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FOSC302 > Food extrusion > Flashcards

Food extrusion Flashcards

1
Q

define extrusion

A

combination of mixing, cooking, kneading, shearing, shaping and forming to transform raw material into a continous specific shape product

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2
Q

advantages of extrusion

A

adaptability
product characteristics
energy efficiency
use of conventional ingredients
new foods
high product quality
reduced cost
no effulent

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3
Q

explain adaptability

A

minor changes in ingredients and conditions results in a variety of products

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4
Q

explain product characteristics

A

a vairety of shapes, textues and colous can be produced

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5
Q

explain energy efficiency

A

processing of relatively dry viscous material results in less re-drying

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6
Q

explain high quality

A

good control of thermal changes of food
improved textural and flavour characteristics
minimal degradation of nutrients
destroys anti-nutritional compounds
inactivates mircroorganisms
detoxifies aflatoxin

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7
Q

what anti-nutritional compounds can extrusion inibit

A

trypsin inhibitors
undesirable enzymes(lipases and lipoxygenase)

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8
Q

explain reduced cost

A

lower processing costs
continuous high productivity

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9
Q

functions of an extruder
-name at least 4

A

agglomeration
degassing
dehydration
expansion
gelatiniation
grinding
homogenisation
mixing, shaping and shearing
texture alteration
thermal cooking

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10
Q

what is an exytruder

A

a screw that pumps continously and simultaneously transports, mixes, shears, cookes and shapes material under elevated temperatures and pressure

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11
Q

describe the mechanism of an extruder

A

-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

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12
Q

classes of factors affecting extrusion (2)

A

operating parameters
rheological properties

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13
Q

explain operating parameters

A

temperature
pressure
diameter of die
shear rate

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14
Q

explain rheological properties

A

moisture
physical state of material
chemical composition

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15
Q

how are single screw extruders classified?

A

extend of shear: high, medium or low
heat generation: adiabatic, isothermal, polytropic
screw configuration: solid, interrupted flight, segmented

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16
Q

what is shear

A

how fast the screw moves

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17
Q

explain the types of heat generation

A

adiabatic: no input or extraction of heat, heat is generated through friction
isothermal: consistent temp
polytropic: heat and water transfer

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18
Q

explain solid screw configurations

A

screw itself is on the shaft

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19
Q

explain interrupted flight screw configurations

A

fabricated from different zones, flights have gaps that allow it
-increases kneading

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20
Q

explain segmented screw

A

different sections where the flights are different

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21
Q

classes of twin screw extruders

A

counterrotating: rotate in opposite directions
corotating: rotate in the same direction

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22
Q

mechanism of transportation for SSE

A

centrifugal force
-the flights push the mixture upwards
-fan/flight is not impacted by blocking discharge hole

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23
Q

mechanism of transportation for TSE

A

positive displacement pump
-what goes in must be transported
-if exit is blocked it can damage shaft.

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24
Q

three spaces in the barrell

A

feed
transition
metering

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25
Q

describe the feed zone

A

deeper flights of greater pitch

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26
Q

what is a preconditioner and what happens

A

Chamber
-can be atmospheric or pressurised
- steam/moisture can be added but most often it is dry

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27
Q

the function of a preconditioner

A

increase residence time: mena extuder wont run out of material
reduce mechanical power consumption: semi-mixing in hopper
increase capacity: extruder constantly has material

28
Q

descirbe the transition section

A

material is compressed and converted to a uniform plasticised dough

29
Q

describe the metering section

A

shallow flights, reduced helix angle

30
Q

what is the compression ratio

A

how much material is in the start comapred to metering

31
Q

compression ratio for pressureised and non pressurised

A

P: 1:5
not 1:1

32
Q

describe the flow profiles inside the barrel

A

drag: foreward
pressure induced: backward
leakage flow: backwards

33
Q

how does low moisture and viscous material effect flow profile

A

increases pressure = higher output and low leakage flow

34
Q

how does small die holes material effect flow profile

A

greater resistance flow = more pressure and lower output

35
Q

how does cooling material effect flow profile

A

higher viscosity = higher output less leakage flow and better drag flow

36
Q

how many ways can two parallel screws be configured

A

8

37
Q

why are TSE useful

A

overcome the effects of slip at wall

38
Q

key points of differientation for TSE

A

direction of screws rotatinf
degree of intermeshing

39
Q

describe fully intermeshign counter rotating screws

A

prevent cylindering effect
extremely high pressure
ppoor mixing
no back mixing
ease of moisture extraction

40
Q

application of fully intermeshign counter roating screw

A

low viscosity system

41
Q

describe intermeshing co-rotating self-wiping screw

A

open lengthwise, closed crosswise

42
Q

type of flow in intermeshing co-rotating self-wiping screw

A

drag flow and positive displacement

43
Q

is there cylindering effect in intermeshing co-rotating self-wiping screw

A

no

44
Q

best materials for intermeshing co-rotating self-wiping screw

A

sticky, wet and oily materials

45
Q

TSE vs SSE sugar content

A

T: 40%
S: 10

46
Q

TSE vs SSE fat content

A

T: 20:
S: 4

47
Q

TSE vs SSE moisture content

A

T: 65%
S: 30%

48
Q

advantages to intermeshing co-rotating self-wiping screw

A

forward or reverse conveying
enhanced mixing capability
increased heat transfer

49
Q

restrictive mechanisms used to back fill twin screw extruders

A

die
reverse pitch screws
reduced pitch screws
cut flight screws
Dam

50
Q

what is a dam

A

variable restriction which inhibits the transfer of ingredients between screws in conveying sections

51
Q

what are reverse pitch screws

A

forward conveying sections fill to force material thorugh -vely pitched screw sections

52
Q

application of cooking extrusion

A

ready to eat
puffing at die
pellets
flaked or sheets

53
Q

evaluation of extrudate properties

A

expansion and cellular structure
water absorption index
water solubility index
extrudate strength

54
Q

when does expansion occur

A

when internal vapour pressure forces are greater than physicla strength of product structure

55
Q

how is expansion measured

A

specific volume = 1/bulk density
product and die diameter ratio

56
Q

relationship between expansion and temperature

A

increases

57
Q

how to increase temp

A

increase mechanical energy input
higher steam injections
higher barrel temp

58
Q

what is water absorption index

A

weight of gel per g of dried sample

59
Q

what is water solubility index

A

percent of dry matter in the supertanant

60
Q

effecct of extrusion on starch

A

classical gelatinisation
extrusion

61
Q

properties starches contribute to in extrudate products

A

puffing
binding
caloric value
viscosity
functionality

62
Q

two protein modifications during extrusion

A

physico-chemical changes
non-enzymatic browning

63
Q

explain the physico-chemical changes

A

proteins become plasticised
continous protein matrix contaiing a dispersed phase

64
Q

what happens to solubility of proteins during extrusion

A

proteins denatrue and solubility is reduced

65
Q

results of increased protein in expanded cereals

A

decreased expansion
increased crunchiness
increased firness
decreased water absorption (pasta)

66
Q

result of high levels of fat in extruded products

A

increased cell size
smoother surface
less gelatinisation
lower mechanical energy dissipated as heat (slip/friction)
increased chance of rancidity

67
Q

effect of lipids in extrusion

A

increase palatability
calcoric density
cell structure
surface textures

FOSC302 (14 decks)

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