Ascorbic acid destruction in aqueous model systems: an additional discussion

The influence of different non‐electrolytes (fructose, xylitol, glucose/mannitol mixture) and electrolytes (NaCl, KCl) on anaerobic L ‐ascorbic acid (AA) degradation in an aqueous model system (pH 3.5, a_w 0.94) was studied to assess the effect on that reaction of substances commonly used to diminish the water activity of fruit or vegetable juices, as well as its relation with non‐enzymatic browning (NEB) development, at processing (70, 80, 90 °C) and storage (24, 33, 45 °C) temperatures. AA degraded as a function of time with a behaviour that could be described by first‐order kinetics at all temperatures studied, and activation energies could be calculated using the Arrhenius law. The presence of humectants enhanced AA destruction, with D ‐fructose promoting the fastest L ‐ascorbic acid destruction and browning development at processing temperatures. The influence of humectants on NEB seemed to determine differences between their influence on AA degradation. Water activity decrease by humectant addition produced higher AA stability in solution at storage temperatures. The differential effect of each humectant used to decrease the water activity seemed to be related to its influence on solvent structure. © 2001 Society of Chemical Industry     

Studies on non-enzymic browning in orange juice using a model system based on freshly squeezed orange juice

An orange juice based model system was set up wherein variables such as the addition of glucose, lysine, sorbitol and L -ascorbic acid and changes in pH and storage temperature were introduced to study their effects on L -ascorbic acid degradation and on browning. The addition of glucose, lysine and sorbitol did not have a significant effect on L -ascorbic acid degradation, whereas added L -ascorbic acid acted solely as a reserve material. In terms of browning, a change in pH and storage at elevated temperature produced a synergistic effect in the development of brown colour which was enhanced by the introduction of additives. Based on the results obtained, L -ascorbic acid appears to be the precursor in the non-enzymic browning occurring in this type of sample due to the formation of reactive carbonyl compounds produced upon its degradation which tend to polymerise, or to react with nitrogenous compounds, to give brown pigments.     

Quality maintenance of minimally processed Chinese cabbage with low temperature and citric acid dip

Chinese cabbage (Brassica campestris L pekinensis group) was minimally processed using best preparation techniques and stored at 0 and at 5°C with and without dips in either citric acid, calcium chloride or ascorbic acid, all at 10 g litre⁻¹. The visual quality, degree of chilling injury, pH and taste were evaluated. The most deleterious effects on quality were produced by black speck (gomasho) and browning. Citric acid inhibited the development of black speck and extended storage life from 10 days of the control to 14 days at 5°C. At 0°C the storage life was not extended by any dip, but citric acid improved quality by reducing black speck. Minimally processed Chinese cabbage treated with citric acid showed only a slight reduction of pH from 6·3 of the control to 6·1 (P⩽0·05) and taste was not significantly affected (P>0·05). Microbial spoilage was not apparent during storage at 0°C for 35 days and 5°C for 21 days under any treatment. © 1997 SCI.     

Proteolytic degradation of myofibrillar components by carp cathepsin L

Carp cathepsin L, which is the best candidate to produce textural change in the arai-treated carp fillet, exhibited maximum hydrolytic activity for Z-Phe-Arg-MCA and soluble casein at pH 5·0–5·5. The proteolytic action of the enzyme was evaluated by complete degradation of various carp myofibrils at pH 5·0 over 30 min and by potent degradation of the same proteins at pH 5·5–6·0 over 20 h. All myofibrillar components were partially degraded by the enzyme at pH 6·5–7·0, but varying amounts of them remained undegraded after 20 h. These findings indicate that carp cathepsin L degrades not only carp myofibrillar components but also their resultant products between pH 5·0 and 7·0 and that it markedly acts on myosin heavy chain, α-actinin and troponin-T and -I. Carp cathepsin L likely contributes to postmortem muscle tenderisation of carp fillet over an extensive pH range during storage. © 1998 SCI.     

Free radical scavenging capacity of Maillard reaction products as related to colour and fluorescence

The free radical scavenging activity of Maillard reaction products (MRPs) produced by heating glucose or lactose with lysine, alanine or glycine was estimated directly by means of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH·) method. Model systems of sugar (lactose and glucose) and amino acid (alanine, glycine, and lysine) were heated without pH control at 100°C up to 24 h. Fluorescence (347 and 415 nm), browning (absorbance at 420 nm), colour parameters in the CIELab scale, and free radical scavenging activity were determined. Browning is not directly related to the free radical scavenging properties of MRPs formed at prolonged heating conditions. This study shows that fluorescence measurement of heated sugar/amino acid systems is more effective than browning to follow the formation of MRPs with free radical scavenging activities. This approach could be applied to optimise the formation of natural antioxidants during food processing where browning is not desirable for consumers.     

贮藏过程中溶解氧对荔枝汁非酶褐变的影响

本文研究了荔枝汁在不同初始溶解氧浓度及有顶空氧存在的贮藏条件下的非酶褐变情况,采用PLSR进行分析揭示不同溶解氧浓度对荔枝汁非酶褐变的影响规律。结果表明:在0~3 d的贮藏期内,随着DOC的快速下降,低溶解氧和中溶解氧荔枝汁的L-AA进行有氧降解;在3 d~17 w的贮藏期内,DOC下降缓慢,趋于稳定,L-AA进行有氧降解和无氧降解,并以无氧降解为主。随着贮藏前期DOC和顶空氧浓度的快速下降,高溶解氧荔枝汁的L-AA持续进行有氧降解并在第4 w时基本全部降解完毕。三种溶解氧浓度的荔枝汁中的蔗糖均不断降解,但果糖和葡萄糖含量呈现先快速增加后减少的趋势。5-HMF和褐变指数不断地增加,溶解氧浓度越高,增加的越快。表儿茶素不断的降解,而芦丁含量比较稳定。在不同溶解氧浓度下,荔枝汁的非酶褐变是以L-AA降解及酚类物质氧化聚合为主,美拉德反应为辅。     

Interaction Between Potassium Sorbate and Aspartame in Aqueous Model Sugar Systems

ABSTRACT: The interaction between potassium sorbate and aspartame in aqueous model sugar systems of 0.97 water activity and pH 3.0 was studied. Aspartame addition showed a protective effect over sorbate destruction in an aqueous system of approximately a_w 1, as well as in a system containing glucose; but, in the presence of sucrose, it enhanced degradation. In all systems studied, addition of K sorbate and depression of water activity to 0.97 increased aspartame destruction. The combined use of K sorbate and aspartame exhibited strong nonenzymatic browning potential. The study demonstrated that nonenzynmatic browning can limit shelf life; and it is not recommended to use potassium sorbate together with aspartame.     

Use of Active Packaging for Increasing Ascorbic Acid Retention in Food Beverages

ABSTRACT: A study on the influence of the type of container and storage temperature on ascorbic acid degradation and browning in a model system simulating a citrus juice is presented. The model system was packaged in glass jars, polyethylene terephthalate (PET) bottles, bottles of PET containing different percentages of oxygen scavengers, and bottles of PET blended with 6% nylon. Samples were stored at 5 °C to simulate the usual storage condition and at 35 °C to simulate a temperature abuse. The slowest ascorbic acid degradation and browning were obtained with PET + 3% oxygen scavengers. Glass and PET gave the worst results: the 1st due to the presence of pro-oxidants and the latter due to the oxygen permeability. In particular, concerning ascorbic acid degradation in glass containers at 35 °C, it was hypothesized that there are 2 simultaneously occurring reactions: the 1st directly related to aerobic oxidation and the 2nd primarily due to the anaerobic consumption of ascorbic acid, which becomes predominant when oxygen is totally exhausted. Results also demonstrated the possible replacement of the glass containers with polymeric ones in the beverage industry.     

Leuconostoc fallax,an acid and ethanol tolerant lactic acid bacterium

Leuconostoc fallax, known to be present in sauerkraut, was reisolated from exudates of Gerbera jamesonii. The identity of the isolates with L fallax was demonstrated by sequence analysis of the first 600 bases of the 16S rRNA. L fallax utilised a small number of sugars and showed a remarkable resistance to lactic acid. The final pH in glucose broth was 3·9. Moreover it was able to grow in the presence of ethanol (9·0% v/v) and salt (5·5%), but it was unable to carry out a malo-lactic fermentation.     

Effect of Initial Dissolved Oxygen Levels on the Degradation of Ascorbic Acid and the Browning of Lemon Juice during Storage

The effect of different initial dissolved oxygen concentrations (0.41, 1.44, and 3.74 mg/L) on rate of deteriorative quality changes [ascorbic acid degradation, browning, hydroxymethylfurfural (HMF) and furfural production) in lemon juice stored at 36°C was investigated. No significant effects on rate of ascorbic acid degradation and furfural formation could be attributed to the different oxygen levels. Degradation of ascorbic acid appeared to be predominantly anaerobic. Lag period before browning increased depended on oxygen level. Zero-, first- and second-order kinetic models were fitted to various degradative reactions occurring in lemon juice during storage. Highly significant correlations were obtained between browning index, HMF, and furfural formation, suggesting that all three would be suitable as chemical indices of storage temperature abuse in lemon juices.     

A comparative study on the transport of L(-)malic acid and other short-chain carboxylic acids in the yeast Candida utilis: Evidence for a general organic acid permease

Cells of the yeast Candida utilis grown in medium with short-chain mono-, di- or tricarboxylic acids transported L(-)malic acid by two transport systems at pH 3·0. Results indicate that probably a proton symport for the ionized form of the acid and a facilitated diffusion for the undissociated form were present. Dicarboxylic acids such as succinic, fumaric, oxaloacetic and α-ketoglutaric acids were competitive inhibitors of the malic acid for the high-affinity system, suggesting that these acids used the same transport system. In turn, competitive inhibition uptake studies of labelled carboxylic acid in the low-affinity range indicated that this system was non-specific and able to accept not only carboxylic (mono-, di- or tri-) acids but also some amino acids. Additionally, under the same growth conditions, C. utilis produced two mediated transport systems for lactic acid: a proton symport for the anionic form which appeared to be a common monocarboxylate carrier and a facilitated diffusion system for the undissociated acid displaying a substrate specificity similar to that observed for the low-affinity dicarboxylic acid transport. The mediated carboxylic acid transport systems were inducible and subjected to repression by glucose. In glucose-grown cells the undissociated dicarboxylic acids entered the cells slowly by simple diffusion. Repressed glucose-grown cells were only able to produce both transport systems if an inducer, at low concentration (0·5%, w/v), was present during starvation in buffer. This process was inhibited by the presence of cycloheximide indicating that induction requires de novo protein synthesis. If a higher acid concentration was used, only the low-affinity transport system was detectable, showing that the high-affinity system was also repressed by high concentrations of the inducer.     

Influence of water activity and storage conditions on survival and growth of proteolytic Clostridium botulinum in peanut spread

Outgrowth of Clostridium botulinum spores followed by toxin production in peanut spread at A_w0·98, 0·96, 0·94 and 0·92 stored at 30°C under anaerobic or aerobic conditions for 0, 3, 7 and 16 weeks or 0, 1, 9 and 16 weeks, respectively, was investigated. Botulinal toxin was not detected in peanut spreads stored under anaerobic conditions for 16 weeks. Peanut spreads at A_w0·98 and two of three samples at A_w0·96 stored aerobically became toxic after 9 and 16 weeks, respectively. Clostridium botulinum in peanut spread at A_w0·98 and 0·96 grew to populations of 10⁶and 10⁵cfu g⁻¹, respectively, within 16 weeks. Lactic acid bacteria grew within 3 days in peanut spread at A_w0·98 and 0·96 stored under aerobic or anaerobic conditions. Regardless of A_w, populations of aerobic and anaerobic micro-organisms decreased in peanut spread stored under anaerobic conditions. Only slight decreases occurred in samples stored under aerobic conditions. The pH of inoculated and uninoculated peanut spread at A_w0·98 and 0·96 increased from 4·8 to 7·0 within 16 weeks and was attributed to growth of Penicillium andMucor spp. Similarly, redox potential (Eh) of peanut spread stored under anaerobic conditions for 3 weeks, decreased as the A_wwas increased. Significantly lower Eh values in peanut spread samples at A_w0·98 or 0·96 stored under aerobic conditions occurred within 1 week and/or 9 weeks compared to peanut spread at A_w0·92 or 0·94. Peanut spreads were judged inedible due to growth of lactic acid bacteria and molds which resulted in ‘off’ aromas before toxicity developed, thus greatly minimizing the likelihood of consumption.     

Effects of temperature,pH, organic acids,and sulfites on tagatose browning in solutions during processing and storage

Tagatose browning in solutions might be unacceptable to certain products containing tagatose during processing and storage. The objective of this study was to evaluate the effects of temperature, pH, organic acids, and sulfites on tagatose browning in solutions. Tagatose showed the fastest browning reaction, followed by fructose, xylose, glucose, and sucrose. Tagatose browning was temperature and pH dependent. As temperature and pH increased, tagatose browning was increased. Tagatose browning was slow at temperatures lower than 80°C and pH of 5. Organic acids, such as ascorbic acid, citric acid, and lactic acid, activated tagatose browning. Sodium bisulfite and potassium metabisulfite inhibited tagatose browning, whereas sodium sulfite had no effect on the inhibition of tagatose browning. During storage at 25°C, tagatose browning did not occur regardless of the pH, tagatose concentration, and storage time. It was suggested that tagatose browning in solutions might be prevented by controlling the temperature and pH.     

高产琥珀酸重组大肠杆菌的构建及厌氧发酵

以野生型大肠杆菌Escherichia coli W为出发菌株,利用Red同源重组系统分别敲除了乳酸脱氢酶基因(ldhA)、乙醇脱氢酶基因(adhE)、丙酮酸甲酸裂解酶基因(pflB)、丙酮酸氧化酶基因(poxB)和乙酸激酶基因(ackA),再通过无氧生长进化筛选过程,构建得到在厌氧条件下能有效生长,并以琥珀酸为主要发酵产物的重组大肠杆菌WS100(△ldhA,△adhE,△pflB,△poxB,△ackA)。利用15 L发酵罐进行厌氧发酵测定显示,经72 h发酵,菌体密度OD600最大值可提高至6.48,琥珀酸产量达到70.13 g/L,琥珀酸的生产强度为0.98 g/(L.h),葡萄糖-琥珀酸转化率为76%。发酵液中副产物含量低,乙酸含量为5.34 g/L,乳酸产量仅为0.15 g/L,未检测到甲酸和乙醇生成。结果表明,厌氧条件下,该工程菌可有效利用低营养成分的无机盐培养基,在不表达任何外源基因的条件下可稳定高产琥珀酸,具有极大的工业化开发前景。     

Color Degradation in an Ascorbic Acid-Anthocyanin-Flavanol Model System

Interactions between ascorbic acid, pelargonidin-3-glucoside, and catechin were studied in pH 3.4 citrate-phosphate buffer at 20°C under anaerobic and oxygenated conditions. Changes in anthocyanin pigment, ascorbic acid, dehydroascorbic acid, and catechin were quantitatively measured during 130 days storage along with Hunter color parameters, browning, and polymeric color. Ascorbic acid had a statistically significant effect on all parameters, including anthocyanin loss under both oxygen and nitrogen environments, catechin loss, increase in browning and polymeric color, decrease in Hunter “a” value and increase in Hunter L value. Ascorbic acid bleached anthocyanin pigment and also induced browning. There is evidence that anthocyanin pigment and ascorbic acid degrade through a direct condensation mechanism.     

Factors affecting the extreme acid resistance of Escherichia coli O157:H7

When stationary phase Escherichia coli O157:H7 cells were subjected to extreme acid shock (pH 2·0, 6 h, 37°C) cell survival was as great as 10%, but culture conditions greatly affected the acid resistance. Anaerobic cultures were more resistant to extreme acid shock if the glucose concentration of the growth medium was high, acids accumulated, and pH declined. By varying pH and acetate concentration, it was possible to demonstrate a high correlation (R²=0·86) between undissociated acetate and extreme acid resistance. Because dissociated acetate and extreme acid resistance were poorly correlated (R²<0·01), it appeared that the pH effects were being mediated via acetate dissociation. Propionate and butyrate were as effective as acetate, but formate, lactate, benzoate and the uncoupler, carbonylcyanide m -chlorophenylhydrazone (CCCP), were much less effective in promoting extreme acid-resistance. Acetate, propionate, butyrate, benzoate and CCCP all decreased the intracellular pH of E. coli O157:H7, but the correlation between intracellular pH and extreme acid resistance was low (R²<0·01). Cultures grown aerobically only needed half as much acetate to induce extreme acid resistance as those grown anaerobically, and the addition of the reducing agent, cysteine, to anaerobic media made the stationary phase cells less responsive to acetate. An rpoS mutant of E. coli O157:H7 was at least 100-fold more sensitive to acid shock than the wild-type, and large amounts of acetate were needed to promote even a small increase in viability.     

Purification and properties of polyphenol oxidase in head lettuce (Lactuca sativa)

Polyphenol oxidase (EC 1.10.3.1) in head lettuce (Lactuca sativa L) was purified by ammonium sulphate fractionation, ion exchange chromatography and gel filtration. The enzyme was found to be homogeneous by polyacrylamide gel electrophoresis. The molecular weight of the enzyme was estimated to be about 56 000 amu by Sephadex G-100 gel filtration. The purified enzyme quickly oxidised chlorogenic acid (5-caffeoyl quinic acid) and (—)-epicatechin. The K^m values for the enzyme, using chlorogenic acid (pH 4·5, 30°C) and (—)-epicatechin (pH 7·0, 30°C) as substrate, were 0·67 mM and 0·91 mM, respectively. The optimal pH of chlorogenic acid oxidase and (—)-epicatechin oxidase activities were 4·5 and 7·8, respectively, and both activities were stable in the pH range 6–8 at 5°C for 20 h. Potassium cyanide and sodium diethyldithiocarbamate markedly inhibited both activities of the purified enzyme. The inhibitory effect of metallic ions such as Ca²⁺, Mn²⁺, Co²⁺ and Ni²⁺ for chlorogenic acid oxidase activity was stronger than that for (—)-epicatechin oxidase activity.     

Mechanism of Browning in Fresh Highbush Blueberry Fruit (Vaccinium corymbosum L). Partial Purification and Characterisation of Blueberry Polyphenol Oxidase

The most efficient method for polyphenol oxidase (PPO) extraction from Highbush blueberry fruits was the preparation of an acetone powder. No activity was detected after direct extraction with phosphate buffer (pH 6·5) and detergents such as Triton X-100. PPO has been purified 19-fold, by ultrafiltration, ammonium sulphate precipitation and hydrophobic chromatography. Native-PAGE of the purified fraction revealed the presence of two isoforms. PPO has an observed optimum pH at 4·0, followed by a shoulder at pH 5·0. Caffeic acid is the best substrate (100%), followed by chlorogenic acid (60%) and pyrocatechol (32·5%). No activity was detected towards catechin, protocatechuic acid, resorcinol and monophenols. © 1997 SCI.     

美拉德反应产物抑制冷冻甘薯片的褐变

氨基酸与葡萄糖形成的美拉德反应产物（MRPs）对甘薯中的多酚氧化酶（PPO）以及冷冻甘薯片的褐变均具有一定的抑制作用.氨基酸与葡萄糖在不同的反应条件下（氨基酸的种类、pH值、反应时间以及氨基酸与葡萄糖之比）所形成的美拉德反应产物对甘薯中多酚氧化酶的抑制作用不同.其最佳的美拉德反应条件是反应pH值为3.0,L-半胱氨酸,氨基酸与葡萄糖的质量比为1：2,反应时间为5 h.     

乳酸乳球菌L9产类细菌素lactococcin GJ-9发酵条件的研究

对产类细菌素乳酸乳球菌L9的发酵条件进行了研究。结果表明 :产类细菌素最适培养基为MRS,培养基的最适初始pH值为 6 5 ;产类细菌素最适温度为 32℃ ;0 2 %Tween 80最适类细菌素的产生。并通过正交试验确定L9产类细菌素的最佳培养基为 :大豆蛋白胨 1 %、酵母膏1 5%、葡萄糖 1 2 5 %、K2 HPO40 2 %、NaAc 0 5 %、MgSO4·7H2 O 0 0 58%、柠檬酸三铵 0 2 %、MnSO4·4H2 O 0 0 0 5 %、Tween 80 0 2 % ;初始 pH6 0。经过优化 ,发酵液效价提高了 61 0 7%。