Extrusion Flashcards
What are the processes involved in extrusion?
Mixing
Cooking
Kneading
Shearing
Shaping
What is the system used for extrusion?
Rotating within a barrel where material is compressed and sheared at the end
How does the material change during the extrusion?
Hydration and loss of starch crystallinity
Melting of fat
Denaturation and reorientation of proteins
Inactivation of enzymes
Destruction of anti-nutritional factors
What happens to the fluid as it reaches the end of the barrel?
Cools to an aerated structure as pressure lowers and water is vaporised
Why does it become an aerated product?
Bubbles expand to form a foam
What happens if the viscosity of the liquid is too high?
Expansion is prevented and bubbles coalesce and collapse
How is viscosity controlled?
Processing conditions
Starch type and conversion
How are snacks now made?
Pellets are made from extrusion at low temperature
Pellets are fried
How do pellets expand when fried?
Water expands and vapour is made and held
What are mycotoxins?
Contamination of grains by fungi
What are acrylamides?
Found in products at high temperature
Formed by reducing sugars and L-asaparagine
What is staling?
Chemical and physical changes during storage caused by amorphous starch becoming crystalline
How does staling effect the crumb?
Polar lipids form complexes with starch and moisture migrates to edges
What is retrogradation?
Re-association of gelatinised starch
What does retrogradation depend on?
Starch structure
Temperature, moisture, pH
Time
