Food Rheology

FOOD RHEOLOGY Dr. Mukund V. Karwe Dept. of Food Science, Rutgers University

Study of deformation and flow (no matter how unlikely) “Everything flows…..” Greek philosopher Heraclitus of Ephesus (536–470 BCE)

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RHEOLOGY Rheology is the study of the flow of materials. Oil and water flow in familiar, normal ways, whereas mayonnaise, peanut butter, chocolate, bread dough, and Silly Putty flow in complex and unusual ways.

Rheology is the science of deformation and flow of matter when external force is applied. Knowledge of rheology is important in pasta and cereal production, bread/cake making, chocolate coating, making salad dressing. How one perceives the texture of a food product in mouth also depends upon the rheology of that product.

Rheology defines the consistency of different food products

Consistency is described by two components •

the elasticity (springiness)

•

the viscosity (thickness, lack of slipperiness)

Elasticity: When force is applied on a spring - it elongates or gets compressed When force is removed from spring - it goes back to its original shape

Similarly, there are food materials that show spring-like behavior and are called elastic. Most solid food materials show some elasticity. Examples: Gummy Bears, Bread dough, Chewing Gum

Some food materials start to flow as a result of the external force These materials such as water and air are called fluids and are associated with a property called viscosity. Viscosity represents the resistance of a fluid to flow. Some food materials have relatively constant viscosity (e.g. air, water, honey, cooking oil, apple juice) at a given temperature but are independent of the applied force or the intensity of mixing. Such materials are called Newtonian. There are many food materials that do not have a constant viscosity value. The materials for which viscosity value decreases or increases, when more and more external force is applied, are called non-Newtonian.

Approximate Viscosity Values at 20 C (68 F) Newtonian Materials Air

0.00001 Pa.s

Water

0.001

Pa.s

Olive oil

0.1

Pa.s

Glycerin

1.0

Pa.s

Honey

10.0

Pa.s

Heavy Corn Syrup

100.0

Pa.s

Glass

1040

Pa.s

In

other words, water is 100 times more viscous than air, olive oil is 100 times more

viscous than water, and honey is 100 times more viscous than olive oil. Therefore, honey is million times more viscous than air. Did you know that glass is a very viscous fluid?!! Pa.s stands for pascal-seconds which is a unit of viscosity in the international system of units (SI).

Additional Viscosity Data for Various Newtonian Materials Material

T( C)

 (Pa.s)

Water

0

0.001787

"

20

0.001002

"

90

0.0003147

Mercury

20

0.001554

"

90

0.001268

Glycerin

20

1.49

"

25

0.954

"

30

0.629

Corn Oil

25

0.0565

Honey (Sweet Clover)

24.7

7.2

Milk, homogenized

20

0.002

Cream (30 % fat)

40

0.00395

Sucrose solution 20% 40% 60%

25 25 25

1.71 5.206 44.02

Corn Syrup (80 % dry substance)

26.7

126.0

Head & Shoulders Shampoo

25

4

Source: Rheological Methods in Food Process Engineering, James F. Steffe, 1996, second edition, Freeman Press, pp. 361-373.

Luckily, most of the food materials are such that their viscosity decreases when subjected to a more vigorous application of external force (chewing, spreading, kneading). These are called shear-thinning materials.

Many of the cosmetic lotions and pastes are shear thinning: when you rub them, they spread easily.

Non-Newtonian Shear-thinning materials (materials that become less viscous with more vigorous mixing or stirring or chewing) Melted chocolate Apple Sauce Banana Puree Orange Juice Peach Puree Tomato Juice Conc. Peanut Butter Oil of Olay Colgate Toothpaste Vaseline Intensive Care Lotion Nail enamel Synthetic Polymers

Materials that simultaneously exhibit fluid-like (viscous) and solid-like (elastic) behavior are called as viscoelastic. Examples of viscoelastic food materials are chocolate brownie batter, peanut butter, cream, and silly putty.

Have you wondered why the frosting on the sides of a cake does not fall off? This is mainly due to what is called as yield stress. Yield stress is the minimum force that has to be applied to a material before it yields or deforms. In the case of the frosting, the weight of the frosting is not enough to overcome the yield stress and pull it off the cake. The lighter (foamier) the frosting, the thicker the layer you can use before it begins to fall off. Other examples of products that have yield stress are yogurt, chocolate syrup, tomato ketchup and Jell-O.

Contact area A W

Cake

L

Frosting Thickness 

 = 0 / (f g) 0 Yield Stress f Density of frosting g gravitational acceleration

g

Experiments Demonstrating Yield stress, Shear thinning, Viscoelastic behavior YIELD STRESS Make Jell-O and stir it with a rotating shaft or a mixer blade. You can also use a kitchen hand mixer. Jell-O near the rotating shaft will rotate with the shaft but the Jell-O away from the shaft will be stationary. We will then do the same experiment with cooking oil which does not have any yield stress and therefore, all of the oil in the container will start moving with the rotating shaft. You can also try using applesauce or chocolate syrup and see what happens. SHEAR THINNING Peanut butter at room temperature is a very viscous mass. Take some peanut butter and stir it with the mixer blade. The longer and faster you stir, the thinner the peanut butter becomes. This is due to the shear-thinning effect of the material.

SHEAR THICKENING (viscosity increases as more and more force is applied) Corn starch and cold water form a mixture that is called dilatant or shear thickening. Mix cold water at room temperature and corn starch in 1:1 proportion. Mix it slowly with a thick glass rod or metal spoon. Let it sit for about 20 minutes. Now try to gently stir the mix with the spoon, note how much resistance you feel. Now try to stir it rapidly and note how much resistance you feel.

How does this compare with peanut butter?

Viscoelastic materials (materials that behave like solid and liquid at the same time)

Filament test for viscoelasticity: Place a small amount of the fluid in question between your thumb and index finger, and then pull them apart. The fluid is viscoelastic if a filament forms. Example: Pizza or bread dough

Source: Rheological Methods in Food Process Engineering, James F. Steffe, 1996, second edition, Freeman Press, pp. 361-37

Viscous fluid

Viscoelastic fluid

Weissenberg effect

Tubeless siphon

Die / Jet swell

Example: Corn Syrup mostly consists of fructose, glucose and some water. Corn syrup is a Newtonian liquid. When you add carboxymethylcellulose (CMC), a long chain molecule derived from cellulose (part of plants), to corn syrup, it becomes non-Newtonian, even at small concentrations (< 1%). In fact it becomes viscoelastic.

VISCOELASTIC EFFECT Prepare chocolate brownie batter and stir with the mixer blade. You will notice that the batter tries to climb on the rotating shaft, showing viscoelastic behavior. This phenomenon is called as Wiessenberg effect. This effect is very prominent when you use corn syrup mixed with CMC (~3.0% w/w). Demonstration of viscoelastic fluid: Weissenberg (rod climbing) effect of Corn Syrup + 3% CMC Material climbs the mixer shaft