A Comparative Study of Low-Pressure Superheated Steam and Vacuum Drying of a Heat-Sensitive Material

Using carrot cubes as a model heat-sensitive material, experimental investigations were conducted to examine the drying kinetics and various quality parameters of the dried product undergoing both low-pressure superheated steam and vacuum drying. Effects of operating parameters such as pressure and temperature on the drying characteristics as well as quality attributes, i.e., volume, shrinkage, apparent density, color, and rehydration behavior, of the dried product underwent the two drying processes were also evaluated and compared. Although low-pressure steam drying required longer dwell time to achieve the same final moisture content than vacuum drying, some of the quality attributes were superior to those obtained in vacuum drying.     

Pulsed vacuum drying of wolfberry: Effects of infrared radiation heating and electronic panel contact heating methods on drying kinetics,color profile,and volatile compounds

ABSTRACT

The drying kinetics and quality attributes of wolfberry were investigated under pulsed vacuum drying based on two different heating ways of far-infrared radiation (PVD-FIR) and electronic panel contact (PVD-EPC) heating. They were operated at different drying values of heating panel temperatures (60, 65, and 70°C) with 15 and 2?min as the constant vacuum pressure and atmospheric pressure duration, respectively. Drying time for wolfberry dried by PVD-FIR was lower by 17–19% compared with that by PVD-EPC at the same drying temperature. The effective moisture diffusivity (D_eff) determined by Weibull distribution model ranged from 3.72?×?10^?10 to 6.59?×?10^?10?m²/s and 3.34?×?10^?10 to 6.88?×?10^?10?m²/s for PVD-FIR and PVD-EPC, respectively. The drying activation energy was 54.30 and 68.59?kJ/mol for the samples dried by PVD-FIR and PVD-EPC, respectively. The color parameters L*, a*, and b* of wolfberry dried by PVD-FIR were higher than those dried by PVD-EPC. The product dried by PVD-FIR contained more vivid luster compared to that dried by PVD-EPC. The contents of aldehydes, esters, phenols, and the heterocyclic compound in PVD-FIR sample were higher than those in PVD-EPC samples. Additionally, the alcohols, ketones, and acid contents in PVD-FIR sample were lower than those in PVD-EPC sample. In summary, PVD-FIR is more suitable for wolfberry drying as it enhances drying rate and product’s quality compared with PVD-EPC.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

Drying Foodstuffs with Superheated Steam

Abstract

A thin-layer superheated steam drier was constructed with the objective of determining the drying characteristics, drying rates, and the effect of superheated steam on product quality in thin-layers. Results from superheated steam drying experiments with sugar-beet pulp, potatoes, Asian noodles, and spent grains indicate that drying times and rates increase with increasing steam temperature. For sugar-beet pulp it was also found that these changes were more significant than increases seen by hot-air drying under the same conditions and that drying rates were not affected by velocity for hot air but were increased for superheated steam. When quality aspects were examined, superheated steam dried Asian noodles saw both beneficial changes to recovery, adhesiveness, and gumminess while parameters of maximum cutting stress, resistance to compression, and surface firmness saw deleterious effects. Spent grains saw high levels of starch gelatinization and retention of fibre content.

     

PULSED MICROWAVE-VACUUM DRYING OF FOOD MATERIALS

ABSTRACT

Microwave drying of food materials has been investigated over several years as a potential means for reducing the total drying time. However, some quality loss almost always accompanied when foods were dried completely using microwaves due to nonuniform temperature and moisture distribution. Some strategies used to improve dried product quality include combination of microwave and conventional hot air drying. pulsed or intermittent drying, and microwave-vacuum drying. Combination of pulsing and vacuum drying is a useful technique to maximize energy use efficiency and product quality especially for temperature sensitive products such as fruits. Some results of pulsed, microwave-vacuum drying of cranberries are presented. Pulsed drying is more energy efficient than continuous drying. In pulsed drying, the longer the pulsing ratio (i.e. longer power-off time in relation to power-on time) was more energy efficient. The quality of pulse-dried product was also generally better than that of continuous-dried product. The cycle power-on time and pulsing ratio should be carefully selected to obtain maximize the benefits of pulsed, microwave vacuum drying     

Experimental Investigation and Practical Application of Superheated Steam Drying Technology for Softwood Timber

Abstract

Due to several advantages, superheated steam drying of timber has attracted great attention. However, the technology is still restricted to some special cases, partly due to the lack of fully understanding of the drying process. In this work, experiments were conducted to dry radiata pine timber using superheated steam under vacuum and at pressure. In the first part of the experiments, softwood timber was dried in a superheated steam kiln with drying rates, steam temperature across the stack and wood temperature being measured during drying. In the second part of the work, experimental studies were performed to investigate potential applications of the superheated steam drying at ultra-high temperatures (UHT) and pressurized steam conditioning of kiln dried timber. Compared to normal drying temperatures, the UHT drying can reduce the drying time by a factor of 5 to 10 and it is also more energy efficient. The pressurized steam conditioning has been proven to be a promising technology to relieve drying stresses and to reduce twist of the dried timer.

     

Studies on Hot Air and Microwave Vacuum Drying of Wild Cabbage

Abstract

Wild cabbage was dehydrated by a combination of hot-air drying and microwave vacuum drying. It showed that the total drying time was reduced by about half while the retention values of the nutrient components and chlorophyll were improved significantly. Microwave/vacuum-dried products had a more porous texture. Finally, microwave drying showed effective bactericidal action in the product with acceptable quality of dried product.     

Optimizing Drying Conditions for Vacuum-Assisted Microwave Drying of Green Peas (Pisum sativum L.)

Green peas were dried in a vacuum-assisted microwave drying system. The effects of microwave power levels (100–300 W) and system vacuum (50–400 mm Hg) on drying parameters (viz. drying efficiency and drying time) and some quality attributes (viz. linear shrinkage, apparent density, green color, rehydration, and sensory attributes) of dehydrated peas were analyzed by means of response surface methodology. A face-centered central composite design was used to develop models for the responses. Analysis of variance showed that a second-order polynomial model predicted well the experimental data. The system microwave power level strongly affected quality attributes of dehydrated peas and drying parameters. A higher vacuum during drying resulted in a better quality product. Microwave power of 237.31 W and a 360.22 mm Hg vaccum were found to be optimum drying conditions for vacuum-assisted microwave drying of green peas.     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

Abstract

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Dehydration of asparagus cookies by combined vacuum infrared radiation and pulse-spouted microwave vacuum drying

ABSTRACT

The restructured cookies were made from old stalks of asparagus (Asparagus officinalis). Combination of vacuum infrared radiation drying and pulse-spouted microwave vacuum drying (VIRD–PSMVD) was studied as a potential mean for dehydration of restructured cookies. The samples were first dried by VIRD and then by PSMVD to final moisture content less than 6%. Expansion ratio, texture, color, flavor, and other attributes of samples dried by this method were evaluated and compared with those dried by PSMVD and VIRD. Results showed that the samples dried by VIRD–PSMVD showed higher overall acceptability, crisper texture, and higher expansion ratio.     

Microwave Drying of Grapes in a Single Mode Cavity at 2450 Mhz - 11: Quality and Energy Aspects

ABSTRACT

Drying studies were conducted to optimize process parameters (air temperature T. microwave power density P and air velocity u) for quality and energy in microwave drying of grapes using a single mode cavity applicator at 2450 MHz. Thc quality was assessed by several attributes like colour. damage, darkness, crystallized sugar, stickiness and non-uniformity. Colour and damage attributes provided more preeise predietability when compared to the other quality attributes. Microwave dried raisins were lighter in colour and hence were superior to hot air dried samples. Quality attributes were sipnificantlv dependent on T. P and u. Increase in air velocitv resulted in better Quality raisins where as P and T had the opposite effect. Optimum selection of T, P and u was found to be critical to achieve an energy efficient process for a quality product. Response surface models in terms of temperature m d micowave power were obtained for each of the quality attributes. Process optimization of the operating variables through response surface method was performed by imposing certain constraint levels on quality attributes     

SUPERHEATED STEAM IMPINGEMENT DRYING OF TORTILLA CHIPS

ABSTRACT

Low-fat snack products are the driving forces for the drying of tortilla chips before frying. Super-heated steam impingement drying of foods has the advantage of improved energy efficiency and product quality. The temperature profile, drying curves, and the physical properties (shrinkage, crispiness, starch gelatinization and microstructure) of tortilla chips dried at different superheated steam temperatures and heat transfer coefficients were measured. Results indicated that the steam temperature had a greater effect on the drying curve than the heat transfer coefficient within the range of study. The microstructure of the samples after steam drying showed that higher steam temperature resulted in more pores and coarser appearance. The modulus of deformation and the shrinkage of tortilla chips correlated with moisture content. A higher steam temperature caused less shrinkage and a higher modulus of deformation. The pasting properties showed that samples dried under a higher steam temperature and a higher heat transfer coefficient gelatinized less during drying and had a higher ability to absorb water. Comparison of the superheated steam drying and air drying revealed that at elevated temperatures the superheated steam provided higher drying rates. Furthermore, there was a less starch gelatinization associated with air drying compared to superheated steam drying.     

Pulsed vacuum drying (PVD) technology improves drying efficiency and quality of Poria cubes

The objective of this research was to examine the drying characteristics of pulsed vacuum drying (PVD) system as an alternative drying technology for a functional food—Poria cocos. Poria cubes in thin layer were dried at different temperatures (55, 65, and 75°C), vacuum duration (5, 15, and 25?min), and atmospheric pressure duration (4, 8, and 12?min) to evaluate the operating conditions on the drying kinetics and quality attributes of P. cocos. The quality attributes included volume shrinkage ratio, firmness, water-soluble polysaccharide content. Volume shrinkage ratio was determined by image processing technology and analyzed by Bigot’s curves. The material internal temperature was also recorded during the whole drying process. Considering the drying rate and quality attributes, drying temperature of 65°C, vacuum pressure duration of 15?min, and atmospheric pressure duration of 4?min were proposed as the most favorable drying conditions for drying Poria cubes. The current work indicates PVD is a promising technology for P. cocos drying as it not only enhances the drying process but also improves the quality attributes of it.     

Kinetic and Quality Study of Mushroom Drying under Microwave and Vacuum

Abstract

The aim of this work is to model the drying kinetics of mushrooms under several operational conditions, to evaluate the effective diffusivity coefficient of moisture removing by a drying model and inverse calculus method in finite differences and to study the effect on the final quality of dehydrated mushrooms. Different ways of microwave vacuum drying were compared to freeze-drying. Results show that a decrement of the applied pressure produces a certain increase in the drying rate together with a lower moisture in the dehydrated product at the end. Temperature control inside the sample helps to ensure a better quality in the dehydrated product, than when controlled at the surface. Diffusivity coefficients show a correspondence with product temperature during drying. The microwave dried samples obtained with moderate power and temperature control of product shown an important degree of quality similar to that obtained by freeze-drying.     

Effects of Drying Methods and Tea Preparation Temperature on the Amount of Vitamin C in Indian Gooseberry Tea

Indian gooseberry is a rich source of vitamin C. The fruit can be consumed either fresh or after processing into different products including Indian gooseberry tea. Both drying and tea preparation steps affect quality of the gooseberry tea drink made using dried gooseberry. This study investigated the effects of different drying methods, i.e., hot air drying, vacuum drying, and low-pressure superheated steam drying, on the retention and degradation of vitamin C (in terms of the total ascorbic acid content, TAA) in dried gooseberry flakes. In addition, the effect of temperature of water used to prepare the tea on the release of TAA and on its later degradation was also investigated.     

Some Aspects of Freeze Drying of Dairy Biomaterials

ABSTRACT

Different theoretical and experimental aspects of freeze drying of dairy biomaterials are reported. The following dairy biomaterials were chosen for investigations: three kefir bacterial cultures and one bacterial concentrate of a dairy vaccine. Experiments were performed both at one-region conductive or radiative heating and at combined two-region conductive-radiative heating. Emphasis was put on quality interactions between physical and biological properties of the material to be dried and freeze drying process parameters. Qualitative estimations of the following properties of biomaterials: a) colour, b)suppleness to caking and c) suppleness to scraping born the hot surface in dependence with the process parameters were performed using four-step subjective scales. On the basis of this estimations a synthetic quality criterion SQC was defined whose numerical value reflects general product quality. The results of mathematical modelling, numerical solution of the model and comparison with experimental data indicated that the process was heat transfer controlled and that the main factors resulting in good product quality and high SQC are low temperature of the conductive heated surface and vacuum chamber pressure affecting sublimation temperature. The influence of the sample freezing parameters on the elimination of the foaming or puffing and on the product quality was observed and examined. Furthermore, preliminary measurements of the dauy biomaterials viability after freeze drying were presented.     

Drying of Fruits and Vegetables: Retention of Nutritional/Functional Quality

In the past, research and development in drying has focused on the process and technology and food drying was performed mainly to extended the shelf life without much importance on retaining quality attributes. Recently, however, efforts have been made to develop high-quality dried foods. This is achieved by utilizing novel drying technologies, by improving and optimizing existing drying methods, and by maximizing quality attributes such as structure, color, flavor, nutrition, etc. In an effort to highlight quality aspect of dried foods and biomaterials, a special issue of Drying Technology [2005, 23(4)] was published. The objective of this article is to present an overview of quality attributes normally considered in the drying of food and biomaterials and highlight the recent advances in drying methods for the retention of nutritional and functional properties of fruits and vegetables.     

THE EFFECT OF TEMPERING-INTERMITTENT DRYING ON QUALITY AND ENERGY OF PLANT MATERIALS

Abstract

Squash slices were dried in a laboratory scale vibro-fluidized bed in batch operation. The optimal drying condition and degradation mechanism of β-carotene was investigated. A mathematical model was established to simulate and predict the quality protection. The changes of other tissue parameters, such as appearance and rehydration ability of the dried product, were analyzed and discussed. Tempering-intermittent drying is proved to reduce degradation of β-carotene, shorten drying time, and reduce energy consumption.     

Study of Intermittent Low-Pressure Superheated Steam and Vacuum Drying of a Heat-Sensitive Material

Low-pressure superheated steam drying (LPSSD) has recently been applied to drying of various heat-sensitive foods and bioproducts with success. Several studies have shown that the quality of LPSSD-dried products is superior to that obtained using conventional hot air or vacuum drying. However, drying time and energy consumption for LPSSD is generally greater than that for vacuum drying. Therefore, it is necessary to examine different methodologies to improve the energy efficiency of LPSSD. An intermittent drying scheme is one possible method to reduce the energy consumption of the process while maintaining the desired product quality. In this study, the effect of intermittent supply of energy (through an electric heater and steam injection to the dryer) and vacuum (through the use of a vacuum pump) at various intermittency values or on:off periods (10:5, 10:10 and 10:20 min in the case of intermittent supply of energy and 5:0, 5:5, and 5:10 min in the case of intermittent supply of vacuum) at the on-period setting temperatures of 70, 80, and 90°C on the drying kinetics and heat transfer behavior of the drying samples (banana chips) was studied. The effects of these intermittent drying schemes and conditions on the quality parameters of dried banana chips; i.e., color, shrinkage, texture, and ascorbic acid retention, were also studied. Finally, the energy consumption values for intermittent LPSSD and vacuum drying were monitored through the effective (or net) drying time at various intermittent drying conditions and compared with those using continuous LPSSD and vacuum drying.     

Effects of Different Drying Methods on the Quality Changes of Granular Edamame

In this article, the quality changes of the granular fruits and vegetables dried by vacuum microwave drying, freeze drying, hot air drying, and combined hot air–vacuum microwave drying are investigated, and the quality parameters compared on the basis of vitamin C and chlorophyll contents, shrinkage and rehydration capacity, color, texture, and microstructure changes. The quality parameters of products dried by vacuum microwave drying are slightly lower than those obtained by freeze drying, but much better than those obtained using conventional hot air drying. The quality characteristics of product dried by combined hot air–vacuum microwave are significantly improved compared to those simply hot air–dried.