Extrusion Flashcards
Describe extrusion process
Where moistened starch and/or protein materials are plasticised and cooked in a tube by a combination of pressure, heat and mechanical shear
What effect does it have on starch and protein
Starch - Gelatinisation
Protein - denaturation, stretching and structuring
Basic principles (3)
1) Granular or flour materials are fed into an extrusion cooker in a continuous manner at controlled rates
2) The material may be pre-conditioned with steam at moderate temperatures (80 degrees)
3) Moisture is added to meal or flour before extrusion or in the feed zone before the product enters the extrusion screw region
What does changing the die do?
Alter the final shape of the product
Describe the 3 extrusion zones of a single screw extruder
1) Transport section - Atmospheric pressure
2) Compression section - Low pressure
3) Holding section - High pressure
Describe the process
In the barrel the food material is forced under high temperature and pressure against a restricting die by means of a rotating screw
How can you enable different temperature zones in the barrel
By water jacketing the barrel
Describe the first and second sections
First - melt zone - 100 degrees C - the dough is converted to a colloid
Second - pump and compression zone - the temperature is increased by propulsion and friction heat to 100-175 degrees C to cook and expand the product
What is the term for the material that has been extruded through a die?
Extrudate
The movement of material from a region of high pressure to atmospheric pressure is equivalent to?
applying a vacuum
As the product is extruded what happens to the water content?
Water in the hot material leaving the extruder flashes off as steam, creating an expanded spongy product, which dries to about 8% moisture.
What is the term to describe the setting of the extrudate within a few seconds of final extrusion?
Thermo-setting
What are the two classifications of extruders?
Heat generated or Pressure developed
Describe heat generated extruders?
a) Autogenous extruders which generate their own heat
b) Isothermal extruders in which constant temperature is maintained by cooling
c) In between the two
Name the two types of pressure developed extruders
Direct or positive displacement type and Indirect or viscous drag type
Describe Direct of Positive displacement extruders
- Piston type - e.g. sausage stuffing
- Intermeshing twin screw (higher shear) subjects material to shear and drag e.g. chocolate and chewing gum manufacture
Describe Indirect or viscous drag type
- Extruder develops viscous drag, low pressure
- Roller extruder for sticky substances e.g. confectionery
- Single screw extruder
- Non-meshing twin screw extruders for dough sheeting
What is the most important part?
The screw
What is the material subjected to as it is conveyed along the screw channel?
mixing, shearing and heating
Describe the material in two words
Thermoelastic and viscoelastic
What assumptions are made for the equations used to predict the response of processing condition to changes in operating variables (3)
1) Extruder is isothermal - constant temp
2) Screw geometry is simple and constant
3) No leaks in the system
Describe 2 types of screw design
a) Full flighted screw with either a constant or a varying pitch and either a constant or varying channel depth
b) Torpeado type screw with either a constant pitch, a smooth pitch or a varying pitch
The net flow or output Q may be divided into two components
Q = QD + QP
What does the pitch of the screw mean?
Constant pitch = the same distance between the grooves on the screw / decreasing pitch = grooves getting closer together
What does QD mean:
QD - represents drag flow, the forward component
The drag rate is:
- proportional to the average velocity in the channel
- dependent on the screw speed and geometry
- independent of pressure and velocity
What effect does cooling have on the drag flow?
Cooling generally retards drag flow and reduces output whereas heating increases drag flow
The output increases when :
Drag flow is larger then pressure flow
What does QP mean:
Pressure flow - can be visualised as a non-rotating screw with fluid flowing backwards from the high pressure discharge end (can be used to emulsify)
What does pressure rate depend on?
The pressure gradient that is sometimes encouraged for homogenisation
Q = Net output - What is the formula?
Q = k1 x A x N - k2 x W x h3 x P
k1 and k2 are
constants
A
area
N
rate of resolutions
W
width of channel
h
Height of channel
P
pressure gradient
Describe the pressure peaks etc
The pressure reaches a maximum at the end of the screw, then drops to zero as the material passes through the die.
The flow rate through the die is
a function of viscosity, die geometry (resistance) and pressure drop (driving force)
There are two types of twin screws:
a) closely intermeshing screws
b) non-intermeshing screws
both of which may be counter or co-rotating
Name 4 ways in which the screw may vary:
1) Flight thickness
2) Number and pitch of flights
3) Pitch, route and intermeshing of the screws
4) Tapering of the screws
Where can leakages occur?
Between opposing routes and flights and between adjacent flights
Describe cold forming extrusion
Applied to press and preform a dough mass, and extrudate at normal mild process conditions under pressure and shear forces. Pre-cooked strachy ingredients are extruded to a completely hydrated mass, dried, baked or deep-fried.
Describe low pressure cooking and forming extrusion (indirect extrusion)
cooks uncooked starchy or blended materials with a high moisture content of 30%. The extruder operates at low speeds for a long gentle cook. they are externally heated. The extrudate has a non-expanded cell structure and is dried, baked or deep fat fried.
Describe high pressure cooking and forming extrusion (direct extrusion)
Direct extrusion or HTST cooking is used to cook low moisture (15%) materials to produce puffed cereals and snacks. The extruder is run at high speeds to generate heat.
Forming extruders from high moisture materials
They run cool be minimising heat generation, through barrel and screw design, and external cooling, and generate high pressures required for shaping. Grooves can be used in barrels to reduce flow rate by increasing heat generation, and to reduce slip at the barrel surface.
SIngle screw: Describe a pasta extruder
has a smooth barrel, no feed section, constant screw geometry and is isothermal. It resembles the idealised extruder closely
Single screw: Describe a high pressure forming extruder
is used for compressing and shaping a pregelatinised dough feed into products which require further processing. e.g. deep fat frying (third generation snacks) and gun puffing (cereals). The barrel is grooved, increasing the product temperature.
Single screw: Describe the low shear cooking extruder
used as a continuous cooker for high moisture dough/feed. The cooked product is further processed by forming and drying. Because of the high moisture external heating is applied. The extruder has a wide variety of applications.
Single screw: Describe The Collett extruder
produces puffed snack products from dry granular feeds. It is a high shear short residence time extruder. A high temperature is obtained by the high applied shear rate, grooved barrels and high viscosity which causes flashing of the moisture creating a puffed dry product (2nd generation snacks) It produces a narrow range of products.
Give an example of first generation snacks
Crisps
Single screw: Describe High shear cooking extruder
similar to the Collett extruder except that the residence time is greater an excess heat is removed be cooling the barrel. It is more flexible and can produce a variety of products e.g. cereals, snacks, dry mixes. The product has a higher moisture contact (20%) which is dried after extrusion to about 8%. There is less violent expansion, but products are browner, harder and have a cooked flavour.
Single screw: Describe steam textursation
the extruder utilises the injection of steam into the tube to increase the temperature to 121-160 degrees C. The residence time of the material, moisture content 20-30% is a fraction of a second compared to a few seconds in other extruders. No drying process is required as the product has a low moisture content 8-10%. The products by this process have a lighter colour and blander flavour.
Process control is required to produce expanded products with reproducible properties like
Product dimension, expansion ratio and texture properties
Process controls are controlled by process variables:
temperature, pressure and moisture content and extruder variables such as thee screw speed, number of screws, screw and die diameter and compression ratio.
What needs to correlate to maintain a regular and continuous feed?
Screw speed of the main engine and the feed screw
What does the screw speed affect?
the residence time which in turn affects expansion and output of the extruder
A low screw speed gives
a smaller output but increased expansion
What is the expansion index?
the ratio of the diameter of the extruded cylindrical pellet to the diameter of the die
what is the compression ratio?
the ratio of the clearances between the barrel and screw root e.g. a compression ratio 1:3 or 1:4 is commonly used for the extrusion of starch. With ungelatinised modified starch the pressure has to be increased.
Production capacities of food extrusion are high - what are they for 1) Second generation snacks and light breakfast cereals? 2) Third generation snacks 3) Gelatinisation of corn meal
1) 150-200 kg/h
2) 1200 kg/h
3) 10000 kg/h
What effect does HTST have on nutrition and bacteriological counts?
Nutritional value is high and bacteriological counts are low
How is instantised hot breakfast porridge produced by extrusion?
Extrusion cooking a flour or mixture of cereal flours at high temperatures to produce extradate readily. The extrudate is dried, cooled and reduced to a granular particle size which can be rehydrated instantly with water or milk.
What different shapes can be made from extrusion for breakfast cereals?
Beads, half beads, rough textured agglomerates - cereals extruded in the form of beads may be rolled while still moist into flakes, dried and cooled.
Describe two ways in which snacks are produced
1) Extruding a starch based granule under high pressure which explodes into the required shape, is then toasted in an oven, sprayed with oil and flavour
2) Extruding starch granules under low pressure to produce a shaped product which is gelatinised, hardened and quick dried, achieving its shaoe through the frying process.
Define first generation snacks
These are not extracted - include sliced or rolled snacks
Define second generation snacks
Include extrusion cooked corn curls, thick cips, scoop rings, beads and bite size wafers which have a long shelf life as the moisture content is less than 10%
Define third generation snacks
are produced by extrusion cooking wet milled starches and passing these cooked extrudates (moist) directly into a sizing extruder to produce onion rings, twists and scoops which are later fried, flavoured and packaged. The shelf life is 2-3 weeks as the moisture content of the product is high. These products may be kept frozen.
Describe an example of extruded confectionary
Gum pastilles - sugar, glucose syrup and modified starch and flour are cooked in a continuous extruder at 125-135 degrees C and cooled to 70 degrees C for the product to gel.
Describe how infant foods are made using extruders
extrusion cooking of protein-enriched cereal flours with minerals, micro-encapsulated vitamins and added vegetable or meat proteins - high temperature extrusion cooking of the mixtures gives a water uptakes and smoothness equal to that of drum dried baby foods - due to HTST process these foods have a low bacterial count, minimum damage to protein quality, amino acid availabilty and vitamin stability.
Describe how pasta products are produced by the traditional extrusion process
Extrusion cooking can be used to pre-cook pasta products which are quick cooking
Describe blended foods
- Provides cheap protein of good quality in developing countries and reduces home cooking. The pioneering product Incaparina was based on ingredients available in Central America - cotton seed, corn, yeast, minerals and vitamins
- may be extrusion cooked to increase the utilisation of the cereal protein of to destroy heat labile growth inhibitors in soy and other oil seeds
- can be used as weaning foods, snacks, breakfast cereals - can be reduced in particle size by impact milling to produce a beverage powder, soup stock and instant hot breakfast products
Textured vegetable protein - discuss?
Defatted soya flour and water are mixed and homogenised by the screw to produce a plastic mass which is extruded through the die giving a layered product with vaculoes caused by steam leaving the product. The chunks are rough cut whilst wet and can be reduced to granules in a different machine and then dried to 8% moisture e.g. ultrasoy, textrasoy
Describe Bontrae
is produced by the steam texturisation process. A ball is passed along the barrel into which steam is injected. The residence time is only a fraction of a second compared to a normal extruder time of a few seconds. No drying process is required as the final product contains 8-10% moisture. The powder has a low density, and rehydrates rapidly in 30 seconds.
Meat analogues
Extruded breakfast products have been devloped from soy flours and concentrates and isolates with cereals, egg albumen, veg oil, meat-like flavours, colour and yeast
Describe process for meat analogues
- The first extruder discharges a moistened, heated and denatured protein into a second low profile extrusion cooker where the proteins are reheated and stretched axially into strands and transversely into sheets to form layers
- The layers are extruded in the tight parallel conformation which causes it to resemble skeletal muscle. The pieces are cut and conveyed to driers to reduce the moisture content to 8%.
Name two manufacturing benefits
Require less floor space and require less labour per ton
What processes can extruding carry out simultaneously?
Gelatinisation, texturisation, blending of proteins and cereals and expansion of product minimises labour and processing costs and product contamination
