Formulation requirements on moisture and starch for extrusion
Maximizing nutrition while optimizing the processing of extruded products can be a challenging operation.
Among the huge variety of pet food formats now available, there are differing requirements for formulation, and the parameters and measurements necessary to create good product are different for each. For the process of extrusion there are numerous points requiring careful control to ensure the desired effects on the finished product. Moisture and starch are two such parameters.
Starch gelatinization
Starch is the primary carbohydrate form found in pet foods. It is an important energy source; however raw starch is undigestible by dogs and cats. Extrusion is a process which, in its simplest form, is starch cooking.
Starch granules within carbohydrate ingredients swell and break during extrusion in a process known as starch gelatinization. The granules lose their crystalline structure and fragment due to heating and shear force respectively. This resulting gelatinized starch is soluble and can absorb large quantities of water.
The degree of gelatinization affects the viscosity of the extrudate, in turn affecting the physical and chemical characteristics of the kibble. The effect of starch gelatinization on different ingredients varies. For tapioca starch, as well as barley and corn starches, gelatinization improves digestibility. For wheat and oat bran starches, digestibility is unaffected.
Settings during extrusion also affect the degree of gelatinization. Examples of this include high screw speeds (400rpm) decreasing starch gelatinization, as well as extrusion under high-moisture and high-pressure conditions leading to incomplete gelatinization and an increased availability of in-vivo and in-vitro starch.
Starch gelatinization plays a key role in kibble expansion. It gives the stretchy, gas-holding capacity of the extrudate, as for instance gluten in a bread dough does. Studies report that for good kibble expansion, starch content should exceed 60-70%. Starch content of 30% in cat foods, puppy foods and high-meat foods is typically seen, with 40% starch content more applicable to other adult dog foods.
Use in binding
Starch also contributes to binding ingredients in the final product, with amylose contributing greater binding properties than amylopectin. This indicates that formulators requiring greater cohesion of the final product should choose amylose-rich starches, such as potato and tapioca, to achieve this.
Starch retrogradation
Retrogradation of starch occurs when gelatinized starch realigns to its crystalline structure. This is associated with cooling of starches in the post cooking step. Retrograded starch is a resistant starch: it is less digestible than gelatinized starch and behaves similarly to a fiber. Varieties of retrograded starches carry similar properties to fiber, such as benefits to gut health and fermentation to produce short-chain fatty acids.
As the extrusion and cooking processes used for particular ingredient combinations can manipulate the presence of retrograded starches, ongoing research considers how this may be implicated for benefits to health. Formulators should consider the implications of resistant starch in the gut alongside other nutrients being supplied to ensure the balance supports a favourable gut environment, rather than digestive disturbance.
Amylose-lipid complexes
Another interaction of starch found to occur during extrusion is the formation of amylose-lipid complexes (ALC). ALC are a type of resistant starch hypothesized to slow down digestion of the product. They also impact glycaemic index and insulin response to the product, among other health implications.
ALC formation is dependent on the quantity and varieties of lipids and starch present in a formula, as well as the degree of starch gelatinization. As the quantity of fresh meat ingredients within a product increases, the formation of ALC reduces.
Impact of moisture
Moisture also contributes to kibble expansion: with increased moisture comes a reduction in expansion. This is explained by the ‘shrinkage and collapse of the extrudate’ after maximum expansion is achieved. This lack of adequate expansion seen with increasing moisture levels in extrusion is also connected with kibble hardness. Research from the West Bengal University of Animal and Fishery Sciences found that as water increases, kibble hardness also increased for every extrusion temperature tested.
Expansion limits
As extruded pet foods vary so massively in their composition, there are additional challenges to formulation. Each set of ingredients, and the analysis of each, means that performance through extrusion varies. Ingredients included in pet food are continuously changing, yet pet owners expect kibble quality and appearance to be maintained, no matter the ingredients included.
The trend of increasing meats, proteins and fat components of the pet diet at the expense of starch- containing ingredients means that extrusion is challenging and often very little expansion of the kibble is possible. When a particular bulk density is requested, this dictates the starch varieties and levels required in the formulation, as increasing starch decreases the bulk density of the final extruded product.
Searching for more solutions
As trends in pet food make extrusion more difficult, we look to other solutions to include the desired ingredients and maintain good kibble quality. Often, expansion is the compromise.
A thermal twin extrusion process can provide greater flexibility when producing kibble with high meat inclusion or low carbohydrate inclusion. By increasing the thermal cook, and reducing the mechanical cook, the extrusion process can be adapted to give greater options.
Research regarding the functions of starches is ongoing and will be important to best understand how to maximize the nutrition of extruded products while optimizing the processing of the final product.
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