Comparison of Vapor and Liquid Isotherms for Casein and Casein-Sucrose Mixture

Hydration by vapor sorption and liquid water addition for the effect on sorption isotherms of casein and a casein-sucrose mixture was compared. Both isotherm data and NMR relaxation rates were obtained from the same samples. Data showed biphasic linear plots with breaks at the same water activity (a_w). At a given moisture content, liquid hydration of casein showed a lower a_w and a lower NMR mobility than the vapor sorbed samples. The casein-sucrose mixture showed the opposite. As a_w was increased above 0.90, casein alone showed decreasing difference between vapor and liquid hydration while the mixture showed rapidly diverging lines. These isotherm differences between vapor and liquid hydration were corroborated by NMR.     

Non-enzymatic browning kinetics analysed through water–solids interactions and water mobility in dehydrated potato

Non-enzymatic browning (NEB) development was studied in dehydrated potato at 70 °C. It was related to the macroscopic and molecular properties and to water–solid interactions over a wide range of water activities. Time resolved ¹H NMR, thermal transitions and water sorption isotherms were evaluated. Although non-enzymatic browning could be detected in the glassy state; colour development was higher in the supercooled state. The reaction rate increased up to a water content of 26 g/100 g of solids (a_w = 0.84) and then decreased at higher water contents, concomitantly with the increase of water proton mobility. The joint analyses of NEB kinetics, water sorption isotherm and proton relaxation behaviour made it evident that the point at which the reaction rate decreased, after a maximum value, could be related to the appearance of highly mobile water. The results obtained in this work indicate that the prediction of chemical reaction kinetics can be performed through the integrated analysis of water sorption, water and solids mobility and the physical state of the matrix.     

Characterization of Polymer and Solute Bound Water by Pulsed NMR

Pulsed NMR signals from water in combination with two polymers and two solutes, individually and in mixtures, was measured over a_w range 0.75–0.95. Both spin-lattice (T₁) and spin-spin (T₂) relaxation curves showed negligible slopes for water with starch and casein (polymer water) and large for water with sucrose and salt (solute water). Mixtures of polymer and solute waters showed intermediate slopes. Dilution of a sucrose solution gave T₁ and T₂ responses approaching that of pure water. T₂ from NaCl indicated less and from sucrose more water structure than pure water. NMR data coincided with sorption isotherms. It was concluded that polymer and solute waters show different NMR responses and can coexist in a food.     

Evaluation of Water Binding Capacity (WBC) of Food Fiber Sources

The Baumann capillary suction apparatus, the AACC standard centrifugation method, moisture sorption isotherm values and freezing point by cryoscopic osmometry were used to measure the water binding capacity (WBC) of several fiber sources. Although each method was able to distinguish between ingredients of different WBC's, the values obtained by the four methods were very different from each other. A significant correlation was only found between the Baumann data and the moisture sorption data for fruit, cereal and legume fibers, while no method correlated with the WBC as determined by the cryoscopic method. This suggests that the different methods evaluate different mechanisms of water binding and that one must carefully choose the proper method in predicting the water binding functionality of fibers in foods.     

Phase transitions of dairy proteins,dextrans and their mixtures as a function of water interactions

The water sorption behaviour and phase transitions of dairy proteins (β-casein and β-lactoglobulin), dextrans (dextran6 and dextran500) and their mixtures were studied at low water content. Freeze-dried polysaccharide samples containing between 20 and 80% dairy protein were equilibrated at different water activities (a_w) between 0.11 and 0.75, at 25 °C. Water sorption isotherms of pure compounds and mixtures, as well as glass transition at different water activities were determined. Crystallization of polysaccharides was also investigated. BET and Gordon and Taylor equations were used to model water adsorption isotherms and glass transition temperature behaviour, respectively. Polysaccharides showed a higher water adsorption capacity than dairy proteins in the range of a_w studied, which decreased with the addition of protein. The addition of β-casein decreased the Tg values of dextran systems. This effect was attributed to water migration from β-casein to the polysaccharide fraction following the formation of β-casein hydrophobic interactions. Likewise, dairy proteins provoked an increase in the temperature of dextran crystallization and a decrease in the enthalpy. This effect did not reflect the increase of dextran molecular mobility in the presence of β-casein but could be masked by other factors, like steric hindrance. The effect of dairy proteins, especially β-casein, on the phase transitions of polysaccharides should be considered for controlling the Maillard reaction, as well as physical and chemical changes that occur during processing and storage of food systems.     

Pulsed electric fields effect on mechanical and sorption properties of dried apple tissue

Electroporated and dried apples were characterised in terms of physical properties, such as sorption profile and mechanical test, in order to understand the treatment effects. Thermodynamic relationship between water activity and equilibrium moisture content at constant temperature and pressure has been mathematically expressed by sorption isotherm. High coefficient of determination of BET equation fitting was obtained, ranging from 0.974 to 0.990 (RMSE 0.07–0.08). The highest electric field strength (1.5 kV/cm) reveals a change in sorption isotherm shape. The III type of isotherm observed indicates a major presence of crystalline molecules increasing a water surface interaction.Compression test were applied to the samples at different hydration level and PEF intensity applied. The plasticizing effect was observed as a function of hydration level raise and involved a reduction of the maximum force from about 14.85 (±2.44) N to 8.45 (±0.59) N. The opposite anti-plasticizing effect was a consequence of electric filed strength increase. Particularly, at low hydration level, where plasticizing effect did not produce softening of structures, PEF treatment induced a toughness and stiffness increase, revealing the anti-plasticizing effect. Fermi equation showed high coefficient of determination (0.902–0.959) and Y₀ value change from 17 to 35, as a function of PEF treatment intensity increase, confirming the structural differences in the analysed samples.Thus, main results revealed the modification of water holding capacity induced by PEF and drying. Sorption profile and mechanical properties well described plasticizing and anti-plasticizing induced effect, underling the enhanced mobility of the system promoted by treatments.     

Effect of microbial transglutaminase on NMR relaxometry and microstructure of pork myofibrillar protein gel

The effect of microbial transglutaminase (MTG) on nuclear magnetic resonance relaxation (NMR) behaviour, water holding capacity (WHC) and microstructure of pork myofibrillar protein (PMP) gel was studied. The enzymatic protein preparation had significantly lower values of spin–spin relaxation time (T ₂), higher WHC and more porous microstructure in comparison with the control system. T ₂ was reduced from 226 ms (peak value) of the PMP gel containing no MTG to 188 ms of the PMP gel containing 2 U/g protein. However, no further decrease was shown when the concentration of MTG increased. The reduction was attributed to reduced mobility of water protons in the system. Scanning electron micrographs (SEM) showed the mobility of water in the proteins gel network was related to gel microstructure.     

Effects of Water-Glycerol and Water-Sorbitol Interactions on the Physical Properties of Konjac Glucomannan Films

Konjac glucomannan (KGM)‐edible films were prepared with different amounts of glycerol or sorbitol as a plasticizer. Films were characterized by moisture sorption isotherm, and following conditioning at different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbitol and glycerol) were found to plasticize KGM‐based films with respect to their tensile properties. However, thermal properties and water sorption capacity (WSC) of polyolplasticized KGM films were found to vary with water activity (a_w), namely at low a_w (< 0.6), WSC and melting enthalpy were decreased with increasing in polyol content and the opposite was true at higher a_w (>0.6). This was attributed to extensive interactions between plasticizer and KGM that reduced the available active site (‐OH groups) for water adsorption. The presence of polyols at low a_w appeared to suppress crystalline structures due probably to restricted molecular mobility. These effects were diminished when the moisture content was >20%.     

Scaling approach to water sorption isotherms of hydrogels and foods

Mechanisms affecting water sorption isotherms of foods, at high water activities, were evaluated by the processes of swelling and shrinking of model systems of uncharged and charged hydrogels. Their water activity was found to be determined by the combined effect of their osmotic pressure and the network pressure of the polymeric network. The scaling approach in polymer science indicates that in an uncharged network the osmotic pressure is affected by the mobility of the polymer segments and thus by the degree of cross-linking. However, in charged gels, which behave in a similar manner to many food systems, the counterions play the dominant role in determining the osmotic pressure. The network pressure may shift from a positive to a negative value during shrinking (dehydration), thus counteracting or co-operating with the osmotic pressure in decreasing the water activity. A scaling sorption isotherm, at high water activity, is formulated for a food system with physically significant parameters.     

Desorption Isotherms and Isosteric Heat of Three Tunisian Date Cultivars

The sorption isotherm of three Tunisian cultivars, Allig, Kentichi, and Deglet Nour, were obtained at 40, 60 and 80 °C. The static gravimetric method was used, according to the COST 90 recommendations. The overall shape of the curves describing the water content as a function of water activity was typical of sugar-rich materials. A difference between varieties exists for the sorption isotherm, thus for the drying behavior. Data were compared with respect to several isotherm models. It was found that the GAB and the BET equations could satisfactorily represent the sorption data up to about 0.9 relative humidity. Through the BET equation, monolayer moisture content was calculated for the isotherm zone corresponding to monomolecular adsorption. In addition, isosteric heat of sorption was determined from sorption data using the Clausius–Clapeyron equation.     

Examination of Starch-Water and Cellulose-Water Interactions With Near Infrared (NIR) Diffuse Reflectance Spectrospocy

Starch-water and cellulose-water interactions were examined with NIR diffuse reflectance spectroscopy. Samples of native potato, wheat, and corn starches and microcrystalline cellulose (MCC) were equilibrated to water activities (a_w) ranging from 0.11 to 0.94 a_w. A modified B.E.T. model was applied to the water vapor sorption isotherm (T=25°C) data. NIR spectra (1000 to 2640 nm) revealed no abrupt changes in molecular structure. However, as has been suggested in the literature, there appear to be two species of non-freezable water whose relative quantities change as a_w varies. It is hypothesized that the first species represents water within the monolayer, and the second species represents multilayer water. NIR spectra of starch suggests that some multilayer water is present before the monolayer is completed. Spectra of MCC corroborate the existence of two species of higly associated water. The isotherm model was highly successful in describing sorption behavior over the a_w range examined.     

Examination of Starch-Water and Cellulose-Water Interactions With Near Infrared (NIR) Diffuse Reflectance Spectroscopy

Starch-water and cellulose-water interactions were examined with NIR diffuse reflectance spectroscopy. Samples of native potato, wheat, and corn starches and microcrystalline cellulose (MCC) were equilibrated to water activities (a_w) ranging from 0.11 to 0.94 a_w. A modified B.E.T. model was applied to the water vapor sorption isotherm (T=25°C) data. NIR spectra (1000 to 2640 nm) revealed no abrupt changes in molecular structure. However, as has been suggested in the literature, there appear to be two species of non-freezable water whose relative quantities change as a_w varies. It is hypothesized that the first species represents water within the monolayer, and the second species represents multilayer water. NIR spectra of starch suggests that some multilayer water is present before the monolayer is completed. Spectra of MCC corroborate the existence of two species of highly associated water. The isotherm model was highly successful in describing sorption behavior over the a_w range examined.     

Empirical Model for Water Uptake and Hydration Rate of Food Powders by Sorption and Baumann Methods

The empirical model q = Q t/B + t, where q is the amount of water taken up at time t was useful to describe water uptake of several food powders determined either by the Baumann or the sorption isotherm method at 75.6 and 100% RH. The equilibrium values (Q) and the specific rate constant (K) derived from this model as (QB)^-1 could be used to characterize food materials for hydration capacity and rate. Large differences were observed in Q values between methods and between food powders. The Baumann Q values and sorption isotherm Q values at 100% RH ranked similarly water takeup of food powders (R = 0.97, p < 0.001). Baumann K values and sorption isotherm K values at 100% RH also correlated highly. However, little differences were observed among the K and Q values of different food powders determined by the sorption isotherm method at 75.6% RH.     

Moisture‐Absorption and Water Dynamics in the Powder of Egg Albumen Peptide,Met‐Pro‐Asp‐Ala‐His‐Leu

Moisture absorbed into the powder of Met‐Pro‐Asp‐Ala‐His‐Leu (MPDAHL)—a novel egg albumen antioxidant peptide—profoundly affects its properties. In this study, we elucidated water dynamics in MPDAHL using DVS, DSC, and low‐field ¹H NMR. Based on the DVS data, we found that MPDAHL sorption kinetics obey a parallel exponential model. DSC results indicated that both water and heating could change the microstructure of MPDAHL. The T₂ parameters of NMR reflected the different phases of moisture absorption revealed that there were 4 categories of water with different states or mobility in the MPDAHL during the moisture absorption process. The fastest fraction T_2b mainly dominated the hygroscopicity of MPDAHL and the absorbed water significantly changed the proton distribution and structure of MPDAHL. Thus, this study shows that DVS, DSC, and low‐field ¹H NMR are effective methods for monitoring water mobility and distribution in synthetic peptides. It can be used to improve the quality assurance of functional peptides.     

Equilibrium Water Content and the State of Water in Dehydrated White Cabbage

The water sorption isotherms for freeze-dried cabbage were determined at 15°C and 23°C, and the state of the water was studied using differential scanning calorimetry (DSC). The water sorption capacity of the freeze-dried cabbage depended on the temperature. The effects of free sugars were clear in both adsorption and desorption. The thermal behavior of freeze-dried cabbage was highly moisture dependent. In the capillary condensation range (>0.9 a_w) solution of low molecular components of the freeze-dried cabbage affected its freezing behavior.     

大豆7S和11S球蛋白对面团特性及馒头品质的影响

本文研究了大豆浓缩蛋白(CSP)、7S和11S球蛋白的添加对小麦粉面团的粉质、糊化、水分分布、迁移等特性和对馒头质构及感官品质的影响。结果表明:添加CSP可使面团的稳定时间增加,弱化度减小,而添加7S和11S可使面团的稳定时间减小,弱化度增加;添加CSP、7S和11S混合粉面团的抗老化程度、持水性有强到弱依次为11S、7S、CSP;馒头的硬度、咀嚼性均有所降低,由大到小依次为CSP、7S、11S;添加3%的11S球蛋白的馒头形态、质构、口感、风味良好,感官评分最高(90分)。本研究可为CSP、7S和11S在馒头中的应用提供理论依据。     

Physicochemical Properties of 7S and 11S Protein Mixtures Coagulated by Glucono-δ-lactone

ABSTRACT: Blends of 7S and 11S proteins with added glucono-δ-lactone were investigated to study the effects of protein composition on gelation. The pH, water-holding capacity, textural, and color properties of the gels formed were studied at a constant temperature as a function of time. Generally high 11S to 7S ratios produced gels of higher hardness, cohesiveness, gumminess, and L values than those of the rest. 11S formed faster acid-induced gels compared with those containing low proportions of 11S. From the data, it was predicted that fractions of 7S:11S differing by 1:10 will form gels with similar physicochemical properties when the coagulating times (at 60 °C) differed by 20 min.     

Microencapsulated Lactobacillus rhamnosus GG powders: relationship of powder physical properties to probiotic survival during storage

Freeze-dried commercial Lactobacillus rhamnosus GG (LGG) were encapsulated in an emulsion-based formulation stabilized by whey protein and resistant starch and either spray-dried or freeze-dried to produce probiotic microcapsules. There was no difference in loss of probiotics viability after spray drying or freeze drying. Particle size, morphology, moisture sorption, and water mobility of the powder microcapsules were examined. Particle size analysis and scanning electron microscopy showed that spray-dried LGG microcapsules (SDMC) were small spherical particles, whereas freeze-dried LGG microcapsules (FDMC) were larger nonspherical particles. Moisture sorption isotherms obtained using dynamic vapor sorption showed a slightly higher water uptake in spray-dried microcapsules. The effect of water mobility, as measured by nuclear magnetic resonance (NMR) spectroscopy, at various water activities (a(w) 0.32, 0.57, and 0.70) and probiotic viability during storage at 25 °C was also examined. Increasing the relative humidity of the environment at which the samples were stored caused an increase in water mobility and the rate of loss in viability. The viability data during storage indicated that SDMC had better storage stability compared to FDMC. Although more water was adsorbed for spray-dried than freeze-dried microcapsules, water mobility was similar for corresponding storage conditions because there was a stronger water-binding energy for spray-dried microcapsule. This possibly accounted for the improved survival of probiotics in spray-dried microcapsules.     

Texture and rehydration properties of texturised soy protein: analysis based on soybean 7S and 11S proteins

Soy protein, one of the most commonly used raw materials for texturised vegetable protein, has an important influence on texturised soy protein (TSP) with its 7S and 11S fractions. In this study, soy 7S and 11S proteins were extracted from soybean isolate and added back to the raw material to prepare TSP and analyse the effect of both on the physical properties of TSP. The results showed that the addition of 5% soy 7s or 11s protein increased the water-holding capacity (up to 9.04%) and rehydration rate (up to 25.71%) of TSP. Compared with adding soy 11s protein, adding soy 7s protein has a faster rehydration rate at a lower temperature (30 and 45 °C). After extrusion, the content of free sulphhydryl groups, total sulphhydryl groups, and disulphide bonds was significantly reduced (P < 0.05). The extrusion treatment caused degradation of the protein chains, and the proteins mainly formed insoluble polymers. Electrophoretic analysis revealed that the sodium dodecyl-sulphate (SDS) reducing the extractable rate of the precipitate after SDS non-reduction extraction of the TSP added with 5% soy 7S and 11S proteins were lower than that of the control. The proportion of different soybean protein components in TSP could change its texture, water-holding, and rehydration characteristics of it, which provides a new method for the characteristics design of TSP.     

Moisture adsorption isotherms of Tarhana at 25 °C and 35 °C and the investigation of fitness of various isotherm equations to moisture sorption data of Tarhana

Moisture adsorption isotherms of Tarhana were determined at 25 °C and 35 °C. As the temperature increased, the adsorbed moisture content decreased at water activities between 0.2 and 0.9. The experimental sorption data obtained were applied to the Halsey, Harkins‐Jura, Smith, BET and Freundlich isotherm equations to test fitness of these equations to Tarhana. The order of best fit or sorption data obtained for Tarhana was Halsey > Harkins‐Jura > Smith > Henderson > BET > Freundlich. The obtained sorption data fitted the Halsey, Harkins‐Jura and Smith isotherm equations quite well in the 0.2–0.9 water activity range; However, the best fit was obtained in the Freundlich, BET and Henderson isotherm equations in this water activity range. © 2000 Society of Chemical Industry