Rehydration of Freeze‐Dried and Convective Dried Boletus edulis Mushrooms: Effect on Some Quality Parameters

ABSTRACT: Quality of rehydrated products is a key aspect linked to rehydration conditions. To assess the effect of rehydration temperature on some quality parameters, experiments at 20 and 70 °C were performed with convective dried and freeze‐dried Boletus edulis mushrooms. Rehydration characteristics (through Peleg's parameter, k₁, and equilibrium moisture, W_e), texture (Kramer), and microstructure (Cryo‐Scanning Electron Microscopy) were evaluated. Freeze‐dried samples absorbed water more quickly and attained higher W_e values than convective dried ones. Convective dehydrated samples rehydrated at 20 °C showed significantly lower textural values (11.9 ± 3.3 N/g) than those rehydrated at 70 °C (15.7 ± 1.2 N/g). For the freeze‐dried Boletus edulis, the textural values also exhibited significant differences, being 8.2 ± 1.3 and 10.5 ± 2.3 N/g for 20 and 70 °C, respectively. Freeze‐dried samples showed a porous structure that allows rehydration to take place mainly at the extracellular level. This explains the fact that, regardless of temperature, freeze‐dried mushrooms absorbed water more quickly and reached higher W_e values than convective dried ones. Whatever the dehydration technique used, rehydration at 70 °C produced a structural damage that hindered water absorption; consequently lower W_e values and higher textural values were attained than when rehydrating at 20 °C.     

PRODUCTION OF A LOW-CHOLESTEROL SHRIMP USING SUPERCRITICAL EXTRACTION

A low‐cholesterol shrimp was produced using supercritical extraction. The processing sequence included freeze drying, cholesterol extraction and rehydration. The shrimp was freeze dried, kept under vacuum until an experimental central composite rotatory design was applied using Response Surface Analysis for the supercritical extraction process. Three variables at five levels each were tested during the experiment (pressure, volume and temperature). After the extraction procedure, various rehydration and cooking conditions were applied to obtain a processed product with characteristics similar to those of the natural shrimp. Two sensory analyses were performed: one which compared the attributes of fresh shrimp with those of the freeze‐dried and rehydrated products, and another one which compared the acceptability between fresh shrimp and low‐cholesterol shrimp after freeze drying, supercritical extraction and rehydration. Under the conditions of 310 bar, 1875 L of carbon dioxide and 37C, it was possible to obtain a low‐cholesterol shrimp with acceptable organoleptic properties.     

Effect of Rehydration Temperature on Functional Properties,Antioxidant Capacity and Structural Characteristics of Apple (Granny Smith) Slices in Relation to Mass Transfer Kinetics

Apple slices dried at 60C were rehydrated at 20, 40 and 60C to analyze the influence of processing temperature on quality attributes and rehydration kinetics. Diffusion coefficient increased with process temperature from 1.36 to 2.37 × 10^?9 m²/s. The Weibull model obtained the best fit quality for the experimental data based on statistical test, chi square. Color was not recovered during rehydration and the results indicated that the use of low temperatures is more adequate. Water‐holding capacity decreased, while rehydration ratio increased with increasing rehydration temperature, indicating structural modifications. Increasing rehydration temperatures led to a reduction in the glass transition temperature and hardness values of samples. The radical‐scavenging activity showed higher antioxidant activity at higher rehydration temperatures rather than at lower temperatures. It was found that rehydration temperature modifies the cell structure and antioxidant capacity of final product.     

Kinetics of water absorption and soluble‐solid loss of hot‐air‐dried carrots during rehydration

Cubes of carrots dried at 60, 70, 80 and 90 °C were rehydrated at 20 and 95 °C. Kinetics of water gain and soluble‐solid loss were monitored and described by Peleg model. Dry matter holding capacity, water absorption capacity and rehydration ability were derived to assess the rehydration process. It was found that leaching flow was higher for the samples dried at higher temperatures. Rehydration ability and dry matter holding capacity of the samples rehydrated at 95 °C were higher than those observed for the samples soaked at 20 °C. Swelling of carrots during rehydration neither depended on temperature of drying nor on the temperature of the water. The results indicate that the use of low drying temperatures is suitable for preserving the quality attributes of carrots after rehydration.     

Structural and physical properties of dried Anaheim chilli peppers modified by low‐temperature blanching

The effect of low‐temperature blanching and drying processes on the ultrastructural and physical properties of Anaheim chilli pepper was studied and optimum conditions to provide a final product with maximum firmness were determined. Lots of Anaheim pepper were blanched in water for 4 min at 48, 55, 65, 75 and 82 °C and maintained for hold times of 35, 45, 60, 75 and 85 min, blanched again for 4 min at 96 °C and dehydrated at 53, 60, 70, 80 and 87 °C. After treatment the samples were rehydrated in water at 30 °C. Rehydration ratio, texture and structural changes were evaluated. Optimisation used a second‐order rotatable central composite design. Texture and rehydration ratio were affected by blanching temperature and the interaction of blanching temperature with hold time (p ≤ 0.05); drying temperature did not show a significant effect. The best results, ie those which gave greatest firmness, were obtained by blanching at 64 °C for 4 min, holding for 55 min after blanching, followed by a second blanching at 96 °C for 4 min and then drying at 70 °C. Evaluation of the rehydrated dried pepper by microscopy showed that low‐temperature blanching close to the optimum conditions provided a protective effect in maintaining cell wall integrity. The results of processing increased firmness in the rehydrated product by a factor of 1.97. Copyright © 2003 Society of Chemical Industry     

干燥方式对复水香菇感官、质构及营养品质的影响

为获得复水性较好的干香菇,研究了热风干燥（hot-air drying,HAD）、中短波红外干燥（infrared radiation drying,IRD）和真空冷冻干燥（freeze drying,FD）香菇的复水特性及复水后的综合品质。结果表明：FD香菇具有均匀的多孔结构,使得干香菇的复水速率明显高于HAD香菇和IRD香菇,复水60?min即可达最大复水体积比（9.22）,且复水香菇的外观与鲜样接近。同时,复水后FD香菇的剪切力（1?352.99?g）和咀嚼性（232.54?g）最小,表明其质地柔软、易被切分。此外,FD复水香菇和其他两种复水香菇具有相似的电子鼻响应值曲线,表明其香气无差异。复水时FD香菇的蛋白质和VB2的溶出率最高,分别为0.42?mg/g和0.71×10－3?mg/g,说明FD复水香菇富含营养成分,且加工时较易溶出。综上,FD可作为一种较好的干燥方式来制备复水性较好的高品质香菇。     

Rehydration Studies on Pretreated and Osmotically Dehydrated Apple Slices

ABSTRACT: The influence of different pretreatments (freezing, blanching, high electric field pulse, and high pressure) and osmotic dehydration (OD) times (0 to 6 h) on some characteristics of rehydrated apple was investigated. Pretreated apple slices were osmotically dehydrated, oven dried, and rehydrated in distilled water at room temperature. Rehydration capacity (RC) increased with OD time. Blanched and prefrozen samples had higher RC, firmer rehydrated samples, and greater dry-matter loss than the other pretreated samples. There was no change (P > 0.05) in the color of the samples before and after rehydration. The electrical conductivity of the immersion medium increased with rehydration time but decreased with OD time.     

APPLICATION OF RESPONSE SURFACE METHODOLOGY FOR THE OSMOTIC DEHYDRATION OF CARROTS

Osmotic dehydrations of carrot cubes in sodium chloride salt solutions at different solution concentrations, temperatures and process durations were analyzed for water loss and solute gain. The osmotically pretreated carrot cubes were further dehydrated in a cabinet dryer at 65C and were then rehydrated in water at ambient temperature for 8–10 h and analyzed for rehydration ratio, color and overall acceptability of the rehydrated product. The process was optimized for maximum water loss, rehydration ratio and overall acceptability of rehydrated product, and for minimum solute gain and shrinkage of rehydrated product by response surface methodology. The optimum conditions of various process parameters were 11% salt concentration, 30C osmotic solution temperature and process duration of 120 min.     

Survival and growth of B. cereus group and presumptive Cronobacter spp. contaminated naturally in rehydrated sunsik

Survival and growth of Bacillus cereus group and presumptive Cronobacter in the rehydrated sunsik were investigated to control the pathogens. Five sunsik contaminated naturally by the pathogens were rehydrated with hot waters of 50, 60, 65, and 70°C and kept for 24 hr at 5 and 25°C. Survival and growth of B. cereus seemed to be influenced not by rehydration water temperature, but by storage temperature and time after rehydration. However, survival and growth of Cronobacter was influenced by both rehydration temperature and storage conditions. Especially, storage temperature and time seemed to be more important than the rehydration temperature to limit the growth of Cronobacter. Therefore, it is required to take immediately the sunsik after rehydration with at least 65°C water, otherwise within 6 hr at room temperature for the risk reduction of B. cereus and Cronobacter.     

AMINO ACID COMPOSITION AND IN VITRO PROTEIN DIGESTIBILITY OF DRIED SQUID REHYDRATED IN WATER AND ALKALI

Crude protein content, amino acid composition and in vitro protein digestibility of fresh, dried and rehydrated Argentina squid (Illex argentinus) were studied. Most of the nonamino acid nitrogenous compounds in the mantle muscle dissolved in water during rehydration. Rehydration improved the nutritive value of protein in dried squid. Rehydration decreased the concentration of proline relative to total amino acids in muscle. Aromatic amino acids were limiting essential amino acids in fresh and dried squid but not in rehydrated squid. Amino acid scores increased with the extent of rehydration, more so in alkali than in water. The alkali rehydration also increased the in vitro protein digestibility of dried squid.     

Rehydration of air-dried Shiitake mushroom (Lentinus edodes) caps: Comparison of conventional and vacuum water immersion processes

Air-dried Shiitake mushroom (Lentinus edodes) caps were rehydrated using conventional and vacuum techniques, at different temperatures (30, 40, 50 and 80 °C) and times (1, 2, 3, 6, 9, 12, 15, 18, 21, 120, 150 and 210 min). Three empirical models were used to model the rehydration process: 1st order kinetics, Peleg’s model and the Weibull distribution function. To determine the effects of temperature and pressure on the mushroom caps, the texture of the rehydrated material was determined, evaluating peak shear force and microstructure using cold-stage scanning electron microscopy (cryo-SEM). The rehydration of the mushrooms was found to be temperature and pressure-dependant. The speed of vacuum rehydration was faster compared to conventional rehydration. When rehydrated at 80 °C, a loss in rehydration capacity was observed in both cases, which can be explained by the structural damage and cell shrinkage which occurred during air-drying. Also, the vacuum rehydrated samples had the lowest toughness values. It was concluded that vacuum rehydration of dried Shiitake mushroom caps could be a feasible alternative to the conventional rehydration process, resulting in a lower immersion time and in a desirable texture.     

Microwave-Assisted Pulse-Spouted Bed Freeze-Drying of Stem Lettuce Slices—Effect on Product Quality

Drying uniformity, microstructure, apparent density, rehydration, and texture properties were measured to evaluate the quality of stem lettuce slices dried in a pulse-spouted bed microwave freeze dryer. Drying was carried out in a 5-cm (o.d.) vacuum chamber at 80?±?5 Pa, mean microwave power level of 3.2 W?g^?1 and pulse-spouting time interval of 10 min. Results show that microwave freeze-dried products in the pulse-spouted mode dried more uniformly as compared to those dried in steady spouted bed mode. Pulse-spouted bed mode also resulted in dried stem lettuce slices with lower discoloration, more uniform and compact microstructure, higher rehydration capacity (RC) as well as greater hardness after rehydration over shorter drying time relative to those obtained in a steady spouting condition.     

EFFECTS OF BLANCHING, CHEMICAL TREATMENTS AND FREEZING METHODS ON QUALITY OF FREEZE-DRIED MUSHROOMS

SUMMARY –Mushrooms, Agaricus bisporus, were cut into slices about 5 mm thick. The polyphenoloxidase activity in the sliced mushrooms was inhibited by: (a) dipping in sodium metabisulfite solution containing 200 ppm SO₂; (b) dipping in 2% NaCl solution; and (c) blanching in boiling water for 2 min followed by evaporative cooling. The products were frozen with Freon-12 at –22°F for 60 sec. The frozen mushrooms were dried to 3% moisture in a Stokes freeze drier and sealed under vacuum in No. 2–1/2 cans. The freeze-dried product made by the Freon-dipping process was better in quality and firmer in texture than that made by the slow-freezing process. Blanching the fresh mushrooms in boiling water for 2 min prior to freezing results in a lighter color in the freeze-dried products after rehydration. However, the product was less attractive in flavor and texture. The blanching process caused considerable loss of water-soluble solids and ascorbic acid. The effects of processing variables on the chemical, physical and organoleptic quality of the rehydrated products are presented.     

复水温度对香菇复水特性及品质的影响

香菇是中国产量最高和市场消费量最大的食用菌。干制是香菇的主要加工方式,而干香菇需复水后才能作为原料被烹调或进一步加工。本研究将热风干燥的香菇分别在25,45,65℃和85℃下复水,研究香菇复水过程中复水比、外观、质构和风味品质的变化。结果表明,在45℃复水270 min,干香菇吸水后表面的褶皱较为舒展,微观结构显示其菌丝恢复膨胀且伸展。同时,其复水比稳定保持在3.88,剪切力(1 290.28 g)、弹力(0.66 g)和复原力(0.34 g)适中,咀嚼性(656.02 g)最大,说明香菇复水完全且质地柔软适中,易被切开。此外,45℃复水香菇所含特征含硫化合物的含量较高,以1,2,3,5,6-五硫杂环庚烷(67.02%)和1,2,4-三硫环戊烷(39.06%)为主,风味较浓郁。综上,复水温度显著影响香菇复水后的食用品质,将干香菇在45℃复水270 min可获得品质较好的复水香菇。     

Determination of microwave drying and rehydration kinetics of green peppers with the bioactive and textural properties

The aim of this study was to determine the effect of microwaves on drying and rehydration kinetics of green pepper varieties (sweet, green, and bell) and the bioactive and textural properties of dried and rehydrated peppers. Drying was performed at 90, 180, and 90 + 180 W and dried peppers were rehydrated at 25, 50, and 70°C. The best drying fits were obtained using the equations of both Sigmoid and Hii et al. for drying, and two term exponential decay was the most appropriate model for rehydration. The lowest and highest final rehydration ratio values were obtained for the peppers dried at 180 and 90 W, respectively at each temperature and pepper variety. Among peppers, bell pepper showed the lowest color change (ΔE) and it was the most potent to recover initial color in rehydration. Peppers had softer texture in both drying and rehydration treatment. Total phenolic content of peppers was reduced around 56–65% compared to the fresh pepper by drying and this decrement was reached up to 87% in rehydrated peppers. Results showed that drying at 180 W and rehydration at 70°C provided faster drying and rehydration as well maintained the quality characteristics of green peppers.     

An evaluation of the retention of quality characteristics in fresh and freeze-dried alpine strawberries

Alpine strawberries (Fragaria vesca) are used in fruit juice, marmalade, or jam production and as a result have economical importance in food sector. The fresh alpine strawberries have a tendency to lose their quality in a few days as a result of high water loss and spoilage. In this paper, the results of a study on the effects of freeze drying process on the characteristics of the alpine strawberries, such as firmness, sugar content, pH, colour, weight loss, dissolved solids, anthocyanin and vitamin C content with reference to the fresh, are reported. Freeze‐drying indicated no difference in the characteristics of the alpine strawberries when compared with the fresh. It is found that a slight acid or base addition onto the rehydrated alpine strawberry juice preserved the stability of pigments and the colour. In addition, the rehydrated alpine strawberry juice exhibited an antimicrobial activity towards an important foodborne pathogen, Enterobacter faecium ATCC 6057.     

Stability of anthocyanins in frozen and freeze-dried raspberries during long-term storage: in relation to glass transition

Abstract: Anthocyanins, natural plant pigments in the flavonoid group, are responsible for the red color and some of the nutraceutical benefits of raspberries. This study explores anthocyanin degradation in frozen and freeze‐dried raspberries during storage in relation to glass transition temperatures. Frozen raspberries were stored at ?80, ?35, and ?20 °C, while freeze‐dried raspberries were stored at selected water activity (a_w) values ranging from 0.05 to 0.75 at room temperature (23 °C) for more than a year. The characteristic glass transition temperatures (T′_g) of raspberries with high water content and glass transition temperature (T_g) of raspberries with small water content were determined using a differential scanning calorimeter. The pH differential method was used to determine the quantity of anthocyanins in frozen and freeze‐dried raspberries at selected time intervals. The total anthocyanins in raspberries fluctuated during 378 d of storage at ?20 and ?35, and ?80 °C. Anthocyanin degradation in freeze‐dried raspberries ranged from 27% to 32% and 78% to 89% at a_w values of 0.05 to 0.07 and 0.11 to 0.43, respectively, after 1 y. Anthocyanins were not detectable in freeze‐dried raspberries stored at a_w values of 0.53 to 0.75 after 270 d. First order and Weibull equations were used to fit the anthocyanin degradation in freeze‐dried raspberries. The 1^st‐order rate constant (k) of anthocyanin degradation ranged from 0.003 to 0.023 days^?1 at the selected water activities. Significant anthocyanin degradation occurred in both the glassy and rubbery states of freeze‐dried raspberries during long‐term storage. However, the rate of anthocyanin degradation in freeze‐dried raspberries stored in the glassy state was significantly smaller than the rate of anthocyanin degradation in the rubbery state.     

Viscoelastic behaviour of dehydrated products during rehydration

Viscoelastic behaviour of dehydrated products during rehydration of apple, banana, carrot and potato was examined under uniaxial compression tests. Samples were dehydrated with four different drying methods: conventional, vacuum, freeze and osmotic-freeze drying and after that they were rehydrated in an air dryer at 50°C and 80% air humidity. Compression tests were performed during rehydration for various moisture contents, ranging from 0.01 to 1.5 kg/kg dry basis. The viscoelastic behaviour of dehydrated products during rehydration were examined comparing the values of the four parameters incorporated into the stress–strain model, during rehydration with those of dehydrated products. The four examined parameters: maximum stress, maximum strain, elastic parameter and viscoelastic exponent seem to show a hysteresis phenomenon. It can be concluded that dehydrated product do not keep their viscoelastic behaviour after rehydration due to structural damages that occur during drying. More specifically, freeze dried materials present the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pretreatment seems to help freeze dried materials to keep their elastic nature, probably due to solids gain. Air and vacuum dried materials showed the smallest hysteresis tendency, keeping their viscoelastic characteristics during rehydration close to those of dried materials.     

Pulsed electric field treatment of carrots before drying and rehydration

BACKGROUND: In this study the effects of pulsed electric field (PEF) pretreatment were evaluated during drying and rehydration of carrots. Carrots pretreated with an electric field intensity of 1 kV cm^?1 (capacitance 0.5 µF, 20 pulses) or 1.5 kV cm^?1 (capacitance 1 µF, 20 pulses) as well as blanched (100 °C, 3 min) carrots were used for the study. Following pretreatment, samples were oven dried at 70 °C and then rehydrated in distilled water (1:30 w/v) at room temperature (24 ± 1 °C). RESULTS: PEF pretreatment increased the drying rate of carrots. However, the rehydration rate of PEF‐pretreated carrots was lower than that of blanched carrots. There were no colour differences between PEF‐pretreated and blanched carrots before drying and after rehydration. In terms of texture, PEF‐pretreated carrots were firmer than blanched carrots. PEF pretreatment reduced the activity of peroxidase by 30–50%, while blanching completely inactivated the enzyme (>95%). CONCLUSIONS: Overall, the results suggest that PEF could be an effective pretreatment during drying and rehydration of carrots. Copyright © 2009 Society of Chemical Industry     

Physicochemical properties and storage stability of spray‐dried soya bean sprout extract

Amylase is a very important enzyme due to its wide food applications. To preserve amylase activity in soya bean sprout extract (SSE), SSEs were spray‐dried with 10% maltodextrin and 0–3% alginic acids, and their physicochemical properties and storage stability were compared with freeze‐dried one. SSE exhibited maximum amylase activity at pH 7.0 and 60 °C, with the most active substrate specificity towards soluble starch. Spray‐dried SSEs exhibited higher water solubility index (WSI) and in vitro relative amylase activity but lower water vapour sorption (WVS) and smaller particle size than freeze‐dried SSE. For spray‐dried SSEs, particle size, WSI and in vitro relative amylase activity increased while WVS decreased with increasing % alginic acid. This study demonstrated that spray drying of SSE, especially with 10% maltodextrin and 2% alginic acid, was effective in keeping amylase active and stable during 7‐week storage at room temperature (25 °C).