Mill juice is the name used in Hawaiian pineapple processing to designate juice that is produced from pineapple skin. This juice is used to top off cans of pineapple slices and chunks.

In contrast, pineapple beverage juice is made from Ginaca strip slices, core, and eradicator meat. The beverage juice from these parts of the fruit typically has 12º Brix. Cake remaining after extraction of beverage juice is directed into the third stage press used in mill juice production.

The juice pressed from pineapple skins has a Brix of 8º to 10º Bx. The juice is produced by first shredding the ends (top and bottom) along with the lateral skin and cull fruit. A high-horsepower shredder, with a coarse screen, is used. Once shredded, the skin is first, second and third pressed. That is, the cake from the first pressing goes directly, without any water addition, to second pressing; and cake from second pressing goes directly to third pressing. The cake after third pressing, called silage, is sold as animal feed.

Recent testing with a Vincent Twin Screw press, Model TSP-12, provided the following data: Skin at the shredder (before pressing), 88% moisture; after first pressing, 80% moisture; after second pressing, 78% moisture; after third pressing (silage), 78% moisture. Brix of all press liquors, 9º.

Only screw presses of the interrupted flight design (Vincent, Jones, Rietz, and Schwarz) were used on the pineapple skin. The one Rietz press in service had the rotating cone feature; however, this appeared unnecessary as there was no channeling tendency and there was negligible press liquor coming through the cone screen. Cake discharge was uniform and non-channeling in all the presses.

Dramatic improvement in juice production was realized by steam injection. Steam was injected both at the inlet hoppers of the screw presses and where stationary resistor teeth protruded into the main screen portion of the presses. Press liquor temperature was held at about 130º F.

The juice from the presses is first filtered in a static screen. Then it is heated to 190º F for three minutes and screened in a second set of static screens. Then it is cooled and filtered in a Vincent Fiber Filter. The filtrate from this machine is sent to a Westfalia horizontal centrifuge, while the Fiber Filter sludge goes to a Westfalia decanter. Filtrate from both Westfalia’s goes to ultrafiltration and deionization. Color and Ph are corrected in the deionizer.