Aqueous Model Systems Research Articles

Mechanism of Deactivation of Triplet-Excited Riboflavin by Ascorbate, Carotenoids, and Tocopherols in Homogeneous and Heterogeneous Aqueous Food Model Systems

Tocopherols (alpha, beta, gamma, and delta) and Trolox were found to deactivate triplet-excited riboflavin in homogeneous aqueous solution (7:3 v/v tert-butanol/water) with second-order reaction rates close to diffusion control [k2 between 4.8 x 10(8) (delta-tocopherol) and 6.2 x 10(8) L mol(-1) s(-1) (Trolox) at 24.0 +/- 0.2 degrees C] as determined by laser flash photolysis transient absorption spectroscopy. In aqueous buffer (pH 6.4) the rate constant for Trolox was 2.6 x 10(9) L mol(-1) s1 and comparable to the rate constant found for ascorbate (2.0 x 10(9) L mol(-1) s(-1)). The deactivation rate constant was found to be inferior in heterogeneous systems as shown for alpha-tocopherol and Trolox in aqueous Tween-20 emulsion (approximately by a factor of 4 compared to 7:3 v/v tert-butanol/water). Neither beta-carotene (7:3 v/v tert-butanol/water and Tween-20 emulsion), lycopene (7:3 v/v tert-butanol/water), nor crocin (aqueous buffer at pH 6.4, 7:3 v/v tert-butanol/water, and Tween-20 emulsion) showed any quenching on the triplet excited state of riboflavin. Therefore, all carotenoids seem to reduce the formation of triplet-excited riboflavin through an inner-filter effect. Activation parameters were based on the temperature dependence of the triplet-excited deactivation between 15 and 35 degrees C, and the isokinetic behavior, which was found to include purine derivatives previously studied, confirms a common deactivation mechanism with a bimolecular diffusion-controlled encounter with electron (or hydrogen atom) transfer as rate-determining step. DeltaH for deactivation by ascorbic acid, Trolox, and homologue tocopherols (ranging from 18 kJ mol(-1) for Trolox in Tween-20 emulsion to 184 kJ mol(-1) for ascorbic acid in aqueous buffer at pH 6.4) showed a linear dependence on DeltaS (ranging from -19 J mol(-1) K(-1) for Trolox in aqueous buffer at pH 6.4 to +550 J mol(-1) K(-1) for ascorbic acid in aqueous buffer pH 6.4). Among photooxidation products from the chemical quenching, lumicrome, alpha-tocopherol quinones and epoxyquinones, and alpha-tocopherol dimers were identified by ESI-QqTOF-MS.

Interactions between aspartame, glucose and xylitol in aqueous systems containing potassium sorbate

The interaction between aspartame, glucose and xylitol in aqueous model systems of pH 3.00 and containing potassium sorbate was studied. Potassium sorbate degradation diminished with the increment of aspartame level from 0.050 to 0.500 g/100 g of system. Xylitol was the humectant that minimized aspartame degradation and non-enzymatic browning development. In general, as expected, presence of aspartame, xylitol or glucose and their mixtures increased the sweetness and they also diminished the sourness of the systems. The addition of 0.050 g of aspartame/100 g of system to the system containing xylitol produced a synergistic effect on sweetness intensity. Based on that trend, it could be concluded that the use of more than one sweetener might allow diminishing the amount of each one of them to assure a specific sweet level. These results stand out the advantage of the use of xylitol as well as the importance of an appropriate choice of the additives and food ingredients to use in the formulation of modified products with lower sugar content to optimize their quality.

Binding of 2-Nonanone and Milk Proteins in Aqueous Model Systems

Interactions of the model flavor compound 2-nonanone with individual milk proteins, whey protein isolate (WPI), and sodium caseinate in aqueous solutions were investigated. A method to quantify the free 2-nonanone was developed using headspace solid-phase microextraction followed by gas chromatography with flame ionization detection. Binding constants (K) and numbers of binding sites (n) for 2-nonanone on the individual proteins were calculated. The 2-nonanone binding capacities decreased in the order bovine serum albumin > beta-lactoglobulin > alpha-lactalbumin > alpha s1-casein > beta-casein, and the binding to WPI was stronger than the binding to sodium caseinate. All proteins appeared to have one binding site for 2-nonanone per molecule of protein at the flavor concentrations investigated, except for bovine serum albumin, which possessed two classes of binding sites. The binding mechanism is believed to involve predominantly hydrophobic interactions.

Structure−Reactivity Relationships of Flavan-3-ols on Product Generation in Aqueous Glucose/Glycine Model Systems

Ring structure-reactivity relationships of three flavan-3-ols [epicatechin (EC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG)] and three simple phenolic compounds (1,3,5-trihydroxybenzene, 1,2,3-trihydroxybenzene, and methylgallate as the analogous individual A, B, and C benzene rings of EGCG) on product generation in an aqueous glucose-glycine reaction model system (125 degrees C and 30 min) were investigated. The addition of EC, ECG, or EGCG to a glucose-glycine model was reported to similarly significantly reduce the formation of pyrazine, methyl-substituted pyrazines, and cyclotene. All three flavan-3-ols were also reported to generate phenolic-C2, C3, C4, and C6 sugar fragment adducts and to statistically reduce the concentration of glyoxal, glycolaldehyde, methylglyoxal, hydroxyacetone, diacetyl, acetoin, and 3-deoxyglucosone during the reaction time course, except for the EGCG reaction where 3-deoxyglucosone was not statistically different from the control after 20 min. For the simple phenolic compounds, methylgallate followed by 1,2,3-trihydroxybenzene was the least reactive, while 1,3,5-trihydroxybenzene was reported as the most reactive phenolic structure for quenching or reducing the concentration of the alpha-hydroxy- and alpha-dicarbonyl sugar fragments during the reaction time course. These results imply that the main mechanism flavan-3-ols reduced product generation was phenolic-sugar fragment carbonyl trapping reactions primarily on the A ring (the meta-polyhydroxylated benzene ring) or not due to the alteration of the reaction reduction potential.

Investigation of the Lactosylation of Whey Proteins by Liquid Chromatography−Mass Spectrometry

Heat treatment of milk induces a reaction between the milk proteins and lactose, resulting in lactosylated protein species. The lactosylation of the two major whey proteins alpha-lactalbumin and beta-lactoglobulin was investigated by reversed phase liquid chromatography-mass spectrometry (LC-MS). Three sample series, consisting of aqueous model solutions of each whey protein separately and in mixture and whole milk, were heated for different time periods, and the progression of the lactosylation reaction was monitored. The observed degrees of lactosylation and the reaction kinetics showed that the lactosylation of beta-lactoglobulin was not influenced by the presence of other components, whereas the lactosylation of alpha-lactalbumin was enhanced in whole milk compared to the aqueous model systems. An in-depth evaluation of the LC-MS data yielded information regarding changes of physicochemical properties of the whey proteins upon lactosylation. Whereas retention time shifts indicated changes in hydrophobicity for both alpha-lactalbumin and beta-lactoglobulin, changes in the charge state distribution denoting conformational alterations were observed only for beta-lactoglobulin. The analysis of different liquid and solid milk products showed that the lactosylation patterns of the whey proteins can be used as indicators for the extent of heat treatment.

Effect of thermal processing on the flavonols rutin and quercetin

The current research involves the study of the thermal treatment of quercetin and rutin in an aqueous model system (cooking). These substances were heated and their degradation was followed by high-performance liquid chromatography/diode-array detection (HPLC/DAD). The influence of pH and the involvement of oxygen in the degradation were studied. HPLC/electrospray ionization multi-stage mass spectrometry (ESI-MS(n)) was used for the structural characterization of the compounds produced. The influence of the degradation of the phenolic compounds on their antioxidant properties was elucidated by a electron spin resonance (ESR) spectrometry study of the reaction samples mixed with the stabilized radical, Fremy's salt. Strong degradation of the model substances took place under weak basic and oxidative conditions. Quercetin showed the most intense degradation. Protocatechuic acid could be identified as a cleavage reaction product by analyzing its retention time and molar mass during the degradation of quercetin. The structure of a second cleavage product could be identified on the basis of ESI-MS(n) fragmentation data. Also, several structures for reaction products of oxidized quercetin are suggested. The ESR analysis showed a decrease in the antioxidant activity of the reaction samples after heat treatment in aqueous solution.

Free glutamine as a major precursor of brown products and fluorophores in Maillard reaction systems

Glutamine is one of the most abundant free amino acid found in raw food. In this study, the contribution of free glutamine to nonenzymatic browning and fluorescence was investigated using an aqueous model system with methylglyoxal. The results indicated that glutamine contributed to the Maillard reaction via two pathways. First, the hydrolysis of the amide bond of glutamine led to the release of ammonia which was implicated in the formation of brown color and fluorescence. Among other nitrogen donors tested (asparagine, glutamic acid and urea) our results demonstrated that free glutamine was a major source of ammonia during heating. When heated at 120 and 180 degrees C, 100% of ammonia was released from glutamine after 60 and 10 min, respectively. The second pathway involved a direct Maillard reaction with the alpha-amino group of glutamine. Both pathways led to a rapid and complete destruction of glutamine when heated in the model systems. With reference to the Maillard browning (absorbance at 420 nm) glutamine turned out to be the most reactive amine, followed by asparagine, glutamate, ammonia and urea. Maximum fluorescence (excitation and emission wavelengths at 330 and 450 nm, respectively) was also observed with glutamine followed by urea and ammonia. Overall this study suggested that free glutamine predominantly contributes to the color and fluorescence formations of foodstuffs.

Stability of patulin in a juice-like aqueous model system in the presence of ascorbic acid

In the present study, the stability of patulin in an aqueous juice-like model system was investigated. At acidic pH, the presence of ascorbic acid reduced the stability of patulin. After 34 days, patulin was reduced to 30% of its initial concentration in the presence of ascorbic acid compared to 68–71% in samples without ascorbic acid. Conditions during storage (presence of light, oxygen and/or metal ions) influenced the stability of patulin. Furthermore, it was possible to induce degradation of patulin by either generating hydroxyl radicals or by adding the rather stable radical diphenyl-1-picrylhydrazyl (DPPH). Data from the present study indicate that patulin is decomposed by free radicals generated by oxidation of ascorbic acid to dehydroascorbic acid. Rapid oxidation of ascorbic acid in the presence of oxygen, catalysed by free metal ions, resulted in a decrease of patulin. After complete oxidation of ascorbic acid, no further patulin degradation was observed. In contrast, slow oxidation of ascorbic acid in the presence of metal-chelators induced a continuous, slow oxidation of patulin. Due to low oxygen content in the headspace of a food package, addition of ascorbic acid to products such as apple juice, prior to filling, cannot be considered as an effective decontamination strategy.

Effect of sweet solutes and potassium sorbate on the thermal inactivation of Z. bailii in model aqueous systems

The effect of solute type (glucose and polyols) and potassium sorbate (KS) on Zygosaccharomyces bailii thermal inactivation was evaluated in acidified aqueous model systems. Thermal inactivation curves obtained were fitted with Baranyi equation and parameters of this model were estimated and used to establish the effect of water activity ( a w), solute added and KS on Z. Bailii survival. Results obtained showed that addition of KS (0.025%, w/w) in general promoted an increase in the rate of heat inactivation. The use of polyols to depress a w to 0.985 in the absence of KS produced no effect on rate of heat inactivation. On the contrary, the use of glucose (10%, w/w) enhanced it. This trend was also observed when a w was depressed to 0.971 by glucose or xylitol or glucose-polyols. Moreover, a w depression in systems containing KS produced different effects on thermal inactivation rate depending on system composition. A synergic effect on the rate of inactivation of Z. bailii was observed by the combined use of KS and sorbitol, xylitol or glucose to depress a W to 0.985–0.988. This behavior might allow to decrease the severity of the thermal treatment with no detrimental effect on sterility.

Toward a Kinetic Model for Acrylamide Formation in a Glucose−Asparagine Reaction System

A kinetic model for the formation of acrylamide in a glucose-asparagine reaction system is pro-posed. Equimolar solutions (0.2 M) of glucose and asparagine were heated at different temperatures (120-200 degrees C) at pH 6.8. Besides the reactants, acrylamide, fructose, and melanoidins were quantified after predetermined heating times (0-45 min). Multiresponse modeling by use of nonlinear regression with the determinant criterion was used to estimate model parameters. The proposed model resulted in a reasonable estimation for the formation of acrylamide in an aqueous model system, although the behavior of glucose, fructose, and asparagine was slightly underestimated. The formation of acrylamide reached its maximum when the concentration of sugars was reduced to about 0. This supported previous research, showing that a carbonyl source is needed for the formation of acrylamide from asparagine. Furthermore, it is observed that acrylamide is an intermediate of the Maillard reaction rather than an end product, which implies that it is also subject to a degradation reaction.

Reactivity of Epicatechin in Aqueous Glycine and Glucose Maillard Reaction Models: Quenching of C2, C3, and C4 Sugar Fragments

Mechanisms of how epicatechin alters the pathways of the Maillard reaction were investigated. Carbon-13 and nitrogen-15 labeling studies were utilized to define the reactivity of epicatechin with glucose, glycine, and/or reaction products in an aqueous model (pH 7, 125 degrees C for 30 min) via GC, GC/MS and HPLC/MS analysis. Quantification of the volatile reaction product isotopomers by GC/MS from a 1:1 labeled to unlabeled glucose (carbohydrate module labeling technique) plus glycine model system indicated the formation of 2,3-butanedione and acetol were primarily formed via intact C4 and C3 sugar fragments, whereas pyrazine, methylpyrazine, 2,5-dimethylpyrazine, 2,3,5-trimethylpyrazine, and cyclotene were primarily formed via intact C2/C2, C2/C3, C3/C3, C3/C3, and C3/C3 sugar fragment pairs, respectively. The formation of these seven compounds was also reported by GC analysis to be dramatically inhibited when epicatechin was added to the glucose/glycine model system (observed 9-113-fold reduction). HPLC/MS analysis of both the glucose-labeled and glycine-labeled model systems with and without epicatechin indicated that epicatechin reacted directly with C2, C3, and C4 sugar fragments, while epicatechin did not report any direct reactivity with glycine. In conclusion, the quenching of sugar fragmentation products via epicatechin was correlated with the observed inhibition on volatile compound formation when epicatechin was added to a glucose/glycine aqueous reaction model system.

Thermal Degradation Kinetics of Bixin in an Aqueous Model System

The kinetics of the thermal degradation of the natural cis carotenoid bixin in a water/ethanol (8:2) solution was studied as a function of temperature (70-125 degrees C), using high-performance liquid chromatography. The curves for the decay of bixin and formation of products (e.g., di-cis and all-trans isomers and a C17 degradation compound) did not adjust well to a first-order rate law, but very good fits were obtained using a biexponential model. This mathematical modeling gave the rate constant values for the formation of the primary products from bixin, and the energy barrier for each step was obtained. The di-cis isomers were formed immediately (15 kcal/mol) together with the decay of bixin, followed by a slow consumption, indicating their role as reaction intermediates. In fact, the di-cis isomers could easily revert to bixin (Ea approximately 3 kcal/mol) or yield the primary C17 degradation product, with an energy barrier of 6.5 kcal/mol. In turn, 24 kcal/mol was necessary for the Bix --> all-trans step, explaining its slower formation.

Investigation of the Retronasal Perception of Strawberry Aroma Aftersmell Depending on Matrix Composition

The development of strawberry aroma aftersmell after consumption in aqueous and milk model systems was followed. A recently developed sensory approach was used to seize the qualitative, quantitative, and temporal aspects of aroma profile development. The results obtained from sensory evaluation were then correlated to analytical data, obtained by means of intraoral aroma detection using the buccal odor screening system (BOSS). A clear correlation was found between the sensory persistence of single odor impressions and the intraoral detectability of odorants by BOSS. For the strawberry aroma mixture it could be shown that BOSS is capable of selecting those odorants of a complex aroma mixture that elicit a certain aftersmell impression developing after a certain period of time in the oral cavity. Also, the changes in in vivo odorant persistence depending on changes in food matrix composition could be directly followed by means of BOSS.

Influence of amylose-flavor complexation on build-up and breakdown of starch structures in aqueous food model systems

Starch, in particular the linear amylose, is able to form inclusion complexes with a wide variety of flavor compounds. Build-up and breakdown of starch/flavor structures in aqueous food model systems were followed with iodometric titration, oscillatory measurements, light microscopy and differential scanning calorimetry (DSC). Amylose-flavor complexation promotes gelation and spherulitic crystallization in simple starch dispersions as well as in complex food systems including milk proteins and sugars. Crystalline starch/flavor spherulites and amylose-flavor complexes are very slowly attacked by α-amylase in mouth while fast breakdown of amylose-flavor complexes becomes possible in the gastrointestinal tract. It is concluded that starch/flavor interactions influence the sensory properties of starch based food systems in particular texture quality and flavor release. Amylose-flavor complexation might be an encapsulation principle for food where a controlled flavor release is desired.

Cu(II) Recovery from Aqueous Systems by Flotation

Ionmolecular and colloidal inorganic particles representing both pollutants and sources of useful substances require the separation from aqueous systems by modern techniques that also permit their removal and recovery. Ion flotation and precipitate flotation are considered as separation methods with wide application, especially for the small concentration ranges of noxious components, indicating high environmental and economical performances. The present paper presents the results of a research study concerning Cu(II) recovery from aqueous model systems by the precipitate flotation method, dissolved air flotation (DAF), with an anionic-type collector C18H33O2Na—sodium oleate (NaOL). The separation mechanism by the collector–Cu(II) interaction at optimum conditions was studied. A comparison of chemical analysis results with electronic and vibration spectra and thermal analysis for isolated species was also made. The results of the experimental data plead for the Cu(II) recovery separation from aqueous systems and its recovery as oxide after thermal decomposition of the species contained in the foam.

Loss of Redness (a*) as a Tool To Follow Hemoglobin-Mediated Lipid Oxidation in Washed Cod Mince

Instrumental measurement of redness loss (decrease in a* value) was evaluated as a tool to follow hemoglobin (Hb)-mediated lipid oxidation in fish muscle. Two washed cod mince model systems were used (prepared at pH 6.5 and 5.5), both fortified with 15 micromol/kg of trout Hb and adjusted to pH 6.5 and 81% moisture. The rate of oxidation was varied through pH alterations (pH 6.1 and 6.9) and addition of an antioxidative cod muscle press juice. During ice storage, TBARS, painty odor, and a* values were followed. In all "oxidizing" samples, a* values correlated well with TBARS and painty odor development; r = -0.95 and -0.77, respectively. In press juice containing samples, the correlation was lower (0.55 for a vs TBARS) because there was a slight a* value decrease even in the absence of measurable lipid oxidation. a* values distinguished between "oxidizing" and stable samples within 1 day, before any lipid oxidation products could be chemically detected. It was confirmed in an aqueous phosphate buffer model system that the redness loss corresponded to a buildup of brownish met-Hb at the expense of oxy- and deoxy-Hb. The a* value data were best used as a lipid oxidation index by calculating the rate of decrease (k value) in the "initial phase" of the redness loss (before accumulation of lipid oxidation products) or in the "differentiation phase" (during the exponential raise in TBARS/painty odor). Calibration to lipid oxidation products must, however, be made for each specific sample type. Washing method, pH, Hb-type, etc., all affected both k values and absolute a* readings. Small yellowness (b*) increases also occurred along with a value losses, possibly the result of polymerized Schiff bases.

방사선 조사에 의한 Phytic Acid의 분해특성 및 항산화 활성

방사선 조사에 의한 phytic acid의 분해특성 및 항라디칼, 항산화 활성을 평가하기 위하여, phytic acid를 수용액 모델에서 방사선 조사(0~20 kGy) 후 기존 항산화제(ascorbic acid, tocopherol, BHA)와의 항산화 특성을 비교하였다. 방사선 조사 후 phytic acid의 조사분해를 확인할 수 있었고, 조사 선량이 증가함에 따라 그 분해정도가 유의적으로 증가하였다. 또한 phytic acid의 농도에 따라 방사선에 의한 영향도 다르게 나타났는데, phytic acid의 농도가 증가함에 따라 방사선 조사분해 정도가 감소하였다. 항산화 특성의 측정결과, 방사선 조사된 phytic acid의 경우 DPPH 라디칼 소거능이 형성되었으며, 조사선량 및 phytic acid의 농도가 증가함에 따라 그 활성이 증가하는 것으로 나타났다. 저장동안 유지산 패의 억제효과는 기존 항산화제보다 phytic acid의 항산화성이 유의적으로 높았으며 방사선 조사에 의해 유지 혹은 다소 상승되는 것으로 나타났다. 따라서 방사선 조사시 phytic acid의 조사분해와 동시에 항산화 특성을 증대시킬 수 있는 것으로 나타났다.

Structure formation in thermoresponsive microgel suspensions under shear flow

Shear-induced structures of concentrated temperature-sensitivepoly(N-isopropylacrylamide) (PNiPAM) microgel suspensions have been studied employing smallangle neutron scattering (rheo-SANS). The interaction potential of swollen PNiPAMmicrogels could be varied from repulsive at temperatures below the lower critical solutiontemperature to attractive at temperatures above the lower critical solution temperature. Incontrast to the case for suspensions of rigid spheres, the effective volume fractioncould be changed by means of temperature while the mass concentration andparticle number density were kept constant. Thus, aqueous PNiPAM microgelsare interesting model systems with unique colloidal properties. Complementaryinformation about shear-induced changes of both the internal particle structure and theoverall microstructural phenomena were obtained from rheo-SANS experimentswith PNiPAM microgels with different particle sizes. The shear-induced particlearrangements strongly depended on the particle–particle interaction potential. When theinteraction potential was repulsive at temperatures below the lower critical solutiontemperature, no significant deformation of the swollen PNiPAM particles was observedeven at high shear rates. Shear-induced ordering was found at high shear ratesresulting in the formation of two-dimensional hexagonal close packed layers thataligned along the flow direction giving rise to shear thinning. The formation ofsliding hexagonal close packed layers under shear flow is therefore proposed to be ageneral property of colloidal dispersion independent of the internal structureof the particle. At temperatures near the lower critical solution temperature,when the particle interaction potential is not yet strongly attractive, shear flowinduces the collapse of an individual particle in concentrated suspension at highshear rates. A so-called butterfly scattering pattern indicates the shear-inducedenhancement of concentration fluctuations along the flow direction leading tosolvent being squeezed out of the particles until phase separation occurs finally.

Complexation of Oleuropein and trans-Cinnamic Acid with Cyclodextrins

The complexation of oleuropein and trans-cinnamic acid with α-, β-, and γ-cyclodextrin has been studied in aqueous model systems by light scaterring. The influence of various parameters (pH, concentration, reaction time, nature of cyclodextrin) has been thoroughly examined. The formation of binary (1:1) inclusion complexes and the higher inclusion ability of β-CD for both compounds has been indicated. Trans-cinnamic acid was extracted from olive olive oil following its complexation with β-CD at satisfactory recovery levels.

Ascorbic acid oxidation in sucrose aqueous model systems at subzero temperatures.

The reduction of Tempol by ascorbic acid in concentrated sucrose solutions was measured by electron paramagnetic resonance (EPR) at temperatures ranging from 16 to -16 degrees C. This method allowed the determination of the rate constants (k) of this fast reaction, by recording the Tempol reduction as a function of time. The two reactants were initially separated and had to migrate for the reaction to occur. The experimental findings were compared with predicted values according to the equation for diffusion-controlled reaction proposed by Atkins. The experimental reaction rate constants were observed to be lower than the calculated ones. However, the experimental values were found to be controlled by the temperature and viscosity changes of the reaction media, as expected for a diffusion-controlled reaction.