Rheological Comparison Between Synthetic And Food Polymers

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It was not until the early 1980s that food scientists, led by Drs. Harry Levine and Louise Slade, realized that synthetic polymer principles are applicable to food systems (1-8). Various thermal analysis techniques have demonstrated the similarity between them (9-11). Figure 1 shows the five regions of viscoelasticity of a synthetic polystyrene: AB glassy region, BC glass transition region, CD

Viscoelastic Curve
Figure 1 Five regions of viscoelasticity, illustrated by using a polystyrene sample. Also shown are the strain (e)-time curves for stress applied at x and removed at y: (a) glassy region, (b) leathery state; (c) rubbery state; and (d) viscous state. (From Ref. 12.)

rubbery plateau region, DE rubbery flow curves for stress applied at x and removed at y. In the glassy state (AB), the material behaves like an elastic solid. In the viscous state (EF), it is a liquid, while it behaves like a viscoelastic material in the temperature ranges from B to E. In The modulus-temperature curve is very sensitive to many structural factors, such as molecular weight, degree of cross-linking, percentage crystallinity, copolymerization, plasticization, and phase separation (13).

The glass transition temperature (Tg) is a function of product composition, molecular weight of the continuous structural matrix, degree of branching, degree of cross-linking, crystallinity, and degree of plasticization. For un-cross-linked molecules, the drop in modulus is about three decades near Tg. The magnitude of drop in modulus in the glass transition region decreases as the degree of cross-linking or molecular entanglement increases, which is the case for low-moisture gluten samples. As shown in Figure 2, for a gluten sample equilibrated in 65% relative humidity (RH), the drop in modulus is less than two decades (13). The degree of viscosity drop at a constant (T - Tg) has been used as an index of ''fragility.'' Unfortunately, organic glass usually is more fragile than inorganic glass. Thus, there is minimal opportunity for applying this concept in stabilizing food systems. A high-crystallinity sample has lower modulus drop at Tg due to

Rubbery Plateau Shrink
Figure 2 Typical DMTA plot for gluten (RH = 65%), showing tan S, log loss modulus (E"), and log elastic modulus (E') as a function of temperature. (From Ref. 13.)

the reduced amount of amorphous region, and is directionally higher in Tg. Plasti-cizers decrease both Tg and the rubbery modulus of a PVC-diethylhexyl succinate system (Fig. 3), while Figure 4 shows that water is a powerful plasticizer for gluten (13). Crispy breakfast cereal has modulus of around 109 Pa. The modulus for glucose glass is 8 X 109 Pa, while it is 8 X 108 Pa for glucose/sucrose glass at 2-3% moisture, regardless of their ratios (15).

As shown in Figure 1, the modulus in the rubbery plateau region is relatively constant, and its temperature range (CD) is a function of molecular weight and the number of entanglements per molecule. Thus, as shown in Figures 2 and 5, a gluten sample at 65% relative humidity, due to its high molecular weight and degree of entanglement, has very long rubbery plateau, while sorbitol has almost no visible one (15). A slice of microwave-heated bread has the modulus of 106-107 Pa at room temperature, which is in between the glass transition and rubbery plateau regions as typified by its leathery-rubbery texture.

Glass Transition Viscosity Food

Temperature PC)

Figure 3 Dynamic shear modulus of polyvinyl chloride plasticized with various amounts of diethylhexyl succinate plasticizer. (From Ref. 14.)

Temperature PC)

Figure 3 Dynamic shear modulus of polyvinyl chloride plasticized with various amounts of diethylhexyl succinate plasticizer. (From Ref. 14.)

In contrast to sorbitol, which goes into rubbery and viscous flow right after the glass transition region, the gluten DMTA curve lacks the rubbery flow and viscous flow regions. This is probably due to the relatively high degree of cross-linking and/or entanglement. Molasses, honey, and batters are examples of food systems in the viscous flow region at room temperature.

A blend of incompatible polymers will phase-separate and show more than one Tg, as indicated in Figure 6 for sodium caseinate-water and fructose-water systems (16). From all these observations, we can conclude that food systems can indeed be viewed as synthetic polymer systems.

Air cells or pockets help soften some products, such as breakfast cereals, that would otherwise be too hard. Gas cells inside some products can be from many sources: entrapped air during mixing, yeast leavening, chemical leavening, injected inert gases, carbon dioxide, ethanol. Those gas cells will eventually have the same gaseous composition as that of the packaged environment. One area that differs from the synthetic polymer system is the presence of fat in food matrices. In bakery products, fats and emulsifiers do not affect Tg, but they do decrease the rubbery modulus (17). That is why they are sometimes called tender-izers instead of plasticizers. The tenderizing effect at serving temperature is a

Pvc Dmta Plasticizer

Figure 4 DMTA plot for gluten samples stored under different RH values. (From Ref. 13.)

Figure 4 DMTA plot for gluten samples stored under different RH values. (From Ref. 13.)

function of the solid fat index, fat content, and fat crystalline form. Due to their lubricating effect, fats and emulsifiers also enhance the perceived moistness of bakery products. Moistness also results partly because the true moisture content of the nonfat portion is higher than the apparent moisture content, which is based upon the total system. No wonder that fat-free bakery products usually taste dry and not as tender.

III. EFFECT OF RHEOLOGICAL PROPERTIES ON THE STABILITY OF MATERIAL BEFORE, DURING, AND AFTER PROCESSING

In addition to having high hedonic quality during consumption, any successful product in the marketplace needs to be stable throughout distribution. During manufacturing, ingredients and in-process intermediates should not have caking,

Polymer Glass Transition
Figure 5 Annular shear test for amorphous sorbitol: (S) storage modulus G, ( + ) loss modulus G", and (*) loss tangent. (From Ref. 15.)

shrinkage, or other physical instability problems. In new-product development, significant effort is on stabilizing the ingredient during storage, the processing intermediates during manufacturing, and the finished products during storage, consumer preparation, and consumption. Directly and indirectly, rheological properties affect physical, chemical, and microbiological stabilities. In this chapter, we will focus more on the physical stability aspect.

How do rheological properties affect stability? Well, in the previous section we discussed the effect of physical state on rheological properties. Thus, it is physical state that determines stability. A state diagram shows Tg as a function of water content and solubility as a function of temperature. It also shows information on various physical changes that may occur due to the metastable state of amorphous food solids and their approach toward equilibrium (18). Roos (19) summarized the methods of applying state diagrams in food processing and product development. LeMeste et al. (20) reviewed the relationship between physical states and the quality of cereal-based foods. A product in the glassy state during storage will be more stable, while products stored at temperatures above the Tg curve will undergo physical changes at a rate according to William-Landel-Ferry (WLF) kinetics (4-8). The Gordon-Taylor equation can relate the effect of composition on Tg (19, 21). Thus, by combining the WLF and Gordon-Taylor

Tan S

Tan S

Dmta Curve

Figure 6 DMTA plot of sodium caseinate and fructose at a ratio of 2:1 stored at 75% RH (16% water). (From Ref. 16.)

Figure 6 DMTA plot of sodium caseinate and fructose at a ratio of 2:1 stored at 75% RH (16% water). (From Ref. 16.)

equations, one can express rheological properties, such as viscosity, of food materials as a function of temperature and moisture content. However, the coefficients in those equations are not yet readily available, even for common food systems. Some of the stability problems encountered in product development will be discussed here. These include caking during storage and spray-drying, structural collapse during processing, and loss of crispness during storage.

A. Caking During Storage and Processing

As described in Chapter 7, caking, or the loss of free flow, is one of the most often encountered stability problems associated with spray-dried or freeze-dried amorphous ingredients. Ingredient caking affects plant operation efficiency. Caking of wheat flour usually is of no concern if it is stored properly. However, under humid conditions, e.g., at 90% relative humidity, mold growth or infestation might occur even at room temperature (22).

Caking is a mobility-related phenomenon associated with the physical state of the continuous structural matrix on the state diagram. When either storage temperature is too high (e.g., above Tg) or moisture content is too high (e.g., higher than Wg), a powder ingredient might loss its free-flowing property. Usually, caking problem is worse for lower-molecular-weight amorphous carbohydrate powders with relatively low Tg values, such as 42DE corn syrup solids or brown sugar when stored at high temperature or under humid conditions. When an ingredient absorbs enough moisture to depress Tg below storage temperature, caking may occur. In the extreme case, such as brown sugar, some dissolved sucrose will recrystallize and the matrix will harden.

Using a sample of sucrose/fructose glass at 7:1 ratio, Roos (23) has shown that the sticky point is about 20-25°C above the onset temperature of the increase in heat capacity, which is close to the end point of glass transition as measured by DSC at a heating rate of 5°C/min. This corresponds to the critical viscosity of 1010 mPa • s for caking (24). The critical moisture content is the moisture content at which the ingredient or product has critical viscosity, which is only slightly higher than Wg. When viscosity is too low due to high temperature or moisture content, the particle surfaces tend to cement together through liquid bridging, resulting in caking (25). If the critical moisture content is known, then from the moisture absorption isotherm, one can find the critical relative humidity above which caking will occur during storage at room temperature.

Mannheim (26) suggested the following formulation and processing methods for minimizing the caking problem during storage: (a) drying to low moisture content, (b) treating powders at low-humidity atmospheres and packaging in high-barrier packages, (c) storing at low temperatures, (d) in-package desiccation, (e) agglomeration, (f) adding anticaking agents. This confirms again the importance of temperature and moisture content on controlling caking. Other solutions include modifying the design of the hopper and agitator for conveying or using agglomerated ingredients (27-28).

Caking during spray-drying is the same phenomenon, except it occurs at higher temperature during a shorter time. From the Tg curve and the temperature profile inside the spray-dryer, one can determine the maximally allowable moisture content beyond which stickiness will occur. In terms of formulation, by adding 10-15DE maltodextrin to orange juice prior to spray-drying, one can reduce caking during processing and storage (29). In Sec. IV.C, we will cover dryer design modification to reduce caking.

B. Structural Collapse During Processing

Phase transitions in foods have a strong impact on food behavior during processing as well. As expected, understanding rheological properties of in-process intermediates can facilitate product formulation and process design for achieving optimum food quality and stability. During manufacturing, structural collapse can occur if the product is not properly formulated or processed. Collapse in amorphous material is usually related to the glass transition (4-8). Levi and Karel (30) show that volume shrinkage occurs in freeze-dried carbohydrates above their Tg. The rate of shrinkage is a strong function of (T - Tg) and can be modeled using the WLF equation, if the coefficients C1 and C2 are known. The effect of moisture is due to Tg depression and the resultant increase in (T - Tg) at constant temperature. Structural shrinkage during extrusion and cake baking can be predicted and manipulated through rheological control.

1. Shrinkage of Extrudates During Cooling

The role of rheological properties on extrudate expansion has been extensively reviewed (31). In the glass transition region, the elastic modulus can range from 106 to 109 Pa, with viscosity ranges of 1011-1015 mPa • s, which is highly dependent on temperature, moisture content, and measurement techniques. The estimated elastic modulus for collapse is about 105-106 Pa, with viscosity ranges of 108-10n mPa • s (32). The rheological properties of extrudates will dictate the die design and extruding temperature. The extrudate temperature will in turn affect the final product moisture content and the degree of shrinkage during cooling. Breakfast cereal extrudates with high sugar content will shrink during the initial stage of cooling due to the plasticization effect of sugar on the continuous polymeric matrix, resulting in lower elastic modulus (G') when the extrudate is still hot. To reduce shrinkage, in addition to formulating a higher Tg matrix, one can modify the exit temperature and cooling rate so that the product does not stay too long in the rubbery state or warmer, i.e., in the range of (Tg + 30°C) to exit temperature (33). Thus, rheological measurements in the proper temperature range can help product scientists determine the critical temperatures above which product expands during heating and shrinks during cooling.

2. Collapse of Cake During Cooling

In high-ratio cakes, due to an excessively high amount of sucrose and a relatively lower amount of water, the effectiveness of the aqueous phase in batter in plasti-

cizing the starch polymer is significantly reduced. Thus, during baking, not enough starch was gelatinized to provide structural strength for converting the closed foam in batter into an open foam, and the cake collapses upon cooling.

Chlorinated flour is usually used to minimize this problem, probably by providing higher G during the later stage of baking (34). Dea (35), reported that a mechani cal spectrometer can be used to measure rheological properties during the baking of cake batter. The technique of mechanical spectroscopy is uniquely suited to the rheological characterization of a material with a dynamic temperature profile during baking. It typically shows that batters that produce a collapsed cake after cooling usually do not have a high enough G at temperatures higher than 80°C.

C. Loss of Crispness During Storage or Consumption

As mentioned in Chapter 9, crispness is a desirable textural attribute of some cereal-based products. However, during storage or consumer preparation, moisture pickup from the air (cotton candy), from other food components (ice cream sandwich), or from milk (breakfast cereal) will significantly decrease the crispness score and acceptability. Most of the crisp materials are in glassy state with modulus of 109-1010 Pa, e.g., gliadin glass with 1-3 X 109 Pa (36), glucose glass with 8 X 109 Pa (37), wheat grain glass with 2 X 109 Pa (38), and sodium caseinate glass with 1.8 X 109 Pa (39). However, as the product absorbs enough moisture, Tg is depressed to below serving temperature, and modulus drops dramatically (40).

The Fermi distribution function describes a normalized distribution of Y ranging from 0 to 1 as a function of temperature with midpoint at Tc and the spread factor "a" that defines the shape of the curve (41). It is useful in describing the sensitivity of normalized Y values around Tc. Peleg adapted it to describe the temperature, water activity, and/or moisture content dependency of Young's modulus or crispness score (42). For example, the following equation describes Young's modulus as a function of moisture content:

where Y(m) is Young's modulus at moisture content m, Yg is Young's modulus in the glassy state, a is the spread factor, and mc is the critical moisture content, i.e., the moisture content at which Y is half of Yg.

It is important to note that the critical temperature, critical water activity, and critical moisture content are material dependent (43). For extruded bread, a significant loss of crispness at room temperature was observed between 9 and 10% moisture. The brittle-ductile transition, defined as the temperature or moisture content above which the product texture changes from brittle to ductile, happens between 9 and 13.7% while Wg is 15% (44). This means the loss in crispness happens at a temperature below Tg.

''Moisture toughening'' means the modulus or stiffness increases with moisture content at moisture content below mc. This happens in extruded bread (44) and cheese puffs (45), and a linear term of moisture needs to be added to the numerator of the original equation, leading to the following equation:

where b is the slope factor at a moisture lower than mc.

Young Modulus Foods
Figure 7 Effect of water on Young's modulus of extruded bread. (From Ref. 44.)

Figure 7 shows that, in extruded bread, the apparent Young's modulus vs. moisture curve has a peak. The modulis at 4% and 11% moisture are both 6.5 MPa, but the modulus is 11 MPa at 9%, then it drops to 5 MPa at 13.7% and 3 MPa at 15.3%, respectively. A significant loss of crispness already occurs between 9% and 10%. However, if we use 11 MPa at 9% for the modulus before the brittle-ductile transition, then the critical moisture content, the moisture content at which the modulus is decreased to 5.5 MPa, would be between 11% and 13.7% moisture. Does this mean that a slight decrease in modulus can significantly decrease crispness or that modulus alone cannot explain the complex sensory attribute of crispness? Vickers (46) reported that sensory crispness contains both hardness and fracturability and has a strong correlation with the peak force and Young's modulus. Roos et al. (43) showed that as moisture content increases, the initial slope of the force-deformation curve determined by a snap test decreases at a rate in between crispness scores measured by a bite task panel and a bite-and-chew task panel, with the crispness measured by the bite-and-chew task panel decreasing the fastest.

How can one apply all this rheological information to product development? It seems that products with Tg or the brittle-ductile transition well above serving temperature will stay crisp after moisture pickup; i.e., the product modulus will remain high even at relatively high moisture content. How do we keep a breakfast cereal crisp as long as possible in a bowl of milk? Well, this is one of the new areas for potential technological innovation in the food industry. One way to increase the bowl life of cereal is to reduce the bulk density of the cereal so that it will float in the milk.

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Why Gluten Free

Why Gluten Free

What Is The Gluten Free Diet And What You Need To Know Before You Try It. You may have heard the term gluten free, and you may even have a general idea as to what it means to eat a gluten free diet. Most people believe this type of diet is a curse for those who simply cannot tolerate the protein known as gluten, as they will never be able to eat any food that contains wheat, rye, barley, malts, or triticale.

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