脂类的催化氧化

对脂类催化氧化作了全面、深入的研究。指出光敏性物质的存在是光氧化发生的直接因素，第一类光敏物可将底物活化成自由基，第二类光敏物诱发的反应为单线态氧和底物的协同反应，其氢过氧化产物不同于自动氧化的产物；脂氧酶能有选择性的催化多不饱和脂肪酸，且具有专一性，来自不同植物中的脂氧酶催化反应的产物也不同；金属催化的本质是自由基反应，过渡金属因其价态发生变化及其无处不有的特点，在脂类的催化氧化反应中，发挥着极其重要的作用，尤其三价铁是很强的自由基诱发剂。光氧化、脂酶氧化和金属氧化是启动脂类自动氧化的三个主要因素     

油脂氧化酸败研究进展

油脂氧化酸败与风味劣变、营养丧失、生物学损害、组织老化紧密相关。该文概述油脂氧化研究以自动氧化自由基链式反应学说为核心发展历程;阐述自动氧化、光敏氧化、酶促氧化之间关联性与研究进展。自动氧化在油脂氧化中处主导地位,而光敏氧化和酶促氧化很可能在诱发氧化前期起关键作用;光敏氧化既可循II型单线态氧化途径,又可循I型自由基链式氧化途径,取决于激发态光敏剂是底物脱氢、还是直接攻击不饱和双键;酶促氧化在有氧条件下类似自由基链式反应,无氧条件下脂氧酶无活性。     

食品抗氧化剂及其进展(Ⅰ)

抗氧化剂是指能防止食品成分因氧化而导致变质的一类食品添加剂。主要用于防止油脂及富脂食品的氧化酸败,以及由氧化所导致的褪色、褐变、维生素破坏等。食品中的油脂可发生两种化学变化;水解和氧化。水解一般受脂肪酶催化而使油脂水解为甘油、单双甘油醋和游离脂肪酸,可通过加热、精炼,以破坏或消除脂肪酶而达到保护作用。更普通和重要的是油脂的氧化,这是一个十分复杂的过程。也是使用抗氧化剂的主要目的。１　油脂氧化基本过程脂类的氧化主要是脂类与氧分子的直接反应,称“自动氧化”,另有光敏氧化及酶氧化。其中最主要的是自动氧…     

烤肉中脂类氧化及抗氧化研究

肉类食品中脂类的氧化会产生许多不利的影响,如使食品风味发生变化以及产生有害人体健康的物质等。本文综述了烤肉中脂类的氧化,分析了影响脂类氧化的因素,并总结了抗氧化的方法。     

氧化·疾病·抗氧化(ⅩⅢ)

<正> 4.2.1类胡萝卜素对单线态氧消除作用 类胡萝卜素能消除单线态氧,已于1968年由Foote和Denney[13]得到证实。由叶绿素之类光敏剂吸收光能后,引发三重态氧激发态,使三重态氧3O2引发反应而产生单线态氧1O2。类胡萝卜素吸收1O2能量而使之回复至三重态氧,自己成为吸附能量类胡萝卜素,经放出热能后重新回至普通类胡萝卜素,同时消除1O2(见图6)。这种消除1O2能力只能在有9个以上共轭双键的类胡萝卜素才具备,如只有五个共轭双键的视黄醇就无法消除1O2[14]。     

碳纤维膜负载组合光敏剂去除重革废水中S2- 、有机物的研究

提出了一种利用碳纤维膜负载组合光敏剂、光敏氧化处理重革废水的新技术。结果显示 :光敏剂的不同组合及碳纤维膜负载不同量的光敏剂 ,对制革废水中S2 -、COD的去除率是不同的 ,其中以负载复合型光敏剂 ,即当吖靛橙 (RO)∶亚甲基蓝 (MB) =3∶2 ,总负载量为 1 80mg/m2 为最佳。在光照强度为 3 0 0 0lx ,面积负荷为94L/m2 ·HRT ,S2 -的去除率在 90 .0 %以上、COD去除率也可达 5 9.7% :对有机物的最大去除能力为 1 65 g(COD) /m2 ·HRT ,并且这种负载型光敏剂可重复使用。并对光敏氧化处理过程的机理进行了探讨     

纸浆光催化漂白反应中的脱木素机理

首先介绍了纸浆光催化漂白中的光源的选择，以及由光敏剂产生的单线态氧(^1O2)和光照射半导体产生的电子-空穴对和UV照射脱木素机理。讨论了这些自由基和中间产物的形成、脱木素反应的特点及选择性。     

血红素和原卟啉光敏作用的研究

研究了肌红蛋白的组成成分血红素和原卟啉的光敏能力,同时分析了不同波长光的照射下血红素和原卟啉对猪肉脂肪氧化的影响。结果表明:原卟啉产生单线态氧(1O2)的能力强于血红素,它的光敏能力强于血红素。在光照强度相同、波长不同的光的照射下血红素和原卟啉均会加快猪肉脂肪氧化的速度,且随着光波长的减小,这种影响越大。     

纤维素纤维负载锌酞菁催化剂光敏氧化碱性绿-1的研究

纤维素纤维因丰度大,与多种物质亲和力强,被广泛用作废水处理催化剂的载体。合成了一种对可见光有响应的纤维素纤维负载锌酞菁催化剂[纤维素纤维负载四(2,4-二氯均三嗪)氨酞菁锌],并在有和无印染助剂的情况下,用于光敏氧化印染废水中的碱性绿-1。与传统的自由基氧化染料方式不同,此负载催化剂将三线态氧激发为单线态作为氧化剂。在有印染助剂存在下,负载催化剂的光敏氧化能力显著增强。     

脂类物质在火腿风味形成中的作用

脂类物质在火腿风味形成中起重要作用,他们既是形成风味物质的前体,也是风味化合物蓄积的溶剂。火腿中的脂类物质主要是甘油脂和磷脂,磷脂对风味物质形成的贡献更突出。腌制过程中,脂类物质主要发生了水解、氧化,氧化产物还可以通过美拉德反应与其它化合物进一步作用形成风味物质。本文综合论述了脂类物质在火腿加工过程中所发生的变化及变化机制,讨论了这些变化在火腿风味形成中的作用。     

Front-face fluorescence measurement of photosensitizers and lipid oxidation products during the photooxidation of butter

This paper shows that fluorescence spectroscopy can measure both degradation of photosensitizers and formation of lipid oxidation products in light-exposed butter. The photosensitizers were already notably degraded after 4 h of light exposure, whereas fluorescent lipid oxidation products were detected after 5 d. The fluorescence measurements were highly correlated with sensory assessments of acidic and rancid flavor. Photosensitizer degradation is therefore a promising indirect indicator of the onset of lipid oxidation in butter. Sensory analysis and measurement of peroxide value showed that the level of lipid oxidation was significantly higher for butter stored in air compared with butter stored in nitrogen (N₂). This might be explained by the formation of singlet oxygen from direct photooxidation and type II photosensitized oxidation. Addition of the singlet oxygen quencher β-carotene reduced the rancid flavor intensity in the air and N₂ packages from 9.0 to 4.9 and from 6.5 to 4.7, respectively. Results indicate that lipid oxidation in the butter stored in N₂ was mainly caused by type I photosensitized reactions, because addition of β-carotene had little effect on the rancid flavor intensity.     

Chemistry and Reaction of Singlet Oxygen in Foods

Singlet oxygen is a highly reactive, electrophilic, and nonradical molecule. It is different from diradical triplet oxygen in its electron arrangement. Photosensitizers can form singlet oxygen from triplet oxygen in the presence of light. The reaction rate of singlet oxygen with foods is much greater than that of triplet oxygen due to the low activation energy. Singlet oxygen oxidation produces undesirable compounds in foods during processing and storage. However, carotenoids and tocopherols in foods can minimize singlet oxygen oxidation. The in‐depth scientific knowledge on the formation, reactions, quenching mechanisms, and kinetics of singlet oxygen can greatly improve the quality of foods by minimizing the oxidation during processing and storage. The single oxygen oxidation of foods has contributed to the explanation of several important chemical reactions in the reversion flavor in soybean oil, sunlight flavor in milk products, and the rapid losses of vitamin D, riboflavin, and ascorbic acid in milk under light storage.     

血红素和原卟啉光敏作用的研究

研究了肌红蛋白的组成成分血红素和原卟啉的光敏能力,还分析了在不同光照强度下贮存期间血红素和原卟啉对猪肉脂肪氧化的影响情况。结果表明:原卟啉产生单线态氧的能力强于血红素,它的光敏能力强于血红素。加入原卟啉样品的脂肪氧化指标在430lux以上就与其他组出现了显著性差异(P<0.05),而加入血红素的样品组在630lux以上时才出现了显著性差异(P<0.05),且随着光强度的增加,这种差异越显著。     

Mechanisms of Antioxidants in the Oxidation of Foods

ABSTRACT: Antioxidants delay or inhibit lipid oxidation at low concentration. Tocopherols, ascorbic acid, carotenoids, flavonoids, amino acids, phospholipids, and sterols are natural antioxidants in foods. Antioxidants inhibit the oxidation of foods by scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen and photosensitizers, and inactivating lipoxygenase. Antioxidants show interactions, such as synergism (tocopherols and ascorbic acids), antagonism (α‐tocopherol and caffeic acid), and simple addition. Synergism occurs when one antioxidant is regenerated by others, when one antioxidant protects another antioxidant by its sacrificial oxidation, and when 2 or more antioxidants show different antioxidant mechanisms.     

Activated oxygen species and oxidation of food constituents.

Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions known to occur in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents can ultimately yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. The generation, detection, measurement, reaction, and inhibition of reactions of active oxygen species are surveyed in this review.     

Activated oxygen species and oxidation of food constituents

Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions known to occur in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents can ultimately yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. The generation, detection, measurement, reaction, and inhibition of reactions of active oxygen species are surveyed in this review.     

Kinetics for Singlet Oxygen Formation by Riboflavin Photosensitization and the Reaction between Riboflavin and Singlet Oxygen

ABSTRACT: The formation of singlet oxygen by riboflavin and the kinetics and mechanisms of riboflavin degradation in aqueous solution under light were determined. The singlet oxygen formation rate by riboflavin was 2.31 μmole oxygen/mL headspace/h of serum bottle. The degradations of riboflavin were 66% in D₂O and 40% in H₂O, respectively, under light after 24 h. The results indicate that singlet oxygen is involved in riboflavin destruction under light. The riboflavin destructions were 94.0% and 15.7% with 0 mM or 160 mM ascorbic acid, respectively, under light after 96 h. The reaction rate between riboflavin and singlet oxygen was 1.01 × 10¹⁰/M/s, which is a diffusion-controlled reaction rate. This explains the extremely fast degradation of riboflavin in foods under light. Ascorbic acid and sodium azide reduce the degradation of riboflavin under light with different quenching mechanisms. Ascorbic acid quenched both singlet oxygen and excited triplet riboflavin. Sodium azide quenched only the singlet oxygen in riboflavin solution with a quenching rate of 1.547 × 10⁷/M/s. With the involvement of both the Type-I and Type-II mechanisms in the riboflavin degradation under light, singlet oxygen quencher alone could not protect the riboflavin from degradation completely. Addition of ascorbic acid can protect riboflavin oxidation in foods exposed to light.     

Effects of solubility characteristics of sensitiser and pH on the photooxidation of oil in tuna oil-added acidic O/W emulsions

The effects of sensitisers and pH on the oil oxidation of acidic O/W emulsions were studied under light by measuring hydroperoxide content and headspace oxygen consumption in the emulsions. The emulsions consisted of canola and tuna oil (2:1 w/w, 32%), diluted acetic acid (64%), egg yolk powder (4%), chlorophyll b or erythrosine (5 μM), and/or diazabicyclooctane (DABCO) or sodium azide (0.5 M). The emulsion pH values were 2.67, 3.68, and 6.27. Chlorophyll increased oil oxidation in the emulsion under light via singlet oxygen production while erythrosine did not. DABCO significantly decreased photooxidation of the oil containing chlorophyll, suggesting singlet oxygen involvement. However, sodium azide increased photooxidation of the oil containing chlorophyll possibly via azide radical production under acidic conditions. The oil photooxidation was higher in the emulsion containing chlorophyll at pH 6.27 than at pH 2.67 or 3.68, primarily by singlet oxygen and secondarily by free radicals produced from hydroperoxide decomposition.     

Chemical Reactions and Stability of Riboflavin in Foods

ABSTRACT: Riboflavin is relatively stable during thermal and nonthermal food processing and storage but is very sensitive to light. It can accept or donate a pair of hydrogen atoms. It can act as a photosensitizer (through either Type I or Type II mechanism) or a prooxidant for food components under light. Photosensitization of riboflavin causes production of reactive oxygen species such as superoxide anion, singlet oxygen, hydroxy radical, and hydrogen peroxide. Radicals and reactive oxygen species accelerate the decomposition of proteins, lipids, carbohydrates, and vitamins, and could cause significant nutrient loss in foods. Carbohydrates are less sensitive to riboflavinphotosensitized oxidation than proteins, lipids, or vitamins. Riboflavin is an excellent photosensitizer for singlet oxygen formation and a superb reactant for singlet oxygen, with the reaction rate of 1.01 ± 10¹⁰/M/s.     

Indirect photodegradation of dissolved free amino acids: the contribution of singlet oxygen and the differential reactivity of DOM from various sources

The role of photochemically generated singlet oxygen (1O2) in the DOM-sensitized degradation of eighteen dissolved free amino acids was investigated. The fraction of total sensitized degradation due to reaction with 1O2 was determined through a kinetic analysis based on a measured reaction rate constant for each amino acid coupled with measured 1O2 concentrations and was confirmed through quenching experiments. Only four of the eighteen free amino acid residues examined were found to be photolabile under environmentally relevant conditions: histidine, methionine, tyrosine, and tryptophan. The fraction of Suwannee River Humic Acid (SRHA)-sensitized degradation due to reaction with 1O2 ranged from an upper value of 110 +/- 10% for histidine to 8 +/- 1% for tryptophan, with 26 +/- 3% contribution for methionine and 33 +/- 4% for tyrosine. In addition to degradation through reaction with 1O2, other reactive intermediates involved in the SRHA-photosensitized degradation of these amino acids were identified. Methionine was thought to be additionally degraded through reaction with H2O2 and triplet excited-state DOM, and 67% of tyrosine's indirect photodegradation was assigned to an oxygen-dependent type I photooxidation reaction. The majority of tryptophan indirect degradation was due to reaction with 3DOM. Photodegradation experiments with various DOM sources including Pony Lake (Antarctica) fulvic acid and a synthetic estuarine sample, as well as Minnesota freshwater samples (lakes Itasca, Superior, Josephine, and the St Louis River), demonstrated distinct reactivity patterns, indicating that DOM's 1O2-generation efficiency is not strongly coupled to its ability to promote other photooxidation pathways. These four amino acids highlightthe differential photoreactivity of DOM from various sources.