Extrusion, Food Education

Factors Affecting Extrusion Cooking

Factors Affecting Extrusion Cooking

1.Feed Properties
a.       Moisture level
Lowering water content of the feed material results in higher viscosity, which causes throughput rate to decrease, pressure drop to increase, and power consumption to increase. However, in extrusion of starch, gelatinization occurs more readily at higher moisture contents. Further, with less moisture content in the product exiting the extruder, the degree of expansion is typically reduced and, hence, higher bulk density and smaller volume.
b.      Type of feed materials
The type and nature of feed material, interm of protein, starch, lipid, and moisture plays a significant role on the nature of extruded products. In addition to differences in viscosity between materials made from different grains, the internal structure of the extruded products is also caused by different constituents. For example, adding whey proteins to grain-type extruded products will significantly alter the gel structure during extrusion cooking and, hence a different product exiting the die.
c.       pH of ingredients
Adjusting the pH of the feed material may influence the state of proteins during extrusion and thus influence physical characteristics of the final product. Study has shown that increasing pH in the 5.5-7.5 range results in an increase in volatile compounds in cystein/reducing sugar/starch mixture during extrusion. Further, in extrusion cooking of defatted soy flour, an optimum in tensile strength of the extruded product was found when the material was fed at pH of about 7. Both higher and lower pH values caused reduction in tensile strength. Changing pH may also cause changes in color and nutritional content of extruded products.
In using an extruder as a bioreactor, an optimum pH range needs to be carefully chosen. For example, in production of maltodextrin from starch using enzyme alpha-amylase, an optimum pH for the enzyme is 5.0-8.0. Lowering pH value in this case will inhibit the enzyme activity.
d.      Particle size of feed materials
Starch having particle size greater than 14 mesh will not be readily gelatinized. Small particles (40-120 mesh) are more easily hydrated and cooked than larger particles.
e.       Other ingredients
Ingredients such as oil and emulsifier may also be added to the feed material to decrease viscosity of the raw material. This also helps lubricate the extrusion cooking process of such material and decrease the amount of viscous heat dissipation.

2.Extruder operating parameters
a.      Type of extruder
Typically, a single-screw extruder is the best choice for simple products. However, a single-screw extruder provides limited flexibility for new and unique products. Twin-screw extruders are able to provide multiple processing steps (i.e., kneading, mixing). They also produce higher production rates.
b.      Feed rate
Feed rates are normally kept low enough that the extruder operates under “starved-fed” conditions. That is, the flights of the screw in the feed section are not completely filled. As the screw root diameter is increased in the transition zone and the channel size is reduced, the screw becomes completely filled as material enters the metering zone.
c.      Screw geometry
In a single-screw extruder, parameters that can be adjusted include the pitch of the screw, the diameter of the screw, the clearance between the top of the flight and the barrel, and the number of flights on the screw. For a twin-screw extruder, the options for screw geometry and range of configurations are numerous. Parameters that can be varied in a twin-screw extruder include pitch of the screw, number of flights, root diameter, angle of pitch of the screw, degree of intermeshing of the two screws, and the type of mixing device.
d.      Extruder length
The length of the extruder determines the magnitude of pressure build-up at the die and the extent of reactions occurring within the food mass. Longer extruders typically produce higher pressures as the metering section is increased. Longer screws also result in longer residence time of the food mass within the extruder.
e.      Screw speed
Screw speed affects the degree of fill within the screw, residence time distribution of product flowing through the extruder, heat transfer rates and mechanical energy input in the extruder, and the shear forces exerted on the materials. Screw speed typically falls in the range of 100-500rpm. For RTE cereals, pet foods and snacks where the total moisture content of the material inside the barrel is in the range of 14-20%, screw speeds in excess of 250 rpm are normal.
The normal minimum screw speed range is 70-100 rpm. Below this, the volumetric capacity would be severely limited and make the majority of food extrusion products costly to manufacturers. Most manufacturers chose to run at the maximum speeds mechanically tolerable, usually 400-500 rpm.
Usually, the screw speed is set so that the length of barrel fill (BF) is greater than 1D (D is the die diameter) from the die. If BF is less than 1D from the die, the system is prone to instability. This can be seen as product surging from the die together with rapid die pressure fluctuations. Reducing the die are and screw speed helps fix this problem.
f.       Die characteristics
The die material can be bronze, bronze alloys, stainless steels, etc. the bronze dies are useful because any alteration to them can be done easily. They also have better suited thermal conductivities when die heating is required but wear out quickly. When a smooth surface is required on third generation snacks or confectionery such as candy sticks, Teflon-coated inserts can be used. However, they do not last long and can absorb moisture over a period of time causing them to distort with heat. For direct expanded cereal/proteins, through hardened steels are used.
The operating pressure for direct expanded products is in the region of 300-2500 psi. For low viscosity liquid such as liquorice, fruit leather, caramels, etc., die pressure is usually less than 300 psi. The die restrictiveness should be such that sufficient barrel length is filled.
g.      Barrel temperature
Most extruders operate with temperature control. The pressure differentials and shear stress forces influence reaction rates and generate frictional heat. Barrel heating generates conductive and convective heat in the filled and partially filled zones and the proportion of each heat source depends on the physical (i.e., specific heat, phase transition temperature, moisture content, density, particle size, and gelatinization enthalpy) and rheological properties of the feed, the barrel temperature profile and the available motor power.
When direct expanded products are extruded, the moisture content within the barrel is normally 12-18% depending on the sugar and fat content. The frictional heat generation requires the barrel to be cooled with air or water. Extrudate temperature can reach 180°C. In order to prevent material from burning on the hot barrel surface, or inhibit excessive maillard browning, or limit the degree of protein denaturation, the barrel jacket can be fed with chilled water. Reducing temperature in the material can also be done by increasing water or oil content or reducing the degree of shear, which can be done by reducing the screw speed or the severity of the screw configuration.
Temperature instability causes unstable extrudate flow and varied quality. Problems creating product temperature fluctuations could include:
·       Scale formation in jacketed barrels
·       Varying thermal loading
·       Poorly set-up controllers
·       Climatic temperature fluctuations
·       Metal wear
·       Ingredient variations

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