Multi-flash Drying Research Articles

Trends in quality assessment and drying methods used for fruits and vegetables

Studies on drying of fruits and vegetables by means of different methods are numerous. This review aims at presenting the most usual quality analyses carried out in dried fruits and vegetables over the last five years along with the most used drying methods. It was prepared based on a search on Scopus database with the term ‘drying fruit vegetables’. Results show that color and microstructure can be preserved, apparent density can be reduced, and high rehydration capacity can be achieved by using mild drying methods like freeze-drying and innovative drying methods like conductive multi-flash drying. Shrinkage and water activity can be minimized by using pretreatments like osmotic dehydration, ultrasound, and pulsed electric fields. Such pretreatments also produce crispy texture, which can also be obtained with the use of hot air drying followed by microwave-vacuum drying. Antioxidant activity, phenolic compounds and vitamin C content can be maximized by using innovative pretreatments like immersion in solutions rich in bioactive compounds and short drying time. Microbial counts can be decreased by using pretreatments like immersion in ethanol, and modified atmosphere packaging. High sensory acceptability can also be achieved by using innovative drying methods, like instant controlled pressure drop drying.

Temperature control for high-quality oil-free sweet potato CHIPS produced by microwave rotary drying under vacuum

Healthy and convenient foods have drawn consumers' attention and represent an opportunity for the food industry. This study is about the production of sliced sweet potato snacks by microwave vacuum drying (MWVD) in a rotary container with temperature control by power modulation. The average temperature of samples' surfaces was measured with an infrared sensor and controlled at 60 °C by a PID controller. The resulting product properties were compared to those from freeze- (FD), air- (AD), and conductive multiflash (KMFD) drying. Rotary container coupled to the temperature control guaranteed uniform heating and resulted in chips without burnt spots in MWVD.MWVD and KMFD produced sweet potato chips in less than 3 h, while AD and FD required 16 h. The drying method influenced chips' porosity: 58% (FD), 44% (MWVD), 39% (KMFD), and 18% (AD). AD chips had a compact structure, while FD chips were spongy with small pores. MWVD and KMFD expanded the material during fast water evaporation under vacuum, creating large and irregular pores. The puncture tests of these chips resulted in jagged force-penetration curves, typical of crispy products, with higher acceptance. Therefore, MWVD and KMFD have the potential for the industrial production of high-quality oil-free and crispy chips.

Mechanical-acoustical measurements to assess the crispness of dehydrated bananas at different water activities

Banana slices were dehydrated by Conductive Multi-Flash Drying (KMFD) and stored at different relative humidities. A sensory panel determined the bananas' crispness and PCA correlated these results to mechanical and acoustic properties from compression and puncture tests. The BET model predicted the monolayer moisture as 0.0723 g/g with aw 0.388 as a supposed condition for good storage stability. However, based on crispiness, the sensory panel suggests a critical aw between 0.225 and 0.327. In the Spectrum crispness scale (intensity 0 to 15), the trained panel rated samples at aw 0.035 to 0.529 from 9.06 to 0.12, respectively. The number of acoustic peaks for the SPL bands depended on type and speed of the mechanical test. In compression, the parameters that correlated well to sensory crispness were moisture, number of force peaks, linear distance, and all acoustic parameters, independently of the acoustic filter (Arimi or FIR). In puncture test, the number of acoustic peaks highly correlated with sensory crispness. The striking correlation between the mechano-acoustic analyses and the sensory panel allows the reliable use of instrumental analyses in predicting the banana snacks crispness at different aw. It directly applies to industrial quality control since instrumental analyses are cheaper and faster than sensory panels.

Effects of vacuum and multiflash drying on the microbiota and colour of dried yellow mealworm (Tenebrio molitor)

Larvae ofTenebrio molitor (yellow mealworm; YM) are an alternative to conventional animal protein as food and feed thanks to their high nutritional value and low environmental impact for production. Since some studies have reported high water activity (aw ) with different microbial contamination levels, blanching and drying can be employed to reduce microbial load, enzymatic activity, moisture content, and water activity, ensuring safety and preserving YM for longer periods. This study applied rinsing or blanching followed by vacuum- or multiflash drying, aiming to preserve YM by reducingaw and the microbial load, enabling its use as food and feed. The processing impact on YM colour was evaluated, and they were also subjected to anEscherichia coli contaminated wheat bran to perform a challenge test to evaluate the effectiveness of the decontamination. Drying curves were determined for each situation. Multiflash drying procedure was faster than vacuum drying for reducing YM moisture. Additionally, blanched samples required a shorter period to reach the targeted water activity (0.2-0.3) than rinsed samples. Blanching plus vacuum drying reduced more than 5 logE. coli load in YM in the challenge test. Vacuum drying preserved YM brightness the most. In conclusion, this study showed that drying could provide shelf-stable YM. Besides, only 15 s of blanching before drying gives better brightness and leads to a greater microbial reduction after a relatively short drying time.

Conductive drying methods for producing high-quality restructured pineapple-starch snacks

Restructured fruit pulps can be dried to obtain new added-value products. In the present study, restructured pineapple pulp was dehydrated by two innovative methods, cast-tape drying (CTD) and conductive multiflash drying (KMFD). The snack produced was evaluated by physical, chemical, and sensory aspects. CTD required a half time length (30 min) than KMFD (60 min) for drying. The drying processes did not affect the ascorbic acid content present in the fresh fruit but decreased its antioxidant activity in 50–70%. The natural volatile compounds were also changed. The product from KMFD was preferred by consumers, which may be a consequence of a more expanded structure with larger pores created by the vacuum pulse and flash water evaporation. KMFD resulted in more jagged acoustic-mechanical curves, which is a characteristic of crispy foods. Therefore, KMFD can be considered the most promising process to produce restructured pineapple snacks in approximately one hour with excellent quality. Industrial relevanceIn recent years, a growing number of consumers have been looking for healthy, convenient, and ready-to-eat foods. Dried restructured fruits can be healthy snacks with very attractive sensory characteristics depending on the drying method. Cast-tape drying (CTD) and conductive multiflash drying (KMFD) are novel technologies with great potential to produce high-quality dried products at an industrial scale in a relatively short time. This study provides important results regarding the most suitable drying processes setup for producing high-quality dried restructured pineapple.

Producing crispy chickpea snacks by air, freeze, and microwave multi-flash drying

This study aimed to obtain and characterize dehydrated chickpea snacks by sequential hydration, cooking, and drying processes. Hydration was performed at 4, 25, and 60 °C until reaching the maximum water uptake by the grains. Then, they were boiled until 80% of the grains reached firmness minor than 15 N. Cooked chickpeas were peeled and dried by three methods, air-drying (AD), freeze-drying (FD), and microwave multi-flash drying (MWMFD). The grains moisture was reduced from 1.681 ± 0.076 g/g to less than 0.050 ± 0.003 g/g and a w below 0.270. The constant drying rate was observed for all the three drying processes, followed by a falling drying rate period. MWMFD required a much shorter processing time (0.4 h) when compared to AD (10 h) and FD (15 h). The microstructure, and texture of the product were determined. Freeze-dried chickpeas were 1.2 and 2 times more porous than the MWMFD and AD samples, respectively. Micrographs showed small and uniform pores in FD samples, while MWMFD and AD samples had irregular porous matrix. MWMFD and AD samples exhibit jagged strain-force curves, characteristic of crispy foods. Therefore, MWMFD is a suitable process to obtain crispy chickpeas in short time.

An innovative hybrid-solar-vacuum dryer to produce high-quality dried fruits and vegetables

Food drying uses much energy and is considered a high carbon footprint process. Solar drying is sustainable, but the dryers must be improved to produce high-quality foods in short times. In this study, a hybrid-solar vacuum dryer was designed, built, and applied to fruit dehydration. A solar heater was coupled to a thermally insulated reservoir with electrical resistances to provide hot water at 90 °C as the indirect heating medium. Drying chambers with a full capacity of 4 kg of sliced food were operated under vacuum or vacuum pulses (conductive multiflash drying). Banana slices could be dried with the most basic equipment configuration (vacuum chamber, vapor condenser, and vacuum pump), resulting in homogeneous and crispy snacks. The dryer was also tested for persimmons and carrots. The drying of all the fruit and vegetable samples resulted in dried and crisp products in 3–4 h, depending on the drying temperature, and the use of continuous electric support for temperatures higher than 70 °C. The hybrid solar dryer operating under vacuum is an innovative and clean technology that results in high-quality products and has excellent potential to be used in small farms.

Effect of drying processes on the final quality of potimarron pumpkin (Cucurbita maxima) powders

This research work focuses on a comparative study of the impact of four drying processes (Air Flow Drying (AFD), Freeze-Drying (FD), Vacuum Multi Flash Drying (VMFD), and Swell-Drying (SD) coupling Instant Controlled Pressure-drop (DIC) to standard Air Flow) on the biochemical, physicochemical, thermochemical, and biological properties of the pumpkin potimarron (Cucurbita maxima) powders. The main results showed that the availability of bioactive substances was much more related to the drying process and attributed to the porosity. VMFD, SD, and FD drying processes had the best preservation of Total Polyphenols (TPP) and Total Flavonoids (TF) much better than AFD. It is worth noting that FD and SD improve the availability thanks to the presence of multi-particulate pores, while VMFD keeps better preservation of thermo-sensitive products.

Oil–free potato chips produced by microwave multiflash drying

The present work is about the production of oil-free potato snacks using successive cycles of microwave heating-vacuum pulses and was therefore called microwave multiflash drying (MWMFD). Drying kinetics and product physical properties were assessed, as well as the influence of blanching (BL) and vacuum impregnation (VI) as pre-treatments before drying. The drying curves presented three periods: a heating period, a high drying rate period promoted by the vacuum pulses, and a falling rate period. The drying time of 400 g of potatoes was approximately 60 min, reducing gradually the microwave power from 800 to 200 W. The vacuum-pulse step changed the material, resulting in a porous and fragile microstructure, characteristic of crispy products, as well resulting in a product surface with a golden yellow color. BL and VI combined was the best pre-treatment for obtaining very crispy oil-free potato snacks. The results of this study indicated MWMFD as a suitable process to produce crispy potato snacks, free of oil, in only 60 min.

Assessment of texture and storage conditions of mangoes slices dried by a conductive multi-flash process

Glass transition temperature (Tg) and sorption isotherms can be used to establish state diagrams that help to assess storage conditions to keep the food as an amorphous material in a glassy state. In this study, state diagrams and mechanical properties were assessed to predict the storage conditions of mangoes dried by 1) air-drying; 2) freeze-drying; 3) vacuum-drying and 4) conductive multi-flash drying (KMFD - four cycles of heating/vacuum pulse followed by vacuum-drying). Dried mangoes showed a typical sorption isotherm behavior of hygroscopic materials. In the relative humidity (RH) range investigated (between 11.3% and 90.3%), Tg of processed samples decreased from 44 °C to −68 °C, due to the plasticizing effect of water. Dried mangoes stayed structurally stable at room temperature (25 °C) if storage RH was kept at 25%, in this conditions the moisture content was 0.036 g g −1 (d.b.). The puncture tests results confirmed that, at room temperature, samples stored at RH higher than 30% lost their crispness.

Effect of multi-flash drying and microwave vacuum drying on the microstructure and texture of pumpkin slices

The influence of different drying methods on the microstructure, porosity, and texture of pumpkins slices was assessed, aiming to find the best process conditions. Pumpkins (Cucurbita moschata) were selected, peeled, sliced, blanched and dehydrated by different methods: i) microwave multi-flash drying (MWMFD), ii) microwave vacuum drying (MWVD), iii) conductive multi-flash drying (KMFD), iv) freeze-drying (FD), and v) air-drying (AD). Micrographs of dried samples showed the compacted structure of AD samples and the spongy structure of FD samples. Otherwise, large pores and a highly porous structure were observed in samples dried by KMFD, MWVD, and MWMV. Porosity MWMFD samples were 2.6 times higher than those observed in AD pumpkins and slightly higher than the observed in FD samples. Puncture test results from KMFD, MWVD and MWMFD samples showed force oscillation that is typical of crispy products, which was not observed from AD and FD samples. Moreover, KMFD, MWVD, and MWMFD samples showed low color changes. Thus, microwave under vacuum and both multi-flash drying processes result in dried-and-crisp pumpkins, with high porosity and big pores. Therefore, these drying processes are suitable for the production of dried-and-crisp pumpkins.

Microwave vacuum drying and multi-flash drying of pumpkin slices

In this study, the effect of different drying methods on the drying kinetics, product structures, rehydration kinetics and rehydration indices of dehydrated pumpkin slices was evaluated. For that, pumpkin slices were dehydrated by five different methods: i) Microwave multi-flash drying (MWMFD), ii) Microwave vacuum drying (MWVD), iii) Conductive multi-flash drying (KMFD), iv) Freeze-drying (FD), and v) Air-drying (AD). For reaching a moisture content of 0.022 g g−1 (dry basis, db) and water activity of 0.438, the drying times were 1.28 h for MWVD, 1.32 h for MWMFD, 3.50 h for KMFD, 16.33 h for AD and 16.67 h for FD. All the drying methods presented a constant drying rate period and a falling rate period. The highest constant drying rate was observed for MWVD, which was 1.3, 8, 22 and 53 times higher than the observed for MWMFD, KMFD, AD, and FD, respectively. Stereo micrographs of samples from multi-flash drying processes (KMFD and MWMFD) and from MWVD showed large pores in a highly porous structure. Rehydration of dried pumpkin slices was studied at 25 °C and 80 °C. Water temperature influenced samples rehydration ratio (RR = mass of rehydrated sample/mass of dried sample). RR up to 15 was observed at 25 °C, while RR up to 12 was observed at 80 °C. Samples from MWMFD and MWVD, rehydrated at 25 °C presented higher moisture after rehydration than those observed from AD, FD and KMFD samples. At 80 °C, FD samples showed the highest RR. From the whole results, one can conclude that MWVD and MWMFD are the suitable processes for producing dehydrated pumpkin slices in very short drying times.

Empirical and diffusion models of rehydration process of differently dried pumpkin slices

Abstract The present study investigated the rehydration kinetics of dried pumpkin slices issued from different drying operations, namely airflow drying (AFD), freeze-drying (FD), Vacuum Multi Flash Drying (VMFD) and Swell-drying (SD) which inserts Instant Controlled Pressure Drop (DIC) Texturing between two AFD stages (AFD + DIC + AFD). Rehydration process depends on dehydration methods. It has been noticed that the slowest rehydration process has been observed for AFD whilst VMFD, FD, and SD averred short rehydration time. Experimental rehydration curves were performed through empirical and diffusion models. Hence, it has been noticed that Weibull has provided the best rehydration fitting curve. In other hand, Fick's diffusion models were also used to describe kinetics of rehydration process. They were characterized by two effective diffusivity coefficients D eff 1 and D eff 2 for AFD and VMFD, and three effective diffusivity coefficients in the case of FD and SD (AFD + DIC + AFD). The temperature dependence of the diffusivity coefficients was also described by Arrhenius-type relationship, with adequate activation energy levels.

Conductive multi‐flash drying of mango slices: Vacuum pulse conditions on drying rate and product properties

In multi-flash drying, the pressure drop applied during the vacuum pulse and the number of heating vacuum pulses cycles (HVP) influence the fruit microstructure and texture. In this work, the effects of conductive multi-flash drying (KMFD) process variables on the drying kinetics, physical properties, and microstructure of mango slices were assessed. Different slopes of pressure drop (vacuum pulses of 0.98, 1.59, and 2.55 kPa s−1) and the number of HVP cycles (14 and 18 cycles for KMFD and a process with the application of 4 cycles followed by vacuum-drying KMFD-VD) were investigated. The number of HVP cycles allows obtaining both soft and crispy mangoes. Higher pressure drop slopes resulted in fruits with increased porosities (≈60%), compared with samples dried using the lower pressure drop slope (≈50%). The KMFD and KMFD-VD drying curves were similar, indicating that KMFD-VD presents operational advantages for obtaining dried fruits, reducing the number of heating-vacuum pulses. Practical applications Some fruits and vegetables have a short shelf life that limits their commercialization as fresh products and increase postharvest losses. Dehydration is an interesting alternative to the development of new products of fruits and vegetables with added value. In particular, the multi-flash drying is an innovative drying process based on the application of multiple cycles of heating-vacuum pulse to a solid food. Fruits and vegetables samples are heated (i.e., by heat conduction for the conductive multi-flash drying—KMFD) at moderate temperatures and submitted to a vacuum pulse, which causes flash drying and sample cooling. This method allows maintaining in dried products good physical–chemical properties and improving its texture. Moreover, according to the number of heating-vacuum pulse cycles and the pressure drop applied during these vacuum pulses, it is possible to obtain soft or crispy fruits. Thus, these variables can be used to control texture and produce high quality dried fruit.

Improving quality of dried fruits: A comparison between conductive multi-flash and traditional drying methods

Conductive multi-flash drying (KMFD) is an innovative drying process based on the application of multiple cycles of heating-vacuum pulse to a food to be dried. This method allows obtaining crispy-and-dried products with appealing characteristics. The objective of this study was to compare different drying methods (KMFD, air-drying, vacuum drying and freeze-drying) with respect to drying kinetics and physical properties of dried mangoes (porosity, microstructure, color, and mechanical properties). The influence of these drying methods on carotenoids concentration and rehydration were also investigated. Mango samples were dried from 5.30 g/g moisture (dry base) to 0.01 g/g (aw = 0.293) in 2.25 h by the KMFD, which was much shorter than the drying times observed for freeze-drying (16 h), air-drying (12 h) and vacuum drying (8 h). Porosity and texture of samples dried by KMFD were similar to the observed for freeze-dried samples, despite the different microstructures. Carotenoids concentration of freeze-dried samples was 1.6 times higher than that from KMFD, which was in turn 1.27 times higher than that from air-dried samples. KMFD is a new and innovative drying process that allows the production of crispy mangoes in reduced drying times, with carotenoids contents and rehydration ability similar to freeze-dried samples.

A microwave multi-flash drying process for producing crispy bananas

A comparative study of different drying strategies to produce dried bananas with controlled microstructure and texture properties was conducted. The result of applying microwave heating coupled with vacuum pulses on the crispness of dried bananas was investigated. Banana slices were dehydrated by microwave vacuum (MWVD), a microwave multi-flash drying process (MWMFD) and freeze drying. Drying kinetics for MWVD and MWMFD showed three drying periods, i.e. i) a very short heating period, ii) constant drying rate period and iii) falling rate period. The higher nominal microwave power led to larger drying rates. The three different magnetron powers applied to the fruits did not result in different microstructures and textures, for a same drying procedure. On the other hand, MWMFD resulted in dried fruits with larger pores and porosities 20–50% higher than those produced by MWVD. Furthermore, puncture tests of MWMFD samples resulted in more irregular force–strain curves (jagged curves) that are characteristic of crisp products. The results show that it is possible to create dried-and-crisp fruits by applying successive cycles of heating and vacuum pulses in a microwave field. The MWMFD process is more effective than the MWVD process for producing dried-and-crisp bananas, with very short drying times when compared to freeze-drying times.

Processes for controlling the structure and texture of dehydrated banana

ABSTRACTThis 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.

A convective multi-flash drying process for producing dehydrated crispy fruits

The aim of this work was to evaluate the application of a convective multi-flash drying process (CMFD) to producing dehydrated and crisp fruits. To accomplish this process, samples of banana (Musa sapientum L.) or mango (Mangifera indica L.) were heated to 60°C by hot air, and a vacuum pulse was applied, which resulted in dehydration by a combination of convective drying and flash evaporation. Banana processed by CMFD had a moisture content of 0.293g/g (dry basis) and aw=0.272 after 3h of processing. Mango had a moisture content of 0.09g/g and aw=0.359 after 4h of processing. Puncture tests on fruits dehydrated by CMFD and on commercial freeze-dried fruits showed strain–force curves with many peaks (jagged curves). For CMFD much smaller global shrinkage was observed. These results indicate that the CMFD process can be applied for producing crispy fruits and is an alternative to the freeze-drying process.