Conclusiones

CEREAL GRAINS AND THEIR PRODUCTS

As mentioned before, tocopherols and tocotrienols are easily oxidized in the presence of light and metals and at high temperature or at alkaline pH, and they are sensitive to ionizing radiation, but tocopheryl esters are much more stable than tocopherols.

1.7.1 S^TORAGE S^{TABILITY IN} O^{ILSEEDS AND} C^EREAL G^RAINS

Tocopherols and tocotrienols in intact grains are relatively stable under proper storage conditions. For example, tocopherols in rapeseeds were stable during storage at 40°C for 24 weeks in low availability of oxygen, but some degradation occurred in open containers (Goffman and Möllers, 2000). Corn grains stored in the dark at room temperature lost only 5% of their total tocopherols in six months (Weber, 1987).

1.7.2 P^{RODUCTION OF} V^EGETABLE O^ILS

Refining of vegetable oils is a process during which phospholipids, free fatty acids, oxidized lipids, most prooxidants and impurities in crude oils are reduced as much as possible in order to produce oils with good oxidation stability and sensory quality

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(Young et al., 1994). Unfortunately, tocopherols and tocotrienols that have a positive effect on the end product are also lost to some extent. The refining processes are divided into chemical and physical ones. In the chemical process, free fatty acids are neutralized using an alkaline reagent, and the soaps thus formed are removed by washing. In the physical process, free fatty acids are distilled off. Other important unit processes in refining are degumming, bleaching and deodorizing. In physical refining, the distillation stage in deodorization is especially important, because it should remove more free fatty acids than in chemical refining. The source of oil to be refined and the quality of raw material determine which processing conditions are needed. Because chemical and physical parameters in each stage may vary considerably, there are differences in losses of tocopherols and tocotrienols.

When crude soybean oil was refined in a commercial refinery, the process removed 31.8% of total tocopherols (Jung et al., 1989). The effects of degumming, alkaline refining and bleaching were small, 5.4%, 2.0% and 4.8%, respectively, compared to deodorization, during which 19.7% of initial tocopherols were lost.

Similarly, degumming and alkaline refining had only minor effects on the total tocopherol content of rapeseed and linseed oils, whereas deodorization of rapeseed oil decreased it by 12% (Schöne et al., 1998). Greater losses of tocopherols were found in refining rapeseed press oil at laboratory scale; namely, 20% and 40%, on degumming stage and on bleaching stage, respectively (Prior et al., 1991). Fewer tocopherols were lost during bleaching, using bleaching medium with a lot of acid sites for purifying alkali-refined vegetable oils, than when standard active clays were used. For example, for rapeseed oil, the decreases were 7% and 44% for acid and standard clay bleaching, respectively (Boki et al., 1992). Great losses of tocopherols and tocotrienols occur during refining of rice bran oil, because it contains up to 20%

of free fatty acids and a high amount of unsaponifiable matter and thus needs a thorough process. Total loss of tocopherols and tocotrienols of physically refined rice bran oil after a combined degumming-dewaxing pretreatment compared to crude oil was >75% and was markedly increased by increasing the degumming-dewaxing temperature from 10°C to 35°C (De and Bhattacharyya, 1998).

The relative composition of different vitamers remained constant during refining of soybean oil (Jung et al., 1989) and rapeseed oil (Schöne et al., 1998), but in one study, α-tocopherol was selectively adsorbed on the bleaching clay (Prior et al., 1991).

1.7.3 PROCESSINGOF CEREAL AND OIL GRAINS

When flour for baking bread is manufactured, the germ and some of the outer layers are removed, and the levels of tocopherols and tocotrienols together with many other vitamins are reduced. By removing germ fraction, significant amounts of tocopherols are lost, because the germ, in general, is especially rich in E-vitamers.

Bran fractions contain more tocotrienols than the germ and endosperm fractions (Barnes, 1983) (Table 1.4).

Any processing of grains subjects tocopherols and tocotrienols to oxidation.

When linseeds and rapeseeds to be used for poultry feeding were crushed and stored either at room temperature or in a cold room, a 50% reduction in the tocopherol content occurred in 30 days (Gopalakrishnan et al., 1996).

Tocopherols and Tocotrienols from Oil and Cereal Grains 23

Tocopherols and tocotrienols are sensitive to irradiation. To reduce the effects of irradiation, it should be performed at very low temperatures and in the absence of oxygen (Ottaway, 1993). They are also sensitive to extrusion cooking, steam-flaking, autoclaving and drum-drying (Jägerstad and Håkansson, 1988; O’Brien and Robertson, 1993). Stability of vitamin E during drum-drying could be markedly improved by using wholemeal wheat flour that was freshly milled (Wennermark et al., 1994).

1.7.4 PRODUCTION OF FOOD AND COOKING

Cooking of porridges of rolled oats and rye meal implies only minor effects on their tocopherol and tocotrienol contents. About 4–9% of tocopherols and 0–5% of toco-trienols were lost in rolled oats and rye meal porridges, when the cereals were of good quality (Piironen et al., 1987). Malting had no effects on tocopherol and tocotrienol concentrations of barley and brewers’ coproduct of barley rich in these vitamers (Peterson, 1994).

Baking of whole wheat bread (200°C, 30 min) destroyed 25% of α-tocopherol and tocotrienols and 12% of β-tocopherol and tocotrienols. In baking of rye bread (200°C, 50–60 min), the losses were greater at 45% and 35%, respectively, which can be explained by both longer dough-making and baking times (Piironen et al., 1987). Jägerstad and Håkansson (1988) confirmed that vitamin E was consumed during the dough-making and baking steps but not during the fermenting step of bread making. Industrial-scale bread making (220–270°C, 22–25 min) caused a reduction in α-tocopherol content of 56–65% in French bread and of 28–40% in rye bread compared with the flours and other ingredients (Wennermark and Jägerstad, 1992). Yet, both tocopherols and tocotrienols were equally stable, and α-vitamers were less stable than β-vitamers during these food preparation procedures. Breads fortified with α-tocopherol acetate retained nearly the same percentage of 67% within the large range of 200–1600 IU/pound added to the ingredients (Ranhotra et al., 2000).

In deep-fat frying, the temperature reaches >170°C, and tocopherols are unstable.

The order of stability of different tocopherols remains similar to that at lower temper-atures, i.e., α < γ < δ (Gordon and Kourimska, 1995; Lampi and Kamal-Eldin, 1998). However, an opposite order of stability has also been found. During deep-fat frying, the relative stabilities of natural tocopherols and tocotrienols in soybean oil were α-tocopherol > δ-tocopherol > β-tocopherol > γ-tocopherol, in corn oil were α-tocopherol > γ-tocopherol > δ-tocopherol > γ-tocotrienol, and in palm oil were α-tocopherol > δ-tocotrienol > α-tocotrienol > γ-tocotrienol, respectively (Simonne and Eitenmiller, 1998). The authors assumed that tocotrienols were less stable than tocopherols, because they acted more effectively as antioxidants.

Tocopherol losses during microwave cooking are mainly caused by the effect of high temperature and not by microwaves as such. When sunflower oil was subjected to microwaves discontinuously for 120 min at two constant temperatures;

namely, 170°C and <40°C, tocopherol losses were 72% and 21%, respectively (Albi et al., 1997). During continuous microwave cooking of soybean oil, the temperature increased and reached 170°C after 8 min and 210°C after 25 min. This caused greater losses of tocopherols, i.e., 5% and 10–30% after 8 and 25 min of heating,

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respectively (Yoshida and Takagi, 1999). During microwave cooking, α-tocopherol is less stable to heating than β-, γ- and δ-tocopherols (Yoshida et al., 1991; Yoshida and Takagi, 1999), which is similar to conventional heating at frying temperatures (Barrera-Arellano et al., 1999). Other studies have confirmed that α-tocopherol (Ha and Igarashi, 1990; Fuster et al., 1998; Lampi et al., 1999) and α-tocotrienol (Lehman and Slover, 1976) are consumed before other tocopherols and tocotrienols, also at temperatures ≤100°C.

At high temperatures such as during deep-fat frying and microwave heating, the rate of tocopherol consumption in the oil was greater in highly unsaturated oils than in less unsaturated oils (Yuki and Ishikawa, 1976; Yoshida et al., 1991; Simonne and Eitenmiller, 1998), which is opposite to that at moderate temperatures <100°C.

It is suggested that at low temperatures, less unsaturated oils do not oxidize, while they participate in thermooxidation at high temperatures.

1.7.5 STABILITY DURING STORAGE OF FOOD

Both food matrix and storage conditions have an effect on the stability of tocopherols and tocotrienols in food. In wheat and rye flours, storage at room temperature caused significant losses in tocopherol and tocotrienol contents during two months, while in cookies, they were stable for a year (Piironen et al., 1988). Thus, vitamers are prone to oxidation in flours exposed to air and protected in cookie matrix with a low surface-to-volume ratio. Similar to food preparation procedures, α-vitamers were less stable than β-vitamers during storage. In flours after 2 and 12 months of storage at room temperature, losses of α-tocopherol and α-tocotrienol were 20% and 80%, respec-tively, and those of β-tocopherol and β-tocotrienol were 10% and 60%, respectively (Piironen et al., 1988). The same difference in stability was found in another study, in which wholemeal flour, white flour, bran and germ of wheat were stored under similar conditions. Despite major differences in the profile of tocopherols and tocot-rienols in these milling fractions, 40–45% and 25–35% of α-vitamers and β-vitamers were lost during one year in all of them (Jägerstad and Håkansson, 1988).

Storage stability of α-tocopherol in crisps was mainly dependent on exposure to oxygen, the storage temperature and the oil used for frying. When crisps (deep-fried in soybean oil) were stored at room temperature for 25 weeks and at 60°C for 11 days, α-tocopherol content decreased by 12% and 34%, respectively. The loss during storage was smaller in products deep-fried in more saturated high-oleic sunflower oil and palm olein compared with conventional sunflower oil (Martin-Polvillo et al., 1996). Because the life of fresh bread is limited, tocopherols and tocotrienols are stable as long as the bread is consumed before the end of the shelf life (Piironen et al., 1988; Ranhotra et al., 2000).