Nutritionists, like me, when they formulate animal feeds, especially complete feeds for swine and poultry, seldom — if ever — look at the dry matter level of the feed. We assume all ingredients have the dry matter they need based on good manufacturing practices and move on to more important matters.
So, it was logical for a junior nutritionist that I was training to assume that 2,000 pounds of mixed feed would produce 2,000 pounds of pellets. He was suspicious when we ended with less weight in pellets, but he was 100% certain he had weighed all ingredients correctly – and in fact he had. So, what happened? The answer is simple: shrinkage. Let me explain.
Steam from conditioning adds to existing mass moisture
Most cereals come in at 86% to 88% dry matter, with soybean meal closer to 88% plus. All other dry ingredients contain over 90% dry matter. When a complete feed is mixed, we end with an average level of dry matter, if we were to properly sample the complete batch.
Let’s assume the mixed mash contained 88% dry matter. During pelleting, the mash undergoes a process called conditioning where steam (moisture) is added under pressure, and it is slowly mixed to allow the steam to penetrate the mass. Afterward, this more pliable mass is pressed through the die openings to produce pellets. The excess steam adds to the existing moisture of the mass, and it needs to be removed.
This all depends on the amount of moisture added. With good quality steam, we add 1% moisture for every 16 degrees Celsius we increase mass temperature. For a final temperature of 70 degrees Celsius (an average pelleting temperature), we add about 4% extra moisture.
In total, then our mass contains 88 – 4 = 84% dry matter, or 16% moisture. During pellet drying, pellets are heated thoroughly to ensure proper drying to the core. This results in most cases to a final dry matter of 90% to 92%. It was 92% dry matter that we measure at the finally cooled pellets in the above real case scenario.
With simple calculations, we had removed 4% water from the total mass through evaporation. We started with 2,000 pounds with 88% dry matter (1,760-pouind dry matter) and 12% moisture (240-pound water).
At the end, we had a product that contained the same amount of dry matter (1,760 pounds) but with only 8% moisture (155-pound water). The total was 1,760 + 155 = 1,915 pound (numbers have been rounded up for this discussion).
In essence, we were short 85 pounds of water and there was no way we could retrieve them. Not only that, but also if that feed were to be sold, then we would have had to calculate prices and margins considering that shrinkage, which when multiplied with the hundreds of tons a pellet machine can produce daily comes to a hefty sum lost as evaporated water.
Experienced pellet machine operators can minimize but never zero such shrinkage. Conversely, it is possible to hydrate very dry raw materials (such as cereals brought in from a drought-hit harvest) to bring them up to standard commercial levels.
I have often received complaints from customers who received one or two bags less when ordering two or three tons of a specific product code. I have always tried to explain that (a) feed mills cannot sell partial bags, and (b) it is better to have one or two bags less – without paying for them of course – than having a couple of “filler” bags containing dubious material (often leftovers from a previous batch – not recommended). Of course, with much larger orders, feed mills can account for shrinkage by increasing batch size accordingly, something they are not so keen doing when it is for very small orders.
Respiration of cereals during storage
Another source of invisible shrinkage is respiration of cereals during storage. Cereal grains are living organisms and continue to undergo post-harvest physiological processes, some of which result in starch being converted into carbon dioxide lost in the air. This process diminishes as time passes but it does not really stop unless the kernel is totally dried up.
Those who buy large volumes of cereals to last them for a year (from harvest to harvest) will invariably experience some dry matter loss due to respiration. This is also one of the factors that cause cereal prices to increase as we move further from the prior harvest season. In contrast, when buying and transporting very dry cereals over regions of high moisture or storing them for prolonged periods of time in high moisture regions, then some moisture may be gained, but there is always the risk of spoilage due to mold growth.
Other invisible losses causing shrinkage
Finally, there are the obvious losses of raw materials that are adding to the invisible losses all accounting for total feed mill shrinkage. These include mixing errors, broken bags, spoilage, errors in bag filling (overfilled bags), errors in weighing, dust lost during receiving and transferring ingredients across the mill, feed lost during transportation to the farms, and even outright theft or accounting errors.
Quite often, one or more of the above factors can contribute more to shrinkage losses than pelleting alone – but that depends on the specifics of each feed mill.
In general, a feed mill under good manufacturing practices should experience a total shrinkage of below 1% annually. Anything above that figure requires immediate fixing, whereas small improvements below 1% need to be justified due to the expenses often involved.
As it happens, higher losses are experienced in smaller mills operating outdated equipment than in modern high-tech feed mills with high levels of automation. Equally, labor expertise can greatly help reduce visible and invisible losses of raw materials.