Changes in pectin methylesterification and accumulation of methanol during production of diced tomatoes

The degree of methylesterification (DM) of the pectins in tomatoes affects the firmness of diced products and the consistency of juices. We examined the changes in DM that occur during commercial production of diced tomatoes packed in tomato juice. Ripe processing tomatoes contained low amounts of free methanol (<20 μg g fresh weight⁻¹) and had a high degree of pectin methylesterification (60%). During production of diced tomatoes, the level of free methanol increased while the degree of pectin methylesterification decreased. Diced tomatoes canned in tomato juice contained about 200 μg methanol g fresh weight⁻¹, and had a DM of about 35% in the dice and less than 25% in the juice. Similar results were obtained for aseptically processed bulk packed tomatoes. Low-temperature blanching of canned diced tomatoes caused additional pectin de-esterification in the diced tomatoes and improved firmness. Heating of the diced tomatoes prior to mixing with topping juice, first to temperatures that maximally activate PME then to temperatures that inactivate PME and other enzymes, is proposed as a way to both improve dice firmness and preserve the consistency of the topping juice.     

Alginate submicron beads prepared through w/o emulsification and gelation with CaCl2 nanoparticles

A simple method for preparing gelled alginate beads with a diameter smaller than 5 μm is described. A 1% alginate solution and a medium chain triglyceride (MCT) oil are used to prepare a water-in-oil (w/o) emulsion, stabilized by polyglycerol polyricinoleate. CaCl₂ nanoparticles with dimensions in the nano-range (6–400 nm), dispersed in MCT oil, are then added to the emulsion. Energy-dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES) show that these nanoparticles migrate to the emulsion droplet interface, where they dissolve into the aqueous alginate phase and cause gelation, forming beads. Gelation of the beads was confirmed with a novel technique using Congo red as an indicator. A color change occurs upon the addition of CaCl₂ to a Congo red solution and we believe this is due to formation of a Congo red–calcium complex. Scanning electron microscopy shows that alginate beads are mostly in a size range around 1 μm, but beads as small as 200 nm and smaller were also found. Extending the size range of alginate beads into the submicron range, while maintaining relatively mild pH conditions in the interior of the bead, will significantly extend the range of applications for this type of beads.     

Firming of fruit tissues by vacuum-infusion of pectin methylesterase: Visualisation of enzyme action

Apple pieces were vacuum-impregnated with either a pectin methylesterase (PME) and calcium solution or with water prior to pasteurization. Pasteurized apple pieces impregnated with PME and calcium showed a significantly higher firmness. Moreover, solid state ¹³C NMR spectroscopy of apple cell wall residues revealed an increase of their molecular rigidity. Exogenous PME addition involved a decrease from 82% to 45% of apple pectin degree of methyl-esterification. Microscopic observations of apple slices immunolabelled with antibodies specific for pectins showed that (i) demethyl-esterification was more intense in the cell wall region lining intercellular spaces (demonstrating a key role for these intercellular channels in the enzyme penetration in the tissue during vacuum-infusion) and that (ii) the number of calcium-dimerized deesterified homogalacturonan chains increased. The results corroborate the hypothesis that vacuum-impregnated PME action liberates free carboxyl groups along pectin chains that could interact with calcium, increasing the rigidity of pectins and finally the mechanical rigidity of apple tissue.     

Effects of high pressure CO2 treatments on microflora, enzymes and some quality attributes of apple juice

The inactivation of microorganisms and enzymes of cloudy apple juice, and some physico-chemical properties of the juice were investigated after continuous high pressure carbon dioxide (HPCD) at 25 MPa and 43 °C, for 2 min and at 22 MPa and 60 °C for 3, 5 and 10 min, respectively. The coliform bacteria were completely inactivated in all the cases. Total aerobic bacteria was reduced by 3.72 log cycles, pectin methylesterase was reduced by 54.3% and polyphenol oxidase was completely inactivated after 10 min treatment at 22 MPa and 60 °C. The yeasts and molds were completely inactivated and the turbidity increased at 22 MPa and 60 °C regardless of time, while the L and a value reduced, but browning degree did not change. The particle size distributions of the juice changed after HPCD but were regained as treatment time was prolonged. The pH was reduced at 22 MPa and 60 °C for 3 or 5 min.     

Quality attributes and cell wall properties of strawberries (Fragaria annanassa Duch.) under calcium chloride treatment

Effects of CaCl₂ (0%, 1% and 4%) treatment on quality attributes and cell wall pectins of strawberry fruits stored at 4 °C for 15 d were investigated. Strawberry firmness was not significantly affected by CaCl₂ treatment. Compared to the other groups, the 1% CaCl₂ group had better quality attributes, including decay rate, weight loss and soluble solids content. The treatment with 4% CaCl₂ inhibited weight loss but caused phytotoxicity. During storage, the chain widths and lengths of water-soluble pectin (WSP), chelate-soluble pectin (CSP) and sodium carbonate-soluble pectin (SSP) decreased. Strawberry softening seemed to be due to modifications of CSP and SSP, especially the side chains. CaCl₂ treatment significantly slowed the breakdown of CSP and SSP chains by strengthening the ionic crosslinkages among these pectin molecules. These results illustrate the fundamental CaCl₂ effects and will help improve the application of CaCl₂ to postharvest fruits.     

Efficacy of non-thermal technologies and sanitizer solutions on microbial load reduction and quality retention of strawberries

The effect of non-thermal technologies (ozone in aqueous solution, ultrasound and ultraviolet C radiation) and washings with chemical solutions (sodium hypochlorite and hydrogen peroxide) on safety and quality features of strawberries was studied. These treatments were applied before fruit storage at two different temperatures (4 and 15 °C). The overall impact on microbial loads (total mesophiles, and yeasts and moulds) and selected quality attributes (colour, firmness, pH, total anthocyanins and ascorbic acid content) was assessed.During storage under refrigerated temperature, washing with hydrogen peroxide solutions resulted in strawberries with lower microbial loads, when compared to the other treatments. However, it produced significant key quality attributes losses, such as colour and total anthocyanins content.The results presented show that ozone and ultrasound are promising alternatives to thermal treatments. The application of such technologies, before refrigerated storage of strawberries, allowed a satisfactory retention of all quality characteristics analysed, while being efficient in controlling microbial contamination.     

Minimizing texture loss of frozen strawberries: effect of infusion with pectinmethylesterase and calcium combined with different freezing conditions and effect of subsequent storage/thawing conditions

Vacuum infusion (VI), freezing, frozen storage and thawing conditions were optimized in order to minimize the texture loss of frozen strawberries. Slow freezing caused severe loss in textural quality of the strawberries. This quality loss could not be prevented by the application of VI prior to slow freezing, or by the application of rapid, cryogenic or high-pressure shift freezing conditions on non-infused fruits. A remarkable texture improvement was noticed when infusion of pectinmethylesterase (PME) and calcium was combined with rapid or cryogenic freezing. The highly beneficial effect of PME/Ca-infusion followed by HPSF on the hardness retention of frozen strawberries was ascribed to the combined effect of the infused PME (53% reduction in degree of esterification (DE) of the strawberry pectin) and the high degree of supercooling during HPSF. During frozen storage, textural quality of PME/Ca-infused high-pressure frozen strawberries was maintained at temperatures below −8 °C, whereas the texture of PME/Ca-infused strawberries frozen under cryogenic freezing conditions was only preserved at temperatures below −18 °C. Thawing at room temperature seemed to be an appropriate method to thaw strawberries. Fast thawing by high-pressure induced thawing (HPIT) did not prevent textural quality loss of frozenstrawberries.     

Identification of phenolic compounds in strawberries by liquid chromatography electrospray ionization mass spectroscopy

Strawberry (Fragaria x ananassa Duch.) fruits contain phenolic compounds that have antioxidant, anticancer, antiatherosclerotic and anti-neurodegenerative properties. Identification of food phenolics is necessary since their nature, size, solubility, degree and position of glycosylation and conjugation influence their absorption, distribution, metabolism and excretion in humans. Freeze-dried whole strawberry fruit powder and strawberry fruit extracts were analyzed by liquid chromatography electrospray ionization mass spectrometry (LC–ESI–MS) methods. Phenolics were identified as ellagic acid (EA), EA-glycosides, ellagitannins, gallotannins, anthocyanins, flavonols, flavanols and coumaroyl glycosides. The anthocyanidins were pelargonidin and cyanidin, found predominantly as their glucosides and rutinosides. The major flavonol aglycons were quercetin and kaempferol found as their glucuronides and glucosides. LC–ESI–MS/MS methods differentiated EA from quercetin conjugates since both aglycons have identical molecular weights (302 g/mol). The identification of strawberry phenolics is necessary to generate standardized materials for in vitro and in vivo studies and for the authentication of strawberry-based food products.     

冰点调节剂对西兰花冰点的控制研究

本文研究不同冰点调节剂对西兰花冰点的影响。实验选取葡萄糖、维生素C、氯化钙和山梨醇四种冰点调节剂,利用单因素比较实验法,系统研究了冰点调节剂种类及其使用条件对西兰花冰点的影响。结果表明,氯化钙是较好的冰点调节剂;3％氯化钙浸泡30rain对西兰花的冰点调节效果较好,使西兰花冰点从-0．8℃降到．1℃。     

氯化钙对机械伤番茄果实生理特性的影响

为研究氯化钙处理对具有机械伤的番茄果实生理特性的影响,本实验用1 mmol/L氯化钙(CaCl₂)处理机械伤的番茄果实,将处理后的番茄于20℃贮藏,测定贮藏期间番茄生理品质和抗氧化酶活性的变化。结果表明:经过CaCl₂处理含有机械损伤的番茄果实,可以维持番茄在贮藏期间的感官品质,降低腐烂率;与对照组相比,CaCl₂处理可维持番茄果实的硬度和色度,延缓贮藏期间维生素C(V_C)含量的下降;同时,抑制丙二醛(MDA)的积累,提高过氧化物酶(POD)、抗坏血酸过氧化物酶(APX)、多酚氧化酶(PPO)的活性。因此,1 mmol/L CaCl₂处理具有机械损伤的番茄可以有效维持其在贮藏期间的品质,延长货架期。     

Influence of Ultrasound on Freezing Rate of Immersion-frozen Apples

The use of power ultrasound within the food industry is an innovative subject. Application of sound to monitor a process or product is common, e.g. in quality assurance. However, the use of ultrasound to directly improve processes and products is less popular in food manufacturing. In the present work, ultrasound-assisted immersion freezing was investigated on apple samples. Because the apple parenchyma is mechanically anisotropic, the influence of applying ultrasound on radial or tangential orientated samples was also examined. Apple cylinders were immersed in an ultrasonic bath system, which operates at 40 kHz frequency. Experiments were carried out at a power level of 131.3 W (0.23 W/cm²), and ultrasound was applied intermittently for different times from temperatures below and close to the initial freezing point. Results showed that ultrasound application at 0°C or −1°C for 120 s in total, with 30 s intervals, significantly improved the freezing rate represented by the characteristic freezing time up to 8% (P < 0.05), compared to immersion freezing without ultrasound. Results of the effect of ultrasound waves applied on radial or tangential cut samples sonicated for 120 s from −1°C and/or 0°C indicated that at the power level considered there were no significant differences among the ultrasonic radial or tangential irradiated samples of these treatments, though the freezing rates were enhanced and different (P < 0.05) from the control treatment. Some evidence of the influence of ultrasound to induce primary nucleation was also observed.     

Characteristics of enzymatically-deesterified pectin gels produced in the presence of monovalent ionic salts

Pectin methylesterases (PMEs) from Valencia orange (p-PME) and Aspergillus aculeatus (f-PME) were used to produce pectin gels in the presence of 0.2 M monovalent ionic salts. At pH 5.0, pectin gels were induced following enzymatic deesterification of high methoxy pectin, with greater deesterification observed using p-PME compared to f-PME. Under these conditions, the deesterification limit of f-PME ended up with a pectin of DE 30.5–31.9 which did not gel at the PME reaction completion, while p-PME reduced the pectin's DE to 16.0–17.2, resulting in gel formation. The pectin gel induced by KCl was significantly stronger than the NaCl-induced gel, but LiCl did not induce pectin gelation. The gel strength was influenced by both DE and species of monovalent cation. The KCl-induced gels released less water than NaCl-induced gels. A synergistic effect on gel strength was observed from the pectin treated with a combination of (p + f)-PMEs, producing even more stable gels. These results indicated that the pectin gelation of our system would be enhanced both by using larger monovalent cation and by lowering the DE value, which would presumably be attributed to the different action patterns recognized for p- and f-PMEs. This pectin gelation system could provide a useful alternative to acid-sugar or calcium cross-linked gels in food and other industrial applications.     

Use of pectinmethylesterase and calcium in osmotic dehydration and osmodehydrofreezing of strawberries

Strawberry samples of two varieties (Camarosa and Elsanta) were dehydrated using different osmotic solutions (60% glucose, fructose, sucrose and raftilose) and subsequently frozen by rapid and high-pressure shift freezing (HPSF). The effect of pectinmethylesterase (PME) and calcium (Ca⁺⁺) added to the osmotic solutions on several compositional parameters and the textural/structural quality of dehydrated and osmodehydrofrozen-then-thawed samples was studied. Due to the presence of PME and Ca⁺⁺ in the osmotic solutions, weight reduction upon dehydration was slightly decreased, which was correlated to a small positive effect on the net uptake of sugars and depression of the initial freezing point. Except for the Camarosa samples treated with sucrose, PME and Ca⁺⁺ in osmotic sugar solutions positively affected the relative hardness of dehydrated fruits, which was ascribed to the effect of PME and Ca⁺⁺ on the cell wall strength of the tissue. No cell wall damage and tissue particle alterations were observed upon dehydration. The effect of osmotic dehydration (OD) using different sugar solutions without PME and Ca⁺⁺ on the texture and structure of frozen-then-thawed samples was limited and sometimes negative. The added PME and Ca⁺⁺ however positively influenced the volume and shape of the thawed samples, which could be related to slightly higher relative hardness values and, for the Elsanta strawberry fruits, also to the reduced (up to 81%) drip loss upon thawing. Upon freezing the dehydrated fruits, no cell wall disruption was observed. Tissue distortion caused by freezing and indicated by a decrease in particle size, convexity and roundness, was compensated by the use of PME and Ca⁺⁺ during OD.     

Effect of temperature, pressure and calcium soaking pre-treatments and pressure shift freezing on the texture and texture evolution of frozen green bell peppers (Capsicum annuum)

The firmness of green bell pepper (Capsicum annuum) was studied under different processing conditions. Thermal texture degradation kinetics of pepper tissue between 75 and 95 °C could be accurately described by a fractional conversion model. The firmness of pre-processed pepper increased when the samples were submitted to several heat, pressure, and combinations of heat/pressure and calcium soaking pre-treatments. Pre-heating at 55 °C during 60 min and mild heat/high-pressure treatments (200 MPa at 25 °C, 15 min) yielded the best results, which were further improved when combined with calcium soaking. These pre-treatments significantly slowed down thermal texture degradation of pepper at 90 °C, a typical temperature used for pepper blanching prior to freezing. The above-mentioned pre-treated samples showed a significant reduction in firmness when frozen by regular freezing at 0.1 MPa. The same samples showed no changes in firmness when frozen by high-pressure shift freezing at 200 MPa. When freezing was carried out by high-pressure shift and after frozen storage (−18 °C) for 2.5 months, pressure pre-treated pepper showed a better retention of texture than thermal pre-treated pepper.     

The kinetics of inactivation of pectin methylesterase and polygalacturonase in tomato juice by thermosonication

The ultrasonic inactivation kinetics of polygalacturonase (PG) and pectin methylesterase (PME) in tomato juice were studied at a frequency of 20 kHz, amplitude of 65 μm and temperatures between 50 and 75 °C. Thermal treatments at the same temperatures were conducted to separate the effects of heat and ultrasound. The thermal inactivation of PG was described by a fractional conversion model with PG 1 remaining stable, whereas the inactivation of PG by combined ultrasonic and heat treatment (thermosonication) was best described by first order biphasic kinetics, with both PG1 and PG2 inactivated at different rates. The thermal and thermosonication inactivation of PME was described well by first order kinetics. The inactivating effect of combined ultrasound and heat was synergistic. Thermosonication enhanced the inactivation rates of both PME and PG. The inactivation rate of PME was increased by 1.5–6 times and the inactivation rate of PG2 by 2.3–4 times in the temperature range 60–75 °C, with the highest increase corresponding to the lowest temperature.     

CaCl2处理对采后甜樱桃果实生理代谢和品质的影响

以"砂蜜豆"品种甜樱桃为研究材料,经不同浓度的CaCl2(0,10,20g/L)浸泡15min后,在(0±1)℃下进行包装贮藏,每9d测试果实颜色、硬度、可溶性固形物及相关酶活性。结果表明,不同浓度CaCl2处理均可延缓果皮的着色进程,防止果实软化,保持原有风味,抑制果实的PPO、POD和LOX的活性,减少MDA的产生。适当浓度的CaCl2处理能延长甜樱桃的货架期,保持较好的口感与风味,有利于甜樱桃的贮藏保鲜。     

Development of an immunological toolbox to detect endogenous and exogenous pectin methylesterase in plant-based food products

An immunological toolbox consisting of five monoclonal antibodies (MA) was developed for the detection of endogenous as well as exogenous pectin methylesterase (PME) in plant-based food products. Based on cross-reactivity with plant PME, the MA were subdivided into two groups. MA of group one recognise exclusively PME from Aspergillus aculeatus (fPME), which could be visualised upon infusion. Three different infusion techniques were compared for three different types of plant tissue, revealing a homogenous distribution of exogenous PME upon pressure-and vacuum-assisted infusion. The MA of group two recognise fPME as well as PME from different plant sources like tomato, carrot, strawberry, broccoli and apple. The use of these antibodies allowed detection of endogenous PME in tomato which revealed an overall presence of PME in the pericarp of tomato fruit. Moreover, the PME detected by the MA of group two in the plant cell wall of tomato coincided with the localisation of two isoenzymes of tomato PME, as detected by isoenzyme-specific MA. The obtained results highlight the versatility of the developed antibodies as probes to detect PME in the context of food processing.     

Comparison of the inactivation kinetics of pectin methylesterases from carrot and peach by high-pressure carbon dioxide

The inactivation of pectin methylesterases (PMEs) from carrot and peach in buffer by high-pressure carbon dioxide (HPCD) at 55 °C was investigated. The two PMEs were effectively inactivated by HPCD, their residual activity (RA) decreasing with increasing pressures. The RA of the two PMEs exhibited a fast decrease firstly and reached a constant after a prolonged treatment time; their inactivation kinetics was adequately modelled by a fractional-conversion model. The non-zero RA(A_∞)of the two PMEs was 6–7%, with increasing pressures the kinetic rate constant, k, increased and the decimal reduction time, D, decreased for the HPCD-labile fraction of the two PMEs. The labile fraction of carrot PME was more susceptible to HPCD than that of peach PME; the activation volume, V_a, and Z_P (the temperature increase needed for a 90% reduction of D) was −1079.37 cm³/mol and 5.80 MPa for carrot PME, and −130.51 cm³/mol and 48.31 MPa for peach PME.     

免疫亲和层析净化-高效液相色谱法同时测定牛奶和奶粉中黄曲霉毒素B1、B2、G1、G2、M1、M2

建立牛奶和奶粉中黄曲霉毒素B1、B2、G1、G2、M1、M2的免疫亲和层析净化和柱后衍生高效液相色谱测定方法。样品经溶解、离心、过滤后,通过免疫亲和柱,黄曲霉毒素特异性抗体选择性地与存在的黄曲霉毒素抗原键合,形成抗体-抗原复合体。甲醇-乙腈混合溶液(4:5,v:v)洗脱,带荧光检测器的高效液相色谱仪经柱后衍生测定,外标法定量。标准曲线线性良好,添加回收率在57.0%～88.7%,相对标准偏差在3.37%～16.9%,牛奶中各黄曲霉毒素检出限:B1为2ng/kg,B2为1ng/kg,G1、G2为3ng/kg,M1、M2为5ng/kg;奶粉中B1为20ng/kg,B2为10ng/kg,G1、G2为30ng/kg,M1、M2为50ng/kg,检测低限能够满足各国对牛奶和奶粉中黄曲霉毒素的限量要求。该方法准确、快速、灵敏度高,适用于牛奶和奶粉中黄曲霉毒素的测定。     

纳米Ag@TiO2、Fe@TiO2和Zn@TiO2的抑菌活性比较研究

探索常见金属离子掺杂对抑菌片的抑菌性能的影响,为新型纳米TiO₂抑菌片的制备与改善提供技术基础。选择固定质量分数的Ag⁺、Zn²⁺和Fe³⁺掺杂TiO₂,通过抑菌圈、MIC与MBC方法综合评价其抑菌性能,对微观结构进行表征分析,探讨抑菌性能差异的原因。结果表明:抑菌性能大小顺序为Ag@TiO₂> Zn@TiO₂> Fe@TiO₂,Zn@TiO2 的抑菌活性较好但抑菌稳定性差Ag@TiO2对大肠杆菌和金黄色葡萄球菌的MIC、MBC均是10、20 mg/L,Zn@TiO₂对大肠杆菌和金黄色葡萄球菌的MIC、MBC均是30、50 mg/L。表征发现抑菌活性强弱主要归因于纳米TiO₂表面与金属离子共同产生的活性位点数量及稳定性,与孔结构或表面宫能团的关联不大。