This phase involves evaluation of how the ingredient affects the manufacturing of the feed and the result of the ingredient on the quality of final pellet. The functionality of an ingredient is very important to feed manufacturers since it can increase operating costs or decrease product quality. Five criteria are evaluated to give a complete picture of the functionality of an ingredient and these include:
- feed particle durability,
- expansion of the feed during extrusion,
- oil absorption capacity of the final pellet,
- the effect of the ingredient on energy consumption during extrusion (specific mechanical energy),
- the stability of the pellet in water is important for some aquatic species.
Commercial aquafeeds must be durable enough not to breakdown during handling whether bagged and palletized or handled in bulk through pneumatic or mechanical conveying systems. There are many methods used by researchers and feed companies to estimate the durability of a feed pellet. The two method used in this laboratory include both mechanical (photo) and pneumatic tests (photo).
During cooking extrusion the carbohydrate present in the feed gelatinizes and expands (under the proper conditions) holding the feed together and creating spces or porese throughout the feed. These spaces help to make the feed float and to hold oil that is coated on the feed after the pellet is dried. If the feed does not expand and sufficient pores space is not present, the oil needed for energy and to make a healthy product for humans will not be absorbed into the pellet. So determining the effect of an ingredient on the expansion of a feed pellet is vital. This is done by setting the conditions on extruder to produce a good quality, expanded pellet using a base formula. Then a certain percentage, typically 30%, of the base diet is replaced with the test ingredient. The percentage change in the diameter of the test pellet from the base diet is the quantitative measure of expansion.
Oil absorption capacity of a pellet is a specific component of expansion of that pellet. Some feeds will expand a great deal but have large variability in pore structure and will not hold as much oil (initially or over time) as a feed with a consistent pore structure. Maximum oil absorption (MOA) is determined using the equipment that is used when oil is commercially “top-coated” with oil (photo). Vacuum coating of oil is done by pulling a vacuum in a mixer like piece of equipment, spraying oil onto the pellets and then slowly releasing the pressure. As the pressure return to the mixing chamber the oil is pushed deep into the pellet. To determine MOA, a 200 gram sample of pellets is totally covered in oil inside the vacuum coater and the pressure cycle is conducted as if a normal application was being made. After the pressure has been released the pellets are drained for 10 minutes. The increase in weight is the percentage of oil that has been absorbed into the pellet.
Specific mechanical energy (SME) is the amount of energy in the form of electricity, and heat that is applied to a feed mixture in order to produce a pellet. If the SME required to make a high quality pellet is increased when a new ingredient is added to the diet, the ingredient will be less attractive to feed manufacturers.
For some aquatic species that feed slowly, such as shrimp and sturgeon, the water stability of the pellet is an important consideration. If an ingredient causes the pellet to disintegrate in water its application will be limited to fast feeding animals. Amylo-pectin is a type of starch with many branch points and it absorbs water very well. As a result ingredients that have a high content of amylo-pectin, such as waxy grains and cassava root, will disintegrate quickly in water as they absorb water. This makes the feed unavailable to the slow feeding animal. Water stability is typically determined by putting a weighed sample of feed pellets in a flask with water on mixing table for a predetermined time period. The time period typically varies anywhere from 30 minutes to 24 hours.