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.     

Effect of Infrared Blanching on Enzyme Activity and Retention of β‐Carotene and Vitamin C in Dried Mango

The objectives of this work were to evaluate infrared (IR) dry blanching in comparison with conventional water blanching prior to hot air drying of mango to inactivate polyphenol oxidase (PPO) and ascorbic acid oxidase (AAO) enzymes, and to study its effect on color change and retention of vitamin C and β‐carotene. Mango cylinders were blanched under similar temperature–time conditions either by IR heating or by immersion in a water bath during 2 min at 90 °C (high‐temperature‐short‐time—HTST) or for 10 min at 65 °C (low‐temperature‐long‐time—LTLT). After blanching mango was hot air dried at 70 °C. PPO was completely inactivated during the blanching treatments, but AAO had a moderate remaining activity after LTLT treatment (～30%) and a low remaining activity after HTST treatment (9% to 15%). A higher retention of vitamin C was observed in mango subjected to IR dry blanching, 88.3 ± 1.0% (HTST) and 69.2 ± 2.9% (LTLT), compared with water blanching, 61.4 ± 5.3% (HTST) and 50.7 ± 9.6% (LTLT). All‐trans‐β‐carotene retention was significantly higher in water blanched dried mango, 93.2 ± 5.2% (LTLT) and 91.4 ± 5.1% (HTST), compared with IR dry blanched, 73.6 ± 3.6% (LTLT) and 76.9 ± 2.9% (HTST). Increased levels of 13‐cis‐β‐carotene isomer were detected only in IR dry blanched mango, and the corresponding dried mango also had a slightly darker color. IR blanching of mango prior to drying can improve the retention of vitamin C, but not the retention of carotenoids, which showed to be more dependent on the temperature than the blanching process. A reduction of drying time was observed in LTLT IR‐blanching mango.     

Polyphenol oxidase inactivation and vitamin C degradation kinetics of Fuji apple quarters by high humidity air impingement blanching

In this study, polyphenol oxidase (PPO) and vitamin C were used as the indicators of enzymes and nutrients to evaluate the apple quality during high humidity air impingement blanching (HHAIB) process. The PPO can be completely inactivated within 7 min at 90–120 °C and can retain relatively more vitamin C in the case of PPO fully inactivation. PPO inactivation followed zero‐order kinetics model at 90 and 100 °C, and followed first‐order fraction model at 110 and 120 °C. Activation energy (E_a) of PPO inactivation was between 11.61 and 13.66 kJ mol^?1 by Arrhenius equation. Vitamin C degradation under all processing temperatures was well described by first‐order model and its E_a value was 26.69 kJ mol^?1. Therefore, the HHAIB process was proved to be an effective pretreatment for Fuji apple quarters to inactivate PPO fast and meanwhile to maintain produce quality.     

Lactobionic acid as a suitable food preservative for yacon juice

Yacon root is known for its antioxidant, prebiotic and insulin modulator benefits. However, like the root, yacon juice exhibits rapid enzymatic browning, requiring treatments such as acidification. Lactobionic acid (LBA) is a promising food ingredient, which contains not only preservative but also nutritional properties. The first goal of this study was to evaluate the influence of combined treatments (blanching, acid pretreatment) and LBA on refrigerated yacon juice (8 °C; 48 h). The analysis of pH, antioxidants, total polyphenols and color, were used to select the optimal combined conditions, which were steam blanching (10 min) + LBA 1%. Secondly, the LBA was compared with citric, malic, tartaric, and ascorbic acids (at 1% levels), where the kinetics of polyphenol oxidases were determined in refrigerated storage of the juice at 8 °C for 48 h. Results showed that LBA, malic and tartaric acids maintained the color of yacon juice and preserved bioactive compounds. LBA and malic acid prevented enzymatic browning with PPO reductions of 80.4% and 79.4%, respectively. This was the first time that LBA has been tested in a refrigerated juice showing significant potential for preservation of color and bioactive compounds in food products.     

Inactivation kinetics of peroxidase and polyphenol oxidase in peach juice treated with gaseous ozone

The effectiveness of gaseous ozone for inactivating peroxidase (POD) and polyphenoloxidase (PPO) in peach juice was investigated. The suitability of first‐order and Weibull models to describe inactivation kinetics was also analysed. Peach juice was exposed to ozone (0.11 and 0.20 mg O₃ min^?1 mL^?1) in a bubble column up to 12 min at 20 ± 1 °C. Enzyme activities were reduced due to treatments. The magnitude of the inactivation increased with ozone level and exposure time. Reductions in activity after 12 min of treatment ranged between 99.5% and 99.8% for POD and between 93.9% and 97.3% for PPO, depending on ozone concentration. Inactivation curves were successfully fitted with the first‐order and Weibull models; although, based on the root‐mean‐square error, the corrected Akaike and the Bayesian Schwarz criterion, the Weibull model showed stronger capability in all cases.     

Infrared and Microwave as a dry blanching tool for Irish potato: Product quality,cell integrity,and artificial neural networks (ANNs) modeling of enzyme inactivation kinetic

This study evaluated the use of Infrared (IR) and microwave (MW) dry blanching technology as an alternative to the conventional hot water (HW) blanching of Irish potato slices. Product quality, cell integrity, and Peroxidase (POD) inactivation kinetics were investigated. Also, the POD inactivation kinetics curve was fitted with both mathematical models and artificial neural networks (ANNs). The result showed that MW blanching technique had the shortest POD inactivation time (5 and 7 mins), when compared with HW blanching (7 and 9 mins) and IR blanching (18 and 21 mins). IR blanched samples had the lowest colour change, slowest moisture loss, and lower microbial activity when compared with HW and MW blanching, although the level of vitamin C retention was similar with MW blanched samples. Furthermore, MW blanching had lower electrolyte leakage and microstructure damage, and better texture, when compared with HW and IR blanching. ANNs (R² = 1.000, RMSE = 1.1597e-14, and SSE = 2.8994e-15) outperformed Weibull distribution (R² = 0.9914, RMSE = 0.0348, and SSE = 0.0073) in fitting the POD inactivation curve. In conclusion, IR and MW technology were efficient in dry blanching of Irish potato, and ANNs allows for smart industrial control and monitoring of dry blanching equipments, since it can combine multiple process variables.     

Influence of ohmic heating/osmotic dehydration treatments on polyphenoloxidase inactivation,physical properties and microbial stability of apples (cv. Granny Smith)

The combined effect of ohmic heating (OH) and osmotic dehydration (OD) with vacuum impregnation (VI), on the polyphenoloxidase (PPO) inactivation, physical properties and microbial stability of apples stored at 5 °C or 10 °C was analyzed. The treatments were performed using a 65% (w/w) sucrose solution and with ohmic heating at 13 V/cm at 30 °C, 40 °C or 50 °C for 90 min. Examination of the dehydrated samples showed that the water loss and the solid gain were greater with the OD/OH and VI/OH treatments at 50 °C. PPO was completely inactivated by the OD/OH and VI/OH treatments at 50 °C. There was a correlation between the PPO activity, the color change and the browning index of the treated and stored samples; the values for these parameters were stable when PPO was inactivated. The lowest loss of firmness and color was obtained with the VI/OH treatment at 50 °C. The shelf-life of the apples treated with VI/OH at 50 °C and stored at 5 °C was extended to more than 4 weeks. Therefore, the VI/OH treatment at 50 °C was determined to be the best process for dehydrating apples.Industrial relevanceThe aim of this research was 1 to study the combined effect of ohmic heating (OH) and osmotic dehydration (OD) with vacuum impregnation (VI) on the polyphenoloxidase inactivation and microbial stability of osmotically dehydrated apples stored at either 5 °C or 10 °C. Two technologies, OH and OD were performed at 30, 40 or 50 °C with an electric field intensity of 13 V/cm and conventional heating for 90 min. The results showed a correlation between the PPO activity, the color change and the browning index of the treated and stored samples; the values were stable when PPO was inactivated. PPO was completely inactivated by the OD/OH and VI/OH treatments at 50 °C. The shelf-life of the apples treated was extended to more than 4 weeks. Under the investigated conditions, VI/OH treatment at 50 °C and stored at 5 °C may be considered the better minimal processing that preserves the fresh-like properties.     

High pressure thermal processing of pears: Effect on endogenous enzyme activity and related quality attributes

The effect of high pressure-thermal (HPT) processing (600 MPa, 20–100 °C) on the activity of pear enzymes and related quality attributes was investigated. HPT processing at 20 °C for 5 min resulted in 32%, 74% and 51% residual activities of polyphenol oxidase (PPO), peroxidase (POD) and pectin methylesterase (PME), respectively. Increasing processing temperature to 40 and 60 °C reduced the level of PPO and POD inactivation, with the maximum residual activities of 64% and 123%, respectively observed after 3-min treatments at 40 and 60 °C. Overall, HPT at 20 to 60 °C had minimal effect on quality, although enzymatic browning was observed upon air exposure. HPT at 80 to 100 °C caused almost complete inactivation of PPO and POD with 90% and 92% inactivation respectively after 3-min processing at 100 °C, which reduced browning upon air exposure. Nevertheless, the lowest texture retention of 22% was observed under this condition.Industrial relevanceThe study examined the effects of combined high pressure thermal processing on quality related pear enzymes and related instrumental quality attributes such as colour and texture. The study enabled identification of processing regimes for enzyme inactivation and quality retention. The excellent quality retention following HPP at 20 to 40 °C makes this condition suitable for ‘fresh-like’ small portion products for immediate consumption after unpacking that do not require complete PPO and POD inactivation. On the other hand, the almost complete inactivation of oxidative enzymes PPO and POD at 100 °C makes this condition more appropriate for the production of bulk products for food service applications or pureed ingredients for baby food, or pear pieces for yoghurt, that require PPO inhibition but not necessarily high firmness retention.     

Thermal inactivation kinetics parameters of browning enzymes in starfruit (Averrhoa carambola L.) juice

This study aimed to evaluate the thermal inactivation kinetics of polyphenol oxidase (PPO) and peroxidase (POD) in starfruit juice. It followed the Malaysia Food Regulations 1985 and CODEX STAN 247-2005. Glucose, fructose and sucrose were the main sugars in starfruit juice. The total soluble solids, pH, titratable acidity, and total phenolics content of the starfruit juice produced were 8.13 ± 0.25 °Brix, 3.80 ± 0.05, 0.43% ± 0.02% malic acid, and 93.67 ± 4.96 mg GAEL⁻¹, respectively. Thermal inactivation kinetics of PPO and POD followed the first-order kinetic model. The decimal reduction time at 83.6 °C (D_83.6) of PPO and POD was 198.48 and 98.4 s, respectively, while the thermal resistance constant (z value) of PPO and POD was 12.8 and 5.4 °C, respectively. In conclusion, PPO might be a suitable signal for thermal processing on starfruit juice since it has higher heat resistance than POD.     

Effect of Pretreatments on Quality Characteristics of Dehydrated Ivy Gourd (Coccinia indica L.)

Ivy gourd (Coccinia indica L.) is an important tropical vegetable cultivated in India. It belongs to the Cucurbitaceae family and is attributed with neutraceutical properties such as hypoglycemic effect and contained a fair amount of ascorbic acid. The effect of blanching and different pretreatments on the quality characteristics of dehydrated ivy gourd slices was optimized. The treated ivy gourd slices were dehydrated in a cross flow hot air drier at 50 ± 1 °C to a final moisture content of 4.6%. The dehydration ratio of dried ivy gourd slices ranged from 17.1:1 to 18.7:1. Ivy gourd slices blanched at 98 °C in water, dipped in 0.2% potassium metabisulphite (K₂S₂O₅) solution and dried at 50 ± 1 °C for 5–6 h resulted in best product. Dehydrated ivy gourd slices obtained from the optimized treatment had good color, texture with rehydration ratio of 1:8.6. Equilibrium relative humidity of dehydrated slices was 34.18% with an initial moisture content of 4.6% at 28 ± 1 °C. Dehydrated slices were packed in low density polyethylene (LDPE) and metallized polyester polyethylene (MPP) pouches and stored at room temperature (27–35 °C) for 6 months. The dehydrated slices were analyzed for changes in physicochemical composition and sensory quality characteristic such as color and appearance, texture and overall quality during storage. Dehydrated ivy gourd slices remained acceptable during storage of 4 and 6 months in LDPE and MPP pouches, respectively, at room temperature.     

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.     

Biochemical characteristics and thermal inhibition kinetics of polyphenol oxidase extracted from Thompson seedless grape

Polyphenol oxidase (PPO) was isolated from Thompson seedless grape (Vitis vinifera ‘Thompson Seedless’), and its biochemical characteristics were studied. The PPO showed activity to catechol and D, L-DOPA, but not towards monophenol l-Tyrosine, diphenols guaiacol and caffeic acid, and triphenols pyrogallic acid and gallic acid. Apparent Michaelis–Menten constant (K _m) and maximum velocity of the reaction (V _max) values were 45.0 ± 0.05 mM and 500.0 ± 15.3 OD_400 nm/min for catechol, and 34.6 ± 0.03 mM and 384.6 ± 11.7 OD_478 nm/min for D, L-DOPA, respectively. The obtained similar specificity values of V _max/K _m ratio of catechol and D, L-DOPA indicated their similar affinity to Thompson seedless PPO. The most effective inhibitor was l-cysteine, followed in decreasing order by ascorbic acid, sodium metabisulfite, EDTA, NaCl, and citric acid. It was discovered that metal ions of Mg²⁺ and Cu²⁺ increased, while Zn²⁺ and K⁺ reduced the PPO activity. Sugars showed inhibition on the PPO activity, with higher effect by sucrose and lower effect by fructose and glucose. Optimum pH and temperature for grape PPO activity were 6.0 and 25 °C with 10 mM catechol as substrate. The enzyme was heat stable between 10 and 25 °C, but showed significant activity loss at temperatures higher than 40 °C and completely inactivation at 70 °C for 10 min. Thermal inactivation of PPO showed a first-order kinetic with an activation energy (E _a) of 146.1 ± 10.8 kJ/mol at pH 6.0.     

Inhibition of enzymes and Pseudomonas tolaasii growth on Agaricus bisporus following treatment with surface dielectric barrier discharge plasma

In this study the applicability of cold plasma, produced by surface dielectric barrier discharge (SDBD), to inactivate Pseudomonas tolaasii (P.tolaasii), polyphenol oxidase (PPO) and peroxidase (POD) enzymes, as well as its impact on quality parameters such as, color, texture, pH and weight loss were evaluated. The study evidently shows that treating with combination of 30% hydrogen peroxide vapor)with flow rate of 0.47 mL min⁻¹) and argon (H₂O₂ + Ar) for 180 s is capable of reducing the activity of PPO (0.17 U min⁻¹ g⁻¹ FW) and POD (0.21 U min⁻¹ g⁻¹ FW) and increasing the SOD enzyme (16.29 U g⁻¹ FW) in a 21-day storage period compared to control samples. This is while the quality characteristics of button mushroom are preserved during storage after such treatment.Industrial relevanceThis study provides information of A. bisporus storage during 21-day period after SDBD plasma treatment, which is rarely. Greater inactivation of P. tolaasii after 180 s treatment with combination of hydrogen peroxide vapor and air (H₂O₂ + air) was shown compared to control samples, but this treatment caused also slightly degradation of button mushroom color. A better reduction of PPO and POD enzyme activity as well as further increase of SOD enzyme activity was observed following treatment with 180 s of H₂O₂ + Ar gas in a 21-day storage period. This research work contributes to the understanding SDBD plasma induced effects on the shelf-life of button mushroom and could be a basis for a possible industrial implementation.     

Nonthermal inactivation of polyphenol oxidase in apple juice influenced by moderate electric fields: Effects of periodic on-off and constant exposure electrical treatments

Inactivation of polyphenol oxidase (PPO) in fresh apple juice was studied in moderate electric field (MEF) treatments at field strengths in the range of 85–110 V/cm. The apple juice was subjected to periodic on-off treatments at 50 °C and constant exposure MEF treatments at 50 and 65 °C using two different electro-processing devices, specially constructed for the purpose. PPO inactivation by the electric fields was determined with respect to the corresponding control (nofield) treatments. The application of periodic on-off treatments resulted in about 100% increase (P ≤ 0.05) of the thermal inactivation rate constant of PPO at 50 °C. A similar relative increase of the thermal inactivation rate constant was observed with the constant exposure MEF treatment at 50 °C. The electric field influence was improved significantly (P ≤ 0.05) in the constant exposure MEF treatment at 65 °C. Differences in electrical energy input among the MEF treatments were observed. Results indicate both the electric field strength and the treatment temperature play critical roles in enzyme inactivation by moderate electric fields.Industrial relevanceThis study shows polyphenol oxidase enzyme in fresh apple juice can be significantly inactivated by moderate electric fields applied at below thermal pasteurization temperatures. Therefore, moderate electric field treatment applications are beneficial for the minimally processed fruit and vegetable industry.     

Kinetics of Crude Peroxidase Inactivation and Color Changes of Thermally Treated Seedless Guava (Psidium guajava L.)

Thermal treatment of seedless guava (Psidium guajava L.) cubes was carried out in the temperature range of 80–95 °C. The kinetics of peroxidase inactivation and color changes due to thermal treatments were determined. Peroxidase inactivation followed a first-order kinetic model, where the activation energy was 96.39 ± 4 kJ mol⁻¹. Color was quantified in terms of L, a, and b values in the Hunter system. The color changes during processing were described by a first-order kinetic model, except total color difference which followed a zero-order kinetic model. The temperature dependence of the degradation followed the Arrhenius relation. The activation energies (E _a) for L, a, b, and total color difference (ΔE) were 122.68 ± 3, 88.47 ± 5, 104.86 ± 5, and 112.65 ± 5 kJ mol⁻¹, respectively. The results of this work are a good tool to further optimize seedless guava thermal treatment conditions.     

Effects of combined pretreatments on drying kinetics and quality of potato chips undergoing low-pressure superheated steam drying

Due to an increasing demand from health-conscious consumers more attention has been placed on investigating alternative techniques to replace conventional deep-fat frying to produce health-friendly snack products including potato chips. Recently, low-pressure superheated steam drying (LPSSD) has proved to have potential to produce fat-free potato chips if performed in combination with appropriate pre-drying treatments. In this study, the influences of various pretreatments and drying temperature on the LPSSD drying kinetics and quality parameters of dried potato chips were investigated. LPSSD of potato chips underwent various combined pretreatments, i.e., (a) blanching, (b) combined blanching and freezing, (c) blanching followed by immersion in glycerol solution and then freezing, and (d) combined blanching, immersion in monoglyceride solution and freezing, were carried out at different drying temperatures (70, 80, and 90 °C) at an absolute pressure of 7 kPa. The quality of the dried chips was then evaluated in terms of colors, texture (hardness, toughness and crispness) and microstructure. In terms of the drying behavior and the dried product quality, LPSSD at 90 °C with combined blanching and freezing pretreatments was proposed as the most favorable conditions for drying potato chips.     

Use of high-power ultrasound combined with supercritical fluids for microbial inactivation in dry-cured ham

The aim of this study was to analyze the application of supercritical carbon dioxide combined with high-power ultrasound (SC-CO₂ + HPU) and the use of a saline solution (SS; 0.85% NaCl) on the microbial inactivation and the quality of dry-cured ham. The effect of temperature, pressure and treatment time was studied using RSM. Physicochemical analyses were carried out after the treatments and during refrigerated storage (30 days / 4 °C). The most significant inactivation of Escherichia coli (3.62 ± 0.20-log CFU/g) was obtained using the SC-CO₂ + HPU + SS (25 MPa, 46-°C and 10-min), with temperature being the most important process variable. Fat content showed a significant (p < 0.05) reduction (46%) after the SC-CO₂ + HPU treatment. The breakage of the muscle fibers, the disorganization in the myofibrils, as well as the enlargement of the interfibrillar spaces led to the ham softening (avg 26.5%). No significant (p > 0.05) changes in color, texture or pH were found during storage. Thus, ultrasonic-assisted SC-CO₂ could be used, in combination or not with SS, to improve the shelf of dry-cured ham.Industrial relevanceSupercritical carbon dioxide (SC-CO₂) inactivation technology has been shown to be highly efficient at reducing different bacteria in liquid media with minimum effect on food quality. This technology is barely applied to solid products and its use is limited by the long processing times and reduced inactivation capacity. The application of high-power ultrasound (HPU) leads to a shorter process time. This technology is useful for the inactivation of ham microbiota and inoculated E.coli. A liquid medium surrounding the treated solid can enhance microbial inactivation for the purposes of improving the effect of ultrasound cavitation, while only minimally affecting the quality of the samples (color, texture, fat and moisture contents).     

Kinetics of Thermal Inactivation of Peroxidase and Color Degradation of African Cowpea (Vigna unguiculata) Leaves

Cowpea leaves form an important part of the diet for many Kenyans, and they are normally consumed after a lengthy cooking process leading to the inactivation of peroxidase (POD) that could be used as an indicator for the potential shelf life of the vegetables. However, color degradation can simultaneously occur, leading to poor consumer acceptance of the product. The kinetics of POD in situ thermal (for thermal treatments in the range of 75 to 100 °C/120 min) inactivation showed a biphasic first‐order model, with Arrhenius temperature dependence of the rate constant. The kinetic parameters using a reference temperature (T_ref) of 80 °C were determined for both the heat‐labile phase (k_ref = 11.52 ± 0.95 × 10^?2 min^?1 and E_a of 109.67 ± 6.20 kJ/mol) and the heat‐stable isoenzyme fraction (k_ref = 0.29 ± 0.07 × 10^?2 min^?1 and E_a of 256.93 ± 15.27 kJ/mol). Color degradation (L*, a*, and b* value) during thermal treatment was investigated, in particular as the “a*” value (the value of green color). Thermal degradation (thermal treatments between 55 and 80 °C per 90 min) of the green color of the leaves followed a fractional conversion model and the temperature dependence of the inactivation rate constant can be described using the Arrhenius law. The kinetic parameters using a reference temperature (T_refC = 70 °C) were determined as k_refC = 13.53 ± 0.01 × 10^?2 min^?1 and E_aC = 88.78 ± 3.21 kJ/mol. The results indicate that severe inactivation of POD (as an indicator for improved shelf life of the cooked vegetables) is accompanied by severe color degradation and that conventional cooking methods (typically 10 min/100 °C) lead to a high residual POD activity suggesting a limited shelf life of the cooked vegetables.     

Effects of low temperature blanching on texture,microstructure and rehydration capacity of carrots

The effect on quality of stepwise blanching (first 10 min at 65 °C, 50 min holding time, and 1 min at 95 °C) prior to drying of carrot slices was compared with conventional blanching (95 °C, 1 min). As quality parameters, rehydration capacity (effective diffusivity D_e, equilibrium moisture content W_e), texture (puncture test) and microstructure (SEM and Cryo‐SEM) were chosen. Non‐significant differences were found between the W_e of stepwise blanched samples and conventionally blanched ones, both values being similar to the moisture content of fresh carrots. D_e of conventional blanched carrots was higher than for stepwise blanched ones. Maximum force of stepwise blanched samples was higher than those of conventionally blanched ones and not significantly different from fresh carrots. Microstructural observations showed that stepwise blanching preserved cell to cell contacts better than conventional blanching; this fact was attributed to pectin‐methyl‐esterase activation at 65 °C, resulting in the differences in the maximum force between both treatments. Conventionally blanched samples tended to separate along their cell walls, forming voids among the phloem parenchyma cells. These voids would be filled with water during rehydration, thus showing the slightly higher W_e and the higher D_e for conventionally blanched carrots. Copyright © 2005 Society of Chemical Industry     

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.