Laboring on bitterness

Wines have good years and bad years. And so do oranges. With some crops the oranges mature well and the juice has the right balance of sweetness and tartness. But for most, the sweetness may be off or the variety has delayed biterness. Nothing like the crop from 1999. And if you are reading this in 2015, the crops in the last few years have not been good in the US. Climate variability has not been helping.

Variation

The secret to removing the bitterness are plastic beads. For the beads to work, the orange juice needs to be separated into pulp and liquid. The liquid has most of the limonin, the bitter compound in orange juice. When the beads come in contact with the liquid, the limonin molecules get trapped in the beads (along with some other useful chemicals, unfortunately).

After some time the columns of beads need to be cleaned so as not to loose their capacity to trap the limonin. That is done with ethanol, the alcohol in wine or beer. The ethanol is then removed by washing the beads in stages with hydrochloric acid (HCl), sodium hydroxide (NaOH), and water. The idea is that the hydrochloric acid and the sodium hydroxide neutralize each other. After the removal of the limonin (a process called de-bittering), orange juice is reconstituted by adding back some of the pulp and maybe even some orange oil extracted from the peel.

De-bittering

Some of the tricks used to de-bitter citrus juices may work for the cook. From filtering techniques, to adding chemicals or microorganisms, many different ideas have been tried. Most ideas fail in practice because they either remove too many components from the citrus juice, affecting flavor or nutritional value; or because the method does not work in practice for the millions of gallons of juice that need to be processed. But on the smaller scale, these ideas may be practical.

The beads used for orange juice are a made from a styrene divinylbenzene cross-linked co-polymer resin. A long word but not a complicated concept. A polymer is a large molecule made linking together many copies of a few groups of atoms. The groups that get repeated are monomers and often differ from a molecule by the position of a few bonds. Styrene is one of those molecules that by the alteration of one bond can join with another styrene molecule to form a chain of molecules: the polymer polystyrene. Sometimes two different monomers will repeat to form a large molecule, which is then called a co-polymer. Polymers chains can be linked together with the introduction of atoms or small molecules. This freezes the co-polymer chains in a rigid network of molecules, such as when sulfur helps link the polymers of isoprene to form the more durable vulcanized rubber.

Rohm and Haas, a chemical company of Plexiglas and Morton Salt fame, introduced in the 1980s a new kind of cross linked co-polymers. They create the co-polymers in a solution containing a filler material that eventually gets washed away leaving behind large a network of tunnels in the beads. These tunnels are 0.1 to 0.3 micron in diameter, large for the gaps in a cross-linked polymer, but sufficiently small to trap other molecules. With this new polymer it was possible to absorb the limonin from orange juice and many other bitter compounds.

Jucing

How the oranges get juiced helps with the flavor. The machine of choice is the FMC juice extractor. It has been around long enough to be mentioned in McPhee’s classic book on oranges. Used commercially since 1947, the FMC machine does not slice the orange nor does it mix the peel oil with the juice. Instead, the orange is punctured by a pipe with sharp edges. A plunger then presses the orange, forcing the pulp into the pipe. Within the pipe the pulp is squeezed and the juice is collected in a lower tank. While this is taking place, the oils from the peel have been collected by washing the peel while its being pressed. All this takes place in less than a second.