Introduction:

The pelleted feed manufacturing process has undergone a significant evolution in recent decades due to the demand for higher standards of physical quality, nutritional value and microbiological hygiene, as well as greater flexibility in the incorporation of new and varied raw materials.

What is pelleting?

Pelleting improves the characteristics of balanced feeds due to the effect of cooking on the digestibility of starches and nutrient absorption: that is, it increases the digestibility of the nutrients and so improves the feed conversion ratio and production index of the farm unit.

By subjecting starches to heat and moisture, water absorption takes place and the starch granule loses its crystalline structure, a process that is catalyzed by a rapid increase in temperature. This causes the starch granule to burst, producing a gelatinous mass: that is, gelatinization of the starches occurs.

In addition to the gelatinization of starches, the use of heat also leads to an increase in the absorption of nutrients and a reduction in any pathogens that could be contaminating the feed, while eliminating anti-nutritional factors and allowing for the incorporation of a higher range and quality of difficult to granulate raw materials. The absorption of moisture also increases the lubrication, softening and cooking of the feed.

The often cited inconveniences of pelleted feed include the need for specialized machinery (conditioners, granulators and chillers), and a lengthier and apparently more costly manufacturing process.

This increase in costs is attributable to:

- amortization of investments in machinery,

- higher energy consumption,

- maintenance of equipment,

- and the loss of raw materials during the manufacturing process (loss of moisture during storage, grinding, granulation, etc).

This can be mitigated by the use of real time on-line humidity technology, to ensure constant moisture levels in the mixer and obtain the target humidity in each batch, thus obtaining pellets of sufficiently high quality to withstand the whole manufacturing and transportation process, up to arrival at the final destination - the feeding trough on the farm -, reducing the formation of fines and avoiding unnecessary losses.

Pellet quality as a key performance indicator

The quality of the pellet is important not only for commercial reasons but also for its effect on the production index, which is the basis for good livestock performance.

Quality here is a combination of several factors, namely: durability, hardness and appearance, - appearance includes colour, surface texture, uniformity of size, percentage of fines and palatability.

Some of these factors are subjective, but others are objective (measurable) and should be taken into account:

Durability:

This is probably the most important factor to assess, and refers to the ability of the pellet to withstand transport and handling without breaking, and with the minimum possible percentage of fines.

It is calculated by subjecting the feed to a Standardized Durability Test and is expressed by means of the Pellet Durability Index (PDI), which is the percentage of the mass of the pellets remaining intact with respect to the total mass of pellets.

Uniform Size:

The variability of pellet size (length and diameter) not only affects the appearance of the feed but also the durability or consumption.

Percentage of fines:

This should be kept to the minimum, assuming adequate sieving of the pellets has been carried out.

Hardness:

This refers to the weight (in kg) that the pellet is able to withstand without breaking. It has to be sufficient to withstand storage and transportation to the farm. Hardness and durability are not always correlated.

It should be borne in mind that there are numerous factors that affect the quality of the pellet: (Figure 1) raw materials, particle size, conditioning, granulator matrix, cooling and drying, condition of equipment, etc.

Therefore, one of the most important factors that determine pellet quality is the moisture level in the conditioning process, which must sufficient to guarantee thorough cooking and most important, good gelatinization of the starches.

Factors that influence pellet quality

1. Raw materials:

The cereal grain used (corn vs wheat) and its inclusion percentage will influence quality since, depending on the origin of the starch, the target temperatures needed to achieve gelatinization will be different (see Table 1).

Another factor is the inclusion of fats (more than 1%), regardless of the source (animal or vegetable), which can greatly damage the pellet quality.

Table 1. Gelatinization temperatures of various raw materials (Source: Ross Tech 07/45: Physical quality of feed pellets)

Figure 1. Factors affecting pellet quality

2. Particle size:

The size of the particle is another factor that affects quality. As a general rule, the finer the particle is ground, the higher the quality of the pellet obtained as the particles are more exposed to the conditioning process and the pellet obtained is more compacted.

3. State of the equipment:

The state of the manufacturing equipment also determines the quality of the pellet. This is so obvious that it is often not taken into account as much as it should be in the daily routine of the factory.

The wear and tear of the hammers, the dies, the rollers, etc., or the incorrect positioning or orientation of the blades or steam injection valves, are all detrimental to the quality of the pellet.

4. The granulation process:

Good granulation is essential, because it directly determines:

- Durability

- Amount of fines

- Efficiency of the pelletizing process (amount produced and energy consumption)

A conditioning temperature greater than 80-85°C is the most suitable for the gelatinization of starch.

The steam used, after leaving the boiler and before entering the conditioner, must be regulated so that the quantity, quality and energy are adequate to achieve good results.

Managing steam quality, together with moisture content and retention time in the conditioner, is critical to the successful granulation process. The steam extracts the essential oils in the cereal grains and lubricates the feed through the matrix, reducing friction and wear of the machinery and increasing pelletizer production rates (tonnes/hour).

In addition to controlling and modifying everything relating to the above variables correctly, to optimize the pelletizing process, on-line technology such as Pellet Plus On-Line, a technology developed by Adiveter, can be used to continuously measure the moisture levels in the mixer and adjust in real time the dose of hydrating solution (water with a surfactant such as Re-Hydra Pro®) necessary to optimize the process and reach the following objectives:

Higher yields: increase in production (↑tonnes feed/hour) and decrease in energy consumption (↓kWh/t)

- Higher pellet quality (↑PDI)

- Improved digestibility, etc.

More attention should be paid in the day-to-day routine to controlling the multiple factors that affect pellet quality, and the all too common mistake of trying to solve the problems of quality by using a high compression matrix or by the use of binders should be avoided, as all this will result in higher production costs and, sometimes, doubtful efficiency and quality.

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Article made possible through the contribution of Adiveter S.L.

Improving pellet quality, the key to good performance