Mobility and Stability Characterization of Model Food Systems Using NMR, DSC, and Conidia Germination Techniques

Mold conidia germination was used as a microbial probe of food stability in sucrose, starch, and sucrose/starch systems. A group of nuclear magnetic resonance (NMR) and differential scanning calorimeter (DSC) techniques were used to fully characterize the water and solid mobility and glass transition temperature (Tg) of the systems, respectively. Water content, a_w, and ²H NMR R₁ and R₂ relaxation rates did not predict mold germination time. We concluded that the self-diffusion coefficient, (translational mobility of water), the DSC Tg (overall system mobility), and to a more limited extent, the ²H NMR R*₂ relaxation rate and the ¹³C T_1p (solids mobility), could provide alternative measures to supplement a_w for predicting food stability and safety.     

Adsorption kinetics of BSA at air–sugar solution interfaces as affected by sugar type and concentration

The influence of glucose, sucrose and fructose at concentrations ranging from 0 to 40 wt.% on the adsorption kinetics of bovine serum albumin (BSA) at air–aqueous solution interfaces at 20 °C was measured using drop shape analysis. The rate of adsorption of proteins decreased in the presence of sugars. The diffusion coefficient of BSA in 40 wt.% sucrose solutions, calculated from the initial period of protein adsorption, was two orders of magnitude lower than the diffusion coefficient of BSA dissolved in pure water. The decrease of the diffusion coefficient with sugar concentration (δD_eff/δc_sugar) was appreciably higher at low sugar concentrations (<10 wt.%). Diffusion coefficients in the presence of sucrose were smaller than in the presence of glucose or fructose. Results were attributed to an increase in solution viscosity, preferential interactions of sugars with protein surfaces and an increased hydrophilicity of protein surfaces due to preferential hydration.     

Mobility of Water in Starch-Sucrose Systems Determined by Deuterium and Oxygen-17 NMR

The molecular mobility of water in a starch-sucrose system was determined by deuterium (²H) and oxygen-17 (¹⁷O) high-field Nuclear Magnetic Resonance (NMR) spectroscopy. The starch-sucrose system was composed of freeze-dried starch:sucrose (FSS) (90:10) in deuterium oxide or enriched oxygen-17 water over the solids concentration range of 10 to 93% solids. The NMR transverse relaxation rate (R₂) and peak behavior were analyzed in terms of solids concentration and water activity. R₂ of the FSS system is compared to the R₂ of starch and sucrose systems alone. The relaxation behavior of the ²H and ¹⁷O were compared for FSS, starch and sucrose. The results led to the conclusion that the best NMR methodology for the investigation of water mobility is provided by ¹⁷O NMR.     

Mobility of Water in Starch Powders Determined by Nuclear Magnetic Resonance

The molecular mobility of water in corn starch powders (71 to 96% solids) was determined by deuterium (²H) and oxygen-17 (¹⁷O) high-field Nuclear Magnetic Resonance (NMR) spectroscopy, in deuterium (D₂O) and oxygen-17 enriched water (H₂¹⁷O), respectively. The NMR transverse relaxation rate (R₂) was related to both the concentration and activity of water (a_w). The concentration plot showed two regions of water mobility; trapped and monolayer water. The water activity plot showed two linear regions of water mobility; Region A, from 0.99 to 0.23 a_w, and Region B, from 0.23 to 0.11 a_w. The linear relationship in Region A between water mobility, as measured by R₂ and water activity is hypothesized to be due to the hydrophobic nature of the starch granules which results in diffusion limited water motion. Relaxation rates for ²H and ¹⁷O at high a_w were compared with one another and literature data.     

Temperature dependence of water activity in aqueous solutions of sucrose

A comprehensive experimental data analysis was performed to evaluate the effect of temperature on the water activity coefficient and selected excess thermodynamic functions for aqueous solutions of sucrose. A four-suffix Margules equation with temperature-dependent parameters was used to fit thermodynamic data such as the vapor pressure, boiling point, osmotic coefficient, freezing point, sucrose solubility, heat of dilution and specific heat of solution. The proposed equation gives an adequate representation of the available literature data on sucrose solutions for temperatures from −15 to +150 °C and sucrose concentrations up to 98% wt. The isotherms of water activity coefficient exhibit a characteristic minimum at about 96% wt. sucrose which is then followed by a dramatic increase to values well exceeding 1, as it was suggested before by some theoretical models [Starzak, M., & Mathlouthi, M. (2002). Water activity in concentrated sucrose solutions and its consequences for the availability of water in the film of syrup surrounding the sugar crystal. Zuckerindustrie, 127, 175–185; Van Hook, A. (1987). The thermodynamic activity of concentrated sugar solutions. Zuckerindustrie, 112, 597–600]. The effect of temperature on water activity, almost negligible for dilute solutions, was found significant for very concentrated solutions (above 80% wt. sucrose). The new water activity equation should find numerous applications in the food technology and sugar industry.     

The effect of shear rate on the viscosity of solutions of guar gum and locust bean gum

The apparent viscosities of aqueous solutions of guar gum and locust bean gum have been measured over a range of shear rates from 14 to 1142 s^?1. Comparable measurements were also made on gum solutions to which glucose, sucrose or glucose syrup had been added. For all the solutions the variation of relative viscosity with shear rate fits a power law equation. Addition of sugar has no effect on the non-Newtonian behaviour of the gum solutions. The relative viscosity of the gum in the sugar solutions is lower than in water. The ratios of the relative viscosities closely correspond with the ratios of the intrinsic viscosities. This is interpreted in terms of a smaller extension of the gum molecule in the sugar solution.     

1H NMR studies of molecular mobility in wheat starch

Pulsed ¹H NMR was used to determine moisture content and molecular mobility of wheat starch suspensions at water activity (a_w) values between 0 and 0.93. Moisture determination by NMR agreed well with gravimetric determinations (R²=0.99). Transverse proton relaxation (T₂) of the systems were identified using single pulse and CPMG experiments. T₂ associated with starch molecules increased linearly from 7.1 to 10.1 μs as a_w increased from 0 to 0.93, due to plasticizing effects of water. T₂ associated with water molecules showed a linear increase up to a_w of 0.69. Results indicated that ¹H NMR could be employed to determine moisture content of starch suspension and to identify and quantify the molecular motion of starch chains and water molecules of starch suspensions at low moisture levels.     

Changes in Structure, Rheology, and Water Mobility of Apple Tissue Induced by Osmotic Dehydration with Glucose or Trehalose

The impact of osmotic dehydration to water activity (a _w) at 0.97 or 0.94 with glucose or trehalose solutions on structure (optical and transmission electronic microscopy observations), rheological properties (small-scale dynamic oscillatory and creep/recovery measurements and large-scale compression force-deformation testing) and water mobility (¹H-NMR spectra) of parenchymatous apple tissue was investigated. In general, the nature and the concentration of sugar employed significantly affected the material properties and the structure of apple tissue. A dramatic loss in rigidity (E _d); an increase in deformation at rupture (?? _R ^R ), creep compliances (J ₀, J ₁, and J ₂), and fluidity (1/?? ₀) and a decrease in storage (G??) and loss (G??) moduli, true rupture stress (?? _R ^R ), and proton transverse relaxation times (T _2i) were induced by osmotic treatments. ?? _R ^R , C ₁, and T _2i parameters allowed to discriminate between the sugars used as osmotic agents while the different a _w levels for each sugar resulted in changes in ?? _R ^R , W, and T _2i values. Loss of turgor due to plasmolysis or rupture of membranes and desorganization/degradation of walls allowed explaining, at least partially, the changes in material parameters.     

Distribution of Water in Fresh Cod

Low-field ¹H nuclear magnetic resonance (NMR) transverse relaxation was used to measure water mobility and distribution of water in fresh cod fillets. The NMR relaxations were analysed with the so-called SLICING method giving uni-exponential profiles from which the transverse relaxation time (T₂-values) and the relative sizes of the water populations were calculated. Two water populations with the T₂-values of 50 and 94 ms were obtained. The shortest relaxation time was primarily found near the head, and water with the longest relaxation time was primarily found near the tail. This variation can be explained by the smaller muscle cells and muscle fibers in the tail, which may influence the distributions of water into the different pools. The amount of one of the water populations was correlated to the overall water content with a correlation coefficient of −0.94.     

The effect of sugars on the diffusion of water in starch gels: a pulsed field gradient NMR study

The self-diffusion coefficient of water was measured in starch–sugar–water systems of various compositions. The starch was either waxy maize or potato starch and the sugar was either sucrose or xylose. The diffusion results obtained from sugar solutions (i.e. with a zero polymer concentration) suggested that a free water behaviour should only be expected after a bilayer coverage of the sugar molecule. The presence of sugars was found to decrease considerably the translational mobility of water in starch gels. This reduction was proportional to the sugar concentration in the system but showed little dependence on the type of sugar. An attempt was made to relate the self-diffusion coefficients of water in starch–sugar–water gels to those obtained for the individual binary systems (starch–water and sugar–water). A prediction based on the additive effects of sugar and starch only corresponded with experimental results in dilute systems.     

核磁共振研究蔗糖对面团中分子流动性的影响

利用脉冲梯度场核磁共振技术(PFG-NMR)研究蔗糖对面团自旋-自旋弛豫特性的影响。分子的自旋-自旋弛豫特性与分子的流动性呈正相关。采用CPMG脉冲序列检测面团中质子的自旋-自旋弛豫时间(T2)。在面团中流动性不同的水分,表现出不同的弛豫时间。实验结果表明,蔗糖的亲水性能够显著改变面团的自旋-自旋弛豫时间。通过磁共振图像可以发现蔗糖能够明显促进面团中质子的均匀分布,增强其流动性。当水分含量一定时,面团的自旋-自旋弛豫时间随蔗糖含量的增加而增加。     

Diffusion of Aqueous Sugar Solutions as Affected by Locust Bean Gum Studied by NMR

Time domain proton NMR was used to study translational diffusion and molecular mobility of sugar and water molecules in simple aqueous solutions to determine how the addition of hydrocolloid stabilizers affect their mobility. Results showed that addition of Locust Bean Gum (LBG) did not affect the diffusion or mobility of either the sugar or water molecules over distances up to 10μm in unfrozen solutions. The diffusion properties of the sugar and water molecules were not affected by either biopolymer cryogelation or the presence of ice. We hypothesized that retardation of the ice crystal growth rates in aqueous sugar solutions caused by the presence of LBG was not a bulk property of the solution. The most likely cause is a modification of the local interfacial region between the ice crystal surface and the surrounding unfrozen solution.     

Monitoring changes in feta cheese during brining by magnetic resonance imaging and NMR relaxometry

Magnetic resonance imaging (MRI) and NMR relaxometry were used to monitor changes in feta cheese during 169 h of brining at 4.8%, 13.0% and 23.0% salt solutions. Image and relaxation data were acquired to study salt uptake and water loss due to dehydration of cheese during brining. Saturation recovery and Carr-Purcell-Meiboom-Gill (CPMG) sequences were used to determine the longitudinal relaxation (T₁) and the transverse relaxation (T₂) times, respectively. Signal intensities of T₂ weighted images decreased during 169 h of brining. An excellent linear correlation between the average signal intensity and the water content was obtained (R² = 0.984). The T₁ values of cheese brined at 4.8% were almost constant but T₁ values decreased for both 13.0% and 23.0% salt brined cheeses. Analysis of the CPMG decays gave relaxation spectra containing two components which decreased during brining. The short component T_2a was highly correlated with salt content (R² = 0.974). Results showed that NMR and MRI can be used to follow salt uptake and changes in water content in cheese during brining.     

The effect of MgCl2 on the kinetics of the Maillard reaction in both aqueous and dehydrated systems

The modification of Maillard reaction kinetics induced by MgCl₂ was evaluated in both liquid and dehydrated model systems with special emphasis on the interactions of the salt with water and/or the sugars. In liquid trehalose systems, browning is accelerated by the presence of MgCl₂ due to the increased sugar hydrolysis and to the reduction of water mobility caused by the salt (shown by the decrease in ¹H NMR relaxation times T₂), counteracting the inhibitory effect of water on the reaction. In water restricted trehalose systems, MgCl₂ inhibited the Maillard reaction. The salt–sugar interactions, manifested by the delayed sugar crystallization, decreased the reaction rate by affecting the reactivity of reducing sugars. Molecular and supramolecular effects in the presence of MgCl₂ have been observed in the present work, and must be taken into account considering high technological interest in finding strategies to either inhibit or enhance the Maillard reaction depending on the application.     

Mobility of Water in Sucrose Solutions Determined by Deuterium and Oxygen-l7 Nuclear Magnetic Resonance Measurements

The molecular mobility of water associated with sucrose was determined by deuterium (²H) and oxygen-17 (¹⁷O) high-field Nuclear Magnetic Resonance (NMR) spectroscopy. Systems of sucrose and water or deuterium-oxide over the solids concentration range of 5 to 80% were studied. Supersaturation to 70% sucrose in deuterium oxide was included. The relation between ¹⁷O and ²H NMR measurements and sucrose concentration indicated three regions of successively decreasing water mobility upon increasing solute concentration. Solutesolvent and solute-solute interactions via hydrogen bonding are suggested as the mechanism to explain the observed decrease in water mobility. The water showed rapid rotational mobility according to NMR correlation times. which ranged between 9.5 and 100 psec depending on the hydration values used.     

Ripening of banana fruit monitored by water relaxation and diffusion 1H-NMR measurements

Water transverse relaxation times T₂ and self-diffusion coefficients D have been measured on banana samples at different ripening stages. Relaxation data have been interpreted on the basis of a chemical and diffusive exchange model proposed by Belton and Hills [Mol. Phys. 61(4) (1987) 999] and Hills et al. [Mol. Phys. 67(4) (1989) 903]. According to that model the observed increase of T₂ values of both cytoplasmatic and vacuolar water may be mainly attributed to the decrease of starch concentration during the ripening process. On the other hand, the observed water self-diffusion coefficient decrease is related to sugar accumulation as starch hydrolysis proceeds. At the early stages of ripening, the individual self-diffusion coefficient values of cytoplasmatic and vacuolar water differ from one another and have been calculated through the analysis of relaxation time-separated pulsed field gradient nuclear magnetic resonance experiments [J. Magn. Reson. A 112 (1995a) 237].     

Modification of Transverse NMR Relaxation Times and Water Diffusion Coefficients of Kiwifruit Pericarp Tissue Subjected to Osmotic Dehydration

The objective of the present study was to evaluate cellular compartment modifications of kiwifruit (Actinidia deliciosa) outer pericarp tissue caused by osmotic treatment in a 61.5 % sucrose solution, through the quantification of transverse relaxation time (T ₂) and water self-diffusion coefficient (D _w) obtained by low field nuclear magnetic resonance means. Raw material ripening stage was taken into account as an osmotic dehydration (OD) process variable by analyzing two different kiwifruit groups, low (LB) and high (HB) °Brix. Three T ₂ values were obtained of about 20, 310, and 1,250 ms, which could be ascribed to the proton populations, located in the cell walls, in the cytoplasm/extracellular space, and in the vacuoles, respectively. According to T ₂ intensity values, vacuole protons represented between 47 and 60 % of the total kiwifruit protons, for LB and HB kiwifruits, respectively. The leakage of water leading to vacuole shrinkage seemed to cause concentration of solutes, retained by the tonoplast, making the vacuole T ₂ value decrease along the OD. As expected, the D _w values of raw kiwifruits were lower than the value of the free pure water. The water mobility (and hence D _w), depending on the kiwifruit distinctive cellular structures and solutes, decreased even more during OD due to water loss and sugar gain phenomena. D _w represents an average value of the diffusion coefficient of the whole kiwifruit tissue protons. In order to obtain D _w values specific for each cellular compartment, a multiple component model fitting was also used. According to these results, the vacuole water self-diffusion coefficient (D _w,v) was much higher than D _w.     

Rheological properties of some starch-water-sugar systems

Summary When certain amounts of starch were blended with saturated, aqueous solutions of d-fructose, d-glucose or sucrose, shear thickening, easily pourable semi-solids were formed. The amount of starch necessary to cause this rheological effect depended, at least in part, on the starch variety. The conditions necessary to observe this effect using potato and corn starch blends with d-fructose, d-glucose and sucrose are presented. This method may be useful in studies of starch granule morphology. Gelation characteristics and differential scanning calorimetry (DSC) of potato and corn starch in saturated aqueous d-fructose, d-glucose, and sucrose solutions showed that both starch varieties interacted differently with the environment. Starch more readily gelatinised in sucrose solutions than in solutions of d-fructose and d-glucose.     

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.     

Molecular Mobilities of Instant Starch Gels Determined by Oxygen-17 and Carbon-13 Nuclear Magnetic Resonance

The molecular mobility of both the water component and the starch component of three instant starches was determined by proton decoupled oxygen-17 (¹⁷O) and proton decoupled carbon-13 (¹³C) Nuclear Magnetic Resonance (NMR) spectroscopy. The effect of starch concentration and various storage conditions were studied. The molecular mobility of the water associated with each of the instant starches over all concentrations studied as measured by ¹⁷O NMR transverse relaxation rates was not much slower than that of free or bulk water. The effect of storage conditions on the molecular mobility and conformation of the starches, as monitored by proton decoupled ¹⁷O and ¹³C NMR, was in most cases small, despite the observed differences in retrogradation among the starches.