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    GEA - by Marina Sgarbossa

    Extrusion Expertise Improves Nutritional Value of Kibbles

  • During the GEA Pet Food Technology Day, which took place at the end of 2021 as an online event, GEA provided an in-depth look at extrusion technology, explaining how the extrusion process influences pet food digestibility and texture.

    The global pet food market is continually growing and is dominated by the dry pet food segment. About 95 percent of the dry and semi-moist products are manufactured by extrusion. The main question is: What makes a kibble good for pets and ultimately a good value for their owners? Marina Sgarbossa,
    Analytical Laboratory Technician from GEA, explains: “A healthful kibble with a high-quality nutritional composition and easy digestibility is of crucial
    importance because it indicates the real availability of nutrients in a diet. On the other hand, kibbles must be palatable and have a good taste, aroma and
    texture. Nutritional composition, taste and aroma are determined mainly by the recipe, while digestibility and texture are affected also by the processing method.

    The influence of the extrusion process on digestibility and texture

    A pet is usually fed one type of food that must satisfy all of their daily nutritional needs for proteins, carbohydrates, lipids,
    fibers and minerals. Kibbles contain all of these components, making them a very complex matrix to process.

    When talking about carbohydrates, starch is not an essential nutrient for dogs and cats but is a source of energy and can impact health depending on the quantity and type of starch included and how it is processed. It is also a very important structural ingredient, since it is necessary for kibble expansion and texturization. The starch in the extruder is not subject to normal gelatinization. The gelatinization process takes several minutes and requires heat and more than 60% of water content. The extrusion process takes a few seconds and is generally carried out at an average of 20 to 30 percent of moisture content in the barrel. What happens in the extruder is a cooking due to the rupture of the starch granules through a combination of heat, moisture, pressure and mechanical shear. The rotation of the screw and barrel causes the product to stretch and the starch granules to degrade. When
    the melt comes out through the die, the difference between the internal pressure and the ambient pressure causes the water to evaporate immediately and the kibble to expand. This means that the more mechanical and thermal energy applied in the process, the more the kibble expands outside the die.

    Extrusion analysis study
    For this customer event in particular, using one recipe for dogs and one recipe for cats, GEA manufactured different kibbles with different textures: one of higher quality and one of lower quality. The demonstration showed how the process parameters could be changed to obtain the desired texture and how
    the process affects gelatinization and digestibility of the product. In the GEA in-house laboratory the participants had an opportunity to study the state of the starch as it cooked by analyzing the viscosimetric profile with a RVA (Rapid Visco Analyzer) and by viewing the starch granules under a microscope. As previously mentioned, starch starts to gelatinize in the presence of heat and water, increasing its viscosity. The RVA detects peak viscosity when the raw starch gelatinizes.

    The “degree of the starch gelatinization” is the official analysis for quantifying gelatinized starch. The principle behind this analysis is that gelatinized starch allows for a better attack by digestive enzymes, therefore it is measured the amount of glucose produced by a glucoamylase which digests the starch. Then the percentage of gelatinized starch, i.e. digested starch, is calculated on the total starch content. The four samples were sent to an accredited, independent laboratory to measure the degree of gelatinization and they discovered that the gelatinization rate for all samples was very high regardless of consistency. For further confirmation that these kibbles were of high nutritional value, GEA also sent the same samples to the University of Bologna for in vitro digestion analysis. All of the nutrients were deemed highly digestible, which means that using GEA extruders guarantees a very high level of starch gelatinization and digestibility, making the product healthy for pets regardless of the desired consistency.

    Having demonstrated the wholesomeness of kibble, the next aspect to focus on is palatability. Palatability concerns other factors such as taste, aroma and mouthfeel (the physical sensation produced in the animal’s mouth by the product, the particle size of the raw material and the texture).

    Texture is the sensory manifestation of how the structure responds to applied force. This is a major topic in the pet food industry because texture is very difficult to analyze, and for this very reason there is no agreement on which measurement methods to use or if these methods correlate to the actual sensation perceived by pets. So the next question is, „How do we measure whether a particular texture is good or bad?”.

    Texture is analyzed first by measuring bulk density. GEA sizes the samples here in a calibrated container that measures the density of one liter of the product. It is a simple but very important analysis because a compact and unexpanded internal structure determines a high bulk density, while an expanded internal structure determines a lower density. Then, a texture analyzer can be used to measure the hardness, which indicates the force of the animal’s teeth needed to break the kibbles. Using a probe with the shape of a dog and cat’s tooth we can imitate the penetration impressed on the
    kibble during the chewing of animals.

    The results of the dog kibble analysis show that the samples have almost the same moisture content, but the internal structure is different: the sample with the good consistency has a very expanded internal structure, with a lower apparent density and lower hardness. The sample with the poor texture has a very compact structure and is therefore harder and heavier than the first sample. Applying the same methodology to the sample of cat kibble produced a similar result: the difference between the two samples is quite evident because the sample with the good texture is really expanded and
    its hardness level is very low.

    The last step in the study was to test whether from a pet perspective the samples that are considered to have good consistency are really better than the others. GEA sent the four samples to a well-known Dutch institute, De Morgenstond, to do a comparative palatability test with a qualified panel of 30 dogs and 30 cats. Each pet was offered 2 bowls of kibble, and observers noted which samples the animals ate first and the amount of each sample they consumed. For each sample, the test was performed over 2 to 3 days, changing bowls from left to right and vice versa.

    De Morgenstond’s test results were very positive, because the percentage of the sample chosen first and the percentage of consumption were higher for the sample with good texture for cats as well as for dogs. The animals confirmed the study: GEA xTru Twin extruders can handle different consistencies and produce highly palatable products for pets.

    Ultimately, GEA xTru Twin extruders guarantee a consistent, high level of starch gelatinization and digestibility for various dry pet food recipes as well as ease in managing process parameters to achieve the desired products, texture and density, while also providing process consistency and stability over time. In addition, the in-house laboratory offers GEA customers analytical support while they develop their products.

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