Interaction of Sucrose with Starch During Dehydration as Shown by Water Sorption

Component interactions in the sucrose-starch-water system were investigated. Starch, sucrose, a mechanical starch:sucrose mixture and four powders obtained by freeze drying dilute aqueous mixtures were equilibrated to a_w ranging from 0.33–0.93. Both raw and gelatinized starch were included. Sucrose-starch interaction was determined by measuring the reduction in water sorption as compared to theoretical. Only the freeze-dried mixtures showed interaction. Interacted sucrose, calculated as the sucrose that did not bind water, (1) decreased from a maximum at 0.86 a_w to zero at 0.936 a_w, and (2) rose sharply with increasing sucrose-starch ratio to a maximum at a ratio of 0.4 and decreased to zero with further increase in ratio to 1.5. More sucrose interacted with gelatinized than raw starch.     

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.     

Crystallinity of Sucrose by X-ray Diffraction as Influenced by Absorption versus Desorption, Waxy Maize Starch Content, and Water Activity

Crystallinity of sucrose in a mixture with waxy maize starch as affected by absorption and desorption in the range 0.33 to 0.97 a_w was studied by X-ray diffraction. Ratio of sucrose to starch peak areas was linear with weight ratios as expected. Sucrose crystallinity as determined by X-ray diffraction was found to be in excellent agreement with that calculated from sorption data. Sucrose in a mixture equilibrated by absorption to 0.33 and 0.75 a_w was about 80% crystalline. On absorption to 0.84 a_w, the mixture showed sucrose crystallinity reduced to 50%. However, on desorption to 0.33–0.84 a_w, the sucrose crystallinity was reduced to zero regardless of sucrose content.     

Sorption Behavior of Mechanically Mixed and Freeze-Dried Sucrose/Casein Mixtures

Interactions between sucrose and casein were investigated by adsorption isotherms. Mechanically mixed and freeze-dried sucrose/casein mixtures at sucrose percentages of 1, 5, 10, 15 and 20% (wet basis) were compared to calculated isotherms using the mass balance equation, over the a_w range 0.23–0.93 at 20°C. The mechanical mixtures did not exhibit solute/polymer interactions. The freeze-dried systems showed positive interaction (i.e., sorbed more water than calculated) for 1 and 5% sucrose at all a_w values. For the 10, 15 and 20% sucrose mixtures, positive interaction continued below 0.86 a_w; however, above 0.86 a_w, no interaction was apparent. That is, experimental sorption values equaled calculated values.     

Branched Oligosaccharides Concentrated by Yeast Fermentation and Effectiveness as a Low Sweetness Humectant

A branched oligosaccharides (BOS) mixture was prepared from liquefied corn starch using Bacillus lichenifrmis maltogenic amylase (BLMA). A highly concentrated BOS was prepared by removing glucose and maltose through yeast fermentation, which increased the BOS content. Water activity (a_w) of bread went from 0.87 to 0.82 when 10% (w/w) or to 0.78 when 20% (w/w) BOS was added. BOS lowered a_w more effectively as concentration of BOS increased. The water sorption isotherm of BOS was much higher than that of sucrose at a_w, 0.1–0.8. BOS also prevented starch retrogradation in bread, probably due to steric hinderance and to the state of water retained by BOS. Relative sweetness of BOS was ± 17.5% of sucrose. The BOS mixture produced by BLMA might provide a new type humectant for foods that is low in sweetness and retards retrogradation.     

Moisture Hysteresis is Due to Amorphous Sugar

Hysteresis, the moisture discrepancy between absorption and desorption, of sucrose-waxy maize starch mixture was studied. Starch was raw, partially and completely gelatinized. Hysteresis of raw starch was insignificant. Gelatinized starch showed slight hysteresis, 0.02g w/g starch, in the range 0.57 to 0.84 a_w. Sucrose showed no hysteresis. However, sucrose-starch mixtures showed a large hysteresis, e.g., 0.15g w/g mix, which occurred only at a_w below 0.86, the saturation a_w for sucrose. Hysteresis showed a close linear correlation with the amount of amorphous sucrose (X-ray diffraction) present. The amount of water sorbed by amorphous sucrose as calculated from the experimental data was in a good agreement with literature data.     

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.     

Correlation of Proton T1 with Polymer and Solute Waters in Starch-Sucrose Mixtures

Proton pulsed NMR methodology was applied to correlate the observed spin-lattice (T₁) and spin-spin (T₂) relaxation times with the amount of water associated to sucrose (SOL) and that to starch (POL) in a sucrose-starch model system containing both. Sucrose, starch and four mixtures were equilibrated to four water activities (a_w, 0.86-0.97). Calculations for T₂ were invalid due to an undeterminable amount of molecular diffusion caused by field inhomogeneity in the magnet. Model equations were developed for calculating SOL and POL from T₁ and a_w; These values showed high correlations (R²>0.97) with SOL and POL as determined from sorption data for sucrose and starch. This validated the use of this instrument with the mathematical models developed.     

Interaction of Solutes with Raw Starch during Desorption as shown by Water Retention

The objective was to study interaction of salt and sucrose with raw starch as determined from water retention at a given a_w. NaCI-starch mixtures were 5:95, 10:90, 15:85, and 25:75. Sucrose-starch mixtures were 1:9, 2:8, 1:1, and 9:1. Equilibration was against salt slushes over the water activity range 0.33 to 0.97. At each a_w, as the solute concentration increased above zero, the amount of interacted solute rose to a maximum and then, in contrast to previous findings with protein, declined to zero. For each solute, saturation a_w showed the highest amount of interaction at any solute concentration; the interaction decreased with increasing a_w. At maximum, 87.2% of total NaCl and 56.3% of the total sucrose interacted with starch. These desorption data showed less interaction than previously found for absorption.     

Crystallinity of Waxy Maize Starch as Influenced by Ambient Temperature Absorption and Desorption, Sucrose Content and Water Activity

Crystallinity of waxy maize starch was determined by X-ray diffraction. Absorption and desorption over the range 0.33-0.84 a_w, were applied to raw, partially gelatinized, and completely gelatinized starches mixed with0–50% sucrose. Increasing aw and moisture content in-creased crystallinity in case of all three starch treatments. A critical moisture content was required for recrystallization of starch. Gelatinized starch showed the greatest crystallinity hysteresis which was much greater than water sorption hysteresis. Sucrose had no effect on starch crystallinity with absorption. With desorption of completely gelatinized starch-sucrose mixtures, the sucrose caused a sharp decrease in crystallinity at0–10% sucrose while, at higher sucrose content, the crystallinity increased.     

Interaction of Sucrose with Gelatin, Egg Albumin and Gluten in Freeze-Dried Mixtures as Shown by Water Sorption

The objective was to study sucrose-protein interactions in freeze-dried mixtures as shown by water binding data. Gelatin, egg albumin and gluten were included. Sucrose content was varied. Each sample was mixed with water and dried. The powder was equilibrated against known water activity (a_w) ranging from 0.58 to 0.93. The experimental water sorption data at a given a_w was then compared to calculated values obtained from the Lang-Steinberg mass balance equation. Contrary to other systems, samples sorbed more rather than less water than calculated. This was ascribed to sucrose-protein interaction which increased water binding by protein. Amount of interaction at a given a_w was found to be linearly related to the sucrose content. A critical sucrose content was needed to trigger the interaction.     

Measurement of Dielectric Constant to Quantitate Solute Water in the Sucrose-Starch-Water System

A quantitative analysis for the measurement of solute water (SOL) in the presence of polymer water (POL) by dielectric measurements was developed. Sucrose, starch and their mixtures at known moisture content and water activity (a_w) were measured. Dielectric constant (K) was relatively insensitive to polymer water (POL) but very sensitive to capillary water (CAP). K was very sensitive to the amount of water in a supersaturated sucrose solution but not to dilution of the saturated solution. A given increment of solute water (SOL) in a mixture gave the same K over the whole a_w range, independent of sorbent composition. A model was developed to quantitate SOL in a mixture with POL by measuring K at 1000 KHz.     

Effect of water content,temperature and storage on the glass transition,moisture sorption characteristics and stickiness of β‐casein 1

Moisture sorption isotherms at +4 °C and +22.5 °C were obtained for β‐casein after isolation and after 9 months of storage at ‐29 °C and +22.5 °C. Glass transition state diagrams (T_g vs. moisture) were determined for β‐casein after storage. The results showed that effects of storage temperature on moisture sorption isotherms were varied; however, at any a_w differences in moisture content were small (< 0.03g H₂O/g solids at high a_w). β‐casein stored at ‐29°C had lower m_o and T_g values than that of β‐casein stored at +22.5°C. The glass transition temperatures for β‐casein were above room temperature, even at a_w = 0.76. Onset of stickiness occurred above a_w = 0.76.     

Empirical Models for Moisture Sorption Isotherms at Very High Water Activities

Published moisture sorption data of selected gums, fibers and starch determined at water activities of up to 0.98-0.99 were fitted by an empirical three parameter model. It had the general mathematical structure of the GAB model but its abscissa's range was expanded from zero to one, to zero to infinity. This was done by substituting In [1/(1 -a_w)] or a_w/(1-a_w) for a_w, terms that could be derived from Henderson's and Oswin's models. This hybrid model with either substitution had a particularly good fit to the isotherms part that accounted for moisture sorption at high water activities. The model with the logarithmic transformation, however, appeared to be more practical and to better account for the intermediate part of the moisture sorption isotherm.     

Enthalpy–entropy compensation in sorption phenomena of starch materials

The enthalpy–entropy compensation theory was applied to the experimental moisture adsorption and desorption isotherm data (water activity (a_w) range 0.006–0.982) of raw potato, potato starch gel, potato starch powder, highly amylopectin corn starch powder and highly amylose corn starch powder in the temperature range 30–60 °C. A linear relation existed between differential enthalpy (ΔH) and differential entropy (ΔS) for all the experimental data considered, thus satisfying the enthalpy–entropy compensation theory. Further analysis of the data indicated an enthalpy-controlled (isokinetic temperature (T_β) > harmonic mean temperature (T_hm)) and spontaneous (−ΔG) sorption process.     

Browning,Starch Gelatinization,Water Sorption,Glass Transition,and Caking Properties of Freeze-dried Maca ( Lepidium meyenii Walpers) Powders

The browning, gelatinization of starch, water sorption, glass transition, and caking properties of freeze-dried maca ( Lepidium meyenii Walpers) powders were investigated and compared with a commercial maca powder. The freeze-dried maca powders had lower optical density (browning) and higher enthalpy change for starch gelatinization than the commercial maca. This resulted from a difference in thermal history. The equilibrium water contents of the freeze-dried maca powders were higher than those of commercial maca at each water activity ( a _w ) because of differences in amorphous part. The glass transition temperature ( T _g ) was evaluated by differential scanning calorimetry. There was a negligible difference in the anhydrous T _g (79.5–80.2 ºC) among the samples. The T _g -depression of freeze-dried maca powders induced by water sorption was more gradual than that of the commercial maca due to a difference in water insoluble material content. From the results, critical water activity ( a _wc ) was determined as the a _w at which T _g becomes 25 ºC. There was negligible caking below a _w = 0.328. At higher a _w , the degree of caking remarkably increased with a large variation depending on the samples. The degree of caking could be described uniformly as a function of a _w / a _wc . From these results, we propose an empirical approach to predict the caking of maca powders.     

Properties and sorption studies of polyethylene oxide–starch blended films

A series of polyethylene oxide (PEO) and starch blends were prepared by extrusion and their films prepared by compression molding. The mechanical, barrier, optical, thermal properties and surface morphology characteristics of PEO–starch blended films were studied. The tensile strength of the blended films decreased whereas the haze values increased with an enhanced starch concentration in PEO–starch blends. The inter-molecular interaction between PEO–starch blends were investigated with FTIR. The thermal properties were also analyzed by differential scanning calorimetric (DSC) and thermo-gravimetric analysis (TGA) techniques. Moisture-sorption characteristics of PEO–starch blended films were carried out at 27 °C for water activity (a_w) ranged from 0.1 to 0.9. The sorption data at different a_w were used to fit different sorption isotherm models as proposed in the literatures. The model constants were determined by linear fitting of the sorption equations. The high coefficient of determination R² (ranged from 0.7 to 1) confirms the applicability of the equations employed. The study on the application of such water activity data of PEO/starch blended films on different model equations will be helpful in prediction of durability and functional behavior of moisture sensitive biopolymeric films.     

Water Mobility in Starch/Sucrose Systems: an Oxygen-17 NMR Study

Sucrose/starch mixtures containing 0, 10, and 20% sucrose were equilibrated with water vapor enriched to 1%¹⁷O, at water activities (a_w) of 0.88, 0.93, and 0.97. In all samples, a significant quantity of water did not appear in the NMR spectrum, presumably because it was immobilized. The amount of water, which was somewhat mobile and appeared in the spectrum, increased with increasing a_w and sucrose content. The relaxation time of this NMR active water was strongly dependent on sucrose content, in a way that could not be explained by any simple model. Thus, the presence of sucrose has a profound effect on interaction of starch with water.     

Moisture sorption and the applicability of the Brunauer-Emmet-Teller (BET) equation for blanched and unblanched mushrooms

Moisture sorption isotherms of blanched and unblanched mushrooms over 0.11–0.75 a_w were determined at 27°C and 37°C by using the static gravimetric method. Adsorption and desorption behaviors of blanched and unblanched mushrooms were compared. The unblanched material adsorbed more water than that of the blanched. In desorption isotherms, the equilibrium moisture contents of the unblanched material were found to be higher than those of the blanched throughout the entire a_w range. The BET equation was tested to fit the experimental moisture sorption data over the 0.11–0.43 a_w, 0.11–0.55 a_w, 0.11–0.64 a_w, and 0.11–0.75 a_w. Nonlinear regression analysis was used for the determination of the parameters in the equation. The quality of the fit of the BET model over each a_w range was judged from the value of the relative percent root mean square (% RMS). The moisture sorption behavior over 0.11–0.43 a_w of mushrooms has indicated that the BET equation is applicable generally up to 0.43 a_w. Monolayer moisture contents and C constants in the BET equation obtained for each a_w interval were reported. The water activities corresponding to the monolayer values have been determined and discussed related to mushroom storage. The net isosteric heats of adsorption and desorption were estimated from equilibrium sorption data, using the integrated form of the Clausius-Clapeyron equation. The heats of adsorption/desorption decreased with increase in moisture content and approached to a constant value. It was concluded that moisture adsorption/desorption occurred by physical mechanisms at high moisture contents, but at low moisture contents, besides physical adsorption, chemisorption was also observed.     

Relations between sensorial crispness and molecular mobility of model bread crust and its main components as measured by PTA,DSC and NMR

Consumer appreciation of brittle cellular foods, like bread crusts, depends on textural properties such as crispness. This crispy character is lost above a certain water activity. It is not known what exactly is happening in these crusts when water enters. So is it unclear whether it is the change in the starch or the gluten that initiates the loss of crispness with ageing time. In this paper the effect of water on the glass transition of model bread crusts was studied using two complementary techniques: phase transition analysis (PTA) and temperature modulated differential scanning calorimetry (TMDSC). The mobility of water was studied with nuclear magnetic resonance (NMR). The results were compared with sensory data. Bread crusts prepared with different types of flour were tested to evaluate the effect of flour composition on the crispness of model crusts equilibrated at different relative humidities. In addition the single flour components starch and gluten were studied. Sensory crispness scores decreased with increasing a_w from 0.55 upwards. At a_w 0.70 sensory crispness was completely lost. Both DSC and PTA showed a transition point at an a_w of 0.70–0.75. NMR gave a transition point in the mobility of the protons of water at a_w 0.58. This supports the hypothesis that loss of crispness starts as a result of processes at a molecular level, before the macroscopic glass transition. This also suggests that the presence of water that is not directly attached to the solid matrix causes the loss of crispness at low a_w. At higher a_w increased mobility of the macromolecules will start to play a role. NMR experiments with the separate flour components indicate that the T₂ transition point in starch samples occurs at a lower RH than for gluten. This could imply that starch loses crispness at lower a_w than gluten. Increased mobility of small components and side chains might induce increased energy dissipation upon deformation of the material resulting in less available energy for fracture propagation and with that in a less crispy product.