Phospholipid oxidation, non-enzymatic browning development and volatile compounds generation in...

Eur Food Res Technol (2007) 225:665–675DOI 10.1007/s00217-006-0462-2

ORIGINAL PAPER

Phospholipid oxidation, non-enzymatic browning developmentand volatile compounds generation in model systems containingliposomes from porcine Longissimus dorsi and selected amino acidsSonia Ventanas · Mario Estevez · Carmen L. Delgado ·Jorge Ruiz

Received: 16 January 2006 / Revised: 25 July 2006 / Accepted: 9 August 2006 / Published online: 15 September 2006C© Springer-Verlag 2006

Abstract Lipid oxidation, non-enzymatic browning devel-opment and volatile compounds generation were studied insterile meat-model systems containing selected amino acidsand/or liposomes during 35 days at 25 ◦C under pro-oxidativeconditions, in order to simulate the ripening conditions ofdry-cured meat products. Liposomes were prepared withpolar lipids (PL) from Longissimus dorsi muscle of out-doors pigs fed on grass and acorns (M) or indoors onesfed on concentrates (C). Thiobarbituric acid reactive sub-stances (TBA-RS) in systems containing both amino acidsand liposomes were higher than in those containing onlyliposomes. The higher susceptibility to lipid oxidation ofliposomes from C animal was reflected in higher TBA-RSthroughout the experiment. All model systems containing li-posomes and/or amino acids showed non-enzymatic brown-ing development, as measured by the yellowness index. Ma-jor volatile compounds were fatty acid oxidation products.Strecker-derived volatile compounds were predominantlydetected in model systems containing both amino acids andliposomes. The chromatographic areas of Strecker-derivedvolatiles increased over time whereas those of lipid-derivedvolatiles decreased. The occurrence of lipid oxidation, non-enzymatic browning and Strecker-type degradation of aminoacids were shown in sterile meat-model systems at lowtemperatures.

Keywords Lipid oxidation . Non-enzymatic browning .

Strecker-type degradation . Liposomes . Model system

S. Ventanas · M. Estevez · C. L. Delgado · J. Ruiz (�)Food Science, School of Veterinary Sciences,University of Extremadura,Caceres, Spaine-mail: [email protected]

Introduction

Ripening conditions (processing time and temperature) andfatty acid compositions of polar lipids (PL) of raw meat, aretwo of the most important factors influencing the profile ofvolatile compounds of dry-cured products and particularlyof Iberian dry-cured hams.

The particular processing conditions (temperatures from15 to 25 ◦C, pH near 6, water activities around 0.85 and aprocessing time up to 30 months) during the processing ofIberian dry-cured hams lead to the occurrence of lipid oxi-dation phenomena [1–3] and could favour the developmentof Maillard reactions, including the oxidative-deamination-decarboxylation of amino acids via Strecker degradation [4].The enormous amount of free amino acids released duringthe drying phase [5], together with the formation of reactivecarbonyls from lipid oxidation [3], could enable the subse-quent development of these type of Maillard reactions, andthus the generation of certain branched aldehydes, such as3-methylbutanal, that have been described as major contribu-tors to the desirable flavour of Iberian ham [6]. Nevertheless,other authors [7, 8] have claimed that Strecker degradationof amino acids is not a possible mechanism for methyl-branched aldehydes formation in dry-cured hams, since thetemperatures reached during ripening are not high enough.Consequently, these authors pointed out that the only possi-ble source of these aldehydes in dry-cured ham is the micro-bial metabolism.

The role of PL in the development of the flavour of meatand meat products has been discussed extensively [9]. PL areconsidered the main precursors of lipid-oxidation productsin meat [10] due to the high degree of unsaturation of theirfatty acids, together with the fact that, as components of cellsmembranes, they are exposed to proteins and other catalystsof lipid oxidative reactions such as metals [11]. Aldehydes,

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666 Eur Food Res Technol (2007) 225:665–675

the main products of lipid oxidation, seem to be the maincontributors to Iberian ham flavour, since they show lowolfaction thresholds and distinctive odour characters (e.g.rancid, sweet, floral notes) [12, 13]. The fatty acid compo-sition of dietary fat influences the fatty acid composition ofPL [14] and thus, the volatile profile in Iberian dry-curedham [1]. The particular PL profile (high monounsaturated(MUFA) content) of Iberian hams from pigs reared outdoorsand fed on grass and acorns, partly explains the high sensoryquality of this type of hams compared to those from pigsreared indoors and fed on mixed concentrates [15].

Several studies have been carried out in order to elu-cidate the influence of the modifications of the fatty acidcomposition of PL through dietary means, on the volatileprofile in different meat-model systems [16, 17]. Moreover,numerous works have studied the occurrence of Maillardreactions and the interactions between Maillard and lipid-derived products in model systems containing amino acidsor proteins and carbohydrates [18, 19] and/or PL [20, 21].Nevertheless, most of these studies were performed at hightemperatures (above 100 ◦C), since they were focused on thestudy of generation of volatile compounds in cooked meatproducts. In addition, most meat-model systems includedsugars in order to study the volatiles generation via the clas-sical Maillard reaction between reducing sugars and aminoacids.

The objective of the present work was to study theformation of volatile compounds, particularly those fromStrecker degradation of amino acids, at low tempera-tures in model systems mimicking dry-cured productsand containing selected amino acids and liposomes fromL. dorsi with different fatty acids profiles. Lipid oxida-tion and non-enzymatic browning development were alsoevaluated.

Materials and methods

Reagents

l-Leucine (minimum 98% TLC), l-methionine (>99%TLC) and l-phenylalanine (minimum 98% TLC) werepurchased from Sigma-Aldrich Chemie (Steinheim, Ger-many). Piperazine-N-N′-bis-(2-ethanosulfonate) solutions(10 mM, 0.15 M NaCl, pH 6.0 adjusted with NaOH 0.1 N)(PIPES buffer) were prepared with Piperacin-1,4-bis-(2-ethanosulfonic) acid supplied by Panreac Quimica S.A.(Barcelona, Spain) and Milli-Q ultra-pure water (18 M�).Sodium L( + )-ascorbate was supplied by Panreac QuimicaS.A. (Barcelona, Spain), whereas extra pure iron(III) chlo-ride hexahydrate (FeCl3·6H2O) was purchased from Schar-lau Chemie (La Jota, Barcelona, Spain).

Table 1 Studied model systems and their composition

LPS-M LPS-C Phe Met Leu Fe(III)/ascorbate PIPES

M ∗ ∗ ∗

M-AA ∗ ∗ ∗ ∗ ∗ ∗

C ∗ ∗ ∗

C-AA ∗ ∗ ∗ ∗ ∗ ∗

AA ∗ ∗ ∗ ∗ ∗

PIPES ∗ ∗

LPS-M, liposomes from PL of L. dorsi from Iberian pigs reared in“montanera” systems and fed on acorn and grass. LPS-C, liposomesfrom PL of L. dorsi from Iberian pigs reared indoors and fed onconcentrates.

The rest of reagents and solvents were of analytical gradeand purchased from Sigma-Aldrich Chemie (Steinheim, Ger-many).

Experimental design

Different meat-model systems were prepared: (i) AA, con-taining three selected amino acids (Phe, Met and Leu); (ii)M and C, containing liposomes made with PL from M andC pigs, respectively; (iii) M-AA and C-AA, containing bothliposomes and amino acids; and (iv) PIPES, containing onlythe PIPES buffer as blank model system. Liposomes wereprepared from L. dorsi muscles from Iberian pigs reared indifferent systems and fed on different diets: free-range rearedand exclusively fed on acorn and grass according to the tra-ditional “montanera” feeding systems (M) or reared indoorsand fed on concentrates (C). The final composition of the sixstudied model systems is presented in Table 1. The formula-tion of the model systems was chosen in order to simulate,in a simplified way, the main chemical components of dry-cured meat products and taking into account that the majorprecursors of Iberian ham flavour are amino acids and fattyacids, especially those from muscle cell membranes PL [1].

The experiment was carried out at 25 ◦C for 35 daysin a pro-oxidant environment (Fe(III)/ascorbate) in order tosimulate the conditions occurring during certain phases ofIberian hams ripening process in which the most relevantchemical reactions lead to volatile compounds generationare taken place.

The experimental analyses: Volatiles from the headspace,TBA-RS and yellowness indices, were carried out at day 0,7, 21, 28 and 35 of the experiment in each studied modelsystems.

Liposome preparation and characterization

Intramuscular total lipids were extracted according to themethod described by Bligh and Dyer [22]. The PL fractionwas separated from total lipid by using NH2-aminopropyl

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Eur Food Res Technol (2007) 225:665–675 667

minicolumns following the method described by Muriel et al.[23]. To eliminate all traces of solvent (methanol), the PLfraction was maintained under a nitrogen stream for 90 minuntil dryness. Polypropylene cone tubes containing PL wereflushed with nitrogen, capped with Teflon-lined screw capsand stored at frozen conditions prior to liposomes preparationin order to avoid oxidative reactions.

To prepare liposome suspensions, 30 ml of PIPES bufferwere added to tubes containing the PL fraction. The mixturewas vigorously stirred for 2 min, maintained in a thermosta-tized bath at 37 ◦C for 1 h and stirred again for 2 min each10 min in order to achieve the distribution of PL as mul-tilamellar vesicles (MLV). Large unilamellar vesicles wereprepared by extrusion of MLV 10 times through sterilizedcellulose acetated membrane filters (0.20 µm diameter ofpore). Final liposome suspensions were stored in sterilizedpropylene cone tubes at 4 ◦C for 12 h, until model systempreparation.

The PL concentration of liposomes suspensions wasquantified by measuring organic phosphorous following themethod described by Barlett [24]. The PL content was cal-culated considering that their average phosphorous contentwas 4% [25].

The fatty acid composition of PL was determined bypreparing their corresponding fatty acid methyl esters(FAMEs) by acidic trans-esterification in presence of sul-phuric acid (5% sulphuric acid in methanol) [26] prior toanalysis by gas chromatography using a Hewlett-PackardHP-5890A gas chromatograph coupled to a flame ionisationdetector (FID). Separation was carried out on a polyethy-lene glycol-TPA modified fused silica semicapillary column(30 m long, 0.53 mm i.d., 1 µm film thickness) maintainedat 225 ◦C. Injector and detector temperatures were 230 ◦C.The carrier gas was nitrogen at a flow rate of 1.8 ml min−1.Individual FAME peaks were identified by comparing theirretention times with those of standards (Sigma, St. Louis).

The peroxidisability index (PI) reflects the relative ratesof peroxidation of each fatty acids and was calculated fromthe following equation as described by Meynier et al. [16]:PI = (% monoenoic × 0.025) + (% dienoic × 1) + (%trienoic × 2) + (% tetranoic × 4) + (% pentaneoic × 6)+ (% hexanenoic × 8).

Model systems preparation

Three amino acid stock solutions (6.5 mg ml−1) (phenylala-nine, methionine and l-leucine) were prepared with PIPESbuffer and stored at 4 ◦C for 12 h until model systems prepa-ration. Three amino acids with different chemical character-istics were chosen: an aliphatic (L-Leu), an aromatic (Phe)and a sulphur (Met) amino acid. Sodium ascorbate and FeCl3stock solutions in ultra-pure Milli-Q water were daily pre-

pared and mixed in equimolar concentrations just beforebeing used.

All the stock solutions (amino acids, PIPES, FeCl3, ascor-bate and liposome suspensions) were filtered through ster-ilized of cellulose acetate membranes (0.2 µm pore diame-ter) before model system preparation, in order to guaranteethe absence of microbial contamination that could interferewith experimental results. All the laboratory material usedto prepare and store liposome suspensions and model sys-tems was sterilized in an autoclave (SELECTA, presoclave75). Moreover, in order to keep sterile conditions, model sys-tems preparation was carried out in a laminar flow chamber(TELSTAR, AV-30/70). All model systems (final volume of6 ml) were prepared in triplicate, in sterilized glass vials(12 ml) sealed with a teflon/silicone septum and contain-ing a final concentration of 0.5 mg PL/ml of liposomes,0.5 mg/ml of each amino acid and 45 µM of pro-oxidant so-lution (Fe(III)/ascorbate). The different components of themodel systems were poured into sterilized glass vials fromthe corresponding stock solutions in the following order:PIPES, liposomes, amino acids and finally the pro-oxidantsolution. All the mixed model systems were kept in an or-bital shaker device (Innova 4000, PACISA, New Bruswickscientific) at 25 ◦C in constant agitation (125 rpm) for 35days.

TBA reactive substances (TBA-RS)

Thiobarbituric acid reactive substances (TBA-RS) were mea-sured according to the method described by Buege and Aust[27] from aliquots of 0.250 ml of each model system. Briefly,2 ml of the TBA reagent [trichloroacetic acid (TCA) 15%,TBA 0.375%, hydrochloric acid (HCL) 0.25 M] and 750 µlof ultra-pure water (Milli-Q) were added to TBA tubes con-taining the aliquot. The reaction mixture was heated at 100 ◦Cfor 15 min. After cooling, the samples were centrifuged at2000 rpm for 15 min. The absorbance of supernatant wasmeasured at 532 nm, against a blank with a spectropho-tometer (Hitachi, U-2000). The results were expressed innmoles equivalent malonaldehyde per milligram PL (nmoleeq. MDA mg−1 PL) using the molar extinction coefficient ofthe TBA–MDA adducts at 532 nm (1.56 × 105 M−1 cm−1).

Browning measurement

The browning of the solutions (1 ml) was determined by us-ing the weighted-ordinate method [28]. Tristimulus values(X, Y, Z) were calculated from the transmittances (T) obtainedby spectrophotometry from 400 to 700 nm at constants inter-vals of 10 nm. Transmittance values were then converted intothe corresponding Tristimulus and into the CIELAB L∗, a∗

and b∗ colour values [29]. The yellowness index at different

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times was calculated according to Francis and Clydeslade[30] as follow: yellowness index = 142.86b∗/L∗.

Analysis of volatile compounds from the headspaceof model systems by SPME-GC-MS

Volatile compounds were analysed by HS-SPME and GC–MS (gas chromatograph Hewlett-Packard 5890 series II cou-pled to a mass selective detector Hewlett-Packard HP-5791A). One millimetre from each model system was poured intoa vial (4 ml) and sealed with a teflon/silicone septum. A mag-netic stirrer was placed into the model systems for stirringduring the extraction. An SPME fibre (85 µm carboxen-polidimethylsiloxane coating) was inserted throughthe septum and exposed to the headspace of thevial.

Vials were preconditioned for 30 min at 37 ◦C. Extractionwas carried out at 37 ◦C for 30 min in a water bath. Af-ter extraction, the SPME fibre was immediately transferredto the injector of the chromatograph (gas chromatographHewlett-Packard 5890 series II) which was in splitless modeat 280 ◦C. The fibre was desorbed for 30 min. The separa-tion of volatile compounds was performed on a 5% phenyl-methyl silicone (HP-5) bonded phase fused silica capillarycolumn (Hewlett-Packard, 50 m × 0.32 mm i.d., film thick-ness 1.05 µm), with helium as carrier gas operating at 6 psiof column head pressure, resulting in a flow of 1.3 ml min−1

at 40 ◦C. Oven program was 40 ◦C for 10 min, 5 ◦C min−1 to200 ◦C, 20 ◦C min−1 to 250 ◦C, and held 250 ◦C for 5 min.Transfer line to the mass spectrometer was maintained at280 ◦C. The mass spectra were obtained using a mass se-lective detector (Hewlett-Packard HP-5791 A) by electronicimpact at 70 eV, a multiplier voltage of 1756 V and col-lecting data at a rate of 1 scan s−1 over the m/z range 30–500.

Identification of volatile compounds

Eighteen volatile compounds were positively identified bycomparing their linear retention indices (LRI) and mass spec-tra with those of standard compounds (Sigma, St. Louis,USA) run in the same conditions. The rest were tentativelyidentified by comparing their mass spectra with those re-ported in the Wiley library and by comparing their LRI withthose reported in the literature [31] and in the NIST database[32]. Results from the volatiles analysis are provided in areaunits (AU).

Microbial analysis

An aliquot of 100 µl was taken from the model systemsthe last day of the experiment (day 35) and inoculated inPetri plates containing Standard plate count agar (PCA) in

Table 2 Fatty acid profile and peroxidisability index (PI) of polarlipids from liposomes included in the studied model systems

∑SFA

∑MUFA

∑PUFA

∑n6

∑n3 PI

LPS-C 33.03 15.44 51.53 48.79 2.32 97.09LPS-M 44.24 16.60 38.21 35.74 2.10 76.55

Results of total SFA, MUFA, PUFA, n6 and n3 are expressed in per-centage of total FAME. LPS-M, liposomes from PL of L. dorsi fromIberian pigs reared in “montanera” systems and fed on acorn and grass.LPS-C, liposomes from PL of L. dorsi from Iberian pigs reared indoorsand fed on concentrates.

order to confirm the absence of microbial growth in themodel systems during the experiment. Plates were examinedafter 48 h of incubation at 30 ◦C for microbial counts. Allthe model systems that presented microbial growth werediscarded.

Results and discussion

Fatty acid profile of PL

Table 2 shows the proportion of total saturated fatty acids(SFA), monounsaturated fatty acids (MUFA), polyunsatu-rated fatty acids (PUFA), n − 6 and n − 3-fatty acids, and thePI of liposomes made with PL from M and C pigs. Overall,the fatty acid composition of PL isolated from L. dorsi mus-cle tended to reflect that of the diets. C liposomes showed ahigher proportion of PUFA, primarily n − 6 fatty acids, and alower of MUFA and SFA than liposomes from M pigs. Con-sistently, liposomes from C pigs had higher values of PI thanthose from M animals (97.09 vs. 76.55) as a consequence ofa higher PUFA content that could affect their oxidative sus-ceptibility. The influence of the feeding systems on the fattyacid composition of PL of L. dorsi muscle will be profuselydiscussed elsewhere.

Liposome oxidation

The evolution of TBA-RS expressed as nmoles of MDA/mgPL from studied model system is shown in Fig. 1. TBA-RSvalues found in this work are similar to those reported inoxidized liposomes from PL of turkey breast [16] and fromPL of porcine muscle [33]. In the present study, the highestTBA-RS detected were those recorded at day 0 whateverthe model system, and thereafter, there was a slight decreaseuntil the end of the experiment. This evolution of TBA-RSagrees with the tendency observed during the Iberian hamprocess, in which an increase in TBA-RS and a subsequentdecrease and stabilization have been described [2]. Neverthe-less, Meynier et al. [16] and Kansci et al. [33], working withoxidized liposomes, reported an increase in TBA-RS num-bers throughout all the experiment. However, these studies

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0.00

5.00

10.00

15.00

20.00

25.00

0 5 10 15 20 25 30 35

Time (days)

nmol

es M

DA

/mg

PL

M C M-AA C-AA

Fig. 1 Liposome oxidation measured as TBA-RS values in the stud-ied model systems. M: system containing only liposomes made withpolar lipids from pigs reared on “montanera” and fed on acorns andgrass. C, system containing only liposomes made with polar lipids frompigs reared indoors and fed on concentrates. M-AA, system contain-ing amino acids and liposomes from M pigs. C-AA, system containingamino acids and liposomes from C pigs

were carried out only for 24 h, whereas in the present work,the second measurement of TBA-RS was carried out at day 7of the experiment, which could explain the lack of TBA-RSincrease over time.

In model systems containing only amino acids (AA) orPIPES buffer, no TBA-RS were detected, and therefore theyare not shown in Fig. 1.

Overall, model systems containing both amino acids andliposomes (M-AA and C-AA) revealed higher TBA-RS thanmodel systems containing only liposomes (M and C sys-tems). It could be that products of oxidized amino acidshad enhanced lipid oxidative reactions leading to higherTBA-RS. Protein oxidation is manifested by free radicalchain reactions similar to those for lipid oxidation, whichinvolve initiation, propagation and termination stages [34].Oxidation products, mainly free radicals and carbonyl com-pounds, of the three amino acids selected for this stud-ied (Met, Leu and Phe) were described by Stadman andBerlett [35]. However, it is surprising that M-AA and C-AA model systems showed higher TBA-RS values thantheir counterparts model systems containing only liposomes(M and C), since the possible formation of derivativesfrom of amino-carbonyl reactions with an acknowledged an-tioxidant activity [36] should have limited lipid oxidativephenomena.

Model systems containing liposomes from C pigs (C andC-AA) tended to show higher TBA-RS values than theirM counterparts (M and M-AA) except in the C-AA modelsystem on days 21 and 28. This could be due to the higher ox-idative susceptibility of the liposomes from C pigs comparedto those from M ones, because of their greater proportion ofPUFA and their higher PIs (Table 2). Accordingly, Meynieret al. [16] reported higher TBA-RS in oxidized liposomes

0.00

10.00

20.00

30.00

0 5 10 15 20 25 30 35Time (days)

YI

M C M-AA C-AA AA

Fig. 2 Browning development in model systems containing onlyamino acids (AA) or only liposomes from M and C pigs (M and C) orboth liposomes and amino acids (M-AA and C-AA) measured as yel-lowness index (YI) (average and standard error). M, system containingonly liposomes made with polar lipids from pigs reared on “montanera”and fed on acorns and grass. C, system containing only liposomes madewith polar lipids from pigs reared indoors and fed on concentrates. M-AA, system containing amino acids and liposomes from M pigs. C-AA,system containing amino acids and liposomes from C pigs

from PL of turkey fed on diets enriched with oils with highPUFA content and high PI values than in those from turkeyfed on tallow-enriched diet.

Browning development

Figure 2 shows the evolution of yellowness index in the stud-ied model systems throughout incubation time. As expected,the control model system (containing only the PIPES buffer)and the systems containing only amino acids (AA) almostdid not develop colour. In contrast, browning developmentwas observed in model system containing both liposomesand amino acids (C-AA and M-AA) and in those with onlyliposomes (C and M).

The M-AA model system exhibited higher values for yel-lowness index than its counterpart model system containingonly liposomes (M). Moreover, M-AA model systems re-vealed the highest yellowness indices throughout the wholeexperiment, except on day 0. The occurrence of lipid oxida-tion derived compounds–amino acid interactions in modelsystems containing both liposomes and amino acids couldexplain these results, since the products of such type of reac-tions have been described as being responsible of “brown”colour development [37].

The development of non-enzymatic browning in modelsystems containing only liposomes could be due to the for-mation of pigments as a consequence of the reaction be-tween the amino group of certain PL of liposomes, par-ticularly phosphatidylethanolamine, with carbonyl lipid-oxidation products [38]. Phosphatidylethanolamine is thesecond major phospholipid (21–22%) in the intramuscularfat of Iberian pigs [39] and, therefore, in the liposomes ofthe studied model systems.

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Volatile compounds

Forty-seven out of 180 volatile compounds detected in theheadspace of M, M-AA, C, C-AA and AA model systems onday 0, 7, 21, 28 and 35 of the experiment were selected andare shown in Table 3 in accordance with their most probableorigin. These compounds were chosen on the basis of theirpresence in most model systems and on their proportionof chromatographic area in the headspace of the studiedmodel systems. Selected compounds included 27 aldehydes,7 ketones, 5 alcohols, 4 hydrocarbons, 4 sulphur compounds,2 furans, 1 acid and 1 thiazole.

Major compounds detected in the headspace of the stud-ied model systems were typical fatty acids oxidation products[9]. Among these, the most abundant lipid-derived volatiles(above 108 AU of chromatographic area at day 7) in M, M-AA, C and C-AA model systems were saturated aldehydes(pentanal, hexanal, heptanal, octanal and nonanal), unsat-urated aldehydes (hept-(Z)-2-enal, oct-(E)-2-enal) and theco-elution of oct-1-en-3-one/oct-1-en-3-ol. Volatiles fromStrecker degradation of the selected amino acids [40–42] (methanethiol, 3-methylbutanal, dimethyl disulphide,dimethyl sulphide, trimethyl trisulphide, methional, ben-zaldehyde and phenylacetaldehyde) were mainly isolatedfrom the headspace of model systems containing both li-posomes and amino acids.

Volatile profiles described from the headspace of the Mand C model systems (Table 3) are similar to those previouslyreported in the headspace of model systems containing lipo-somes from PL from turkey breast [16] and from porcineL. dorsi [33] under similar pro-oxidative conditions. In ad-dition, compounds listed in Table 3 have been previouslyreported as volatile components of Iberian dry-cured prod-ucts, including Iberian hams [6, 23] and other dry-cured meatproducts [43]. Moreover, some of them have been previouslydescribed as odour active compounds in Iberian ham withdifferent odour descriptors [12, 13] such as methanethiol,3-methylbutanal, pent-1-en-3-one, hexanal, hex-(E)-2-enal,heptan-2-one, oct-1-en-3-one/ oct-1-en-3-ol and octanal.

Regarding to lipid-derived volatiles, diet affected volatilecompounds formed in model systems containing liposomes(Table 3). Thus, systems containing liposomes from C pigs,with high levels of PUFA and n − 6 fatty acids (C and C-AA), showed greater chromatographic areas of lipid-derivedvolatiles, particularly of those derived from n − 6 fatty acidssuch as hexanal, deca-(E,E)-2,4-dienal and 2-pentyl-furan,which arise mainly from linoleic acid (C18:2, n − 6) [17, 43].Hexanal was the major volatile whatever the studied modelsystems. Hexanal and deca-(E,E)-2,4-dienal have been as-sociated with the development of warmed over flavour incooked meat [9] and hexanal content has shown good cor-relations with rancid notes in dry-cured ham [1]. Modelsystems containing liposomes from C pigs exhibited higher

chromatographic areas of deca-(E,E)-2,4-dienal than thosecontaining liposomes from M animals, in accordance withstudies in Iberian hams where this volatile compound hasbeen largely reported in hams from pigs fed on concentrates(high PUFA content) than in those from pigs fed on acornsin “montanera” system.

All Strecker-derived volatile compounds were detected inthe headspace of model system containing liposomes and theselected amino acids Leu, Met and Phe. These compounds in-cluded methyl branched aldehydes such as 3-methylbutanaland 3-methylthiopropanal derived from Leu and Met, re-spectively, sulphur compounds such as methanethiol anddimethyl disulphide, derived from Met, and aromatic com-pounds derived from Phe such as benzaldehyde and pheny-lacetaldehyde [4, 40–42]. Dimethyl disulphide is an oxida-tion product of methanethiol and can further react to formdimethyl trisulphide and dimethyl sulphide [43], which havebeen also isolated from the headspace of studied model sys-tems.

The Strecker degradation of amino acids, which is a minorpathway of the Maillard reactions [44], involves the oxidativedeamination and decarboxilation of α-amino acids in thepresence of compounds such as reducing sugars or somelipid-oxidation products [40]. The temperature, pH and pro-oxidative experimental conditions established for the studiedmodel systems for 35 days, similar to those present during theprocessing of dry-cured meat products, seem to encouragethe generation of a great number of lipid-derived carbonylcompounds, particularly volatile aldehydes, that could havereacted with the amino acids from the model systems, leadingto the formation of aforementioned Strecker compounds.Several volatile compounds such as malondialdehyde [45],deca-2,4-dienal [46] and hexanal [47] have been reportedas lipid-oxidation products that can react with the reactivegroups of the amino acids and proteins.

Ventanas et al. [4] have previously hypothesized that re-actions between free amino acids and carbonyls could bean important source of flavour compounds in Iberian dry-cured hams. However, Hinrichsen et al. [7, 8] establishedthat methyl branched aldehydes including 3-methylbutanalcan be generated by secondary metabolism of microorganismand also claimed that Maillard and Strecker reactions onlytake place at higher temperatures than those reached dur-ing dry-cured ham processing. Nevertheless, the detectionof Strecker-derived volatiles in the studied model systemssupports the occurrence of this type of reaction under the es-tablished environmental conditions, similar to those reachedduring dry-cured ham process. Moreover, our results are inagreement with those reported by other authors who havedescribed the occurrence of Strecker reactions at low temper-atures in different model systems [41, 42]. Previous studieshave revealed low microbial counts inside dry-cured hams[48]. Thus, microbial metabolism cannot be considered the

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3.

Springer


only possible source of generation branched aldehydes indry-cured products.

Numerous studies have been aimed to study Strecker-derived volatiles formation in model systems containingamino acids in the presence of ribose [18, 19], glucose [44]epoxy derivates compounds [38, 42] and other carbonyl com-pounds such us diacetyl o pentan-2,3-dione [41]. In otherstudies, PL [21] or lipidic systems such as cocoa butter[40] were included into amino acid–sugar model systems.However, most of these studies were performed at high tem-peratures to simulate the conditions during meat cooking.In some studies, experimental temperatures were low, 25 ◦C[41] or 37 ◦C [38, 42]. Nevertheless, in these papers, the gen-eration of Strecker-derived compounds was achieved usingsynthetic carbonyl compounds and was performed for a shortperiod of incubation (overnight) [41]. To our knowledge, thisis the first study in which the generation of Strecker-derivedvolatile compounds in model systems mimicking meat prod-ucts containing amino acids and lipids without any reducingsugar is shown.

Overall, M-AA systems exhibited greater chromato-graphic area than C-AA ones for methanethiol, dimethyldisulphide and 3-methyl-butanal (Table 3). The majorStrecker-derived volatile compound in M-AA model sys-tems was dimethyl disulphide, whereas in C-AA systems themajor compound was phenylacetaldehyde.

The evolution of chromatographic areas of lipid-derivedand Strecker-derived volatiles in model systems containingboth liposomes and amino acids are shown in Fig. 3. Overall,the variety of lipid-oxidation-derived volatiles (Table 3) andtheir chromatographic areas increased from day 0 to 7 andthereafter decreased slightly until the end of the experiment.Strecker volatiles exhibited the opposite behaviour, partic-ularly in model systems containing both amino acids andliposomes (M-AA and C-AA). In M-AA systems Strecker-derived compounds increased throughout incubation time,whereas lipid-derived volatiles decreased, probably due tothe reaction between carbonyl compounds derived from lipidoxidation and amino acids, leading to Maillard and Streckervolatiles formation. Previous studies in Iberian dry-curedham have described a similar evolution of volatile com-pounds during processing [6, 49]. Although certain Streckercompounds were found in systems without amino acids, theirchromatographic areas were considerably smaller than thosereported in systems containing amino acids. The detectionof Strecker-derived volatiles in model systems containingonly liposomes supports that the amino group of phospho-lipids [36], as those of amino acids or proteins, can re-act with lipid-oxidation products, leading to the formationof amino-cabonyl reactions products and Strecker-derivedcompounds.

The proportion of compounds from lipid oxidation andStrecker degradation could be very different to that of real

0.E+00

1.E+09

2.E+09

3.E+09

4.E+09

5.E+09

6.E+09

0 5 10 15 20 25 30 35

Time (days)

AR

EA

UN

ITS

M-AA ldv M-AA sdv C-AA ldv C-AA sdv

Fig. 3 Evolution of chromatographic areas of lipid-derived volatiles(ldv) and Strecker-derived volatiles (sdv) (identification shown in Table3) in model systems containing amino acids and liposomes from eitherM (M-AA) or C (C-AA) pigs

dry-cured meat samples due to the considered amounts ofprecursors in the model systems and differences with thereal conditions in a meat product (presence of antioxidant andprooxidant substances, presence of micro organisms, oxygenavailability, etc.). In fact, oxygen could have been partiallyconsumed due to lipid oxidation reactions, leading to a lowerrate of lipid oxidation. However, given that total lipid content(in those systems containing liposomes) was 3 mg, and thatthe total volume of air in the systems was more than 6 ml(12 ml vials containing 6 ml of model system), the initialoxygen content was most likely sufficient to allow oxidationto develop during the whole experiment.

Similarly to the results concerning the evolution of theyellowness index over the time (Fig. 2), M-AA systemsexhibited higher chromatographic areas of Strecker-derivedvolatiles than C-AA ones. In fact, in C-AA model systems,no increase in either browning or Strecker-derived volatileswas observed over time. Moreover, the similar behaviour ofbrowning and Strecker degradation throughout the experi-ment suggests that both reactions take place simultaneously,and that Strecker-derived volatiles, together with other Mail-lard reaction products, could contribute to browning devel-opment.

Oxidized phospholipids, particularly oxidized phos-phatidyletanolamine, could also have played an importantrole in inhibiting lipid oxidation phenomena. In fact, Hi-dalgo et al. [36] have reported that the antioxidative activityof phospholipids increases during their oxidation as a con-sequence of amino-carbonyl reactions and the subsequentformation of compounds with antioxidative properties.

Acknowledgements Sonia Ventanas would like to thank the ‘Ministe-rio de Educacion’ for the ‘FPU’ grant support. This study was supportedby the “Ministerio de Ciencia y Tecnologıa” project entitled: “Haciael establecimiento de predictores de calidad en la materia prima y enproductos del cerdo iberico mediante parametros fisico-quımicos (UL-

Springer


TRAFAT)” (Project AGL 2001-6932-01). Authors are grateful to Dr.Francisco J. Hidalgo (Instituto de la Grasa, Sevilla, SPAIN) for hisscientific advices during the development of this study

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