Isomerization of Cholecalciferol through Energy Transfer as a Protective Mechanism Against Flavin-Sensitized Photooxidation

Cholecalciferol, vitamin D3, was found to isomerize to 5,6-trans-vitamin-D3 with a quantum yield of 0.15 ± 0.01 in air-saturated 7/3 tert-butyl alcohol/water (v/v) at 25 °C, increasing to 0.32 ± 0.02 in the absence of oxygen, through quenching of triplet excited state flavin mononucleotide, FMN, rather than becoming oxidized. The quenching was found by laser flash photolysis to have a rate constant of 1.4 × 10(8) L mol(-1) s(-1) in 7/3 tert-butyl alcohol/water (v/v) at 25 °C, assigned to energy transfer from (3)FMN* to form a reactive vit.D3 diradical. vit.D3 forms a 1/1 precomplex with FMN by hydrophobic stacking with ΔH° = -36 ± 7 kJ mol(-1) and ΔS° = -4 ± 3 J mol(-1) K(-1), as shown by single photon counting fluorescence spectroscopy and steady-state fluorescence spectroscopy. Both ground-state precomplex formation and excited-state energy transfer seem important for vit.D3 protection against flavin-sensitized photooxidation of nutrients in food and biological systems.

Synergism between soluble and dietary fiber bound antioxidants

This study investigates the synergism between antioxidants bound to dietary fibers (DF) of grains and soluble antioxidants of highly consumed beverages or their pure antioxidants. The interaction between insoluble fractions of grains containing bound antioxidants and soluble antioxidants was investigated using (i) a liposome-based system by measuring the lag phase before the onset of oxidation and (ii) an ESR-based system by measuring the reduction percentage of Fremy's salt radical. In both procedures, antioxidant capacities of DF-bound and soluble antioxidants were measured as well as their combinations, which were prepared at different ratios. The simple addition effects of DF-bound and soluble antioxidants were compared with measured values. The results revealed a clear synergism for almost all combinations in both liposome- and ESR-based systems. The synergism observed in DF-bound-soluble antioxidant system paints a promising picture considering the role of fiber in human gastrointestinal (GI) tract health.

Evaluation of physical integrity of lipid bilayer under oxidative stress: application of fluorescence microscopy and digital image processing

Membrane damage as a result of oxidative stress is quantified using digital image heterogeneity analysis of single giant unilamellar vesicles (GUVs) composed of soy phosphatidylcholine (PC), which were found to undergo budding when containing chlorophyll a (Chla) as photosensitizer in the lipid bilayer. Based on digital image heterogeneity analysis, a dimensionless scalar parameter "entropy" for the budding process was found to change linearly during an initial budding stage. Photo-induced peroxidation of PC to form linoleoyl hydroperoxides, further leading to domains of higher polarities in GUVs, was suggested to initiate the budding process. The effect on budding process of GUVs was suggested for use in assays for evaluation of potential protectors of lipid bilayer integrity under oxidative stress, and "entropy" seemed to be a valid descriptor of such membranal integrity. The one-step procedure for quantification of prooxidative effects and antioxidative protection provided by drug candidates and potential food ingredients in membranes could be easily automated for direct measurement of oxidative and antioxidative effects on cellular integrity.

Flavonoids protecting food and beverages against light

Flavonoids, which are ubiquitously present in the plant kingdom, preserve food and beverages at the parts per million level with minor perturbation of sensory impressions. Additionally, they are safe and possibly contribute positive health effects. Flavonoids should be further exploited for the protection of food and beverages against light-induced quality deterioration through: (1) direct absorption of photons as inner filters protecting sensitive food components; (2) deactivation of (triplet-)excited states of sensitisers like chlorophyll and riboflavin; (3) quenching of singlet oxygen from type II photosensitisation; and (iv) scavenging of radicals formed as reaction intermediates in type I photosensitisation. For absorption of light, combinations of flavonoids, as found in natural co-pigmentation, facilitate dissipation of photon energy to heat thus averting photodegradation. For protection against singlet oxygen and triplet sensitisers, chemical quenching gradually decreases efficiency hence the pathway to physical quenching should be optimised through product formulation. The feasibility of these protection strategies is further supported by kinetic data that are becoming available, allowing for calculation of threshold levels of flavonoids to prevent beer and dairy products from going off. On the other hand, increasing understanding of the interplay between light and matrix physicochemistry, for example the effect of aprotic microenvironments on phototautomerisation of compounds like quercetin, opens up for engineering better light-to-heat converting channels in processed food to eventually prevent quality loss.

Competitive reduction of perferrylmyoglobin radicals by protein thiols and plant phenols

Radical transfer from perferrylmyoglobin to other target species (myofibrillar proteins, MPI) and bovine serum albumin (BSA), extracts from green tea (GTE), maté (ME), and rosemary (RE), and three phenolic compounds, catechin, caffeic acid, and carnosic acid) was investigated by electron paramagnetic resonance (EPR) spectroscopy to determine the concentrations of plant extracts required to protect against protein oxidation. Blocking of MPI thiol groups by N-ethylmaleimide was found to reduce the rate of reaction of MPI with perferrylmyoglobin radicals, signifying the importance of protein thiols as radical scavengers. GTE had the highest phenolic content of the three extracts and was most effective as a radical scavenger. IC50 values indicated that the molar ratio between phenols in plant extract and MPI thiols needs to be >15 in order to obtain efficient protection against protein-to-protein radical transfer in meat. Caffeic acid was found most effective among the plant phenols.

Spontaneous supersaturation of calcium D-gluconate during isothermal dissolution of calcium L-lactate in aqueous sodium D-gluconate

Continuing dissolution of solid calcium L-lactate pentahydrate in saturated aqueous solutions following addition of solid sodium D-gluconate corresponding to a gluconate/lactate ratio around three was found to result in homogeneous solutions supersaturated with calcium D-gluconate by a factor of seven, from which calcium D-gluconate monohydrate precipitated only slowly. In contrast, dissolution of calcium D-gluconate monohydrate by sodium L-lactate in aqueous solution with the reverse lactate/gluconate ratio also around three did not result in similar homogeneous solutions on the route to solid calcium L-lactate pentahydrate. This increasing supersaturation of calcium D-gluconate during dissolution of calcium L-lactate in aqueous sodium D-gluconate may enhance calcium bioavailability. The dissolution overshooting depends on competitive kinetics and is also of interest in modeling biomineralization and in designing novel food products with increased calcium bioavailability.

Nutritional aspects of β-carotene and resveratrol antioxidant synergism in giant unilamellar vesicles

Giant unilamellar vesicles of soy phosphatidylcholine are found to undergo budding when sensitized with chlorophyll a ([phosphatidylcholine] : [chlorophyll a] = 1500 : 1) under light irradiation (400-440 nm, 16 mW mm(-2)). 'Entropy' as a dimensionless image heterogeneity measurement is found to increase linearly with time during an initial budding process. For β-carotene addition ([phosphatidylcholine] : [β-carotene] = 500 : 1), a lag phase of 23 s is observed, followed by a budding process at an initial rate lowered by a factor of 3.8, whereas resveratrol ([phosphatidylcholine] : [resveratrol] = 500 : 1) has little if any protective effect against budding. However, resveratrol, when combined with β-carotene, is found to further reduce the initial budding rate by a total factor of 4.7, exhibiting synergistic antioxidation effects. It is also interesting that β-carotene alone determines the lag phase for the initiation of budding, while resveratrol supports β-carotene in reducing the rate of the budding process following the lag phase; however, it alone has no observable effect on the lag phase. Resveratrol is suggested to regenerate β-carotene following its sacrificial protection of unsaturated lipids from oxidative stress, modeling the synergistic effects in cell membranes by combinations of dietary antioxidants.

Thermodynamics of dissolution of calcium hydroxycarboxylates in water

Aqueous solubility of calcium l-lactate, calcium d-gluconate, and calcium d-lactobionate increases with temperature (10-30 °C investigated), most significantly for the least soluble d-gluconate, while the calcium ion activity of the saturated solutions decreases with temperature, as measured electrochemically, most significantly for the most soluble d-lactobionate. This unusual behavior is discussed in relation to dairy processing and explained by endothermic binding of calcium to hydroxycarboxylate anions determined to have ΔH°ass = (31 ± 3) kJ·mol(-1) for l-lactate, (34 ± 2) kJ·mol(-1) for d-gluconate, and (29 ± 3) kJ·mol(-1) for d-lactobionate in 1:1 complexes with thermodynamic binding constants at 25 °C of Kass = 49 (l-lactate), 88 (d-gluconate), and 140 (d-lactobionate). Quantum mechanical calculations within density functional theory (DFT) confirm the ordering of strength of binding. The complex formation is entropy driven with ΔS°ass > 0, resulting in decreasing calcium ion activity in aqueous solutions for increasing temperature, even for the saturated solutions despite increasing solubility.

Oxidative stability and chemical safety of mayonnaise enriched with grape seed extract

Grape seed extract (GSE), a by-product from the wine industry, was explored for its use as enrichment to mayonnaise, due to its potential health effects. Mayonnaises were enriched with 0 mg GSE per mL, 0.5 mg GSE per mL (~0.050%), 0.9 mg GSE per mL (~0.10%) and 1.4 mg GSE per mL (~0.15%) during preparation and stored in the dark at room temperature for 8 weeks. The antioxidative capacity of the mayonnaises was evaluated by their ability to scavenge the stable radical TEMPO by electron spin resonance (ESR) spectroscopy. The oxidative stability of the mayonnaises was determined by the content of lipid hydroperoxides (peroxide value, POV), the content of conjugated diene hydroperoxides and the content of thiobarbituric acid reactive substances (TBARS). The highest antioxidative capacity and the lowest content of lipid hydroperoxides and TBARS were found in the mayonnaise with the highest percentage of GSE (0.15%). Therefore, the oxidative stability of the mayonnaises enriched with GSE was slightly improved through storage. However, mayonnaise without GSE had the highest sensorial acceptability compared to mayonnaise enriched with GSE. In the Artemia salina assay, a fast screening method for overall toxicity, the death rate of brine shrimps larvae was found to increase for increasing percentage of GSE. A level of 0.05% GSE in mayonnaise is concluded not to constitute any toxicological risks, but to provide significant protection against oxidation during storage.

Oxidation of carbon monoxide by perferrylmyoglobin

Perferrylmyoglobin is found to oxidize CO in aerobic aqueous solution to CO2. Tryptophan hydroperoxide in the presence of tetra(4-sulfonatophenyl)-porphyrinate-iron(III) or simple iron(II)/(III) salts shows similar reactivity against CO. The oxidation of CO is for tryptophan hydroperoxide concluded to depend on the formation of alkoxyl radicals by reductive cleavage by iron(II) or on the formation of peroxyl radicals by oxidative cleavage by iron(III). During oxidation of CO, the tryptophan peroxyl radical was depleted with a rate constant of 0.26 ± 0.01 s(-1) for CO-saturated aqueous solution of pH 7.4 at 25 °C without concomitant reduction of the iron(IV) center. Carbon monoxide is as a natural metabolite accordingly capable of scavenging tryptophan radicals in myoglobin activated by peroxides with a second-order rate constant of (3.3 ± 0.6) × 10(2) L mol(-1) s(-1), a reaction that might be of importance in cellular membranes of the intestine for protection of tissue against radical damage during meat digestion.

Riboflavin photosensitized oxidation of myoglobin

The reaction of the fresh meat pigment oxymyoglobin, MbFe(II)O₂, and its oxidized form metmyoglobin, MbFe(III), with triplet-state riboflavin involves the pigment protein, which is oxidatively cleaved or dimerized as shown by SDS-PAGE and Western blotting. The overall rate constant for oxidation of MbFe(II)O₂ by ³Rib is (3.0 ± 0.5) × 10⁹ L·mol⁻¹·s⁻¹ and (3.1 ± 0.4) × 10⁹ L·mol⁻¹·s⁻¹ for MbFe(III) in phosphate buffer of pH 7.4 at 25 °C as determined by laser flash photolysis. The high rates are rationalized by ground state hydrophobic interactions as detected as static quenching of fluorescence from singlet-excited state riboflavin by myoglobins using time-resolved fluorescence spectroscopy and a Stern-Volmer approach. Binding of riboflavin to MbFe(III) has K(a) = (1.2 ± 0.2) × 10⁴ mol·L⁻¹ with ΔH° = -112 ± 22 kJ·mol⁻¹ and ΔS° = -296 ± 75 J·mol⁻¹·K⁻¹. For meat, riboflavin is concluded to be a photosensitizer for protein oxidation but not for discoloration.

Electron transfer from plant phenolates to carotenoid radical cations. Antioxidant interaction entering the Marcus theory inverted region

β-Carotene, lycopene, and zeaxanthin are maximally regenerated by plant phenolates from their radical cations formed during laser flash photolysis in 9:1 (v/v) chloroform/methanol for a driving force corresponding to the reorganization energy according to the Marcus theory. For β-carotene, the reorganization energy has values of 0.41 ± 0.04 and 0.40 ± 0.04 eV for the plant phenols in the presence of 1 and 2 equiv of base, respectively, at 23 °C. For a driving force lower than the reorganization energy, regeneration of the carotenoids is less efficient as is seen for m-hydroxybenzoic acid, vanillic acid, and p-coumaric acid. For a driving force above the maximum rate as determined to have kET = 6.3 × 10(9) L·mol(-1)·s(-1) for syringic acid and β-carotene, the reaction becomes gradually slower and regeneration less efficient as is seen for the more reducing caffeic acid, rutin, and quercetin corresponding to an inverted region for the rate of electron transfer. Lycopene and zeaxanthin show a similar behavior for the same series of plant phenols with slightly lower reorganization energy, in agreement with the lower reduction potential of their radical cations, while, for the ketocarotenoids astaxanthin and canthaxanthin, fast reactions with a solvent of radical cations inhibit regeneration from being detected. Intermediate reducing plant phenols accordingly yield maximal protection of carotenoids against photobleaching in foods and beverages.

Antioxidant capacity versus chemical safety of wheat bread enriched with pomegranate peel powder

Pomegranate peel powder (PP), a by-product of the pomegranate juice industry rich in polyphenols, was explored for use in bread production, due to its potential health effects. Wheat bread was prepared using different levels for replacement of flour with PP (0 to 10 g per 100 g flour) resulting in antioxidant levels expressed as Trolox equivalent antioxidant capacity values (TEAC) ranging from 1.8 to 6.8 μmol TEAC per g bread for fresh bread. TEAC remained constant during 5 days of storage in polyethylene bags at room temperature. The oxidative stability was evaluated by detection of radicals by direct electron spin resonance (ESR) spectroscopy, and peroxide value, and the highest capacity of scavenging of radicals (Fremy's salt) and the lowest content of peroxide values were found in bread with the highest percentage of PP. Safety evaluation was performed by the Artemia salina assay. An increased death rate of the brine shrimp larvae was found as a function of the replacement of wheat flour with PP in fortified bread providing a general screening method for the toxicological test of polyphenol fortified bread to be recommended for use in product development in addition to subjective evaluation. Based on both toxicological and subjective evaluations an addition of 2.5% PP is recommended for the actual product.

Free Radical Processes in Non-enzymatic Browning of Glucose and Lysine: Influence of Temperature and Unsaturated Lipids

Formation of dialkylpyrazinium radical cations in aerated 70 % aqueous glycerol solutions of glucose and lysine during heating resulting in browning (90, 110, and 130°C, investigated) was more dependent on temperature than formation of brown colour. Activation energy (Ea) for radical formation was ~83 kJ mol–1, compared with ~70 kJ mol–1 for browning, and was unaffected by methyl linolenate. Low-temperature browning was influenced by non-radical degradation of Amadori products, whereas radical processes were prominent at higher temperatures and were unaffected by unsaturated lipids. In contrast, methyl linolenate reacts with lysine in the absence of glucose to form fluorescent products at a slow rate (Ea 25 kJ mol–1). Glucose increased the rate of formation of fluorescent products (Ea ~60 kJ mol–1), in agreement with Maillard reactions at low temperatures involving glucose as a rate-determining reagent. Lipid oxidation does not have a direct effect on lysine and glucose browning reactions at conditions relevant for food; effects of lipids on Maillard reactions are matrix-related.