Steam Extrusion – the Technology Behind the Feeds
By: Dr Catherine Dunnett BSc, PhD, R.Nutr
Steam extrusion is a relative new process in equine feed manufacture, although it has been used for cats and dogs and indeed our own food for longer. Steam extrusion involves the cooking of a mixture of ingredients and the formation of the characteristic kibbles, or nuggets, which can be a variety of shapes and colours. Ingredients are ground into a meal and then fed into a conditioner that is tightly sealed. The ground ingredients are then treated with hot steam and water under pressure, for a brief but intense time. The involvement of steam and pressure instigates a rapid cooking process, which softens the ground mixture and breaks down the complex structure of particularly starches that are present – this molecular process is known as gelatinisation. The ground mixture is physically by now quite doughy and can be fed through the extruder, which is a steel tube with a rotating auger or screw. Throughout this process pressure is maintained and water and steam continually applied. Finally, the mixture then is forced through the narrow end of a die with cone shaped holes, which allows the mixture to rapidly expand as it emerges through the wider end. A series of rotating knives are then used to slice off the expanded mixture to form the kibbles as they are known, which are then cooled and dried.
The extrusion parameters, in terms of temperature, pressure and time can be modified and this will alter the extent of the expansion or the shape of the kibbles, as well as the nutritional characteristics of the finished product. These extrusion conditions affect the level of gelatinisation and also the interaction between other nutritional components. It is very important to have expert extrusion operators who understand the impact that extrusion condition have not only on the physical look of the product but also on these nutritional characteristics as these are not always desirable.
Steam extrusion, like other means of advanced cooking, has a beneficial effect on feed digestibility through the gelatinisation of starch whilst it has also has been shown to have a beneficial effect on protein digestibility. The cooking process unravels the complicated physical structure of both starches and proteins, making them more accessible and increasing surface area for digestive enzymes to break down in the small intestine. Improved digestion and absorption rates of starches and proteins in the small intestine as a result is very beneficial to the horse. Firstly with respect to proteins, enhanced digestion in the small intestine means that more amino acids will be released and absorbed in the upper digestive tract, which makes them more available for use to support tissue growth and repair. When protein escapes digestion in the small intestine it can be fermented in the hindgut releasing nitrogen for use by the resident beneficial bacteria, however, the majority of the useful amino acids cannot be absorbed from the hindgut and so may be lost for the horse directly.
Turning our attention to starches, extrusion is an efficient method of starch gelatinisation avoiding the problem of poor pre-caecal digestibility. Poor digestibility of starches in the small intestine e.g. from straight cereals, pressure pelleted ingredients, or uncooked ingredients is both economically inefficient and can be a risk for digestive problems including, colic, laminitis, colitis and can even have a negative effect on immune function. This is because starch, like protein, is able to escape being digested thoroughly in the small intestine with a proportion reaching the hindgut, where it can be rapidly fermented contributing to a build up of particular volatile fatty acids including propionic acid and lactic acid. This acidosis, which is measured as a fall in hindgut pH, can precipitate a change to the environment and ‘food source’ to the population of bacteria and other microflora present. This can lead to a detrimental change in the population of microflora, as those that can thrive in these conditions multiply and those that can’t survive reduce in numbers. This essentially means that the friendly bacteria are reduced in numbers and the integrity of the gut mucosa may be compromised as a result of associated inflammation and ‘leakiness’ of the mucosa may increase, allowing otherwise unabsorbed food components to pass through.
Another key benefit of the steam extrusion process for horse feed in comparison to other cooking processes such as micronisation, is that it lends itself to higher additions of oil. This is a benefit as it allows the overall starch content of the feed to be controlled and reduced as oil provides an alternative energy source that is often considered safer from the perspective of maintenance of health and behaviour. The extrusion process itself allows an increased level of oil to be added to the ingredient mix, but also the physical characteristics of the final kibbles produced, means that it is very easy to add a further spray coating of oil post-extrusion to enhance the overall oil content, thus increasing energy density.
The shelf life of horse feed is also an important consideration for both manufacturers and their clients with moulding being a key issue with increased storage time. Steam extruded kibbles (or nuggets) have a very low moisture content and so mould formation during storage can be much less of an issue. In addition, the kibbles also tend to have a very low dust content, which can be advantageous for maintaining respiratory health a key consideration in sports and performance horses.
Steam extruded feeds are also light in weight and so can be produced with a reduced energy density, allowing an increase in meal volume (but not weight) for greedy feeders and those that are overweight. Eating time has been suggested to be longer for extruded feeds compared to other feeds, which again would be an advantage for digestive heath through longer periods of time chewing.
Careful choice of the starch source and extrusion conditions is needed to prevent the formation of what is known as retrograde starch. This is a form of starch that is more resistant to digestion in the small intestine – and in a sense represents an over cooking. Maillard reaction products can also be produced, which are complexes between free amino acids and a particular type of sugar (reducing sugars). Maillard reaction products can contribute towards improved palatability, but may also reduce the availability of free amino acids.
Dry extrusion is a similar process to steam extrusion except as the name suggests no steam is employed in the process. It is sometimes considered to be a harsher process, with the possibility of a higher rate of nutrient degradation. Dry extrusion bypasses the preconditioning element and also doesn’t use hot water and steam, instead relying on the physical force and shear of the feed material to produce heat as it passes through the extrusion barrel. The less controlled high temperatures and increased shear forces involved in dry extrusion may have a negative effect on vitamin stability depending on the temperature and cooking time. Steam extrusion is proposed to be a more gentle and controlled process which limits the amount of nutrient degradation.
Why Keyflow choose Wet Steam Extrusion as their manufacturing process of choice
By: Keyflow Nutritionist, Louise Scott
Wet steam extrusion is the key process behind the beneficial cooking phase of many of the ingredients in Keyflow feeds and balancers. Keyflow have considerable experience with this advanced process, with founder Cam Price being a graduate of one of the largest horse feed companies in Australia, namely Mitavite who are a world leaders and pioneers in extrusion technology for use with horse feeds. Cam boldly introduced this process for broad use in the horse feed sector in the UK and judging from the feedback from our customers it is a trend that is set to continue. Keyflow feel that wet steam extrusion offers an advantage to the horse, in terms of digestibility, and this can be seen clearly in how well the horses that we feed look – a sort of 1 plus 1 equals 2.5 effect. In turn horses don’t require as much to be fed as traditional mixes and cubes/pellets meaning they have more room to take up more fibre. This has a very positive effect on digestive health and the horses well being.
Keyflow’s manufacturing team and partners have a wealth of experience with this unique technology and consistently produce a high quality product, which our clients’ horses give a firm, thumbs up to.