Mustards are several plant species in the genera Brassica and Sinapis. The most common types of mustard are Sinapis alba (also called Brassica alba, yellow mustard, or white mustard), a plant of Mediterranean origin; and Brassica juncea (also called brown mustard or Indian mustard), which is of Himalayan origin. Black mustard (Brassica nigra) fell out of use in commercial mustard products in the 1950s because it was unsuitable for mechanical harvesting.
Mustard seed in its various species of white, brown and black is a close relative of rape seed. It has been favoured for centuries in areas such as India and Pakistan while rapeseed and canola is mainly grown in Canada  .
Mustard seed oil is a very popular oil seed today used in many countries mostly Asians for different application besides its use as cooking oil mostly in India and elsewhere.
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Traditionally, mustard or mustard oil have been used as a treatment for gastrointestinal disorders, as a natural antimicrobial to eliminate food borne bacteria and pathogens, by diabetics, as an emetic, and as a massage oil to improve blood circulation, muscular development, and skin texture. Mustard plaster (a mixture of flour and mustard powder) has been traditionally applied to the chest and abdomen to promote healing  as reported based on a thorough systematic review of the available scientific articles.
Many people use mustard as spices and for sauces, especially in western countries where it is included in hot dog and hamburger for preparation of relish or spice to enhance flavour and enjoyment. The spicy background flavour of mustard seed oil is appreciated by many of Asian countries. Mustard seed is used for edible oil extraction and is a good source of unsaturated fatty acids. However, due to its unsaturated fatty acids, the oil extracted from mustard seed is less stable to oxidation. Amongst oilseeds, mustard seeds, having different varieties, are rich in polyphenols most of which act as anti oxidants, contain tocopherols and carotenoids. A typical Brassica plants compounds, glucosinolates are also present in the seeds. However, upon extraction, very small proportion of these polyphenols is extracted with the oil by the commercial extraction method and they are further reduced by refining1  .
Mustard plant contains broad quantities of chemical components. Chemical composition of leaf and root of Brassica Juncea, brown mustard, is given in the table below.
Carotenes (ï¢ carotene equivalent)(mg)
Source: Source: Perdue University, centre for new crops and plants products.
Mustard seed unlike the leaf and root part of the plant contains substantial amounts of sterols, phenolics, enzymes, fatty acids glycerides and other minor volatile compounds besides those chemical compositions listed in the table above. The mustard seed (Brassica Juncea) composition of the various compounds is depicted in the table below. These compounds are responsible for different characteristics of the mustard seed oil such as taste and odour. Mustard oil is a pungent plant extract from mustard seed and Isothiocyanate compounds are responsible for the mustard's pungency, the main constituent of which is allyl isothiocyanate and Sinigrin is also responsible for the bitter taste of black mustard  . The odour of the oil extracted from the seed is thus due to the various volatile components present in mustard seed.
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Mustard seed composition
9.1% of this is esterified
34.0% of this is esterified
55.2% of this is esterified
D-5 avensterol (%)
These are esterified form
Present in traces
Sinigrin (potassium myronate)
Sinigrin hydrolysis by myrosinase gives allyl isothiocynate, glucose and potassium bisulfate
Allyl isothiocynate is volatile and is about 0.25- 1.4% from mustard seed (B.juncea)
hydrolyses of sinigrin
sinapine is sinapic acid choline ester
erucic, oleic, eicosenoic, arachidic, nonadecanoic, behenic, and palmitic acids
Globulins in seed
Minor volatile components
methyl, isopropyl, sec-butyl, butyl, 3-butenyl, 4-pentenyl, phenyl, 3-methylthiopropyl, benzyl, and b-phenylethyl isothiocyanates
Resulted from enzymatic hydrolysis in the seed
Source: Perdue University, centre for new crops and plants products.
In countries like Nepal, after mechanical pressing of the seed to expel oil, the remaining cake is taken for roasting at 160ï‚°C for about 30minutes with the aim of enhancing the flavor of the final oil. However, this treatment will cause the enzymatic inactivation and other chemical or biochemical changes as well.
The objective of this study is thus to evaluate the oxidative stability of mustard seed oil.
Food Lipid oxidation
Lipid oxidation is the reaction between oxygen and unsaturated fatty acids. Polyunsaturated fatty acids, such as lionoleic, linolenic, and arachidonic acid, are especially susceptible to oxidation. Lipid oxidation causes losses of nutritional and sensorial quality of food lipids. As lipids oxidize, they form hydroperoxides, which are susceptible to further oxidation or decomposition to secondary reaction products such as aldehydes, ketones, acids and alcohols. In many cases, these compounds adversely affect flavor, aroma, taste, nutritional value and overall quality. Production of off-flavour low molecular compounds and destruction of essential nutrients leads to unacceptability of the product for consumers as well.
Moreover, the safety of the oxidized oil and fat containing foods as a result of some toxic compounds formation from the lipid oxidation could become unacceptable. These autoxidation reactions lead to the breakdown of lipid and to the formation of a wide array of oxidation products. The nature and proportion of these products can vary widely between foods and depend on the composition of the food as well as numerous environmental factors the former major factor being the degree of unsaturation of the food lipid while the latter is related with the availability of oxygen, temperature of storage and sunlight exposure. The toxicological significance of lipid oxidation in foods is complicated by interactions of secondary lipid oxidation products with other food components. These interactions could either form complexes that limit the bioavailability of lipid breakdown products or can lead to the formation of toxic products derived from non-lipid sources. On the other hand, secondary products of lipid autoxidation can be absorbed and may cause an increase in oxidative stress and deleterious changes in lipoprotein and platelet metabolism.  Many lipid oxidation by-products been regarded as potential carcinogens and the hydroperoxides are known to damage the DNA and epoxides and hydrogen peroxide by products are also known potential carcinogens.
There are two forms of oxygen which can be involved in lipid oxidation; triplet (3O2) and singlet (1O2). Triplet oxygen is the common and stable form of oxygen. Lipid oxidation by 3O2 occurs in three main stages; initiation, propagation, and termination. During the initiation stage an alkyl free radical is formed from the fatty acid via the removal of a hydrogen molecule.
Singlet oxygen, on the other hand is a high energy, non-radical compound. Singlet oxygen can be created through several different reactions the primary mechanism of its creation in food system being food photosensitization. Photosensitization is a process that involves the excitation of photosensitizers by light energy. When exposed to light, of a specific wavelength, a photosensitizer moves from its ground singlet state to an excited singlet state. Once in the excited singlet state the photosensitizer will attempt to return to its unexcited ground state via internal conversion, emission of fluorescent light, or intersystem crossing  . Internal conversion is the transformation from one excited state to another of the same spin state, resulting in a loss of heat energy. Intersystem crossing is the conversion of an excited singlet state to an excited triplet state. This excited triplet state photosensitizer is reactive with triplet oxygen and produce singlet oxygen, transferring energey themselves being returned to ground state. Therefore, it is this excited triplet state photosensitizer that can cause the photosensitized oxidation of food lipids starting from its reactivity with the triplet oxygen  .
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Tocopherol content of plant oils as determined by HPLC (mg/kg).
*Mean values of total tocopherol content together with standard deviations
Source: Anna GliszczyÅ„ska-ÅšwigÅ‚o, 2007. Tocopherol Content in Edible Plant Oils, Polish journal of Food and Nutrition Sciences
Antioxidants are natural or synthetic compounds that are used for the inhibition of lipid food oxidation and can also be formed during processing. These are substances that are varying widely in chemical structure and possessing different diverse mechanism of action, the most important mechanism being their reaction with lipid free radicals leading to inactive product formation  . These substances delay or inhibit the oxidation at very low concentration compared to the higher contents of lipids and proteins present foods. Replacement of the synthetic antioxidants with natural ones getting attention of scholars for some food applications with could be in response of the consumers' suspicion of the synthetic antioxidants potential effects on their health.
Oilseeds which are consumed directly as such or after roasting may be added to foods in order to increase their stability against oxidation. Antioxidants derived from oilseeds contain both less polar antioxidants soluble in the oil phase, and more polar antioxidants, better soluble in the aqueous phase. The liposoluble antioxidants are extracted in crude oil during oilseed processing, and they are partially recovered in deodorisation sludges. More polar antioxidants remain in expeller cakes or extracted meal, which may be also used as food additives to increase the oxidative stability of foods. Oilseed meal extracted with hydrocarbons may be subsequently extracted with more polar organic solvents to obtain concentrates of phenolic substances, mainly phenolic acids, lignans or flavonoids. These are more active antioxidants, but also more expensive. Pure isolated antioxidants from oilseeds should be tested for their safety. The application should be considered from several aspects, such as antioxidant activity, safety, availability, effect on sensory value, and price 
Phenolic compounds are common in most oilseeds. The content of phenolics in rapeseed/canola is much higher than in other oilseeds, about ten times that in peanut and cottonseed and about 30 times that in soybean  . The most important phenolic compound in rapeseed/canola seeds is sinapine, the choline ester of sinapic acid, which represents from 5.0 to 17.7 g-kg-1 total seed weight  .
Rapeseed contains more phenolic compounds than any other oilseed plant. Mustard seed, being a close variey of rapeseed therefore comprises similar composition of phenolics. The most significant of these are sinapic acid and its derivatives. Sinapic acid however is bound to choline and forms sinapine as a result of which its antioxidative activity is restricted. Mustard seed contains similar antioxidants as rapeseed  .
Antioxidants identified in oilseed extracted meals of cakes
Substances with antioxidant activities
sinapine, benzoic and cinnamic acid derivatives, phenolic acid esters and glycosides
sinapine, esters of phenolic acids
syringic, vanillic, ferulic, salicylic, p-coumaric acids and esters, chlorogenic, caffeic,sinapicacids,isoflavonesandtheirglucosides
phenolic acids and esters, such as p-hydroxybenzoic, p-coumaric, syringic, ferulic, caffeicacids
chlorogenic, caffeic,p-hydroxybenzoic, p-coumaric, cinnamic, m-hydroxy-benzoic, vanillic, syringic, gallic, and vanillic acids, epicatechin, catechin
Evening primrose seed
proanthocyanidines and their gallates, isoflavones
sinapic, p-hydroxybenzoic, coumaric, ferulic acids, lignans and their glucosides
sinapic, ferulic, p-hydroxybenzoic acids, quercetin, rutin
lignans, coumaric, ferulic, vanillic, sinapic acids
Olive fruits, cakes
hydroxytyrosol, secoiridoids, flavonoids,lignans
catechin, epicatechin, procyanidin
Source: Collected from different literatures and tabulated by Stefan Schmidt and Jan Pokorny, 2005: Potential Application of Oilseeds as Sources of Antioxidants for Food Lipids,
Canolol or DMVP (4-Vinyl 2, 6-Dimethoxy Phenol) is a compound whose antioxidative activity was observed in different variety of mustard seeds oil which is extracted after the seed is roasted, canola being the first seed oil from which it was identified  .
Fig: Formation of DMVP by thermal decarboxylation of sinapic acid.
Tocopherols are the unsaponifiable components present in varying concentration in different oil seeds or vegetable oils and are known to be natural antioxidants. Tocopherols are synthesized only by plants and are very important dietary nutrients. These are monophenolic compounds existing in different isomers or homologues varying in molecular structures and biological effectiveness. Their high solubility in oil makes them very important antioxidants in edible oils. Tocopherols have lipophilic character and thus they are found in the membranes of tissues or they are located certainly in the lipid storage tissues. This location of these compounds enables them to be immediately available for the lipid hydroperoxides in non biological systems like food systems. The tocopherols (Î±, Î², Î³, and Î´), which are commonly known as vitamin E, have been showed to have antioxidant activity and thus are potent antioxidants which prevents lipids from oxidation by scavenging the free radical species of unsaturated lipids.
The loss of tocopherols under light is mainly due to singlet oxygen oxidation. The study of H.J. KIM etal revealed that the oxidation of tocopherols showed that the losses of equal concentration of Î±-, Î³-, and Î´-tocopherols under light were higher for alpha followed by gamma tocopherol and the delta tocopherol was found to have less loss up on oxidation. The degradation of Î±-, Î³-, and Î´-tocopherols was undetectable in the absence of chlorophyll under light or in the presence of chlorophyll in dark.
Tocopherols are by far efficient antioxidant than BHT (butylated hydroxytoluene) and the peroxy radicals react with tocopherols than other lipids as a result of which can a single tocopherol molecule protects about 103 to 106molecules of PUFA. Gunstone also explains that the high potency of tocopherols as antioxidants arises from their ability to be transformed from the oxidized form back into the active structure by other molecules such as ascorbic acid citing Tapel, 1968. Tocopherols exist in different isomers, ï¡, ï¢, ï§ and ï¤ tocopherols all of them having different degree of antioxidative potency. Literatures indicate that the antioxidative activity of these isomers decreases in the above order from ï¡ to ï¤ tocopherol  .
The high antioxidative potency of ï¡-tocopherol could be explained by its bond dissociation energy (BDE) which is a measure of a bond strength in a chemical bond. This energy depends on the strength of the O-H strength of the phenolic antioxidant and indicates the lowest BDE for easy donation of hydrogen to lipid peroxy radical by cleaving the O-H bond. ï¡-tocopherol, when compared with the other isomers of tocopherols, has low bond dissociation energy (75.8kcal/mol) while ï¢, ï§ and ï¤ tocopherols have 77.7, 78.2, and 79.8 kcal/mol, respectively. As it can be seen from the chemical structures of the tocopherols isomers, ï¡, ï¢, ï§ and ï¤ tocopherols are methylated at three, two and one position. Therefore, ï¡-tocopherol is the more sterically hindered than the other three as a result of which it is most antioxidant for scavenging the lipid peroxy radicals by easily donating hydrogen atoms  .
Î±- tocopherol Î²-tocopherol
CH2[CH2CH2CH(CH3)CH2]3H CH2[CH2CH2CH(CH3)CH2]3H O O
Î³- tocopherol Î´-tocopherol
Chemical structures of tocopherols and tocotrienol
The antioxidative property of tocopherols also depends on the concentration. For instance, the ï¡ tocopherols activity is much higher in decreasing oxidation than ï§ tocopherols at a concentration of 200 ppm above which its activity declines  . Above this concentration thus, ï¡-tocopherol would act as prooxidant whose probable reason was suggested by Akoh and Min assuming the higher the concentration of tocopherols in lipid would result higher amounts of intermediate radicals formation from the oxidation of tocopherols during the oil storage.
At higher concentration, alpha-tocopherol may produce more alpha-tocopherol peroxy radical, alpha-tocopherol oxy radical, alpha-tocopherolquinone oxy radical, hydroxy radical, and singlet oxygen and promote the oxidation of oils. The prooxidant mechanisms of oxidized alpha-tocopherol is thus presumed to be resulted because of alpha-tocopherol peroxy radical, alpha-tocopherol oxy radical, alpha-tocopherolquinone oxy radical, hydroxy radical, and singlet oxygen. These species are formed during the oxidation of the alpha tocopherol as the food comprising these molecules are kept in storage. It is therefore presumed that these oxidized species of alpha tocopherols, a molecule in which the polar hydroxyl and non polar hydrocarbons are attached, may impart the oxidation of oil by reducing the surface tension of oil as a result of these non polar and polar groups of the molecule and increasing the diffusion of oxygen from air to oil  . Oxidized lipids with hydroxyl and or carbonyl groups were reported to be less soluble in the soybean oil and moved to the surface of oil. The oxidized oils having polar and nonpolar groups decreased the surface tension between air and oil and increased the transportation of oxygen from air to oil  .
Â·O 5 4a 4 O O
OO O O O
O O OÂ·
âˆ’Â· OH O Â·
Â· OO O
Î±-Tocopherol hydroperoxide O O O
âˆ’ Â· OH Â· O O Â· O
O O2 O2
OO OÂ· O
OO O O O O O O
Â·O O O RH
HO O O
âˆ’ Â· OH
OO O O
OOÂ· O OÂ·
O O O
4a,5-Epoxy-Î±-tocopherolquinone 7,8-Epoxy Î± tocopherolquinone
Possible mechanism for the formation of alpha-tocopherolquinone (A), 4a,5 epoxy-alpha tocopherolquinone (B), and 7,8-epoxy-alpha-tocopherolquinone (C) from alpha tocopheryl radical with triplet oxygen.
Amount of tocopherols in final oil is influenced by the processing operations. During oil extraction, deodorization and refining, the concentration of these compounds decreases significantly. In canola oil, the cold pressed canola oil was reported to have lowest tocopherols content where as pressing at increased temperature doubled the content of tocopherols  .
Roasting mustard seeds prior to oil extraction and thereafter using the oil for frying has been practiced in Nepal from long time. Seed roasting before oil extraction gives it a characteristic good flavor and improves the stability of the oil. The former could be due to the formation of some of the maillard reaction products and the later due to decarboxylation of the sinapic acid, a phenolic acid giving rise to the formation of canolol. However, the responsible maillard reaction product compounds for the antioxidant activity have not been clearly determined to date  and the mechanisms of action are unknown. Because of the complexity of the Maillard reaction, the vast number and variety of Maillard Reaction Products (MRPs), and the diversity of the model systems that can be studied, has proved difficulty of identification of the compounds responsible for antioxidant activity  . Roasting gives rise to change in colour of the oil that is extracted from the roasted seed of the vegetable oil seeds. This of course could contribute to the formation of the brown colour compounds whose identification and manner of action in the oxidative stability relationships is complex. In a study conducted upon sunflower roasting by microwave, the colour of the oil extracted from the microwave roasted sunflower was found to be changed from the light yellow to yellow after which it was changed to brown upon elongated roasting time indicating the formation of the brown compounds, most likely maillard reaction products.
Roasting effect on tocopherol content
Tocopherols are known in foods, particularly in edible oils for the antioxidative properties. This property however is also dependent of temperature. In many of the literatures, it has been reported that at high temperatures, the loss of these compounds, even if variation exist among the homologues, is faster as the degree of oil unsaturation decreases. At low temperatures however, this trend not reflected indicating that the mechanism of the tocopherol change and their action is dependent of the temperature. With respect to the relative stability of alpha-, beta-, gamma- and delta-tocopherol at high temperature, there is full agreement that alpha-tocopherol is less stable than delta-tocopherol, while beta- and gamma-tocopherol degrade at an intermediate rate.
Tocopherol content (mg kg-1) of initial natural oils
ND, not detected.
Roasting of oil seeds and vegetable oils prior to extraction of the oil gives change in tocopherol content in different extent. Tocotrienols are also studied in different oils whether the change is observed before and after roasting of the oil seed as this is common practice of some countries where the need of the roasted seed oil extraction is popular. However, since the tocopherols and tocotrienols were found in different oil seeds at varying quantity and isomers, the change resulted from the roasting of different oil seeds could give different pattern of respective isomers as well. The tocopherols or tocotrienols isomers in oil seeds are ï¡, ï¢, ï§ and ï¤ tocopherols or tocotrienols. The content of these isomers vary in oils. Mustard seeds contain ï¡, ï§ and ï¤ tocopherols, gamma tocopherol making up the most abundant followed by alpha tocopherol while the delta tocopherol is found in mustard seeds with low concentration. The beta tocopherol is absent in these seeds  . The heat stability of these isomers however is higher for delta tocopherol than alpha tocopherol the gamma tocopherol being less stable to heat which also shows that among the tocopherol isomers, delta-tocopherol degradation rate is lowest and alpha- and gamma-tocopherols are more heat-sensitive as indicated by the same study. The stability of delta tocopherol could be explained partly by its high bond dissociation energy for O-H in comparison to the alpha and gamma tocopherols as was reported by DiLabio etal  .
Canola can be definied as a rapeseed cultivar that contains less than two percent of erucic acid in oil and less than 30ïmol/g of glucosinolates in its defatted meal. In the seed oil of canola, mainly alpha and gamma tocopherol are found with an alpha to gamma tocopherol ratio of about 0.5. In canola tocopherol proportions of 65% gamma tocopherol and 35% of alpha tocopherols are commonly found in the seed oil. Amounts of delta tocopherol are less than one and beta tocopherol is absent  . Roasting of the canola seed at temperature of 165oC for five minutes does not show significant increase as is reported by Wakamatsu etal. However, the derivative of a phenolic Sinapic Acid, Canolol, was found to be increased by ten up to 100 fold increase at this treatment.
Among the isomer homologues of tocopherols and tocotrienols, alpha, beta and gamma tocopherols are found in safflower as well as gamma and delta tocotrienols, the most abundant homologue being alpha tocopherol. However, no delta tocopherol and alpha and beta tocotrienols are found in the seeds  . The study of In-Hwan Kim etal reported that the tochopherol homologues are gradually increased by roasting as the temperature of roasting increases all the three tocopherol homologues showing the same trend. The tocotrienols however, shows no difference. Contents of alpha tocopherol in safflower oil increased as roasting temperature increased up to 160oC but further increase of temperature up to 180oC decreases the content. 
Sesame seeds oil contains less amount of tocopherol than other vegetable oils. The only tocopherol homologues present in the sesame seed is gamma tocopherol. Others are not detected in these seed oil. The prevalent constituent of fresh sesame seed is gamma tocopherol where as the sesamol, hydrolysis product of sesamolin, dominates after 1 to 2 hours of heating at frying temperature  . Roasting of sesame seed was reported in the same study to increase the gamma tocopherol content nearly two fold in refined sesame oil as well as in the crude sesame oil the increase is still significantly high. However, the reason and mechanism is rarely clear. Roasting also results in the formation of sesamol, the hydrolysis product of sesamolin which is a major lignin found in the sesame seeds. 
The tocopherol content of sunflowers is different in homologues composition. Alpha tocopherol is abundant and the delta tocopherol is found in small quantity while the gamma tocopherol is nearly undetectable  . By roasting of sunflower seeds the tocopherol contents change. Alpha tocopherol on microwave oven roasting gives significant reduction while delta tocopherols were also reported to be destroyed after 15 minutes of roasting. The alpha tocopherol thus shows rapid and high rate of loss up on microwave roasting. This trend of tocopherol loss is also found in other plant oils like soybean oil.  ,