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Influence of steam-conditioning on chemical changes of feed

There are several advantages of conditioning such as improving pellet durability index (PDI), eliminating feed pathogens, eliminating insects’ egg, improving feed conversion ratio, reducing processing costs, and increasing starch gelatinization. However, it has some chemical effects on starch, vitamins and protein.


Amir Attar – CEO
Alireza Abbasipour – Chairman of the Board
Samira Hassanpour- Scientific Research Member
New Millennium Feed Processing Co.

Feed processing is defined as any changes and processing conducted on feed ingredients before is ingested by animals. The common feed processing for poultry is pelleting. One of the important steps of pelleting is conditioning which is adding steam to the mash feed. In fact, in the later process, heat and moisture are added to the mash feed in order to prepare feed for further physical and chemical changes in next steps. Conditioning pursues two aims: 1. destroying microorganism, 2. beginning the process of gelatinization. Conditioning requires time, moisture, heat and pressure.
Adding moisture to the pelleting is a sensitive step, because mash feed should have about 18% moisture in the conditioner. This amount of moisture cannot be reached only in the conditioner and the primary moisture content of ingredients also plays very important role. Adding moisture to the low moisture content feed ingredients leads to high quality pellet. In fact, moisture helps feed passing easily through the die and increasing pellet density. Generally, the ideal moisture content in conditioned feed is 16.5-17%.

Nowadays, retention time in the conditioner is two minutes or less, while this period has been decreased to 30 seconds in some feed mills.

The most important purpose of every feed mill is to use the maximum capacity of manufacturing facilities, while the feed has high quality standards. One of the effective factors on pellet quality is feed conditioning. Several advantages have been reported for conditioning as follows:
• Improving pellet durability index (PDI)
• Eliminating feed pathogens
• Eliminating insects’ egg
• Improving feed conversion ratio
• Reducing processing costs
• Increasing starch gelatinization
• Increasing nutrient digestibility
In conditioning, mash feed is mixed with steam and pressure for a defined period of time. During conditioning, feed moisture increases 4 to 5 percent and feed temperature to 70 to 90 °C.

Nowadays, the conditioner is one of the main components of animal feed manufacturing. Time and temperature are different from one conditioner to the other one. Also, conditioners differ from various points like length, diameter, type, number of equipment, place, angle, steam entering, the presence of blades and their positions. Any changes in these factors can affect the retention time of materials in the conditioner. Since, there are various factors influencing performance and maintenance of conditioners:
• Controlling steam stream from boiling pot to the conditioner
• Appropriate steam pressure
• Appropriate position of pedals
• Durability
Generally, first two points includes steam controlling, while second two points considers the conditioner performance. The most important aspect of feed processing is adjusting steam flow. The latter process is performed through the effective control of steam from steam pot to the conditioner. Steam monitoring in its way to the conditioner is a routine job in the industry. The aim of controlling steam is to provide clean and adjusted steam for the conditioner.

Starch (500 to 700 g/kg in most cereals) is the most available source of energy for poultry that is in insoluble granules comprising unique chemical and physical structure. Starch is a type of glucan containing polysaccharides of amylose and amylopectin. The structure of both polysaccharides is based on 1→4 chain that connects to each other with α-glucose bands. Amylose has linear structure, while amylopectin has branched chain that a 1→6 branch can be observed after 20 to 25 carbons. Amylopectin comprises about 70-80% and amylose does about 20-30% of starch structure.

Although starch granules are insoluble in the water, they can solve in water under mild heat. This solubility is usually reversible, unless it receives to a specific threshold that the solubility would be irreversible and starch structure would change. This process is called “Gelatinization” and the temperature which gelatinization takes place is called “Gelatinization Temperature”. Starch granules would spread in water molecules in this temperature. In fact, gelatinization opens starch granules and enzymes can enter to the starch structure and significantly increases amylose degradation. Temperature and moisture affect hydrogen bonds in polysaccharide chain in the starch granule that leads to gelatinization. These granules are destroyed by moisture and then water can make link with free hydroxyl groups. The latter phenomena results in swollen starch and more degradation of granules with increasing water activity. Melting crystalline section of starch leads to eliminating double-sided reactions which are irreversible.

Gelatinization improves nutrients availability. Some animals cannot digest unprocessed starch, so starch cooking is necessary for such animals. In addition, gelatinized starch can serve as a binder to enhance pellet durability.Also, mechanical processing like milling and grinding may increase gelatinization that depends on different factors such as raw materials, production rate, equipment situation and product moisture.

There are some factors like heat, friction, steam (moisture), pressure and oxidation which can affect vitamins stability. Researches have shown that adding moisture has more negative effects on vitamins destruction compared to conditioning temperature. Increasing moisture may attenuate vitamins coverage against oxygen and other destructive components which subsequently increases vitamins destruction. Oxidation-reduction reactions are in relation with vitamins activity.

An increase or a reduction in hydrogen atoms with double bands or hydroxyl groups can reduce vitamins stability. The destruction range of vitamins varies in various conditioning processes. It has been reported that vitamin A, Biotin and Folic acid are more susceptible to heat compared to other vitamins. Vitamins A, D3and C are more intended to oxidation. Vitamin K and pantothenic acid are more susceptible to moisture. Methods like reducing temperature, moisture and pressure in the conditioner can increase vitamin stability, but the latter reduction is not possible due to controlling pathogens and pellet quality. In prevalence of liquid addition after pelleting, it is logical to add vitamins after pelleting process.

Protein has a sensitive structure to the heat. Protein structure may usually change with temperature and moisture that causes changes in structure, performance and nutritional properties of a specific protein. Every change in protein structure is called “Denaturation”.
Most proteins may change their structures and forms when confront with wet temperature or pressure on the feed. If the thermo-mechanical process is stopped before complete protein condensation, protein can return to primary form. If mentioned temperature or pressure is increased, non-covalent bonds making third type structure (second, third and fourth structures) may break which leads to irreversible denaturation of proteins. If thermo-mechanical process continues, covalent bonds like disulfide bonds also degrade. If heat, moisture and pressure are considered as main factors affecting denaturation, retention time, pH and presence of materials like lipids and carbohydrates are less effective. When processes performed on the feed are a combination of pressure, temperature, retention time and moisture, it is possible that denaturation takes place in the protein that increases protein digestibility. While stability improvement has not been reported with pelleting process, temperature can improve protein digestibility through deactivating inhibitor enzymes and protein denaturation.

Feed processing may result in Maillard reaction in which various factors interfere. High temperature and low moisture in thermo-mechanical treatment are known as Maillard inducing factors. Free aldehyde groups in reducing carbohydrates like glucose, lactose, and mannose and also free amine groups in amino acids and epsilon group in lysine may produce melanoide in presence of heat and moisture that can increase viscosity. The latter reaction is called “non-enzymatic browning” which has negative consequences on nutrient digestibility. Since lysine is the first limiting amino acids in cereals, losing this amino acid through browning has negative effects on animal. Starch and reducing carbohydrates like sucrose may change to reducing carbohydrates in feed processing, especially extrusion, and lose the lysine. High processing temperature may lead to degradation of cysteine which is the most sensitive amino acids to the heat following by lysine, arginine, threonine and serine.

We also suggest you to read our previous article titled "Effective and chemical-free means to control Salmonella in feed".

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