Controlled starch cook and bulk density of dry petfood - Is this for real?

Published on: 10/5/2006
Author/s : Andritz Sprout A/S

In this article Andritz Sprout will present the latest patented technological developments designed to enable continuous control of product quality and bulk density by means of simple adjustment parameters.

Wouldn’t the production of dry petfood be much easier, if you did not have to concentrate on the physical quality as well as the bulk density of the products, but could solely focus on optimising the production costs?

FLEXTEX system - opportunities

The task of extruding petfood is primarily a matter of cooking the starch. Typically the degree of starch cook of commercial petfood is approx. 80-90%. By making a specific screw configuration of the extruder one can to a certain extend supply the specific mechanical energy (SME) necessary to produce a given product with an optimum degree of cook. The degree of cook is decisive from both a nutritional as well as a physical product quality point of view. The better starch cook, the better quality!

The physical quality of a petfood product can be defined by:

  • Density
  • Shape and size
  • Uniformity
  • Crispness and texture

The nutritional quality of petfood can be defined as:

  • Palatability
  • Digestibility

On the most technologically updated extruders an optimum screw configuration can be established in order to apply the SME quantity necessary for matching a specific product, e.g. petfood with a high content of fat/oil, meat slurry or similar products with high nutritional value.

A screw configuration can be optimised to apply more SME by implementing the following components:

  • Screw design and geometry
  • Shearlocks
  • Kneading blocks
  • Reverse elements

An optimum screw configuration is not always capable of applying sufficient SME to produce a given product. Frequently it is also necessary to optimise other parameters, which can be used to increase the SME supply in the extruder:

  • Screw speed
  • Open area of venturi die
  • Open area of die plate
  • Extruder capacity

The task of producing petfood primarily consists of combining an optimal product quality (optimum cook) with minimal production costs. Changes in screw configuration and other measures, which may contribute with higher or lower SME values, are all operations, which usually result in down time and increased production costs. Changeover procedures between ½-2 hours to adapt an extruder to a specific product are not unusual.

Fig. 1 shows the difference in SME typically generated in a high shear extruder configuration, compared with the SME range used when operating the FLEXTEX system.

The FLEXTEX system (patent pending) – working principle

Technological advances which focus on both cost savings in the form of reduced down time as well as optimal flexibility in terms of capacity and quality have been in demand.

With the development of the FLEXTEX system the following advantages have been observed:

  • No need for changing extruder configuration regardless of SME requirement
  • Consistent capacity of extruder
  • Reduced number of adjustment parameters during the extrusion process
  • Basically only the SME is changed (set-point)

The FLEXTEX system (patent pending) is based on being able to continuously control the SME applied in the extruder during operation without changing the extruder configuration or other parameters. The system adjusts the opening area in the venturi die plate, which is placed between the last screw and the die plate in the extruder. The venturi die is used in many extruders in a stationary design to decrease or increase the kneading zone in the extruder in order to control the SME applied.

This is done by adjusting the size of the hole and thus the opening area and the pressure against it. The smaller the hole, the larger the pressure and thus the more energy consumption from the main motor (see fig. 2 and 3).

With the FLEXTEX system the opening area of the venturi die can typically be adjusted from 3000 mm2 to 100 mm2 (4.65 to 0.15 in2) depending on throughput (see fig. 4).

With the FLEXTEX system the extruder operator can determine how much specific mechanical energy (SME) the product needs. From a control system, for example a separate control or alternatively a control integrated in the extruder control, the operator can make a set-point, e.g. kW/ton (HP/ton) dry matter. By Venturi 100% restricted (small open area) Venturi 0% restricted (large open area) Fig. 4 Fig. 2 Fig. 3 means of a hydraulically controlled piston the FLEXTEX system automatically adjusts in relation to the set-point by decreasing or increasing restriction of the venturi die by moving a piston.

The FLEXTEX system - which in principle is a dynamic venturi die - controls and adjusts the SME supply continuously during operation. In short: the change of only this one parameter means the following for the production of petfood:

  • The starch cook (up to 100%) is completely controlled during operation
  • The bulk density of the product can be reduced by up to 30% and can be controlled with an accuracy of ±5 g/l (0.3 lbs/ft3)
  • Higher addition of e.g. oil/fat and meat slurry without significant influence to bulk density and product quality

The FLEXTEX system – mechanical design

In principle the FLEXTEX system consists of 3 parts:

  • The PLC control system
  • The venturi die and the piston system
  • The hydraulic station

The FLEXTEX system (patent pending) is designed with focus on simplicity and consists of a few components only. The critical part of the system is however the piston, which - besides being used as a restriction for the meal flow - also distributes the meal to the die plate. When restricting the meal flow it is essential that this takes place synchronically in order not to hinder the flow ability. Changes in the meal flow will influence the visual quality of the product due to an uneven pressure at the die plate. The piston in the FLEXTEX system is moved axially and at the same time it is conical, so that the meal flow is not negatively influenced.

The FLEXTEX system - Performance

Documented tests supported by experience from systems in full scale operations have shown significant opportunities for the FLEXTEX system.

During a series of tests a super premium dog food formula was used (27% protein, 18% fat, 4% fi bre), where 4% additional oil was added into the pre-conditioner. The following tests were made:

A. Starch cook with and without FLEXTEX
B. Influence on bulk density with and without FLEXTEX
C. Influence on bulk density with and without FLEXTEX when adding oil (sunfl ower)
D. Influence on bulk density with and without FLEXTEX when adding meat slurry

Test A
Changes in starch cook (see fig. A)

By increasing the SME in the extrusion process by approx. 13 kW/t (18 HP/Mt) the cook rate was increased by 9,9% from 90,1% to 100%.

Fig. A

Test B
- Changes in bulk density (see fi g. B)

By increasing the SME in the extrusion process by approx. 13 kW/t (18 HP/Mt) the density was decreased from 391 g/l to 316 g/l (24,4 to 19,7 lbs/ft3).

Fig. B

Test C
Oil addition (see fi g. C)

By increasing the SME it was possib le to add 5% additional oil compared to a normal extruder set-up without influencing the bulk density and physical quality.

Fig. C

Test D
- Meat slurry addition (see fi g. D)

By increasing the SME it was possible to add 10% more meat slurry compared to a normal extruder set-up without influencing the bulk density and physical quality. No additional oil added.

Fig. D

The FLEXTEX system - Summary

The FLEXTEX system provides significant flexibility in the production of dry petfood. By continuously controlling the SME during operation it is possible to achieve an optimal physical quality. By operating the extruder using less parameters than one would normally use for controlling the density and the cook rate, the extrusion process becomes less demanding.

The advantages of the system can be summarised to:

  • Increase starch cook by 10-15%
  • Decrease bulk density by 20-30%
  • Increase bulk density by 0-5%
  • No change of screw configuration (reduced down time)
  • Only one parameter needed for controlling starch cook and reduced bulk density

Increased density – Expansion Control System (ECS)

The FLEXTEX system is the ideal tool to control starch cook and subsequently the reduction in bulk density. By using the FLEXTEX system an increase in bulk density is controlled in the same way as in all conventional extruders. All parameters, which have a positive influence on increased bulk density, have a negative influence on starch cook, as increasing the bulk density in petfood in principle is a matter of reducing the SME and thus the starch cook.

To be able to control the bulk density of products in a wide range and at the same time obtain an optimum product quality, the ECS concept (patent patented) can be used. The ECS concept is based on controlling the expansion in the extruder knife house without infl uencing the product quality. Thus all desirable parameters can be used in the extruder without regard to expansion. Main focus is product quality!

By adding compressed air in the knife house one can control and adjust the pressure, which is made possible by mounting an airlock under the knife house (see fig 5). An increased pressure in the knife house results in a reduction of flash-off and thus an increase of bulk density (less expansion). The larger the overpressure, the larger the density (less expansion).

In a petfood formula the ECS system can increase the bulk density by additional 70 to 100 g/l (4,3 to 6,2 lbs/ft3). In a super premium dog food formula (27% protein, 18% fat, 4% fibre) the bulk density developed as shown in fig. 6.

Fig. 6

The FLEXTEX system and ECS – Combined

The FLEXTEX and the ECS systems can either be installed separately or as a combined concept. This is solely a question of each producer’s requirements in the production. Most frequently a FLEXTEX system is installed as a result of a requirement for either more starch cook, increased quantity of meat slurry, or more oil in the process.

Combining the systems means that on a standard petfood formula bulk density can be controlled from approx. 251 g/l to 431 g/l (15,6 to 27 lbs/ft3) with 100% gelatinisation by using only two (2) parameters being SME (FLEXTEX) and pressure (ECS), as shown in fig.7

Fig. 7

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