Effect of post‐harvest treatments on the level of glucosinolates in broccoli

Broccoli is a very perishable vegetable with a high water content (around 88%) which leads to rapid dehydration and probably to an alteration in composition if conditions after harvest are not controlled. This study evaluates the glucosinolate pattern and glucosinolate levels in the principal and secondary inflorescences of fresh broccoli cv ‘Tokyodome’, and after being submitted to some situations which are likely to occur during or after harvest: room temperature (±20 °C) for 5 days, kept in the fridge at 4 °C for 5 days, and frozen after blanching. Another set of material was harvested 5 days later, simulating a post‐maturation stage, and analysed. The highest total glucosinolate content was found at commercial maturation with 20 888 and 20 355 µmoles kg⁻¹ DW in the principal and secondary inflorescences, respectively. Keeping the inflorescences at room temperature caused the most significant (P < 0.05) reductions in total and individual glucosinolates, except for 4‐hydroxyindol‐3‐ylmethyl‐, 2‐hydroxy‐2‐phenylethyl‐ and 2‐phenylethyl‐, when compared to the other situations. The highest levels (10 925 µmoles kg⁻¹ DW) of 4‐methylsulphinylbutyl‐, the precursor of the anti‐cancer isothiocyanate sulphoraphane, were found in the inflorescences freshly harvested at commercial stage. Refrigeration at 4 °C and freezing were shown to be the best preservation processes for maintaining high levels of these and other glucosinolates in contrast with the other situations. © 1999 Society of Chemical Industry     

Effects of pulsed electric field on selected properties of L‐tryptophan

The effects of pulsed electric fields (PEF) treatment on the solubility, surface tension and fluorescence spectra of L‐tryptophan were systematically investigated using a range of PEF intensities (0–50 kV cm^?1) and a number of pulses (0–216). PEF frequency and pulse width used in this experiment were 1000 Hz and 40 μs, respectively. Under these conditions, the solubility, pH value and maximum relative fluorescence value (RFV) of L‐tryptophan were generally increased and then decreased with both increasing the PEF intensity and pulse number. Conductivity of the solution was insensitive to changes but still increased slightly with increasing the PEF intensity. In addition, surface tension was decreased at higher PEF intensity and pulse number. Results from the mass spectrum analysis confirmed that after the PEF treatment, the basic structure of L‐tryptophan remained unchanged. These results suggested that PEF treatment induced some function alteration to the isolated L‐tryptophan without changing the basic structure.     

Influence of light on health‐promoting phytochemicals of broccoli sprouts

BACKGROUND: Broccoli (Brassicaceae) is a rich source of phytochemicals (glucosinolates and phenolic compounds) and micronutrients (vitamins and minerals). Germinated broccoli sprouts contain much higher levels (10–100 times) of aliphatic (glucoraphanin) and indolic glucosinolates than the inflorescences. This quality characteristic of broccoli sprouts plays an important role in human health and disease prevention. Although it is known that genetic and environmental factors can affect the composition of broccoli inflorescences, the influence of such factors on the seeds and sprouts has not been widely reported. Therefore the aim of this study was to determine the effect of light versus dark growth conditions on the phytochemical composition (vitamin C, phenolic compounds and glucosinolates) of broccoli sprouts. RESULTS: Broccoli sprouts grown in the light were found to have much higher concentrations of vitamin C (by 83%), glucosinolates (by 33%) and phenolic compounds (by 61%) than those grown in the dark. During a 7 day period there was a clear and analogous trend in both treatments, with a general reduction in concentrations over time. Among the different organs studied (seeds, cotyledons, stems and roots), the cotyledons contained the highest levels of bioactive compounds, while the roots contained the lowest. CONCLUSION: Light treatment of sprouting broccoli seeds increased their concentration of health‐promoting phytochemicals, mainly during the first 3–5 days of development. Therefore the younger broccoli sprouts are a better source of bioactive compounds for the consumer than the inflorescences. Copyright © 2008 Society of Chemical Industry     

脉冲电场对蛋白质与金属离子螯合作用的影响

采用拉曼光谱研究脉冲电场对卵清蛋白与金属离子(Cu~(2+)、Ba~(2+)、Mn~(2+)或Ca~(2+))螯合作用的影响。试验结果表明:(1)PEF处理时间和金属离子种类都能影响金属离子和蛋白质的螯合作用,随着脉冲时间的延长,金属离子与卵清蛋白的螯合作用增强,PEF处理时间为1 695μs时,Mn~(2+)和Cu~(2+)与蛋白质的鳌合开始减弱;(2)蛋白质分子和金属离子在1 200~1 700cm~(-1)位置出现不同程度的螯合:与Mn~(2+)鳌合最强,Cu~(2+)和Ca~(2+)较弱,Ba~(2+)最弱。     

脉冲电场对玉米淀粉成膜性能的影响与优化

为探讨脉冲电场对玉米淀粉成膜性能影响机理及最优工艺参数,采用多目标非线性优化方法分析了电场强度、作用时间和脉冲数对膜断裂伸长率、抗张强度、透光度和水蒸气透过系数的影响规律,优化了影响淀粉成膜性能的脉冲电场工艺参数。结果表明,作用时间、脉冲数分别影响膜抗张强度和透光度;脉冲强度主要影响膜断裂伸长率和水蒸气透过系数。在脉冲电场为脉冲强度890 V/cm,作用时间85μs,脉冲数62个的最佳工艺参数条件下,淀粉膜的断裂伸长率为62.34%,抗张强度为1.03 N/mm~2,水蒸气透过系数为0.10 g/(cm~2·24 h),透光度为1.339%,为脉冲电场在玉米淀粉膜生产中的应用和推广提供理论依据。      

“Cold” electroporation in potato tissue induced by pulsed electric field

This work compares PEF-induced effects in potato tissue at temperatures below and above ambient (T = 2–45 °C). The potato (Agata) was selected for investigation. The PEF treatment using electric field strength E = 200–800 V/cm and bipolar pulses of near-rectangular shape with pulse duration t_p (=100 μs) was applied. The PEF experiments were done under non-isothermal conditions with temperature increase owing to the effect of ohmic heating. The linear temperature dependencies of electrical conductivity of potato tissue with different values of the electrical conductivity disintegration index, Z, were observed. However, the values of the conductivity temperature coefficient, α, at the reference temperature T_r = 25 °C were noticeably different for the intact (α_i = 0.0255 ± 0.0003 °C⁻¹) and completely damaged (α_d = 0.031 ± 0.009 °C⁻¹) potato tissues. This difference was explained by the impact of temperature on the structure of the damaged tissue. The non-isothermal PEF treatment was shown to be an effective tool for electroporation at low temperatures (below ambient). For initial temperature T_i = 2 °C, the most power saving was the PEF treatment at E = 200 V/cm (W ≈ 20–30 kJ/kg), and the PEF treatment at E = 400–800 V/cm required more power consumption (W ≈ 50–80 kJ/kg). The PEF treatment at the fixed value of E (=400 V/cm) showed that the total power consumptions (accounting for PEF treatment and thermal heating), required for high level of tissue disintegration, Z ≈ 0.9, were comparable for initial temperatures T_i = 2 °C (W ≈ 50–80 kJ/kg) and T_i = 20 °C (W ≈ 80 kJ/kg) and were noticeably higher for initial temperature T_i = 40 °C (W ≈ 150 kJ/kg).     

Reduction of protease activity in milk by continuous flow high-intensity pulsed electric field treatments

High-intensity pulsed electric field (HIPEF) is a non-thermal food processing technology that is currently being investigated to inactivate microorganisms and certain enzymes, involving a limited increase of food temperature. Promising results have been obtained on the inactivation of microbial enzymes in milk when suspended in simulated milk ultrafiltrate. The aim of this study was to evaluate the effectiveness of continuous HIPEF equipment on inactivating a protease from Bacillus subtilis inoculated in milk. Samples were subjected to HIPEF treatments of up to 866 micros of squared wave pulses at field strengths from 19.7 to 35.5 kV/cm, using a treatment chamber that consisted of eight colinear chambers connected in series. Moreover, the effects of different parameters such as pulse width (4 and 7 micros), pulse repetition rates (67, 89, and 111 Hz), and milk composition (skim and whole milk) were tested. Protease activity decreased with increased treatment time or field strength and pulse repetition rate. Regarding pulse width, no differences were observed between 4 and 7 micros pulses when total treatment time was considered. On the other hand, it was observed that milk composition affected the results since higher inactivation levels were reached in skim than in whole milk. The maximum inactivation (81%) was attained in skim milk after an 866-micros treatment at 35.5 kV/cm and 111 Hz.     

Comparative study on antioxidant properties of carrot juice stabilised by high‐intensity pulsed electric fields or heat treatments

BACKGROUND: The effect of high‐intensity pulsed electric field (HIPEF) processing (35 kV cm^?1 for 1500 µs using 6‐µs bipolar pulses at 200 Hz) on the antioxidant features (vitamin C, β‐carotene, total phenolic compounds and antioxidant capacity) of carrot juice as well as on peroxidase activity was investigated and compared to the observed in heat pasteurised juices (90 °C for 60 s or 30 s) having the fresh juice as a reference. RESULTS: HIPEF and heat‐treated carrot juices had higher β‐carotene and lower vitamin C contents than the untreated juices immediately after processing. The antioxidant capacity of the juices was significantly modified neither by HIPEF nor by thermal treatments. POD activity decreased drastically (≥93.3%) after processing irrespective of the treatment applied. Vitamin C and β‐carotene content decreased throughout the storage following an exponential trend (R² = 0.801–0.984) with degradation rates between 1.7 × 10^?2 and 3.5 × 10^?2 day^?1. Vitamin C and β‐carotene contents were better maintained in HIPEF‐treated than in heat‐pasteurised juices throughout the storage. Total phenolic content and the antioxidant capacity of the HIPEF‐treated juice did not substantially differ from that of the thermally treated juice for 56 days. CONCLUSION: HIPEF processing may help to achieve fresh‐like carrot juices with increased amounts of health‐related phytochemicals. Copyright © 2009 Society of Chemical Industry     

高压脉冲电场和热处理对蓝莓汁品质的影响

以蓝莓汁为原料,比较了高压脉冲电场(PEF)杀菌和热力杀菌处理对蓝莓汁中微生物和理化指标的影响,以及不同处理的蓝莓汁在贮藏期间的品质变化规律。结果表明:PEF可以有效杀灭蓝莓汁中的微生物;PEF杀菌对蓝莓汁色泽参数影响程度小;杀菌处理后,贮藏期内PEF处理和热处理对蓝莓汁的还原糖、总酸、可溶性固形物含量、总酚含量影响较小,PEF处理较热处理能更好地保持蓝莓汁中的VC和花青素含量。      

Impact of high-intensity pulsed electric field variables affecting peroxidase and lipoxygenase activities of watermelon juice

The effects of pulse frequency (50-250 Hz), pulse width (1.0-7.0 μs) and polarity (monopolar or bipolar) of high-intensity pulsed electric field (HIPEF) treatments (35 kV/cm and 1000 μs total treatment time) on peroxidase (POD) and lipoxygenase (LOX) activities were evaluated using a response surface methodology. Second-order expressions were accurate enough to fit experimental results. HIPEF bipolar treatments resulted to be more effective than monopolar treatments in reducing POD and LOX activities. Watermelon juice LOX was more resistant to HIPEF than POD within the range of assayed conditions. HIPEF treatments set at 50 Hz for 1.0 μs pulse width could attain minimum residual POD activity values up to 10%, whereas the highest POD activity reduction was reached by combining high frequencies and pulse widths. Thus, POD could be totally inactivated by applying 7.0-μs bipolar pulses at frequencies higher than 114 Hz. In addition, the effect of frequency on the LOX activity was highly affected by the pulse width of the treatment. Thus, treatments conducted at 220 Hz with bipolar pulses of 1.0 μs led to the lowest residual LOX activity (50%).     

Effects of pulsed electric field on colloidal properties and storage stability of carrot juice

The effects of pulsed electric fields (PEF) and high‐temperature short time (HTST) on the colloidal properties of carrot juice initially after PEF processing and throughout storage period were studied. Compared with HTST treatments, PEF treatments have less effect on the colloidal properties, such as cloud stability, zeta potential, viscosity and particle size distribution (PSD). Pectin methyl esterase (PME) could influence the colloidal properties of carrot juice by de‐esterification of the pectin. In the present work, PME activity was reduced significantly as electric‐field strength (from 15 to 30 kV cm^?1) and treatment time (from 100 to 800 μs) increased. The results also showed that the emergency of a new peak in PSD of samples occurred when the electric intensity exceeded 30 kV cm^?1. Images of transmission electron microscopy showed that particles slightly gathered after PEF treatment. However, under HTST treated, these smaller particles aggregated into net‐like structures that further gathered larger particles. At the micro level, both PEF treated (30 kV cm^?1, 800μs) and HTST treated kept good stability for more than 1 month.     

Impact of pulsed electric fields and post‐mortem vacuum ageing on beef longissimus thoracis muscles

The impact of pulsed electric field (PEF) processing (0.2–0.6 kV cm^?1, 1–50 Hz, 20 μs) followed by vacuum ageing (1 and 3 days) on the quality traits of beef longissimus thoracis (LT) was assessed. The results show that pH, colour stability (L*, a* and R630/580 values) and cooking loss were unaffected by PEF treatments, whilst moisture content significantly (P < 0.05) decreased by 0.7–3.6%. No significant (P > 0.05) difference was found in shear force between PEF treated and untreated samples. However, the shear force values significantly reduced in response to ageing times regardless of the PEF treatments. After 3 days post‐treatment ageing, the shear force decreased by 20–22% compared to 1 day ageing for all samples. Furthermore, Cryo‐SEM results suggest that PEF treatments have led to more porous tissue structure leading to more water loss. Protein profile was unchanged by PEF treatments applied.     

Changes in quality attributes throughout storage of strawberry juice processed by high-intensity pulsed electric fields or heat treatments

The effects of high-intensity pulsed electric field (HIPEF) processing (35 kV/cm for 1700 μs applying 4-μs pulses at 100 Hz in bipolar mode) on color, viscosity and PME and PG activities in strawberry juice were studied and compared to those of heat treatments (90 °C for 60 s or 30 s) through 63 days of storage. L^∗ and viscosity values of the HIPEF-processed juices were higher than those found in the thermally treated. In addition, HIPEF-treated juice exhibited lower 5-(hydroxymethyl)-2-furfural (HMF) concentration and browning index than heat-treated juices throughout storage. On the other hand, HIPEF-treated juice maintained low residual pectin methylesterase (PME) activity (13.1%) for 63 days, whereas in the case of the thermally treated, 22.2 and 48.8% was retained after 60 s and 30 s, respectively. Strawberry juice treated by HIPEF achieved lower residual polygalacturonase (PG) activity (73.3%) than those of heat-processed at 90 °C for 60 s (76.2%) or 30 s (96.8%). Thus, HIPEF could be a feasible alternative to thermal processing to minimize browning and viscosity loss in strawberry juice during storage.     

Critical electric field strengths of onion tissues treated by pulsed electric fields

The impact of pulsed electric fields (PEF) on cellular integrity and texture of Ranchero and Sabroso onions (Allium cepa L.) was investigated. Electrical properties, ion leakage rate, texture, and amount of enzymatically formed pyruvate were measured before and after PEF treatment for a range of applied field strengths and number of pulses. Critical electric field strengths or thresholds (E(c)) necessary to initiate membrane rupture were different because dissimilar properties were measured. Measurement of electrical characteristics was the most sensitive method and was used to detect the early stage of plasma membrane breakdown, while pyruvate formation by the enzyme alliinase was used to identify tonoplast membrane breakdown. Our results for 100-μs pulses indicate that breakdown of the plasma membrane occurs above E(c)= 67 V/cm for 10 pulses, but breakdown of the tonoplast membrane is above either E(c)= 200 V/cm for 10 pulses or 133 V/cm for 100 pulses. This disparity in field strength suggests there may be 2 critical electrical field strengths: a lower field strength for plasma membrane breakdown and a higher field strength for tonoplast membrane breakdown. Both critical electric field strengths depended on the number of pulses applied. Application of a single pulse at an electric field up to 333 V/cm had no observable effect on any measured properties, while significant differences were observed for n≥10. The minimum electric field strength required to cause a measurable property change decreased with the number of pulses. The results also suggest that PEF treatment may be more efficient if a higher electric field strength is applied for a fewer pulses.     

Effect of nitrogen and sulfur fertilization on glucosinolates in the leaves and roots of broccoli sprouts (Brassica oleracea var. italica)

Broccoli (Brassica oleracea L. var. italica cv. Marathon) sprouts are a rich source of glucosinolates, particularly 4‐methylsulfinylbutyl glucosinolate (glucoraphanin), the precursor of the chemoprotective isothiocyanate, sulforaphane. Sulfur and nitrogen fertilization have been shown to influence the pattern and levels of glucosinolates in mature broccoli, but little information is available on the fertilization of sprouts, a transient stage of broccoli growth, which have been recommended for salads. Therefore, an experiment was set up to evaluate the effect of N and S fertilization on the glucosinolate content of the aerial part and roots of broccoli sprouts. Nitrogen was tested at 0, 45.5, 91.0 mg L^?1 and sulfur at 0, 14.6 and 29.2 mg L^?1. The results showed that total glucosinolates in the aerial part were significantly higher (P < 0.001) than in the roots. The major glucosinolates in the aerial part were 4‐methylsulfinylbutyl and 3‐methylsulfinylpropyl whereas in the roots they were 2‐phenylethyl and 4‐methylthiobutyl. Fertilization of broccoli sprouts had a significant (P < 0.001) detrimental effect on the levels of aliphatic glucosinolates whereas the opposite was noted for indole and aromatic glucosinolates, for some of the fertilization combinations tested. Overall, the results indicate that broccoli sprouts do not benefit from fertilization. Copyright © 2006 Society of Chemical Industry     

Metabolite profiling of phenolic and carotenoid contents in tomatoes after moderate-intensity pulsed electric field treatments

A metabolite profiling approach was used to study the effect of moderate-intensity pulsed electric field (MIPEF) treatments on the individual polyphenol and carotenoid contents of tomato fruit after refrigeration at 4 °C for 24 h. The MIPEF processing variables studied were electric field strength (from 0.4 to 2.0 kV/cm) and number of pulses (from 5 to 30).     

Changes of health-related compounds throughout cold storage of tomato juice stabilized by thermal or high intensity pulsed electric field treatments

The effect of high intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1500 μs in bipolar 4-μs pulses at 100 Hz, with an energy density of 8269 kJ/L) on the main bioactive compounds and antioxidant capacity of tomato juice was investigated and compared to heat pasteurization (90 °C for 1 min or 30 s) having the fresh juice as a reference. HIPEF and heat treated tomato juices showed higher lycopene and lower vitamin C levels than the untreated juice. However, no significant changes in the total phenolic content and antioxidant capacity were observed between treated and fresh juices just after processing. Lycopene, vitamin C and antioxidant capacity of both treated and untreated juices decreased exponentially during storage following a first order kinetics (R² = 0.763–0.987), whereas tomato juices maintained their initial phenolic content. HIPEF-treated tomato juice maintained higher lycopene and vitamin C content than the thermally treated juices during the storage time. Hence, the application of HIPEF may be appropriate to achieve nutritious and fresh like tomato juice.

Industrial relevance

HIPEF processing can lead to tomato juice with higher nutritional value than that thermally processed. HIPEF-treated (35 kV/cm for 1500 μs with 4-μs bipolar pulses at 100 Hz, energy input of 8269 kJ/L) tomato juice shows greater lycopene, vitamin C and antioxidant capacity just after the treatment and during the storage time than heat treated (90 °C­30 s and 90 °C­60 s) tomato juice. Therefore, HIPEF technology is a feasible alternative to thermal treatment to obtain tomato juice with a high presence of health-related compounds.