Peel oil (cold-pressed oil) recovery

The oil-water emulsion, or oil frit, from the ex- trac tion process is sent to the peel oil recovery sec tion. Apart from the oil and water, other fruit substances are present in the emulsion. These in clude particles of peel and pulp, and soluble pec tin and sugars. The aim of the peel oil re cov ery system is to recover pure oil by removing all oth er substances with as little oil loss as pos si ble.

Peel oil is commonly referred to as cold- pressed oil.

5.8.1 STRAINING AND CONCENTRATION STEP The fi rst step involves using a fi nisher as a strain- ing method to remove large bits of peel and other parts of the orange that must not enter down- stream centrifuges (see Figure 5.19). After straining, the oil emulsion containing about 0.5–2.0 % oil enters the fi rst stage centrifuge (also called a desludger or concentrator). The cen tri fuge concentrates the oil up to 70–90 %.

The fi rst centrifuge is a three-phase ma chine. The light phase is concentrated oil, the heavy phase is water, and the third phase is re sid u al particulate matter. The control of solids discharge from the sludge space is critical to the overall performance of the oil recovery system. If the discharge frequency is set too high then prod uct is lost, but if the sludge space is allowed to fi ll up then separation effi ciency is lost.

The water stream is often recycled back to the oil extraction system as spray water, al though it is important that some water is re moved from the system to allow additional fresh water to enter it. Microbiological problems may occur if the same water is continuously re cy cled. Moreover, the cen- trifuged water contains undesirable components such as soluble pectin.

As the concentration of these com po nents builds up in the emulsion, the oil sep a ra tion ef fi cien cy decreases, thereby resulting in lower oil yields. Again, this limits the amount of water re cy cling possible.

The centrifuged water also contains mi cro scop ic particles of oil that are too small to be sep a rat ed by the centrifuge. As this level of oil builds up with water recycling, the effectiveness of the water to extract oil from the peel de creas es. This will also lead to an overall drop in effi ciency of oil recovery.

The type of oil extraction used and the per- form ance of the centrifuges will determine the amount of water that can be recycled. The clean er the peel oil emulsion, the higher the oil yield of the peel oil recovery system and the larg er the recy- cling of water. The oil extraction system upstream of the reamer-type juice ex trac tor is claimed to give a “less contaminated” oil emulsion than the one-step squeezer-type ex trac tion system.

For oil recovery, the hermetic centrifuge has several advantages over the open bowl type de sign. The fully fl ooded bowl in the hermetic ma chine ensures that oil does not come in con tact with air. The precise manner in which the interface between oil and water is controlled leads to higher separation effi ciency.

Fig. 5.19 Flow chart of peel oil recovery.

Peel oil emulsion from extractors

Winterisation tanks Second stage centrifuge

First stage centrifuge Oil finisher

Reclaimed water returned to extractors

Drum storage Bulk transport

A hermetic centrifuge for concentration of peel oil emulsion is illustrated in Figure 5.20. 5.8.2 POLISHING

The concentrated oil stream then passes to a sec ond stage centrifugation process (polishing). With in this machine the oil is further con cen trat ed

to >99 % purity. The fl ow rates are ex treme ly small (1–2 %) compared with the fl ow rates in the fi rst stage, or with fl ow rates used in juice clar i fi ca tion and deoiling of single-strength juice.

Since the product has already undergone one centrifugation process, virtually no solid par ti cles remain in the product. For smaller ca pac i ties, a sol id bowl machine is used and the water and oil are continuously discharged. Pe ri od ic takedown re moves any material that col lects in the bowl pe riph ery. For larger fl ow rates, a sol ids-ejecting pol ish er is used in which water and oil leave the ma chine under pressure. Ac cu mu lat ed solids are dis charged about once or twice per hour.

One tonne of fruit typically yields 200–300 l emulsion to the fi rst centrifuge and 3–6 l of con- cen trat ed oil to the polisher.

5.8.3 THE WINTERISATION PROCESS

The polished oil contains trace amounts of dis- solved wax derived from the peel of the fruit. At temperatures above 15 or 20 °C, the wax is to tal ly

The winterisation process involves pre cip i tat ing the wax by causing it to crystallise and then settle. The oil is stored in tanks at 1 °C or lower, which

causes the waxes to come out of solution and sediment. The process typically takes 30 days or more, although at lower tem per a tures this period may be considerably short er. The winterised oil is then decanted from the tank. Larger processors collect the sludge from different winterising tanks so that once suffi cient material has accumulated, the waxes can be removed by centrifugation to re cov er residual oil.

The winterised oil is packed in 200 l (55 US gal.) drums or road tankers. Normally the oil is stored under refrigeration (–10 °C) and is traded as Cold-Pressed Oil, CPO. Sometimes it is called Cold-Pressed Peel Oil, CPPO. It is used as a raw ma te ri al in the fl avour manufacturing industry, and by concentrate blending houses and drink- base manufacturers.

5.8.4 D-LIMONENE RECOVERY SYSTEMS The heavy phase from the peel oil fi rst-stage

centrifuge is usually recirculated to extractors as “yellow water”. Part of this fl ow (10 to 50 %) needs to be replaced with fresh water in order not to concentrate too much insoluble material like pectin present in this poor oil emulsion before it’s sent back to the juice extractors.

The nonrecirculated fraction of the yellow water has some residual oil content (0.5–1.0 %) that, instead to be sent to the feed mill or waste water treatment, is separated using specifi c equip- ment dedicated to recovering the residual oil. A traditional d-limonene recovery system heats the yellow water and further vaporises a mixture of water vapour and d-limonene in one or more in- series fl ash chambers. The vapour is subsequently condensed to recover the terpenic fraction.

A more effi cient new design eliminates the in- dividual fl ash effects by incorporating a specially designed fractionation column to perform the va- pour enrichment. This results in better d-limonene quality and yields, and a more compact recovery unit. Both systems can usually be fed with sludge

Fig. 5.20 Hermetic centrifuge for peel oil concentration.