Evaluation of high pressure pretreatment for enhancing the drying rates of carrot,apple, and green bean

Preservation of fresh produce by drying dates back to ancient times and is still an indispensible technique. Conventional drying of fruits and vegetables is often accompanied by changes in color, texture, and taste. Suitable pretreatments can improve the drying process by reducing the drying time, yielding higher-quality products, and energy savings. In this study, two varieties of apples, Amasya and red delicious, green beans and carrots were pretreated with high hydrostatic pressure (HHP) at different pressure–time–temperature combinations (100–300 MPa for 5–45 min at 20 and 35 °C) prior to drying. The drying experiments were carried out by using a hot-air tunnel dryer at different temperatures (27–85 °C) and air velocities of 0.4 and 0.8 m/s with constant external conditions. Improving the drying conditions by increasing the drying temperature generally masked the effect of HHP pretreatment on drying rate. Generally, pressures of more than 100 MPa caused cell permeabilization resulting in higher drying rates. Among 14 models, the modified Page model was found to best explain the drying behaviors and model constants were evaluated accordingly. The Tukey multiple comparison test was applied on characteristic drying times to evaluate the relative effects of different pretreatments and drying conditions.     

苹果微波干燥的变温控制方法研究

在苹果脱水处理中,微波干燥可以有效缩短干燥时间,提高干燥品质。试验采用6种设定温度下的恒温微波干燥方案、3种变温微波干燥方案,研究苹果在不同方案下的干燥特性及品质。试验结果表明:"恒温"干燥方案中,干燥速率在干燥中期缓慢降低,会造成制品的品质较低。"干燥速率控制"方案中,干燥速率在干燥中期控制为恒速,但在干燥初期控制存在缺陷;"线性温度控制"方案中,干燥速率在干燥中期基本保持恒速,并且干燥耗时、耗能合理,感官质量、色泽品质等均优于恒温干燥;"三段温度控制"方案中,简化了"线性温度控制"方案所用设备的复杂程度,虽制品品质指标略有下降,但设备简化,更利于工业应用。该研究为微波干燥技术应用于苹果的干燥理论提供了技术依据。     

Optimal power control strategies in microwave drying

A microwave drying system, which can automatically and continuously adjust microwave power, control sample temperature and measure sample mass, was developed and used in apple drying. At three drying temperatures, i.e., 75, 65, 55 °C, applied microwave power was recorded and analyzed, and a relationship of the power with respect to drying rates was built. With this relationship, apples were further dried at the same three temperatures but using variable power during drying process based on the online obtained drying rates. A better temperature control was achieved and less energy was consumed with this method. Depending on the resulted power profiles, a linear and a three-step power control methods were further developed to simplify the control system. With the developed methods, mass measurements were no longer required and power control could be easily implemented, while temperature control and energy consumption were still acceptable.     

Infrared thermography assisted control for apples microwave drying

A microwave based system was developed for drying of apple slices while on-line controlling their temperatures. An infrared thermography assisted control system allowed to realize temperature control by detecting the instantaneous maximum temperature among the samples under test instead of a chosen slice. The effectiveness of the system at hand in realizing temperature control both in space and in time was proven for three temperatures: 55, 65 and 75 °C. Temperature fluctuations became larger both with time increasing and with rising temperature levels thus causing a negative impact on dried fruit quality. In fact apples slices dried at 75 °C by microwave showed L^∗ (lightness) and white index (WI) values significantly lower (p < 0.05), when compared to the fresh samples and to the apples dried by hot air. To overcome such behaviour a mode stirrer was introduced resulting in halving the temperature spatial oscillations of sample slices.     

Effect of ultrasound on banana cv Pacovan drying kinetics

The aim of this work was to study and to model the drying kinetics of fresh and ultrasonic pretreated banana cv Pacovan using the diffusional model (Fick’s second law) and an empirical two parameters model (Page model). The pretreatment was carried out in an ultrasonic bath at 30 °C. The drying process was carried out in a fixed bed dryer at two different temperatures (50 and 70 °C) and 3.0 m/s air velocity. Page empirical model provided the best simulation of the drying curves. The diffusional model was used to describe the moisture transfer and the effective diffusivities of water were determined and were in the order of 10⁻⁹ m²/s. These diffusivities increased with increasing temperature and with the application of ultrasound, while the process time reduced, which can represent an economy of energy, since air drying is cost intensive.     

Carrot volatiles monitoring and control in microwave drying

Volatiles emanating from carrot [Daucus carota L. cv. ‘Enterprise’] samples dried in a microwave oven were monitored throughout the drying process with a zNose™. Drying temperatures were first set to 50 °C, 60 °C, 70 °C, and 80 °C. Six typical volatile peaks were recorded every 4 min for each of the three replicate runs. Color and sensory evaluation were performed after drying. Samples could be dried in a short time at high temperatures but the interior of some samples was charred. Longer time and more energy were required at low temperature drying. A fuzzy logic control system was then designed and employed to control the drying temperature throughout the whole drying process, using detected volatile peaks as variables. Based on the fuzzy logic controlled temperature curve, a simple linear control method was developed. It was shown that carrot quality was intact and time and energy were saved with these new control strategies.     

Shrinkage, vitamin C degradation and aroma losses during infra-red drying of apple slices

The goal of this study was to determine the effect of drying temperature on the shrinkage, ascorbic acid (vitamin C) degradation and aroma retention of apples. Apple samples were found to shrink continuously until a water content value of 2 kg/kg d.b. The degradation of ascorbic acid followed a pseudo-first-order reaction and the degradation rate constant increased when temperature increased from 40 to 70 °C. The loss of aroma volatiles increased with temperature and drying time.     

Study of the drying behaviour in peeled and unpeeled whole figs

Aim of this study was to estimate the effect of peeling on drying kinetics and effective diffusivity D_eff of figs (Ficus carica L. var. tsapela) during air-drying. For this purpose three temperatures (45, 55 and 65 °C) were tested. The Logarithmic model was chosen to describe the drying curves among seven drying models. The estimated drying constants were associated with the drying temperature by an Arrhenius type equation. The ratio of peeled to unpeeled relaxation times was found to be 0.54 ± 0.16. The D_eff of figs (peeled and unpeeled) was estimated by the method of slopes. The D_eff of the peeled figs was higher than this of the unpeeled figs presenting smaller differences as drying temperature was increased. This behaviour was attributed to the case hardening effect which is faster developed as the drying rate increases during high temperature drying.     

Effects of conventional and multistage drying processing on non-enzymatic browning in tomato

The effect of different drying temperatures on the development of non-enzymatic browning in tomato was studied. In particular, the influence of product moisture and temperature on the colour change during drying at various temperatures in the range of 50–90 °С was investigated. The extent of browning was evaluated spectrophotometrically as tomato extracts absorbance at 420 nm. As a result of analysis of experimental data, a multistage drying process of tomato was developed. In this method, drying was realized at decreasing air temperature, arranged such, that the temperature inside the fruit did not exceed acceptable level of 55 °С. Higher product quality and shorter drying time in comparison with the conventional drying at constant air conditions were observed.     

Study of banana dehydration using sequential infrared radiation heating and freeze-drying

The drying and quality characteristics of banana slices processed with a sequential infrared radiation and freeze-drying (SIRFD) method were investigated. Cavendish bananas slices with 5 mm thickness were predehydrated using IR heating at each one of three radiation intensities, 3000, 4000, and 5000 W/m² or hot air at 62.8 °C. The predehydrated samples with 20% and 40% weight reductions obtained using 4000 W/m² IR intensity were then further dried using freeze-drying for various times to determine the effect of predehydration on the drying rate during freeze-drying. To improve the quality of dried banana chips, the banana slices were also treated with a dipping solution containing 10 g/l ascorbic acid and 10 g/l citric acid before the IR predehydration. Control samples were produced using regular freeze-drying without the predehydration. The quality characteristics of dried banana chips, including color, thickness shrinkage and crispness, were evaluated. The predehydration results showed that the drying rate of IR heating was significantly higher than the hot air drying and increased with the increase of IR intensity. For example, it took 10 and 38 min to achieve 40% weigh reduction by using IR at 4000 W/m² and hot air drying, respectively. However, the banana slices with IR predehydration dried slower during freeze-drying compared to the samples without predehydration, which was due to texture changes that occurred during the predehydration. Acid dipping improved product color and also reduced freeze-drying time compared to non-dipped samples. It has been concluded that SIRFD can be used for producing high crispy banana chips and additional acid dipping improved product color and reduced required freeze-drying time.     

Electrohydrodynamic (EHD) drying of tomato slices (Lycopersicon esculentum)

The drying rate of tomato slices (Early Urbana L.) treated by a high voltage electrostatic field (HVEF) was investigated with an oven-drying and open-room drying sample, under identical conditions. ANOVA showed that the drying methods had a significant effect (P ? 0.01) on decreasing moisture content, surface temperature, and apparent color of tomato slices, but no difference was observed in shrinkage. Average drying rate for 3, 4, and 5 kV cm⁻¹ electrostatic fields over a period of 780 min increased by 1.3, 1.43, and 2 times, respectively, compared to that of the air-dried. Tomato slices dried with EHD⁺, which consumed only 16.5 mJ g⁻¹ of electric power, exhibited a better appearance (lightness and red color) with lower surface temperature than the air-dried and oven drying samples.     

Effect of drying methods on the phenolic constituents of meadowsweet (Filipendula ulmaria) and willow (Salix alba)

The objective of this study was to investigate the effect of drying conditions on the phenolic constituents and colour of extracts of organically grown white willow and meadowsweet for incorporation into a functional beverage with potential anti-inflammatory properties. The herbs were freeze-dried, air-dried, oven or tray-dried at 30 or 70 °C. The drying kinetics of the herbs was first determined. Both drying temperature and method had a significant effect (p ≤ 0.05) on the drying rate, the samples tray-dried had a faster drying rate than those oven-dried. Results show that for meadowsweet and willow, freeze-drying and oven or tray drying at 30 °C had no significant effect on the phenolic constituents (e.g. total phenols, salicylates, quercetin) or the colour of the extracts in comparison to traditional air-drying. Although increasing the drying temperature to 70 °C resulted in an increase in the drying rate of both herbs it also led to the loss of some phenolic compounds. Also, the extracts from both herbs dried at 70 °C were significantly (p ≤ 0.05) redder than the other drying methods. Therefore, tray drying these herbs at low temperatures may reduce drying time without having a significant effect on the phenolic content and colour of the extracts.     

Drying kinetics and survival studies of dairy fermentation bacteria in convective air drying environment using single droplet drying

The drying and survival kinetics of Lactococcus lactis ssp. cremoris in a convective air drying environment were measured using single droplet drying experiments. Tests were carried out at five different drying temperatures (45–95 °C) at a constant air velocity (0.5 m/s) and within 2.4–11% relative humidity. The effect of protective agents (10% w/w) of lactose, sodium caseinate and lactose:sodium caseinate (3:1) was also evaluated. The thermal inactivation kinetics parameters in convective air drying and isothermal water bath heating were determined and compared. The results showed that the final temperature attained by the droplet affected the survival of the bacteria significantly, however, most of the bacterial death occurred in early stage of drying while evaporative cooling kept the drop temperature relatively low. At higher droplet temperatures (?65 °C) the bacterial cultures were inactivated by both dehydration and thermal stresses. At lower droplet temperatures (?55 °C) the rate of change in droplet moisture content had much stronger effect on the bacterial survival. Lactose and sodium caseinate, as protective agents, enhanced the survival of bacterial cells significantly at all the test conditions. The lactose:sodium caseinate (3:1) mixture synergistically enhanced the survival of the bacterial cultures. The death of these bacteria followed first-order kinetics during convective single droplet drying as well as during isothermal water-bath heating. However, the inactivation energy in convective single droplet drying (181.3 kJ/mol) was much higher than the inactivation energy in isothermal water bath heating (16.8 kJ/mol) within the medium temperature of 45–95 °C.     

Drying of yellow pea starch on inert carriers: Drying kinetics,moisture diffusivity,and product quality

This study aimed at investigating the drying of yellow pea starch dispersions on inert solid carriers and determining the drying kinetics, moisture diffusivity and the product quality, quantified through damage index and final moisture content. Drying kinetics accomplished in a convective drying tunnel show that the overall mass transfer is controlled by internal migration of moisture within the starch particles. For a given inlet air temperature from 100 to 180 °C, the apparent diffusion coefficient derived from the drying curves increases exponentially with the instantaneous moisture content, with values ranging from 4 × 10⁻¹¹ to 3 × 10⁻⁸ m²/s. Due to low diffusivity and the thin coat formed on the surface of solid carriers, the resistance to internal diffusion is negligible as compared to the overall mass transfer resistance when drying of starch dispersions takes place on inert solid carriers. Drying of yellow pea starch dispersion on Teflon particles as inert carriers was studied in laboratory and pilot fast spouted bed dryer for inlet air temperatures from 140 to 240 °C, and initial solid content of 38%, mass (d.b.). The starch damage index for targeted product moisture content was below 2.5% in the inlet air temperature range from 120 to 210 °C, when atomizing from the bottom of dryer.     

Baking kinetics of muffins in convection and steam assisted hybrid ovens (baking kinetics of muffin…)

Effects of oven type and baking temperature on acrylamide concentration, surface browning, temperature profiles and drying rates of muffins were investigated. Muffins were baked in convection and steam assisted hybrid ovens at 145, 160 and 175 °C for different baking times. For all oven types, the acrylamide concentration of muffins increased with increasing baking time and temperature (p < 0.05). The formation was considered as the first order reaction kinetics except for the lowest baking temperature at natural convection and steam assisted hybrid ovens. The reaction rate constant, k was found to be in the range of 0.027–0.078 (min⁻¹). For the forced convection oven, the effect of baking temperature on acrylamide concentration followed the Arrhenius type of equation; with activation energy of 36.35 kJ/mol. The minimum drying rate was observed by the steam assisted hybrid oven, at all conditions. Steam assisted baking resulted in lower acrylamide concentration at all baking temperatures, while providing the average moisture content not significantly different.     

Effect of hot air drying and sun drying on color values and β-carotene content of apricot (Prunus armenica L.)

In this study, sulphurated and nonsulphurated Hacihaliloglu apricots (Prunus armenica L.) which is the most widely produced cultivar in Turkey were used to study the effects of different hot air drying temperatures (50, 60, 70, and 80 °C) and sun drying on color and β-carotene content of apricot. The time required to obtain the desired final dry matter in hot air drying was lower than sun drying. Sulphuration also decreased drying time at all drying conditions. Color values and β-carotene content of hot air dried samples were favorable in comparison to air drying. β-carotene content in dried apricots at 70 and 80 °C was 7.14, 7.17 mg 100 g⁻¹ dry matter and 6.12, 6.48 mg 100 g⁻¹ dry matter for sulphurated and nonsulphurated apricots, respectively. A good relationship was found between treatments (drying temperatures and drying times) and β-carotene content for sulphurated and nonsulphurated apricots (R²=0.9422 and 0.9129, respectively).     

Effect of temperature on drying characteristics and quality of cooked rice

A shorter drying time, together with acceptable product quality, is preferred in food processing. To accomplish this requirement, the present work was studied on the effect of temperature on drying kinetics and quality of cooked rice. The long grain-jasmine rice was chosen as a raw material. The experiments were carried out at temperatures of 50, 60, 80, 100 and 120 °C. Effective diffusion coefficient was determined experimentally using a method of slopes. An empirical equation was developed to describe the effective diffusion coefficient as a function of temperature and moisture content. The developed equation was utilized to predict moisture content as the drying progressed and the prediction agreed with the experimental result relatively well. For the quality, the drying temperature affected the color of cooked rice, but it insignificantly affected shrinkage and rehydration capability of dried cooked rice. As examined by scanning electron microscope, morphologies of cooked rice dried at temperatures were similar, but they were relatively different to that of freshly cooked rice, with slightly more porous material after processing.     

Drying of ultrasound pretreated apple and its selected physical properties

The aim of this work was to investigate the utilization of ultrasound as a mass transfer enhancing method prior to drying of apples tissue. Ultrasound power was provided at a frequency of 35 kHz for 10, 20 and 30 min in the ultrasound bath. Apple cubes were dried using convection method in 70 °C and at air velocity of 1.5 m/s. The effects of ultrasound pre-treatment upon drying were investigated.     

Drying and quality characteristics of fresh and sugar-infused blueberries dried with infrared radiation heating

We evaluated the finished product quality and infrared (IR) drying characteristics of fresh and sugar-infused blueberries dried with a catalytic infrared (CIR) dryer. IR drying tests were conducted at four product temperatures (60, 70, 80, and 90 °C) to evaluate the drying rate and the color and texture of the finished product. Fresh blueberries dried with convective hot air drying at 60 °C were used as control for comparison. The experimental data of moisture changes during IR drying were modeled with eight different models, including Page, modified Page, Thompson, Newton, Wang and Singh, and Henderson and Pabis, and two models developed in this study. The Thompson model showed the best fit to all experimental data. The CIR drying produced firmer-texture products with much reduced drying time compared with hot air drying. For fresh blueberries, CIR drying conserved drying time by 44% at 60 °C. The effective moisture diffusivity ranged from 2.24 × 10⁻¹⁰ to 16.4 × 10⁻¹⁰ m²/s and from 0.61 × 10⁻¹⁰ to 3.84 × 10⁻¹⁰ m²/s for fresh and sugar-infused blueberries, respectively.