Extrusion, Food Education

Process Instability in Food Extrusion

Process Instability in Food Extrusion

It is important to diagnose extrusion problems quickly and accurately to minimise off-quality product and downtime losses. For effective diagnosis it is necessary to have both adequate process instrumentation and good process understanding. Lack of instrumentation makes the exercise a guessing game that may not resolve the problem quickly. Good process understanding comes with experience.

The food extrusion process involves mixing, physical and chemical modification and forming. The temperature and pressure profile, residence time distribution and the balance of mechanical and thermal energy input to the product constitute the operating conditions of the process. Changes in feed material specification and /or the operating conditions are likely to affect the extruded product specification or the extruder stability performance. Problems are generally more severe in cooking extrusion operations due to increased sensitivity of the process at high temperatures.

Possible Causes Of Extruder Instabilities

Irregular feed composition resulting from inadequate batch mixing causes variations in the material viscosity as it is processed through the extruder. Particle size segregation and changes in fat and moisture distribution are the most common problems associated with inadequate pre-blending. Variations from batch to batch in ingredients from suppliers can affect the material friction properties and their viscosity. Both affect extruder performance and product quality. Proper quality control procedures should be adopted and precise specifications agreed with all ingredient suppliers.

Irregular feed rate of the dry and liquid components of the feed material can cause changes in the filling pattern of the extruder. This will result in a different temperature, pressure and residence time distribution with consequent fluctuations in extruder performance and product quality. In some cases, generated process steam and entrapped air can cause feeding problems. Cooling of the feed section can minimise the backward flow of steam, and subsequent condensation, which causes feed hopper bridging problems. Proper air venting is necessary when incorporating low density feed materials, especially very fine powder fractions.

Heating or cooling fluctuations can be caused by either severe cycling of the barrel temperature controller or by changes in the external environment. This is normally more significant in small extruders since heat transfer area per unit of throughput is greater than in larger machines. Material friction properties and viscosity may be affected and this may influence the filling pattern and the mechanical energy input into the product. This normally shows up as changes in motor torque and die pressure.

Irregularities in conveying and mixing lead create a wide residence time distribution, which, together with high temperature can result in product degradation and fluctuations in melt consistency. In some cases the extruder screw design may not match the desired process characteristics. The screw design should provide a balanced transfer between conveying, mixing and forming functions. The mixing function is particularly important when significant levels of fat are included in the recipe, Inadequate mixing may result in the fat forming a second phase and the extruder becoming severely unstable. In extreme cases, the extruder will stall.

Product burning will result in blockage of the diffuser or die plate holes. This can be caused by:

(i) The presence of large particle size ingredients in the feed
(ii) Wide residence time distribution. This is more noticeable at low moisture (high viscosity), high temperature extrusion conditions.
(iii) Dead zone spots. These can occur within the die or with screw or barrel wear. The die design should be streamlined to ensure minimum flow velocity. Wear in extrusion components takes place gradually and the effect on product quality can be reduced by slight adjustments to overall extrusion conditions.
(iv) When a cooking extrusion process results in low die pressure, near or below the steam saturation pressure, steam superheating takes place. This results in two phase flow (steam and extrudate) through the die causing die flow instability. Steps to increase the die pressure, such as increasing the flow rate, the extrudate viscosity or the die resistance need to be taken.
(v) Irregular screw feed causes irregular conveying, mixing and shear rate.

Other process upsets may result, not in unstable extrusion, but in a new steady state with a different final product specification.

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