FOOD ENGINEERING

Lesson 5. RHEOLOGICAL PROPERTIES OF FLUID FOODS

Module 1. Rheology of foods

Lesson 5

RHEOLOGICAL PROPERTIES OF FLUID FOODS

5.1 Introduction

It is necessary to study properties of fluid food products for designing and lay-outing of transport system (piping and pumping layout). For the fluid food products, the design of transport system mainly depends on the type and description of flow characteristics of the product. Some of the properties are interdependent and some are dependent on the fluid food composition and therefore it is necessary to measure dependant properties and we can predict its rheological properties.

Most important dependant fluid food property is viscosity i.e. resistance against flow, generally indicated by μ i.e. dynamic viscosity / ή kinematic viscosity ( ή= μ/ ρ ). In food industry μ is broadly used to describe a single parameter known as ‘consistency’. But this approach may lead to confusion in many cases due to non-Newtonian behaviour of many fluid food products. The rheological classification of food is given in Fig-5.1. The stress and rate of shear diagram indicate varieties of food products classified under different categories, which is considered to be non-Newtonian as shown in the figure 5.2.

5.2 Classification

The fluids can be classified into following categories depending on the response to the applied shear force.

5.2.1 Newtonian fluids

Newtonian fluids are fluids which exhibit a linear increase in the shear stress with the rate of shear. These fluids exhibit a linear relationship between the shear stress and the rate of shear. The slope ' µ ' is constant therefore, the viscosity of a Newtonian fluid is independent of the rate of shear. These fluids exhibit a pure viscous flow i.e. the product begins to flow with the slightest force and the rate of flow is proportional to the magnitude of force applied. The examples of Newtonian fluids are milk, clear fruit juices, sucrose solution, most types of honey, corn syrup etc. The equation for characterizing Newtonian fluid is

Τ = µ (-dv/dx) ----- (Eq-1)

Where, Τ = shear stress, µ = dynamic viscosity (η = µ/ρ) , -dv/dx = velocity gradient

5.2.2 Non-newtonian fluids

A non-Newtonian fluid is broadly defined as one for which the relationship between shear stress and shear rate is not a constant. When the shear rate is varied, the shear stress doesn't vary in the same proportion. These fluids exibit either shear thinning or shear thickening behaviour and some exhibit a yield stress. The two most commonly used equations for characterizing non-Newtonian fluids are the power law model (Eq-2) and Herschel-Bulkley model (Eq-3) for fluids.

Τ = K ( γ )ⁿ -------(Eq-2)

Τ = Τ ₀ + K ( γ )ⁿ -------(Eq-3)

Where, Τ = shear stress, K = consistency constant, γ = shear rate, n = flow behaviour index,

Τ_{0 =} yield stress

There are several types of non-Newtonian flow behaviour, characterized by the way a fluid viscosity changes in response to variation in shear rate (Fig-5.2). The most common non-Newtonian fluids are:

(A) Time-independent flow of non-Newtonian foods

The fluid foods whose viscosity is not influenced by the shearing time at a constant shear rate show two distinct patterns of stress – shear rate relationship i.e shear-thinning and shear-thickening.

(i) Pseudoplastic/shear-thinning fluids

This type of fluids will display a decreasing consistency with an increasing shear rate. Probably the most common of the non-Newtonian fluids, psedo-plastic include emulsions and dispersions of many types. This type of flow behaviour is some times called shear-thinning. The shear stress ('Τ' or 'σ') versus shear rate (γ) curve is convex toward the stress axis. The shear thinning behaviour of a fluid or semi-solid food is expressed by the power law model or de Waele's model:

Τ = K ( γ )⁺ⁿ ---- (Eq-4)

Where, Τ = shear stress, K = consistency constant (Pa s)ⁿ, γ = shear rate, n = flow behaviour index ( 'n' has a positive value between zero and unity)

Protein concentrates, skim milk concentrate, milk ultrafiltration retentates, concentrated fruit juices such as unpectinized apple juice (50-65 ۫ Brix), orange juice (50-65 ۫ Brix) etc., melted chocolates, thawed frozen egg, fruit and vegetable purees and gum solutions are the examples of psedo-plastic fluid food products.

(ii) Dilatant/shear-thickening flow behaviour

This type of fluid will display an increasing viscosity with increase in shear rate. Dilatancy is frequently observed in fluids containing high level of deflocculated solids, such as candy compounds, cooked corn starch paste, certain types of honey etc. Dilatancy is also referred to as shear-thickening flow behaviour. The stress shear rate curve is concave toward the stress axis and the value of 'n' in the power law (Eq-4) is negative.

(B) Time-dependent flow of non-Newtonian foods

Certain non-Newtonian fluids show a time-dependent stress-shear relationship which can be one of the following types:

(i) Thixotropy:- When at a constant shear rate, the stress decreases over a period of time due to structure breakdown until eventually it reaches a steady value, the product is said to be thixotropic. Aged condensed milk, cream and ice cream mix, egg white etc. revel thixotropy.

(ii) Rheopectic:- This essentially the opposite of thixotropic behaviour, in which the fluids viscosity increases with time as it is sheared at a constant rate. Rheopectic fluids are rarely encountered. Both thixotropic and rheopectic may occur in combination with any of the previously discussed flow behaviour, or only at certain shear rates.

(iii) Plastic:- This type of fluid will behave as a solid under static conditions. A certain amount of force must be applied to the fluid before any flow is induced, this force is called yield value. Tomato ketchup is a good example of this type fluid, its yield value will often make it refuse to pour from the bottle until the bottle is shaken or struck, allowing the catchup to flow freely. Once the yield value is exceeded and flow begins, plastic fluids may display Newtonian, pseudoplastic, or dilatant flow characteristics.

5.3 Important Properties of Fluid Foods

· Bounce: The resilience rate at which the sample returns to its original shape after partial compression.

· Chewiness: It is the number of chews (@1 chew/sec.) needed to masticate the sample to a state ready for swallowing.

· Coarseness: Degree to which mass feels coarse in the mouth.

· Cohesiveness: Degree to which the sample deforms before rupturing during biting.

· Denseness: It is a compactness of cross-section of the sample after complete biting.

· Dryness: Degree to which the sample feels dry in the mouth.

· Fracturability: Force with which the sample shatters/breaks. It encompasses crispiness, crumbliness, crunchiness and brittleness.

· Graininess: Degree to which a sample contains small grainy particles.

· Gumminess: Energy required to disintegrate a semisolid food to a state ready to swallow.

· Hardness: Force required to deform the product to a given distortion. Generally the distortion is between tongue and palate.

· Heaviness: Weight of product perceived when first placed on the tongue.

· Moisture absorption (related to sensory): Amount of saliva absorbed by the product in the mouth.

· Moisture released: Amount of wetness and juiciness from the product.

· Mouth coating: Type and degree of coating in the mouth after mastication.

· Roughness: It is the degree of abrasiveness of product surface which is perceived by the tongue.

· Slipperiness: It is the degree to which the product slides over the tongue.

· Smoothness: It represents the absence of any particle, lumps, etc. in the product.

· Springiness/sponginess: Degree to which the product returns to its original position i.e. shape and size after partial compression (without failure). Here compression is between the palette and teeth.

· Uniformity: Degree to which the sample is even throughout.

· Uniformity of bite : Evenness of force applied on the product while biting.

· Uniformity of chew : The degree to which the chewy characteristics of the product are even throughout the mastication

· Viscosity : Force required to draw a liquid from spoon over to the tongue.

· Wetness : Amount of moisture perceived on the product surface.