Agricultural and Food Science, Vol. 13 (2004): 88–99 88 © Agricultural and Food Science Manuscript received June 2003 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 13 (2004): 88–99. Review article Role of lipid reactions in quality of oat products Pekka Lehtinen and Simo Laakso Helsinki University of Technology, Laboratory of Biochemistry and Microbiology, PO Box 9400, FIN-02015 TKK, Finland, e-mail: Pekka.Lehtinen@hut.fi In traditional oat processing practice the control of lipid reactions relies largely on empirical experi- ences and dogmatic principles rather than on profound understanding of the underlying mechanisms. However, in today’s global food markets, the industry faces strict challenges in the development of new processes and applications where the prior experience is unsatisfactory or insufficient. The stor- age stability of novel oat products can be greatly enhanced by taking the mechanisms of lipid deteri- oration into account, and by adjusting the processing conditions accordingly so that these reactions can be minimized. The lipid reactions in oat products result in two different unwanted properties: bitter, astringent, taste or a rancid flavor. Chemically, these properties are associated to enzymatic hydrolysis of ester bonds and non-enzymatic oxidation of unsaturated fatty acyl chains respectively. The processing history oat product has a huge impact on which of these reactions predominates in oat products. The review focuses on the reactions of lipids in processed oat products, and identifies fac- tors that are critical for enhanced shelf-life. Key words: oat, lipids, storage stability Introduction Even though many cereals are palatable as har- vested, they are usually processed to consumer products with increased nutritional, technologi- cal or commercial value. However, such process- ing will inevitably induce changes in the natural organization of seed lipids. Many of these chang- es render lipids more susceptible towards dete- riorative reactions. On the other hand, process- ing techniques based on novel information on lipid reactions can also be used to enhance the stability of cereal lipids. This can be achieved not only by inactivating the deteriorative en- zymes, but also by introducing processing steps by which the favorable molecular organization and phase distribution of lipids are enhanced. Processing induced changes in the molecu- lar organization between lipids and other flour components are enforced by mechanical and thermal energy. Cereal lipids themselves are liq- uid over the ambient temperature range, and thus no phase changes are expected to occur in the continuous lipid phase during processing. The viscosity of lipids does, however, change dra- mailto:Pekka.Lehtinen@hut.fi 89 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 13 (2004): 88–99. matically in the temperature range encountered during cereal processing (Abramovic and Klof- utar 1998). The low viscosity in the elevated tem- peratures increases the mobility of lipids and thus increases the chances of disintegration of native molecular organization. Concurrently, the dis- ruption of grain structure upon milling brings the lipolytic enzymes and lipids into close contact. Especially non-inactivated oat is known to form a bitter taste due to enzymatic deterioration in- duced by milling. In the typical milling process- es, subcellular components such as starch gran- ules and oil-bodies are likely to remain intact. However, heating, introduction of extensive me- chanical energy, enzymatic treatments and vari- ous aqueous treatments can cause the individual components to lose their integrity and as such lead to changes in molecular mobility. Reactions of cereal lipids during processing can be divided into reactions which are catalyzed by the enzymes present in cereals, and reactions which occur without the involvement of en- zymes. Most of the enzymatic reactions docu- mented in the scientific literature are related to hydrolytic or oxidative pathways or to non-oxi- dative isomerization of carbon-carbon double bonds structures. Non-enzymatic reactions are limited to the reactions related to oxidative path- ways and isomerizations, as the non-enzymatic lipid hydrolysis is exceedingly slow at ambient pH and temperature values typically encountered during cereal processing. For certain lipid com- pounds intramolecular acyl migration within glycerol structures has been reported to occur without enzyme activity also at ambient condi- tions (Plueckthun and Dennis 1982). However, the relevance of such acyl migration for the qual- ity of lipid containing food products is likely a small or non-existent. Enzymatic hydrolysis Most of the fatty acids found in plant seeds are esterified to a specific alcohol molecule, glycer- ol. The trans-esterification reaction in which the acyl group is transferred from the glycerol to water is generally referred to as lipid hydrolysis (Fig. 1). This, as well as the reverse reaction, the synthesis of acylglycerols from glycerol and free fatty acids, is catalyzed by lipase enzyme (EC 3.1.1.3). R2COO C H CH2 CH2 OCOR1 O P O CH2CH2N +(CH3)3 O O - PHOSPHOLIPASE D PHOSPHOLIPASE C PHOSPHOLIPASE A2 PHOSPHOLIPASE A1 O O O O O O OH OH OH HO HO HO O O O LIPASE 3 x H20 Fig. 1. Hydrolysis of polar membrane lipids and neutral storage lipids. 90 A G R I C U L T U R A L A N D F O O D S C I E N C E Lehtinen, P. and Laakso, S. Role of lipid reactions in quality of oat products The water activity has an utmost important role in determining lipase activity. Water affects both the enzyme activation and the thermody- namical equilibrium of the reaction. Different lipases have different water activity values at which activation is established and the literature on microbial lipases suggests that these values are well below a water activity of 0.3 (Wehtje and Adlercreutz 1997). The water amount re- quired is much smaller than for most other en- zymes, corresponding roughly to a mono or multiple adsorption layer of water surrounding the enzyme (Caro et al. 2002). As water is also a reactant in the lipase reaction, it also affects on the equilibrium of the reaction. The reaction equilibrium has been reported to change from the synthesis to the hydrolysis of esters in the water activity range of ca. 0.2 to 0.3 (Svensson et al. 1994). However, the equilibrium is also a function of other substrates involved, namely free fatty acids, glycerol and different acylglyc- erols. Thus the reaction equilibrium in a cereal matrix can not be deduced solely based on the water content. It is, however, evident that in most situations encountered upon cereal processing, where water activities lie well above 0.4, the li- pid hydrolysis is a thermodynamical downhill. Consequently, the hydrolysis of esterified lipids in enzyme active cereal products can easily pro- ceed to an extent that is perceived as sensory flaw. In water activities above 0.8 the amount of free water becomes remarkable, and the lipase catalyzed hydrolysis can either increase or de- crease as a function of water activity, depending on the substrate concentration, distribution of substrates between aqueous and oil phases and probably also on the source of the lipase (Adler- creutz et al. 2002, Ma et al. 2002). Different enzymes are responsible for the hydrolysis of neutral triacylglycerols and polar phospho- and glycophospholipids. Cereal seeds contain apparently only 1 or up to 3 different isoenzymes of lipases acting on storage lipids (Baxter 1984, O’Connor et al. 1989, Peterson 1999, Edlin et al. 2002). On the other hand, the hydrolysis of phospholipids in plant membranes is a far more controlled process involving the induction of plant defense system, and multiple isoenzymes of each phospholipase classified in Figure 1 (Wang 2001). During various process- es, cereal lipids may also be exposed to micro- bial lipase and the role of microbial enzymes in cereal lipid reactions is a controversial subject (O’Connor et al. 1992). The synthesis of cereal hydrolytic enzymes is induced by hormone signals from embryo tis- sue, leading eventually into protein synthesis or proenzyme activation in the aleuronic and ap- parently partly also in the endosperm tissues (Tavern and Laidman 1969, Laidman and Tav- ern 1971, Gallie and Young 1994). Mature oat grains have a remarkable lipase activity even if the germination is not started (O’Connor et al. 1992). During fractionating of oat, the lipase activity has been found to be present both in the aleuronic rich bran fraction as well as in the en- dosperm fractions obtained from the inner parts of grains (Hutchinson et al. 1951, Ekstrand et al. 1992, Lehtinen et al. 2003). The presence of high lipase activity in the endosperm fraction of non-germinated oat is puzzling. Assuming that the presence of the activity is related to incipi- ent germination, the presence of induction, syn- thesis and transport mechanisms would be ex- pected also for other hydrolytic activities. In non- germinated oat these activities are, however, not detected consistently with lipase activity. Also the fact that neither lipid hydrolysis nor an in- crease in lipase activity is observed during early germination, suggests that the lipase activity present in mature oat grain is not related to the germination process (Peterson 1999, Outinen 1999). More likely, the activity represents either a residual activity originating from the lipid syn- thesis upon the seed development or is related to some other biological function such as defense systems (Urquhart et al. 1983). Many microbial lipases discriminate between the different acyl groups and have different af- finities for fatty acids acylated to different OH- groups of glycerol. For wheat and oat lipases the provided data is somewhat conflicting. When endogenous lipolysis in oat products is followed, no such specificity has been reported (O’Connor 91 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 13 (2004): 88–99. et al. 1992, Heiniö et al. 2002). In these cases the slight difference in the proportion of fatty acids moieties in triacylglycerol and free fatty acid pools is likely a sign of further oxidative reactions of unsaturated free fatty acids rather than of lipase specificity towards unsaturated fatty acids (Warwick et al. 1979). Furthermore, the lipid hydrolysis in oat proceeds apparently without any accumulation of di- or monoacylg- lycerols. Rather, once the triacylglycerols are accessible to lipase, all three acyl groups are subsequently rapidly converted to free fatty ac- ids (Liukkonen et al. 1993). However, when the hydrolysis of supplied 1,2,3-trihexanoylglycer- ol by oat lipase was studied, a strong positional specificity of hydrolysis was observed (Yasuhide et al. 1997). Even though the hydrolysis of neutral stor- age lipids in oat is faster than in other cereals, the hydrolysis of polar lipids during oat process- ing and storage is minimal and detailed infor- mation on the hydrolysis of oat polar lipids is sparse (Liukkonen et al. 1992). However, in bar- ley and especially in barley malt, the hydrolysis of polar lipids occurs swiftly once the seed is milled and the water content is increased. In such a case phospolispases show notable specificity for different acyl groups in such a manner that unsaturated fatty acids are most easily hydro- lyzed (Kaukovirta-Norja et al. 1998). Enzymatic oxidation of acylglycerols Fatty acids in cereal lipids contain 0–3 double bonds. The presence of these double bonds adds extra reactivity to the lipid compounds, and enoic groups can undergo different isomerization or oxidization reactions (Fig. 2). Lipoxygenase (EC 1.13.11.12) is an abundant enzyme in plants and catalyzes the non-reversible oxidation of cis,cis- 1,4-pentadiene moieties in acyl groups to a re- spective acyl hydroperoxide. The lipoxygenase reaction rate in different cereals varies greatly and barley and wheat have high lipoxygenase activity, whereas in rye and oat the reaction is slow (Fretzdorff and Jördens 1986, Lehtinen et al. 2000). Cereals contain multiple isoenzymes with lipoxygenase activity and cDNA sequenc- es of two lipoxygenases from germinating bar- ley have been determined (Shiiba et al. 1991, Hugues et al. 1994, Van Mechelen et al. 1999). These isoenzymes have apparently different sub- strate specifity, different distribution in various tissues and produce different hydroperoxide iso- mers in different proportions, but otherwise the biological role of these isoenzymes is unknown (Schmitt and Van Mechelen 1997, Feussner and Wasternack 1998). Enzymatic oxidation of double bonds in acyl chains consists of series of reactions. In many plant tissues the hydroperoxides formed by the lipoxygenase reaction are further cleaved by hydroperoxide lyase (EC 4.1.2.-), an enzyme which has been extensively studied in cucum- ber, tomato and beans (Matsui et al. 2000, Suur- meijer et al. 2000, Noordermeer et al. 2001). The presence of hydroperoxide lyase in cereal grains has not been published in the scientific litera- ture, but the presence of typical reaction prod- ucts of this enzyme suggests that it is abundant also in cereals (Sjövall et al. 2000, Parker et al. 2000, Sides et al. 2001). In oat, a lipoperoxidase (EC 1.11.1.-) activity is responsible for the con- version of hydroperoxides to relevant hydroxy- acids (Biermann and Grosch 1979). These hy- droxyacids are suggested to be partially respon- sible for the bitter taste associated with the en- zymatically active oat (Biermann et al. 1980). As a consequence of these reactions, wide spectrums of products are formed. In many ce- reals linoleic acid is the most abundant substrate for the lipoxygenase reaction. The lipoxygenase reaction produces mainly two different isomers of hydroperoxide linoleic acid, namely 9- and 13- hydroperoxide linoleic acids. The cleavage of 9-hydropreoxide linoleic acid further, yields mainly 8–10 carbon monoenoic compounds, whereas the cleavage of 13 hydroperoxide lino- leic acid yields 5–7 carbon compounds with no 92 A G R I C U L T U R A L A N D F O O D S C I E N C E Lehtinen, P. and Laakso, S. Role of lipid reactions in quality of oat products (CH2)7 COOHHCO CH3 (CH2)4 CH=CH CH=CH CH (CH2)7 COOH OOH H3C(CH2)4CH CHCH2CHO CH3 (CH2)4 CH=CH CH=CH CH (CH2)7 COOH OH CH3 (CH2)4 CH CH=CH CH=CH (CH2)7 COOH OOH H2C CH=CH (CH2)7 COOHOCHH3C(CH2)4CHO CH3 (CH2)4 CH CH=CH CH=CH (CH2)7 COOH OH HYDROXY ACIDS 9-HYDROPEROXIDE LINOLEIC ACID 13-HYDROPEROXIDE LINOLEIC ACID PEROXIDASE HYDROPEROXIDE LYASE LIPOXYGENASE HYDROPEROXIDE LYASE 3-NONENAL OXOCARBOXYLIC ACID (NON VOLATILE) OXOCARBOXYLIC ACID (NON VOLATILE) HEXANAL ISOMERASE DEHYDROGENASE DEHYDROGENASE LINOLEIC ACID MOLECULAR OXYGEN CH3 (CH2)4 CH=CH CH=CH (CH2)7 COOHCH2 Fig. 2. Simplified scheme of the reactions in the enzymatic oxidation of linoleic acid in cereals (modified after Noordermeer et al. 2001 and Biermann et al. 1980). double bonds. Upon cleavage both isomers yield also the non-volatile oxo-carboxylic acid com- pound with 8 to 12 carbons (Galliard and Mat- thew 1977, Olias et al. 1990). Non-enzymatic reactions Whereas the enzymatic oxidation of enoic struc- tures uses molecular, triplet state oxygen as a primary substrate, the non-enzymatic oxidation occurs only after pre-formation of reactive acyl or oxygen species. Thus non-enzymatic oxida- tion is initiated by factors such as radicals, met- al-ions and photons with an energy level capa- ble of triggering endogenous photosensitive molecules. However, once the reaction is initi- ated, the reaction itself can provide the radicals that will cause the reaction to continue. The pres- ence of endogenous antioxidants in cereals has a marked effect on the onset of non-enzymatic oxidation due to their capability to quench these reactive molecule species into non-reactive form. Many products of the non-enzymatic oxida- tion are the same as in the enzymatic oxidation (Fig. 3). However, the fatty acid hydroperoxides may accumulate if neither hydroperoxide lyase nor lipoperoxidase are present. In this case, the 93 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 13 (2004): 88–99. rate of hydroperoxide decomposition is set by the molecular environment of fatty acid hy- droperoxides, i.e. the presence of antioxidants and metal-ions or other radical forming com- pounds, phase distribution and presence of ami- no acids and sugars (Nishiike et al. 1999, Mc- Clements and Decker 2000, Mäkinen and Hopia 2000). Certain antioxidants slow the decompo- sition of hydroperoxides, which can reduce the overall rate of lipid oxidation, as the decompo- sition of hydroperoxides will provide less radi- cals for the initial H* abstraction from the fatty acid. However, the role of antioxidants is not straightforward, as in some cases antioxidants can actually increase the decomposition of hy- droperoxides, possibly by reducing metal ions into more active form (Mäkinen et al. 2001). Lipid may also form polymeric compounds upon (CH2)7 COOHHCO CH3 (CH2)4 CH=CH CH=CH CH (CH2)7 COOH OOH H3C(CH2)4CH CHCH2CHO CH3 (CH2)4 CH CH=CH CH=CH (CH2)7 COOH OOH H2C CH=CH (CH2)7 COOHOCHH3C(CH2)4CHO 9-HYDROPEROXIDE LINOLEIC ACID 13-HYDROPEROXIDE LINOLEIC ACID 3-NONENAL OXOCARBOXYLIC ACID (NON-VOLATILE) OXOCARBOXYLIC ACID (NON-VOLATILE) HEXANAL LINOLEIC ACID SCISSION REACTIONS PENTYLFURAN 2-NONENAL H3C(CH2)4C O RADICALS, METAL-IONS, ANTIOXIDANTS CLEAVAGE OF OH . PHOTOSENSITIZER + MOLECULAR OXYGEN 1O2 RADICALS, METAL-IONS PHOTOSENSITIZER ABSTRACTION OF H . MOLECULAR OXYGEN hv CLEAVAGE OF OH . H3C(CH2)5CH CH CHO SCISSION REACTIONS 1O2 Fig. 3. Simplified scheme of the reactions in the non-enzymatic oxidation of linoleic acid in cereals (modified after Min and Boff 2002). In addition to 9-and 13-hydroperoxides also 10- and 12-hydroperoxides are formed upon photo-oxidation of linoleic acid. 94 A G R I C U L T U R A L A N D F O O D S C I E N C E Lehtinen, P. and Laakso, S. Role of lipid reactions in quality of oat products oxidation. These are formed mainly in the high- ly unsaturated oils such as fish or linseed oils, or in oils that have been excessively heated such as frying oils (Neff et al. 1988, Shukla et al. 1991). The lipid oxidation products may also under- go different isomerization reactions. The occur- rence of trans-2-nonenal (T2N) is associated with the cardboard flavor in beer (Jamieson and Van Gheluwe 1970, Noeel et al. 1999). The forma- tion of the T2N has been suggested to involve an isomerase reaction in which the 3-nonenal is isomerized to T2N (Galliard et al. 1976). How- ever, neither 2- or 3-nonenal products have been reported in rancid oat or wheat germ or, if present, are detected at very low abundance (Sjövall et al. 2000, Heiniö et al. 2001). Instead, 2-pentyl furan is detected in stored cereals and may represent the major product from the cleav- age of 9-hydroperoxide linoleic acid. The con- version of straight chain structure of nonenal into cyclic furan structure can be initiated by the sin- glet oxygen as shown by Min and Boff (2002), and it seems plausible, that the oxidative stress in cereal matrix is responsible for the instability of 3-nonenal. Interestingly, studies in which pu- rified 9-hydroperoxide linoleic acid has been used as a substrate, the 3-nonenal formed via the hydroperoxide lyase reaction, appears to be sta- ble at least for analytical purposes (Gargouri and Legoy 1998). The enoic structures found in unsaturated fatty acids do not undergo spontaneous isomeri- zation at the ambient processing conditions met during cereal processing. However, in the pres- ence of catalyst such as transition metal ions or at temperatures above 200˚C, both cis/trans and positional isomerization can occur in unsaturat- ed fatty acids (Wolff 1993, Kemeny et al. 2001). During thermal processing of cereals tempera- tures are usually well below 200˚C and for ex- ample upon extrusion, the formation of trans fat- ty acids is relatively low, 1–1.5% of total un- saturated fatty acids (Maga 1978). Relevance of lipid reactions to quality of cereal products The most significant result of lipid reactions is their effect on sensory and rheological proper- ties of cereal products (Cumbee et al. 1997, Jacobsen 1999). A loss of nutritive value (An- dersen et al. 1986) and even cytotoxicity (Ester- bauer 1993) has also been associated with ex- tensive oxidation of unsaturated fatty acids (Fig. 4). Formation of rancid flavor due to lipid oxidation is a relatively well known phenome- non, whereas relevance of lipid hydrolysis, dis- ruption of cellular structures and lipid interac- tion with other flour components is less evident. Analytically the extent of lipid oxidation in food materials is characterised by using param- eters such as the amount of remaining intact un- saturated fatty acids, presence of fatty acids hy- droperoxides and presence of secondary oxida- tion products. These parameters are valuable tools for studying the lipid oxidation and for developing products with increased storage sta- bility. However, the relevance of these parame- ters in explaining the sensory properties is lim- ited, as the sensory impact of different food prod- ucts differs widely (Jacobsen 1999). Hexanal is the most abundant and easily detectable second- ary oxidation product and thus often used as a marker of lipid oxidation (Fritsch and Gale 1977, Frankel et al. 1989). Still, it is clear, that neither hexanal nor any other volatile lipid oxidation product is solely responsible for the perceived rancidity. Rather, flavors such as paint- or card- board-like odor associated with the rancidity are apparently caused by a combination of different volatile carbonyl compounds (Heydanek and McGorrin 1981, Zhou et al. 1999, Heiniö et al. 2002). These compounds are reported also in rancid oat products in which their concentration in headspace of cereal sample increases approx- imately 2 to 100 fold during storage (Sjövall et al. 2000, Heiniö et al. 2002). The consequences of the hydrolysis of tria- cylglycerols to free fatty acids and partially es- 95 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 13 (2004): 88–99. Fig. 4. Lipid reactions causing sensory and rheological changes in cereals. LIPID HYDROLYSIS OXIDATION OF UNSATURATED FATTY ACIDS BITTER TASTE INCREASED WATER SOLUBILITY AND EMULSIFICATION CAPABILITY INCREASED SUSCEPTIBILITY TOWARDS DETERIORATION RANCID FLAVOUR CO-OXIDATION OF PROTEINS AND PHENOLICS DECREASE IN NUTRITIVE VALUE CHANGES IN THE RHEOLOGICAL PROPERTIES terified glycerols are reflected both on changes in technical properties and on changes in per- ceived sensory properties. The prior dogma has been that the hydrolysis of acylglycerols renders the unesterified fatty acids moieties susceptible towards oxidation and thus increases the risk for rancidity. The free fatty acids in cereals formed via hydrolysis have such long carbon chains that they are virtually non-volatile. The bitter taste of free unsaturated fatty acids, mainly linoleic and li- nolenic acids, has been observed in the aqueous emulsions at concentrations above 0.7 and 0.1 mg g-1 respectively (Stephan and Steinhart 2000). In dry oat material, in which extensive lipid hydrolysis has occurred, the concentrations of these acids is ca. 10 and 0.5 mg g-1 respec- tively (Sippola 2002). However, the bitter taste in such oat is suggested to be due to the pres- ence of long chain hydroxyacids rather than to free fatty acids (Biermann et al. 1980). Biermann et al. (1980) also postulated that the bitter taste associated with non-heat treated oat products results from the formation of a hydroxygroup in the carbon chain of monoglyceridelinoleate. However, the hydrolysis of neutral storage lip- ids is well characterised in the scientific litera- ture, and it appears that the hydrolysis proceeds rapidly to glycerol and free fatty acids. The ac- cumulation of partially hydrolyzed triglycerols, such as monoglyceridelinoleate has not been observed (Liukkonen et al. 1992). One possibil- ity is that the hydroxy fatty acid is formed while the fatty acid is still acylated to glycerol and that the formed hydroxyacylglycerol is discriminat- ed by oat lipase and thus accumulates into the monoglyceride pool. Prevention of unwanted lipid reactions during processing and storage of oat products The hydrolysis reaction of acylglycerols can be very rapid once the oat is milled, and enzyme active oat products develop a characteristic bit- ter taste within weeks or months after milling. 96 A G R I C U L T U R A L A N D F O O D S C I E N C E Lehtinen, P. and Laakso, S. Role of lipid reactions in quality of oat products Conventionally this reaction is prevented by in- activating the enzyme activities either from whole kernels or from the product obtained short- ly after milling. The enzyme inactivation is most easily achieved by moist heat. If this enzyme inactivation is adequate, then oat can be proc- essed even by aqueous processes without the development of bitterness. However, the enzyme inactivation posses a risk, that it will promote the non-enzymatic ox- idation of unsaturated fatty acids during subse- quent storage of oat product. Compared to the bitter taste associated with enzyme activated oat products, the consequences of non-enzymatic oxidation are perceived as rancid flavor. It ap- pears that the development of rancidity is de- pendent on the severity of heat treatment (Leh- tinen et al. 2003). Thus an optimum enzyme in- activation scheme should enable the inactivation of lipolytic enzyme and yet simultaneously pre- vail the endogenous resistance towards oxida- tion. This requires an effective control over se- verity of heat treatment. Extrusion seems to be promising in this respect, due to the rapid heat transfer, that enables the strict control over the intensity of heat treatment. One strategy to reduce the risk for lipid re- lated storage problems is to produce products with lower lipid content than normal oat prod- ucts. This can be achieved by choosing oat vari- eties with lower lipid content or by removing part of the lipids by extraction process. However, the effect of extraction process on the other func- tional properties of oat is not clear (Hoover et al. 1994). Conclusions The highly valuable nutritional properties of oat flour makes it an interesting alternative compo- nent for various food products. However, the lim- ited storage stability can reduce the usability of oat products. To ensure the sufficient shelf life, the lipid reactions during and after oat process- ing must be minimized. This can be achieved by inactivating endogenous lipid degrading en- zymes and by avoiding oxidative processing steps such as excessive heating. Also the pack- ing material of product, especially the permea- bility for UV-radiation, moisture and oxygen, can have a huge impact on the storage stability. Two different kind of deterioration that can be linked to lipid reactions are typical for oat products: 1) bitter, astringent, taste that occurs after enzymatic hydrolysis reactions and 2) ran- cid, paint-a-like, flavor that results from non- enzymatic oxidation. Separately these reactions are relatively easy to control, but often the ac- tions taken to prevent the other, may actually promote the other. For example if oat kernels are heated excessively in order to ensure com- prehensive inactivation of lipase, the heating can instead promote unwanted oxidation reactions. However, by implementing processing steps that are easily monitored and that can be effectively controlled, it is possible to produce oat products with similar stability compared to correspond- ing products from other cereals. Extrusion is an attractive solution for enzyme inactivation, as the heat exchange is fast and easily controlled. Acknowledgment. This review is partly based on the au- thor’s academic thesis “Reactivity of lipids during cereal processing” available online at http://lib.hut.fi/Diss/2003/ isbn9512265575/. References Abramovic, H. & Klofutar, C. 1998. The temperature de- pendence of dynamic viscosity for some vegetable oils. Acta Chimica Slovenica 45, 1: 69–77. Adlercreutz, D., Budde, H. & Wehtje, E. 2002. Synthesis of phosphatidylcholine with defined fatty acid in the sn-1 position by lipase-catalyzed esterification and transesterification reaction. Biotechnology and Bio- engineering 78, 4: 403–411. Andersen, J., Oluf, E., Bjoern, O. & Jacobsen, I. 1986. The significance of products of lipid oxidation upon the nutritive value of feed. In: Marcuse, R. (ed.). Lipid oxidation: biological and food chemical aspects. Lip- idforum/SIK. p. 148–153. Baxter, E.D. 1984. 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Tie- don tarve korostuu markkinoiden kehityttyä maail- manlaajuiseksi samalla kun tiukentuva lainsäädäntö ja muuttuvat kulutustottumukset asettavat kauratuot- teiden säilyvyydelle aikaisempaa huomattavasti suu- rempia vaatimuksia. Kauran rasvojen aiheuttamat aistittavat ongelmat ilmenevät hyvin monimuotoisesti. Niiden primääri- senä aiheuttajana voidaan kuitenkin pitää kahta ras- vojen perusreaktiota, esterisidoksen hydrolyysiä ja tyydyttymättömien rasvahappoketjujen hapettumista. Edellisen tunnusmerkkinä pidetään kitkerää, poltta- vaa makua, kun taas jälkimmäinen on tunnistettavis- sa eltaantuneena, maalimaisena hajuna. Näiden reak- tioiden yhteisvaikutuksena kauran aistittavat ongel- mat voivat kuitenkin esiintyä hyvin monimuotoisena. Laadukkaaseen kauran prosessointiin tulee siten kuu- lua rasvojen hydrolyysin ja hapettumisen esto sekä tuotteiden formulointi siten, että edellytykset näille reaktioille ovat mahdollisimman vähäiset. Tässä kat- sauksessa keskitytään rasvojen reaktioihin prosessoi- duissa kauratuotteissa, ja käydään läpi prosessointiin liittyviä tekijöitä, jotka ovat kriittisiä kauratuotteiden säilyvyyden kannalta. Role of lipid reactions in quality of oat products Introduction Enzymatic hydrolysis Enzymatic oxidation of acylglycerols Non-enzymatic reactions Relevance of lipid reactions to quality of cereal products Prevention of unwanted lipid reactions during processing and storage of oat products Conclusions References SELOSTUS