Mechanical relaxation times as indicators of stickiness in skim milk-maltodextrin solids systems

Thermal and water plasticization and glass transition of amorphous components often result in changes in physico-chemical properties of food solids including stickiness and component crystallization. The glass transition temperatures (T_g) of skim milk-maltodextrin solids systems were measured by differential scanning calorimetry (DSC) and a sticky point test was used to determine the sticky-point temperatures (SPT). The mechanical α-relaxation around glass transition was measured by dynamic-mechanical analysis (DMA). The Guggenheim-Anderson-deBoer (GAB) model was fitted to water sorption data and sorption isotherms were established. The glass transition and water sorption properties were dependent on the dextrose equivalent (DE) of the maltodextrin and the skim milk-maltodextrin solids system composition. Maltodextrins and lactose in skim milk solids were miscible and lactose crystallization was delayed in solids containing maltodextrins. The mechanical α-relaxation behavior was governed by the skim milk-maltodextrin solids system composition and was related to the development of stickiness. The α-relaxation at varying frequencies occurred at higher temperatures in solids with increasing maltodextrin contents and the sticky point was governed by the glass transition of the carbohydrates phase.     

Coupling of dielectric and mechanical relaxations with glass transition and stickiness of milk solids

Dielectric and mechanical α-relaxations of milk solids with varying milk protein contents were determined by dielectric (DEA) and dynamic-mechanical (DMA) analysis, respectively. The frequency dependence of α-relaxations occurring around and above the glass transition was modeled by the Vogel-Fulcher-Tamman (VTF) relationship. The α-relaxations were governed by the amorphous lactose and shifted to higher temperatures when protein contents of milk solids were increased at all water activities, a_w. Increasing protein contents lowered the overall molecular motions of milk solid components which were shown by small changes in dielectric and mechanical properties above the glass transition. The α-relaxations were strongly frequency-dependent. At temperatures around and above the glass transition, the primary α processes were recorded and the corresponding relaxation times were determined. The temperature dependence of the relaxation times was affected by glass transition, solid composition and water. The relaxation times decreased with increasing temperature and water content as a result of thermal and water plasticization. Higher protein contents increased the relaxation times measured for milk solids. This was associated with small changes in α-relaxation characteristics and reduced powder stickiness. Data on the compositional effects on the relaxation times of milk solids were related to flow characteristics and powder stickiness around and above the glass transition.     

Mechanical α-relaxations and stickiness of milk solids/maltodextrin systems around glass transition

BACKGROUND: Stickiness correlates with changes in mechanical α‐relaxation properties and often results from glass transition and plasticisation of amorphous food components. In this study, milk solids with maltodextrins with different dextrose equivalents (DE9 and DE17) were analysed for glass transition (T_g), α‐relaxation (T_α) and sticky point (SPT) temperatures using differential scanning calorimetry, dynamic mechanical analysis and a sticky point test respectively. RESULTS: At the same maltodextrin contents, T_g and T_α were lower for milk solids with the higher‐DE maltodextrin. Increasing maltodextrin contents gave T_g, T_α and SPT at higher temperatures, and the magnitudes of α‐relaxations with high maltodextrin (DE9 and DE17) contents were less pronounced. CONCLUSION: Stickiness was governed by glass transition and affected by skim milk/maltodextrin composition. Stickiness was reduced with increasing maltodextrin content as a result of maltodextrin miscibility with skim milk solids, particularly lactose, which changed the relaxation behaviour above the glass transition. The mixes of milk solids with low‐DE maltodextrin may show improved dehydration characteristics and powder stability resulting from increased T_g, T_α and SPT. Copyright © 2011 Society of Chemical Industry     

Influence of milk proteins on the development of lactose-induced stickiness in dairy powders

The stickiness behaviour of a range of spray dried dairy powders differing in protein/lactose ratio was determined using a fluidised bed apparatus. Powders with higher protein/lactose ratios were less susceptible to sticking. Stickiness was related to both the glass transition temperature (T_g) and the temperature increment by which T_g must be exceeded before sticking occurred (T?T_g). T?T_g values of approximately 10, 22, 29, 45 and 90 °C were found for powders containing 15.5, 26.9, 39.5, 55.7 and 83.4% protein respectively. Composition had different effects on T_g and T?T_g. The rate at which water was sorbed and desorbed by powders increased with protein content. With increasing protein content, preferential sorption of water by non-amorphous constituents delayed the rate at which lactose underwent the requisite change from the ‘glassy’ to the ‘rubbery’ form in order that powder particles became sticky.     

Effect of protein concentration on the surface composition,water sorption and glass transition temperature of spray-dried skim milk powders

The effects of dilution of protein content in skim milk (34–8.5% protein content), by lactose addition, on the surface composition, water sorption property and glass transition temperatures of spray-dried powders were investigated. The X-ray photoelectron spectroscopy (XPS) study of spray-dried powders showed preferential migration of proteins toward the surface of the milk particles whereas the lactose remained in the bulk. Sorption studies showed that the lower protein concentration in milk powders is linked to an increased water adsorption property and lowering of water activity (a_w) for lactose crystallization. Analysis of glass transition temperature (T_g) of the powders sorbed at different humidities showed no distinct change in T_g values, indicating the dominant effect of lactose on the glass transition temperature of all the powders.     

Water Plasticization and Crystallization of Lactose in Spray-dried Lactose/Protein Mixtures

ABSTRACT: Water plasticization led to depression of the glass transition causing significant changes in the physico-chemical and crystallization properties in storage of lactose and lactose/protein (3:1) mixtures. Glass transition (Tg) and crystallization temperatures (Tcr) were determined using differential scanning calorimetry. Whey protein isolate (WPI), albumin, and gelatin increased the Tg of dry powders; when Na-caseinate was used, a decrease was observed. In the presence of proteins and water, a decrease of Tg at aw ≤ 0.23 was observed. At aw ≤ 0.33, proteins increased the Tg In the anhydrous state, Tcr decreased in the presence of proteins possibly because of browning. WPI, Na-caseinate, albumin, and gelatin delayed lactose crystallization in humidified samples, with albumin and gelatin delaying it more than WPI at all storage humidities. Temperature difference between an observed instant crystallization and glass transition (Tcr to Tg) was larger for humidified samples containing proteins than for lactose. Various proteins and water affect crystallization behavior of amorphous lactose differently in spray-dried powders. This should be considered in evaluating sugar crystallization properties in food products including dairy powders.     

Analysis of particle-gun-derived dairy powder stickiness curves

The stickiness curves of a range of dairy powders were measured using a particle-gun rig. The stickiness curves for the powders were shown to run parallel but above the curve of the glass transition temperature (T_g) of amorphous lactose. By assuming that the amorphous lactose at the surface of the powder was in equilibrium with the exit conditions of the air from the particle gun, it was found that for any particular dairy powder sample, the amount of powder deposition measured on the particle-gun target disc collapsed into a single function of the temperature difference by which the amorphous lactose T_g at the surface was exceeded. The x-axis intercept of these plots was calculated and designated as (T−T_g)_crit, characterizing the conditions for initiation of stickiness of the powder. The sensitivity of each powder to stickiness problems when placed in conditions where the critical T−T_g value at the surface is exceeded was quantified with the slope of the plot. These results show that it is the amorphous lactose component that is probably the main cause of stickiness in dairy powders and demonstrates how the particle-gun rig can be used to characterize the stickiness behaviour of powders over a wide range of conditions with two parameters.     

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.     

Retention of Diacetyl in Milk during Spray-Drying and Storage

Spray-drying of milk in a Leaflash dryer was studied in relation to volatile aroma loss in drying and during storage of the powders. Diacetyl was used as the volatile model, and the contribution of the various milk constituents to retention was determined. Proteins retain more diacetyl than does lactose or fat. Conditions leading to a powder with more amorphous lactose result in higher diacetyl loss during drying. The rate of diacetyl loss during storage depended on relative humidity and on the T — T_g parameter and was strongly enhanced by crystallization of amorphous lactose.     

Stickiness curves of high fat dairy powders using the particle gun

High fat (>42%) dairy powders are inherently sticky due to their high levels of liquid surface fat. Incorrect operating conditions when spray drying these powders can rapidly lead to blockages. The particle gun was used to characterise the stickiness curves of high fat cream and cheese powders. Stickiness was shown to increase with increasing temperature to a maximum at 50 °C after which it decreased until no stickiness was observed above 68 °C. A dramatic increase in stickiness for the powders was found when the relative humidity of the air was increased past a certain critical point for each temperature. This was attributed to the lactose component of the powder exceeding its glass transition temperature by a critical amount. Best estimates of the (T−T_g)_crit. values for White Cheese Powder, Low Fat Cream Powder and High Fat Cream Powder were 28, 37 and 38 °C, respectively.     

Crystallization of Amorphous Lactose

Crystallization of amorphous lactose was studied at constant water content and at constant relative humidity above glass transition temperature (Tg). The rate of crystallization was followed using differential scanning calorimetry. Crystallization at constant water content increased the amount of water in the remaining amorphous matrix, which decreased T_g and accelerated crystallization. At constant relative humidity crystallization proceeded at a rate determined by T -T_g which increased with crystallinity. Temperature dependence of time to complete crystallization was confirmed to follow Williams-Landel-Ferry (WLF) equation. The results can be used to evaluate diffusion and crystallization kinetics of amorphous carbohydrates.     

Milk powders ageing: effect on physical and functional properties

Milk powders are now considered as food ingredients, mainly because of the functional properties of milk proteins. During the storage of milk powders, many physicochemical damages, mainly dependent on lactose glass transition occur. They have important consequences on physical (flowability) and functional properties (solubility, emulsifying, and foaming properties) of milk powders. First, lactose crystallization modifies the microstructure and chemical composition of the surface of powder particles. Thus, milk powders flowability is decreased. Since the structure of milk proteins is destabilized, its solubility is damaged. Moreover, particle collapse and caking occur and mainly decrease the physical properties of milk powders (density and flowability). The mechanical stresses involved may also enhance proteins unfolding, which is detrimental to solubility. Finally, molecular mobility is favored upon ageing, and both chemical (Maillard reaction) and enzymatic reactions occur. Maillard reaction and oxidation enhance protein interactions and aggregations, which mainly lessen milk powders solubility. Maillard reaction also decreases emulsifying and foaming properties. Storage temperature and relative humidity have been considered as the predominant factors involved, but time, milk components, and their physical state also have been implied.     

Thickening of molten white chocolates during storage

When white chocolates are kept molten in storage tanks, problems can arise due to uncontrolled thickening and solidifying of the chocolate mass. The thickening of molten white chocolate was simulated on a laboratory scale using a rotational rheometer under static conditions, interrupted by short shear periods to measure the increasing viscosity. Several chocolates having different dairy components and fat contents were investigated for their tendency to thicken. In addition, sorption isotherms for white chocolates were obtained using dynamic vapour sorption at different temperatures. The sorption isotherms showed the presence of amorphous lactose in all the chocolates that were manufactured from milk powders. Moisture that is released during the crystallization of amorphous lactose causes stickiness and agglomeration of the neighbouring particles and starts the thickening process. This process is highly temperature-dependent. On elevating the temperature the lactose crystallization occurs at lower relative humidities. In order to reduce the tendency of white chocolate to thicken, a high free-fat level should be maintained, based on a high total fat content and on the use of high free-fat milk powders, preferably roller-dried whole milk powders or the combination of skimmed milk powder and anhydrous milk fat.     

Water sorption and time-dependent crystallization behaviour of freeze-dried lactose-salt mixtures

Water sorption properties of freeze-dried lactose, lactose/CaCl₂, lactose/NaCl, lactose/MgCl₂, and lactose/KCl mixtures in their molar ratio of (9:1) were investigated. Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models were used to model water sorption properties. Water is known to function as a plasticizer, depressing the glass transition and facilitating crystallization. Crystallization in the present study resulted in loss of sorbed water from lactose. The crystallization of pure lactose and lactose/salt mixtures was observed at RVP?44.0% within 24 h. At RVP?54.4% water contents were higher in lactose/CaCl₂ and lactose/MgCl₂ mixtures than in pure lactose, lactose/NaCl, and lactose/KCl.Water content in pure lactose after crystallization was ?5.0%, suggesting that lactose crystallized as a mixture of α-lactose monohydrate and various anhydrous forms of α/β-lactose crystals. Anhydrous lactose/CaCl₂ and lactose/MgCl₂ had higher glass transition temperatures than lactose, but other salts (NaCl and KCl) with lactose gave lower glass transition than amorphous lactose. It seems that bivalent salts in mixtures with lactose gave a higher T_g than smaller monovalent ions. Salts delayed lactose crystallization. The effect on lactose crystallization was highest with calcium chloride (CaCl₂) and lowest with potassium chloride (KCl). It seems that different salts interacted with lactose to different extents. For water sorption, GAB model gave a better fit than BET model. Water sorption and time-dependent crystallization properties of lactose/salt mixtures should be considered in manufacturing and storage of dairy-based dehydrated materials.     

Structural strength and crystallization of amorphous lactose in food model solids at various water activities

Freeze-dried lactose and lactose/whey protein isolate (WPI) mixtures were used as amorphous food models at various a_w, and the effects of temperature and water and WPI contents on physical state were analyzed. Thermal behavior and mechanical properties were studied and Williams-Landel-Ferry (WLF) model was fitted to structural relaxation times (τ). The WLF-analysis gave a strength parameter (S) that was used to describe structural strength of the food solids. Our results showed that lactose and WPI in mixtures exhibited fractional water sorption. Thermal properties and structural strength of the solids were affected by water and WPI while T_g measured for the lactose/WPI systems followed that of the lactose component and showed phase separation of lactose and proteins. A relationship between S and water content was established, whereas the crystallization of amorphous lactose was more rapid in systems with a smaller S. Therefore, S provided a simple and convenient measure of τ controlling structure formation in food processing as well as to control lactose crystallization.Industrial relevanceSugars are common ingredients and often used as a mixture with other components, e.g., proteins, in the food and pharmaceutical industries. Thus, understanding the physical state and thermal behavior of sugar containing food materials has a great importance in the development of processing and shelf life control procedures for such ingredients and relevant products. This study provides physicochemical information about thermal and mechanical properties of freeze-dried lactose/whey protein systems used as food models at various water activities. Data on water sorption, time-dependent lactose crystallization, calorimetric glass transition and crystallization temperatures, and structural relaxation times can be used to understand and predict structural changes during processing and storage of relevant foods. Moreover, the structural strength concept, described in this study, allows of the control of crystallization behavior as a physical state and time-dependent phenomenon, and therefore, stability of food and pharmaceutical materials     

Effect of caking and stickiness on the retention of spray‐dried encapsulated orange peel oil

Flavour microcapsules containing amorphous carbohydrate as wall material can undergo changes such as crystallisation, clumping, sticking and caking during handling and storage. Such physical changes may lead to the release of entrapped flavours. The objective of this study was to investigate the effect of storage temperature and water activity on caking, stickiness and glass transition temperatures and to evaluate the relative degree of protection provided to orange peel oil entrapped in mesquite (Prosopis juliflora) gum by spray drying. The powders were stored at water activities (a_w) ranging from 0.108 to 0.972 at 25 and 35 °C. The surface caking temperature (T_sc) and advance caking temperature (T_ac) were determined by the modified ampoule and sealed glass tube methods respectively. The glass transition temperature was determined by differential scanning calorimetry. Changes in the amount of encapsulated oil were determined by Clevenger hydrodistillation. As expected, both T_sc and T_ac decreased with increasing storage a_w. Above a_w 0.628 the powders caked and collapsed during storage at 35 °C. Below a_w 0.628 the capsules were not damaged and high retention levels (above 90%) were obtained. Increasing a_w in the range 0.743–0.972 caused progressive dissolution of the wall polymer, and the retention level dropped sharply. The volatiles are protected and retained by mesquite gum as long as the capsule structure remains intact. Copyright © 2003 Society of Chemical Industry     

Water sorption and glass transition temperature of spray dried açai (Euterpe oleracea Mart.) juice

Sorption isotherms and glass transition temperature (T_g) of powdered açai juice were evaluated in this work. Powders were produced by spray drying using different materials as carrier agents: maltodextrin 10DE, maltodextrin 20DE, gum Arabic and tapioca starch. The sorption isotherms were determined by the gravimetric method, while the T_g of powders conditioned at various water activities were determined by differential scanning calorimetry. As results, experimental data of water adsorption were well fitted to both BET and GAB models. Powders produced with maltodextrin 20DE and gum Arabic showed the highest water adsorption, followed by those produced with maltodextrin 10DE and with tapioca starch, respectively. With respect to the glass transition temperature, Gordon–Taylor model was able to predict the strong plasticizing effect of water on this property. Both a_w and T_g were used to determine the critical conditions for food storage, at which powders are not susceptible to deteriorative changes such as collapse, stickiness and caking.     

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.     

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.     

Glass transition and caking of spray-dried lactose

Simple test methods to determine the onset temperatures of viscous flow and caking of spray-dried amorphous lactose are described. The extent of viscous flow was measured as an increase in the density of lactose plugs within a cylindrical aluminium compaction apparatus after incubation for 3h at a specified temperature. Caking was characterized by an increase in the hardness of the lactose plugs formed. The onset temperature of viscous flow decreased with increasing water activity, A_w, and corresponded to the onset temperature of glass transition, T_g1. Glass transition temperatures were determined using both differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spin relaxation. The results suggest that elevation of the powder temperature above T_g1 promotes viscous flow and increases the potential for caking of amorphous food powders.