Improvement of frozen banana (Musa cavendishii, cv. Enana) colour by blanching: relationship between browning, phenols and polyphenol oxidase and peroxidase activities

 Browning in banana (Musa cavendishii, cv. Enana) processed products is a result of phenol oxidation catalysed by polyphenol oxidase (PPO) and peroxidase (POD) or of other non-enzymatic reactions (Maillard and Strecker mechanisms). Microwave and steam blanching significantly reduced PPO and POD activities and phenol levels in banana flesh, steam blanching being the most effective method for enzyme inactivation. Freezing/thawing processes produced a significant increase in phenol levels in all samples, due to cellular breakdown. After microwave heating browning processes occurred while steam-treated samples did not exhibit a significant colour change. Extractable PPO and POD activities in all banana samples increased as a consequence of freezing/thawing: steam-blanched slices exhibited lower residual activities. High correlations occurred between phenols and browning (r=0.86) in control samples. Blanched samples (microwave or steam) only exhibited correlations between PPO (r=0.80) and POD (r=0.80) activities and browning. Received: 22 February 1996     

Improvement of frozen banana (Musa cavendishii, cv. Enana) colour by blanching: relationship between browning, phenols and polyphenol oxidase and peroxidase activities

 Browning in banana (Musa cavendishii, cv. Enana) processed products is a result of phenol oxidation catalysed by polyphenol oxidase (PPO) and peroxidase (POD) or of other non-enzymatic reactions (Maillard and Strecker mechanisms). Microwave and steam blanching significantly reduced PPO and POD activities and phenol levels in banana flesh, steam blanching being the most effective method for enzyme inactivation. Freezing/thawing processes produced a significant increase in phenol levels in all samples, due to cellular breakdown. After microwave heating browning processes occurred while steam-treated samples did not exhibit a significant colour change. Extractable PPO and POD activities in all banana samples increased as a consequence of freezing/thawing: steam-blanched slices exhibited lower residual activities. High correlations occurred between phenols and browning (r=0.86) in control samples. Blanched samples (microwave or steam) only exhibited correlations between PPO (r=0.80) and POD (r=0.80) activities and browning. Received: 22 February 1996     

Effect of nitric oxide on pericarp browning of harvested longan fruit in relation to phenolic metabolism

The effects of nitric oxide (NO) on enzymatic browning of harvested longan fruit in relation to phenolic metabolisms were investigated. Fruits were dipped for 5 min in 1 mM sodium nitroprusside (SNP), a nitric oxide donor, then packed in 0.03 mm thick polyethylene bags, and finally stored for 6 days at 28 °C. Changes in pericarp browning and pulp breakdown were evaluated, while total phenol content, activities of phenolic-associated enzymes, polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonia lyase (PAL), and concentrations of total soluble solids, titratable acidity and ascorbic acid were measured. SNP treatment delayed pericarp browning, inhibited activities of PPO, POD and PAL and maintained a high total phenol content of longan fruit during storage. Furthermore, NO showed a significant inhibition of the in vitro activities of PPO and POD, indicating that the beneficial effect of NO was direct. Moreover, application of NO resulted in a lower pulp breakdown and maintained relatively high levels of total soluble solids and ascorbic acid.     

Fruit quality and physiological responses of litchi cultivar McLean's Red to 1-methylcyclopropene pre-treatment and controlled atmosphere storage conditions

The effect of 1-MCP pre-treatment and two different controlled atmosphere storage conditions (CA-1, 17% O₂ + 6% CO₂; CA-2, 7% O₂ + 3% CO₂) on fruit quality parameters and physiological changes with respect to pericarp browning in ‘McLean's Red’ litchi were investigated. Fruits were pre-treated with 1-MCP (500 nl/l) and held at CA-1 or CA-2 for 21 d at 2 °C and at 90% RH. Stand-alone CA-1 or stand-alone CA-2 and the commercially adopted sulphur dioxide (SO₂) treatment were included in this study for comparison. Of the five treatments 1-MCP + CA-1 was most effective in preventing browning, loss of red colour (colour value a*) of the pericarp, ascorbic acid content; and retaining acceptable SSC/TA and taste. Fruit from 1-MCP + CA-1 showed higher overall acceptance after 21 d storage without any off-flavour according to the sensory panel data.1-MCP + CA-1 reduced the polyphenol oxidase (PPO) and peroxidase (POD) activity, retained membrane integrity and anthocyanin content during storage. Although SO₂ treatment prevents browning it showed negative effects on SSC/TA, taste and membrane integrity. Stand-alone CA-2 condition indicated higher pericarp browning, PPO, POD activity and loss of membrane integrity. Therefore, 1-MCP pre-treatment and CA-1 retains overall fruit quality for up to 21 d.     

The role of polyphenol oxidase and peroxidase in the browning of water caltrop pericarp during heat treatment

The mechanism of browning involving enzymatic browning was investigated in the pericarp of water caltrop, an Asian vegetable popular for its taste and medicinal properties. Polyphenol oxidase (PPO) and peroxidase (POD) activities were determined in pericarp at various times and temperatures. Water caltrop consisted of 44.22% moisture content, 37.23% crude fibre, and 2.63% crude protein. PPO and POD activities dropped from 62 and 38 units/g sample, respectively, as water temperature was increased from 30 to 80 °C. Optimum pH and temperature for PPO activity was at pH 5.0, 25–45 °C, and POD activity peaked at 60 °C. High PPO and POD activities at 40–50 °C resulted in degradation of phenolic compounds, which led to increased aggregation of browning pigments and discolouration (lower L-values) of the pericarp. Enzymatic browning was determined as the major factor in the browning discolouration of heat-treated water caltrop pericarp.     

热烫方式对桃微观结构、酶促褐变及相关品质的影响

本实验采用热水与蒸汽两种热烫方式对桃进行预处理,研究其对微观结构、酶促褐变及相关品质的影响,确定桃脯前处理的最佳热烫条件。结果表明:随热烫时间（0~80 s）的增加,两种处理方式对桃品质的影响基本一致,表面微观结构塌陷、多酚氧化酶（PPO）和过氧化物酶（POD）钝化失活、质构下降、褐变度减弱、V_C含量降低、总酚含量及抗氧化能力上升。相关性分析和聚类分析结果表明:PPO、POD、硬度、褐变度、V_C间呈极显著相关（P<0.01）;总酚、DPPH·清除能力、亚铁离子还原能力间呈极显著相关（P<0.01）。热水与蒸汽热烫前40 s内各理化品质变化接近,具有较高的相关性,而热水与蒸汽热烫60 s和80 s的品质变化相关程度较高。总之,相比于热水处理,蒸汽处理能较好地维持桃的品质。当蒸汽处理80 s时,在有效钝化PPO和POD的基础上,仍能维持相对饱满的组织结构和较好的色泽品质,且V_C含量也保持在较高水平,是适宜桃脯前处理的最佳热烫条件。     

Effects of hot-water and steam blanching of sliced potato on polyphenol oxidase activity

The worldwide potato production is considered the fourth-most important food sector due to the increasing use of potatoes as raw materials for high-convenience food. Enzymatic browning, due to polyphenol oxidase (PPO), is related to unacceptability by consumer. Among antibrowning agents, thermal treatments are viable alternatives. In this study, the efficacy of hot-water and steam blanching at 80–90 °C of potato slices (1-cm thick) was evaluated in terms of colour changes as well as PPO inactivation kinetics, substrate specificity and transition state parameters. In general, all treatments [1] bleached the slices, [2] inactivated PPO and [3] reduced its kinetic efficiency. Results from thermal inactivation kinetics promoted hot-water blanching at 90 °C for approx. 2 min as the fastest treatment to obtain enzymatic-stable potato slices. Moreover, steam blanching required more energy (53.93 ± 1.24 kJ mol⁻¹) than hot-water treatment (41.41 ± 4.51 kJ mol⁻¹) to reach the transition state and then to unfold the PPO enzyme.     

Kinetic characterisation and thermal inactivation study of polyphenol oxidase and peroxidase from table grape (Crimson Seedless)

Polyphenol oxidase (PPO) and peroxidase (POD) were extracted from a table grape (Crimson Seedless) using Triton X-114 and characterized using spectrophotometric methods. Both PPO and POD were activated by acid shock. However, in the presence of the anionic detergent sodium dodecil sulphate (SDS), PPO was activated whereas POD was inactivated. The enzymes were kinetically characterized and both followed Michaelis–Menten kinetics, although with different values of their kinetic parameters. The V_m/K_m ratio showed that Crimson Seedless grape PPO presents a similar affinity for 4-tert-butyl-catechol (TBC) whether activated by acid shock (0.018 min⁻¹) or SDS (0.023 min⁻¹). With regards to POD, the K_m and V_m values for 2,2′-azinobis(3-ethylbenzothiazolinesulphonic acid) (ABTS) were 0.79 mM and 1.20 μM/min, respectively. In the case of H₂O₂, the K_m and V_m value were 0.4 mM and 0.93 μM/min, respectively. PPO and POD showed similar thermostability, losing >90% of relative activity after only 5 min of incubation at 78 °C and 75 °C, respectively. In addition, PPO´s activation energy was similar to that obtained for POD (295.5 kJ/mol and 271.9 kJ/mol, respectively).     

Effects of aqueous chlorine dioxide treatment on browning of fresh-cut lotus root

Effect of aqueous chlorine dioxide (ClO₂) treatment on browning of fresh-cut lotus root (FLR) was investigated to explore the feasibility to apply ClO₂ for browning inhibition of fresh-cut products. Cut lotus roots were treated in ClO₂ solutions at different concentrations (10, 50 and 100 mg/l) for different time (5, 10 and 15 min), followed by chilled storage for 8-10 days at 4 °C. Color parameters (L^∗, a^∗ and b^∗), polyphenol oxidase (PPO) activity and overall visual quality (OVQ) were measured at one-day interval during storage. Results showed that higher ClO₂ concentration and longer treatment time can provide better inhibitory effects on the browning of FLR. ClO₂ concentration, treatment time and storage time were three significant factors (P < 0.05) and some significant interactions were observed. PPO activities were largely inhibited by 100 mg/l ClO₂ treatment for 10 min. The 100 mg/l ClO₂ treatment maintained high OVQ scores during 10-day storage; while 50 mg/l ClO₂ treatment was acceptable for maintaining OVQ during 4-day storage. ClO₂ treatment was demonstrated to be a promising alternative approach to control browning and improve OVQ of FLR.     

Microwave inactivation of red beet (Beta vulgaris L. var. conditiva) peroxidase and polyphenoloxidase and the effect of radiation on vegetable tissue quality

The inactivation of polyphenoloxidase (PPO) and peroxidase (POX) in red beet by traditional and microwave (MW) blanching was studied. Microwave heating effects on color and texture were also studied.Red beet subjected to MW blanching for 5 min at 100-200 W resulted in large weight losses accompanied by a high degree of shrinking. POX was the more heat resistant enzyme. The 90% destruction (D value) of the activity of both enzymes could be achieved only at 200 W within the 5 min frame employed for the tests.When the samples were immersed in water and both the food sample and the water were submitted to MW exposure at 250-450 W or variable power with a maximum at 935 W, shrinking was avoided. The D value at 90 °C (reference; D_Tref) and z could be determined after time-temperature corrections, and it was observed that, in general, D_Tref values for POX were smaller than for PPO. The microwave treatment (maximum power: 935 W) designed to provide a similar temperature profile to the one observed for traditional blanching (immersion in water at 90 °C), showed the smallest D_Tref value for POX inactivation. All treatments reduced elastic characteristics and changed the color of the tissues showing a shift to blue mainly in the case of microwave processes.     

Polyphenol oxidase activity,phenolic acid composition and browning in cashew apple (Anacardium occidentale, L.) after processing

This study describes the extraction and characterisation of cashew apple polyphenol oxidase (PPO) and the effect of wounding on cashew apple phenolic acid composition, PPO activity and fruit browning. Purification factor was 59 at 95% (NH₄)₂SO₄ saturation. For PPO activity, the optimal substrate was catechol and the optimum pH was 6.5. PPO K_m and V_max values were 18.8 mM and 13.6 U min⁻¹ ml⁻¹, respectively. Ascorbic acid, citric acid, sodium sulphite and sodium metabisulphite decreased PPO activity, while sodium chloride increased PPO activity. Wounding at 2 °C and 27 °C for 24 h increased PPO activity but storage at 40 °C reduced PPO activity. Gallic acid, protocatechuic acid and cinnamic acid (free and conjugate) were identified in cashew apple juice. Cutting and subsequent storage at 40 °C hydrolysed cinnamic acid. 5-Hydroxymethylfurfural content in cashew apple juice increased after injury and storage at higher temperatures, indicating non-enzymatic browning.     

Light exposure inhibiting tissue browning and improving antioxidant capacity of fresh-cut celery (Apium graveolens var. dulce)

Fresh-cut celery is perishable and susceptible to tissue browning during storage. In this study, the effect of continuous light exposure (2000 lux) on browning related enzyme activity of fresh-cut celery was investigated during 8 d storage at 7 °C using darkness (0.2 lux) as control. Light exposure significantly suppressed polyphenol oxidase (PPO) and peroxidase (POD) activities, and subsequently decreased soluble quinone accumulation and browning index (BI) evolution during storage. In addition, phenylalanine ammonia lyase (PAL) activity, total phenol (TP) content, and antioxidant capacity (AC) values were all higher when the fresh-cut celery samples were exposed to light than in darkness during storage. A significant positive correlation between TP and AC was observed at both light (R = 0.884, P < 0.01) and dark (R = 0.705, P < 0.01) conditions.     

Effects of various pretreatments and drying methods on Salmonella resistance and physical properties of cabbage

The combined effects of pretreatment and drying methods on the resistance of Salmonella attached to vegetable surfaces as well as some physical properties, in terms of color and shrinkage, were investigated. Cabbage was used as a test vegetable and Salmonella Anatum was used as a test microorganism. Cabbage leaves were pretreated either by soaking in 0.5% (v/v) acetic acid for 5 min, blanching in hot water for 4 min or blanching with saturated steam for 2 min prior to either hot air drying, vacuum drying (10 kPa) or low-pressure superheated steam drying (10 kPa) at 60 °C. Based on an initial Salmonella contamination level of approximately 6.4 log CFU/g, soaking in acetic acid, hot-water and steam blanching resulted in 1.6, 3.8 and 3.6 log CFU/g reduction in the number of Salmonella, respectively. Drying without pretreatment could not completely eliminate Salmonella attached on the cabbage surfaces, while no Salmonella was detected on the pretreated samples at the end of the drying process. Volumetric shrinkage was not affected by the pretreatment and drying methods. Dried blanched samples exhibited greener and darker color than the dried acetic acid pretreated and untreated samples.     

Polyphenoloxidase Activity and Color of Blanched and High Hydrostatic Pressure Treated Banana Puree

The effects of blanching and high hydrostatic pressure (HHP) treatments on natural flora evolution, polyphenoloxidase (PPO) activity and color of banana puree adjusted to pH 3.4 and water activity (a_w) of 0.97 were evaluated during 15 days storage at 25°C. Standard plate as well as yeast and mold counts of HHP treated purees were <10 CFU/g throughout storage. Blanching time was found to affect (P<0.05) puree color. HHP treatments retained the initial color of the banana purees. Longer browning induction times and slower browning rates were observed when a longer blanching time was combined with a 689 MPa pressure treatment. A residual PPO activity < 5% was observed in the puree when a 7 min blanch was followed by HHP treatment at 689 MPa for 10 min.     

Inhibition of browning on the surface of peach slices by short-term exposure to nitric oxide and ascorbic acid

The effect of 0.2% ascorbic acid (AA), 5 μM nitric oxide (NO), and the simultaneous use of 0.2% AA and 5 μM NO solutions on inhibiting surface browning of fresh-cut peach slices stored at 10 °C and RH 95% was investigated. The browning index, relative leakage rate, microstructure, total phenol content, and activity of the phenol metabolism-associated enzymes phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO) and peroxidase (POD) were evaluated. The results indicate that treatment with 0.2% AA, 5 μM NO and simultaneous use of 0.2% AA and 5 μM NO resulted in higher total phenol content, inhibition of PPO and POD activity, reduced membrane permeability and protection of cell microstructure to maintain compartmentation between enzymes and their substrates. In addition, NO increased PAL activity. The causes of inhibition in the browning of peach slices by NO are discussed.     

Maintaining the quality of sugarcane juice with blanching and ascorbic acid

The physicochemical changes in fresh sugarcane juice stored at 10 °C were studied by determining juice yield, color, reducing sugar, titratable acidity, viscosity, pH, polyphenol oxidase (PPO), sucrose neutral invertase (SNI) and total microbial count. Results showed that blanching of stems before squeezing effectively prevented degreening and/or browning, and reduced activities of PPO and SNI in fresh sugarcane juice. Added ascorbic acid delayed the increase of reducing sugar, titratable acidity, viscosity and total microbial count, and also prevented degreening and/or browning with reduced PPO and SNI activities in fresh sugarcane juice during storage. Addition of 0.1% ascorbic acid seemed to be more effective than blanching of sugarcane stems, and was able to maintain the quality of fresh sugarcane juice for up to 5 days at 10 °C. Deterioration of fresh sugarcane juice was demonstrated as a rapid increase of titratable acidity and viscosity with a obvious browning.     

Inactivation kinetics and secondary structural change of PEF-treated POD and PPO

Effects of pulsed electric fields (PEF) on the activity of peroxidase (POD) and polyphenol oxidase (PPO) in buffered solution were studied while the corresponding changes to their secondary structures was demonstrated by far-UV Circular dichroism (CD). The relative residual activity of POD and PPO decreased with the increase in electric field strength and treatment time, and PPO was more susceptible than POD to PEF treatment. The greatest reduction of the activity was achieved for POD at 25 kV/cm for 1740 μs and PPO at 25 kV/cm for 744 μs with reductions of 32.2% and 76.2%, respectively. The inactivation kinetic parameters D-value and Z_E value were calculated. The D-values of PPO were smaller than those POD at higher electric field strength, and Z_E values of POD and PPO were 36.9 and 16.2 kV/cm, respectively. The secondary structures of the two enzymes were changed following treatment by PEF. The intensity of negative peaks in the CD spectra decreased, and the CD spectra of PPO changed more significantly than that of POD; the reduction of the relative α-helix fractions for POD at 25 kV/cm for 124 μs was 22.63% while it was 50.72% for PPO at 25 kV/cm for 52 μs. The inactivation of PEF-treated POD and PPO was in close agreement with their secondary structure changes.     

Biochemical properties and potential endogenous substrates of polyphenoloxidase from chufa (Eleocharis tuberosa) corms

Polyphenoloxidase (PPO) was partially purified from chufa corms through ammonium sulphate precipitation and dialysis. Biochemical properties of chufa PPO were analysed using exogenous substrate catechol. Optimal pH and temperature for PPO activity were 5 and 45 °C. Ethylenediaminetetraacetic acid disodium salt and l-cysteine could not inhibit the PPO activity. However, sodium thiosulphate pentahydrate exhibited the strongest inhibiting effect, followed by ascorbic acid and anhydrous sodium sulphite. Except for K⁺, other metal ions such as Zn²⁺, Cu²⁺, Fe³⁺, Ca²⁺, Fe²⁺ and Na⁺ accelerated the enzymatic reaction between catechol and PPO. Kinetic analysis showed that the apparent K_m and V_max values were around 10.77 mM and 82 units/ml min. In addition, (−)-gallocatechin gallate, (−)-epicatechin gallate and (+)-catechin gallate isolated and identified from chufa corms were supposed to be the potential endogenous PPO substrates due to their ortho-diphenolic or pyrogallolic structures. These polyphenols might be catalysed by PPO, resulting in the browning of chufa corms after fresh-cut processing.     

Colour degradation and quality parameters of sonicated orange juice using response surface methodology

Freshly squeezed orange juice was subjected to sonication at amplitude levels ranging from 40 to 100% at a constant frequency of 20 kHz for different times (2-10 min) and pulse durations of 5 s on and 5 s off. Hunter colour values (L^∗, a^∗ and b^∗), pH, °Brix, titratable acidity, cloud value and browning index were measured. Response surface methodology (RSM) based on two factor five level central composite design was applied to determine the effect of amplitude level and sonication time on juice quality parameters. There was no significant difference (p < 0.05) on pH, °Brix and titratable acidity. The model predictions for critical quality parameters of Hunter colour values (L^∗, a^∗ and b^∗), cloud value and browning index were closely correlated to the experimental results obtained. RSM was demonstrated to be an effective technique to model the effect of sonication on juice quality while minimising the number of experiments required.     

Inactivation of peroxidase, polyphenoloxidase, and lipoxygenase in paprika and chili powder after immediate thermal treatment of the plant material

Residual activities of lipoxygenase (LOX), peroxidase (POD), and polyphenoloxidase (PPO) were determined in paprika and chili powder after immediate thermal treatment of the fresh plant material. Heat treatments comprised water and steam blanching of the integral fruits and pasteurization of the comminuted tissues, respectively. PPO showed the lowest heat stability and was completely inactivated by heating at 80 °C for 10 min. Inactivation of LOX was also largely accomplished by heating at 90 °C for 5 min and at 100 °C for 5 min, whereas up to 3.5% (paprika) and 3.3% (chili) of the initial POD activities were retained even when applying rigorous time–temperature regimes. The results demonstrate that substantial inactivation of deteriorative enzymes was ensured by the recently suggested process, thus facilitating the production of high quality spice powders.