by Detlef Bunzel and Andreas Lemme, Evonik, Germany

Pelleting is a core process in compound feed production for feed mill operators as it increases bulk density and stabilises the mixture. The resulting uniform particle size improves storage and handling properties which, in turn, also means lower transportation costs for mill operations.

Compacting feed also enhances its nutritional value by increasing energy density and preventing selective eating. Animals are unable to avoid or reject individual ingredients, due to changes in palatability if components in the diet are altered for nutritional and/or cost reasons. This reduces waste, losses and production costs for farms.

Alongside achieving these benefits, operators must ensure they are meeting food and feed safety requirements. In Europe, Regulation (EC) No 178/2002 lays down requirements for feed safety in Article 15:

1. Feed shall not be placed on the market or fed to any food-producing animal if it is unsafe.

2. Feed shall be deemed to be unsafe for its intended use if it is considered to:

have an adverse effect on human or animal health;

make the food derived from food producing animals unsafe for human consumption.

Pellet quality, as well as size, should therefore be a key issue for operators. Studies have shown that feed formulation (40 percent), particle size distribution (20 percent) and mash conditioning (20 percent) have the highest impact on standards. If we assume formulation and particle size distribution are constant in the production process, conditioning of the mash is the most significant process variable that feed mill operators can influence to improve the quality.

Chemical preservatives are subject to restrictive regional regulations (e.g. 70/524/EG in the European Union), so heat treatment is a focus for feed producers when reducing and controlling bacterial contamination of feed mash in the production process.

Equipment manufacturers have developed several approaches to deal with this challenge. All of them consider conditioning temperature and time as relevant parameters for the successful reduction of bacteria in the process. By inducing more thermal energy into the mash, mechanical influence factors, such as changes in raw material properties and particle size distribution, can be better balanced in the compacting or pelleting process.


Adapt conditioning according to formulation

In feed production, a wide variety of raw materials and formulations are pelleted. With raw materials of agricultural origin, handling and processing properties vary over time based on provenance, weather conditions during growth and harvest, pre-processing and storage conditions, as well as shelf life.

Besides bulk density and particle size distribution, humidity or water content is the most prominent physical property affecting feed processing. Among chemical properties of raw materials, protein, fat and starch contents, as well as ash and fibre contents, have the most impact on nutrition and processing.

Researchers and feed mill practitioners have found approaches, over the years, to address different physical and chemical properties of raw materials in diets by adapting the conditioning process. Assuming that inclusion of liquids is defined in the diet, theoretically the only parameters that pellet mill operators could adjust besides the feed rate would be steam pressure and temperature.

As a rule of thumb, approximately 0.6 percent of dry steam added into the conditioner will raise the temperature of the mash by 10°C. In practice, steam consumption will be affected by steam pressure and quality of the steam-jack including insulation, function of condensate traps, pressure reduction ratio and thermal losses in the conditioner.

In this context, it is also important to remember that, with dry steam, pressure and temperature are strictly related. With pressure, steam temperature increases. Therefore, less high-pressure steam is needed to increase mash temperature. On the other hand, more low-pressure steam and moisture will be added to the mash to reach a certain temperature in the conditioner before the pellet mill.

Keeping this in mind helps operators to understand recommendations of researchers and practitioners to use different steam pressure to optimise conditioning of certain diet types. For example, in diets with high starch contents, low-pressure steam provides not only temperature increase but also humidity that must be present to support starch modification.


Examples of sanitation processes

Several equipment suppliers present solutions developed on the design of the barrel type conditioner: ripening conditioners subsequent to the steam conditioner or molasses machine provide volume and retention time for the mash. The size of the equipment will be chosen to meet customers' requirements for throughput and retention time.

Two minutes at 80 – 85°C is generally considered as a good starting point. It is important to note that, by design, the barrel shaped ripening conditioners ensure first-in, first-out for the mash in the process so that all particles are exposed to high temperature treatment for the same time.

Enough insulation of the steam conditioners and retention conditioners must be in place to prevent thermal losses and condensation on the inner surface of the barrel, as this would result in encrustations and cross contamination. At the same time, easy access for maintenance and cleaning is necessary.

For longer residence times (up to eight minutes and more), Kahl Group suggests their concept 'Retention Plus' including a vertical ripening vessel, the so-called long-term conditioner. Due to the long residence time in this process, higher inclusion rates of liquids such as molasses are possible without compromising pellet quality. As the ripening vessel operates under ambient pressure, conditioning temperatures of up to 100°C are possible.

A third example for a different technological approach in sanitation is the expander. Expanders operate with short retention times in the range of several seconds. As product is pressed through a ring die in the outlet, process pressure can be adjusted up to 80 bar. Steam can be injected directly into the barrel and process temperatures of up to 150°C are possible.


Nutritional aspects of conditioning processes

The main purpose of steam addition is to condition the mash for the compacting process while choice of length and diameter of the die will also result in more or less heat. The sanitary value is of high importance as it directly affects animal health by managing or controlling pathogens. However, the nutritional value can also be affected by heat.

One example is the availability of dietary energy. All organic compounds in feed can deliver energy to the metabolism of the animals. With respect to the macro-nutrients (protein, lipids, carbohydrates) which all can be energetically utilised by the animals, particularly different fractions of carbohydrates should be distinguished. While dietary crude fibers are only little digestible and, thus, only little available to monogastric animals such as chicken or swine, they are much better utilised by ruminants.


Impact of heat on nutritional additives

Over-processing will negatively impact the nutritional value. So, any compound susceptible to higher temperature will suffer. Prominent examples are certain vitamins, enzymes and unsaturated fatty acids, which will be oxidised or destroyed. Therefore, respective feed additives are usually added after the conditioning and pelleting process or with vacuum coating after extrusion.

Amino acids, in contrast – are added into the mixer before conditioning and are, thus, exposed to heat. In an experiment we conducted, stability and recovery of MetAMINO®, Biolys®, ThreAMINO® and TrypAMINO® was examined at increasing extrusion temperatures ranging from 100°C up to even 190°C, lasting for about 15 seconds. Concentrations of supplemental amino acids in the feed mixture were not reduced compared to initial levels, even at the harshest condition of 190°C.

A further investigation focused on various processing techniques for shrimp feed. Shrimp feed was either extruded using a single-screw or twin-screw extruder or was pelleted. It can generally be stated that total protein and amino acids were not susceptible to damage under the conditions tested.

Recovery of amino acids ranged between 95 percent and 102 percent. At the same time, analysis of free amino acids revealed high recovery, too, and suggested high stability during feed processing. Free amino acids do not behave differently from protein bound amino acids.

Therefore, these studies suggest high stability of amino acids under the conditions tested. On the other hand, it is well established that especially over-heating negatively impacts amino acid digestibility which in turn would reduce their availability to the animals and thus the nutritional effectiveness of the diet.


Benefits and drawbacks of heat regarding nutritional value

The impact of heat treatment on amino acid digestibility and availability has been investigated in the context of raw material processing rather than in compound feed, although the consequences and principles behind are similar.

In addition to beneficial effects on feed hygiene, heat treatment is required to destroy or at least to reduce anti-nutritional factors (ANF) while over-heating compromises digestibility of amino acids to the animals. ANFs include e.g. trypsin-inhibitors (e.g. soybean) which impair protein digestion, glucosinolates (e.g. rapeseed), gossypol (cotton seed), or lectins (e.g. lupins). These examples are all heat sensitive.

While heat treatment is needed to reduce ANF, exceeding optimal heat exposure can cause impaired amino acid availability. A series of trials we conducted with broiler and swine confirmed this for several ingredients. Initially, soybean products as well as Distiller's dried grains with solubles (DDGS), were systematically and harshly heat treated in an autoclave at 135°C up to 30 minutes (Fontaine et al. 2007).

Amino acid analyses revealed losses, particularly for lysine, arginine and cysteine – amino acids known to be heat sensitive. However, not only total amino acids but also reactive lysine was determined. The free amino group of lysine tends to react with sugars under heat exposure forming so-called Amadori compounds, which cannot be cleaved in the digestive tract.

Thus, this lysine is not available any more to the animals. Reactive lysine represents the fraction, which did not undergo this Maillard reaction. In the above experiment by Fontaine et al. (2007), levels of reactive lysine in low (43 percent) and high (47 percent) protein soybean meal, full-fat soybeans as well as in low (23 percent) and high (27 percent) DDGS diminished stronger than total lysine, indicating a much stronger impact on the nutritional value than suggested by total amino acid analysis – although considered in raw material analysis the latter can alleviate the performance depression of the animals to a certain degree.

Ongoing research focused on amino acid digestibility generally revealed that over-heating impairs the digestibility of all amino acids in both broilers and swine. While the magnitude of response differs between amino acids within, as well as between, animal species, it can be concluded that over-heating affects all amino acids resulting in a more or less severe reduction of the nutritional value.

Evonik Nutrition & Care developed a rapid method (WO 2018/146295 A1) to quantify the impact on the nutritional value due to over-heating, at least for a couple of raw materials. Respective data can be used in the feed formulation process. However, for compound feed production this method is not available. Overall, it is concluded that exceeding certain temperature loads should be avoided in order to avoid reductions of animal performance.

Overall, it can be summarised that ingredients and feed mixtures are exposed to heat during processing. While on the one hand heat is required to a certain extent for conditioning for example for the pelleting process as well as for sanitary reasons and reduction of anti-nutritional factors. On the other hand, over-processing will have detrimental effects, which at first glance might not be obvious but will affect animal performance.

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