Developments in glass transition determination in foods using moisture sorption isotherms

The food polymer science (FPS) approach has been effectively used to investigate the physical stability of amorphous food materials. The glass transition, a key FPS parameter, has traditionally been determined using thermal techniques that scan temperature while holding the plasticizer (moisture) content constant. Moisture sorption isotherms provide information about the physical properties of food as the plasticizer level is adjusted and temperature is held constant. New automatic isotherm generators can be used to produce high resolution, dynamic isotherms much faster than traditional static methods. Dynamic isotherms for a small selection of amorphous materials have been investigated and shown to experience distinct inflection points in the water activity region where the glass transition temperature is close to the experimental temperature. Several studies on amorphous spray dried milk powder and amorphous polydextrose indicate very good agreement between glass transitions determined using thermal techniques and dynamic isotherm methods. This agreement suggests that dynamic isotherms are a viable alternative to traditional thermal methods for investigating glass transitions of amorphous foods.     

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.     

Gelatinization and Thermal Properties of Modified Cassava Starches

The thermal properties of seven commercial modified cassava starches, including oxidized, acetylated, cross‐linked, and combined acetylated and cross‐linked starches were studied by differential scanning calorimetry (DSC) in the glassy and rubbery states. Increase in gel hardness in the rubbery state during storage was also monitored, as well as gelatinization behavior. The modified starches were prepared from granular starch and had a degree of substitution in the range 0–0.053. The glass transition temperatures (T_g) of the modified starches were 3–6°C significantly lower than that of the non‐modified starch. The physical aging peak temperatures were also significantly reduced by 2–3°C, compared to the non‐modified starch, while aging enthalpies increased. Starch modifications did not decrease amylopectin retrogradation significantly. During storage, the oxidized starch gel became significantly harder than the non‐modified starch gel, while the hardness of the acetylated and/or cross‐linked starch gels was significantly reduced, which confirmed that acetylation or cross‐linking can decrease hardness, even when the extent of modification is limited. Different modifications controlled different properties of the starch system, with cross‐linking and acetylation influencing the gelatinization behavior and the changes in starch gel texture during storage, respectively.     

Phase and State Transitions in Granular Starches Studied by Dynamic Differential Scanning Calorimetry

Phase and state transitions of granular starches heated in water are studied by dynamic differential scanning calorimetry (DDSC). The DDSC results of various granular starches show a stepwise heat capacity (C_p) change in the storage C_p curve, which starts at the beginning of melting or immediately preceding gelatinization and ends before the melting reaches its peak temperature. Annealing reduces the magnitude of this stepwise change in storage C_p. After the amorphous phase is selectively removed by acid, the stepwise storage C_p is greatly reduced. The stepwise storage C_p increase is not observed when recrystallized linear short chains from enzyme‐debranched waxy maize starch are heated in water. The results show that the stepwise change in C_p observed on the granular starch is a unique property associated with the phase transition of granular structure and reflects a glass transition of the rigid amorphous regions in starch granules. The glass transition of the mobile amorphous phase in granular starch is believed to extend over a broad range of temperatures before gelatinization. A small glass transition below 0°C is directly detected by DDSC and is attributed to part of the mobile amorphous fraction in granular starch.     

Moisture content in honey determination with a shear ultrasonic reflectometer

An ultrasonic method, based on the measurement of the complex reflection coefficient at an interface (elastic material)/(viscoelastic material) has been developed for shear waves and an operating frequency of 10 MHz. With this technique, the dynamic elastic moduli G′ and G′′ can be obtained with a high accuracy. For various honeys and moisture contents we show that the temperature for which (G′′/G′) = 1 undergoes an important variation (25–3 °C) for moisture contents ranging from 15% to 20%. With this technique one can distinguish two honeys with moisture contents difference less than 0.2%. Regarding the high sensitivity of the method, considering the fact that ultrasonic waves can propagate through optically opaque materials and is a nondestructive approach, we propose a discussion about the application of this technique for honey quality control.     

Non-enzymatic browning kinetics analysed through water–solids interactions and water mobility in dehydrated potato

Non-enzymatic browning (NEB) development was studied in dehydrated potato at 70 °C. It was related to the macroscopic and molecular properties and to water–solid interactions over a wide range of water activities. Time resolved ¹H NMR, thermal transitions and water sorption isotherms were evaluated. Although non-enzymatic browning could be detected in the glassy state; colour development was higher in the supercooled state. The reaction rate increased up to a water content of 26 g/100 g of solids (a_w = 0.84) and then decreased at higher water contents, concomitantly with the increase of water proton mobility. The joint analyses of NEB kinetics, water sorption isotherm and proton relaxation behaviour made it evident that the point at which the reaction rate decreased, after a maximum value, could be related to the appearance of highly mobile water. The results obtained in this work indicate that the prediction of chemical reaction kinetics can be performed through the integrated analysis of water sorption, water and solids mobility and the physical state of the matrix.     

Effects of Extrusion Temperature and Plasticizers on the Physical and Functional Properties of Starch Films

Cornstarch, at 20% moisture content (dry basis, d.b.), was mixed with glycerol at 3:1 ratio to form the base material for extruded starch films. Stearic acid, sucrose and urea, at varying concentrations, were tested as secondary plasticizers for the starch‐glycerol mixture. The ingredients were extruded at 110 and 120°C barrel temperatures to determine the effects of extrusion temperature, plasticizer type and their concentrations on the film‐forming characteristics of starch, as well as their effects on selected physical and functional properties of the films. The physical and mechanical properties of the films were studied by scanning electron microscopy (SEM) and tensile testing, while the glass transition and gelatinization properties were analyzed using differential scanning calorimetry (DSC). The interactions between the functional groups of starch and plasticizers were investigated using Fourier‐transform infrared (FTIR) spectroscopy. The water vapor permeability (WVP) properties of starch films were determined using ASTM standard E96‐95. Scanning electron micrographs exhibited the presence of native and partially melted starch granules in the extruded films. The tensile stress, strain at break and Young's modulus of starch films ranged from 0.9 to 3.2 MPa, 26.9 to 56.2% and 4.5 to 67.7 MPa, respectively. DSC scans displayed two glass transitions in the temperature ranges of 0.1 to 1°C and 9.6 to 12°C. Multiple melting endotherms, including that of amylose‐lipid complexes, were observed in the thermoplastic extrudates. The gelatinization enthalpies of the starch in the extruded films varied from 0 to 1.7 J/g, and were dependent largely on the extrusion temperature and plasticizer content. The shift in the FTIR spectral bands, as well as the appearance of double‐peaks, suggested strong hydrogen bonding interactions between the starch and plasticizers. The WVP of starch films ranged from 10.9 to 15.7 g mm h^‐1 m^‐2 kPa^‐1, depending on the extrusion temperature and the type of plasticizer used.     

Moisture sorption isotherms of nutritious supplementary foods prepared from cereals and legumes for feeding rural mothers and children

 Four types of supplementary foods (FSs) based on popped wheat (Triticum vulgare), ragi (Eleusine coracana), bajra (Pennisetum americanum) and sorghum (Sorghum bicolor) blended with soy (Glycine max Merr) and bengalgram (Cicer arietinum) and fortified with vitamins and minerals were prepared on a pilot plant scale. All the FSs contained 10–20% proteins and 350–380 kcal of energy. Moisture sorption characteristics of these FSs were evaluated at room temperature (27  °C) with a view to designing a functional package for such products. The results revealed that a moisture content of about 10% (equilibrium relative humidity 68–70%) is optimum to give a good storage stability to the products. Above the 10% moisture level the products were found to be vulnerable to microbial spoilage (mould growth), leading to quality deterioration. In order to protect these types of products from being spoiled by mould growth during storage, use of moisture-proof packaging materials based on high density polyethylene and polypropylene may be considered. Received: 13 August 1999     

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.     

Overview on mitigation of acrylamide in starchy fried and baked foods

Acrylamide in fried and baked foods has the potential to cause toxic effects in animals and humans. A major challenge lies in developing an effective strategy for acrylamide mitigation in foods without altering its basic properties. Food scientists around the world have developed various methods to mitigate the presence of acrylamide in fried food products. Mitigation techniques using additives such as salts, amino acids, cations and organic acids along with blanching of foods have reduced the concentration of acrylamide. The use of secondary metabolites such as polyphenols also reduces acrylamide concentration in fried food products. Other mitigation techniques such as asparaginase pre‐treatment and low‐temperature air frying with chitosan have been effective in mitigating the concentration of acrylamide. The combined pre‐treatment process along with the use of additives is the latest trend in acrylamide mitigation. © 2018 Society of Chemical Industry     

Physiological and mathematical aspects in setting criteria for decontamination of foods by physical means

In heat processing, microbial inactivation is traditionally described as log-linear. As a general rule, the relation between rate of inactivation and temperature is also described as a log-linear relation. The model is also sometimes applied in pressure and in pulsed electric field (PEF) processing. The model has proven its value by the excellent safety record of the last 80 years, but there are many deviations from log-linearity. This could lead to either over-processing or under-processing resulting in safety problems or, more likely, spoilage problems. As there is a need for minimal processing, accurate information of the inactivation kinetics is badly needed. To predict inactivation more precisely, models have been developed that can cope with deviations of linearity. As extremely low probabilities of survival must be predicted, extrapolation is almost always necessary. However, extrapolation is hardly possible without knowledge of the nature of nonlinearity. Therefore, knowledge of the physiology of inactivation is necessary. This paper discusses the physiology of denaturation by heat, high pressure and pulse electric field. After discussion of the physiological aspects, the various aspects of the development of inactivation models will be addressed. Both general and more specific aspects are discussed such as choice of test strains, effect of the culture conditions, conditions during processing and recovery conditions and mathematical modelling of inactivation. In addition to lethal inactivation, attention will be paid to sublethal inactivation because of its relevance to food preservation. Finally, the principles of quantitative microbiological risk assessment are briefly mentioned to show how appropriate inactivation criteria can be set.     

Elastic properties of extruded starchy melts containing wheat bran using on-line rheology and dynamic mechanical thermal analysis

Addition of wheat bran reduces the expansion volumes of extruded cereals. This may be explained by the influence wheat bran has on the elastic properties of the melt. The elastic properties of extruded refined wheat flour supplemented with wheat bran were investigated using a complementary on-line and an off-line rheological technique. The Bagley pressure (end pressure), reflecting extensional properties of the melt, was measured on-line using the orifice die method (dies with different lengths). The storage modulus of the extruded samples was measured using dynamic mechanical thermal analysis (DMTA). Increasing the bran concentration to an intermediate level significantly increased the Bagley pressure likely due to a reduced mobility of starch molecules. Further increasing the bran concentration significantly decreased the Bagley pressure. This might be caused by physical rupture of the melt at the die exit due to the low adhesion properties between starch and bran particles. This change in Bagley pressure indicates that the elastic properties of the composite melt may be reduced by bran particles. The storage modulus measured by DMTA was decreased when increasing the bran concentration, further supporting a potential decrease of the elastic properties of the melt with bran.     

Effect of moisture content on glass transition and sticky point temperatures of sugarcane, palmyra-palm and date-palm jaggery granules

Sorption isotherms of sugarcane (SC), palmyra-palm (PP) and date-palm (DP) jaggery granules were obtained at 25 and 35 °C following isopiestic vapour transfer technique. Glass transition temperatures ( T _g) and sticky point temperatures ( T _sc) at different moisture contents were determined using DSC and rotational viscometer, respectively. All sorption isotherms fitted well to the GAB equation while no apparent difference in equilibrium moisture content (EMC) was seen among them at either of the temperatures. T _g values for dry granules were 316.89, 335.24 and 325.07 K for SC, PP and DP respectively. T _g could be predicted well with both Gordon and Taylor equation, and combined Gordon and Taylor and GAB equations. T _sc for palmyra-palm was highest (347.3 K), followed by DP (339.6 K) and SC (334.9 K). A generalised linear equation correlates T _g and T _sc. Critical moisture contents for SC, PP and DP are 1.0–2.0, 3.1–4.3 and 1.9–2.9% db, respectively.     

Interactions of heat and mass transfer in steam reheating of starchy foods

Steaming is a common practice in both household and industrial food processing; however, the process of heat transfer during steaming has not been fully explored. In this study, mathematical models that consider coupled energy, water and vapor transport were developed, and were solved using finite element software (COMSOL Multiphysics) for their application to the steam reheating of non-porous starch gel radish cake and steamed buns with porous matrices. Our simulation results indicated that the reheating of non-porous starch gel is a heat conduction process. During the reheating of the porous starch matrix, the infusion of condensate and evaporation–condensation mechanism contribute to steep sigmoid-shaped temperature profiles that are different from those of normal heat conduction. The validity of our mathematical model is corroborated by our experimental work of the steam reheating of samples with and without PE film wraps in a rice cooker. It shows a good agreement between the simulation and experimental results in terms of temperature and increased moisture. The implications of these models can provide a basis for future elucidation of more complicated food steaming processes.     

Fat oxidation in freeze-dried grouper during storage at different temperatures and moisture contents

The effects of moisture content levels and storage temperature on the lipid oxidation in freeze-dried grouper were studied. Three trends were observed in the changes of PV (peroxide value) depending on the moisture content and storage temperature. An exponential rise of PV was observed (p < 0.05) at lower temperatures (−20 °C and 5 °C for samples at moisture 5 g/100 g; and −20 °C for samples at moisture 20 g/100 g). At temperatures 25 °C for samples at moisture 5 g /100 g, and 5 and 25 °C for samples at 20 g moisture/100 g peaks of PV were observed (p < 0.05). At higher temperatures (40 °C for both moisture content levels) an exponential decay instead of increase were observed (p < 0.05). Moisture content levels had no significant effect (p > 0.05) on PV stored at temperature of −40 °C. The monolayer moisture for freeze-dried grouper was estimated as 6.2 g/100 g dry solids using BET-isotherm. Differential scanning calorimetry (DSC and modulated DSC) thermogram line showed two shifts (i.e. glass transition) and two endothermic peaks, one for melting of oil and another for decomposition. The validity of the water activity and glass transition concepts were evaluated. It is concluded that glass transition concept may be used to explain the process more adequately.     

Food Stability Beyond Water Activity and Glass Transtion: Macro-Micro Region Concept in the State Diagram

The water activity concept proposed that a food product is the most stable at its monolayer moisture content. Recently, the limitations of the water activity concept were identified and the glass transition concept was proposed in order to overcome the limitations of water activity. Based on the glass-transition concept, a food is the most stable at and below its glass transition point. Recently it has also become evident that the glass transition concept is not universally valid for stability determination when foods are stored under different conditions. The glass transition concept was used to develop the state diagram by drawing another stability map using freezing curve and glass line. Currently, other components indicating different characteristics are being included in the state diagram. It is being emphasized in the literature to combine the water activity and glass transition concepts. In this paper, an attempt is made to combine these two concepts in the state diagram and to propose a macro-micro region concept for determining the stability of foods.     

Water Sorption and Plasticization Behavior of Spray-dried Lactose/Protein Mixtures

ABSTRACT: Water sorption properties, effects of proteins on glass transition temperature, and time-dependent lactose crystallization of spray-dried lactose and lactose in lactose/WPI (3:1), lactose/Na-caseinate (3:1), lactose/albumin (3:1), and lactose/gelatin (3:1) mixtures were investigated. Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models were used to model water sorption. Lactose/protein mixtures sorbed high amounts of water at low relative vapor pressure (RVP) up to 23.1%. Above 23.1% RVP levels, water sorbed by pure lactose was higher, up to 44.1% RVP, except in the case of the lactose/gelatin mixture. Lactose/ gelatin also sorbed a high amount of water at 33.2% RVP. Loss of sorbed water resulting from crystallization of amorphous lactose was observed. Crystallization of pure lactose and lactose crystallization in lactose/protein mixtures occurred at RVP ≥ 44.1% within 24 h. After crystallization at RVP ≥ 54.5%, water contents remained higher for lactose/protein mixtures than for pure lactose. The rate of lactose crystallization was less in all lactose/protein mixtures than was observed for pure lactose. WPI had the lowest effect on lactose crystallization. Crystallization occurred most slowly in lactose/gelatin mixtures. Both GAB and BET models fitted to water sorption data up to 0.441 a_w. It seems that different proteins interact with lactose differently. Water sorption and time-dependent lactose crystallization of lactose/protein mixtures have important consequences to processing and storage behavior of lactose-protein based products.