DEHYDRATION OF PEACHES WITHOUT SULPHUR DIOXIDE

ABSTRACT

The experimental drying behaviour of clingstone peach halves dehydrated without sulphites was investigated. Fruit treatments such as peeling and peeling followed by blanching led to greatly increased drying rates in comparison to unpeeled halves during isothermal dehydration. Superficial product temperature during dehydration was also affected by fruit treatment. Drying rates during isothermal dehydration were found to increase as linear air velocity increased from 2 to 4 m.s^?1. Fruit size had a pronounced effect on drying rates, with halves obtained from the smallest fruit (54 mm.), showing the highest rates. Experimental drying studies were     

DEHYDRATION OF PEARS WITHOUT SULPHUR DIOXIDE

The experimental dehydration behaviour of Bon Chretien pears dehydrated without sulphites was investigated during isothermal (67°C) and simulated counter-flow dehydration (48°→60°C and 58°→67°C). Dehydration of peeled halves and quarters treated with sodium chloride retarded browning during isothermal dehydration relative to untreated halves. This retarding effect was enhanced by using dehydration conditions favouring high drying rates, low superficial product water activity and low superficial product temperatures Ascorbic acid-treated samples exhibited considerable browning, practically to the same degree as untreated samples-Objective colour measurements of dried products subjected to different dehydration conditions are presented. Constant rate drying periods were observed in counter-flow simulations (48°→60°C) at higher humidity (30% RH), but not at relatively low humidity (20% RH). It was found that enzymatic browning reactions could be controlled during dehydration by selecting dehydration conditions favouring low superficial product temperature and water activity (low humidity) in conjunction with a suitable inhibitor of polyphenol oxidase. Thus, it was found that simulated counter-flow dehydration (48°→60°C and 58°→67°C) of pear quarters gave significantly lighter coloured products when dehydrated at 20% RH in comparison to dehydration at 30% RH     

Ultrasound-Assisted Osmotic Dehydration of Strawberries: Effect of Pretreatment Time and Ultrasonic Frequency

Pretreatment of fruits prior to drying has shown success in reducing drying time and costs. In this work, ultrasound-assisted osmotic dehydration has been implemented as a method to increase water diffusivity and reduce drying time in strawberries. Strawberry halves were immersed in distilled water and in two different concentrations of sucrose solutions while pretreatment time and ultrasonic frequency levels were varied to determine their effect on drying time, water loss, and soluble solids gain. A microscopic analysis was carried out to evaluate the formation of microchannels and other changes to the fruit tissue structure. Greater sucrose concentration used in ultrasound-assisted osmotic dehydration resulted in greater water loss with greatest loss observed for the strawberry halves pretreated for 45 min in a 50% w/w sucrose solution. The pretreatment carried out for 30 min employing an osmotic solution of 50% w/w of sucrose resulted in the highest drying rate among the pretreatments. Osmotic dehydration used alone during pretreatment increased total processing time, whereas osmotic dehydration combined with ultrasonic energy during pretreatment reduced total processing time and increased effective water diffusivity. Cell distortion and breakdown were observed not only in pretreatments employing ultrasound-assisted osmotic dehydration but in conventional osmotic dehydration. Formation of microchannels through ultrasonic application and effects of osmotic pressure differential were considered to be largely responsible for reducing drying time for strawberry halves.     

Processes for controlling the structure and texture of dehydrated banana

This study is about how to control the microstructure of dehydrated banana during drying by a multi-flash drying process. The influence of different drying methods and process variables on the formation of fruit microstructure and texture were evaluated. These processes included a conductive multi-flash drying process, conductive multi-flash drying combined with classical vacuum drying, convective drying in an oven, and vacuum drying. Density, porosity, and shrinkage of dehydrated fruit were strongly influenced by the drying process. Results showed that it is possible to control the porous structure of dehydrated banana by choosing the correct dehydration process.     

Aerated Apple Leathers: Effect of Microstructure on Drying and Mechanical Properties

Fruit leathers are restructured products made from drying fruit purees until a leathery consistence is achieved. Aerated apple leathers (AAL) were created in order to incorporate air into the fruit structure and decrease the caloric density. AAL were prepared using gelatin as a foaming agent. Effects of gelatin concentration (0, 0.5, 1.0, and 1.5%) and whipping time (3, 5, 7, and 9 min) on microstructure, drying behavior, and mechanical properties of AAL were studied. Increasing the gelatin content and whipping time increased the gas hold-up up to 42.3% and decreased the mean bubble size of the foam. The time of drying decreased as the aeration level was increased. Samples with 1.5% gelatin and seven minutes’ whipping presented the highest drying rate, taking 2.8 h of drying to reach a moisture content of 0.13 kg H₂O/kg dry basis. The drying behavior of samples was satisfactorily modeled with a semi-empirical relationship. Mechanical properties showed differences among aerated products elaborated with different whipping times and gelatin contents. Aeration conferred unique characteristics to apple leathers in terms of appearance and mechanical properties.     

Osmo-Convective Drying of Fruits and Vegetables: Technology and Application

With reference to typical food preservation processes which consist of changing the water binding to material, the technology and application of osmotic dehydration were discussed as an initial treatment before convection drying of fruit and vegetables. Particular attention was paid to the possibilities to produce more shelf-stable food while keeping the high quality of final product. The course of osmotic dehydration of plant tissue as well as its influence on convection drying and on the properties of preserved fruit and vegetables were presented.     

AN EXPERIMENTAL FRUIT DEHYDRATION SYSTEM

An experimental dehydration system comprising two dehydration units, and in which control and data acquisition functions were computerized, was developed. Product mass is continuously monitored using a built-in weighbridge which facilitates drying rate studies. Product temperatures are also monitored on a continuous basis. Control of wet bulb temperature is achieved by varying the rate of recirculation of air, in conjunction with humidification. Dehydration under isothermal conditions as well as programmed temperature control is supported. In the latter mode temperature programming is performed according to change in mass to simulate the typical air temperature profiles encountered in commercial dehydration tunnels during parallel- and counter- flow operations.     

AN EXPERIMENTAL FRUIT DEHYDRATION SYSTEM

ABSTRACT

An experimental dehydration system comprising two dehydration units, and in which control and data acquisition functions were computerized, was developed. Product mass is continuously monitored using a built-in weighbridge which facilitates drying rate studies. Product temperatures are also monitored on a continuous basis. Control of wet bulb temperature is achieved by varying the rate of recirculation of air, in conjunction with humidification. Dehydration under isothermal conditions as well as programmed temperature control is supported. In the latter mode temperature programming is performed according to change in mass to simulate the typical air temperature profiles encountered in commercial dehydration tunnels during parallel- and counter- flow operations.     

Loss of fatty acid esters from grape surfaces during drying

Fatty acid esters have been shown to reduce the drying time of grapes by interacting with the waxy surface of the grapes. Laboratory scale procedures were developed to determine the fate of esters during dehydration. Thompson seedless grapes were dipped in a water emulsion containing 2% fatty acid ester, 2% potassium carbonate and an emulsifier. Grapes were dried at ambient, 43 C and 71 C. Drying rates vs. fatty acid ester concentration on the grape surfaces were plotted. The fate of fatty acid esters during dehydration was determined by running the dryer exhaust through a cold trap and extracting the condensate. These studies showed that the large loss of esters during drying lowered the drying rate considerably. An additional dip during drying reduced the drying time significantly. The ester losses appeared to be caused by vapor distillation of the esters during drying.     

POLYPHENOL OXIDASE ACTIVITY IN BANANA CHIPS DURING OSMOTIC DEHYDRATION

Various treatments were used to determine polyphenol oxidase activity changes during osmotic dehydration of banana slices. A temperature of 35 °C and 5.0 pH were used as the most adequate conditions for polyphenol oxidase activity determination when 4-methyl catechol was used as a substrate. Enzyme activity change was determined in the central and edge region of fruit. The central part of the banana slice showed 2.4 times higher activity than in the edge region. A difference in the kinetics of enzyme activity decrease also occurred during four hours of drying. When the drying was carried out at 70 °C, the enzyme was completely inactivated after 60-90 minutes of specific osmotic treatments. After four hours of osmotic drying at 50 and 60 °C, polyphenol oxidase activity was still present; the residual activity varied from 1.3 to 15 %.     

Numerical Study of Drying Stresses in Agar Gel

A numerical model of isothermal drying of deformable substances is proposed. The model is applied in a uni-directional case to analysis of water transport in a sphere of agar ge. The simulation program makes it possible to calculate the water content fields and def rmations and stresses in the sphere during drying. Validation of the water content profile results is proposed by the experimental study of the dehydration of gel spheres in a sugar solution. The sensitivity of internal stresses to drying conditions is then analysed.     

On the Development of Osmotically Dehydrated Seabuckthorn Fruits: Pretreatments,Osmotic Dehydration,Postdrying Techniques,and Nutritional Quality

Osmotic dehydration of whole seabuckthorn berries, followed by convective or vacuum drying, was investigated. First, different pretreatments were applied to the fruits in order to accelerate the rate of osmotic dehydration: immersion in liquid nitrogen, steam blanching, or freeze cycles. Immersion in liquid nitrogen was found to be the best pretreatment to maximize dehydration rate and to increase sugar gain during osmotic dehydration. An evaluation of moisture loss and sugar gain kinetics during osmotic dehydration of seabuckthorn fruits pretreated with liquid nitrogen, followed by vacuum or hot-air drying, was then performed. Loss of nutritional compounds due to processing was also measured. Sugar intake and partial dehydration of seabuckthorn samples increased with osmosis time and reached an equilibrium value after 4 h treatment. The finish drying methods (vacuum or convective) applied after OD showed a marked impact on the remaining moisture content of seabuckthorn samples. Concentration of some nutritional compounds was, however, dramatically reduced after the combined osmotic dehydration/drying processes.     

Process-Based Drying Temperature and Humidity Integration Control Enhances Drying Kinetics of Apricot Halves

The effects of drying temperature (50, 60, and 70°C) and absolute humidity (65, 90, 115, 140 g/kg at the initial stage) on drying kinetics and color attributes of apricot halves under process-based drying temperature and humidity integration control (PDTHIC) were investigated. Results indicated that appropriate PDTHIC could reduce the drying time by 18.75% compared to the control group. The absolute humidity parameter should be controlled well rather than continuously dehumidify for traditional practices. The moisture effective diffusivity (D_eff) at 70°C presented drastic fluctuations with increasing absolute humidity parameters, calculated using the Weibull distribution model. The activation energy (E_a) of samples treated by continuous dehumidification and PDTHIC were 31.40 and 74.18 kJ/mol, respectively. The variation trend of color parameters was different from the conventional hot air drying probably due to the dehydration mechanism. Observation of the microstructure of the dried samples indicated that the PDTHIC process can enhance the drying rate of apricot through generating a larger pore network compared to continuous dehumidification at the same temperature. The findings of the current work clearly indicated that process-based drying temperature and humidity integration control enhances the drying kinetics of apricot halves, leading to a promising technology for energy savings.     

POLYPHENOL OXIDASE ACTIVITY IN BANANA CHIPS DURING OSMOTIC DEHYDRATION

ABSTRACT

Various treatments were used to determine polyphenol oxidase activity changes during osmotic dehydration of banana slices. A temperature of 35 °C and 5.0 pH were used as the most adequate conditions for polyphenol oxidase activity determination when 4-methyl catechol was used as a substrate. Enzyme activity change was determined in the central and edge region of fruit. The central part of the banana slice showed 2.4 times higher activity than in the edge region. A difference in the kinetics of enzyme activity decrease also occurred during four hours of drying. When the drying was carried out at 70 °C, the enzyme was completely inactivated after 60-90 minutes of specific osmotic treatments. After four hours of osmotic drying at 50 and 60 °C, polyphenol oxidase activity was still present; the residual activity varied from 1.3 to 15 %.     

Quality Evaluation of Microwave-Dried Packham's Triumph Pear

Abstract

The objectives of this work were to carry out the microwave and microwave-convective drying of pear, in order to preserve the quality by reducing the drying time of the fruit; to compare the kinetic with convective drying mode data and to fit the comparative data to models from literature. The effect of osmotic dehydration and the variation in the microwave power were analyzed at constant values of air temperature and velocity. The experimental data fit well the proposed kinetic model. The processed pear was evaluated with respect to water content, quantity of calcium, phosphorus and potassium, water activity and apparent density. The best results were found for the samples osmotically dehydrated and dried in the microwave-convective dryer. The osmotic dehydration and the short drying times were important to maintain sample characteristics.     

Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.     

Effect of two-stage dehydration on retention of characteristic flavor components of flue-cured tobacco in rotary dryer

Two-stage dehydration method of flue-cured tobacco in rotary dryer was experimentally evaluated and compared to the single-stage dehydration process. The characteristic flavor components in dried cut tobacco, including major Maillard reaction compounds and carotenoid degradation products, as well as drying rate were analyzed for different two-stage dehydration operations. The results showed that both the pre-drying temperature and intermediate moisture content had a significant effect on the overall drying rate of cut tobacco during two-stage drying. On the other hand, the retention of characteristic flavor components in tobacco, especially Maillard reaction compounds retention, was influenced significantly by the final drying temperature as well as intermediate moisture content. The 10°C increase in the final drying temperature from 100 to 110°C reduced the retentions of Maillard reaction compounds and carotenoid degradation products by 14.5 and 9.4%, respectively. Change of intermediate moisture contents from 19 to 15% reduced the Maillard reaction compounds retention by 14.7%. Given the consideration of characteristic flavor components’ retention and drying process efficiency, two-stage dehydration of cut tobacco in rotary dryer could reach a better performance as compared with the single-stage dehydration. The cylinder-wall temperature combination of 140/100°C with the 17% intermediate moisture content could be used as the optimal two-stage condition for the investigated flue-cured tobacco.     

Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

ABSTRACT

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.     

Use of Ultrasound as Pretreatment for Dehydration of Melons

This work examined the influence of the ultrasonic pretreatment prior to air drying on dehydration of melon (Curcumis melo L.). Ultrasonic pretreatment for air drying of fruits was studied and compared with osmotic dehydration. This study allowed estimate of the effective diffusivity water in the air-drying process for melons submitted to ultrasonic pretreatment. Results show that the water effective diffusivity increases after application of ultrasound causing a reduction of about 25% in the drying. During ultrasonic treatment the melons lost sugar, so such a pretreatment stage can be a practical process to produce dried fruits with lower sugar content. Compared to osmotic dehydration, the use of ultrasonic pretreatment performed better when large amounts of water need to be removed from the fruit, since the combined processing time (pretreatment and air drying) is shorter.     

Recent advances in drying at interfaces of biomaterials

A better insight in heat and mass transport across interfaces of biomaterials with their environment, particularly at the microscale, is a key element in improving dehydration processes. Recent advances in interfacial drying are targeted, including evaporation from microscopic pores, droplets or microperforated membranes, and drying of soft cellular tissue such as fruit. Manufacturing of thin biopolymer layers, such as (edible) films and coatings, is discussed as well as their performance as barriers at product–environment interfaces. The physical processes at play are illustrated, recent insights are highlighted and a future outlook is given. These interfacial processes are critical for controlling the processing conditions during drying and for tailoring the structure and quality of biomaterials.