In 1995, “they” said the best place for the coater was after the dryer “because it absorbs better”. In 2006, I was told the same words after a customer moved our machine from being after the cooler. “They” is a powerful force all over the world. And it is difficult for one vendor to suggest some changes. This Guide might also be called “the mistakes we have made and what we wouldn’t do again”. The conflicting information and perception in the market originates with the fact that the extruder companies have a fountain of knowledge on how to make many diets, while the vacuum coater suppliers are more machinery manufacturers whose knowledge is related just to coating and mistakes we have made. Among all of us, there is little if any exchange of knowledge because we think we have a competitive edge that we don’t want to give up. The double rotor paddle mixers are the best devices to use because they will coat the entire surface given enough liquid. The operation is to use vacuum to remove all the air from a porous pellet, coat the surface and let atmospheric pressure push the oil inside. If the pellet is not completely coated, then air takes the path of least resistance leaving liquid on the surface. Another part of the puzzle could be that potential purchasers don’t have real facts about their operation such as what the real temperature is when ready for coating. The importance of this is potential moisture loss and what to do to prevent it. Part of this also is because we don’t ask enough questions particularly when language is a perceived problem. This Guide is intended to answer the questions and explain why we need information.
What vacuum coating is and the differences between vacuum coating & mixture.
Vacuum coating or vacuum infusion is using atmospheric pressure to push liquids inside the pellet. You need to use it when the level of liquid is high enough that you cannot go straight to packaging because it wicks or pools in the bags. Some examples of that level could be 8% on cat food, or 12% on dog or fish feed. The coater is generally placed inline with the extruder, after the dryer or cooler, and has to coat atmospherically as well. Between 5 & 10% generally we can make a good appearance. Below 5% may start to get spotty or what we call the “salt & pepper effect”. The appearance does not affect the quality, because when you are not getting all the surface coated, you just don’t have enough liquid. I will concede we may get a worse appearance than a reel because we are taking what liquid you have and applying it over the entire surface, where a reel may just put it on the sides. Just to apply some judgement, if you had one solid piece, like a rock, that weighed the same as your batch, would the liquid called up by the nutritionist be enough to paint it with a brush? If it is, we will try to do it in our mixer. A myth is that vacuum will help appearance inside and out. Myth is that pigment will make pellets look the same inside. The reason for this has a simple explanation: mixing is randomization, conditioning is randomization, therefore there is randomization in the pellets. The moisture is also random. There is nothing in a vacuum coater to overcome this randomization of the starting product. There are things that can be done to improve outside appearance. With the double rotor mixer we are using high particle movement for mixing and also distributing the liquid. If the liquid will stay on the surface for just a few seconds, we can spread it around. If it the pellets are cool, it slows the absorption. The other option is to slow down the application of liquid. All particles pass a given point every 6 to 10 seconds, so if you hold a spray in multiples of that, say 20 to 30 seconds, then you can hit all the particles directly. We have had instances where we have had to spray for over a minute to get good appearance. A system needs flexibility. You cannot use atomizing sprays in these mixers because there are too many air currents during atmospheric coating that deposits liquid on the walls of the mixer and not on the product. Under vacuum, atomizing can cause evaporation. You will see a measurable amount of liquid exhausting through the vacuum pump and it should be piped outside or to a sump. If you are seeing more than 6 litres per shift per year, then I would suspect the method of spraying.
How to measure quality
A quick measurement is to weigh 6 samples of a given size and calculate the standard deviation and coefficient of variation (Cv.). A Cv of 5% is considered statistically perfect, and if you have up to 7%, you have excellent product. The other way, of course, is to do a fat content analysis. We know the techniques mentioned under heading 2 work. We did an installation in England applying 5% fat and 22 grams per tonne of enzyme. The enzyme was cut 5:1 so we had about ½ liter to spray. We put it through one nozzle in the center and toward the end of the mixer. The nozzle was sized to hold a hydraulic spray (not atomized) for 20 seconds. BASF did the acceptance analysis and had a Cv. of 6.4. (Test report is available.) To show this in another context, if you prorate the surface area to be equal to a U.S. football field, then the liquid would be equal to eight 8 oz. (240 ml) glasses of water. We spray the 8 glasses onto a foot ball field, and take 6 samples the size of a sheet of wall board, and the variation among them was Cv. 6.4.