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.     

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.     

Effect of water activity on sugar crystallization and β-carotene stability of freeze-dried mango powder

Storage water activity (a_w) affects the stability of freeze-dried food. The sugar crystallization and storage stability of β-carotene in freeze-dried mango powder was investigated following storage under various relative vapor pressures (11.3-80.9%). Sugar crystallization was revealed by the loss of sorbed water in the water sorption experiment. However, the sorption isotherms showed unclear divergence between the experimental and predicted values. X-ray powder diffraction and scanning electron microscopy were used to confirm the crystallization. The results showed that increased a_w resulted in higher sugar crystallization. The loss of β-carotene was monitored by high-performance liquid chromatography with a diode-array detector and fitted to a first-order reaction. The rate constant decreased as a_w increased up to 0.43, due to the collapse of the mango powder. An increase in the rate constant above this value of a_w coincided with pronounced sugar crystallization. Choosing the appropriate value of a_w for storage can prevent sugar crystallization and enhance β-carotene stability in freeze-dried fruit powder.     

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.     

Predicting Water Activity in Solutions of Mixed Solutes

The Ross equation is a multiplicative procedure for estimating the water activity (a_w) of aqueous unprocessed food preparations that contain only solutes, only food polymers, or mixtures of solutes and polymers. Using accurate a_w values of single solute solutions in the Ross equation resulted in prediction accuracies to ± 0.01 a_w for 118 of 120 multiple-solute solutions. The exceptions at ±0.02 a_w occurred within a range of concentrated solutions containing NaCl, KCl, and CaCl₂.     

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.     

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.     

Predicting Water Activity from 0.30 to 0.95 of a Multicomponent Food Formulation

The objective was to develop an equation to calculate the a_w of a mixture of known composition at a given moisture content. A simple equation was derived by combining the Smith isotherm with the Lang-Steinberg mass balance: log (1–a_w) = [MW–Σ(a_iw_i)]/Σ(b_iw_i) where M is moisture content of the mixture, W is total dry weight of the mixture, a_i and b_i are Smith isotherm parameters for the individual ingredient, and w_i is the dry weight of the ingredient. For comparison of experimental and calculated a_w values, an experimental value was used to calculate M from the mixture isotherm and this M was used to calculate a_w, from the model. Four binary and one ternary mixtures of two macromolecules, two solutes and one complex ingredient were included. Twenty-one comparisons between calculated and experimental a_w, over a_w 0.30–0.95 resulted in a maximum error of only 1.86% and a mean error of -0.25%, showing excellent agreement. Only one constraint was found; when the mixture contains i solute (sugar or salt) and the a_w, is below the a_w of saturation for that solute, that solute is deleted from the summations in the Model.     

Isotherm Equations for Starch, Sucrose and Salt for Calculation of High System Water Activities

The mass balance equations previously developed for formulated foods were extended to the starch-sucrose-salt system at high a_w. For starch, the Smith isotherm was extrapolated; sucrose and salt data were divided into two a_w regions below and above 0.925 with the second region extrapolated on the Smith isotherm to 0.9998 a_w. Isotherm parameters and a_w calculated for representative mixtures were determined. Comparison of the calculated values with those measured by the isopiestic method showed a correlation coefficient of 0.968.     

A Corrected Ross Equation

A simple method for integrating and extending the binary form of the Gibbs-Duhem equation for routine application in food technology is given. The integrated equation was used to show that considerable solute-solute interaction occurs in mixed solutions which, when corrected for in the Ross equation, results in a revised equation whose predictions of the a_w of mixed solutions more closely agree with measured values. The revised equation indicates that one can obtain accurate estimates of the a_w of a mixed solution by combining estimated water activities with estimated interactions based on the number and concentrations of the solution components.     

Mixed Saturated Salt Solutions as Standards for Water Activity Measurement in the Microbiological Growth Range

Present work explores the feasibility of using mixed saturated salt solutions to provide additional solutions of known water activity (a_w). The mixed saturated salt solutions studied were: K₂SO₄-KNO₃, K₂SO₄-MgSO₄, KCl-KNO₃, NaCl-KCl and NaCl-NaNO₃. The a_w of the ternary saturated salt mixtures was predicted using a theoretical model of the thermodynamic properties of mixed aqueous electrolyte solutions. Prediction of a_w compared very well with experimental determinations.     

Effect of preservatives on the functional properties of tapioca starch: analysis of interactions

The effect of different levels of potassium sorbate or sodium benzoate addition on the water sorption behavior and on the mechanical properties of freeze-dried tapioca starch gel was studied.It was observed that the addition of potassium sorbate or sodium benzoate to starch before the gelatinization process increased the amount of water sorbed by the system, but experimental moisture content of the mixture was lower than the one predicted by a mass balance, revealing the existence of an interaction between starch and the preservatives. In general, in the range of water activity 0.755-0.976, most of the preservative was bounded to starch. Mechanical properties of the powder obtained after freeze drying of gels were also influenced by the addition of the preservatives: an increase in bulk density, higher cohesiveness and initial Young modulus was observed after antimicrobial addition.Results obtained showed that sorbate and benzoate interact with tapioca starch and that this interaction has a great influence on system behavior.     

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.     

Water activity of multicomponent mixture of solutes and non-solutes

An equation has been derived to calculate the water activity (A_w) of multicomponent mixtures of solutes and non‐solutes. The equation is based on a previous Caurie adsorption equation and it depends on the number of component ingredients and the initial moisture contents of the ingredients in the mixture. Solute and non‐solute/water interaction effects in solution were identified as playing important roles in determining accurate equilibrium A_ws of the aqueous mixtures. The new equation allows for interaction effects in mixed solution/suspensions and has been tested on model foods (the data was taken from the literature) containing up to five solutes and non‐solute components. The equation displays a high degree of accuracy. Unlike the Ross equation, which applies only approximately to mixtures of dilute solutions, the new equation applies to mixtures at all solution concentrations and to wet solids.     

Influence of water activity on microbial development in minced,cured pork,in relation to gas headspace composition

The water activity (a_w) of a cured minced meat mixture was reduced with NaCl or glycerol + NaCl and the products stored in air, N₂ or CO₂ at 4°C. The lower the a_w, the lower was the growth rate of the total microflora except when a_w was reduced below 0·98 with glycerol and products were stored in CO₂, when the growth rate was independent of a_w. At a_w 0·94 NaCl was more effective than glycerol + NaCl in reducing the growth rate. In air the final microflora of products stored at a_w ≤0·96 consisted primarily of yeasts while at 0.98 a_w, both Lactobacillus (62%) and yeasts (38%) were found. In CO₂ and N₂Lactobacillus predominated at 0·94–0·98 a_w when 2% NaCl or NaCl + glycerol were used. At 0·94 a_w, produced using 6% NaCl, a heterogenous final microflora was found, including Micrococcus and Staphylococcus. An increase in the microbial shelf-life of meat products may be obtained by reducing the a_w, particularly for use with storage in N₂- and CO₂-atmospheres. The type of solute chosen to regulate a_w may affect the solubility of CO₂, which in turn affects the shelf-life.     

Chlorophyll Degradation in a Spinach System at Low and Intermediate Water Activities

Chlorophyll degradation in blanched freeze-dried spinach puree was studied in model systems as a function of water activity (a_w), pH, temperature and also in the presence of glycerol, a water-binding agent. For a_w > 0.32 the most important mechanism of chlorophyll degradation is the transformation into pheophytin. The reaction can occur at very limited water concentration and has a first order dependence on pH, water activity and pigment concentration. The addition of glycerol increases the rate of degradation of chlorophyll. At temperatures higher than 56.7°C and a_w= 0.11, the energy of activation increased, indicating different mechanisms. For temperatures below 38°C the rate tended to increase instead of decreasing with decreasing temperatures because of changes in the sorption isotherm (increase of water content) caused by the shift in the temperature.     

Characterization of Physical and Mechanical Properties of Miscible Lactose‐Sugars Systems

Lactose‐sugars systems were produced by spray drying. They were lactose, lactose–glucose (4:1) mixtures, lactose–maltose (4:1) mixtures, lactose–sucrose (4:1) mixtures, lactose–trehalose (4:1) mixtures, and lactose–corn syrup solids (CSS) (4:1) mixtures. The physical characteristics, water sorption behavior, glass transition, and mechanical properties of miscible lactose‐sugars systems were investigated. Lactose–glucose mixtures had larger particle size than other lactose‐sugars systems after spray drying. The presence of glucose or sucrose in lactose‐sugars mixtures decreased the glass transition temperatures of amorphous systems, while the presence of maltose and trehalose had only minor impact on the glass transition temperatures. Moreover, glucose accelerated the crystallization of amorphous system at 0.44 a_w, but its presence delayed the loss of sorbed water at higher water activities (≥0.54 a_w). Mechanical property study indicated that glucose and sucrose in amorphous system could result in an increase of molecular mobility, while the presence of CSS could decrease the free volume and maintain the stiffness of the miscible systems.     

A Factorial Approach to Modeling aw of a Multicomponent Food in the High Moisture Range (aw 0.90–1.00)

Water activity (a_w) depression in a multicomponent food such as fish sausage was investigated using a 4x3x3 factorial design on combinations of NaCl, nonfat dry milk (NFDM) and lard added to a fixed amount of minced fish. The effects of NaCl, NFDM, lard and the interaction NaCl x NFDM, were significant. The responses across levels of the above factors were linear. From the magnitude of effect of each factor, two extended models for predicting a_w were derived in terms of molal salt concentration and two empirical parameters called water activity depressing capacity, d_i, and water binding capacity, WBC, respectively. Both were demonstrated to predict accurately a_w of newly formulated fish sausage.     

Glass transition and crystallization behaviour of freeze-dried lactose-salt mixtures

Glass transition temperatures were determined for dehydrated lactose/salt mixtures with various water contents and water activities, and state diagrams were established. Crystallization behaviour was studied for pure amorphous lactose stored at various relative water vapour pressures (RVP). Furthermore, glass transitions temperatures and time-dependent lactose crystallization of freeze-dried lactose and lactose/CaCl₂, lactose/NaCl, lactose/MgCl₂ and lactose/KCl mixtures in molar ratios of 9:1 were determined. Glass transition temperatures (T_g) of lactose powder as determined by differential scanning calorimetry (DSC) was lower than that of lactose/CaCl₂ (9:1)_, and lactose/MgCl₂ (9:1), but it was slightly higher than the T_g of lactose/NaCl (9:1), and lactose/KCl (9:1). Lactose/KCl had the lowest glass transition temperature, but it had about the same crystallization temperature as lactose/NaCl, and lactose/MgCl₂. The glass transition temperatures decreased as water contents increased. The critical water contents and water activities at 23 °C were predicted using data on glass transition temperature and water sorption. Pure lactose had a different critical water activity and water content from lactose/salt mixtures. The critical values of lactose/CaCl₂ (9:1) were the highest. Loss of sorbed water, indicating lactose crystallization, was observed in lactose and lactose/salt mixtures stored above the critical RVP.     

On the selection of relevant environmental factors to predict microbial dynamics in solidified media

Several studies have shown that food structure causes slower growth rates and narrower growth boundaries of bacteria compared to laboratory media. In predictive microbiology, both a_w or corresponding solute concentration (mainly NaCl) have been used as a growth influencing factor for kinetic models or growth/no growth interface models. The majority of these models have been based on data generated in liquid broth media with NaCl as the predominant a_w influencing solute. However, in complex food systems, other a_w influencing components might be present, next to NaCl. In this study, the growth rate of Salmonella typhimurium was studied in the growth region and the growth/no growth response was tested in Tryptic Soy Broth at 20 °C at varying gelatin concentration (0, 10, 50 g L⁻¹ gelatin), pH (3.25–5.5) and water activity (a_w) (0.929–0.996). From the viewpoint of water activity, the results suggest that NaCl is the main a_w affecting compound. However, gelatin seemed to have an effect on medium a_w too. Moreover, there is also an interaction effect between NaCl and gelatin. From the microbial viewpoint, the results confirmed that the a_w decreasing effect of gelatin is less harmful to cells than the effect of Na⁺ ions. The unexpected shift of the growth/no growth interface to more severe conditions when going from a liquid medium to a medium with 10 g L⁻¹ gelatin is more pronounced when formulating the models in terms of a_w than in terms of NaCl concentrations. At 50 g L⁻¹ gelatin, the model factored with NaCl concentration shifts to milder conditions (concordant to literature results) while the model with a_w indicates a further shift to more severe conditions, which is due to the water activity lowering effect of gelatin and the interaction between gelatin and NaCl. The results suggest that solute concentration should be used instead of a_w, both for kinetic models in the growth region and for growth/no growth interface models, if the transferability of models to solid foods is to be increased.