All about conditioning

An important unit operation of feed pelleting is the conditioning step. This involves blending a mix of dry granular components (feed mash) with steam to soften those particles and facilitate bonding of particles into feed pellets. Additionally, the water and heat present in the steam will modify components like starch and protein and thereby affect nutritional characteristics of the feed. Feed components like starch can be gelatinized. Feed additives are, mostly, negatively affected by the effects of heat and moisture, resulting in a reduction in the activity or level of these bio-actives.

Heat and moisture diffuse at different speeds through feed mash particles. Heat diffusion is generally very quick and typical associated times are less than 1 second, after which the particle is homogeneously warm. Moisture diffusion is about 100 times slower than heat diffusion, therefore during the conditioning and subsequent pelleting process, a different amount of moisture can be observed in the feed mash particle, depending on location (Figure 1).

Therefore, transformations of components like starch and protein and inactivation of bio-active components are affected by three major components:

For (amorphous) bio polymers these transitions can be marked out in a state-phase diagram (Figure 2).

Both examples (Figures 1 and 2) show that transformation and inactivation are functions of temperature and moisture present. The concentration of the latter in a particle is highly dependent on the size of the particle, the moisture diffusivity and the residence time in the subsequent unit operation (conditioner, pelletizer, cooler). This, therefore, affects the extent of transformation of nutritional relevant components like starch and protein and affects the activity of bio-active, nutritionally relevant raw materials like vitamins, yeast, enzymes and bacteria.

For the inactivation of bio-compounds (vitamins, yeasts, enzymes, bacteria) an example is given in Figure 3. This shows, that at constant temperature level, higher water activities lead to inactivation of bio-actives (in this case Typhimurium DT104) in shorter time spans.

In brief, monitoring the feed mash before pelleting in relation to pellet quality is performed at the conditioning step. During this relatively short step (20 s.), steam is added to the feed mash, with fast heat diffusion (<1s.), and slow water diffusion (100 s.). Consequently, water is always inhomogeneously distributed over a feed mash particle and diffusing from wet, outer region into dry core of the particles. This moisture gradient is affected by size and type of particles (starch rich, fat rich). Therefore, depending on the parameters used, the conditioning step affects functional changes in components as protein, starch and bio-actives. State phase diagrams (SPD) allow to connect raw material properties to processing conditions. It supplies information on changes in feed components relevant to nutritionist as the melting temperature (Tm) for starch, the denaturation temperature (Td) for protein, and information on bioactive losses. Last, but not least, the glass transition temperature (Tg) gives information on physical changes relevant to processing related to changes in structure (hard, brittle vs. soft, rubbery, elastic). The transformation of these components in the raw materials will subsequently affect the compaction behaviour of the feed during pelleting.