TECHNOLOGY REPORT: Vacuum infusion or vacuum confusion?
A general guide to understanding and trouble shooting problems associated with vacuum infusion of oils into extruded salmon feeds
Vacuum infusion of salmon feeds is the application of liquid marine or vegetable oil to dry extruded feeds under a negative air pressure. During the vacuum cycle air is removed from the coating chamber and from the air filled pellet voids. By allowing the pressure to release back to that of atmospheric, oil is forced throughout the interior voids of the pellet. The pellets are infused with oil.
The principals are simple, however, the variables that can affect the process are many. Over time we have experienced many questions and tackled many problems directly and indirectly related to vacuum infusion of extruded salmon feeds. Many clients who are new to vacuum infusion often have misconceptions and misunderstandings about what is possible with vacuum infusion. They may blame the VIS for not achieving high enough oil levels, when the real culprit is not enough pellet expansion, or they wonder why the vacuum target takes too long and the vacuum pump oil is continually being contaminated with water, the culprit here is too high a moisture in the product to be vacuum infused normally. There are many more such variables to be considered.
Basic operating recommendations for vacuum infusion:
Vacuum Infusion Systems (VIS) are used to infuse liquid ingredients into extruded pellet or shaped feeds. For salmon and trout feeds this generally means the infusion of marine and or vegetable oils.
What are the ideal pellet conditions for vacuum infusion? Over the last ~ 15 years many VIS experiments have been conducted with varying results, the best of these results have been applied in production. Vacuum infusion although relatively new, is well past its infancy and is now a useful and necessary production tool for salmon (fin fish) feeds and pet foods production. Universities and private companies continue to fine tune vacuum infusion techniques and processes. (UAS has provided Vacuum Infusion Systems and technical support to 3 universities including the University of Santiago Chile, Texas A&M, & University of Adelaide Australia).
Much has been learned regarding best practices for vacuum infused salmon feed production. In general; vacuum infusion as part of the entire process relies on quality production from selection of raw materials, formulation, grinding, extrusion, expansion, drying, cooling, temperature, moisture content, and product conveying. The old saying “Garbage in = Garbage out” applies here and so Quality in must = Quality out!
- Starch and protein ingredients can affect grinding and extrusion as well as nutrition.
- Raw ingredients must be maintained at a specific quality if consistent product is to be produced.
- Uniform fine grinding of feeds to 70% particle sizes under 0.25 mm and 100% particle sizes below 0.4 mm generally results in acceptable to good quality extruded feed products. By use of hammer mill technology an industry rule of thumb for salmon feed production is “the finer the grind the better”.
- It has been learned that pellet durability increases with uniform fine grinding of high protein formulations as used in Salmon feed production.
- Finer ground starches gelatinize easier. Finer ground proteins bind more readily.
- Uniform extruded pellets will result from use of quality equipment operated properly.
- Good preconditioning from 2 - 3 minutes helps to ensure maximum gelatinisation of starches during the extrusion process.
- Poorly extruded products can vary in uniformity of size and structural integrity resulting in a product that will not dry evenly, will fracture easily, will not absorb liquid additives uniformly and may have poor overall appearance.
- Extruded pellets expand as they leave the extruder and are released to atmospheric pressure.
- Over expanded feeds may be more fragile. Over expanded feeds can float even after vacuum infusion due to too low a bulk density.
- Under expanded feeds will result in feeds that cannot absorb the total required oil formulated in the diet. They will also be physically harder and generally sink faster due to a higher bulk density.
- Under expanded, higher density feeds, will not dry as easily as greater expanded feeds, after vacuum infusion they may appear oily / wet.
- Expansion is critical to vacuum infusion and must be controlled to produce pellets with the desired characteristics needed to allow for the right amount of free area within the pellet in order to absorb the total required oil as formulated in the diet. Designed and accurately produced expanded feeds are essential to provide a balanced slow sinking salmon feed.
- Drying must be uniform from pellet to pellet, in order to provide a balanced diet. As an industry rule of thumb, variations in moisture from pellet to pellet greater than 1% - 1.5% are considered below modern industry drying standards.
- Good practice indicates that the dryer be capable of drying all that the extruder can produce and a little extra capacity is recommended.
- Average high quality salmon feeds are dried to a range from 6% to 9%. The dryer must be guaranteed to meet all drying requirements at the required capacities. Too small a dryer will cause problems in drying uniformity, moisture target, vacuum infusion, and ultimately product appearance and nutritional quality.
These concepts are well founded in our modern salmon feed industry.