1. EXTRUSION, HOMOGENIZATION & MIXING OF FOOD.

2. INTRODUCTION FOOD EXTRUSION
Extrusion is defined as a process in which material is pushed through an orifice or a die of given shape.
The pushing force is applied using a piston or a screw.
In food applications, screw extrusion is predominant.
Today’s extruder consists of one or more screws encased in a metal barrel, attached to a drive motor.
A hopper at one end is used to feed raw materials, while a die on the other gives shape to the product.

3. Flow chart for pasta processing
Ingredient storage Ingredient feeding Mixing Drying Cutting Packing

4. Advantages of extrusion
Extrusion is a continuous process.
It is flexible because on-line process adjustments can be made to achieve desired product characteristics.
The process has no effluents and is energy-efficient.
It can be used to process relatively dry, highly viscous materials as well as moist or wet materials.

5. Food extrusion is a high-temperature–short-time (HTST) process.
During extrusion, the food material may be exposed to temperatures as high as 200 °C.
The exposure to such high temperatures lasts only 1–10 seconds.
In a typical corn cooker, the operating temperature is about 100 °C, and exposure time is about an hour.

6. Flow diagram for the production of corn flakes using conventional & extrusion processes  

7. Extruded Products for Human Consumption
Type
Example
Directly expanded
Breakfast cereals, corn curls
Unexpanded
Pasta
Half-products
Potato pellets
Co-extruded
Fruit-based cereal, jelly-filled cores
Modified
Starches, fat mimics
Candy
Licorice, chewing gum
Texturized
Meat analogs

8. CLASSIFICATION OF EXTRUDERS:
Extruders are classified into two types
Based on operation: Hot and cold extruders.
Based on type of construction: Single screw and twin screw extruder

9. The Principles of Operation
Raw materials are fed into the extruder barrel and the screw(s) then convey the food along it.
Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food.
As a result, it fills the barrel and the spaces between the screw flights and becomes compressed.
If the food is heated above 100ºC the process is known as extrusion cooking (or hot extrusion).
In Cold extrusion, the temperature of the food remains at ambient and is used to mix and shape foods such as pasta and meat products.

10. The food is then passed to the section of the barrel having the smallest flights, where pressure and shearing is further increased.
Finally, it is forced through dies at the discharge end of the barrel as the food emerges under pressure from the die, it expands to the final shape and cools rapidly as moisture is flashed off as steam.
A variety of shapes, including rods, spheres, doughnuts, tubes, strips, squirls or shells can be formed.
Typical products include a wide variety of low density, expanded snack foods and ready-to-eat (RTE) puffed cereals.

11. Extrusion cooking is a high-temperature short-time (HTST) process which reduces microbial contamination and inactivates enzymes.
The main method of preservation of both hot- and cold-extruded foods is by the low water activity of the product (0.1–0.4), and for semi-moist products in particular, by the packaging materials that are used.
Cold extrusion, in which the temperature of the food remains at ambient is used to mix and shape foods such as pasta and meat products.
Low pressure extrusion, at temperatures below 100ºC, is used to produce, for example, liquorice, fish pastes, surimi and pet foods.

12. The equipment consists of a cylindrical screw that rotates in a grooved cylindrical barrel.
The length to diameter ratio of the barrel is between 2:1 and 25:1.
The screw speed is one of the main factors that influence the performance of the extruder: it affects the residence time of the product, the amount of frictional heat generated, heat transfer rates and the shearing forces on the product.
Typical screw speeds are 150–600 rpm, depending on the application.

13. Compression is achieved in the extruder barrel by :
Back pressure, created by the die.
Increasing the diameter of the screw and decreasing the screw pitch.
Using a tapered barrel with a constant or decreasing screw pitch.
Placing restrictions in the screw flights.

15. Single screw extruder claficassion
High shear: High speeds and shallow flights create high pressures and temperatures that are needed to make breakfast cereals and expanded snackfoods.
Medium shear: For breadings, texturised proteins and semi-moist petfoods.
Low shear: Deep flights and low speeds create low pressures for forming pasta, meat products and gums.
Single-screw extruders have lower capital and operating costs and require less skill to operate and maintain than twin-screw machines do. They are used for straightforward cooking and forming applications, when the flexibility of a twin-screw machine is not needed.

16. HOMOGENIZATION
Homogenization is the term used to describe the operation in which the desired reduction in size of droplets of internal phase is brought by forcing the curde emulsion through narrow opening at high velocity.
A pressure homogenizer consists of a homogenizing valve & pressure pump. The valve provide an adjustable gap, of order of a few thousandths of an inch, through which a crude emulsion is pumped at press. up to p.s.i
On entering the gap of the liquids are greatly accelerated and the droplets on internal phase shear against each other are deformed & disrupted.

17. The sudden drop in pressure as the liquids leave the gap and collapse of bubbles due to cavitation probably also contribute to the reduction in droplet size.
The common type of valve design, the poppet valve is shown
The liquid travel between the valve and its seat causing it to lift against a strong spring.
Adjustments of tension on the spring provides the means of altering the homogenizing pressure.
On leaving the gap the liquid impinge on breaker ring.
In some cases the valve is made to rotate backward and forward in its seat.
The liquids are forced through the multitude of small chanels in the body.

18. Positive displacement pumps are necessary to supply feed to the valve.
The most common system used multiple cylinder type plunger pump.
The feed introduced to the homogenizer as a crude premixed emulsion.
Droplet sizes of the order of μ are common and may be low as 0.02 μ.
These pressure homogenizer is applied in milk to reduce fat globule size down to 1or 2 μ.
Low fat cream, cream evaporated milk and sterilized milk are also homogenized.
Ice cream mix exhibits good body & texture when homogenized.

19. Mixing
It can be defined as that state in which all samples are found to contain the components in same proportions as in whole mixture.
Mixing is the dispersing of components, one throughout the other .
It also occurs in innumerable instances in food industry and is probably the commonly encountered of all process operations.

20. SPECIAL FEATURES OF FOOD MIXING
Mixing involves a whole spectrum of materials from dry free flowing to thin, viscous liquids viscous pastes.
Mixing involves to many components existing in different physical state, which have widely differing and time dependent properties.
Energy requirements for each component can also differ widely.
Food mixing can involve dispersion of air or gas bubbles into liquids and pastes.
The effect of mixing can continue well after the mixing action has ceased &it could be quite some time after the end point.

21. Energy Input
Quite substantial quantities of energy can be consumed in some type of mixing, such as in mixing plastic solids.
There is no necessary connection between energy consumed and progress of mixing.
In well designed mixers energy input does relate to mixing progress, though the actual relationship has normally to be determined experimentally.
In the mixing of flour doughs using high speed mixers, the energy consumed or the power input at any particular time, can be used to determine the necessary mixing time.

22. Types of mixers Mixers for low or moderate viscosity Paddle agitators
Turbine agitators
Propeller agitators
Mixing vessels
Mixers for high viscosity pastes and plastic solids
Pan mixers
Horizontal trough mixers
Continuous paste mixers

23. Mixers for dry solids
Horizontal trough mixers
Vertical screw mixers
Fluidized bed mixers

24. Applications for mixing equipments in food industry:
Low viscosity fluids : Impeller agitator is applied for blending of oils & cooking of fats, diluting concentrated solutions, manufacturing fruit squqshes.
High viscosity liquids & pastes: Pan mixer Z- blade mixers are applied for dough and batter mixing in bread, cake & biscuit manufacture, preparation of meat paste.
Dry solids : Tumbler, ribbon & screw mixer are applied for blending of grains prior to milling, blending of flour & incorporation of additives to flour, preparation custard powders.