Increasing interest in high-quality food products with increased shelf life and reduced environmental impact has encouraged the study and development of edible and/or biodegradable polymer films and coatings. Edible films provide the opportunity to effectively control mass transfer among different components in a food or between the food and its surrounding environment. The diversity of proteins that results from an almost limitless number of side-chain amino-acid sequential arrangements allows for a wide range of interactions and chemical reactions to take place as proteins denature and cross-link during heat processing. Proteins such as wheat gluten, corn zein, soy protein, myofibrillar proteins, and whey proteins have been successfully formed into films using thermoplastic processes such as compression molding and extrusion. Thermoplastic processing can result in a highly efficient manufacturing method with commercial potential for large-scale production of edible films due to the low moisture levels, high temperatures, and short times used. Addition of water, glycerol, sorbitol, sucrose, and other plasticizers allows the proteins to undergo the glass transition and facilitates deformation and processability without thermal degradation. Target film variables, important in predicting biopackage performance under various conditions, include mechanical, thermal, barrier, and microstructural properties. Comparisons of film properties should be made with care since results depend on parameters such as film-forming materials, film formulation, fabrication method, operating conditions, testing equipment, and testing conditions. Film applications include their use as wraps, pouches, bags, casings, and sachets to protect foods, reduce waste, and improve package recyclability.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1750-3841.2007.00636.x

Thermoplastic Processing of Proteins for Film Formation—A Review

The unique properties of wheat reside primarily in its gluten-forming storage proteins. Their intrinsic viscoelastic behavior is responsible for the characteristics of different wheat-based foods and for the use of wheat gluten proteins in different food products. Wheat-based food processing generally develops and sets the gluten protein network. Heat-induced gluten aggregation proceeds through cross-linking within and between its protein fractions. Prominent reactions include sulfhydryl (SH) oxidation and SH-disulfide (SS) interchange, which lead to SS cross-links. Other covalent bonds are also formed. Gluten functionality can be (bio-) chemically impacted. We focus on bread making, in which gluten proteins contribute to dough properties, bread loaf volume, and structure, and on pasta production, in which gluten proteins generate the desired cooking quality. Furthermore, it is speculated that the structure and texture of soft wheat products are also, at least to some degree, shaped by the heat-induced changes in the gluten protein fraction.

Wheat Gluten Functionality as a Quality Determinant in Cereal-Based Food Products

The last decade has seen the development of an alternative chemistry, which intends to reduce the human impact on the environment. The polymers are obviously involved into this tendency and numerous bio-sourced plastics (bioplastics), such as polylactide, plasticized starch, etc., have been elaborated. However, even if a lot of commercial products are now available, their properties (mechanical properties, moisture sensitivity) have to be enhanced to be really competitive with the petroleum-based plastics. One of the most promising answers to overcome these weaknesses is the elaboration of nano-biocomposites, namely the dispersion of nano-sized filler into a biopolymer matrix. This review reports the last developments in nano-biocomposites based on polysaccharides and nanoclays. The main elaboration strategies developed in starch, chitosan, cellulose acetate and pectin based nano-biocomposites elaborated with montmorillonite as the nanofiller are exposed herein. The corresponding dispersion state and properties are discussed.

Progress in nano-biocomposites based on polysaccharides and nanoclays

Increasing interest in competitive, sustainable, and biodegradable alternatives to petroleum-based plastics has encouraged the developmentof protein-based plastics. The formation of a homogeneous protein melt during extrusion occurs through: denaturation, dissociation, unraveling, and alignment of polymer chains. The presence of covalent cross-links is unfavorable, decreasing chain mobility, increasing viscosity and preventing homogenization. Proteins have high softening temperatures, often above their decomposition temperatures. To avoid degradation, the required chain mobility is achieved by plasticizers. By understanding a protein's physiochemical nature, additives can be selected that lead to a bioplastic with good processability. The final structural and functional properties are highly dependent on the protein and processing conditions.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mame.200900167

Extrusion Processing and Properties of Protein-Based Thermoplastics

The Arrhenius equation has been widely used as a model of the temperature effect on the rate of chemical reactions and biological processes in foods. Since the model requires that the rate increase monotonically with temperature, its applicability to enzymatic reactions and microbial growth, which have optimal temperature, is obviously limited. This is also true for microbial inactivation and chemical reactions that only start at an elevated temperature, and for complex processes and reactions that do not follow fixed order kinetics, that is, where the isothermal rate constant, however defined, is a function of both temperature and time. The linearity of the Arrhenius plot, that is, Ln[k(T)] vs. 1/T where T is in °K has been traditionally considered evidence of the model's validity. Consequently, the slope of the plot has been used to calculate the reaction or processes’ “energy of activation,” usually without independent verification. Many experimental and simulated rate constant vs. temperature relation...

The Arrhenius Equation Revisited

Gluten-glycerol dough was extruded under a variety of processing conditions using a corotating self-wiping twin-screw extruder. Influence of feed rate, screw speed, and barrel temperature on processing parameters (die pressure, product temperature, residence time, specific energy) were examined. Use of flow modeling was successful for describing the evolution of the main flow parameters during processing. Rheological properties of extruded samples exhibited network-like behavior and were characterized and modeled by Cole-Cole distributions. Changes in molecular sizes of proteins during extrusion were measured by chromatography and appeared to be correlated to molecular size between network strands, as derived from the rheological properties of the materials obtained. Depending on operating conditions, extrudates presented very different surface aspects, ranging from very smooth-surfaced extrudates with high swell to completely broken extrudates. The results indicated that extrudate breakup was ca...

Extrusion of wheat gluten plasticized with glycerol: Influence of process conditions on flow behavior, rheological properties, and molecular size distribution

The rheological behaviour of a gluten plasticized with glycerol has been studied in oscillatory shear. The mixing operation in a Haake batch mixer leads to a maximum torque for a level of specific energy (500–600 kJ/kg) and temperature (50–60 °C) quite independent of mixing conditions (rotor speed, mixing time, filling ratio). The gluten/glycerol dough behaves as a classical gluten/water dough, with a storage modulus higher than the loss modulus over the frequency range under study. A temperature increase induces a decrease of moduli, but the material is not thermorheologically simple. Glycerol has a plasticizing effect, which can be classically described by an exponential dependence. Mixing conditions influence the viscoelastic properties of the material, mainly through the specific mechanical energy input (to 2000 kJ/kg) and temperature increase (to 80 °C). Above 50 °C, specific mechanical energy highly increases the complex modulus. The aggregation of proteins, as evidenced by size-exclusion chromatography measurements, occurs later as the dough temperature reaches 70 °C. The nature of network interactions and the respective influence of hydrophobic and disulphide contribution is discussed. A general expression is proposed for describing the viscous behaviour of a gluten/glycerol mix, which could seem simplistic for such a complex rheological behaviour, but would remain sufficient for modelling the flow behaviour in a twin screw extruder.

Rheological properties of gluten plasticized with glycerol: dependence on temperature, glycerol content and mixing conditions

Thermal properties of raw and processed wheat gluten in relation with protein aggregation

During pasta processing, structural changes of protein occur, due to changes in water content, mechanical energy input, and high temperature treatments. The present paper investigates the impact of successive and intense thermal treatments (high temperature drying, cooking, and overcooking) on aggregation of gluten protein in pasta. Protein aggregation was evaluated by the measurement of sensitivity of disulfide bonds toward reduction with dithioerythritol (DTE), at different reactions times. In addition to the loss in protein extractability in sodium dodecyl sulfate buffer, heat treatments induced a drastic change in disulfide bonds sensitivity toward DTE reduction and in size-exclusion high-performance liquid chromatography profiles of fully reduced protein. The protein solubility loss was assumed to derive from the increasing connectivity of protein upon heat treatments. The increasing degree of protein upon aggregation would be due to the formation of additional interchain disulfide bonds.

Mechanisms of heat-mediated aggregation of wheat gluten protein upon pasta processing.

Size exclusion high-performance liquid chromatography analysis was carried out on wheat gluten−glycerol blends subjected to different heat treatments. The elution profiles were analyzed in order to follow the solubility loss of protein fractions with specific molecular size. Owing to the known biochemical changes involved during the heat denaturation of gluten, a mechanistic mathematical model was developed, which divided the protein denaturation into two distinct reaction steps:  (i) reversible change in protein conformation and (ii) protein precipitation through disulfide bonding between initially SDS-soluble and SDS-insoluble reaction partners. Activation energies of gluten unfolding, refolding, and precipitation were calculated with the Arrhenius law to 53.9 kJ·mol-1, 29.5 kJ·mol-1, and 172 kJ·mol-1, respectively. The rate of protein solubility loss decreased as the cross-linking reaction proceeded, which may be attributed to the formation of a three-dimensional network progressively hindering the rea...

Joëlle Bonicel

Papers

Extrusion of wheat gluten plasticized with glycerol: Influence of process conditions on flow behavior, rheological properties, and molecular size distribution

Rheological properties of gluten plasticized with glycerol: dependence on temperature, glycerol content and mixing conditions

Thermal properties of raw and processed wheat gluten in relation with protein aggregation

Mechanisms of heat-mediated aggregation of wheat gluten protein upon pasta processing.

Polymerization kinetics of wheat gluten upon thermosetting. A mechanistic model.