Food extrusion Flashcards
define extrusion
combination of mixing, cooking, kneading, shearing, shaping and forming to transform raw material into a continous specific shape product
advantages of extrusion
adaptability
product characteristics
energy efficiency
use of conventional ingredients
new foods
high product quality
reduced cost
no effulent
explain adaptability
minor changes in ingredients and conditions results in a variety of products
explain product characteristics
a vairety of shapes, textues and colous can be produced
explain energy efficiency
processing of relatively dry viscous material results in less re-drying
explain high quality
good control of thermal changes of food
improved textural and flavour characteristics
minimal degradation of nutrients
destroys anti-nutritional compounds
inactivates mircroorganisms
detoxifies aflatoxin
what anti-nutritional compounds can extrusion inibit
trypsin inhibitors
undesirable enzymes(lipases and lipoxygenase)
explain reduced cost
lower processing costs
continuous high productivity
functions of an extruder
-name at least 4
agglomeration
degassing
dehydration
expansion
gelatiniation
grinding
homogenisation
mixing, shaping and shearing
texture alteration
thermal cooking
what is an exytruder
a screw that pumps continously and simultaneously transports, mixes, shears, cookes and shapes material under elevated temperatures and pressure
describe the mechanism of an extruder
-feeding of raw material
-conveying and compression using screw
-working/transformation of raw material to semi-solid using thermal and mechanical energy
-extrusion of the food through die
-release of material using rotating knives
classes of factors affecting extrusion (2)
operating parameters
rheological properties
explain operating parameters
temperature
pressure
diameter of die
shear rate
explain rheological properties
moisture
physical state of material
chemical composition
how are single screw extruders classified?
extend of shear: high, medium or low
heat generation: adiabatic, isothermal, polytropic
screw configuration: solid, interrupted flight, segmented
what is shear
how fast the screw moves
explain the types of heat generation
adiabatic: no input or extraction of heat, heat is generated through friction
isothermal: consistent temp
polytropic: heat and water transfer
explain solid screw configurations
screw itself is on the shaft
explain interrupted flight screw configurations
fabricated from different zones, flights have gaps that allow it
-increases kneading
explain segmented screw
different sections where the flights are different
classes of twin screw extruders
counterrotating: rotate in opposite directions
corotating: rotate in the same direction
mechanism of transportation for SSE
centrifugal force
-the flights push the mixture upwards
-fan/flight is not impacted by blocking discharge hole
mechanism of transportation for TSE
positive displacement pump
-what goes in must be transported
-if exit is blocked it can damage shaft.
three spaces in the barrell
feed
transition
metering
describe the feed zone
deeper flights of greater pitch
what is a preconditioner and what happens
Chamber
-can be atmospheric or pressurised
- steam/moisture can be added but most often it is dry
the function of a preconditioner
increase residence time: mena extuder wont run out of material
reduce mechanical power consumption: semi-mixing in hopper
increase capacity: extruder constantly has material
descirbe the transition section
material is compressed and converted to a uniform plasticised dough
describe the metering section
shallow flights, reduced helix angle
what is the compression ratio
how much material is in the start comapred to metering
compression ratio for pressureised and non pressurised
P: 1:5
not 1:1
describe the flow profiles inside the barrel
drag: foreward
pressure induced: backward
leakage flow: backwards
how does low moisture and viscous material effect flow profile
increases pressure = higher output and low leakage flow
how does small die holes material effect flow profile
greater resistance flow = more pressure and lower output
how does cooling material effect flow profile
higher viscosity = higher output less leakage flow and better drag flow
how many ways can two parallel screws be configured
8
why are TSE useful
overcome the effects of slip at wall
key points of differientation for TSE
direction of screws rotatinf
degree of intermeshing
describe fully intermeshign counter rotating screws
prevent cylindering effect
extremely high pressure
ppoor mixing
no back mixing
ease of moisture extraction
application of fully intermeshign counter roating screw
low viscosity system
describe intermeshing co-rotating self-wiping screw
open lengthwise, closed crosswise
type of flow in intermeshing co-rotating self-wiping screw
drag flow and positive displacement
is there cylindering effect in intermeshing co-rotating self-wiping screw
no
best materials for intermeshing co-rotating self-wiping screw
sticky, wet and oily materials
TSE vs SSE sugar content
T: 40%
S: 10
TSE vs SSE fat content
T: 20:
S: 4
TSE vs SSE moisture content
T: 65%
S: 30%
advantages to intermeshing co-rotating self-wiping screw
forward or reverse conveying
enhanced mixing capability
increased heat transfer
restrictive mechanisms used to back fill twin screw extruders
die
reverse pitch screws
reduced pitch screws
cut flight screws
Dam
what is a dam
variable restriction which inhibits the transfer of ingredients between screws in conveying sections
what are reverse pitch screws
forward conveying sections fill to force material thorugh -vely pitched screw sections
application of cooking extrusion
ready to eat
puffing at die
pellets
flaked or sheets
evaluation of extrudate properties
expansion and cellular structure
water absorption index
water solubility index
extrudate strength
when does expansion occur
when internal vapour pressure forces are greater than physicla strength of product structure
how is expansion measured
specific volume = 1/bulk density
product and die diameter ratio
relationship between expansion and temperature
increases
how to increase temp
increase mechanical energy input
higher steam injections
higher barrel temp
what is water absorption index
weight of gel per g of dried sample
what is water solubility index
percent of dry matter in the supertanant
effecct of extrusion on starch
classical gelatinisation
extrusion
properties starches contribute to in extrudate products
puffing
binding
caloric value
viscosity
functionality
two protein modifications during extrusion
physico-chemical changes
non-enzymatic browning
explain the physico-chemical changes
proteins become plasticised
continous protein matrix contaiing a dispersed phase
what happens to solubility of proteins during extrusion
proteins denatrue and solubility is reduced
results of increased protein in expanded cereals
decreased expansion
increased crunchiness
increased firness
decreased water absorption (pasta)
result of high levels of fat in extruded products
increased cell size
smoother surface
less gelatinisation
lower mechanical energy dissipated as heat (slip/friction)
increased chance of rancidity
effect of lipids in extrusion
increase palatability
calcoric density
cell structure
surface textures
