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.     

Release of deuterated nonenal during beer aging from labeled precursors synthesized in the boiling kettle.

The use of labeled nonenal enabled the demonstration that the appearance of the cardboard flavor in finished beer comes from lipid auto-oxidation during wort boiling and not from lipoxygenasic activity during mashing. Free trans-2-nonenal produced by linoleic acid auto-oxidation in the kettle disappears, owing to retention by wort amino acids and proteins. This binding linkage protects trans-2-nonenal from yeast reduction but is reversible, allowing release of the compound at lower pH during aging. Labeled trans-2-nonenal is detected after aging when deuterated precursors form in the boiling kettle. The amount of alkenal released correlates with the concentration of reversible associations in the pitching wort. This work brings new illumination to the formation of trans-2-nonenal and overturns many previous hypotheses. It also explains why a reduction in the beer pH intensifies the cardboard flavor.     

Interactions between methyl ketones and beta-lactoglobulin: sensory analysis, headspace analysis, and mathematical modeling

Interaction of flavor compounds with proteins is known to have an influence on the release of flavor from food. Hydrophobic interactions were found between beta-lactoglobulin and methyl ketones; the affinity constant increases by increasing the hydrophobic chain. Addition of beta-lactoglobulin (0.5 and 1%) to aroma solutions (12.5, 50, and 100 microL L(-)(1)) of three methyl ketones induces a significant decrease in odor intensity. The chosen methyl ketones were 2-heptanone (K(b) = 330), 2-octanone (K(b) = 950), and 2-nonanone (K(b) = 2440). The release of these flavor compounds (50 microL L(-)(1)) was studied by static headspace in water solution (50 mM NaCl, pH 3) with different concentrations of beta-lactoglobulin (0, 0.5, 1, 2, 3, and 4%). Increasing the concentration of protein increases the retention of volatiles, and this effect is greatest for 2-nonanone, the compound with the highest affinity constant, and lowest for 2-heptanone. A mathematical model previously developed to describe flavor release from aqueous solutions containing flavor-binding polymers (Harrison, M.; Hills, B. P. J. Agric. Food Chem. 1997, 45, 1883-1890) was used to interpret the data. The model assumes that the polymer-flavor interaction is reversible and the rate-limiting step for release is the transfer of volatiles across the macroscopic gas-liquid interface. This model was used to predict the equilibrium partitioning properties and the rate of release of the three methyl ketones. Increasing the affinity constant leads to decreased release rates and a lower final headspace aroma concentration.     

热处理强度对猪肉肌球蛋白结构及风味成分吸附特性的影响

为了探究热处理强度对肌球蛋白结构及风味吸附特性的影响,该文选取猪肉中肌球蛋白,采用拉曼光谱等技术,分析了不同温度(45、55、65、75、85℃)对肌球蛋白巯基总量、二级结构和表面疏水性的影响;然后选取8种典型肉品风味成分,建立肌球蛋白-风味化合物作用体系,以同一顶空内不同风味化合物浓度自由比例为指标,采用固相微萃取气质联用技术探究不同热处理强度对肌球蛋白风味吸附作用的影响。结果表明:整个热处理强度增加的过程中肌球蛋白巯基基团含量显著降低(P0.01);温度升至55℃蛋白分子展开,α-螺旋和β-折叠转换成β-转角及无规则卷曲结构;温度升至75℃蛋白分子变性,β-转角及无规则卷曲转换成α-螺旋和β-折叠结构;在温度升高至65℃的过程中蛋白表面疏水性持续增加,温度继续升高略有降低;蛋白对4种醛(3-甲基丁醛、戊醛、庚醛、辛醛)的吸附性在展开过程中增强,聚合过程中减弱;蛋白对2-戊酮的吸附性先降低后升高,45℃处理组最低,85℃处理组最高,对3种酮(2-庚酮、2-辛酮、2-壬酮)的吸附性呈先增加后降低再增加的趋势,蛋白变性时(75℃)显著低于其余温度条件(P0.01),85℃处理时显著高于其余温度条件(P0.01);研究结果为今后关于肉品风味形成机理的研究提供参考。     

热处理及β-环糊精添加量对豆乳风味的影响

风味是评价豆乳产品品质的重要指标,而在豆乳生产中,热处理及 β-环糊精的添加对豆乳风味品质的影响则未可知.因此,该研究首先分析了豆乳加热过程中(30-90℃范围内)关键性风味物质的含量变化,并比较了在不同加热阶段(加热前、加热至40℃时、加热至60℃时、加热后)添加不同质量分数β-环糊精(0.25%、0.50%、0.75%、1.00%)所获得的豆乳的风味品质.研究结果显示,豆乳中的关键性风味物质对温度的敏感度因风味物质的种类不同而异,随着加热过程中温度的升高,豆腥味的强度呈现显著降低的趋势,而非豆腥味的强度则相对增强.添加 β-环糊精能够显著降低关键性风味物质在豆乳体系中的浓度,当添加量为≥0.50%的β-环糊精在豆乳加热至60℃阶段添加时,所得豆乳中己醛、己醇、1-辛烯-3-醇等关键性豆腥味成分含量的降低幅度最为显著(P<0.05),但同时反-2-辛烯醛这种非豆腥味成分的含量也有所损失.豆乳豆腥味、蘑菇味和甜香味的感官评价结果也呈现类似的趋势,另外,根据感官评价的综合得分,当添加量为0.75%β-环糊精在豆乳加热至60℃时添加时,所得豆乳的整体风味品质最佳.由于β-环糊精价格低廉、且安全性高,其在豆乳加工过程中的添加又较易实现,因而在豆乳生产中具有良好的应用价值.     

Identification of character impact odorants of different soybean lecithins.

The potent odorants of standardized, enzymatically hydrolyzed, and deoiled soybean lecithins were characterized systematically by combined gas chromatography/mass spectrometry and olfactometry. Sixty-one odorants were identified; 53 of these odor-active compounds have not previously been reported as odorants of soybean lecithin flavor. By aroma extract dilution analysis and modified combined hedonic and response measurement the following odorants showed the highest flavor dilution factors and CHARM values: (E,E)-2, 4-decadienal (deep-fried), (E)-beta-damascenone (apple-like), 2, 3-diethyl-5-methylpyrazine (roasty, earthy), (E)-2-nonenal (cardboard-like), trans-4,5-epoxy-(E)-2-decenal (metallic), 1-nonen-3-one (mushroom-like), 2-ethyl-3,5-dimethylpyrazine (roasty, earthy), and 1-octen-3-one (mushroom-like). Enzymatic hydrolysis intensified especially the roasty sensation of 2, 3-diethyl-5-methylpyrazine, whereas deoiling effected a general significant decrease in olfactory perception on the nitrogen-containing compounds. In addition, sensory profiles of nasal and retronasal lecithin odor were performed.     

Effect of flavor compound chemical structure and environmental relative humidity on the binding of volatile flavor compounds to dehydrated soy protein isolates

Influence of flavor compound chemical structure, including functional group and stereochemistry, and environmental relative humidity (RH) on the binding of volatile flavor compounds to dehydrated soy protein isolates (SPIs) was evaluated by inverse gas chromatography. Binding of selected volatile flavor compounds differed slightly between SPIs of different origins. Results showed that the flavor compound chemical structure greatly determined its binding potential to SPIs. Binding of nonpolar flavor compounds (hydrocarbon) to soy proteins was attributed mainly to nonspecific van der Waals dispersion forces and was not affected by adsorbed water. The more polar flavor compounds (ester, ketone, aldehyde, and alcohol) exhibited both specific (hydrogen bonding, dipole forces) and nonspecific interactions, and their binding with soy proteins was greatly impaired by adsorbed water in the extremely low humidity region (approaching 0% RH). Further water uptake in the 30 to 50% RH region did not significantly affect the binding of polar compounds, although sorption of alcohol compounds (when present at high levels) further increased.     

Effect of concentration and relative humidity on the transfer of alkan-2-ones through paper coated with wheat gluten

The transport properties of two methyl ketones, 2-heptanone and 2-nonanone, through a paper coated with a wheat gluten (WG) solution were studied as a function of aroma compound concentration differential and relative humidity. Whatever the conditions, coating improved the aroma barrier properties of the paper. Whereas permeability of 2-nonanone through WG coated paper was not affected by the concentration differential, a high concentration of 2-heptanone induced a permeability increase, which can be explained by a slight plasticization effect of this compound. The moisture content of the films markedly affected the barrier properties depending on the aroma compound. For 2-heptanone, the most polar compound, the increase of permeability with relative humidity was related to the increase of diffusivity and solubility coefficient of this compound. With 2-nonanone, permeability and diffusivity coefficients strongly decreased with the increase of moisture content. This decrease could be linked to the hindrance in diffusivity of the less polar compound induced by the presence of water molecules. The different behavior in permeation properties seems to be related to the hydrophobic nature of the aroma compound. At intermediate and high humidity levels, coated papers are found to have better aroma barrier properties than LDPE films.     

Solid-phase microextraction (SPME) technique for measurement of generation of fresh cucumber flavor compounds

Investigations were carried out to determine whether flavor compounds characteristic for fresh cucumbers could be rapidly determined using a solid-phase microextraction (SPME) dynamic headspace sampling method combined with gas chromatography and flame ionization detection. Cucumbers were sampled, during blending, for fresh cucumber flavor compounds (E,Z)-2,6-nonadienal and (E)-2-nonenal. The GC was such that the two target compounds were separated and baseline-resolved. Relative standard deviations for analysis of both (E,Z)-2,6-nonadienal and (E)-2-nonenal using this SPME sampling method were +/-10%. Utility of the analytical method was demonstrated by determining the effect of heat treatments on the ability of cucumbers to produce these flavor impact compounds.     

Fresh cucumber flavor in refrigerated pickles: comparison of sensory and instrumental analysis

The ability of nonacidified, refrigerated pickled cucumbers to produce the fresh cucumber flavor impact compounds (E,Z)-2,6-nonadienal and (E)-2-nonenal declined during storage. Production of these compounds decreased as the pH of refrigerated cucumbers was reduced. Despite the fact that the concentrations of (E,Z)-2,6-nonadienal and (E)-2-nonenal generated were over 10(5)-fold greater than the threshold levels, it was possible for a sensory panel to consistently detect differences in the intensity of fresh cucumber flavor, provided the pH difference between samples was 1 unit or greater. The presence of spices did not interfere with the ability of panelists to detect differences in fresh flavor intensity. There was a linear correlation between sensory scores and the amount of (E,Z)-2,6-nonadienal produced by cucumbers equilibrated at different pH levels.     

Enzymatic hydrogenation of trans-2-nonenal in barley

Conversion of undesirable, taste-active compounds is crucial for using barley as a suitable raw material for beer production. Here, ALH1, a barley alkenal hydrogenase enzyme that reduced the alpha,beta-unsaturated double bond of aldehydes and ketones, was found to convert trans-2-nonenal (T2N), a major contributor to the cardboard-like flavor of aged beer. Although the physiological function of ALH1 in barley development remains elusive, it exhibited high specificity with NADPH as a cofactor in the conversion of several oxylipins-including T2N, trans-2-hexenal, traumatin, and 1-octen-3-one. ALH1 action represents a previously unknown mechanism for T2N conversion in barley. Additional experimental results resolved the genomic sequence for barley ALH1, as well as the identification of a paralog gene encoding ALH2. Interestingly, T2N was not converted by purified, recombinant ALH2. The possibility to enhance ALH1 activity in planta is discussed--not only with respect to the physiological consequences thereof--but also in relation to improved beer quality.     

Influence of the food matrix structure on the retention of aroma compounds

The retention of the aroma compounds in a multicomponent medium like the food matrix is influenced by their affinity with the protein when lipid is present at a low level (0.5%). The effect of the structure of the media is also studied by using two media with the same composition; one was emusified, and the other was not. Among the studied aroma compounds, 2-nonanone and isoamyl acetate present opposite behaviors: the volatility of isoamyl acetate is not affected by the change of the medium structure whereas that of 2-nonanone increases. The decrease of retention of 2-nonanone in an emulsified system would be due to a modification of the fixation site for this compound on the protein or to a competition between the lipid and the aroma compound while the protein is adsorbed at the lipid-water interface.     

Comparison of odor-active compounds from six distinctly different rice flavor types

Using a dynamic headspace system with Tenax trap, GC-MS, GC-olfactometry (GC-O), and multivariate analysis, the aroma chemistry of six distinctly different rice flavor types (basmati, jasmine, two Korean japonica cultivars, black rice, and a nonaromatic rice) was analyzed. A total of 36 odorants from cooked samples were characterized by trained assessors. Twenty-five odorants had an intermediate or greater intensity (odor intensity >or= 3) and were considered to be major odor-active compounds. Their odor thresholds in air were determined using GC-O. 2-Acetyl-1-pyrroline (2-AP) had the lowest odor threshold (0.02 ng/L) followed by 11 aldehydes (ranging from 0.09 to 3.1 ng/L), guaiacol (1.5 ng/L), and 1-octen-3-ol (2.7 ng/L). On the basis of odor thresholds and odor activity values (OAVs), the importance of each major odor-active compound was assessed. OAVs for 2-AP, hexanal, ( E)-2-nonenal, octanal, heptanal, and nonanal comprised >97% of the relative proportion of OAVs from each rice flavor type, even though the relative proportion varied among samples. Thirteen odor-active compounds [2-AP, hexanal, ( E)-2-nonenal, octanal, heptanal, nonanal, 1-octen-3-ol, ( E)-2-octenal, ( E, E)-2,4-nonadienal, 2-heptanone, ( E, E)-2,4-decadienal, decanal, and guaiacol] among the six flavor types were the primary compounds explaining the differences in aroma. Multivariate analysis demonstrated that the individual rice flavor types could be separated and characterized using these compounds, which may be of potential use in rice-breeding programs focusing on flavor.     

Effects of non-covalent interactions with 5-O-caffeoylquinic acid (chlorogenic acid) on the heat denaturation and solubility of globular proteins

The non-covalent interactions between the monomeric phenolic compound chlorogenic acid (5-CQA) and bovine serum albumin (BSA), lysozyme, and alpha-lactalbumin were characterized, and their effect on protein properties was examined. 5-CQA had a low affinity for all three proteins, and these interactions seemed to show a negative cooperativity. 5-CQA-BSA binding decreased with increasing temperature, whereas pH (pH 3.0 compared to pH 7.0) and ionic strength had no pronounced effect. At high 5-CQA/protein molar ratios, both the denaturation enthalpy and temperature of BSA increased; however, covalent bonds were created at high temperatures. The presence of 5-CQA had no effect on the solubility of BSA and alpha-lactalbumin as a function of pH, whereas it decreased lysozyme solubility at alkaline pH due to covalent interactions. These results indicate that the non-covalent interactions with 5-CQA do not have pronounced effects on the functional properties of globular proteins in food systems.     

How low pH can intensify beta-damascenone and dimethyl trisulfide production through beer aging

Flavor quality is of major importance to the consumer, but the flavor characteristics of beer appear to deteriorate greatly with time, at a rate depending on the composition of the beer and its storage conditions (notably pH). Prior to identifying the influence of pH on the development of the most intense staling flavors found in aged lager beers, the corresponding key flavor compounds were determined by aroma extract dilution analysis. In addition to trans-2-nonenal, beta-damascenone seems at least as important in the flavor of aged beer. Ethyl butyrate, dimethyl trisulfide, 2-acetylpyrazine, 3-(methylthio)propionaldehyde, 2-methoxypyrazine, maltol, gamma-nonalactone, and ethyl cinnamate are also relevant to the sensory profile of aged beer. Upon aging, a beer having a higher pH produces less beta-damascenone, because acid-catalyzed glycoside hydrolysis is decreased. On the other hand, it produces more 3-(methylthio)propionaldehyde, owing to Strecker degradation of methionine. Raising the beer pH additionally causes the release of 3-(methylthio)propionaldehyde from sulfitic adducts. These adducts, more stable at a lower pH, protect the aldehyde against premature oxidation to 3-(methylthio)propionic acid, thus making it available for dimethyl trisulfide formation during aging.     

食盐对鸡汤挥发性风味物质的影响

为探究食盐对鸡汤品质和挥发性风味物质的影响,以武定鸡与狄高鸡杂交系F₁为研究对象,采用料水比1:3、炖煮时间2 h进行炖煮鸡汤,通过顶空固相微萃取(SPME)和气相色谱-质谱联用(GC-MS)方法检测不同食盐添加量对鸡汤中挥发性风味物质的影响。结果表明,随着食盐添加量的增加,鸡汤风味成分增多,其中,鸡汤中含量较高的挥发性风味物质主要为醛类物质和醇类物质,醛类物质主要为己醛、庚醛、壬醛、2-庚烯醛、正辛醛、反式-2-辛烯酸等,醇类物质主要为正辛醇和庚醇。感官评价结果显示,当食盐添加量为2%时,鸡汤感官评分最高,且挥发性风味物质组成较为丰富。因此,当料水比1:3,炖煮2 h,食盐添加量为2%时,鸡汤的品质和食用最佳。本研究结果为探究食盐对鸡汤产品的挥发性成分提供了研究方法和科学依据。     

Identification of a stale-beer-like odorant in extracts of naturally aged beer

For a long time, beer staling has been a prime concern in brewery research. Yet, to improve flavor stability, better knowledge of all chemicals involved is still needed. From our aroma extract dilu-tion analyses (AEDA) applied to naturally aged lager beers emerged an old-beer-like odorant at RICP-SIL 5 CB = 1532 and RIFFAP = 2809, with a FD value close to that of trans-2-nonenal (the well-known cardboard off-flavor found in aged beers). Specific phenol extraction, GC cold trapping, and mass spectrometry (electron impact and chemical ionization) enabled us to identify it as 4-vinylsyringol. Although already mentioned in some fresh beers, this compound had never been highlighted as involved in the aging process of lager beers.     

Aroma properties of a homologous series of 2,3-epoxyalkanals and trans-4,5-epoxyalk-2-enals

A few odor-active epoxyaldehydes, formed during lipid peroxidation, have recently been reported as intense aroma compounds in foods. However, very little is known about their flavor properties in general. Syntheses of homologous trans-2,3-epoxyalkanals (C(6)-C(12)) and trans-4,5-epoxy-(E)-2-alkenals (C(7)-C(12)) followed by structural characterization using mass spectrometry (MS/EI; MS/CI) and (1)H NMR measurements were performed. An evaluation of their odor qualities and odor thresholds by gas chromatography-olfactometry revealed the following: within the trans-2,3-epoxyalkanals, the odor quality changed from grassy for the compounds with six and seven carbon atoms to citrus-like or soapy for aldehydes with eight and more carbon atoms. The odor thresholds lay in the range of 3-15 ng/L (in air) and were nearly identical within the series; however, a slight minimum was measured for trans-2,3-epoxyoctanal to trans-2,3-epoxydecanal. In the series of the trans-4,5-epoxyalk-(E)-2-enals the C(10) compound was characterized by the lowest odor threshold of 0.6-2.5 pg/L of air. However, all trans-4,5-epoxy-alk-(E)-2-enals smelled intensely metallic.     

Quantitation of odor-active compounds in rye flour and rye sourdough using stable isotope dilution assays

Application of the aroma extract dilution analysis on a flavor distillate prepared from freshly ground rye flour (type 1150) revealed 1-octen-3-one (mushroom-like), methional (cooked potato), and (E)-2-nonenal (fatty, green) with the highest flavor dilution (FD) factors among the 26 odor-active volatiles identified. Quantitative measurements performed by stable isotope dilution assays and a comparison to the odor thresholds of selected odorants in starch suggested methional, (E)-2-nonenal, and hexanal as contributors to the flour aroma, because their concentrations exceeded their odor thresholds by factors >100. Application of the same approach on a rye sourdough prepared from the same batch of flour revealed 3-methylbutanal, vanillin, 3-methylbutanoic acid, methional, (E,E)-2,4-decadienal, 2,3-butanedione, and acetic acid as important odorants; their concentrations exceeded their odor thresholds in water and starch by factors >100. A comparison of the concentrations of 20 odorants in rye flour and the sourdough made therefrom indicated that flour, besides the fermentation process, is an important source of aroma compounds in dough. However, 3-methylbutanol, acetic acid, and 2,3-butanedione were much increased during fermentation, whereas (E,E)-2,4-decadienal and 2-methylbutanal were decreased. Similar results were obtained for five different flours and sourdoughs, respectively, although the amounts of some odorants in the flour and the sourdough differed significantly within batches.     

Influence of sucrose on the thermal denaturation, gelation, and emulsion stabilization of whey proteins

The influence of sucrose (0-40 wt %) on the thermal denaturation and functionality of whey protein isolate (WPI) solutions has been studied. The effect of sucrose on the heat denaturation of 0.2 wt % WPI solutions (pH 7.0) was measured using differential scanning calorimetry. Sucrose increased the temperature at which protein denaturation occurred, for example, by 6-8 degrees C for 40 wt % sucrose. The dynamic shear rheology of 10 wt % WPI solutions (pH 7.0, 100 mM NaCl) was monitored as they were heated from 30 to 90 degrees C and then cooled to 30 degrees C. Sucrose increased the gelation temperature and the final rigidity of the cooled gels. The degree of flocculation in 10 wt % oil-in-water emulsions stabilized by 1 wt % WPI (pH 7.0, 100 mM NaCl) was measured using a light scattering technique after they were heated at fixed temperatures from 30 to 90 degrees C for 15 min and then cooled to 30 degrees C. Sucrose increased the temperature at which maximum flocculation was observed and increased the extent of droplet flocculation. These results are interpreted in terms of the influence of sucrose on the thermal unfolding and aggregation of protein molecules.