Php

Solvent extractor \

Source: Refs. 50 and 51.

Degumming 1

Degummed crude canola oil

Figure 5. Outline of the primary processing of canola.

erally undergo mild pressing or prepress to reduce oil content from 42 to 16-20%, while compressing the thin flakes into large cake fragments. Canola cake fragments are solvent extracted with normal hexane to remove the remaining oil. This is achieved by countercurrent movement of the cells of pressed canola cake and hexane, thus interfacing the oil in the flake or cake with a rich solvent-oil solution (57). The solvent is recovered from the oil-hexane solution by the conventional distillation system that ensures the oil and meal are solvent free.

The solvent-extracted oil (43-47% of the total oil) is combined with the prepressed oil (53-56% of the total oil) to form the crude oil fraction, as outlined in Figure 5. The crude oil contains a variety of minor constituents (Table 4) detrimental to oil quality that are removed by a series of unit processing steps including degumming, alkali refining, bleaching, hydrogenation, and deodorization.

Degumming

Conventional degumming is carried out in most plants by treating the crude oil with either hot water (85-90°C) or steam (2-5%) while mixing intensively from 1 to 30 min (59). This precipitates the water-hydrated phospholipids, which are then removed by centrifugation. The major drawback to this type of degumming process is that it only removes hydratable phospholipids and still leaves 100 to 200 ppm of phosphorus (0.25-0.5% phosphatides) in the oil. To achieve better results, most Canadian processing plants now carry out acid degumming using citric, phosphoric, or malic acid. This is followed by steam, which results in an oil with phosphorus levels of 50 ppm or less. This product is referred to as acid degummed or super de-gummed oil.

Table 4. Minor Constituents in Crude, Degummed Canola Oil

Constituents Amount

Unsaponifiables (sterols and sterol esters), % 0.1-0.8

Color bodies, ppm 20-30 Sulfur compounds, ppm 2-10 Iron, ppm 2 Copper, ppm 5

Refining

The crude degummed oil is then subjected to refining, which removes free fatty acids, phospholipids, color bodies, iron and copper, as well as some sulfur compounds. The major type of refining in Canada is alkali refining, although there is a shift towards physical refining due to the fewer environmental problems associated with the latter process.

Five major steps are involved in alkali refining.

1. Initial pretreatment of the crude degummed oil with phosphoric acid (0.2-0.5% of 85% phosphoric acid) at 40°C for a minimum of 30 min. This conditions or acidifies the nonhydratable phospholipids (gums) and permits their separation during the refining process. In addition to precipitating nonhydratable phosphatides, chlorophyll as well as pro-oxidant metals such as iron and copper also are removed (60,61).

2. The second and major step in alkali refining, neutralization with sodium hydroxide (8-12%), depends on the free fatty acid content of the oil and the amount of phosphoric acid used during pretreat-ment. A slight excess is normally added to ensure complete saponification of these fatty acids.

3. The addition of sodium hydroxide in step 2 produces a soap-stock phase that contains precipitated non-hydratable phospholipids as well as free fatty acids and other insoluble impurities. This soap stock is separated from the oil by centrifugation.

4. Any residual soap stock in the oil is further reduced to 50 ppm or less by washing with water at 86 to 95°C. Citric or phosphoric acid may be added to the washed oil to remove remaining traces of soap stock.

5. During this stage the oil is heated at 105°C with agitation until dry.

Most oil processors use a continuous alkali refining process, although batch refining is still used by some processors. The overall result of alkali refining is a marked reduction in free fatty acids, phosphorus, chlorophyll, and sulfur, as summarized in Table 5.

An alternative method to alkali refining is physical refining, which removes free fatty acids from canola oil by steam distillation. This avoids the production of a soap stock and attendant disposal problems. The oil is first acid degummed with citric or phosphoric acid and then bleached to remove phosphatides and trace metals. This method reduces the phosphorus content to less than 5 ppm, as higher levels cause a darkening of the oil during steam distillation. The latter process (distillative deacidification) is carried out in a specially designed deodorizer. An alternative method to alkali refining is physical or steam refining. This method is used widely in Europe and has been only recently introduced into North America.

Bleaching

Before hydrogénation or deodorization, canola oil is bleached with acid-activated bleaching clay (at 0.1252.0%) under vacuum at 100 to 125°C for 15 to 30 min. This is an adsorptive process attracting polar substances to active sites of clay surface particles. These include pigments such as chlorophylls and carotenes, residual soaps, phospholipids, and trace metals, as well as primary and secondary oxidation products in the oil. Of particular impor-

Table 5. Effect of Alkali Refining on the Quality of Canola Oil

Constituents

Crude degummed

Alkali refined

Free fatty acids, %

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