Oli e grassi Distinzione in base allo stato fisico a temperatura ambiente. Grassi, solidi. Oli, liquidi Origine animale: grasso bovino e ovino, lardo (oltre.

Presentazione sul tema: "Oli e grassi Distinzione in base allo stato fisico a temperatura ambiente. Grassi, solidi. Oli, liquidi Origine animale: grasso bovino e ovino, lardo (oltre."— Transcript della presentazione:

1 Oli e grassi Distinzione in base allo stato fisico a temperatura ambiente. Grassi, solidi. Oli, liquidi Origine animale: grasso bovino e ovino, lardo (oltre il 50% di saturi), grasso d’oca, olio di balena e di pesce Origine vegetale: Cacao (oltre il 60% di saturi) Palma (circa il 50% di saturi), oli di semi (girasole, lino, cotone, sesamo), olio di colza (alimenti se privo di acido erucico), Cocco (con C12:0 e C14:0, resiste all’ox e fonde intorno ai 28°C, uso nei gelati)

2 Catene idrocarburiche con un gruppo carbossilico. In genere sono lunghi da 14 a 20 atomi di carbonio quasi sempre in numero pari perché la loro biosintesi e sempre “due a due” Possono essere saturi o insaturi e in questo secondo caso i doppi legami sono sempre “cis”. Diventano trans in seguito ad alcuni processi industriali. In genere i doppi legami sono NON coniugati. Acidi grassi essenziali: sono quelli insaturi con il doppio legame vicini alla fine della catena (oltre il carbonio 9 dal termine) Acidi grassi

3 Nomenclatura: Es. C 18: 2 Numero di atomi di Carbonio Numero di insaturazioni L’ultimo atomo di carbonio viene definito “n” oppure “  “e l’insaturazione (partendo dal termine della catena) viene detta, ad esempio, “n – 3” oppure “  3“ (gli “  3“ sono importantissimi)

4 I nomi degli acidi grassi C14:0MiristicoC14:1Miristoleico C16:0PalmiticoC16:1Palmitoleico C18:0StearicoC18:1Oleico C18:2LinoleicoC18:3Linolenico C20:4Arachidonico

5 Trigliceridi Una molecola di glicerolo esterificata con tre acidi grassi

6 Tripalmitina: tre molecole di acido palmitico (saturo, grassi animali) solido a temperatura ambiente.

7 Infatti la trioleina (che ha tre molecole di acido oleico) è liquida a temperatura ambiente ) L’insaturazione introduce una distorsione nella catena e quindi meno foze di Van der Waals

8 Lo stato fisico della materia grassa dipende dal tipo di acido presente: I grassi saturi essendo lineari, tendono ad impacchettarsi bene molte interazioni quindi stato solido Nei grassi insaturi i doppi legami creano una distorsione della catena. Questo provioca una diminuzione della superficie di contatto e quindi delle forze di Van der Waals. Olio di oliva: stato liquido ma viscoso. Altri oli di semi: liquidi ma meno viscosi SOLIDO O LIQUIDO?

9 Le lipasi Enzimi che rompono il legame estere quindi sono delle esterasi Ne esistono due tipi (le posizioni 1 e 3 sono uguali).

10 Ruolo dei lipidi negli alimenti Contribuiscono in modo decisivo al sapore all’aroma e alla consistenza. Perciò quando si ossidano..... Veicolano le vitamine liposolubili I fattori ossidativi sono luce temperatura metalli enzimi e microrganismi Agiscono soprattutto sugli acidi grassi insaturi innescando un processo autocatalitico La reazione termina quando molecole antiossidanti reagiscono con i radicali liberi RO 2. + AH = RO 2 H + A.

11 ROS: REACTIVE OXYGEN SPECIES Radicaliche e non radicaliche OSSIGENO TRIPLETTOO 2 ANIONE SUPEROSSIDOO 2 - PEROSSIDO D’IDROGENOH 2 O 2 RADICALE IDROSSILICOHO ACQUAH 2 O OSSIGENO SINGOLETTO 1 O 2 ORBITALI  radi cale ossidan te SI NO SI NO

12 ANIONE SUPEROSSIDO O 2 -. Generato: accidentalmente nella catena di trasporto degli e - nei mitocondri e nei microsomi intenzionalmente dalle cellule fagocitarie tramite le ossidasi NADPH-dipendente reattività bassa, diffusione alta (vedi legge di Fick) partecipa alla reazione di Fenton (vedi dopo) trasformato a H 2 O 2 dalla SOD nel citoplasma e nei mitocondri SPECIE RADICALICHE

13 RADICALE IDROSSILEHO RADICALE IDROSSILEHO Generato: nella catena di trasporto degli e - nei mitocondri per azione delle radiazioni elettromagnetiche in presenza di metalli di transizione O 2 - + Fe +++ Fe ++ + O 2 reazione di Fenton Fe ++ + H 2 O 2 Fe +++ + HO + HO - Reattività alta, diffusione limitata SPECIE RADICALICHE

14 PROTEINE: ossidazione gruppi -SH (NO, HO, ONOO - ) ossidazione di alcuni AA (HIS, ARG, LYS, PRO) liberazione del Fe per degradazione degli anelli porfirinici (H 2 O 2 ) perdita di funzionalità EFFETTO SULLE BIOMOLECOLE

15 ACIDI NUCLEICI: Idrossilazione delle basi per addizione del radicale OH o sottrazione di un H dalla molecola saccaridica. EFFETTO SULLE BIOMOLECOLE

16 PEROSSIDAZIONE LIPIDICA E’ la classe di biomolecole più suscettibile all’attacco dei radicali L’autossidazione avviene a carico degli acidi grassi presenti nelle membrane cellulari o nelle lipoproteine La suscettibilità aumenta all’aumentare dei doppi legami La reazione di perossidazione porta alla formazione di prodotti secondari tossici o cancerogeni come aldeidi, chetoni, …. L’iniziatore principale è HO Markers di perossidazione: MDA e 4-idrossinonenale EFFETTO SULLE BIOMOLECOLE

17 Lipid Oxidation The overall mechanism of lipid oxidation consists of three phases: –(1) initiation, the formation of free radicals; –(2) propagation, the free-radical chain reactions; and –(3) termination, the formation of non radical products.

18 Schema generale

19 Lipid Oxidation The important lipids involved in oxidation are the unsaturated fatty acid moieties, oleic, linoleic, and linolenic. The rate of oxidation of these fatty acids increases with the degree of unsaturation. Stearic– speed 1 (per convenzione) Oleic – 10 fold stearic speed Linoleic– 100 fold stearic speed Linolenic – 200 fold stearic speed

20 Lipid Oxidation Where RH is any unsaturated fatty acid; –R· is a free radical formed by removing a labile hydrogen from a carbon atom adjacent to a double bond; ROOH is a hydroperoxide, one of the major initial oxidation products that decompose to form compounds responsible for off-flavors and odors. –Such secondary products include hexanal, pentanal, 4 OH 2 alkenal (4-HDA) and malonaldehyde.

21 Lipid Oxidation Initiation: RH + O2 -->R· + ·OH R· + O2 --> · + ROO· Propagation: ROO· + RH --> R· + ROOH ROOH--> RO· + HO· Termination: R· + R· --> RR R· + ROO·--> ROOR ROO· + ROO· --> ROOR + O2

22 Lipid Oxidation of a Monoenoic Acid oleic acid as an example, a hydrogen could be removed from either C-8 or C-10, as these positions are located alpha to the double bond.

23 Lipid Oxidation of a Monoenoic Acid Using oleic acid as an example, a hydrogen could be removed from either C-8 or C-10, as these positions are located alpha to the double bond. Abstraction from carbon 8 results in the two radicals A and B which are positional isomers of each other stabilized by resonance

24 Lipid Oxidation of a Monoenoic Acid Or abstraction from carbon 11 can occur, resulting in the two radicals C and D:

25 Lipid Oxidation of a Monoenoic Acid Oxygen can be added to each radical to form peroxy radicals at C-8, C-9, C-10 or C-11. Addition to the 8 and 10 positions yield the peroxy radicals shown above

26 Lipid Oxidation of a Monoenoic Acid These radicals may abstract hydrogens from other molecules to yield the hydroperoxides shown

27 Lipid Oxidation of dienoic acid The situation with a dienoic acid is a little different. While there are more positions a to a double bond, there is one position that is at two double bonds. This position is very reactive. For linoleic acid, carbon 11 is at two double bonds and will be removed to yield the free radical

28 Lipid Oxidation of dienoic acid There are two possible resonant structures that can result from this radical. The radical may shift to carbon 14 with the double bond reforming between carbons 11 and 12. The radical may also shift to carbon 9 with the double bond forming between carbons 10 and 11. Both of these cases result in conjugated structures that are at lower energies than are the non conjugated structures they were derived from. For this reason, the oxidation of linoleic acid yields approximately equal amounts of the C 13 and C 9 radical with only traces of the original C 11 radical present. The resonant structures formed are shown

29 Lipid Oxidation Once formed, hydroperoxides may break down through a number of mechanisms. A common breakdown scheme is called dismutation. In this reaction a hydroperoxide reacts with another molecule or radical to form two new compounds.

30 Lipid Oxidation Hydroperoxides are not stable compounds and given time, they will break down. A typical mechanism, as shown below, results in the formation of two radicals from a single hydroperoxide molecule.

31 Lipid Oxidation role of metals Both of these new radicals can initiate further oxidation. Some metals can speed up this reaction.

32 Lipid Oxidation Note that both ions and free radicals were formed. The net reaction is shown above Copper was the catalyst. Copper did not initiate the reaction, but once the hydroperoxides were formed, it sped up their breakdown.

33 Measurement of lipid oxidation Peroxide value Peroxides are the main initial products of autoxidation. They can be measured by techniques based on their ability to oxidize ferrous to ferric ions. Their content is usually expressed in terms of milliequivalents of oxygen per kilogram of fat. Following peroxide formation is realistic only at the early stages of oxidation, During the course of oxidation, peroxide values reach a peak and then decline.

34 Measurement of lipid oxidation Thiobarbituric acid (TBA) TBA is the most widely used test for measuring the extent of lipid peroxidation in foods due to its simplicity and because its results are highly correlated with sensory evaluation scores. The principle of the method is the reaction of one molecule of MDA and two molecules of TBA to form a red MDA-TBA complex, which can be quantitated spectrophotometrically. This method has been criticized as being nonspecific. Other TBA-reactive substances (TBARS) including sugars and other aldehydes could interfere with the malonaldehyde-TBA reaction. Abnormally low values may result if some of the TBA reacts with proteins in an oxidizing system. In many cases, however, the TBA test is applicable for comparing samples of a single material at different states of oxidation.

35 Measurement of lipid oxidation TBA - Thiobarbituric Acid

36 Measurement of lipid oxidation Iodine Value Iodine value is a measure of the unsaturated linkages in fat and is expressed in terms of percentage of iodine absorbed. The decline in iodine value is sometimes used to monitor the reduction of dienoic acids during the course of the autoxidation.

37 Hydrogenation and spreads Treatment of an oil with hydrogen and a suitable catalyst to decrease the number of double bonds and increase the degree of saturation Si usa per fare la margarina… qual’è il senso di questo prodotto?

38 Hydrogenation Rate is determined by: –Nature of substrate –Type and concentration of catalyst –Pressure (Concentration of hydrogen) –Temperature –Agitation

39 Hydrogenation Before After Unsaturated Saturated Liquid Solid Cis Cis/Trans Effects of Hydrogenation

40 Hydrogenation Parameter Selectivity Formation of Trans bonds Reaction Rate Correlation Direction Temperature Positive Pressure Negative Positive Concentration Positive Agitation Negative Positive The effects of processing conditions on hydrogenation

41 Hydrogenation Method Oil is heated with catalyst (Ni), heated to the desired temperature (140-225°C), then exposed to hydrogen at pressures of up to 60 psig and agitated.

42 Hydrogenation Heterogeneous Catalysts Most commonly utilized –Catalysts and reactants exists in different physical states –Hydrogenation reaction takes place on surface of catalyst –Nickel containing catalysts are most frequently utilized

43 Hydrogenation Hydrogenation Limitations –Selectivity is never absolute –Little preference for C18:3 over C18:2 –Important amounts of trans acids are formed –Selectivity and isomerization are linked

44 Hydrogenation Isomerization An equilibrium will be established between positional and geometric isomers in the mixture. Double bonds that are reformed tend to have a trans/cis ration of 2:1. All trans would be expected if there were no steric considerations.