Far-infrared radiation assisted drying of longan fruit

Oversupply of fresh longan (Dimocarpus longan Lour.) from seasonal production in Thailand needs to be preserved in the form of dried product (exported value of over US$70 million in 2009) to prevent it from being damaged by microorganism. Due to well known limitations of hot air (HA) drying, a new technique of using far-infrared radiation (FIR) in combination with hot air and heat pump (HP) dryers for longan is proposed in this study. Longan with initial moisture content of approximately 84–86% w.b. was dried to about 18% w.b. at the drying temperature of 55 °C in the case of combined FIR-heat pump drying and 65 °C in the case of combined FIR-hot air drying with 80% recycled air. In both cases, the electric power supplied to FIR rods was set at 250, 350 and 450 W. The experimental results are presented in terms of the drying kinetics, dried product qualities (color, shrinkage, percentage of rehydration, water activity, texture, and microstructure) and sensory characteristics of the dried product. The results show that FIR in combination with hot air and heat pump drying increases the drying rate of longan by reducing the drying time. FIR also helps to create more porous structure in dried longan, with porosity increasing with the power supplied to the FIR heaters. The formation of porous structure results in a product with lower shrinkage, improved rehydration, lower hardness and lower toughness than samples dried in the absence of FIR. In addition, the FIR dried longan had a stronger red color and appeared darker. Dried longans obtained by HP, HA, combined techniques and the product from the local market, were all rated similarly by a sensory panel for color, shape, flavor and taste. It is worth noting that the overall energy used for FIR-assisted drying processes decreased with the increase in the power supplied to the FIR heaters.     

Low humidity air drying of apple slices: Effect of EMR pretreatment on mass transfer parameters,energy efficiency and quality

The apple slices were treated with electromagnetic radiations (EMR) namely, infrared (IR) and microwave (MW) and dried using low humidity air (LHA) and normal hot air (HA) at 40 °C. The effect of EMR pretreatment on drying kinetics, mass transfer, shrinkage, porosity, energy efficiency and product quality was evaluated. The combination of EMR pretreatment with LHA drying reduced the drying time by 23.4 to 27.3%, and the dried product had lower shrinkage (60 to 65%) as well as higher porosity (0.38 to 0.45) compared to control. Among the EMR pretreatments, IR treated slices retained a higher amount of ascorbic acid (78.68%), phenolic content (74.42%), antioxidant activity (79.54%) and better color compared to MW treated. EMR pretreatment reduced the energy requirement for drying by 26.5 to 37.1%. The mass transfer parameters (moisture diffusivity and mass transfer coefficient) estimated using the analytical model proposed by Dincer and Dost showed higher values for EMR treated samples. Moisture content predicted for both the drying methods showed good agreement with experimental values. The present study demonstrated that for heat sensitive products like apple, a combination of IR pretreatment and LHA drying could be effective for reducing drying time as well as energy requirements and also to obtain better quality products.     

Dehydration of apple slices by sequential drying pretreatments and airborne ultrasound-assisted air drying: Study on mass transfer,profiles of phenolics and organic acids and PPO activity

Pretreatments of either water blanching or NaCl immersion and airborne ultrasound-assisted air drying were combined to dehydrate apple slices. The drying time was found to be reduced by 1.6–69.0% compared with air drying without any pretreatment and sonication. Meanwhile, it was found that sequential water blanching coupled with air drying with sonication gave the shortest drying time. A modified diffusional model incorporating temperature-dependent moisture diffusivity (D_eff) could simulate the apple drying process well. Water blanching pretreatment and airborne sonication were found to lower the activation energy required for the enhancement of moisture diffusive ability, and intensify water exchange on apple surface to speed up the drying process. Both water blanching and NaCl immersion significantly increased the amounts of procyanidin B2 in dehydrated apple slices. However, airborne sonication promoted the loss of procyanidin B2 in apple slices with pretreatments during air drying. In addition, sonication had no obvious influences on apple PPO activity and individual organic acids throughout drying. Overall, the sequential NaCl immersion pretreatment (or water blanching pretreatment) and drying with airborne sonication are effective to improve the quality of dried apple slices.     

Effect of different moisture equilibration process on the quality of apple chips dried by instant controlled pressure drop (dic)‐assisted hot air drying

Moisture equilibration process (MEP) is an important process before controlled pressure drop (DIC) drying to obtain greater products quality and avoid producing bubbles. The aim of this work was to study the effect of MEP on the quality of apple chips dried by instant controlled pressure drop‐assisted hot air drying (DIC‐AD). After predrying by AD, four types of moisture equilibration solutions were applied to equilibrate the moisture of apple chips before DIC. The quality of apple chips equilibrated in water for 10 min was superior to those for 30 and 60 min with better color, brittleness, hardness, puffing degree (PD), and microstructure. Sugar solutions showed positive effect on brittleness and hardness, PD, and soluble solids of apple chips. In addition, calcium chloride was beneficial to the color of apple chips, and the product showed good rehydration and PD possibly due to the reaction between pectin and calcium ions.

Practical applications

Instant controlled pressure drop (DIC) was a new sort of united drying method that can combine with other drying technology, such as hot air drying and freezing drying. In practical production, apple slices are usually dried to low moisture content for long time storage and regained appropriate moisture content before drying. After predrying, moisture equilibration process (MEP) is an important process before DIC drying which made water homogenous distributed in the sample to obtain greater products quality and avoid producing bubbles during DIC drying. In this study, MEP with different solutions including water, fructose syrup, maltitol, and calcium chloride were performed before DIC drying, and the changes of physiochemical properties of dried products were studied. Based on the analysis of experimental results, predried apple slice equilibrated in water for 10 min, 10% fructose syrup and 30% maltitol solution might produce apple chips with better quality.     

Effects of ultrasound and infrared assisted conductive hydro-drying,freeze-drying and oven drying on physicochemical properties of okra slices

Okra (Abelmoschus esculentus) is a nutritious vegetable but it is highly perishable and drying is one of the best methods for its preservation. Ultrasound and infrared assisted conductive hydro-drying (UIACHD) is a new patented drying method that can dry food materials with high quality and high energy efficiency. In this study, okra slices were dried using UIACHD with different settings and also by freeze-drying and oven drying. The UIACHD drying curves, moisture content, color, vitamin C content, hydrocolloids rheology and microstructure of the dried okra sample as affected by drying method were investigated. Drying curves showed that the application of ultrasound and infrared could increase the drying rate. In terms of quality parameters, UIACHD performed significantly better than oven drying and the quality of the products was comparable to freeze-drying. Flow and oscillation rheology tests showed that UIACHD drying could result in okra hydrocolloids with higher apparent viscosity and the application of ultrasound and infrared has preserved the rheological properties of the okra hydrocolloids. The microstructure of the dried okra slices was also well preserved by UIACHD. This study showed that UIACHD is a suitable drying method for maintaining the physicochemical quality of dried okra.Industrial relevanceDried okra can be used as a nutritious and functional ingredient in the formulation of different products such as soups, sauces and other culinary products. This study shows that ultrasound and infrared assisted conductive hydro-drying can be a suitable method for the production of high quality dried okra with well-retained color, nutritional value and rheological properties.     

Effects of different pretreatment methods on drying kinetics,three-dimensional deformation,quality characteristics and microstructure of dried apple slices

The purpose of this study was to evaluate the effects of different pretreatment methods including blanching (30, 60 and 90 s), ultrasound (10, 30 and 60 min), pectinase treatment (15, 30 and 60 min), and osmotic dehydration treatment (0.5, 1.0 and 4.5 h) on the drying characteristics, three-dimensional (3D) deformation, color, total polyphenol content (TPC), antioxidant activity and microstructure of apple slices during drying. The shrinkage and height standard deviation (HSD) were calculated to evaluate the 3D deformation such as surface curl and flatness of the apple slices. The results showed that blanching, pectinase and osmotic dehydration treatment could increase the shrinkage and reduce the surface curl of the samples. Blanching, ultrasound and osmotic dehydration pretreatment could inhibit the color changes of dried apple slices, while pectinase treatment could aggravate the color deterioration of the samples. The apple slices pretreated with blanching for 60 s contained the highest TPC of 4.86 mg/g, and the samples also had the highest antioxidant activity. In addition, microstructure of the pretreated samples was significantly different from those of untreated samples. Therefore, pretreatment technology before drying is of great significance to improve the appearance and quality of products.     

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.     

Comparison of four drying methods for re-structured mixed potato with apple chips

The effects of microwave-freeze drying (MFD), freeze drying (FD), microwave vacuum drying (MVD) and vacuum drying (VD) on texture, color, rehydration, sensory, microstructure and other quality parameters of re-structured chips made by blending apple with potato were studied. MFD chips had the best quality and these products were preferred by consumers. The drying time for MFD was lower than that for FD. The drying time for MVD was the shortest and the energy consumption was also the lowest. Microwave drying not only reduces the drying time but also improves the quality of dried products. Both MFD and MVD techniques are found to be suitable for production of re-structured chips of varying grades.     

Effect of drying methods on volatile compounds,fatty acids and antioxidant property of Thai kaffir lime (Citrus hystrix D.C.)

This study investigated the changes in colour, volatile compounds, fatty acids, total phenolic content (TPC), total flavonoid content (TFC) and antioxidant activity of kaffir lime leaves (makrud in Thai) Citrus hystrix D.C. after three drying treatments, namely hot‐air (HA) drying, low relative humidity (LRH) air drying and far‐infrared radiation (FIR) drying. Overall, when compared with fresh leaves, a slight decrease in L* values of the three dried samples was observed. TPC, TFC and ferric‐reducing/antioxidant power (FRAP) values were increased by LRH and FIR drying (LRH: 39%, 43% and 28%; FIR: 62%, 39% and 39%), while HA drying decreased TPC, TFC and FRAP values (28%, 21% and 1%), compared with fresh leaves. The main volatile compounds in fresh leaves were citronellal, followed by linalool and viridiflorol. Drying brought about an increase in the concentrations of volatile components, such as citronellal with HA, LRH and FIR drying (79%, 83% and 64%, respectively), compared with the results for fresh leaves (56%). LRH and HA drying increased essential fatty acid alpha‐linolenic acid (ALA, 18:3n‐3), while linoleic acid (LA, 18:2n‐6) was increased by FIR drying. The present study has provided useful information for the industrial use of dried kaffir lime production.     

Effect of far-infrared radiation drying of citrus press-cakes on free radical scavenging and antioxidant activities

In this study, far-infrared radiation (FIR) drying was used to convert wet citrus press-cakes (CPCs) into dried form at different temperatures (40, 50, 60, 70, and 80 °C). The dried samples were evaluated for flavonoid composition by HPLC and antioxidant activities by various in vitro assays. Compared with CPCs prepared by freeze drying (FD), all FIR-dried CPCs showed slightly lower extraction yield and total phenolic content, but almost equal total flavonoid contents and antioxidant activities. However, CPCs dried by FIR showed significantly lower drying times. Therefore, FIR drying is effective and economical method. CPCs dried by FIR at 80 °C showed higher antioxidant activities in several assays, and needed very short drying time. Hence, FIR-80 sample was more potential than other FIR-dried samples. The data confirmed that FIR drying is an effective process to convert wet CPCs into dried form which shows strong antioxidant activities.     

Effects of layer arrangement on heating uniformity and product quality after hot air assisted radio frequency drying of carrot

The conventional drying technology prolongs the shelf life of carrots but may cause the serious loss of nutrients. The aim of this study was to investigate the effects of intermittently rearranging layers on the heating uniformity, drying characteristics and quality of carrot slices under the hot air assisted radio frequency (RF) heating. The carrot slices were dried for 270 min using hot air (60 °C) assisted RF heating at the electrode gap of 100 mm firstly, and then followed hot air drying to achieve the final moisture level (0.11 kg/kg (d.b.)). The results showed that the selected hot air assisted RF drying protocol for the carrot slices reduced 30% of the duration compared to the single hot air drying. The carrot slices dried by the combined drying method had the highest total carotenoid value (P ≤ 0.05) except for possessing accepted color and rehydration. Therefore, the combined drying method could improve the drying rate and maintain heat sensitive substances in carrot slices.     

热泵干燥温度对柿子片干燥特性及品质的影响

目的:探明热泵干燥温度对柿子片干燥特性和品质的影响。方法:以柿子为原料,研究干燥温度对柿子片干燥时间、干燥速率,以及干制品色泽、硬度、复水率、单宁、总糖、维生素C和总酸的影响。结果:当热泵干燥温度为65℃时,柿子片干燥速度快,干燥时间短,此时干燥产品色泽鲜艳,与鲜果色泽相近,复水性能好,硬度最低,口感最好,总酸和维生素C含量最高,总糖等营养成分含量适中。结论:柿子片的最适热泵干燥温度为65℃。     

Moisture transfer and microstructure change of banana slices during contact ultrasound strengthened far-infrared radiation drying

To explore water transfer and structure change during contact ultrasound strengthened far-infrared radiation drying (CU-FIRD) on banana slices, the effects of far-infrared radiation (FIR) temperature and contact ultrasound (CU) power on drying process, water transfer change, microstructure and pore evolution were studied. The results showed that the drying time could be shortened obviously by increasing FIR temperature and CU power. The effective moisture diffusivity values of banana slices were between 2.28 × 10⁻¹⁰–6.12 × 10⁻¹⁰ m²·s⁻¹, which increased with the rises of FIR temperature and CU power. The moisture changes in banana slices during CU-FIRD were analyzed by low-field nuclear magnetic resonance (LF-NMR). There were three kinds of water status in banana slices including bound water, immobilized water and free water. Free water was the main water in banana, and the increases of both FIR temperature and CU power could obviously shorten the removal time of free water. The immobilized water in banana slices increased first and then decreased during the drying process, and the improvements of FIR temperature and CU power could accelerate its mobility and migration. There was no significant change for bound water, which was the main water left in banana slices after drying. The SEM and shrinkage ratio results showed that higher FIR temperature and CU power could reduce the structure shrinkage of banana samples, increase the porosity, and generate more diffusion micro-channels with larger size, so as to speed up the migration process of water in banana. CU-FIRD could improve the porous structure of banana samples, enhance the heat and mass transfer process, and accelerate the migration and diffusion of water.     

Dehydration of salmon (Atlantic salmon), beef,and apple (Granny Smith) using Refractance window™: Effect on diffusion behavior,texture, and color changes

The aim of this research was to study the use of Refractance window™ (RW™) in the dehydration of salmon (Atlantic salmon), beef (lean), and apples (Granny Smith) through the analysis of the effective diffusivity determined through the Fick's model and the anomalous model.Salmon, beef, and apple slices were dried at 55 and 95 °C, using either a conventional drying process or RW™. Water activity (a_w) was measured periodically by drying, and the effective diffusion (D_eff) was measured using Fick's second law and an anomalous model. Color changes (∆E) and firmness were also measured.The results showed that mass transfer in salmon and beef was not improved by RW™. In contrast, in apple slices dehydrated using RW™, the D_eff and processing times were significantly (p < 0.05) affected, reflecting a reduced drying time required to attain an a_w of 0.6.RW™ is a powerful tool that allows for the dehydration of fruit using temperatures of 95 °C and provides a 50% reduction in drying time.     

Combined radio frequency-vacuum and hot air drying of kiwifruits: Effect on drying uniformity,energy efficiency and product quality

Two drying methods, both individually and in their combination [RF-vacuum drying (RFVD), hot air drying (HAD) and RFVD+HAD] were evaluated for three-layer drying of 6 mm thick kiwifruit slices in order to shorten drying time, improve energy efficiency and product quality. An electrode gap of 95 mm and vacuum of 20.1 kPa were selected for RFVD applications based on previous studies. Results showed that the total drying time was the shortest (480 min) when using RFVD, followed by RFVD+HAD (600 min; 20% longer) and HAD (900 min, almost double). However, non-uniform drying patterns were observed in both RFVD and HAD methods, but the RFVD+HAD process resulted in a more uniform moisture distribution both within and among the fruit slices. The average energy efficiency of HAD was improved remarkably from 9.92% to 22.93% by applying the RFVD+HAD method. Additionally, the RFVD+HAD process resulted in better product quality attributes like color, shrinkage ratio and rehydration capacity. Therefore, the RFVD+HAD method is recommended as a time and energy efficient drying technology for kiwifruits with a better moisture distribution and product quality.     

不同干燥方法对苹果片品质的影响

不同干燥方法对苹果片品质的影响不同,探讨了普通热风干燥、远红外线干燥及微波干燥3种方法对苹果片品质(VC的含量、总酸度、复水性及感官品质)的影响。试验结果表明:微波干燥对于苹果片品质的影响最小,其次是普通热风干燥,远红外线干燥则对苹果片品质影响最大;综合考虑苹果片中VC的含量、总酸度、复水性及感官品质等的变化,三种干燥方法中微波干燥是苹果片适宜的干制方法。     

不同前处理和冻结方式对苹果片真空冷冻干燥效率及干制品品质的影响

冻干前处理和冷却固化环节是果蔬冻干的关键工序,为进一步缩短干燥时间并提高干制品品质,该研究选用苹果片作为试验对象,采用超声波技术、真空冻结技术、超声波-真空冻结技术,将苹果片分为常规组、超声-常规组、真空冻结组、超声-真空冻结组,比较不同前处理方式对苹果片冻干效率及干制品品质的影响。结果表明,超声-常规组苹果片冻干效率最高,由16.00 h缩短至11.83 h;干制品复水比最大,较常规组增加了62.16%;色泽指标b*值由28.24降至19.46,颜色更加洁白;维生素C保留率最高,可达83.90%;硬度最小且松脆可口,感官评价最好。真空冻结组苹果片冻干时间由16.00 h缩短至14.17 h;干制品复水比较常规组增加了17.99%;色泽指标b*值由28.24降至22.98,颜色较为洁白;酥脆性良好且有较好的咀嚼感,感官评价良好。研究结果为苹果片在冻干前处理环节参数优化及新型果蔬脆片的开发提供参考。     

Impact of pre-treatment methods on the drying kinetics,product quality,and energy consumption of electrohydrodynamic drying of biological materials

Electrohydrodynamic (EHD) drying is an energy-efficient drying method. This novel drying technology operates at room temperature, which makes it particularly suitable for drying biomaterials that contain heat-sensitive compounds. It has a higher drying rate than other low-temperature methods, such as solar and freeze-drying. However, its drying rate is not high enough to compete with other conventional thermal methods, such as hot-air drying. For industrial applications requiring high product throughput, the drying rate of EHD drying should be improved. One way to do this is to combine EHD with pre-treatment methods. Therefore, this study evaluated the impact of different pre-treatment methods on drying kinetics, energy consumption, and product quality attributes of apple slices dried using EHD drying. Pulsed electric fields (PEF), ultrasound, and blanching are the studied pre-treatment methods. Results show that only PEF pre-treatment could significantly decrease the EHD drying time by 39%. However, it resulted in a 26% higher browning index than the untreated EHD-dried apples, which is not appealing to consumers. The applied pre-treatment methods did not significantly affect other quality attributes, such as antioxidant activity, total phenolics, and rehydration ratio. In conclusion, using the studied pre-treatments for EHD drying increases the complexity of the process, whereas it is arguable whether the added values outweigh the energetic and quality downsides or not.     

热风真空组合干燥苹果片试验研究

以苹果为原料,通过试验探讨了在热风真空组合干燥条件下,热风温度、热风时间、真空温度、真空度等因素对热风真空组合干燥苹果片品质的影响。结果表明:热风真空组合干燥较热风干燥而言可有效提高其干制品的质量;热风真空组合干燥苹果片较优的工艺参数为:热风温度60℃、热风时间1h,真空温度50℃、真空度30kPa。     

微波气流预干燥和微波真空组合干燥生产甜瓜片

研究了微波气流预干燥(MAD)和微波真空(MVD)组合干燥甜瓜片,表明这是一种有潜力的干燥方法。样品先通过微波气流干燥脱去一些自由水,然后通过微波真空干燥到最终水分含量低于7%(湿基)。甜瓜片利用微波气流(MAD)和微波真空(MVD)组合干燥后测得Vc保留率和物性(收缩率,色差,质地和复水率)然后同冷冻干燥(FD)、微波气流干燥(MAD)、气流干燥(AD)和常规热风干燥(HAD)分别进行比较,表明,MA+MVD方法干燥的甜瓜片的Vc保留率接近冻干的甜瓜片,大大优于传统的热风干燥。当前方法干燥的样品表现出的复水率、色差、质构与冻干的产品非常相似,但是出现了明显的膨化现象,然而,这正是生产果蔬脆片所期望得到的结果。