A variety of cereal products can be produced simply by varying processing condi- tions. The basic unit operations include mixing, forming, and drying. Each unit operation must be optimized for each product to maintain high final product qual- ity. Processing conditions affect the structure of the cereal product and in turn affect the product’s material properties. The material properties govern the drying kinetics and stress development. Understanding the effects of processing on mate- rial behavior is necessary to optimize each process to reduce breakage.
IV. GLASS TRANSITION AND MATERIAL PROPERTIES
The glass transition phenomenon can be used to describe cereal product behavior during processing. Glass transition governs an amorphous material’s behavior and can be explained using the theory of free volume. Many polymers and macro- molecules in foods, such as proteins, form a cross-linked structure with branches.
The cross-linking and branches prevent the formation of a well-ordered crystal- line structure. The resulting structure is termedamorphous. Amorphous struc- tures can be liquid, semisolid, or brittle.
Free volume, the volume not occupied by the amorphous chains, is affected by temperature and moisture content. The free volume is a minimum at the glass transition temperature. At and below the glass transition temperature, the amor- phous material is glassy, where deformation and diffusion are restricted. Increas- ing the temperature and/or moisture content increases the free volume. Thus, the amorphous chains have more volume in which to move, so shrinkage or expan- sion can occur. Additionally, more volume between chains is available for diffu- sion of small molecules, such as water. Material properties, such as viscosity and diffusion, change by several orders of magnitude at the glass transition. There- fore, the state of a material greatly affects the material properties that influence mass transfer and stress development.
The point of glass transition varies for different raw materials and is repre- sented by a glass transition temperature as a function of moisture content. Above the glass transition temperature curve, the material is rubbery and below the curve glassy. Glass transition temperatures for different proteins vary with season and variety (40), and glass transition temperature depends on molecular weight (41).
Kokini et al. (42) developed the state diagrams for gliadin, zein, and glutenin.
Gliadin had an anhydrous glass transition temperature of 121.5°C, which was lower than that of zein, 139°C, and glutenin, 145°C, and was attributed to its lower molecular weight (42). The variation in glass transition temperature among raw materials is due to structural and molecular weight differences.
Glass transition affects the properties, such as viscosity, diffusion, and stiff- ness, of a material, that are observed in processing phenomena, such as collapse, stickiness, reaction rates, and crispness (43–46). Below the glass transition tem- perature, movement is localized; but above the glass transition temperature, seg- mental motion occurs. Material properties provide a link between the state of a material and processing phenomena.
Viscosity (and other material properties such as the storage modulus) re- flects the ability of a material to flow, which is observed in collapse and sticki- ness. In the glassy state, the viscosity or storage modulus is high, approximately 1012Pa⋅s. The viscosity decreases by several orders of magnitude at the glass transition temperature, so above the glass transition in the rubbery state, the vis- cosity is low. The lower viscosity allows the material to deform or flow. For example, while drying at temperatures above the glass transition temperature, pasta shrinks as moisture is removed to produce a dense product. Stickiness is an important phenomena in particle processing. Above the glass transition tem- perature, the particle surface becomes plasticized, which leads to interparticle bonding (46). Caking is an issue in the handling of particles such as sugar. Higher- molecular-weight materials, such as maltodextrins with high glass transition tem- peratures, are added to the low-molecular-weight particles to reduce caking (45).
Controlling the viscosity of a material controls the collapse and stickiness.
Mass transfer and reaction rates are affected by diffusion. Diffusion de- pends on the solubility and diffusivity of the diffusing species. The diffusing species and other species present must be compatible for diffusion to occur. In addition, a space for the diffusing species is required, which is related to the free volume. The theory of free volume is based on the following (47):
1. The probability that a molecule will obtain sufficient energy to over- come attractive forces
2. The probability that a fluctuation in density will result in a hole of sufficient size to accommodate the diffusing species
The formation of a hole is related to the viscosity of the solid material. In the glassy state, molecular movement is inhibited and the free volume is a minimum.
But in the rubbery state, free volume increases and more movement occurs. Dur- ing pasta drying, Ollivier (48) observed changes in the diffusion coefficient of water corresponding to changes in pasta behavior. Increasing the temperature decreased the moisture content at which the transition occurred. Although not recognized at the time, the pasta went through glass transition during drying, which affected the diffusion coefficient and viscosity. Reaction rates are also affected by glass transition via moisture content, temperature, and diffusion (45).
Browning reactions, enzymatic activity, and nutrient loss reactions decrease with temperatures below the glass transition temperature (46). The diffusion coeffi-
cient reflects the mobility of a diffusing species, which is revealed in mass transfer kinetics and reaction rates.
Young’s modulus, or stiffness, changes with glass transition and is ob- served in crispness. Stiffness decreases with plasticization, which affects stress development and texture. During drying, few stresses and cracks develop if the material remains rubbery, when stiffness is low. Crisp products, such as biscuits, retain their characteristic texture in the glassy state, when stiffness is high. Woll- ney and Peleg (49) studied the effect of plasticization on stiffness of Zwiebacks and cheese balls. Stiffness versus water activity curves for both products re- mained constant over a low water-activity range, then dropped sharply. The abrupt change in stiffness was indicative of glass transition. Nikolaidis and La- buza (50) studied the bending modulus of crackers as a function of moisture content. The bending modulus decreased at the glass transition, as the crackers became rubbery. Comparing the glass transition temperature curve to the iso- therm revealed a critical water activity of 0.65 at room temperature (23°C) at a moisture content of 10.7% (wb), below which the cracker was glassy and above which the cracker was rubbery. The stiffness of a material affects the stress and texture of a material.
The relationship between relative humidity and equilibrium moisture con- tent is given by isotherms. Relative humidity is a critical processing parameter in drying and product storage. High relative humidities plasticize the material, resulting in collapse or loss of crispness. Thus, material behavior is dictated by the equilibrium moisture content at the processing relative humidity and temperature conditions. The state of a material is based on the relationship between the pro- cessing temperature and glass transition temperature of the product at the equilib- rium moisture content. From a plot containing two curves, glass transition temper- ature versus water activity and moisture content versus water activity at the processing temperature, the critical water activity and corresponding moisture content that depress the glass transition temperature to the storage or processing temperature are determined (46). Isotherms are a valuable tool for determining the behavior of a material under a given set of processing conditions.
The glass transition temperature varies among materials, so their behavior under a given set of processing conditions is different. For example, drying at a temperature of 130°C allows zein to remain rubbery to its anhydrous state, but gliadin becomes glassy as it approaches zero moisture content. Generally, higher- molecular-weight materials have higher glass transition temperatures. Increasing the moisture content of a material depresses the glass transition temperature. Dry- ing is a highly dynamic process, since the material constantly changes as moisture is removed and proteins aggregate to form larger molecules. Both of these changes result in an increasing glass transition temperature. Knowledge of the glass transition temperature as a function of moisture content for a given material is important for setting optimal drying conditions. If a dense product is desired,
the drying temperature should remain above the glass transition temperature throughout drying to allow the product to collapse. If a recipe change is made and another raw material is used, optimal processing conditions change since the new material’s glass transition temperature is different than the original raw material’s glass transition temperature. Therefore, processing must be optimized for each product to attain optimal final product quality.
State diagrams relating processing temperature to product glass transition temperature and moisture content can be used to understand the effects of pro- cessing on final product quality. This can be demonstrated by looking at state diagrams for two very different products—pasta and extrusion-puffed cereal.
During pasta drying, collapse is desired to form a dense structure;Figure 1shows the relationship between ideal drying conditions and the glass transition of pasta. Initially, the pasta enters the dryer at a high moisture content and is quickly heated. The drying temperature must be greater than the glass transition temperature so the structure can collapse. Near the outlet of the dryer or in the cooling section, the temperature is less than the glass transition temperature, so the pasta becomes glassy. This sets the structure, and the resulting product is a dense, hard piece of pasta. If the pasta transitions to the glassy region early in the drying process, all mobility is greatly decreased, often by multiple orders of magnitude. The matrix becomes rigid, with a drying front moving from the outer edge of the pasta toward the center. A shell forms, which decreases the moisture mobility and ‘‘locks’’ it into the pasta. The residual moisture in the form of moisture gradients causes the pasta to crack as the moisture equilibrates (34).
Moisture equilibration can occur during later stages of production or once the pasta is packaged and on the store shelf. Therefore, basing the drying protocol on the glass transition curve is vital to the production of uncracked pasta.
Processing conditions of extruded puffed cereals depend on the glass transi-
Figure 1 Phase-state diagram for pasta drying.
Figure 2 Phase-state diagram for cereal: A—increasing moisture content and tempera- ture of flour to form a melt; B—further increase in the temperature of melt; C—exit through the die to a lower temperature and pressure, resulting in moisture leaving rapidly;
D—cool to room temperature.
tion of the material, as shown inFigure 2.In the case of puffed extruded cereal, flour is metered into the extruder at a low moisture content and temperature. The moisture content and temperature are increased with water addition and heating to a temperature greater than the glass transition temperature. This forms a melt that can easily be worked into a dough and shaped. Pressure is also increased to greater than atmospheric. Upon exit through the die into lower temperature and pressure conditions, moisture is flashed off, forcing the product to expand. The puffed product is rapidly cooled to a temperature less than the glass transition temperature to preserve the structure. The resulting product is a light, airy, crunchy cereal. As demonstrated by these two cases, the state of a material under a given set of processing conditions greatly affects the final product texture.
Glass transition governs cereal product behavior during processing. Pro- cessing phenomena, such as collapse, stickiness, and diffusion, depend on the state of the material. The state of the material, rubbery or glassy, is related to the free volume. Moisture content, temperature, and molecular weight influence the free volume. Understanding material behavior as related to free volume is necessary for process optimization.
V. NUMERICAL MODELS: MASS TRANSFER, STRESS DEVELOPMENT, AND MATERIAL PROPERTIES
The most efficient method for process optimization is the use of mechanistic models. Drying, tempering, and cooling can be optimized with regard to kinetics
and product failure through the use of mass transfer and stress development mod- els, which require the input of material properties.