1. Memprediksi perubahan mutu selama pengolahan2. Menduga kemunduran mutu selama penyimpanan3. Menghitung umur simpan/kadaluwarsa produk 4. Menghitung waktu yang dibutuhkan untuk

sterilisasi/pasteurisasi5. Menghitung waktu fermentasi6. Menganalisa pengaruh penambahan pengawet

terhadap orde reaksi kemunduran mutu, atau terhadap laju k sehingga dapat dipilih pengawet yang paling efektif.

7. Menduga kerusakan mutu akibat berbagai suhu pengolahan sehingga dapat memutuskan suhu pengolahan yang tepat.

Setiap makanan atau produk pangan pasti memiliki warna, bau dan rasa.

Demikian pula mereka masing-masing memiliki sifat mekanis yang unik, bisa keras atau lunak, liat atau

empuk, lembut atau kasar, rapuh, renyah, mudah dan tidak mudah mengalir, dan seterusnya.

Ada dua cara yang bisa dilakukan untuk menguji sifat mekanis produk pangan. Pertama, menggunakan

indera manusia, dengan cara menyentuh, memijit, menggigit, mengunyah, dan sebagainya, selanjutnya kita

sampaikan apa yang kita rasakan. Ini yang disebut dengan analisa sensori. Karena reaksi kita sebagai

manusia yang menguji berbeda-beda, maka diperlukan analisa statistik untuk menyimpulkan skala

perbedaan ataupun tingkat kesukaan penguji terhadap produk tersebut. Cara uji kedua dengan pendekatan

fisik, menggunakan instrument atau peralatan tertentu, hasilnya dinyatakan dengan unit satuan meter (m),

kilogram (kg) dan detik (dt). Pendekatan fisik untuk mempelajari sifat mekanis bahan disebut rheology.

RHEOLOGY adalah suatu cabang ilmu fisik yang didefinisikan sebagai ilmu yang mempelajari perubahan

bentuk suatu mat erial. Gesekan antara bahan padat, sifat alir material bentuk tepung, bahkan pengecilan

ukuran suatu partikel seperti pada proses penggilingan, proses emulsifikasi dan atomisasi juga termasuk.

Mengapa kita mempelajari Rheology ?

Pertama, bisa melihat lebih dalam struktur suatu bahan, misalnya hubungan antara

ukuran molekul dan bentuknya dalam suatu larutan terhadap kekentalan, hubungan

antara tingkat cross-linkage polymers dengan elastisitasnya.

Kedua, test reologi sering diterapkan untuk mengontrol bahan dasar dan mengontrol

proses suatu pengolahan. Contohnya: sifat reologi adonan tepung gandum pada

pengolahan roti.

Ketiga, pengetahuan reologi diperlukan didalam mendesain alat tertentu seperti

pompa, pipa-pipa aliran. dan lainnya. Desain akan lebih efektif bila reologi bahan

yang menggunakan pompa atau melalui pipa alir tersebut diketahui.

Keempat, Penerimaan konsumen terhadap suatu produk dipengaruhi oleh sifat

reologinya. Misalnya, mudah tidaknya jam atau selai dioleskan, liat dan empuknya

daging , dan sebagainya.

Kendala dalam mempelajari reologi suatu produk pangan

secara garis besar disebabkan oleh:

Pertama, sangat bervariasinya produk pangan, ada yang bersifat

padat, cair atau gas, dan ada yang dalam bentuk-bentuk

antaranya.

Kedua, disebabkan karena masing-masing produk tersebut

mempunyai sifat berbeda pada kondisi yang berbeda. Contohnya,

sebuah batu bersifat sebagai bahan padat, tapi kumpulan batu

bisa dikategorikan bersifat sebagai bahan cair. Kapankah hal ini

terjadi?

Rheology is the science that studies the deformation of

materials including flow deformation and flow of materials

Rheological properties are defined as mechanical properties that

result in deformation and the flow of material in the presence of a

stress.

Classification of rheology diagram

Consider a fluid between two large parallel plates of area A, separated by a very small distance Y .

The system is initially at rest but at time t = 0, the lower plate is set in motion in the z-direction at a constant velocity V by applying a force F in the z-direction while the upper plate is kept stationary.

At t = 0, the velocity is zero everywhere except at the lower plate, which has a velocity V .

Then, the velocity distribution starts to develop as a function of time.

Finally, steady state is achieved and a linear velocity distribution is obtained.

The velocity of the fluid is experimentally found to vary linearly from zero at the upper plate to velocity V at the lower plate, corresponding to no-slip conditions at each plate.

The force required to maintain the motion of the lower plate per unit area is proportional to the velocity gradient, and the proportionality constant, μ, is the viscosity of the fluid

The microscopic form of this equation is known as Newton’s law of viscosity.

Shear stress and shear rate have two subscripts: z represents the direction of force and y represents the direction of normal to the surface on which the force is acting.

A negative sign is introduced into the equation because the velocity gradient is negative, that is, velocity decreases in the direction of transfer of momentum

Viscosity is defined as the resistance of a fluid to flow.

The unit of dynamic viscosity is (Pa.s) in the SI system and poise (g/cm.s)

in the CGS system.

Viscosity varies with temperature. Viscosity of most of the liquids

decreases with increasing temperature. In most gases, viscosity increases

with increasing temperature.

The effect of temperature on viscosity of liquids can be described by an Arrhenius type equation.

Viscous fluids tend to deform continuously under the effect of an applied stress.

Newtonian Fluids Fluids that follow Newton’s law of viscosity. The slope of the shear stress versus shear rate

graph, which is viscosity, is constant and independent of shear rate in Newtonian fluids (Figs below).

Examples: gases, oils, water, and most liquids that contain more than 90% water such as tea, coffee, beer, carbonated beverages, fruit juices, and milk.

Non-Newtonian Fluids Fluids that do not follow Newton’s law of viscosity Shear thinning or shear thickening fluids obey the

power law model (Ostwald-de Waele equation) For shear thinning (pseudoplastic) fluids, n < 1, For shear thickening fluids, n > 1. Newtonian fluids can be considered as a special case of

this model in which n = 1 and k = μ.

The slope of shear stress versus shear rate graph is not constant for non-Newtonian fluids.

Apparent viscosities of time-independent fluids.

The slope of shear stress versus shear rate graph is not constant for non-Newtonian fluids

For different shear rates, different viscosities are observed. Therefore, apparent viscosity or a consistency term is used for non-Newtonian fluids.

The symbol η is often used to represent the apparent viscosity to distinguish it from a purely Newtonian viscosity, μ.

The ratio of shear stress to the corresponding shear rate is therefore called apparent viscosity at that shear rate:The apparent viscosity and the Newtonian viscosity are identical for Newtonian fluids but apparent viscosity for a power law (Non-Newtonian) fluid is:

Shear Thinning (Pseudoplastic) Fluids. ◦ as shear rate increases friction between layers

decreases. ◦ Shearing causes entangled, long-chain molecules to

straighten out and become aligned with the flow, reducing viscosity.

◦ Examples: paint, the ink in a ballpoint pen, applesauce, banana puree, and concentrated fruit juices

Shear Thickening Fluids◦ as shear rate increases, the internal friction and

apparent viscosity increase◦ Examples: walking on wet sand on a beach, corn

starch suspension

Bingham Plastic Fluids Fluid remains rigid when the magnitude of

shear stress is smaller than the yield stress (τ0) but flows like a Newtonian fluid when the shear stress exceeds τ0.

Example: Toothpaste, mayonnaise, tomato paste, and ketchup.

The apparent viscosities for Bingham plastic fluids can be determined by taking the ratio of shear stress to the corresponding shear rate:

Non-Bingham Plastic Fluids a minimum shear stress known as yield stress must be exceeded

before flow begins. However, the graph of shear stress versus shear rate is not

linear. Fluids of this type are either shear thinning or shear thickening

with yield stress.

Herschel-Bulkley model◦characterized by the presence of a yield

stress term (τ0) in the power law equation:◦ Examples of the models: minced fish paste and

raisin paste. Casson model

◦ expressed as:◦ Examples of the models: Molten milk chocolate

When some fluids are subjected to a constant shear rate, they become thinner (or thicker) with time

Thixotropic fluids Fluids that exhibit

decreasing shear stress and apparent viscosity with respect to time at a fixed shear rate (shear thinning with time)

Examples: Gelatin, egg white, and shortening

Rheopectic fluids shear stress and apparent viscosity increase with time, that is, the

structure builds up as shearing continues (shear thickening with time) Examples: Bentonite–clay suspensions, in food systems, it is rarely

observed.

In the case of solutions, emulsions, or suspensions, viscosity is often measured in comparative terms, that is, the viscosity of the solution, emulsion, or suspension is compared with the viscosity of a pure solvent.

The relative viscosity, ηrel is:

The specific viscosity, ηsp is:

The reduced viscosity, ηred is:

Inherent viscosity, ηinh is:

Intrinsic viscosity, ηint, can be determined from dilute solution viscosity data:

Stress is defined as force per unit area and expressed in Pa (N/m2).

Normal stress (σ) Fig (a) the force applied

perpendicular to the plane per unit area.

Normal stress can be tensile or compressive depending on whether it tends to stretch or to compress the material on which it acts

Shear stress (τ ) Fig (b) the force applied parallel to

the plane per unit area (the stress acts tangential to the surface)

Strain is the unit change in size or shape of a material referred to its original size or shape when a force is applied.

Normal strain (ε) ◦ the change in length per unit length

in the direction of the applied normal stress

Stresses and strains can also be described as either dilatational or deviatoric.

A dilatational stress or train causes change in volume

A deviatoric stress or strain is the one that results in change in shape.

Dilatation can be calculated from the initial (V0) and final (Vf ) volumes of the sample:

Change in strain as a function of stress during loading and unloading can be seen in the figure.

Strain that is not recovered during unloading is called plastic strain, while strain that is recovered is called elastic strain.

The ratio of plastic strain to total strain when a material is loaded to a certain load and then unloaded is called the degree of plasticity.

Similarly, the ratio of elastic strain to total strain is defined as degree of elasticity.

When a stress is applied to a purely elastic solid, it will deform finitely but then it will return to its original position after the stress is removed.

Material showing elastic behavior is known as a Hookean solid.

Young’s modulus or Modulus of elasticity (E ) is defined as the ratio of normal stress (σ) to normal tensile or compressive strain (ε).

In the case of food materials, the apparent modulus of elasticity is used to relate stress to strain since stress–strain curve is not linear and no single E value is obtained.

Shear modulus or modulus of rigidity (G) is used to describe the relationship between the shear stress and shear strain.

If the force is applied from all directions which results in a volume change, modulus is called bulk modulus (K).

When a sample is subjected to uniaxial compression in one direction, it may expand in the other directions. Poisson’s ratio (μ) is defined as the ratio of the strain in the direction perpendicular to the applied force to the strain in the direction of the applied force.

Bioyield point is defined as the point at which an increase in deformation is observed with a decrease or no change of force.

In some agricultural products, the presence of this bioyield point is an indication of initial cell rupture.

Rupture point is a point on the stress–strain or force-deformation curve at which the axially loaded specimen ruptures under a load.

Rupture point corresponds to a failure in the macrostructure of the specimen while bioyield point corresponds to a failure in the microstructure of the sample.

Viscous fluids generally exhibit viscosity while solids exhibit elasticity.

Some foods show both viscous and elastic properties which are known as

viscoelastic materials semi solid food

Examples: wheat flour dough, dairy cream, ice cream mix, marshmallow cream,

cheese, and most gelled products.

There is no simple constant for viscoelastic materials such as modulus because

the modulus will change with respect to time.

If a viscoelastic fluid is stirred by a rotating rod it tends to climb the rod, which is

known as the Weissenberg effect.

Recoil phenomenon: when the flow of viscoelastic material is stopped, tensile

forces in the fluid cause particles to move back. However, viscous fluids stay

where they are when their motion is stopped.

Three different methods to study viscoelastic materials: stress relaxation test,

creep test, and dynamic test.

The deformation distance of spring (x) varies linearly with the force acting on the system (F)

The spring is considered as an ideal solid element obeying Hooke’s law:

The rate of extension varies linearly with the force acting on the system

The dashpot is considered as an ideal fluid element obeying Newton’s law in which force is proportional to rate of extension

The Maxwell model has been used to interpret stress relaxation of viscoelastic liquids, especially polymeric liquid.

The total shear strain can be expressed as the summation of strain in the spring and dashpot:

Total shear stress is espressed as

Creep behavior can be described by the Kelvin-Voigt model.

This model contains a spring and a dashpot connected in parallel

Therefore, it is possible to express all strains as equal to each other.

Total shear stress caused by the deformation is sum of the individual stresses:

The series combination of the Kelvin and Maxwell models: