. The electric field strength affected the drying curves in the range tested at 4 and 60 Hz. Apple juice yield was improved by ohmic pretreatment, with 4 Hz sawtooth samples yielding significantly greater quantities than the 60 Hz sinusoidal pretreatment. Due to increased electrical conductivity at 4 Hz, pretreatments at this frequency require considerably less time than pretreatments at 60 Hz The efficiency of mass transfer processes appears to be significantly dependent on waveform and frequency of alternating current. These phenomena could have useful applications for food processing.