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Preconditioning By Gordon Young, Food Industry Engineering and Dennis Forte, Dennis Forte & Associates

The authors are presenting short courses on “Aquafeed Extrusion Technology”, “Petfood Drying Technology” and “Drying Technology” at Centre for Feed Technology, FôrTek, Norway, in April/May 2017 (

Preconditioning is a unit operation that has long been associated with extrusion processing and with other feed processing methods such as pellet milling. Although a seemingly simple process, there are many design variants available. There are also a number of operational parameters that need to be given careful consideration when selecting and/or designing a process application. The following seeks to present some of the key design and process control criteria, such that process optimisation may be completed in a less empirical manner.

What is Preconditioning? Preconditioning, as applied prior to extrusion, involves adding moisture and heat to the formulation. This involves adding water – normally partially as liquid water, and partially as steam – while mixing (and holding the mix). So the primary objectives of the preconditioning in extrusion of

feeds are to: • Ensure uniform Hydration – this is achieved by adding sufficient water, ensuring it is applied uniformly (which relates to the Mixing Characteristics within the system), and allowing sufficient time for the water to penetrate the feed particles (which also depends on particle size) • Initiate the Cooking process – the cooking reaction, or Degree of Cook, is primarily a function of temperature and time, in the presence of sufficient water.

Note that in this discussion, Cooking is used as a generic term

to describe the various material transformations occurring within the product as it passes through the system. These simple relationships highlight the key variables in

preconditioning. The different manufacturers of preconditioning equipment each have their own approaches to how to control and optimise the key variables. In this article we will discuss key attributes of preconditioning for extrusion.

Benefits of Preconditioning All of the preconditioner equipment suppliers regularly espouse the benefits of this technology. A number of the benefits typically claimed are presented in Table 1. If we examine these benefits, we can see that they all relate to those basic functions of preconditioning: • Achieving required Hydration softens particles and reduces


Table 1: Process Improvements gained via the use of preconditioning

Potential Benefits of Preconditioning Improved Product Uniformity

Higher Output (from a given extruder size or alternatively less barrel length required)

Efficient Heating

Reduced Wear of Extruder Components Improved Process Stability

Reduced Requirement for Starch in the Formulation Improved “Cook” Improved Digestibility

Improved Water Absorption (as measured via Water Absorption Index, WAI)

Reduced Water Solubility (as measured via Water solubility Index, WSI)

Improved texturisation of some protein-based products due to enhanced moisture distribution

extruder wear. As water is part of the cooking reaction, uniform hydration also enhances Degree of Cook. • Achieving required Degree of Cook means digestibility is improved. A high degree of cook also means that binding and pellet durability can be achieved with less starch (with protein binding also contributing subject to the protein quality). If cook is initiated in the preconditioner, less cooking needs to be done in the extruder – therefore improved throughput. But also, in a preconditioner, the cook is achieved under relatively low shear conditions (low mechanical energy) compared to cook in the extruder barrel – therefore reduced water solubility and higher water absorption in the product, again enhancing pellet durability. • Sufficient Mixing ensures the hydration and cooking are uniform – therefore improved product uniformity and process stability.

Key Preconditioner Operating Parameters Temperature and Moisture Diffusion into Particles In order for the cooking reactions to occur within the formulation, two key parameters need to be controlled and optimised – these are the provision of a sufficient amount of water and an elevated temperature. Injection of live steam into the preconditioner provides both heat and moisture simultaneously. The effectiveness of the steam in achieving the objectives depends on two mechanisms. These are: The internal resistance: This relates to the Mass Diffusion (for hydration) and Thermal Diffusion (for heating) properties of the material being

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