Showing report on Oils
The olive is the drupe from the olive tree (Olea europaea L., Oleaceae family). The Mediterranean region is the major olive growing area and accounts for 98% of the olive tree plantations in the world. The olive fruits are consumed as olive oil or table olives. About 90% of the annual olive production is used for olive oil. The olive fruit is harvested at the green stage (‘green olive’), or left to ripen to a dark purple colour (‘black olive’). The oil content of the olive fruit lies between 12-30%, depending on the variety.
Olive fruits are naturally bitter due to the presence of the polyphenol oleuropein, and therefore need to be processed before consumption. Traditional methods use the natural microflora on the fruit and procedures that bring about fermentation of the fruit. This fermentation leads to the leaching out and breakdown of oleuropein and phenolic compounds, the formation of lactic acid and of a complex of flavours. Three main processing methods are distinguished: the Spanish style, the Californian style, and the Greek style. The Spanish-style green olives are treated with sodium hydroxide (NaOH), and fermented in brine (sodium-chloride solution, NaCl) for several months. Californian-style olives are preserved in brine or acidified liquids during several months for fermentation to occur. Then, fruits are repeatedly treated with NaOH, and in between the alkaline treatments the olive fruits are suspended in water through which air is bubbled. Greek-style olives are not treated with NaOH. They are harvested when fully ripe, and fermented and canned in brine. There are a several thousand of different olive cultivars distinguished. Some economically important cultivars are: Manzanillo (world), Picual (Spain), Arbequina (Spain), Kalamata (Spain), Frantoio (Italy), Leccino (Italy), Koroneiki (Greece) and Pecholine (France).
Olive oil has become more and more popular as nutritional oil in the last decade, due to its taste and nutritional properties. It is often recommended for frying. In heating, it preserves largely its nutritional value, because of its antioxidants and its high levels of oleic acid. Whereas industrial classification distinguishes ‘(extra) virgin olive oil’ (physical extraction), ‘refined olive oil’ (chemically treated to improve taste), and ‘pomace olive oil’ (chemical extraction from olive pomace, the by-product remaining after physical extraction of the oil from the fruits), retail classification is different. In stores, olive oils are classified as ‘extra-virgin olive oil’ (first pressing), ‘virgin olive oil’, ‘pure olive oil’ (mixture of refined olive oil and (extra-) virgin olive oil), or ‘olive-pomace oil’ (blend of refined pomace olive oil and virgin olive oil). Currently, most data on polyphenols are available for (extra-)virgin olive oils.
Olives contain a great number of compounds of which some are typical for this food (hence the name of the compound oleuropein). Polyphenols in olives contribute to colour, taste and texture, and enhance auto-oxidative and thermo-oxidative stability of the oil (365). Strong correlations have been found between the dialdehydic forms of the oleuropein aglycones, ligstroside aglycones, and the aldehydic forms of oleuropein aglycones, and the bitterness intensity of virgin olive oil (366).
Total polyphenol contents, as measured by the Folin method, are 117 mg/100 g in black olives and 161 mg/100 g in green olives. For the analysis of polyphenol contents in olives, only data on olive flesh (the fruit without the inner stone) have been retained. In extra virgin and virgin olive oil total polyphenol contents are 55 and 21 mg/100 g. Olive fruits contain several types of polyphenols, mainly tyrosols, phenolic acids, flavonols and flavones, and for black olives, anthocyanins. The most abundant polyphenols are tyrosols, containing in their structure either tyrosol itself (2-(4-hydroxyphenyl)ethanol or p-HPEA) or hydroxytyrosol (2-(3,4-dihydroxyphenyl)ethanol or 3,4-DHPEA). These tyrosols are also secoiridoid glycosides, as the tyrosol moiety is most often esterified to a monoterpene of the secoiridoid class, the most common being elenolic acid or elenoic acid derivative; this secoiridoid is itself often glucosylated as in oleuropein, the most common tyrosol in olives. The contents of oleuropein and its aglycone are respectively 72 and 82 mg/100g in black olives, and 56 and 59 mg/100 g in green olives. Ligstroside, glucopyranosyl coupled to elenolic acid (oleoside) which is esterified to tyrosol, is present in young olive fruits, however decreases as the fruit develops (367). Only traces are present in black olive fruits (368). Free hydroxytyrosol is also a major polyphenol in olive fruits (66 and 56 mg/100 g in resp. black and green olives). Content values of demethyloleuropein are 23 and 13 mg/100 g in black and green olives respectively. Hydroxytyrosol-elenolate (3,4-DHPEA-EA), a derivative of oleuropein-aglycone, is present in amounts of 9 and 18 mg/100g in respectively black and green olives. Content values of tyrosol are 14 and 7 mg/100g in black and green olives respectively. Other tyrosol-derivatives detected in olives are hydroxytyrosol-1-glucoside and hydroxytyrosol-4-glucoside (369, 370), as well as oleoside-methylester. 3,4-DHPEA-EDA, the dialdehydic form of decarboxymethyl oleuropein aglycone, has been detected in olive fruits, but this compound and its comparable structures are more abundant in olive oils (see below) (368, 371).
Verbascoside, the caffeoylrhamnosylglucoside of hydroxytyrosol, is a phenolic acid derivative present in olives. Concentrations are 68 mg/100 g in black olives and 17 mg/100 g in green olives. Verbascoside levels depend on olive type and colour (372, 373, 374). Black and green olives also contain other phenolic acids: sinapic acid (10.8 and 44 mg/100 g respectively), m-coumaric acid (12.5 and 8 mg/100 g respectively), syringic acid (33.1 and 6 mg/100 g respectively) and o-coumaric acid (0.50 and 10 mg/100 g respectively). Other phenolic acids present in olive fruits are protocatechuic acid, p-coumaric acid, 4-hydroxybenzoic acid, p-hydroxyphenylpropanoic acid, 4-hydroxyphenylacetic acid, 3-methoxy-4-hydroxyphenylacetic acid, ferulic acid and caffeic acid.
Black olives contain the anthocyanins cyanidin 3-O-rutinoside and cyanidin 3-O-glucoside, in amounts of respectively 72 and 11 mg/100 g. The contents of flavonols vary among olive varieties. Quercetin 3-O-rutinoside values in black olives are 45 mg/100 g and quercetin 3-O-rhamnoside values are 4.1 mg/100g. Flavones are also present in olive fruits. Luteolin 7-O-glucoside is the most important one (15 mg/100 g in black olives). Luteolin, luteolin 6-C-glucoside, apigenin 7-O-glucoside, and apigenin 7-O-rutinoside have also been detected. Trace amounts of lignans are present in olive fruits. Two phenolic compounds identified in olives, but not quantified, are the methyl acetal of the aglycone of ligstroside, and the ß-hydroxytyrosol ester of methyl malate (375).
In olive oils, tyrosols are also the most abundant polyphenols. The major tyrosols in olive oil are: 3,4-DHPEA-EDA (hydroxytyrosol linked to a dialdehydic form of decarboxymethyl elenolic acid), 3,4-DHPEA-EA (monoaldehydic form of oleuropein aglycone), p-HPEA-EDA (dialdehydic form of decarboxymethyl elenolic acid) and p-HPEA-EA (monoaldehydic form of ligstroside aglycone). Because of keto-enolic tautomeric equilibria involving the ring opening of the secoiridoid, two aldehyde groups are present in the elenolic acid moiety of the oleuropein aglycone. The favoured structure of the decarboxymethyl aglycones is the dialdehydic form (3,4-DHPEA-EDA and p-HPEA-EDA), whereas the monoaldehydic form is favoured for oleuropein and ligstroside aglycones (3,4-DHPEA-EA and p-HPEA-EA) (376). During the crushing, kneading and extraction of olive fruits to obtain olive oil, the glycosidic oleuropein, demethyloleuropein and ligstroside are hydrolyzed by endogenous β-glucosidases, to form aldehydic aglycones. The aglycones become soluble in the oil phase, whereas the glycosides remain in the water phase.
The content values of 3,4-DHPEA-EDA, 3,4-DHPEA-EA, p-HPEA-EDA and p-HPEA-EA in extra virgin olive oil are 25, 7.2, 14 and 3.8 mg/100 g respectively. Tyrosol and hydroxytyrosol are present in minor amounts (1.1 and 0.8 mg/100 g respectively). Oleuropein, ligstroside, and the oleuropein- and ligstroside-aglycones have been detected in olive oil, as well as their dialdehydic aglycone forms, and the decarboxymethyl oleuropein-aglycone.
The lignans 1-acetoxypinoresinol and pinoresinol are present in olive oil (0.67 and 0.42 mg/100 g in extra virgin olive oil). The flavones apigenin and luteolin are also detected (1.2 and 0.4 mg/100 g in extra virgin olive oil). The phenolic acids p-coumaric acid, vanillic acid, 4-hydroxyphenylacetic acid, syringic acid, caffeic acid, cinnamic acid and ferulic acid are present in low amounts in olive oil. Also vanillin, protocatechuic acid, 4-hydroxybenzoic acid, 3-methoxy-4-hydroxyphenylacetic acid, and 3,4-dihydroxyphenylglycol have been measured in small amounts. In contrast to olive fruits, olive oil contains neither anthocyanins nor flavonols.
The polyphenol composition of olive fruits varies during fruit ripening. In the growth phase of the olive fruit the oleuropein content increases. In most varieties oleuropein levels are highest in the spotted olives just before they turn black, and then decrease in black olives. Anthocyanins are responsable for the colour of black olives (373, 377, 378, 379). Oleuropein content reaches concentrations up to 14% on a dry weight basis in young Picholine olives (378, 380). As oleuropein declines with fruit maturity, the demethyloleuropein content increases, however this is strongly variety-dependent (373, 380, 381). Verbascoside was not detectable in young olive fruits, but increases considerably during growing, before its levels fall sharply during ripening and maturation (378). Hydroxytyrosol contents increase during maturation, as well as tyrosol contents, correlating with the hydrolysis of the components with higher molecular weights (382). Olive ripening lasts several months and ripening processes vary according to growing area, variety, water availability, temperature, and farming practices (377). Polyphenol contents, especially of tyrosols, increase in water-deficit stressed olives (383, 384). Lignans, vanillic acid and vanillin however increase in more irrigated olive trees, compared to water-stressed olive trees (370).
Fermentation has a great impact on the polyphenol concentration, and final polyphenol composition depends on the type of fermentation used. During Spanish-style fermentation, hydrolysis of oleuropein into hydroxytyrosol and elenolic acid glycosides occurs through the treatment with NaOH. By leaving the olives in brine, hydroxytyrosol diffuses from the fruits into the brine solution. In the Californian way of fermentation, ripe olives are immediately stored in brine for fermentation to occur. Hydrolysis of oleuropein results in an increase of hydroxytyrosol. Because of the presence of acetic acid in the brines, hydroxytyrosol-acetate and tyrosol-acetate are formed. Cyanidin 3-O-glucoside and cyanidin 3-O-rutinoside contents decrease by about 95% during brining. After washing to remove the brining salts all compounds show a marked decrease (385, 386, 387, 388).
Different olive cultivars show very different polyphenol contents (374), but the polyphenol profiles are rather similar (389). Polyphenol composition of olive oils differs significantly from that of olive fruits due to selective effects of the process on the different polyphenols. Large proportions of water-soluble compounds are lost in the waste water during the extraction of the olive oil. Furthermore, during olive oil production modifications take place as a consequence of cellular destruction and the mixing of cellular contents, like the hydrolysis of glycosides. When comparing the different types of olive oils, the total polyphenol and tyrosol contents appear to be highest in extra-virgin olive oil, and then in descendent order in virgin olive oil and refined olive oil (390, 391). Olive-pomace oil and pure olive oil had a total polyphenol content respectively 15% and 30% lower than that of virgin olive oils. Apart from these differences in polyphenol content, the polyphenol profiles of the different olive oils are similar, although some minor differences have been reported such as the presence of 4-ethylphenol in olive-pomace oils but not in virgin olive oils (392).
The most common ways of domestic heating of olive oil are frying (180°C), microwave heating, and boiling with water. During heating of olive oil, changes in polyphenol composition occur. These changes depend on the way of heating. The frying of olive oil provokes a decrease in the contents of hydroxytyrosol, tyrosol and other tyrosol derivatives. Hydroxytyrosol, 3,4-DHPEA-EDA and 3,4-DHPEA-EA in virgin olive oil are reduced by 40-50% after 10 minutes frying at 180°C, and by about 90% after six frying operations. Tyrosol, p-HPEA-EDA and p-HPEA-EA however are much more stable. Their contents are only reduced by 20% during up to 12 frying operations. Lignan content remains largely constant (393, 394). Microwave heating provokes only minor losses of polyphenols. The boiling of olive oil and water results in the hydrolysis of the secoiridoid aglycones and the release of hydroxytyrosol and tyrosol. These simple phenols then migrate from the oil into the water phase. Also contents of these polyphenols strongly decrease in the oil. Hydroxytyrosol and tyrosol in the water phase were further degraded particularly at sufficiently acidic pH (pH 4 and 5) (394).
Storage of olive oil leads to an increase of hydroxytyrosol and tyrosol likely explained by the breakdown of secoiridoids, because the contents of 3,4-DHPEA-EDA, p-HPEA-EDA, 3,4-DHPEA-EA, and 3,4-DHPEA-AC decrease in the mean time (377). Total polyphenol contents are lower after storage of olive oil. Lignans are stable during storage.
The use of olive oil in cooking is very common in the Mediterranean area. When 60 g of virgin olive oil is used for the preparation of a 4-person meal, about 15g olive oil is consumed per person. This meal will provide about 9.6 mg of tyrosols per person, of which 2.7 mg 3,4-DHPEA-EDA.
Rape seed (Brassica napus L.) is a member of the family Brassicaceae. The closely related Brassica campestris is sometimes included within Brassica napus L.. Rapeseed is grown for the production of vegetable oil for human consumption. Processing of rape seed for oil production provides rapes eed animal meal as a by-product. The leading producers are Europe, Canada, the USA, Australia, China and India. In India, it is grown on 13% of the cropped land. According to the United States Department of Agriculture, rapeseed is the third most important oilseed crop in worldwide oilseed production.
The Brassica rapa variety is called turnip rapeseed. A winter and summer variant exist, sown respectively in late summer or early spring on Northern hemispheres. Yields, as well as fatty acid profiles differ between oil rape seed and turnip rapeseed. Rapeseed oil contains erucic acid, which is mildly toxic to humans in large doses. However, it is used as a food additive in minor doses. Canola, derived from ‘CANadian Oilseed, Low-Acid’ is the trade name for a rapeseed variety low in erucic acid. This variety is also called ‘Double low (00)’. A rather large amount of data exists on polyphenols in rapeseed meal. However, not much content data are available on polyphenols in the oil extracted from the rapeseeds. After the extraction of oil from the rape seeds, most polyphenols remain in the dry meal. Total polyphenol content in rape seed oil is about 18 mg/100 g, with large variations between trace amounts and 106 mg/100 g. Polyphenol content in turnip rape seed oil appears to be low (0.37 mg/100 g) (395).
The main polyphenols in rapes eed oil are 4-vinylsyringol, sinapine (O-sinapoylcholine) and sinapic acid. 4-Vinylsyringol is a decarboxylation product of sinapic acid, and content values are 14 mg/100 g rape seed oil. Contents of sinapic acid and sinapine in rape seed oil, measured without hydrolysis, are 0.83 and 0.64 mg/100 g respectively. After hydrolysis, sinapine is hydrolysed into sinapic acid.
It is not clear to what extent polyphenols present in rape seeds are transferred into the oil during oil extraction processes. Rape seed oil is usually expelled from the seed at high temperatures. Refining removes most of the non-triacylglycerol components, including many sinapic acid derivatives. Polyphenol contents decrease with an increasing degree of refining (396).
Sesame (Sesamum indicum L.) is an oilseed crop from the Pedaliaceae family grown in various tropical and subtropical regions of the world. Sesame seeds are very rich in oil, 50 percent of seed weight, compared to 20% of seed weight for soybeans. Sesame seeds exist in two basic types, white and black. The paler varieties of sesame tend to be more valued in the West and Middle East. The black varieties are of preference in the Far East. Sesame seeds are used in breads, bagels and buns, crackers and cakes. The bakery and confectionary industry usually uses dehulled sesame seeds. Also, sesame seeds are added to snacks and sushi foods. Ground and processed, the hulled sesame seeds are used for tahini (sesame paste) and halva (a sweet confection). Black sesame seeds are used for making the flavouring gomashio.
Sesame oil (with a pale yellow colour) is extracted from the sesame seeds for nutritional use. Salad oil is extracted by an expeller and refined by alkaline treatment, water washing, bleaching with acid clay, and lastly a deodorizing process. Sesame oil and seeds are used in salads. Oils made from roasted or unroasted sesame seeds are used for cooking. The total polyphenol content, as measured by the Folin method, is about 3 times higher in black sesame meal than in white sesame meal (397); this difference is more pronounced in the sesame hulls as,,like in cereal grains, sesame hulls contain higher concentrations of polyphenols than their endosperm.
The major polyphenols in sesame seeds are the lignans. The most abundant lignan is sesamin, with content values in sesame meal of 538 mg/100 g. Another major lignan in sesame is sesamolin (134 mg/ 100 g in sesame meal). Sesaminol, pinoresinol and matairesinol are present with content values of 103, 8.0 and 0.11 mg/ 100 g in sesame meal. Lignan glycosides are present in sesame meal and have been detected after basic hydrolysis (not able to release the aglycon from the glycosides), sesaminol triglucoside being the most abundant (68 mg/100 g in sesame meal). Furthermore, sesaminol diglucoside and sesaminol monoglucoside are found in sesame meals with contents of 11.6 and 8.3 mg/100 g respectively. Other minor lignans (detected after hydrolysis) are lariciresinol (11 mg/100 g), 7-hydroxymatairesinol (7.2 mg/100 g), medioresinol (4.2 mg/100 g) and todolactol A (2.5 mg/100 g). Trace values of other lignans have been found in sesame meal. Amongst them are secoisilariciresinol, iso-hydroxymatairesinol, cyclolariciresinol, conidendrin, syringaresinol and oxomatairesinol.
The oil content of white seeded sesame varieties is higher than that of black seeded varieties (398, 399). Sesame oil contains 421 mg/100 g sesamin. Sesaminol and sesamolin contents are 305 and 243 mg/100 g oil respectively. Sesamolinol is found in lower amounts in oil (56 mg/100 g). Other lignans, episesamin, episesaminol and sesamol, have also been detected in sesame oils. Next to lignans, sesame contains phenolic acids. Both ester and insoluble-bound forms of caffeic acid, p-coumaric acid and ferulic acid are found in sesame meal, the ester forms being more abundant. The proportion of esterified to insoluble bound phenolic acids in defatted sesame meal (DW) is 4:1 (341). Roasted sesame oil contains minor amounts of benzaldehyde. Volatile compounds in roasted sesame seeds are guaiacol, 3-acetylanisole, acetophenone, benzenemethanol and 2-methoxy-5-(1-propenyl)phenol. Oil extracted from light-roasted sesame seeds is richer in these volatile compounds than oil extracted from deep-roasted seeds.
Black sesame seeds are richer in sesaminol than white sesame seeds. Conversely, white sesame seeds are richer in sesamin compared to black sesame seeds. No difference was found between black and white sesame seeds in sesamolin and sesamolinol contents (398).
During the processing of sesame oil the sesaminol content increases. This occurs especially during the bleaching process (400). Furthermore, the heating of sesame seeds to produce oil, and its storage, leads to degradation of sesamolin, and in minor amounts sesamin. This effect is more pronounced in oil from dehulled sesame seeds compared to oil from hulled sesame seeds (401). In use as a cooking oil, lignan contents remain unchanged after short heating (<4 min.). After prolonged heating (>20 min.) sesamol contents increase and sesamolin and sesamol contents decrease (402).
Sunflower (Helianthus annuus) belongs to the Asteraceae family. Russia. Argentina, the European Union, China, India, Turkey and South Africa are all significant producers of sunflower. There are two types of sunflowers. The first is the oilseed type. It has small black seeds that are high in oil content and processed into sunflower oil and meal. The second type is the confectionery sunflower. This is a larger black and white striped seed. Sunflower whole seeds still contain their hull. They are sold as a snack food after roasting. The edible kernel is obtained by (mechanical) removal of the hull. Sunflower oil, extracted from the seeds, is used for cooking. There are three types of sunflower oil depending on their fatty acid profiles: linoleic (the original sunflower oil), ‘mid-oleic’ sunflower oil and ‘high oleic’ sunflower oil.
Not much data on polyphenol contents in sunflower seeds and oils are yet available. Sunflower oil was found to contain 1 mg/100 g of total polyphenols (Folin assay) (single data source).
More data are available on the defatted dry meal of sunflower seeds. Sunflower seeds contain phenolic acids in free, esterified and insoluble-bound forms. Esterified forms are much more abundant than insoluble-bound forms. The major phenolic acid is caffeic acid. 4-Hydroxybenzoic acid, ferulic acid and p-coumaric acid are also present. Minor amounts of syringic acid and vanillic acid have also been detected.
No lignans were detected in sunflower oil. However, matairesinol, lariciresinol, pinoresinol and secoisolariciresinol have been detected in sunflower seeds.