Extrusion is widely used in manufacturing industry (food/feed, plastic, etc.) to produce different shapes, all regards to what customer wants.
Food extrusion system is much more complexed compared to plastic extrusion system. A simple reason to that is a number of ingredients used for forming an extruded product, known as pellets or kibbles so predicting the behaviour of this material is a much more complex.
Typical extrusion system consists of several steps showing its complexity: formulation including basket of different raw materials, grinding, mixing, pre-conditioning also known as pre-moisturizer, extruder, drying, coating, and packing.
Usually ingredients for pet food are selected because of their impact on the nutritional basis of a product, mouth feel and texture or a final product bulk density.
For dry pet food nutrition is very important (including higher starch levels and medium fat levels). However, equally important factor is an effect of chosen ingredients on to physical pellet quality (e.g. texture). Experience says that an effect on nutrition and on physical pellet quality are not often complementary. Young and Forte (2015) putting aside nutritional issues for a moment were highlighting major functions required by extrusion system with respect to final quality. These are;
- Shape and size
- Digestibility (making nutrients more available)
- Don`t destroy nutritional profile (by overwork/overheat)
- Process out any anti-nutritional factors (e.g. soy)
- Destroy harmful bacteria
- Produce consistent product at high speed
Theoretically, an issue with final physical pellet quality could be resolved by going backwards and start changing formulation and until we don`t get a desired product. However, in practice this is not often possible as using this approach we risk missing our target (recipe wise).
This brings us to an another and much practical approach which is balancing these two ends (raw materials with their formulation suited for an end-user and processing as the other end). Knowing more about different ingredients and their functional properties put us into a position to choose best out of each. As an example: we shall choose not only from a protein and/or carbohydrate point of view but rather more from a functional prospective. For instance, pea starch is not only high in starch level but it comes along with high binding properties and high expansion characteristics. Higher amylose content ensures good expansion characteristics required for obtaining desired texture in dry pet food products. This comes to an effect after extruded products are being dried, as one of the main reasons for drying is to provide the required texture, beside the preservation reason.
Furthermore, pea starch has still certain amount of protein level left that contributes both nutritionally and physically. A pea protein, as another example and separate product contributes not only nutritionally but is also functional providing binding characteristics. Some of the main functional characteristics of pea protein are gelation, emulsification, and water/fat absorption.
In addition, having in mind that beside these described effects these products come from peas known as an excellent source of lysine, with its low glycemic index and perceived as consumer friendly (clean label; glucose free, non-GMO, etc.), it appears to be a right choice when choosing functional ingredient for your dry pet food.