Phase transition in foods: effect of pressure and methods to assess or control phase transition

Food systems can be defined as heterogeneous and complex systems in which multiple interactions take place. In addition, food systems are unstable due to chemical and physical reactions. Food components (proteins carbohydrates, lipids, water …) might undergo phase transitions due to a pressure or temperature change. The effect of temperature and pressure on phase transitions of selected food components is discussed with emphasis on high pressure processing of food systems. The possibilities of Nuclear Magnetic Resonance and Magnetic Resonance Imaging in assessing phase transitions in food are highlighted.     

Frozen State Transitions of Sucrose-Protein-Cornstarch Mixtures

ABSTRACT: State transitions of solutions of mixtures of sucrose, albumen, gelatin, and cornstarch were studied using differential scanning calorimetry, and state diagrams were established. Maximum freeze concentration was achieved by annealing of solutions at a temperature T'_m of⁻¹°C. The onset temperatures of glass transitions of maximally freeze-concentrated solutes, T'_m, were lower and onset temperatures of ice melting, T'_g, were higher for mixtures of sucrose, proteins, and cornstarch when compared with those of pure sucrose. Solute concentrations in the maximally freeze-concentrated phase, C'_g, were 80% and 81% for sucrose and sucrose/albumin, respectively. The C'_g of sucrose/gelatin, sucrose /cornstarch, and sucrose/cornstarch/gelatin was 76%. The state diagrams established with experimental and predicted T_g values are useful for characterization of thermal phenomena and physical state of carbohydrate-protein mixtures at various water contents.     

Diffusion behavior of dextrans in dairy systems of different microstructures

The diffusion of solutes through milk and milk-derived media (i.e. dairy systems) is an important factor in the transformation and stabilization of dairy products. In several polymer systems, the diffusion of solutes largely depends on the microstructure within the material. The aim of this work was to establish how the diffusion of solutes is affected by the microstructure of dairy systems. Diffusion measurements were carried out using the technique of fluorescence recovery after photobleaching (FRAP). Diffusion coefficients of a range of 8 fluorescein isothiocyanate (FITC)–dextrans (molecular weights from 4 to 2000 kDa) were measured in dairy systems. These systems had the same casein concentration (130 g kg^− 1) and water activity but their microstructure had been altered by different treatments: heat-treatment and rennet-induced coagulation. Microstructural parameters of these systems were estimated by the image analysis of transmission electron microscopy (TEM) micrographs. The results showed that the diffusion of solutes was affected by the microstructure of dairy systems. The application of heat-treatment hindered the diffusion of all solutes (irrespective of molecular weight), whereas rennet coagulation only increased the diffusion of the larger solute molecules (> 20 kDa). At the microscopic scale, the diffusion of solutes in dairy systems was mainly explained by both the pore size within the structure and by the interface area between the solute molecules and the surrounding matrix. Larger solutes diffused faster in dairy systems with larger pore sizes and less interface area.     

Modeling polychlorinated biphenyl mass transfer after amendment of contaminated sediment with activated carbon

The sorption kinetics and concentration of polychlorinated biphenyls (PCBs) in historically polluted sediment is modeled to assess a remediation strategy based on in situ PCB sequestration by mixing with activated carbon (AC). We extend our evaluation of a model based on intraparticle diffusion by including a biomimetic semipermeable membrane device (SPMD) and a first-order degradation rate for the aqueous phase. The model predictions are compared with the previously reported experimental PCB concentrations in the bulk water phase and in SPMDs. The simulated scenarios comprise a marine and a freshwater sediment, four PCB congeners, two AC grain sizes, four doses of AC, and comparison with laboratory experiments for up to 540 days of AC amendment slowly mixed with sediment. The model qualitatively reproduces the observed shifts in the PCB distribution during repartitioning after AC amendment but systematically overestimates the overall effect of the treatment in reducing aqueous and SPMD concentrations of PCBs by a factor of 2-6. For our AC application in sediment, competitive sorption of the various solutes apparently requires a reduction by a factor of 16 of the literature values for the AC-water partitioning coefficient measured in pure aqueous systems. With this correction, model results and measurements agree within a factor of 3. We also discuss the impact of the nonlinearity of the AC sorption isotherm and first-order degradation in the aqueous phase. Regular mixing of the sediment accelerates the benefit of the proposed amendment substantially. But according to our scenario, after AC amendment is homogeneously mixed into the sediment and then left undisturbed, aqueous PCB concentrations tend toward the same reduction after approximately 5 or more years.     

Kinematic viscosity and water activity of aqueous solutions of glycerol and sodium chloride

During osmotic dehydration, a removal of water from foodstuffs is achieved with a decrease in the water activity of the food sample. This dehydration process involves the utilization of highly concentrated solutions with one or several solutes that increase considerably the viscosity of the liquid phase. This property is fundamental in the studies of mass and momentum transfer processes. Glycerol and sodium chloride are studied as systems employed in this type of process due to the advantages for the final dehydrated product. Kinematic viscosities of binary and ternary aqueous solutions of these solutes were measured at various concentrations (from 0 to up 5.0 mol kg^–1) and temperatures (from 20 up to 50 °C). Water activities for each indicated solution at 25 °C are also reported. Experimental data for both physical properties were simultaneously correlated with concentration and temperature for binary and ternary solutions with a suitable accuracy. Additionally, relationships between kinematic viscosities and water activities were established.     

The Inhibition of Maillard Browning by Different Concentrations of Rosmarinic Acid and Epigallocatechin‐3‐Gallate in Model,Bakery, and Fruit Systems

Rosmarinic acid and Epigallocatechin gallate concentrations were studied as natural inhibitors of Maillard browning in glucose/glycine model systems, and in bakery rolls and applesauce. The concentrations of the inhibitors were varied to determine the highest level of inhibition without a pro‐oxidant/browning effect. UV absorbance and gas chromatography/mass spec (GC/MS) with solid phase microextraction (SPME) sampling was used to study browning in the model systems. Hunter L*, a*, b* was used to analyze the color change results of the inhibitors on applesauce and bakery rolls. It was determined that a 1.0% solution of either antioxidant in the glucose/glycine system produced the greatest inhibition and a synergistic effect was not apparent when the two were combined. Inhibition of browning and a lack of synergy between the antioxidants were also determined in food systems consisting of applesauce and bakery rolls. GC/MS analysis of the model system revealed a high level of pyrazine formation in no‐inhibitor control samples and the absence of pyrazines in inhibitor‐containing samples.     

Effect of Solutes on Rheology of Soy Four and Its Components

The objective was to investigate the effects of solutes on rheology of soybean beverage components. Mixtures of these components with water and various amounts of sucrose were subject to shear stressshear rate analysis and both apparent viscosities and power law parameters were determined. Sucrose addition decreased “a values” (apparent viscosity) and increased “b values” (degree of Newtonian behavior) of all component suspensions. “A values” showed a minimum at sucrose concentrations of 55–60% of the aqueous phase. Glucose behaved like sucrose. On a weight basis, sodium chloride reduced “a values” and increased “b values” to a greater extent than either sucrose or glucose. In general, addition of solutes reduced the high viscosity of concentrated soy beverages.     

Reduction of benzene and naphthalene mass transfer from crude oils by aging-induced interfacial films

Semi-rigid films or skins form at the interface of crude oil and water as a result of the accumulation of asphaltene and resin fractions when the water-immiscible crude oil is contacted with water for a period of time or "aged". The time varying patterns of area-independent mass transfer coefficients of two compounds, benzene and naphthalene, for dissolution from crude oil and gasoline were determined. Aqueous concentrations of the compounds were measured in the eluent from flow-through reactors, where a nondispersed oil phase and constant oil-water interfacial area were maintained. For Brent Blend crude oil and for gasoline amended with asphaltenes and resins, a rapid decrease in both benzene and naphthalene mass transfer coefficients over the first few days of aging was observed. The mass transfer coefficients of the two target solutes were reduced by up to 80% over 35 d although the equilibrium partition coefficients were unchanged. Aging of gasoline, which has negligible amounts of asphaltene and resin, did not result in a change in the solute mass transfer coefficients. The study demonstrates that formation of crude oil-water interfacial films comprised of asphaltenes and resins contribute to time-dependent decreases in rates of release of environmentally relevant solutes from crude oils and may contribute to the persistence of such solutes at crude oil-contaminated sites. It is estimated that the interfacial film has an extremely low film mass transfer coefficient in the range of 10(-6) cm/min.     

STUDIES ON THE MECHANISM OF ACTION OF COPPER FINING AGENTS (κ CARRAGEENAN)

Carrageenan mediated clarification of brewers' wort comprises two reactions: interaction between carrageenan and soluble wort polypeptides and flocculation of particulate material. The dose/response characteristics of interaction between carrageenan and soluble polypeptide material suggested that carrageenan exhibits selectivity with respect to specific polypeptide fractions. Size exclusion chromatography of worts demonstrated that carrageenan treatment resulted in reductions in fractions of relative molecular mass Mr 70 000, 40 000 and 12 000. Fining trials demonstrated that the reactivity towards soluble polypeptides is independent of the presence of insoluble particulate material. Furthermore, both interaction with soluble polypeptides and flocculation of particulate material occurred in systems fined under high (80°C) and low (20–25°C) initial temperatures and this suggested that the presence of carrageenan in a helical conformation is essential to fining action rather than conformational transitions in polysaccharide structure. Additionally, carrageenan has no effect on the levels of low molecular mass wort solutes such as nucleic acid derivatives, aromatic and heterocyclic amino acids.     

Sorption behavior of charged and neutral polar organic compounds on solid phase extraction materials: which functional group governs sorption?

Numerous polar anthropogenic organic chemicals have been found in the aqueous environment. Solid phase extraction (SPE) has been applied for the isolation of these from aqueous matrices, employing various materials. Nevertheless, little is known about the influence of functional groups on the sorption of the solutes onto these materials. Therefore, the sorption interactions of (charged) polar organic solutes to neutral (HLB), cation-exchanging (MCX, WCX), and anion-exchanging (MAX, WAX) OASIS polymers have been studied. For neutral solutes HLB has the highest capacity and affinity. Van der Waals interaction, rather than hydrogen bonding, appears to be the predominant factor determining sorption. For charged molecules, MCX and MAX show by far the highest affinity and capacity. Adsorption is already efficient at low concentrations and the maximum sorption capacity equals the amount of charged functional groups on the material. The results from this study allow semiquantitative predictions if a solute will adsorb on one of the OASIS materials and which functional groups govern adsorption.     

Mobility of solutes in frozen pork studied by electron spin resonance spectroscopy evidence for two phase transition temperatures

The mobility of solutes in frozen pork (longissimus dorsi) was studied by X band electron spin resonance spectroscopy (ESR) using the three water soluble spin probes TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), CAT 1 (4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl) and TEMPO choline (4-(N,N-dimethyl-N-(2-hydroxyethyl))ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl). The mobility was quantified as the observed hyperfine coupling constant A'(zz). The three spin probes, of which only the neutral TEMPOL may penetrate membranes, gave very similar values of A'(zz) as a function of the temperature. A'(zz) was constant at temperatures below -55?°C, whereas A'(zz) decreased as the temperature was increased above -55?°C. This behavior has been interpreted as an increase in the amplitude of the rotational oscillations of the spin probes (and other solutes) with temperature. Above -13?°C the spin probes gave isotropic ESR spectra indicating free rotational mobility in a molten liquid phase. These results are discussed in relation to glass transitions in frozen pork.     

Linear adsorption of nonionic organic compounds from water onto hydrophilic minerals: silica and alumina

To characterize the linear adsorption phenomena in aqueous nonionic organic solute-mineral systems, the adsorption isotherms of some low-molecular-weight nonpolar nonionic solutes (1,2,3-trichlorobenzene, lindane, phenanthrene, and pyrene) and polar nonionic solutes (1,3-dinitrobenzene and 2,4-dinitrotoluene) from single- and binary-solute solutions on hydrophilic silica and alumina were established. Toward this objective, the influences of temperature, ionic strength, and pH on adsorption were also determined. It is found that linear adsorption exhibits low exothermic heats and practically no adsorptive competition. The solute-solid configuration and the adsorptive force consistent with these effects were hypothesized. For nonpolar solutes, the adsorption occurs presumably by London (dispersion) forces onto a water film above the mineral surface. For polar solutes, the adsorption is also assisted by polar-group interactions. The reduced adsorptive forces of solutes with hydrophilic minerals due to physical separation by the water film and the low fractions of the water-film surface covered by solutes offer a theoretical basis for linear solute adsorption, low exothermic heats, and no adsorptive competition. The postulated adsorptive forces are supported by observations that ionic strength or pH poses no effect on the adsorption of nonpolar solutes while it exhibits a significant effect on the uptake of polar solutes.     

The effect of starch gelatinization and solute concentrations on T g′ of starch model system

The effect of starch gelatinization on glass transitions in a starch/water model system and how the concentrations of added solutes (sucrose and sodium chloride) affect the glass transition temperatures of the gelatinized starch solution was investigated. The starch suspension samples were heat treated in a Differential Scanning Calorimeter (DSC) under different time and temperature regimes to achieve different degrees of gelatinization. The gelatinization characteristics (onset, peak and end temperatures and enthalpy) and the glass transition values of a potato starch were determined using the DSC. The results showed that the starch concentrations had no effect on gelatinization characteristics and the T_g′ of the gelatinized potato starch but had clearly increased their ΔC_p in the T_g′ region. Annealing at a temperature slightly below the T_g′ of −5 °C, led to maximal freeze‐concentration in the total/partial gelatinized starch and a higher T_g′ value at about −3 °C was obtained. The T_g′ values of the totally gelatinized starch samples were slightly lower than those of partially gelatinized samples. The T_g′ of the gelatinized starch decreased with increasing concentrations of sucrose or sodium chloride. Sodium chloride had a stronger depressing effect on T_g′ than sucrose. © 2000 Society of Chemical Industry     

Phase Behavior of a Meat-Starch Extrudate Illustrated on a State Diagram

ABSTRACT: The phase behavior of a meat-starch extruded system was illustrated on a state diagram. A mixture of meat and potato granules (1.48:1) was extruded with a twin-screw extruder. The extrudates were equilibrated at relative humidities between 0 to 88% and their glass transitions were determined.
Starch and proteins were phase separated at macromolecular level and retained their own phase transitions. The state diagram of the system showed that proteins dictated the texture of the mixed system, with starch contributing to the high value of the mechanical properties. Water had a plasticizing effect on both biopolymers. At room temperature, the extrudates with aw < 0.32 were glassy, while those with aw > 0.57 were rubbery.     

Impact of xanthan gum,sucrose and fructose on the viscoelastic properties of agarose hydrogels

Mixed carbohydrate systems are of special interest for the food and non-food industry as they offer a versatile range of unique and novel functional properties. However, intense research is required to understand the complex processes occurring in such systems on a molecular level and to be able to modify them aim-oriented. In food, characteristic properties are based on the physicochemical functions of the biopolymers added. Thus, small deformation tests and moisture analysis have been applied to study the impact of xanthan gum and two types of sugar on the viscoelastic properties, the sol–gel transition and the water holding capacity of 1% agarose hydrogels. Agarose gels are very elastic, turbid and prone to synaeresis, which impinges on their mouth feeling. Additions of xanthan gum revealed less elastic gels with an unaffected water holding capacity. Progressive addition of two different types of up to 40% of sugar yield an increase of the elasticity of agarose gels, whereby sugar concentrations of 60% partially result in a structural breakdown and thus a significant lower network structure but better water holding. In ternary systems, the effect of the sugar concentration and sugar type used is diminished by xanthan gum. The gelation mechanism of agarose gels with a distinct amount of co-solutes is presumably mainly affected by the water shortage evolved from the competition for it of all solutes present.     

Changes in apple liquid phase concentration throughout equilibrium in osmotic dehydration

ABSTRACT:  Previous results on apple tissue equilibration during osmotic dehydration showed that, at very long processing times, the solute concentrations of the fruit liquid phase and the osmotic solution were the same. In the present study, changes in apple liquid phase composition throughout equilibrium in osmotic dehydration were analyzed and modeled. Results showed that, by the time osmosed samples reached the maximum weight and volume loss, solute concentration of the fruit liquid phase was higher than that of the osmotic solution. The reported overconcentration could be explained in terms of the apple structure shrinkage that occurred during the osmotic dehydration with highly concentrated osmotic solutions due to the elastic response of the food structure to the loss of water and intake of solutes. The fruit liquid phase overconcentration rate was observed to depend on the concentration of the osmotic solution, the processing temperature, the sample size, and shape of the cellular tissue.     

The effects of xanthan conformation and sucrose concentration on the rheological properties of acidified sodium caseinate–xanthan gels

Xanthan solution transitions caused by annealing at different temperatures were studied by oscillatory rheology. The rheological behaviour of solutions evaluated at 10 °C showed different irreversible xanthan transitions, depending on the annealing temperature. The effect of xanthan conformation and caseinate, xanthan and sucrose concentrations on the mechanical and stress relaxation properties of acidified dairy gels were also investigated. Xanthan transitions produced different structural characteristics in mixed gels, as observed by their relaxation time and residual stress. The amount of sucrose played an important role in the network formation of gels and on the final elasticity. Intermediate concentrations of sucrose yielded gels that were finer than those obtained with low sucrose content, whereas at high amounts of this co-solute, the gels became more fragile. This quadratic effect of sucrose occurred at lower sucrose contents than in protein–sucrose or polysaccharide–sucrose systems. The mechanical properties at equilibrium were affected only by the strength of the interactions between caseinate and xanthan.     

Effect of sucrose on the phase and flow behaviour of polysaccharide/protein aqueous two-phase systems

The phase and flow behaviour of two model polysaccharide/protein aqueous two-phase systems (atps) – locust bean gum (LBG)/sodium-caseinate (NaCas) and guar gum (GG)/NaCas – have been investigated in the presence of varying amounts of sucrose. The phase behaviour of these atps was found to be affected by the concentration of sugar in the system. An increase in the miscibility of these mixtures was initially induced by addition of sugar in concentrations up to 15 wt%, but further increase had the opposite effect, thus increasing the incompatibility between the polysaccharide and protein macromolecules. The rheological behaviour of these atps was found to relate to their microstructure under quiescent conditions. Depending on the phase sense in the quiescent atps the occurrence of a shear-induced phase inversion event was observed; phase inversion occurred only in those atps where the more viscous polysaccharide-rich phase was included. Addition of sugar in concentrations up to 20 wt% did not seem to affect the occurrence of phase inversion, but further increase in the sugar concentration was found to suppress this phenomenon.     

Confounding impacts of iron reduction on arsenic retention

A transition from oxidizing to reducing conditions has long been implicated to increase aqueous As concentrations, for which reductive dissolution of iron (hydr)oxides is commonly implicated as the primary culprit. Confounding our understanding of processes controlling As retention, however, is that reductive transformation of ferrihydrite has recently been shown to promote As retention rather than release. To resolve the role iron phases have in regulating arsenic concentrations, here we examine As desorption from ferrihydrite-coated sands presorbed with As(III); experiments were performed at circumneutral pH under Fe-reducing conditions with the dissimilatory iron reducing bacterium Shewanella putrefaciens strain CN-32 over extended time periods. We reveal that with the initial phase of iron reduction, ferrihydrite undergoes transformation to secondary phases and increases As(III) retention (relative to abiotic controls). However, with increased reaction time, cessation of the phase transitions and ensuing reductive dissolution result in prolonged release of As(III) to the aqueous phase. Our results suggest that As(III) retention during iron reduction is temporally dependent on secondary precipitation of iron phases; during transformation to secondary phases, particularly magnetite, As(III) retention is enhanced even relative to oxidized systems. However, conditions that retard secondary transformation (more stable iron oxides or limited iron reducing bacterial activity), or prolonged anaerobiosis, will lead to both the dissolution of ferric (hydr)oxides and release of As(III) to the aqueous phase.     

The chemical composition and some physical properties of a water-soluble gum from Prosopis africana seeds

A water‐soluble gum was extracted from the endocarp capsule of the seed coat of Prosopis africana. The gum yield varied substantially with the method of dehulling. Maximum yield was obtained after boiling in 0.1 m Na₂CO₃ for 4 h. The effects of two different gum concentrations on the rheological properties of gum solutions were examined. The gum formed viscous solutions at low concentrations and exhibited stable viscosity in the pH range 6.0–8.0. Chemical analyses showed that the purified gum had a composition of 2.4% fat, 1.04% protein, 21.5% crude fibre and a gelation concentration of at least 10%. Galactose and mannose were the major polysaccharides identified. Large reductions in viscosity were observed with the addition of various concentrations of Na⁺ salts. The activation energies of flow for 2 and 3% gum solutions were in the range of 19.2–22.8 kJ mol^?1 and were characteristic of systems with little intra‐ and intermolecular interactions.