Canola Oil: Physical and Chemical Properties by Dr. Roman Przybylski Canola oil produced in Canada is obtained from the seeds of Brassica napus and Brassica rapa. These cultivars, low in erucic acid and glucosinolates, are very different from high erucic acid rapeseed oil in chemical, physical and nutritional properties.
PHYSICAL PROPERTIES Selected physical properties for canola oil are shown in Table 1. Table 1. Physical Properties of Canola Oil
Parameter
Relative Density (g/cm3; 20˚C/water at 20˚C) Refractive Index (nD 40˚C) Crismer Value Viscosity (Kinematic at 20˚C, mm2/sec) Cold Test (15 Hrs at 4˚C) Smoke Point (˚C) Flash Point, Open cup (˚C) Specific Heat (J/g at 20˚C) Thermal Conductivity (W/m˚K)
Value
0.914 - 0.917 1.465 - 1.467 67 - 70 78.2 Passed 220 - 230 275 - 290 1.910 - 1.916 0.179 - 0.188
Relative Density The relative density of canola oil was first reported by Ackman and Eaton in 1977 and later confirmed by Vadke et al. (1988) and Lang et al. (1992). Noureddini et al. (1992) reported a density for high erucic acid rapeseed oil of 0.9073 g/cm3 while Appelqvist & Ohlson (1972) reported a range from 0.906 g/cm3 to 0.914 g/cm3. Ackman and Eaton (1977) indicated that a different proportion of eicosenoic (C20:1) and C18 polyunsaturated acids could be a major factor for the increase in relative density of canola oil. The higher specific gravity of 0.9193 g/cm3 observed for soybean oil can be attributed to the higher content of linoleic acid (Ackman and Eaton, 1977). As for other liquids, the density of vegetable oils is temperature dependent and decreases in value when temperature increases (Figure 1).
Crismer Value The Crismer Value measures the miscibility of an oil in a standard solvent mixture, composed of t-amyl alcohol, ethyl alcohol and water in the volume proportion 5:5:0.27. Crismer value (CV) is one of the specification criteria used for international trade, mostly in Europe. Characteristic values are usually within a narrow limit (AOCS, 1992). The miscibility of an oil is related to the solubility of glycerides, and is affected mainly by the unsaturation and chain length of the constituent fatty acids. Little data is available describing the solubility characteristics of canola oil. Sahasrabudhe (1977) found that the Crismer value decreased from 82.0 to 76.8 with the reduction of erucic acid content from 54 to 0.1%.
Viscosity Viscosity values estimate an oil’s relative thickness or resistance to flow. Viscosity of refined, bleached and deodorized (RBD) canola is higher than soybean oil (Figure 2). Figure 2: Effect of Temperature on Viscosity of Canola and Selected Oils. Adapted from Lang et al. (1992), Vadke et al. (1988) and Noureddini et al. (1992)
Figure 1: Effect of Temperature on Density of Selected Oils. Adapted from Lang, et al (1992) and Noureddini, et al (1992)
Lang et al. (1992) and Noureddini et al. (1992a) found that the viscosity of canola and other vegetable oils, like other liquids, was affected by temperature and proposed an equation to calculate viscosity in the temperature range from 4 to 100˚C. Figure 2 shows the relation between temperature and viscosity for canola and selected vegetable oils. Rapeseed oil exhibited a higher viscosity than canola, corn and soybean oils. This can be directly related to the contribution of saturated fatty acids (Noureddini et al., 1992a).
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Smoke Point Smoke point is the temperature at which a fat or oil produces a continuous wisp of smoke when heated. This provides a useful characterization of its suitability for frying. The Canadian Government specifications define that frying oil should have a smoke point above 200˚C. Table 1 indicates that canola oil fulfills this requirement. A similar smoke point was observed for rapeseed oil (Appelqvist & Ohlson, 1972). The heating technique used in the standard method for smoke point determination is well-defined (AOCS Method Cc 9a-48). Arens et al. (1977) reported that, when measured by different laboratories, the smoke point for the same oil can differ by 10%, causing ±20˚C deviation. Therefore, caution should be exercised when comparing smoke points reported from different laboratories. These variations are related to the subjective determination by an observer as to when “a continuous stream of smoke” occurs, and is not matched with a reference.
Flash Point Flash point defines the temperature at which the decomposition products formed from frying oils can be ignited (AOCS Method Cc 9b-55). This temperature ranges from 275˚C to 330˚C for different oils and fats. Canola oil falls within this range (Table 1).
Cold Test The cold test measures the resistance of an oil to formation of a sediment at 0˚C or 4˚C (AOCS Method Cc 6-25), and is generally used to measure the effectiveness of the winterization process. Compounds with high melting temperatures, mainly waxes and triglycerides with saturated fatty acids, usually cause sediment formation (Przybylski et al., 1993). The cold test reveals whether an oil remains free of clouding when held at 4˚C or 0˚C for 15 hours. The formation of haze in canola oil is not a common occurrence, but may happen on occasion (Mag, 1990). It has been observed that oil produced from seeds grown in dry conditions will develop sediment more quickly. This may be related to the higher content of saturated fatty acids formed as a response to dry stress conditions (Przybylski et al., 1993).
Melting Characteristics, Polymorphism and Crystal Properties Canola oil has a homogeneous fatty acid composition with 95% 18 carbon fatty acids (Ackman, 1990). Canola oil is hydrogenated to produce shortenings and margarines, as the trans isomers formed have higher melting points than cis fatty acids, as is shown in Table 2 (D'Souza et al., 1991). Table 2: Melting Characteristic of Octadecanoic Fatty Acid Family a
Fatty Acid Linolenic (cis 9, 12, 15) Linoleic (cis 9, 12) Oleic (cis 9) Octadecenoic (cis 6) Elaidic (trans 9-octadecenoic) Stearic a - Adapted from Mag (1990)
Melting Point (˚C) -11.2 -5.1 13.2 28.6 43.7 69.6
Polymorphism is a well-known phenomenon associated with the crystallization behaviour of long chain compounds. Fats can crystallize into a number of sub-crystalline forms such as α, β, and β', each differing in size and stability of the crystals (D'Souza et al., 1991). The ability of a fat
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to exist in a number of different crystalline forms depends on how the molecules arrange themselves in the solid state. It has been established that hydrogenated canola oil has a tendency to crystallize in the β-form, which forms large crystals ranging in size from 5 - 25 µm. The formation of these large crystals causes an increase in graininess, which is directly responsible for gritty and crumbly products (Yap et al., 1989). In the manufacturing of margarine the β' crystal form is desired, as it has smaller crystals (less than 1µm in size), thereby giving the formulated product desirable textural characteristics. However, the β' form is less stable, requiring higher amounts of energy for crystals to pack than in the β-form. Therefore, it has a tendency to transform into the lower energy state β form (Sato, 1988). Trans isomers of fatty acids were found to have a tendency to produce products with higher β' stability than cis acids. This was attributed to the sterical effect of these isomers, which hinders the transformation to β form (D'Souza et al., 1991). To stabilize the β' form, a blend of three hydrogenated canola oils is used to increase the heterogeneity of the triglycerides. This prompts the margarine to crystallise in small, needle-shaped crystals, giving the final product a smooth, pleasing mouth feel and good spreadability (Mag, 1990). A more effective approach in avoiding β crystal formation is to use 10 - 15% palm oil or 20 - 25% cottonseed oil to supply triglycerides containing palmitic acid (Mag, 1990). Yap et al. (1989) found that the addition of 10 % palm oil before hydrogenation had a better effect on β' stability than blending it with hydrogenated canola oil prior to product formulation. Increased crystalline stability is directly related to the presence of palmitic acid as it has a tendency to crystallize in β' form (Postmus et al., 1989). The type of hydrogenation was also shown to affect the β' stability. Naguib-Mustafa and deMan (1985) found that selectively hydrogenated canola oil with an iodine value of 70 was more stable in this crystalline form than any oil hydrogenated under nonselective conditions. Addition of crystallization inhibitors such as sorbitan tristearate, in the amount of 0.3% of the oil phase, also prevented β crystal formation. Interesterification of canola oil with palmitic acid containing edible product can also produce stocks with high β' crystallization tendencies (Mag, 1990). Manufacturers often use this process to replace hydrogenation. When melted fat is cooled, the high-melting glycerides (HMG) crystallize first and dictate the polymorphic form in which the solids will crystallize, as well as their future behaviour during storage. It has been established that HMG consist of saturated and monounsaturated fatty acids. The saturates are mainly palmitic and stearic acid, while the unsaturates consist mostly of trans isomers (D'Souza et al., 1991). The rate and extent of β' to β transformation depends on the molecular composition and configurations of the fat, crystallization conditions, temperature, and the duration of storage.
Solid Fat Index (SFI) and Dilatation Curve The solid fat index (SFI) and dilatation curve for hydrogenated fat describe the amount of solid fat remaining at defined temperatures. Individual triglycerides differ in physical properties according to their fatty acid composition. Thus, when a fat is kept at a particular temperature those triglycerides containing unsaturated fatty acids melt first, while those containing the more saturated and trans isomers of fatty acids melt last. An expansion of the solid fat component occurs as temperature increases, reaching a maximum when it melts completely. The expansion of the fat or dilatation can be monitored by measuring the increase in specific volume with temperature and establishing a dilatometric curve. This enables calculation of the percent solid fat at any specific temperature. The spreadability of a margarine or spread can also be predicted from the SFI. To achieve the desired body and melting properties with stick margarines, selectively hydrogenated canola oils (SH) are used, along with nonhydrogenated oils. The most desirable approach is to use SH and SH1 canola oils together with a certain amount of liquid canola oil
(Table 4). The solid fat indices of the oil phase blend must be in the range of 25 - 30 at 10˚C, 14 -18 at 21.1˚C, and 2 -3.5 at 33.3˚C (Moustafa, 1992). The low solid fat index at 10˚C in soft margarines appeared to be responsible for their spreadability at refrigeration temperatures (Table 4). Formulation of these margarines requires the use of 70 -85% of slightly hydrogenated and/or liquid oils, with a minimum of about 11 -15% of highly hydrogenated stock (usually nonselective) (Table 4). Using this stock, the formulated product is characterized by good spreadability at refrigeration temperatures and good emulsion stability so there are no oiling-out problems (Mag, 1990). Typical solid fat indices of soft margarines are 8 - 14 at 10˚C, 5 - 8 at 21.1˚C, and 0.5 -2.5 at 33.3˚C (Moustafa, 1992). In margarines with good mouth-melt characteristics the oil phase in the product melts sharply at body temperature which results in the total breakdown of the emulsion in the mouth with the release of the flavourladen water phase (Moustafa, 1992). If the crystalline structure does not melt rapidly the margarine feels waxy or thick in the mouth. Table 3. Solid Fat Indices of Hydrogenated Canola Stocks and Margarines a Sample Solid Fat Index Fatty acids 10.0˚C 21.1˚C 33.3˚C PUFA MUFA SAT.e TRANS Hydrogenatedb SHc 10.8 1.4 0.1 10.6 78.1 9.1 34.0 SH1 41.3 22.5 15.9 3.5 76.7 17.7 51.9 NSd 6.2 1.8 1.2 14.4 67.5 14.9 24.6 NS1 24.5 13.4 8.2 7.7 67.0 23.1 31.7 Margarines Stick Regular 25 - 30 14 - 18 2 - 4 3 - 10 50 - 70 16 - 25 Stick Hi - Lif 16 - 24 10 - 15 1.5 - 4.0 20 - 40 20 - 50 13 - 23 Soft (70% Liq.) 8 - 14 5 - 8 0.5 - 2.5 30 - 60 15 - 42 10 - 20
with n3 amount of potential molecular species where n is the number of different fatty acids present in the oil (Figure 3). Figure 3:
Structure of Acylglycerides and Phospholipids. FR - Functional Residue such as Nitrogenous or Polyol R1, 2, 3 - Residue of Fatty Acid
CH2 –– 0 –– R1
CH2 –– 0 –– R1
CH2 –– 0 –– R2
CH2 –– 0 –– R2
CH2 –– 0 –– R3
CH2 –– 0 –– R3 –– FR
Acylglycerides
Phospholipids
The TAG molecular species profile represents a key to understanding the physical characteristics of an oil and also is a unique means of identification (Rezanka and Mares, 1991). The position of fatty acids on the glycerol molecule was originally examined in rapeseed oil. Long chain (C20:0-C24:0) and saturated fatty acids occurred mostly in the 1- and 3-positions, while the octadecanoic (C18) fatty acids, especially linoleic and linolenic, are integrated in the 2-position (Kallio and Currie, 1993; Ackman, 1983). Paterson (1981) examined the triglyceride composition of canola oil and found 25% of the total TAG's to be triolein. The triglyceride compositions of modified canola oils are presented in Figure 4. Triglyceride composition is governed by the type and amount of fatty acids present in an oil. As can be predicted, in high oleic acid canola oil the main triglyceride was triolein (Figure 4). In regular canola oil four triglycerides, namely: olein-dilinolein, linolenin-dilinolein, triolein and linolein-diolein were detected in almost equal amounts. Figure 4:
Composition of Triglycerides in Canola Oils. Abbreviations: Ln - Linolenic; L - Linoleic; O - Oleic; P - Palmitic; S - Stearic; LL Canola - Low Linolenic Canola; HOCanola - High Oleic Canola. Adapted from Neff et al (1994)
a - Adapted from Mag (1990) and Moustafa (1992) b - Hydrogenated stocks c - Selective hydrogenation d - Nonselective hydrogenation e - Saturated fatty acids f - Margarine with high content of liquid oil
CHEMICAL CHARACTERISTICS Nature of Edible Oils and Fats Edible oils and fats are composed primarily of triglycerides, which are the ester of one molecule of glycerol and three molecules of fatty acids. Canola oil analyses show that the triglycerides constitute 94.4 to 99.1% of the total lipid (Mag, 1990). The typical composition of canola, rapeseed and soybean oils is presented in Table 4. Table 4: Constituents of Canola, Rapeseed and Soybean Oils a
Component Triglycerides (%) Phospholipids (%) Crude Oil Water-degummed Acid-degummed Free Fatty Acids (%) Unsaponifiables (%) Tocopherols (ppm) Chlorophylls (ppm) Sulfur (ppm)
Canola 94.4 - 99.1
Rapeseed 91.8 - 99.0
Soybean 93.0 - 99.2
up to 2.5 up to 0.6 up to 0.1 0.4 - 1.2 0.5 - 1.2 700 - 1200 5 - 35 3 - 15
up to 3.5 up to 0.8 0.5 - 1.8 0.5 - 1.2 700 - 1000 5 - 35 5 - 25
up to 4.0 up to 0.4 up to 0.2 0.3 - 1.0 0.5 - 1.6 1700 - 2200 Trace Nil
a - Adapted from Mag (1990) and Ying, et al (1989)
Triglycerides Triacylglycerols (TAG) are the most abundant lipid class found in canola oil. The combination of fatty acids on the glycerol moiety is complex,
Jáky and Kurnik (1981) investigated the concentration of linoleic acid in the 1, 3- and 2-positions. They found that in high erucic acid rapeseed oil (HEAR) at least 95% of the linoleic acid was concentrated in the sn-2 position, whereas in canola oil only 54% was in this position. The increased amount of linoleic acid in canola oil was placed by the plant’s enzymatic system into sn -1,3 position to replace erucic acid. Ohlson et al. (1975) indicated that linoleic acid replaced erucic acid in the sn-1 position, and while only present in canola oil at low levels, gadoleic and erucic acids were preferentially esterified in the sn-3 position. The investigators also found that linolenic acid was similarly distributed to linoleic acid. Kallio and Currie (1993) found that triglycerides with 54 carbons and two double bonds consisted of glycerides where stearic acid was
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present predominately at the sn-2 position. Glycerides with saturated fatty acids in this position usually have higher a melting point and poor solubility, (Rezanka, 1989). Additionally high melting glycerides can directly effect the clarity of an oil and stimulate sediment formation (Liu et al., 1993). The hydrogenation of unsaturated fatty acids proceeds more rapidly in the sn-1 and 3 positions than in the sn-2 position (Kaimal and Lakshinarayana, 1979). Therefore, the distribution of fatty acids in canola oil is a factor affecting the selectivity of hydrogenation.
temperature on isomerization of linoleic and linolenic acids is presented in Figure 6 (Wolff, 1993). After heating for two hours at 260˚C about 22% of the linolenic acid was transformed into trans isomers (Figure 6). Measurement of the amount of isomers can be used as an assessment of the deodorization process, where a lack of vacuum is often “replaced” by an increase in temperature to obtain odourless oil. Properly optimized deodorization will produce oil that contains zero or very low amount of trans isomers of linolenic acid. Figure 6:
Thermal Isomerization of Linoleic and Linolenic Acids. Adapted from Wolff (1993)
Fatty Acids Fatty acids are composed of a carboxyl group and a hydrocarbon chain. Individual fatty acids are distinguished from one another by the nature of the hydrocarbon chain (Figure 5). This chain can vary in length from 4 to 24 carbon atoms and can be saturated, monounsaturated (one double bond, MUFA) or polyunsaturated (two or more double bonds, PUFA). The most common fatty acids in edible oils and fats are those containing 18 carbons. These include: stearic acid (a saturated fatty acid), oleic acid (a monounsaturated fatty acid), and linoleic and linolenic acids (polyunsaturated fatty acids containing two and three double bonds, respectively) (Figure 5). Figure 5:
Configuration of Octadecanoic Fatty Acids
Fatty acid abbreviations are made according to the number of carbon atoms in the molecule and the number of cis ethylenic double bonds. The general assumption is that all multiple double bonds are methyleneinterrupted. The chemical nomenclature requires that carbon atoms be counted from the carboxyl end of the fatty acid. However, for biological activity carbon atoms are numbered from the terminal methyl group to the first carbon of the ethylenic bond. Such a classification is designated by the symbol ϖ-x, ϖx, or n-x, nx, where x denotes the position of the double bond closest to the terminal methyl group. For example, linoleic acid with two double bonds, where one is located on the sixth carbon atom counted from the methyl group, will be abbreviated as C18:2n-6.
Geometric Isomerism In the case of unsaturated fatty acids, the carbon chain is bent into a fixed position at the double bond, resulting in several possible geometric isomers. When the portions of the chain are bent towards each other they are called cis; and when bent away from each other, trans (Figure 5). The natural configuration of fatty acids is cis, as shown for oleic acid. The corresponding trans configuration, elaidic acid, results in a straight chain (Fig. 5). From a nutritional point of view the cis isomer is more desirable. However, fatty acids with trans configuration affect the texture and melting properties of fat or oil. Isomerization from cis to trans occurs mainly during the hydrogenation of an oil. Formation of trans isomers of linolenic and linoleic acids may also occur when harsh conditions are applied during refining. During processing of canola oil formation of trans isomers of linolenic and linoleic acids are observed. Oleic acid is less prone to isomerization, trans isomers were detected only when extreme parameters were applied. (Ferrari, 1996). Due to elevated temperatures, deodorization is the stage of processing where isomerization predominantly occurs. The effect of time and
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Fatty Acid Composition of Canola Oil The reduction of erucic acid (C22:1) in rapeseed oil resulted in a marked increase in octadecanoic acids. In fact, 18 carbon fatty acids account for about 95% of canola’s total fatty acids (Table 6). Table 6:
Fatty Acid C10:0 C12:0 C14:0 C16:0 C18:0 C20:0 C22:0 Total Saturated
Comparison of Major Fatty Acids in Some Vegetable Oils (w/w%)a
Canola HEAR LLCAN b HOCANb LTCAN e LLFlaxb Soybean Sunflower Corn 0.1 3.5 1.5 0.6 0.3
4.0 1.0 1.0 0.8
0.1 3.9 1.2 0.6 0.4
0.1 3.4 2.5 0.9 0.5
0.1 38.8 4.1 2.7 1.6 0.4 0.2
0.1 6.3 4.1 0.1 0.1
0.1 10.8 4.0 -
6.2 4.7 -
11.4 1.9 -
6.0
6.9
6.2
7.4
47.9
10.4
14.9
10.9
13.3
0.2 60.1 1.4 0.2
0.3 15.0 10.0 45.1
0.2 61.1 1.5 0.1
0.2 76.8 1.6 0.1
0.2 32.8 0.8 0.5
0.1 16.5 0.1 -
0.2 23.8 0.2 -
0.2 20.4 -
0.1 25.3 -
61.9
70.1
62.9
78.7
34.3
16.7
24.2
20.6
25.4
C18:2n-6 20.1 C18:3n-3 9.6 Total 29.7 PUFAd
14.1 9.1
27.1 2.1
7.8 2.6
11.2 6.3
69.5 1.8
53.3 7.1
68.8 -
60.7 -
23.2
29.2
10.4
17.5
71.4
60.4
68.8
60.7
C16:1 C18:1 C20:1 C22:1 Total MUFAc
a - Adapted from Ackman (1990) b - Adapted from Przybylski, unpublished; LLCAN - Low linolenic acid canola oil; HOCAN - High oleic acid canola oil; LLFlax - Flaxseed oil with reduced content of linolenic acid. c - Monounsaturated fatty acids d - Polyunsaturated fatty acids e - LTCAN - Canola oil with high content of lauric acid (Adapted from Del Vecchio, 1996)
Plant breeders developed canola oil with the linolenic acid content reduced to 2.1% (Scarth et al., 1988) (Table 6). Storage stability of this oil was shown to be better than regular canola oil (Przybylski et al., 1993a). Low linolenic canola oil also exhibited improved frying performance and better storage stability of fried products such as french fries and potato chips (Petukhov et al., 1999; Warner and Mounts, 1993). Canola has been further developed to produce an oil with an oleic acid content raised from 60% to 85% (Wong et al, 1991). The fatty acid composition of high oleic canola is presented in Figure 6. This oil showed improved frying stability and produced better quality fried potato chips (Petukhov et al, 1999). From the health and flavour formation point of view, both low linolenic and high oleic canola oils should provide good quality frying products without the presence of trans isomers (Ackman, 1990). Warner and Mounts (1993) found that some amount of linolenic acid is required for good flavour formation in fried foods. This is due to the formation of oxidation products, which are important flavour compounds. Thus elimination of linolenic acid from oil can cause negative changes in fried product flavour formation. Recently canola oil with an elevated content of lauric acid was developed (Table 6). This oil is being used in confectionery coatings, coffee whiteners, whipped toppings and center filling fats (Del Vecchio, 1996). Calgene has also succeeded in the development of a canola plant that produces oil containing 40% stearic acid. This oil could be used as a replacement for hydrogenated fats in bread and bakery applications (INFORM, 1999).
Minor Fatty Acids Minor fatty acids present often differ from their acid family members by the location of the double bond (Table 6). Most of these acids are present in the 0.01 - 0.1% range, except for C16:1n-7 which is around 0.3%. Most of the minor fatty acids in canola oil are from the n-7 series, but n-9 isomers are also present in varying amounts. (Ackman, 1990). A similar series of minor fatty acids was found in the B. rapa variety Candle (Sebedio and Ackman, 1981). Conjugated C18:2 fatty acids have also been found in canola oils. Some of these compounds are artefacts of refining, although some were observed as natural components in some oil seeds. The refining process itself is a source of artefact fatty acids due to the isomerization of one or more of the double bonds of cis linolenic acid. These trans isomers can be found after refining in any linolenic acid-containing oil, accounting for 1% or more of the parent acid (Ackman, 1990). Canola oil is the only known edible oil that contains one or more fatty acid with sulfur as the integral part of the molecule (Figure 7). The structure of the proposed molecule of this fatty acid suggests the formation or presence of many isomers (Wijesundera and Ackman, 1988). Figure 7:
Isomers 1 2 3
Isomers of a Sulfur-Bearing Cyclic Fatty Acid Found in Canola Oil Adapted from Wijesundera et al. (1988)
m 5 6 7
n 7 6 5
In the sediment from industrial winterization, additional minor fatty acids and alcohols with 26 to 32 carbon atoms have been found in waxes and triglycerides (Przybylski et al., 1993). Most of these compounds are extracted from the seed coat/hull and can initiate sedimentation in canola oil (Hu et al., 1994).
Polar Lipids Sosulski et al. (1981) examined the polar lipids (PL) in several rapeseed cultivars, including a low erucic acid (LEAR) winter cultivar grown in Poland, and found that phospholipids formed the major component (3.6 %) of the total polar lipids, while glycolipids contributed only 0.9%. A more recent study by Przybylski and Eskin (1991) reported changes in phospholipids during the early stages of canola oil processing (Table 7). Table 7: Composition of Phospholipids in Canola Oil During Processing (%)
Oil Sample Solvent Expeller Degummed
Phosphorus (ppm) PCa 529.0 31.2 242.3 34.3 12.2 2.8
PEa 18.8 16.1 10.8
PIa 19.8 18.7 28.9
PAa 21.6 20.3 38.4
PSa 3.1 4.5 14.6
Phospholipids: PC - Phosphatidyl Choline; PE - Phosphatidyl Ethanolamine; PI - Phosphatidyl Inositol; PA - Phosphatidic Acid; PS - Phosphatidyl Serine.
a
Significant amounts of PA were formed during processing, which indicates hydrolysis of other phospholipids due to the hydro-thermal treatment during the conditioning of flaked seeds. Cmolik et al. (1987) observed an increase in the amount of phospholipids from 0.5% to 15% during conditioning of seed flakes. This increase was explained as the result of lipoprotein decomposition in seeds by hydro-thermal treatment. It was reported that hydratable phospholipids such as PC and PE stimulate removal of nonhydratable phospholipids. PI and PA are considered nonhydratable phospholipids and are difficult to remove during degumming. Smiles et al. (1989) examined the effectiveness of different chemical treatments for degumming canola and soybean oils. Phosphoric acid was found to be the most effective degumming agent in terms of reducing the levels of nonhydratable phospholipid (Table 8). Other nonhydratable phospholipids were more effectively removed by water. Water and phosphoric acid were not different in terms of their ability to remove PE. These researchers found that lecithins obtained from these oils by water degumming formed the most stable oil-in-water emulsion. Table 8: Relative Phospholipid Composition of Acetone Insoluble Mixture from Degumming of Canola and Soybean Oils
Oil Canola WDGc Canola PDGd Soybean WDG Soybean PDG
PC 32.5 39.2 32.5 29.1
PE 21.1 17.9 33.3 32.9
PI 15.2 12.6 17.3 16.4
LPCb 4.6 11.8 4.2 14.3
PA 3.2 1.1 3.5 1.8
PG+DPGb 23.4 17.5 9.2 5.6
Adapted from Smiles et al. (1989) LPC - Lysophosphatidyl choline; PG - Phosphatidyl glycerol; DPG - Diphosphatidyl glycerol c WDG - Water degummed oil d PDG - Phosphoric acid degummed oil a b
Sosulski et al. (1981) examined the fatty acid composition of the individual phospholipids in the LEAR varieties from winter rapeseed cultivars (Table 9). Smiles et al. (1988) found a similar fatty acid composition in phospholipids from canola oil, with the exception of slightly higher levels of linolenic acid (Table 9). Phosphatidyl choline contained the highest amount of unsaturated fatty acids, mostly oleic and linoleic acids. The other two phospholipids were rich in palmitic, linoleic and linolenic acids. The presence of highly unsaturated fatty acids in phospholipids is important as they can initiate oxidation of other fatty acids causing accelerated deterioration of the oil. It was reported that phospholipids have a tendency to complex heavy metals and in this form they are a rather stable catalyst which can initiate and stimulate oxidation (Pokorny, 1987).
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Table 9: Fatty Acid Composition of Phospholipids (w/w%) a
Phospholipid 16:0 16:1 18:0 18:1 Phosphatidyl Choline 8.7 0.8 1.2 55.8 Phosphatidyl Inositol 21.8 0.8 1.9 33.6 Phosphatidyl Ethanolamine 17.7 1.8 2.0 47.7 a Adapted from Sosulski et al. (1981) and Smiles et al. (1988)
Figure 8:
18:2 30.9 38.1 27.3
18:3 1.9 3.6 2.7
Structure of Plastochromanol-8 and Isomers of Tocopherol and Tocotrienol
20:1 0.5 0.5
Trace Elements The existing Codex standard for canola provides the maximum permitted levels for iron, copper, lead and arsenic. While these metals are found in other edible oils and are present naturally in the seed, they can be introduced during handling and processing. Diosady et al. (1983) and Elson et al. (1979) examined the effect of processing on trace elements in canola oils. Their results are summarized in Table 10. These oils were all of high quality with respect to cadmium and copper levels. It is clear from the data in Table 10 that processing reduces the amount of toxic and damaging trace elements, particularly lead, iron and sulfur. Iron in the oil acts as a catalyst which can initiate free radical oxidation of unsaturated fatty acids. Table 10: Mineral Element Content in Canola Oilsa (ppm)
Oil Sample Crude Oil Degummed with Water(WDG) Phosphoric Acid(PDG) Bleached WDG PDG Deodorized WDG PDG a
Phosphorus 1190.0
Iron 3.52
Calcium 296.0
Sulfur 6.5
Zinc 2.4
Lead 0.24
Isomers
R1 CH3
R2 CH3
CH3 H
H
γ
R3 CH3 CH3
δ
H
CH3 H
CH3 CH3
α
222.0 117.2
1.32 0.63
169.0 34.8
1.2 1.5
2.1 -
-
0.21 0.19
0.23 0.59
5.6 4.1
0.87
-
-
0.25 0.22
-
-
0.25 0.38
-
0.07
Adapted from Diosady et al. (1983) and Garrido et al. (1994).
As shown in Table 10, the levels of phosphorus and calcium are greatly reduced during processing. Sulfur in canola oil is in the form of organic compounds as the decomposition products of glucosinolates. Although these sulfur components occur in trace quantities, they are known to inhibit catalysts used for hydrogenation as well as impart characteristic odours to the oils. Recent developments in analytical methods for sulfur content evaluation revealed that soybean, sunflower, and even coconut oils all contain sulfur at a level of 2 - 10 ppm. In crude and RBD (refined, bleached and deodorized) canola oils the amounts of sulfur detected were 25 ppm and 9.4 ppm, respectively (Wijesundera et al. 1988; Ying and deMan, 1989). Sulfur components may negatively affect canola oil quality, but they also may improve the stability of the oil. Some sulfur components can act as antioxidants and protect the oil from autoxidation by complexing hydroperoxy radicals with the sulfur to form stable compounds. The other positive action of these compounds is inactivating catalysts involved in the oxidation process (Barnard et al., 1958).
Tocopherols The main nonsaponifiable components in vegetable oils are tocopherols and sterols, which are present in varying amounts depending on the oil. Tocopherols are natural antioxidants and their amount in the plant is probably governed by the content of unsaturated fatty acids. Tocopherols are present in different isomeric forms (Figure 8).
6
Tocopherol and Tocotrienol Isomers
β
Plastochromanol-8 is a derivative of γ- tocotrienol which has a longer side chain. This compound has been detected in canola and flax oils (Zambiazi, 1997).The tocopherol content in canola and some common vegetable oils is summarized in Table 11. Canola oil contains mostly two isomers of tocopherols, alpha and gamma, and the gamma isomer is normally present in higher amounts. Table 11: Tocopherol Contents in Selected Vegetable Oils (ppm)a
Oil HEAR Canola LLCanola HOCanola HOLLCanola Soybean Sunflower Corn LLFlax
α
β
γ
δ
P-8
268.0 272.1 149.8 226.3 285.8 116.0 613.0 134.0 25.8
0.1 34.0 17.0 18.0 -
426.0 423.2 313.6 201.6 607.2 737.0 18.9 412.0 212.6
7.1 2.7 8.2 275.0 39.0 9.2
96.8 74.8 46.5 42.2 82.5 129.3
Adapted from Zambiazi (1997) and Normand (1998). Abbreviations: HEAR - high erucic acid rapeseed; LLCanola - canola oil with low content of linolenic acid; HOCanola - canola oil with high content of oleic acid; HOLLCanola – canola oil with low linolenic acid and high oleic acid; LLFlax - flax oil with low content of linolenic acid; P-8 - Plastochromanol-8. The content of tocopherols in RBD oils is affected by processing, mainly by the extraction procedure and deodorization (Figure 9).
a
Figure 9:
Changes of Tocopherols During Processing. Continuous Deodorization at 245˚C and 3.5 mbar; Batch Deodorization at 180˚C and 3 mbar for 6 Hours. Mixed oil - Mixture of Solvent Extracted and Pressed Oils. Adapted from Willner et al. (1997)
Two of the major sterols (campesterol and sitosterol) are equally distributed in the esterified and free sterol fractions in canola oil (Figure 11B). Twice the amount of brassicasterol is found in the free sterols than in the esterified. The fatty acid distribution in the esterified sterol fraction differs from the fatty acid distribution of canola oil (Figure 11A). A higher palmitic and stearic acid content was observed in the esterified sterols. The total amount of sterols in rapeseed and canola oils ranges from 0.53 to 0.97%. The composition of major sterols in common vegetable oils is presented in Table 12, and their structures are presented in Figure 12. Figure 12: Structure of Phytosterols and Cholesterol
The lowest content of tocopherols was found in cold pressed canola oil. When temperature of pressing was increased, the amount of tocopherols doubled (Figure 9). Solvent extracted oils contain higher amounts of tocopherols than cold pressed oil, and similar amounts as oils from hot pressing. Refining decreased the tocopherol content of canola oil, and deodorization caused the removal of the largest portion of these compounds.
Sterols Sterols are present in canola oil in equal amounts in two forms, free and esterified (Ackman, 1983; Evershed et al., 1987). The composition of fatty acids and sterols in the esterified sterol fraction of canola oil is presented in Figure 10. Figure 10: Composition of Esterified Sterols in Canola Oil. A - Fatty Acids in Esterified Sterol; B - Sterol Composition. Adapted from Gordon et al. (1997)
Brassicasterol is one of the major sterols present in rapeseed and canola, and is also unique to these oils. This sterol is often used to determine the presence of rapeseed or canola oils in other oils (Strocchi, 1987; Ackman, 1990). Sterols are also affected by processing. Significant portions (up to 40%) of sterols are removed from the oil during deodorization. Refining also causes removal and isomerization of these compounds (Kochar, 1983; Marchio et al., 1987). Table 12: Proportions of Major Sterols in Selected Vegetable Oilsa (%).
Sterol Cholesterol Brassicasterol Campestrol Stigmasterol β-Sitosterol ∆5-Avenasterol ∆7-Avenasterol ∆7-Stigmasterol Total(mg/kg) Esterified(mg/kg)
Esterified Sterols Free Sterols a
HEAR CAN LLCAN 0.4 0.1 0.1 13.2 13.8 12.2 34.4 27.6 31.2 0.3 0.5 0.2 47.9 52.3 51.3 2.1 1.9 1.9 1.6 1.1 1.1 2.1 2.3 2.1 8810.0 6900.0 6326.0 4356.8 4231.5 3987.6
HOCAN 0.1 10.8 33.9 0.8 48.7 1.8 1.9 2.1 7102.0 4356.8
HOLLCAN 0.1 16.2 28.8 0.1 50.9 2.1 0.8 2.3 6892.3 4156.2
SOY 0.3 18.1 15.2 54.1 2.5 2.0 1.4 4600.0 576.4
SUN 0.1 7.5 7.5 58.2 4.0 4.0 7.1 4100.0 2068.8
Corn 0.1 17.2 6.3 60.3 10.5 1.1 1.8 9700.0 5654.8
Adapted from Ackman (1990), Strocchi (1987), Zambiazi (1997) and Gordon and Miller (1997)
The amount of total sterols in canola oil is about 50% higher than in soybean oil. Corn oil, which is produced from the corn seed embryo, contains the highest amount of sterols, or roughly two times that found in canola oil. The chemical structure of phytosterols is similar to that of cholesterol, so it is possible that these compounds are involved in oxida-
7
tive reactions (Figure 12). Recently, Przybylski and Eskin (1991) found plant sterol oxidation products formed during the storage of fried food products. Similar oxidation products have been found in soybean oil and wheat flour (Nourooz-Zadeh & Appelqvist, 1992). During the last few years more and more data is also being published showing the positive effect of plant sterols and their derivatives, particularly stanol esters, on human plasma cholesterol (Hendriks et al., 1999).
Pigments Pigments present in canola and other oilseeds are important factors as they can impart undesirable colour to vegetable oils, promote oxidation in the presence of light, and inhibit catalysts used for hydrogenation. A bleaching step is necessary during oil processing to remove chlorophyll-related pigments and other colour bodies. Changes in chlorophyll during canola oil processing are summarized in Table 13. During processing, chlorophyll completely decomposes to derivatives that are much harder to remove during bleaching. A bleaching test showed that pheophytin a and pyropheophytin a are more absorptive than their b isomers. Consequently, smaller amount of b isomers than a isomers are removed from the oil during bleaching. This necessitates the use of much higher amounts of bleaching activated earth in order to achieve similar removal of all chlorophyll derivatives (Suzuki and Nishioka, 1993). Table 13: Chlorophyll Pigments in Canola Oil During Processing (ppm) a
Oil After Expeller Extraction Expeller + Extraction Degumming Alkali Refining Bleaching
Chlor ab 6.27 1.88 1.79 0.27 0.22 -
Pheo ab 4.48 3.31 5.55 7.16 6.27 0.56
Pheo bb 1.79 1.34 1.34 1.07 1.12 0.32
Pyro ab 5.37 16.57 9.76 9.40 9.13 0.21
Pyro bb 0.67 3.13 1.43 1.84 1.79 0.25
Adapted from Suzuki and Nishioka (1993) Chlor a - Chlorophyll a; Pheo a - Pheophytin a; Pheo b - Pheophytin b; Pyro a Pyropheophytin a; Pyro b - Pyropheophytin b
a b
The type and content of chlorophyll present is dependent on the maturity of the seed. In fully matured seed only 2 ppm of chlorophyll was observed, while in physiologically matured seed (35 days before maturity) 1239 ppm was found. Also at maturity only chloropyll a and b were present while all possible isomers/derivatives were observed at other stages of maturation (Figure 12). These changes in the composition and content of chlorophylls can have a direct impact on the processing and quality of canola oil. Figure 12: Changes in Composition and Content of Chlorophylls During Canola Seed Maturation. Adapted from Ward et al. (1994)
In addition to chlorophyll pigments, carotenoids were also found in canola oil. Crude canola oil carotenes are reported to be at a level of 95 ppm, and are composed of 90% xanthophylls and 10% carotenes (Hazuka and Drozdowski, 1987).
Saponification Value Saponification value is defined as the weight of potassium hydroxide, in milligrams, needed to saponify one gram of fat. This parameter is inversely proportional to the molecular weight of the fat. Replacement of long chain fatty acids such as erucic acid (C22:1) in canola oil by eicosenoic (C18) fatty acids increased the saponification number from 168-181 to 188-192 due to the reduction in molecular weight.
Iodine Value Iodine value (IV) is an empirical test indicating the degree of unsaturation of fat or oil. It is defined as the number of grams of iodine absorbed by 100 grams of fat. An iodine value of 97 - 108 was reported for rapeseed oil with 45 % erucic acid, as compared to 110 - 126 for canola oil (Ackman, 1983). The higher value for canola oil is due in part to the replacement of erucic acid with oleic acid, along with an increase in linoleic and linolenic acids. Iodine value can also be calculated from fatty acid composition as proposed by AOCS Method Cd 1c-85 where the content as a percentage is multiplied by the characteristic factor for each unsaturated fatty acid.
Chemical Stability The stability of canola oil is limited mostly by the presence of linolenic acid, chlorophyll and its decomposition products, and other minor components with high chemical reactivity such as trace amount of fatty acids with more than three double bonds (Chapman et al., 1994). The presence of 7 to 11% of linolenic acid in the glycerides of canola oil places it in a similar category to soybean oil with respect to flavour and storage stability. The deterioration of flavour as the result of auto - and photooxidation of unsaturated fatty acids in oils and fats is referred to as oxidative rancidity.
Oxidative Rancidity Oxidation of unsaturated lipids produces components that behave as catalysts for this process, making it autocatalytic. Generally, oxidation occurs when oxygen is present in an oil or in the head-space above the oil. Solubility of oxygen in oil is about three to five times greater than in water. The amount of oxygen present in oil, dissolved during manipulation, is sufficient to oxidize the oil to a peroxide value of around 10 (Przybylski and Eskin, 1988; Labuza, 1971). The rate of oxidation of fats and oils is affected by many factors, including oxygen partial pressure, exposure to oxygen, the degree of unsaturation of fatty acids, the presence of light, temperature, and the presence of antioxidants and prooxidants such as copper, iron and pigments. Oil stability was best when iron and copper contents were below 0.1 and 0.02 ppm, respectively (Smouse, 1994). The phenomenon of flavour reversion is defined as the return to the flavour an oil had prior to deodorization. It does not appear to be related only to the oxidation of linolenic acid, as was originally thought (Smouse, 1994). This characteristic is distinct from rancidity, which is defined as the overall flavour defect noted when an oil first becomes oxidized. The mechanism of autoxidation is outlined in Figure 13.
2
8
48
463
906
Chlorophyll Content (mg/kg)
8
1239
Figure 13: Schematic of Radical and Photoxidation of Fatty Acids. Adapted from Chan (1987)
Figure 14: Formation of Oxidation Products from Unsaturated Fatty Acids. AUnsaturated Fatty Acids; B - Hydroperoxides; C - Non-volatile Products; D - Volatile Compounds (Off-flavour, Rancid). Adapted from Chan (1987)
Photooxidation
Initiation of this process comes about with the abstraction of a hydrogen atom adjacent to the double bond by an excited molecule of catalyser or pigment which received energy from light or heat to form a free radical. The free radical then combines with oxygen to form a peroxy radical, which abstracts a hydrogen from another unsaturated fatty acid to form a hydroperoxy radical. The hydroperoxy radical reacts with another fatty acid molecule to form hydroperoxide and another free radical. This free radical catalyses the reaction causing an autocatalytic effect. The primary reaction products are hydroperoxides, which decompose readily to form a range of secondary oxidation products (Figure 13). These include different carbonyl compounds such as unsaturated aldehydes, which possess strong disagreeable flavours and odours and have very low threshold levels (Hall and Andersson, 1983). The susceptibility of individual fatty acids to oxidation is dependent on their degree of unsaturation. Thus, the rate of oxidation of linolenic acid is 25 times higher than that of oleic acid and twice as fast as that of linoleic acid (Labuza, 1971). Oxidation products are formed according to the progress of this process (Figure 14). Primary products such as hydroperoxides appear first, along with the disappearance of unsaturated fatty acids. Secondary products, including non-volatile and volatile compounds, are produced as the result of primary product decomposition, and their presence can be detected after a certain period of time. The amount of hydroperoxides decreases with time as unsaturated fatty acids become oxidized and the decomposition process starts to be a dominating factor.
Light is an important factor that affects the flavour stability of vegetable oils such as canola and soybean which contain polyunsaturated fatty acids and traces of pigments or pigment derivatives. The degradation of oils and fats due to light exposure is primarily a photo-catalyzed oxidation. During photooxidation, singlet oxygen is generated by the transformation of energy from a light to a sensitizer. Singlet oxygen is an extremely reactive species of oxygen, and reacts with double bonds of unsaturated fatty acids to form peroxides or free radicals. Typical photosensitizers are chlorophylls and their decomposition products formed during maturation of seed and processing, heme compounds and polycyclic aromatic hydrocarbons (Pokorny, 1987). It has been found that chlorophyll degradation products are more active as photosensitizers than chlorophyll itself (Smouse, 1994). Another important factor in photooxidation is the colour of light or its wavelength. It has been established that shorter wavelengths of light, such as UV and blue, have more detrimental effects than longer wavelengths (Sattar et al., 1976). Exposure to light even for a short period of time initiates lipid oxidation, and each additional exposure further accelerates the deterioration of lipids (Chan, 1987). The presence of fluorescent light in many supermarkets, transmitting wavelengths between 350-750 nm, necessitates suitable packaging of oils to ensure that containers are impervious to low wavelength light. Tokarska et al. (1986) found light to be a critical factor in the development of off-flavours in canola oil and they strongly recommended that it be packaged in amber containers.
Antioxidants The role of antioxidants in retarding rancidity is well established, although the efficacy of some of them has recently been questioned. The best antioxidants are the natural components of canola oil such as tocopherols, in particular its γ-isomer (Figure 9). This isomer is present in processed canola oil in an amount twice higher than the alpha isomer (Table 11). Tocopherols are recognized as very effective natural antioxidants, but the isomers have varying antioxidant activity (Figure 16). The antioxidative activity of tocopherol and tocotrienol isomers is structure dependent. If a phenolic compound contains electron-releasing substituents in position ortho and/or para to the hydroxy group this increases the electron density of the active center. This combination
9
facilitates the hemolytic fission of the hydroxyl bond and makes tocopherol a good hydrogen donor, thus improving reactivity with peroxy radicals. α-tocopherol has methyl groups substituted at all positions, making it a very potent hydrogen donor and by structure the most potent antioxidant among all the tocopherol isomers (Figure 9). α-tocopherol has the highest biological activity (Figure 16). In food systems, antioxidant activity decreased in the following order: γ>δ>β>α (Figure 15). Figure 15: Biological (In vivo) and Antioxidant (In vitro) Activity of Tocopherols and Tocotrienols. Adapted from Kamal-Eldin and Appelquist (1996)
for vegetable oils. Hawrysh et al. (1992) showed that a mixture of PG and ascorbyl palmitate (AP) was also effective in the retardation of canola oil deterioration. Citric acid and its salts are used as chelating agents to deactivate metal catalysers present in the oils. Warner et al. (1989) reported that the use of citric acid did improve canola oil stability. The efficiency of antioxidants in inhibiting the development of heated room odors and delaying changes in heated canola oils has been examined. The use of antioxidants in the frying fat did not contribute to stability of the fried products (McMullen, 1988). Normand (1998) has shown that naturally occurring tocopherols need to be present at adequate levels in order to benefit frying oil stability.
In vivo Recent trends to find alternatives to synthetic food additives have caused the search for natural antioxidants to be intensified. Consumers may prefer natural food additives to synthetic compounds because they occur naturally in foods that have been consumed for centuries. Most natural antioxidants of plant origin are phenolic in nature.
In vitro
For years tea has been perceived as a source of antioxidants due to the relatively high content of phenolic compounds. Chen et al. (1996) tested the efficiency of ethanolic tea extracts on the oxidative stability of canola oil. Selected results are presented in Figure 16. Extracts from green, white and yellow tea were the most efficient in protecting from oxidation, while black tea extract was ineffective. Figure 16: Antioxidant Activity of Tea Extracts During Storage of Canola Oil at 100˚C Adapted from Chen et al. (1996)
Tocopherols are excellent antioxidants, about 250 times more effective than BHT (Burton and Ingold, 1989). These compounds seem to be the most efficient lipid antioxidants provided by nature. Lipid peroxy radicals react with tocopherols several magnitudes faster than with other lipids. Consequently, a single molecule of tocopherol can protect about 103 - 106 molecules of polyunsaturated fatty acids. This effectiveness explains why the ratio of tocopherol to PUFA in cells, usually 1:500, is sufficient to provide protection (Patterson, 1981). Tocopherols are also good singlet oxygen quenchers, but are less efficient than carotenoids. A single molecule of tocopherol can quench up to 120 molecules of singlet oxygen (Bowry and Stocker, 1993). Plastochromanol-8, a derivative of γ-tocotrienol but with a longer side chain, was detected in canola oil (Figure 9; Table 11). During evaluation of the antioxidant activity of tocopherols it was established that plastochromanol-8 and α-tocopherol have similar effectiveness in the oil (Zambiazi, 1997). The most commonly used synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG) and tertiary butylhydroquinone (TBHQ). These phenolic compounds react with free radicals to form relatively stable antioxidant free radicals that further degrade to produce quinones, thereby terminating the chain reaction of autoxidation. Hawrysh et al. (1988) found that BHT/BHA was not effective in promoting canola oil stability, while TBHQ substantially improved the stability of canola oil at levels as low as 100 ppm. Recently, TBHQ was approved in Canada as an antioxidant
10
Wanasundara and Shahidi (1994) evaluated the effectiveness of flavonoids as antioxidants, and found that some of these natural components were effective in protecting canola oil (Figure17). Flavonoids are a plant phenolic, and may be found in a variety of plant materials. In particular, quercetin and myricetin were more efficient than BHA/BHT.
Figure 17: Antioxidant Activity of Flavonoids During Storage of Canola Oil at 65˚C. Adapted from Wanasundara and Shahidi (1994)
Figure 18: Antioxidant Activity of Canola Meal Extracts During Storage of Canola Oil at 65˚C. Adapted from Wanasundara and Shahidi (1994)
Wanasundara and Shahidi (1994) also examined the effectiveness of ethanolic extracts produced from canola meal. When added at a level of 100 ppm, these extracts effectively prevented the oxidation of canola oil (Figure 18). The extracts were as effective as BHT and BHA, although they showed lower antioxidative activity than TBHQ. The authors also stated that the canola meal extracts did not impart colour and odour to Dr. Roman Przybylski is a Professor in the Dept. of Foods and Nutrition canola oil. Therefore, these and other extracts produced from plant origin at the University of Manitoba. material are a potential source of natural food antioxidants.
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