Low-pressure Superheated Steam Drying Research Articles

Combined effects of low pressure superheated steam drying and vacuum drying on sugar reduction and quality attribute in mango (Mangifera indica L.) slices

Abstract Consumers paying more attention to physical health has led to an increasing market demand for low-sugar dried fruit products. The quality of products dried via low pressure superheated steam drying (LPSSD) is not only superior to those dried via conventional hot air or vacuum drying (VD), but also has the potential to reduce sugar content. In order to elucidate the mechanism of reducing the sugar of mango slices by LPSSD and obtain low-sugar dried mango slices, the combined effect of LPSSD–VD on mango slices was studied and an evaporation experiment of a sugar solution in a low pressure superheated steam environment was performed. This study revealed that the sugar reduction of mango slices was mainly due to the superheated steam carryover phenomenon in the second half of the constant-temperature stage and the occurrence of Maillard reaction during LPSSD. The quality attributes of mango slices dried using LPSSD–VD was improved compared with LPSSD and VD. As a result, LPSSD–VD could be used to regulate the sugar content in dried fruit and provide a theoretical basis for the production of low-sugar preserved fruits.

The role of emerging technologies in the dehydration of berries: Quality, bioactive compounds, and shelf life.

Berries are among the fruits with the highest nutritional and commercial value. This paper reviews the conventional and emerging dehydration methods most commonly used as postharvest treatment and discusses their efficacy in maintaining and/or improving the nutritional and functional qualities of dried berries. The characteristics of the conventional methods (e.g., convective drying, freeze-drying, spray-drying, osmotic dehydration), their pre-treatments, their combination, and intermittent drying, as well as their potential disadvantages are discussed. The use of emerging dehydration techniques (e.g., electromagnetic radiation drying, explosion puffing drying, heat pump drying, low-pressure superheated steam drying, microwave drying) allows to improve the quality of the dried berries compared to conventional techniques, in addition to reducing drying times, increasing drying speed and energy efficiency. Finally, the use of pre-treatments and the combination of technologies can enhance the quality of the final product as a result of the improvement in the effectiveness of the dehydration process.

Drying Characteristics of a Combined Drying System of Low-Pressure Superheated Steam and Heat Pump

The present study aimed at investigating the use of a drying system combining the concept of low-pressure superheated steam drying and heat pump drying for fish. The effects of various drying medium pressures on the temperature field, airflow field, drying time, equipment performance as well as the power consumption of the drying process were investigated and discussed. Four comparative tests with different initial pressures were carried out according to a specified drying process by the combined drying system. The results showed that when the vacuum was high, the temperature field and airflow field in the drying chamber were more uniform. Due to the poor heat transfer performance of the drying medium at high vacuum, the drying time increased with a decrease in initial pressure. It was also found that with the decrease in drying medium pressure, the power consumption of the heat pump and the axial fans was reduced, while the power consumption of the electric heater went up. Overall, the total power consumption is directly proportional to the drying medium pressure.

Novel drying techniques for controlling microbial contamination in fresh food: A review

Fresh foods such as fruits, vegetables, grains, nuts, and meats are generally rich in water and are susceptible to microbial contamination. Drying and dehydration processes can reduce water content, but low water activity foods may not be microbiologically safe. Several novel drying techniques have demonstrated varying degrees of effectiveness in microbial inactivation, including supercritical CO2 drying, low-pressure superheated steam drying, microwave drying, radio frequency drying, infrared drying, vacuum drying, freeze drying, etc. Some typical microorganisms used as indicators of microbial inactivation including natural present microorganisms (e.g., mesophilic bacteria and their spores, yeast and mold) and various pathogens (e.g., Escherichia coli O157:H7, Listeria monocytogenes and Salmonella spp.). This paper reviewed novel drying techniques for controlling microbial contamination of fresh foods and analyzed their advantages/disadvantages and microbial inactivation capabilities. These novel drying techniques have the potential to ensure microbiological safety during drying and have the prospect of shortening the food processing chain by integrating drying and sterilization/decontamination into the same process.

Effects of a Combined Processes of Low-Pressure Steam Enrichment and Low-Pressure Superheated Steam Drying on the γ-Aminobutyric Acid Content of Japonica Rice

In order to improve the content of GABA in japonica rice, so as to improve the nutritional composition and enhance the added value of rice, this study explored a combined process of low-pressure steam enrichment of γ-aminobutyric acid (GABA) content in japonica rice and low-pressure superheated steam (LPSS) drying of japonica rice. The low-pressure energy and the energy stored in rice grains were used to promote water absorption and activation of rice germs in a steam environment. Under the action of glutamate decarboxylase, the germ glutamate underwent an enzymatic reaction to generate GABA, which was transferred to and stored in the rice endosperm due to the concentration gradient and water gradient. After germination, LPSS was successively applied to dry the rice grains. The response surface method was used to optimize the parameters of low-pressure wet steam enrichment and LPSS drying of the rice grains. The experimental results showed that when the pressure was 0.026 MPa, the temperature was 60.66°C, the time was 5.01 h, and the flow rate was 0.31 L/h; the GABA content produced by low-pressure wet steam germination of japonica rice peaked at 94.1751 mg/100 g. When the germinated and enriched japonica rice was put through LPSS drying with a pressure of 0.021 MPa, a temperature of 66.55°C, a time of 1.57 h, and a flow rate of 0.28 L/h, the GABA content peaked at 104.161 mg/100 g. The results show that the combined process of low-pressure steam enrichment and drying continuously produced GABA in the soaking enrichment and drying stages, and the content of GABA in the drying stage accounted for 10% of the total content, which effectively increased the content of GABA in japonica rice and shortened the enrichment time. This study provides a technical reference for industrialized production of GABA-rich brown rice, embryonic rice, and polished rice.

Study and comparison of different drying processes of porous particle at high temperature

This work presents a comparative study of four drying processes of a porous media: superheated steam drying at atmospheric pressure (APSSD), humid air (HA), low pressure superheated steam drying (LPSSD) and vacuum drying (VD). A single porous particle model has been developed to simulate the four drying processes. The model is based on the method of averaging volume. Spherical porous particles of coal are used as the model material in this paper. The evaporation rates are equal for these processes in point called the inversion temperature. This temperature was calculated during the constant rate period (CRP) and the falling rate period (FRP). A variation in the values of inversion temperature was observed (363-503 K). The effect of drying parameters such as: particle radius, gas mass flux, permeability, porosity and operating pressure were investigated. Several researchers have reported the noticeable variation of the inversion temperature values according to the drying period used to calculate this key parameter. Our results are compared with those obtained from a front model reported in the literature. A good agreement is found.

Drying of pineapple slices using combined low-pressure superheated steam and vacuum drying

Abstract A method of combining low-pressure superheated steam drying (LPSSD) and vacuum drying (VD) was proposed to improve the dried pineapple quality and increase the drying rate. It was found that the inversion temperature in low-pressure superheated steam drying of pineapple was 85.75 °C in terms of the first falling rate period. The combining drying (LPSSD–VD) reduced the maximum material temperature by 9.5 °C and 0.35 °C, and shortened the drying time by 50 min and 90 min compared with LPSSD and VD at the same drying temperature of 90 °C. The vitamin C retention rate of dried pineapple by LPSSD–VD was 29.33% and 15.94% higher than that of LPSSD and VD, respectively. The color of dried pineapple was also improved. Moreover, the sugar content of dried pineapple can be well controlled to meet the health demand of low sugar and ensure the taste of dried pineapple during LPSSD–VD process.

The microbiological quality of various foods dried by applying different drying methods: a review.

With the drying process, the water activity and moisture content of the foods are reduced, so the growth of microorganisms in the foods is largely prevented/postponed. But low-aw foods should not be considered sterile they can be contaminated by fungi and other contaminants during the drying process under unhygienic conditions. If drying is not done to a sufficient degree of moisture during food processing and storage, where dried foods are processed, sometimes the minimum value is reached for the growth of microorganisms. In dry foods, some pathogens, yeast and molds can continue to grow during storage, transport and transportation until the sale and they causing spoilage. They can even cause health problems if enough pathogen or spore cells remain viable. Considering this situation today, it is attempted to obtain high-quality dried foods with good microbiologically and chemically properties. For this purpose, various drying methods have been developed. Most studies suggest that when foods are pre-treated with the ascorbic acid or sodium metabisulfite or applied with various combined methods such as UV irradiation, supercritical carbon dioxide (SCO2), low-pressure superheated steam drying (LPSSD), and infrared (IR) drying, they can be effective on inactivation of microorganisms. We have reviewed in this study how these methods made dried products efficient of microbial inactivation and microbiologically safe.

Profiles of prebiotic fructooligosaccharides, inulin and sugars as well as physicochemical properties of banana and its snacks as affected by ripening stage and applied drying methods

Contents of fructooligosaccharides and inulin along with those of selected sugars in fresh banana of different ripening stages were first determined. Banana was then processed into snacks either by hot air drying (HD) or low-pressure superheated steam drying (LPSSD). Selected physicochemical properties of both fresh banana (moisture content, water activity, flesh-to-peel ratio, total soluble solids content, pH, titratable acidity and firmness) and its snacks (microstructure, texture and color) were investigated. Fresh banana of ripening stage 5 exhibited higher fructooligosaccharides, inulin and sugar contents; its snacks also possessed maximum fructooligosaccharides and inulin contents. Drying methods did not have any significant effect on the contents of these prebiotics. Although snacks from banana of ripening stage 5 possessed higher sugar contents, such contents, when snacks are consumed at the recommended amount, are still lower than the recommend daily intake. Stage-5 LPSSD snack exhibited better texture and color than its HD counterpart, so the former has a potential of being a prebiotic-based snack.

Low pressure superheated steam drying of onion slices: kinetics and quality comparison with vacuum and hot air drying in an advanced drying unit.

Pungency is important characteristics of onion and during processing it is generally reduces. Low pressure superheated steam drying (LPSSD) is gaining importance due to energy and product benefits. It results in better retentions of bioactive components. So, in current study onion slices were dried using low pressure superheated steam, and compared with vacuum and hot air drying at different temperature in NIFTEM advance drying unit. Among the selected models, Page's model gave a better prediction and satisfactorily described drying characteristics of onion slices. The Activation energy was found to be 41.87kJ/mol in LPSSD. Quality of product, i.e. retention of color, rehydration ratio, thiosulphinate content, total phenol content and antioxidant activity, were better at 70°C using LPSSD, at 60°C using VD and HAD, as compared to other drying temperature in respective drying technologies used. Significant differences in quality of the dried product were also observed due to drying temperature in different drying techniques.

Quality Evaluation of Onion Slices Dried Using Low Pressure Superheated Steam and Vacuum Drying

The effect of combined low pressure superheated steam drying (LPSSD) followed by vacuum drying (VD) on quality of onion slices at different temperature (60-80 o C~10 kPa) was determined. In the first stage, the moisture content of onion slices was reduced to 50 % (w.b.) from 88-90 % (w.b.) using LPSSD; and then it was further reduced up to 7-8 % (w.b.) by VD. It was found that among the various drying temperatures studied, retention of colour, rehydration ratio, thio-sulphinate content and antioxidant capacity of dehydrated onion was better at 60 o C, followed by 70 o C, whereas total phenol contents were higher at 80 o C. The results of control dried sample and sample dried at 60 o C had significant differences in time taken for drying, thio-sulfinate content, total phenol content and antioxidant capacity, whereas insignificant effects of drying temperature was observed on rehydration and water activity of dried onion slices.

Drying Kinetics and Quality Attributes of White Radish in Low Pressure Superheated Steam

Abstract In this paper, white radish discs were dried in superheated steam at absolute pressure 95 mbar at four levels of drying temperature from 75 °C to 90 °C. The drying kinetics and various quality attributes of white radish were investigated, compared with the results by vacuum drying. By kinetic modeling of drying processes with Fick’s second law, low pressure superheated steam drying (LPSSD) had slightly lower value of effective moisture diffusivity but higher activation energy than vacuum drying. In comparison with vacuum drying, the rehydration capability of dried samples by LPSSD was better due to porous microstructure. Only 25 % or more the total amount of Vitamin C was preserved after drying to the ultimate moisture content, but it was found that some amount of Vitamin C was taken with the exhaust steam and preserved in the condensate.

Low-Pressure Superheated Steam Drying of a Porous Media

The present study aimed at developing a theoretical model to simulate the drying process of a porous particle in low-pressure superheated steam drying (LPSSD). Spherical and porous particles of ceramic and coal are used as the model material in this work. Experimental data for ceramic and coal particles reported in the literature were used for the model validation. The effect of particle thermophysical properties and operating variables is tested. Results showed that a decrease in pressure and an increase in the fluid temperature during drying lead to an elevation of mass flux during the constant rate period (CRP) and the falling rate period (FRP) of the drying process. Moreover, this increase in mass flux is more significant for particles with the highest effective porosity. Thus, we conclude that decreasing pressure and increasing the drying fluid temperature reduce the mass flow.

Low-Pressure Superheated Steam Drying—Vacuum Drying of a Porous Media and the Inversion Temperature

This article deals with comparative study of low-pressure superheated steam drying (LPSSD) and vacuum drying using coal particles as model material. The focus is made on the inversion temperature, which means that the drying rate in the two drying processes becomes equal. This temperature is of great importance because it allows the choice of drying agent for given operating conditions. The effect of external and internal parameters on the drying kinetics and the inversion temperature were investigated. Several studies have shown discrepancies of inversion temperature values basing on the constant rate period (CRP) and the falling rate period (FRP). This work points out gaps in these temperature values using the two calculation methods.

Bioactive compositions of extracts from cabbage outer leaves as affected by drying pretreatment prior to microwave-assisted extraction

In order to add value to cabbage outer leaves, which are generally considered as a waste product, extraction and use of their health-beneficial bioactive compounds should be made. Among many extraction alternatives microwave-assisted extraction (MAE) is one of the most promising and has indeed been applied to extract bioactive compounds from cabbages. However, study is lacking on the effect of drying prior to MAE, which is sometimes required since extraction cannot always be performed on fresh materials. This study was divided into 2 parts, with the first part investigating the effects of selected drying methods, i.e., hot air drying (HD) and low-pressure superheated steam drying (LPSSD), on selected bioactive compounds, namely, glucosinolates (GLS), sulforaphane, phenolics, as well as the total antioxidant activity (TAA) of the dried cabbages. In the second part the effects of the drying methods on subsequent MAE were assessed. The results suggested that HD but not LPSSD led to a significantly decreased GLS content of the dried cabbages. Sulforaphane and total phenolics contents as well as TAA of the dried cabbages decreased upon both HD and LPSSD. The GLS and total phenolics contents as well as TAA of the extracts from the dried cabbages prepared by HD and LPSSD were lower than those from the fresh sample. The results on the sulforaphane content of the extracts were nevertheless opposite.

Combined effects of drying methods, extract concentration, and film thickness on efficacy of antimicrobial chitosan films.

An idea of using a suitable drying method to minimize the loss of added antimicrobial agent and, at the same time, to modify the structure, and hence the release characteristics of chitosan films was proposed. Chitosan film-forming solution was incorporated with galangal extract (0% and 1.5% w/w) and formed into films with the thickness of 15 and 30 μm via hot air drying (HD) (40 °C) and low-pressure superheated steam drying (LPSSD) (70 °C, 10 kPa). The extract retention, release characteristics, and antimicrobial efficacy of the films were then assessed; fresh-cut cantaloupe was used as the test food material, while Staphylococcus aureus was the test pathogenic microorganism. The retention and release of 1,8-cineole, a major bioactive compound in the galangal extract, was monitored during 5-d storage at 25 °C. The film swelling was also evaluated and their results used to interpret the release characteristics of 1,8-cineole from the films to the cantaloupe. At the same thickness, the films prepared by HD had lower extract retention and higher degree of swelling, thus exhibiting faster extract release and lower antimicrobial efficacy than the films prepared by LPSSD. Within the same drying method, the increased film thickness led to higher extract retention and antimicrobial efficacy. The concentration of the extract in the cantaloupe matched well with the extract retention and release characteristics as well as the antimicrobial efficacy of the films. The results of this work illustrate the feasibility of using a suitable drying technology to improve the quality of antimicrobial films from edible biopolymer and form the basis for future development of an improved antimicrobial film production process.

Composition Profiles and Functional Properties of Dietary Fiber Powder from Lime Residues: Effects of Pretreatment and Drying Methods

Lime residues after juice extraction have proven to be a potential raw material for producing dietary fiber (DF) powder due to their good functional properties. Compositions and antioxidant activity of DF powder from lime residues as affected by selected pretreatment (hot-water blanching and ethanolic soaking) and drying methods, viz. hot air drying, vacuum drying, and low-pressure superheated steam drying (LPSSD) at 60–80°C, were investigated. Fresh lime residues contained significant amounts of vitamin C, phenolic compounds, and flavonoids. Hesperidin was a major flavonoid and only one polymethoxyflavone (i.e., tangeretin) was detected in small amounts. A decrease in the amount of interested bioactive compounds and their antioxidant activity was noted at almost all steps of processing. Higher retention of bioactive compounds was noted when the residues were subject either to vacuum drying or LPSSD; the total antioxidant activities were 61–62% and 81–82% when being assessed by the β-carotene bleaching and DPPH assays, respectively. Vacuum drying at 80°C was the most suitable condition for preparing DF powder from lime residues due to its short required drying time and its ability to retain bioactive compounds. The in vitro analyses imply that DF powder prepared by vacuum drying at 80°C has the potential to reduce blood glucose and cholesterol levels by exhibiting high glucose retardation index (GRI) and bile acid retardation index (BRI).

Bioactive Compounds and Bioactivities of Centella asiatica (L.) Urban Prepared by Different Drying Methods and Conditions

Centella asiatica (L.) Urban has attracted significant research and commercial interest due to its many health-promoting bioactive compounds, especially phenolic compounds and triterpene saponins, which possess several functional capacities, including antioxidant activity and antimicrobial activity. Prior to its use, however, C. asiatica usually needs to be dried to extend its storage life and to prepare the material for subsequent pharmaceutical processing. The present study investigated the effects of selected drying methods and temperature on the drying characteristics, phenolic compounds, triterpene saponins, antioxidant activity, and antimicrobial activity of dried C. asiatica. Hot-air drying (HAD), combined infrared–hot air drying (IR-HAD), and low-pressure superheated steam drying (LPSSD) were carried out at various temperatures (50, 60, 70°C). The results showed that higher drying temperatures led to higher drying rates but to lower levels of total phenolic compounds, total triterpene saponins, antioxidant activity, and antimicrobial activity. At the same drying temperature, IR-HAD resulted in the highest drying rates; this was followed by LPSSD and HAD in a descending order. Nevertheless, LPSSD resulted in the highest levels of the tested bioactive compounds, antioxidant activity, as well as antimicrobial activity. LPSSD at 50°C yielded dried C. asiatica of the best overall quality.

Enhancement of sulforaphane content in cabbage outer leaves using hybrid drying technique and stepwise change of drying temperature

Sulforaphane is a hydrolysis product of glucosinolates, which are plant secondary metabolites found in various kinds of Brassica vegetables including white cabbages (Brassica oleracea L. var. capitata). Sulforaphane is of interest as it is claimed to possess chemoprotective effect against an array of cancers. The formation of sulforaphane is via enzymatic reactions and the rates of formation and degradation are strongly dependent on the temperature. Careful regulation of the material temperature during drying to maximize the formation and minimize the degradation of sulforaphane should therefore be possible. In this study, the effect of stepwise change of medium temperature during hot air drying, vacuum drying and hybrid drying, i.e., low-pressure superheated steam drying (LPSSD) followed by vacuum drying, on the evolution of sulforaphane in white cabbage outer leaves was investigated; the results were compared with those in the cases of constant drying medium temperature. Similar evolution patterns of sulforaphane were noted in all cases; sulforaphane first increased before reaching the maximum and then decreased towards the end of drying. Nevertheless, the cabbages undergone LPSSD at 60°C for 10min and then vacuum drying at 45°C until reaching the final moisture content exhibited the highest amount of sulforaphane.

Physical and mechanical properties of chitosan films as affected by drying methods and addition of antimicrobial agent

Incorporation of an antimicrobial agent into a biopolymer matrix not only affects the antimicrobial property, but also the physical, mechanical and barrier properties of the resulted films. In addition to the effect of antimicrobial agent different drying methods that are used to prepare the films should also be considered. The aim of this study was to investigate the combined effects of drying methods, namely, hot air drying and low-pressure superheated steam drying (LPSSD), and addition of a natural antimicrobial agent, namely, galangal extract, at concentrations of 0%, 0.6%, 0.9%, 1.2% and 1.5% (w/w film forming solution) on selected physical properties (thickness, moisture content, color and opacity), mechanical properties (tensile strength and percent elongation), barrier properties (water vapor and oxygen permeabilities), degree of crystallinity and degree of swelling of antimicrobial films prepared from chitosan. X-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy as well as dynamic mechanical analysis (DMA) were also performed and their results used to explain the observed combined effects. The combined effects were noted to have a statistically insignificant influence on the moisture content, thickness and water vapor permeability of the films. On the other hand, the combined effects had a statistically significant influence on the color, degree of crystallinity, degree of swelling, tensile strength, percent elongation and oxygen permeability of the films. Only the galangal extract concentration had a statistically significant influence on the film opacity. The DMA patterns and FTIR spectra confirmed the formation of crosslinkage interactions between the galangal extract and chitosan matrix.