Specific Mechanical Energy (SME) in EXTRUSION
By Gordon Young, Food Industry Engineering, Australia and Dennis Forte, Dennis Forte & Associates, Australia
The authors are presenting short courses on “Aquafeed Extrusion Technology”, “Petfood Extrusion Technology” and “Drying Technology” at Centre for Feed Technology, FôrTek, Norway, in April/May 2018 (
www.fie.com.au/events)
INTRODUCTION In the most simple of terms, the transformation of Raw Materials into a Product, involves the input of Energy. The means by which the energy is imparted, is defined by the physical design of the equipment and operational parameters of the process. The type of energy has a direct influence upon the Product Characteristics (e.g, Colour, Texture, Flavour, etc.). Let us consider a Generic Cooking Process, shown schematically
in Figure 1. There are three mechanisms for Energy Transfer. These are:
o Mechanical Energy (also referred to as Viscous Dissipation) o Convection Energy (associated with latent heat transfer – Steam Injection)
o Conduction Energy (associated with indirect heat transfer – Conduction from the Vessel Wall)
Figure 1 – The Generic Cooking Process
can also cause breakdown at a molecular level, as will be discussed later.
The shear stress applied to the product is dissipated through the
viscous fluid movement of the melt (“Viscous Dissipation”) leading to an increase in the temperature of the material. The bulk effect of SME is noted as a temperature rise (additional to temperature rise due to added steam). This temperature increase directly drives the cooking reactions
that occur. But SME also affects the feed at the molecular level. Figure 2 shows measured effect of SME on Mean Molecular Weight of the Starch Fraction in an extrusion operation. The shear generated in the process breaks down molecules – the higher the SME, the smaller is the average molecule size. Break-down of proteins results in the generation of peptides and changes to fatty acids. When starches are broken down, dextrins (long-chain sugars) are produced.
Figure 2: Effect of SME on Mean Molecular Weight in starch extrusion
In large-scale extrusion, Conduction Energy is relatively
insignificant – as size of the extruder increases, the reduced surface area to volume ratio in the barrel reduces heat transfer from the barrel wall. Some of the energy in extrusion is commonly added through steam, especially via the preconditioner, so Convection Energy can be significant. But the nature of the process means that a lot of the heating during extrusion is due to Mechanical Energy.
SME (SpEcific MEchanical EnErgy) and EXTRUSION This mechanical energy input, often specified as Specific Mechanical Energy (SME) is critical to the quality characteristics of the extruded feed. The stresses developed in the material (“Shear Stress”) not only convey the product along the barrel, but cause mixing, and under the correct conditions, can break down starches and re-align proteins and develop structure within the dough or melt. But intense shear stress
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This break-down of molecules affects product characteristics in a number of ways: 1. Flavour and nutritional profile – changed amino acids due to break-down of proteins, bitter notes from peptides, sweeter taste from dextrins. The digestibility of certain components can be enhanced (for certain species). 2. Pellet durability – sufficient “cook” is essential to the development of structure via the starches (present in very limited quantities in many aquafeeds) and any functional proteins. However, excessive breakdown of the molecules destroys binding. Therefore with respect to pellet durability, there must be sufficient
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