Model studies on acrylamide generation from glucose/asparagine in aqueous glycerol

Acrylamide formation from asparagine and glucose in different ratios in neutral glycerol/water mixtures was found to increase with decreasing water activity (0.33 < or = aw < or = 0.71 investigated) and increasing temperature (120 degrees C < or = T < or = 160 degrees C investigated). The initial rate of acrylamide formation was found to be approximately proportional to the asparagine concentration for an excess of asparagine, but less dependent on an excess of glucose. A steady-state concentration of acrylamide was established at 160 degrees C after 1 h for aw = 0.33 (30 microg x L-1 for GLU:ASN = 10:1, 11 microg x L-1 for GLU:ASN = 1:1, and 130 microg x L-1 for GLU:ASN = 1:10) and for aw = 0.47 (15 microg x L-1 for GLU:ASN = 10:1 and 80 microg x L-1 for GLU:ASN = 1:10), suggesting a protection by glucose against acrylamide degradation. The energy of activation, as estimated from the temperature dependence of the initial rate, increased with decreasing aw despite a higher rate of formation of acrylamide at low aw. For high aw, water elimination from a reaction intermediate is suggested to be rate determining. For low aw, the increase in energy of activation (and enthalpy of activation) is accordingly counteracted by a more positive entropy of activation, in agreement with decarboxylation as rate determining at low aw.

Amino acid and protein scavenging of radicals generated by iron/hydroperoxide system: an electron spin resonance spin trapping study

Reduction of free radicals generated by Fe(II)/cumene-hydroperoxide (CumOOH) by amino acids (Gly, Cys, Met, His, and Trp) and proteins (bovine serum albumin (BSA), beta-lactoglobulin, and lactoferrin) was followed by electron spin resonance spectroscopy using alpha-phenyl-N-tert-butylnitrone (PBN), 2-methyl-2-nitrosopropane (MNP), and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin traps. The radical species detected were mostly carbon-centered radicals from CumOOH fragmentation (methyl/*H3 and ethyl/*H2CH3), although carbon-centered radicals originated from amino acids could be formed in the presence of Cys, Met, His, or Trp. All proteins and amino acids, except Cys, were effective at inhibiting generation of radicals from the Fe(II)/CumOOH system. Trp was the amino acid with the highest antiradical activity, followed by His > Gly approximately Met. Lactoferrin was the protein showing the most efficient inhibition of radical formation from the Fe(II)/CumOOH system, and BSA and beta-lactoglobulin were not significantly different in their antiradical activities. These results suggest that proteins with higher inhibitory activity on lipid oxidation promoted by transition metal catalytic decomposition of hydroperoxides should be those with elevated metal-chelating and radical-scavenging properties as well as low concentration and accessibility of reducing groups from amino acids capable of activating metals, such as sulfhydryl groups.

Flavin-induced photodecomposition of sulfur-containing amino acids is decisive in the formation of beer lightstruck flavor

Photooxidation of sulfur-containing amino acids and derivatives readily occurs upon visible-light irradiation in the presence of flavins. The sulfur moiety seems pivotal for interaction, as was determined from kinetic analyses using laser flash photolysis spectroscopy. After photooxidation, the resulting radical intermediates were characterized by addition to a spin trap, followed by electron paramagnetic resonance spectroscopy and evaluation of the coupling constants. The presence of the proposed radical intermediates was strongly supported by the identification of the reaction products using mass spectrometry. Accordingly, feasible degradation pathways for various sulfur-containing amino acids and derivatives were proposed. It was finally proven that flavin-induced photoproduction of sulfhydryl radicals and recombination with a 3-methylbut-2-enyl radical, derived from the photodegradation of hop-derived isohumulones, are decisive in the formation of beer lightstruck flavor.

[Antioxidants and health]

The Danish Fitness and Nutrition Council has evaluated the basis for recommendations on the intake of antioxidants and has found limited basis for increasing the recommended intake levels for the antioxidants vitamin C and E. Evidence was insufficient to support recommendations for polyphenol or carotenoid supplementation. Supplementation with high doses of vitamin E and beta-carotene may present a health risk. A high intake of fruit and vegetables is associated with a reduced risk of lifestyle diseases, but there is no evidence that this association is due to an antioxidant effect.

Phenol and terpene quenching of singlet- and triplet-excited states of riboflavin in relation to light-struck flavor formation in beer

Phenolic compounds present in beer were shown by fluorescence spectroscopy and laser flash photolysis to deactivate both singlet- and triplet-excited states of riboflavin with bimolecular rate constants close to the diffusion control ranging from 2.8x10(9) to 1.1x10(10) M-1 s-1 and from 1.1x10(9) to 2.6x10(9) M-1 s-1, respectively. Enthalpies of activation were low (up to 33.2 kJ mol-1), and entropies of activation were positive, ranging from 17 to 92 J mol-1 K-1, as derived from temperature dependence, indicating a compensation effect. From a Stern-Volmer analysis of the singlet-excited riboflavin quenching by phenols it was found that high amounts of phenolic compounds (>0.3 M) would be needed to hinder triplet-excited riboflavin generation. On the other hand, a phenolic content of 0.36 mM is likely to quench 90% of the triplet-excited state. Phenol photodegradation was found to be complex, and using ESI-MS analysis it was not possible to identify specific photooxidation products of the phenolic compounds; only the photoproducts of riboflavin could be detected and structurally assigned. The rate of reaction of triplet-excited riboflavin with phenolic compounds in acetonitrile/citrate buffer (pH 4.6, 10 mM) is 550 times faster than the reaction with iso-alpha-acids from hops, indicating that triplet-excited quenchers such as phenols may be involved in the early steps in light-struck flavor formation in beer through radical formation. Terpenes present in herb-flavored beers were found to be nonreactive toward singlet- and triplet-excited-state riboflavin, and any protection depends on other mechanisms.

Oxidation of bovine serum albumin initiated by the Fenton reaction--effect of EDTA, tert-butylhydroperoxide and tetrahydrofuran

Oxidation of bovine serum albumin (BSA) was investigated using different oxidants: The water-soluble azo-initiator 2,2'azo-bis-(2-amidinopropane) hydrochloride (AAPH), a combination of FeCl(3) and ascorbate or the Fenton oxidant consisting of FeCl(2), H(2)O(2) and EDTA. In addition, the effects of exogenous compounds such as tert-butyl hydroperoxide (tBuOOH) or solvents such as tetrahydrofuran (THF), often used in model systems, was evaluated. The extent of protein damage was studied by measuring protein carbonyl groups and protein hydroperoxides. The interaction between Fenton oxidant and EDTA, THF or tBuOOH was further characterized using spin trapping electron spin resonance (ESR) spectroscopy. The results showed that the extent of protein oxidation depended on the oxidant used. The Fenton oxidant was the most reactive of the initiators tested. However, in the absence of EDTA, the Fenton system produced protein carbonyl groups on BSA equivalent to that obtained with the other oxidants, however, significantly more protein hydroperoxide was produced. Surprisingly, it was also found that addition of tBuOOH or THF to BSA reduced protein damage when the oxidation was initiated with the Fenton oxidant. ESR investigation showed that EDTA played a key role in the generation of free radicals. It was also revealed that in an EDTA containing system both tBuOOH and THF were able to react with radicals without inducing protein damage in effect protecting BSA from oxidative damage.

Molecular mechanism of antioxidant synergism of tocotrienols and carotenoids in palm oil

During repeated deep-fat frying of potato slices at 163 degrees C in yellow or red palm olein of comparable fatty acid profiles, the oxidative stability (peroxide value and anisidine value) of the palm oleins was similar, and in yellow palm olein, the rate of antioxidant depletion decreased in the order gamma-T3 > alpha-T3 > delta-T3 (T3, tocotrienol). In red palm olein, which had a total tocopherol/tocotrienol content of 1260 vs 940 ppm in yellow palm olein and a corresponding longer induction period in the Rancimat stability test at 120 degrees C, only depletion of gamma-T3 was significant among the phenols during frying and slower as compared to that in yellow palm olein. The carotenes in the red palm olein were depleted linearly with the number of fryings, apparently yielding an overall protection of the phenols. In antioxidant-depleted palm olein and in phospholipid liposomes with added increasing concentrations of phenols, gamma-T3 was found to be a better antioxidant than alpha-T3. alpha-T3 and alpha-T (T, tocopherol) had a similar antioxidant effect in antioxidant-depleted palm olein in the Rancimat stability test, while in the liposomes the ordering as determined by induction period for the formation of conjugated dienes was gamma-T3 > alpha-T3 > alpha-T. The addition of 100-1000 ppm beta-carotene to antioxidant-depleted palm olein or liposomes (lycopene also tested) did not provide any protection against oxidation. In the liposomes, synergistic interactions were observed between beta-carotene or lycopene and alpha-T, alpha-T3, or gamma-T3 for carotene/phenol ratios of 1:10 and 1:2 but not for 1:1. In chloroform, carotenes were regenerated by tocopherols/tocotrienols from carotene radicals generated by laser flash photolysis as shown by transient absorption spectroscopy, suggesting that carotenes rather than phenols are the primary substrate for lipid-derived radicals in red palm olein, in effect depleting carotenes prior to phenols during frying. Regeneration of carotenes by the phenols also explains the synergism in liposomes. In the laser flash photolysis experiments, gamma-T3 was also found to be faster in regenerating carotenes than alpha-T3 and alpha-T.

Radical cation generation from singlet and triplet excited states of all-trans-lycopene in chloroform

On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1B(u)-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-beta-carotene in generating the radical cation. The time constant of radical cation generation from the 1B(u)-type state was found to be approximately 0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for beta-carotene. A slow rising phase (20-30 micros) in the bleaching of ground-state absorption was common for both lycopene and beta-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.

Comparison of Descriptive Sensory Analysis and Chemical Analysis for Oxidative Changes in Milk

Oxidation in 3 types of bovine milk with different fatty acid profiles obtained through manipulation of feed was evaluated by analytical methods quantifying the content of potential antioxidants, the tendency of formation of free radicals, and the accumulation of primary and secondary oxidation products. The milk samples were evaluated in parallel by descriptive sensory analysis by a trained panel, and the correlation between the chemical analysis and the descriptive sensory analysis was evaluated. The fatty acid composition of the 3 types of milk was found to influence the oxidative and lipolytic changes occurring in the milk during chill storage for 4 d. Sensory analysis and chemical analysis showed high correlation between the typical descriptors for oxidation such as cardboard, metallic taste, and boiled milk and specific chemical markers for oxidation such as hexanal. Notably, primary oxidation products (i.e., lipid hydroperoxides) and even the tendency of formation of radicals as measured by electron spin resonance spectroscopy were also highly correlated to the sensory descriptors for oxidation. Electron spin resonance spectroscopy should accordingly be further explored as a routine method for detection of early events in lipid oxidation in milk to predict shelf-life.

Mechanism of Radical Cation Formation from the Excited States of Zeaxanthin and Astaxanthin in Chloroform

The C-40 xanthophylls zeaxanthin and astaxanthin were confirmed to form radical cations, Car.+, in the electron-accepting solvent chloroform by direct excitation using subpicosecond time-resolved absorption spectroscopy in combination with spectroelectrochemical determination of the near-infrared absorption of Car.+. For the singlets, the S2(1B(u+) state and most likely the S(x)(3A(g)-) state directly eject electrons to chloroform leading to the rapid formation of Car.+ on a timescale of approximately 100 fs; the lowest-lying S1(2A(g)-) state, however, remains inactive. Standard reduction potential for Car.+ was determined by cyclic voltametry to have the value 0.63 V for zeaxanthin and 0.75 V for astaxanthin from which excited state potentials were calculated, which confirmed the reactivity toward radical cation formation. On the other hand, Car.+ formation from the lowest triplet excited state T1 populated through anthracene sensitization is mediated by a precursor suggested to be a solute-solvent complex detected with broad near-infrared absorption to the shorter wavelength side of the characteristic Car.+ absorption. However, ground state carotenoids are able to react with a secondary solvent radical to yield Car.+, a process occurring within 16 micros for zeaxanthin and within 21 mus for astaxanthin. Among the two xanthophylls together with lycopene and beta-carotene, all having 11 conjugated double bonds, zeaxanthin ranks with the highest reactivity in forming Car.+ from either the S2(1B(u+)) or the ground state. The effects of substituent groups on the reactivity are discussed.

Water activity-temperature state diagram of amorphous lactose

Development in the glass transition temperature, Tg, to reach a stabilized value for amorphous lactose stored at 5, 25, and 38 degrees C at different water activities varying from aw = 0.21 to 0.59 was followed by differential scanning calorimetry. Combinations of stabilized Tg and water activity were used for interpolation of the value of water activity, where Tg was equal to the storage temperature. These values of water activity were used to construct a state diagram in the (aw,T)-plane for lactose in the amorphous state from which critical combinations of water activity and storage temperature may be obtained for optimization of storage conditions of lactose-based dry products.

Heterometallic manganese/zinc-phytate complex as a model compound for metal storage in wheat grains

Myo-inositol-1,2,3,4,5,6-hexakisphosphate, also known as phytate, is a natural metal chelate present in cereals, an important feedstock worldwide. This article reports the characterization of three metal storage model complexes: the homometallic Mn(II) myo-inositol-1,2,3,4,5,6-hexakisphosphate (IP6), the heterometallic Zn(II), Mn(II) analogue Na4MnZn4(C6H6O24P6) x (NO3)2 x 8H2O (MnZn4IP6) and the homometallic Zn(II) metal complex Na3Zn5(C6H6O24P6)OH x 9H2O (Zn5IP6). The techniques of high-resolution 23Na, 13C and 31P NMR, electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) were applied in this study. The complexation of Zn(II) and Mn(II) by phosphate groups of IP6 is demonstrated by NMR and XPS results. 13C NMR results show a conformation for IP6 consisting of five equatorial phosphate groups to one axial group showing only one chemical environment for Zn and two for Mn, when characterized by XPS and EPR, in both Mn complexes. These results support, for the first time, a probable supramacromolecular structure for phytate complexes of transition metals. Based on the similarity between the EPR spectra of wheat seeds and that of the MnZn4IP6 compound, the manganese storage centers in wheat grains can be assigned to similar heterometallic phytate complexes.