Mechanism of formation of sulphur aroma compounds from l-ascorbic acid and l-cysteine during the Maillard reaction

The sulphur aroma compounds produced from a phosphate-buffered solution (pH 8) of l-cysteine and l-, l-[1-¹³C] or l-[4-¹³C] ascorbic acid, heated at 140 ± 2 °C for 2 h, were examined by headspace SPME in combination with GC-MS. MS data indicated that C-1 of l-ascorbic acid was not involved in the formation of sulphur aroma compounds. The sulphur aroma compounds formed by reaction of l-ascorbic acid with l-cysteine mainly contained thiophenes, thiazoles and sulphur-containing alicyclic compounds. Among these compounds, 1-butanethiol, diethyl disulphide, 5-ethyl-2-methylthiazole, cis and trans-3,5-dimethyl-1,2,4-trithiolane, thieno[2,3-b]thiophene, thieno[3,2-b]thiophene, cis and trans-3,5-diethyl-1,2,4-trithiolane, 1,2,5,6-tetrathiocane, 2-ethylthieno[2,3-b]thiophene, 2,4,6-trimethyl-1,3,5-trithiane and cyclic octaatomic sulphur (S₈) were formed solely by l-cysteine degradation, and the rest by reaction of l-ascorbic acid degradation products, such as hydroxybutanedione, butanedione, acetaldehyde, acetol, pyruvaldehyde and formaldehyde with l-cysteine or its degradation products, such as H₂S and NH₃. A new reaction pathway from l-ascorbic acid via its degradation products was proposed.     

The effect of pH on the formation of aroma compounds produced by heating a model system containing l-ascorbic acid with l-threonine/l-serine

The identification of aroma compounds, formed from the reactions of l-ascorbic acid with l-threonine/l-serine at five different pH values (5.00, 6.00, 7.00, 8.00, or 9.55) and 143 ± 2 °C for 2 h, was performed using a SPME–GC–MS technique, and further use of LRI. The results showed 35 aroma compounds. The reaction between l-ascorbic acid and l-threonine/l-serine led mainly to the formation of pyrazines. Many of these were alkylpyrazines, such as 2-methylpyrazine, 2,5-dimethylpyrazine, 2-ethylpyrazine, 2-ethyl-6-methylpyrazine, 2-ethyl-5-methylpyrazine, 3-ethyl-2,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, and 3,5-diethyl-2-methylpyrazine; other compounds identified were furans and aldehydes. More volatiles were generated in l-ascorbic acid with l-threonine systems than in l-ascorbic acid with l-serine systems. The studies showed that furans, such as furfural, 2-furanmethanol, benzofuran, 2,5-furandicarboxaldehyde and 2-furfurylfuran were formed mainly at acidic pH. In contrast, higher pH values could promote the production of pyrazines.     

肌肽对抗坏血酸-半胱氨酸模式反应形成香味化合物的影响

以固相微萃取-气相色谱-质谱联用(SPME-GC-MS)技术对抗坏血酸与半胱氨酸(ASA-Cys)的模式反应产物进行鉴定,研究了肌肽对模式反应形成香味化合物的影响。呋喃、吡嗪、噻吩、噻唑、噻吩并噻吩及脂环硫化物在内的47个香味化合物被鉴定出来,其中,含硫化合物如脂环硫化物、噻吩、噻吩并噻吩是最主要的香味成分。肌肽加入到模式体系中,一方面使一些含硫化合物的产量显著降低,甚至消失;另一方面却促进了几个含氮化合物如甲基吡嗪,乙基吡嗪,2,6-二甲基吡嗪及其它烷基吡嗪的生成。这表明在模式反应中,肌肽抑制了Cys的热降解,同时暗示了肌肽作为氮源在热降解时很可能生成了NH3,NH3与H2S和ASA降解产物发生竞争反应形成含氮化合物如烷基吡嗪,从而导致含硫化合物的产量降低。     

Inhibition of polyphenoloxidase from apple by Maillard reaction products prepared from glucose or fructose with l-cysteine under various conditions of pH and temperature

The effects of Maillard reaction products (MRPs), synthesized from equimolar glucose or fructose with l-cysteine (1 mol l⁻¹) aqueous model mixtures, by modulating pH and temperature of heating, according to a two-factor and five-level experimental design, were investigated on polyphenoloxidase (PPO) activity from apple. Final pH and absorbance measurements at 350 nm were also selected as indicators of the Maillard reaction development and checked. In general, inhibitory potency (IP) of the mixtures increased with the increase in temperature (80-120°C) and the decrease in pH (pH 2.0-12.0) of the reaction medium. A linear relationship between the IP and heating time (0-48 h) or Abs.350 nm (0-70 AU) was demonstrated for glucose/cysteine system heated from 80°C to 120°C. Polarographic and spectrophotometric data were used to calculate kinetic constants and activation energy (Ea) values of inhibitory MRPs formation versus PPO activity and of those compounds absorbing at 350 nm. Ea values for these reactions were close, being 191 and 124 kJ mol⁻¹, respectively. The experimental design allowed to conclude that linear effects of both factors as well as a quadratic effect of pH were significant, leading to optimum conditions for the production of glucose-derived MRPs inhibitors. In most cases, glucose produced MRPs with higher IP compared to counterpart fructose-cysteine MRPs.     

Surface coating of l-glutamine solid microparticles for enteric delivery

The objective of this work was to produce enteric delivery systems to minimize gastric degradation of bioactives, using l-glutamine microparticles as a model, by surface-coating with a layer of enteric polymer (hydroxypropyl methylcellulose phthalate). Suspensions of solid glutamine microparticles in a methanol solution with enteric polymer and different concentrations of plasticizer (glycerol or triethyl citrate) and medium chain fatty acids (MCFA) were spray-dried at 55 °C. Spray-dried powders were characterized for in vitro release kinetics in simulated gastric (pH 1.2) and intestinal (pH 6.8) fluids at 37 °C. At a MCFA:polymer mass ratio of 2:1, formulations with plasticizer used at 30% of polymer mass resulted in <30% glutamine release after 2 h and <40% after 4 h at pH 1.2. At pH 6.8, complete release of glutamine was observed for most samples after 4 h. Our work demonstrated a simple approach for enteric delivery of food and pharmaceutical compounds.     

pH值和缓冲剂对抗坏血酸-半胱氨酸模式反应形成香味化合物的影响

以固相微萃取-气相色谱-质谱联用技术对抗坏血酸与半胱氨酸的模式反应产物进行鉴定,研究pH值和缓冲剂浓度对模式反应形成香味化合物的影响。依赖于不同的pH值条件,模式反应主要产生呋喃、吡嗪、噻唑、噻吩及含硫脂环化合物,这些化合物种类及产量的变化形成各具特色的肉香风味,呋喃类化合物主要形成于低pH值条件下,而吡嗪类化合物则易于在碱性条件下生成,噻唑类化合物的形成与pH值的依赖性不太显著,但过高的酸度会抑制其生成,中性偏碱的条件对噻吩和含硫脂环化合物的生成有利;Na2HPO4-NaH2PO4缓冲剂对模式反应没有催化作用,它们对模式反应形成香味化合物的影响体现于其维持反应体系pH值稳定的能力,0.20mol/L Na2HPO4-NaH2PO4缓冲剂对维持反应体系pH值的相对稳定是合适的。     

Effect of different fermentation parameters on l-lactic acid production from liquid distillery stillage

Expansion of lactic acid applications, predominantly for the preparation of biodegradable polymers increased the research interest for new, economically favourable production processes. Liquid stillage from bioethanol production can be an inexpensive, valuable source of nutrients for growth of lactic acid bacteria. Utilisation of residual biomass with spent fermentation media as a functional animal feed can greatly influence the process value and its ecological aspect. In this paper, the kinetics of lactic acid and biomass production on liquid stillage by Lactobacillus rhamnosus ATCC 7469 was studied. In addition, the impact of temperature, inoculum concentration, shaking and pH control by addition of CaCO₃ was evaluated. Maximal lactic acid yield of 73.4%, as well as high biomass production (3 × 10⁸ CFU ml⁻¹) were achieved under selected conditions (41 °C, 5% (v/v) of inoculum, 1% (w/v) of CaCO₃, initial pH of 6.5 and shaking rate of 90 rpm). These results were achieved without supplementation of the stillage with nitrogen or mineral sources.     

Determination of l- and d-amino acids in smokeless tobacco products and tobacco

Quantities of free l- and d-amino acids were determined by GC-SIM-MS in 25 European snuff tobaccos (from Germany, England and Sweden) and eight chewing tobaccos (from the Philippines, Africa and Denmark) and compared to those of cigar, cigarillo, and freshly harvested tobacco leaves of cultivars of Nicotiana tabacum L. Amino acids were isolated from tobacco samples by treatment with 70% aqueous methanol and purified by a cation exchanger. Next they were converted into their N(O)-pentafluoropropionylamino acid-(2)-propyl esters, and enantiomers separated and quantified by GC-SIM-MS on a Chirasil^®-l-Val capillary column. Among l-amino acids the most abundant were Pro, Asx and Glx in the low milligram range (about 2–6 mg/g) whereas the other l-amino acids were in the submilligram range. Though native tobacco leaves contained low amounts of few d-AAs (0.2–1.9%), all processed tobacco samples had d-amino acids in varying amounts and patterns.     

Evaluation of l-ascorbic acid oxidation on SH concentration in soy-wheat composite dough during resting period

The effect of l-ascorbic acid (l-AA) on free sulfhydryl concentration (SH) was evaluated in soy-wheat composite dough from 100-500 (g/kg) soy flour substitution for wheat flour. Raw soy flour (RSF) and physically modified soy flours (PMSF1 and PMSF2) were used for the preparation of the composite dough with wheat flour. The two physically modified soy flours were prepared by steam flushing (PMSF2) and water boiling (PMSF1) of raw soy beans before flour preparation. Using a timer, dough blends were manually mixed (at approximately 60 rpm) to dough development time after which, dough was sampled for the estimation of free SH groups. l-AA (0.05% w/w) was mixed with the dough after dough development and the dough was sampled after 1 h of resting the dough. The results showed that l-AA (0.05% w/w) acted as a reducing agent by increasing SH levels in all soy-wheat dough blends (P<0.05). After 1 h of resting, soy-wheat composite dough without l-AA had lower concentrations of SH than that with l-AA. A positive correlation was shown between soy flour concentrations and SH concentration before and after dough resting. A negative correlation existed between l-AA consumption and SH concentration for RSF-wheat, PMSF1-wheat and PMSF2-wheat doughs. The results indicated that soy flour weakened wheat flour dough by increasing SH concentration and that l-AA could have a synergistic effect on the reduction of gluten proteins and thus weakening the dough.     

Effect of l-ascorbic acid, sugar, pectin and freeze-thaw treatment on polyphenol content of frozen strawberries

The stability of phenolic compounds of three strawberry cultivars was evaluated for changes during prefreezing treatments, storage and various freezing and thawing conditions. Polyphenol content was determined by HPLC-DAD-FL. The sum of assayed polyphenolic (proanthocyanidin and monomeric flavan-3-ols, anthocyanins, ellagic acid, p-coumaric acid) represented 2858.4 mg/kg in Kent, 2893.1 mg/kg in Elkat, and 2438.0 mg/kg in Senga Sengana. After freezing, 4.5-33.6% of polyphenols were lost; protective effects of prefreezing treatments were seen on anthocyanidins and proanthocyanidins: ascorbic acid was the most effective pretreatment, allowing retention of 3.9-27.5% of anthocyanins, and almost total recovery (3.9-23.9%) when associated with liquid nitrogen freezing. Pectin and sugar only allowed retention of 3.0-25.1% and 5.5-25.4% of the antocyanins, respectively. Thawing of the strawberries in a microwave oven (instead of 20 h at 20 °C) had a further positive effect on retention of anthocyanins, proanthocyanins, (+)-catechin and ellagic acid.     

Light exposure during storage preserving soluble sugar and l-ascorbic acid content of minimally processed romaine lettuce (Lactuca sativa L.var. longifolia)

Minimally processed romaine lettuce (MPRL) leaves were stored in light condition (2500 lux) or darkness at 4 °C for 7 d. Light exposure significantly delayed the degradation of chlorophyll and decrease of glucose, reducing sugar, and sucrose content, and thus preserved more total soluble solid (TSS) content at the end of storage in comparison with darkness. While, it did not influenced starch content that progressively decreased over time. The l-ascorbic acid (AA) accumulated in light-stored leaves, but deteriorated in dark-stored leaves during storage. The dehydroascorbic acid (DHA) increased in all leaves stored in both light and dark condition, of which light condition resulted in less DHA than darkness. In addition, the fresh weight loss and dry matter significantly increased and these increases were accelerated by light exposure. Conclusively, light exposure in applied intensity effectively alleviated MPRL quality deterioration by delaying the decreases of pigments, soluble sugar, TSS content and accumulating AA.     

Oxidation of 6-O-arachidonoyl l-ascorbate microencapsulated with a polysaccharide by spray-drying

The oxidative stability at 37°C and 12% relative humidity of arachidonoyl moiety of 6-O-arachidonoyl l-ascorbate, which was synthesized through lipase-catalyzed condensation with l-ascorbic acid, was higher than that of unmodified arachidonic acid. Arachidonic acid and arachidonoyl ascorbate were microencapsulated with maltodextrin, gum arabic or a soluble soybean polysaccharide by spray-drying. The oxidation of arachidonic acid under the conditions was retarded by its microencapsulation with gum arabic or the soluble soybean polysaccharide. The oxidation of arachidonoyl moiety of the ascorbate encapsulated with gum arabic or the soluble soybean polysaccharide more slowly proceeded than unmodified or encapsulated arachidonic acid. The fractions of the unoxidized arachidonoyl moiety on day 29 were greater than 0.7. Thus, esterification of arachidonic acid with ascorbic acid and subsequent microencapsulation with gum arabic or the soluble soybean polysaccharide significantly improved its oxidative stability.     

The use of cyclic voltammetry to study the oxidation of l-5-methyltetrahydrofolate and its preservation by ascorbic acid

A cyclic voltammetry study of 1 mM l-5-methyltetrahydrofolate (l-5-MTHF) was performed in pH 5.5 Britton-Robinson buffer at room temperature to study the stability of l-5-MTHF alone and in combination with ascorbic acid (AA). The degradation of l-5-MTHF and AA over a period of 12 h both followed first order reaction kinetics. Using this technique, oxidation peaks of l-5-MTHF were identified at +0.17 and +1.18 V, and another oxidation peak appeared after 4 h under air at +0.89 V. Cyclic voltammetry and HPLC quantification enable us to confirm that l-5-MTHF can be highly preserved by the addition of an equimolar concentration of AA. This treatment was equivalent to a purge of nitrogen used to remove oxygen and thus minimise oxidation of l-5-MTHF when present in aqueous solutions. HPLC confirmed the fact that a full regeneration of oxidised l-5-MTHF occurred with the addition of sodium ascorbate, thus denoting that the redox character of l-5-MTHF can be controlled by the presence of reducing agents. Cyclic voltammetry proved to be a sensitive and accurate method for characterising l-5-MTHF oxidation and potential preservation with ascorbic acid. To our knowledge, this is the first study that has demonstrated the number of oxidation sites on l-5-MTHF.     

Optical resolution of n-butyl d- and l-lactates using immobilized lipase catalyst

n-Butyl d- and l-lactates (BuDLa and BuLLa) were incubated with immobilized lipase. ¹H-NMR showed that BuDLa reacted to oligomers, while BuLLa did not react. A mixture containing 90.4% of BuLLa and 9.6% of BuDLa was incubated with the enzyme for 72 h, then distilled. The purity of BuLLa increased to 98.6%.     

Microwave-assisted synthesis of prebiotic di-D-fructose dianhydride-enriched caramels

The synthesis of prebiotic caramels involving the use of microwaves as the activating/heating source has been achieved. The yields in di-fructose dianhydrides (DFAs) in caramels were measured. The aim of this study was twofold: first to check the feasibility of the process, and second to determine the conditions to obtain an optimum response with microwave heating. The study showed that it was possible to obtain a yield of almost 50% of DFAs in a reaction time that was 10 times shorter than a previous study; i.e. 5–10 min for microwave activation compared to 60–120 min for conventional heating. It was shown that the radiation time and the radiation power were linked. The simultaneous determination of the values of these two factors was therefore necessary to obtain significant yields. This technique demonstrates the advantage of activation for mixtures such as caramels.     

反应温度和时间对抗坏血酸/精氨酸Maillard反应体系中挥发性化合物形成的影响

采用顶空固相微萃取-气相色谱-质谱联用技术对抗坏血酸/精氨酸反应体系在不同反应温度和时间下的产物进行鉴定,共鉴定出包括吡嗪、醛、呋喃、醇和吲哚在内的24种化合物,其中14种吡嗪化合物为主要香味化合物。考察了反应温度和时间对产物特别是对吡嗪化合物的影响。结果表明,随着反应温度的升高和反应时间的延长,吡嗪类化合物的总量均增加;其中,随反应温度的升高,相对其他吡嗪化合物2-乙基-6-甲基吡嗪、3-乙基-2,5-二甲基吡嗪的量明显增加;随反应时间的延长,2-乙基-6-甲基吡嗪的量明显增加。     

Studies on the retention of microencapsulated l-5-methyltetrahydrofolic acid in baked bread using skim milk powder

Our aim was to protect l-5-methyltetrahydrofolic acid (L-5-MTHF) from degradation throughout the baking and storage of a fortified white bread using microencapsulation. L-5-MTHF, with or without sodium ascorbate (ASC), was microencapsulated using skim milk powder (SMP) as the coating agent. Recoveries of L-5-MTHF in spray-dried materials were greater than 95 ± 5%. Microencapsulated L-5-MTHF was completely released from the skim milk coating material in simulated gastric fluid within the first 10 min at 37 °C. Incorporation of SMP-L-5-MTHF or SMP-L-5-MTHF + ASC into bread gave recoveries of 81.3 ± 1.3% and 87.1 ± 1.2% (n = 3), respectively, for L-5-MTHF immediately after bread baking. These treatments also showed significantly (p < 0.05) greater L-5-MTHF stability during room temperature storage, compared to the free L-5-MTHF. This study has shown that SMP is an effective microencapsulating agent and in the presence of ASC will produce excellent conditions for stabilising L-5-MTHF in baked bread.     

Myosin is solubilized in a neutral and low ionic strength solution containing l-histidine

Myosin, one of the major myofibrillar proteins, is insoluble at low and physiological ionic strength and soluble at high ionic strength. In this study, the behavior and morphology of myosin solubilized in a low ionic strength solution containing l-histidine (l-His) was investigated. More than 80% of myosin was solubilized in a low ionic strength solution with dialysis against a solution containing 1 mM KCl and 5 mM l-His. Transmission electron microscopy with rotary shadowing demonstrated that the rod of myosin in a low ionic strength solution containing l-His is longer than that of myosin in a high ionic strength solution. The elongation of the myosin rod in a low ionic strength solution containing l-His would inhibit the formation of a filament, resulting in the solubilization of myosin.     

Significant influence of particular unsaturated fatty acids and pH on the volatile compounds in meat-like model systems

The present study aimed to investigate the effects of particular unsaturated fatty acids and pH on volatile compounds in the meat-like model systems containing 21 amino acids and ribose. We have chosen oleic acid (C18:1n-9), linoleic acid (C18:2n-6) and linolenic acid (C18:3n-3) for the present investigation. The effect on the volatile compounds was evaluated at two different pH 5.5 and 6.2. The addition of C18:2n-6 and C18:3n-3 to the Maillard reaction mixtures inhibited the formation of many Maillard compounds or produced in low amount. Whereas, the addition of C18:1n-9 resulted in an increase in the amount of many Maillard compounds. The thiols and thiophenes were preferentially formed at pH 5.5 whereas pyrazines were preferentially favored at pH 6.2. Based on our results, it is suggested that a high level of C18:2n-6 and C18:3n-3 in meat probably causes an undesirable flavor. The pH value of 5.5 was found to be ideal for the formation of volatile compounds associated with desirable flavor of meat.     

Interaction between starch and aroma compounds as measured by proton transfer reaction mass spectrometry (PTR-MS)

The interaction between starch and the complexing aroma compounds hexanal and menthone was investigated by headspace analysis using proton transfer reaction mass spectrometry (PTR-MS). Starch systems with non-modified and modified tapioca starch were prepared containing single aroma compounds (hexanal, menthone) or a blend of the two aroma compounds at low aroma concentrations (6 × 10⁻³ mg/100 g). The headspace intensity of starch–aroma systems was slightly reduced due to starch–aroma interactions compared to aroma water-systems. The combination of hexanal and menthone did not lead to competitive aroma release effects, but hexanal retention increased by increasing the equilibration time from 48 to 96 h. Non-modified and chemically modified starch showed similar aroma release behaviour. It is concluded that the formation of starch–aroma complexes influences aroma release, but that at low aroma concentration the dissociation of the complex is not hindered by the aggregation of amylose–aroma complexes.