Pulsed electric field treatment of carrots before drying and rehydration

BACKGROUND: In this study the effects of pulsed electric field (PEF) pretreatment were evaluated during drying and rehydration of carrots. Carrots pretreated with an electric field intensity of 1 kV cm^?1 (capacitance 0.5 µF, 20 pulses) or 1.5 kV cm^?1 (capacitance 1 µF, 20 pulses) as well as blanched (100 °C, 3 min) carrots were used for the study. Following pretreatment, samples were oven dried at 70 °C and then rehydrated in distilled water (1:30 w/v) at room temperature (24 ± 1 °C). RESULTS: PEF pretreatment increased the drying rate of carrots. However, the rehydration rate of PEF‐pretreated carrots was lower than that of blanched carrots. There were no colour differences between PEF‐pretreated and blanched carrots before drying and after rehydration. In terms of texture, PEF‐pretreated carrots were firmer than blanched carrots. PEF pretreatment reduced the activity of peroxidase by 30–50%, while blanching completely inactivated the enzyme (>95%). CONCLUSIONS: Overall, the results suggest that PEF could be an effective pretreatment during drying and rehydration of carrots. Copyright © 2009 Society of Chemical Industry     

Pilot scale inulin extraction from chicory roots assisted by pulsed electric fields

A pilot study for the pulsed electric fields (PEF) assisted countercurrent diffusion of inulin from chicory roots is presented. The influence of PEF parameters (electric field intensity E = 600 V cm^?1, treatment duration t_PEF = 10–50 ms) and diffusion temperature (varied between 30 and 80 °C) on soluble matter extraction kinetics, inulin content of juice, and pulp exhaustion are investigated. The draft (liquid to solid mass ratio) was fixed at 140%, similar to the industrial conditions. PEF treatment facilitates extraction of inulin at conventional diffusion temperature (70–80 °C), and diffusion temperature can even be reduced by 10–15 °C with comparable juice inulin concentration. Less energy consumption can be achieved by reducing PEF treatment duration to 10 ms, which is observed sufficient for effective extraction.     

Quality of fresh‐cut purple‐fleshed sweet potatoes after X‐ray irradiation treatment and refrigerated storage

The effect of X‐ray irradiation on the quality of fresh‐cut, refrigerated purple‐fleshed sweet potato (PFSP) cubes was investigated. Packaged sweet potato cubes were treated with 0, 250, 500, 750 or 1000 Gy X‐ray irradiation and stored at 4 ± 1 °C for 14 days. After 14 days, total aerobic bacteria counts were 4.1 and 3.2 log₁₀ CFU g^?1, and mould–yeast counts were 3.3 and 3.0 log₁₀ CFU g^?1 in 750 and 1000 Gy treated samples, respectively. Doses up to 1000 Gy did not affect the firmness, moisture content and anthocyanin content of PFSP cubes throughout storage. PFSP cubes' flesh colour did not change during the first week of storage, but lightness (L*) increased after 14 days. Also, irradiation doses at 750 and 1000 Gy decreased saturation (C*) significantly, producing duller flesh colour than controls. Results indicate that X‐ray irradiation treatment at doses up to 1000 Gy can reduce microbial populations while maintaining the physical quality and anthocyanin content of PFSP cubes up to 14 days of storage.     

Thermal transition and thermo-physical properties of potato (<Emphasis Type="Italic">Solanum tuberosum L</Emphasis>.) var. Russet brown

Potatoes are an important food in many regions of the world and are commonly used in a variety of food products. Thermal transition and thermo-physical properties of potatoes are important in order to design efficient food processes and select appropriate storage conditions. In this study, we determined the thermal transitions and thermophysical properties of raw and blanched/par-fried potato for a temperature range of ??32 to 21.1 °C. Using differential scanning calorimetry, we found an initial freezing point (T_f) at ??1.8?±?0.1 °C, an onset of melting (T_m^′) at ??9.9?±?0.2 °C and an unfreezable water content (X^′_w) for maximally freeze-concentrated raw potato at 0.21 kg water/kg potato. Corresponding values for blanched/par-fried potatoes were ??0.9?±?0.1 °C, ??11.0?±?0.2 °C and 0.18 kg water/kg potato. Results show that an increase in solids content decreased T_f of both raw and blanched potatoes. We modelled the relationship between them using the Chen model. The apparent specific heat (C_app) increased around T_f to 31.7?±?1.13 kJ/kg K for raw potato and 26.7?±?0.62 kJ/kg K for blanched/par-fried potato. For frozen raw potato at ??32 °C, thermal diffusivity (α) was 0.89?±?0.01?×?10?^?6 m²/s and thermal conductivity (k), 1.82?±?0.14 W/m K, respectively. These values were higher for frozen raw potato than for the unfrozen raw potato (0.15?±?0.01?×?10?^?6 m²/s and 0.56?±?0.08 W/m K, respectively at 21.1 °C). The apparent density (ρ) of frozen raw potato (992?±?4.00 kg/m³ at ??32 °C) was less than that for unfrozen raw potato (1053?±?4.00 kg/m³ at 21.1 °C), and a similar trend was obtained for blanched/par-fried potato (993?±?2.00 kg/m³ at ??32 °C and 1188?±?7.00 kg/m³ at 21.1 °C, respectively). This study established a correlation between thermo-physical properties and temperature. Findings may be used to inform the design and optimization of freezing processes and frozen storage for potato products.     

Physicochemical Changes in Potato Granules During Storage

Physicochemical properties of freeze-thaw (F-T) and add-back (A-B) potato granules, such as swelling power (SP), water-holding capacity (WHC), degree of starch retrogradation, moisture change, and rate of rehydration were monitored over about 100 wk of storage. SP of both types of granules decreased to minima after 40 wk then increased on further storage, while WHC showed an opposite trend, reaching maxima after 52 wk. Degree of starch retrogradation in the granules increased to maxima after 40 wk then declined. Moisture content of the granules also increased slightly during the first 33 wk then decreased on further storage. These changes caused the reduction in the rehydration rate of the granules from 8.3 × 10^?2 see^?1 to 5.5 × 10^?2 set^?1 (F-T) and 5.2 × 10^?2 sec^?1 (A-B) after 36 wk of storage, after which the rate again increased. Addition of Ca⁺⁺ to the granules during precooking and cooling steps of the process appeared to cause similar changes.     

Impact of high intensity electric field pulses on cell permeabilisation and as pre-processing step in coconut processing

The impact of high intensity electric field pulses (HELP) for cell permeabilisation of coconut was studied. A preliminary investigation of suitable field strength and total energy input for HELP treatment (field strength, 0.1–2.5 kV/cm, number of pulses, 0–200, pulse width, 575 μs and pulse frequency 1 Hz) was carried out. A comparison of the HELP process (E=2.5 kV/cm, n=20 pulses, t=575 μs and frequency 1 Hz) was made with other methods of cell disintegration (e.g. mechanical rupture, thermal treatment, freeze-thaw cycle) on the yield of coconut milk obtained and its protein and fat contents. A combination of HELP treatment and centrifugation (10 000×g for 10 min) was employed as pre-processing step prior to coconut dehydration in a fluidised bed dryer (60°C, air velocity 1 m/s) in an attempt to enhance drying rates. Treatment with HELP under suitable conditions (E=2.5 kV/cm, n=20 pulses, t=575 μs and frequency 1 Hz) resulted in 20% increase in milk yield with reference to the untreated samples. The fat contents of the extracted milk from HELP treated and untreated samples were 58.0% and 61.2%, respectively, and the corresponding protein contents of the aqueous medium after separation by centrifugation were 50.0% and 51.6%. The combination of HELP and centrifugation step resulted in a reduction of approximately 22% of the drying time as compared to the untreated samples which could be of advantage in the production of copra.     

Comparative evaluation of the effects of pulsed electric field and freezing on cell membrane permeabilisation and mass transfer during dehydration of red bell peppers

The extent of cell membrane permeabilisation due to high intensity electric field pulses (HELP) varying number of pulses (1–50) using electric field of 2 kV/cm, 400 μs pulse duration and freezing on mass transfer and vitamin C content during osmotic (50° Brix sucrose at 40 °C) and convective air (60 °C, 1 m/s for 5 h) dehydration of red bell peppers was studied. Total pore area due to HELP increased with number of pulses while freezing resulted in total pore area of almost 6 times as greater as the highest value from the HELP process. Higher water loss was observed for all HELP treated than for prefrozen samples while slow freezing provided samples with the highest solids uptake. The correlation coefficient (R²) of linear regression between water loss and solids gain estimated from either total solids or soluble solids measurement ranged from 0.954 to 0.998 suggesting the possibility of using the soluble solids method in evaluating mass transfer kinetics during osmotic dehydration process. Drying rate during convective air-drying was more enhanced by HELP than by freezing. Electrical conductivity of the osmotic solution increased with the degree of permeabilisation to a given medium value after which no further increase in the release of the intracellular ions was observed. Minimal vitamin C depletion was observed immediately after HELP treatment. The order of magnitude of vitamin C retention was untreated>frozen>HELP pretreated samples with 1 pulse>5 pulses>50 pulses>10 pulses>20 pulses after osmotic dehydration. The reduction in vitamin C content of HELP treated samples after convective drying ranged from approximately 11 to 24% while freezing resulted in approximately 24% decrease compared to the untreated samples.     

Purification and characterization of highly active and stable polyphosphatase from Saccharomyces cerevisiae cell envelope

Saccharomyces cerevisiae cell envelope polyphosphatase was isolated in highly active and stable form by extraction from cells with zwittergent TM-314 followed by chromatography of the extract on phosphocellulose and QAE-Sephadex in the presence of 5 mM -MgCl₂, 0·5 mM -EDTA and 0·1% Triton X-100. The enzyme possessed a specific activity of 220 U/mg and after 30 days retained 87% of its activity at ?20°C. Polyphosphatase molecular mass was determined to be about 40 kDa by gel filtration and polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The enzyme hydrolysed polyphosphates with various chain lengths (n = 3–208), had low activity for GTP and did not split pyrophosphate, ATP and p-nitrophenylphosphate. On polyphosphates with chain lengths n = 3, 9 and 208, K_m values were 1·7 × 10^?4, 1·5 × 10^?5 and 8·8 × 10^?7 M respectively. Polyphosphatase was most active and stable at pH 6·0–8·0. The enzyme showed maximal activity at 50°C. The time of half inactivation of polyphosphatase at 40, 45 and 50°C was 45, 10 and 3 min, respectively. In the absence of divalent cations and also with Ca²⁺ or Cu²⁺, the enzyme showed practically no activity. The ability of divalent cations to activate polyphosphatase was reduced in the following order: Co²⁺ > Mg²⁺ > Mn²⁺ > Fe²⁺ > Zn²⁺. Polyphosphatase was completely inhibited by 1 mM -ammonium molybdate and 50 μM -Zn²⁺ or Cu²⁺ (in the presence of Mg²⁺).     

Effect of blanching on the diffusion of glucose from potatoes

Glucose losses from potato slabs into the water during blanching at 60, 70, 80 and 90 °C were studied after 10, 20, 30, 40, 50 and 60 min. In general, an increase of temperature during the considered times produced a higher glucose loss. The apparent diffusion coefficients (D) for glucose losses from potatoes were calculated under the blanching conditions. The D-values were found to be in the range of 3 × 10^?10 to 8 × 10^?10 m² · s^?1 and could be related to temperature by an Arrhenius equation, having an activation energy of 29.1 kJ/mol. The results will allow the prediction of mean glucose concentration in potato tissues, and hence the overall losses incurred, after a given blanch treatment in this time and temperature range.     

Rehydration Kinetics of High-Pressure Pretreated and Osmotically Dehydrated Pineapple

Rehydration kinetics of high‐pressure pretreated (100, 300, and 500 MPa for 10 min) and osmotically dehydrated pineapple (Ananas comsus) cubes (2 × 2 × 1 cm) were studied at different temperatures (5, 25, and 35°C), and compared with ordinary osmotically dehydrated samples. The effective diffusion coefficients for water and solute were determined, assuming the rehydration process to be governed by Fickian diffusion. Diffusion coefficients for water absorption into the tissue as well as for solute diffusion out of the tissue were found to be lower in the samples subjected to high‐pressure treatment. Further, the diffusion coefficients decreased with increase in treatment pressure. A possible explanation for the observed decrease in diffusion coefficients can be attributed to the permeabilization of cell membranes, the release of cellular components, and structural changes of the cell materials. The diffusion coefficients were correlated with rehydration temperature (T) and treatment pressure (P) by an Eq. of the form D = A exp[–(B.P + C/T)], where A, B, and C are constants.     

Molecular and physicochemical characterisation of starches from yam,cocoyam, cassava,sweet potato and ginger produced in the Ivory Coast

Granule sizes, macromolecular features and thermal and pasting properties of starches from seven tropical sources (Florido, Kponan and Esculenta yams, cocoyam, cassava, sweet potato and ginger) were compared with those of several well‐known cereal, legume and tuber starches. The aim of the study was to characterise some non‐conventional starches with a view to possibly marketing them. Amylose content varied from 148 mg g^?1 in Esculenta starch to 354 mg g^?1 in smooth pea starch. For total starches, weight‐average molar mass (M?_w) ranged between 0.94 × 10⁸ and 1.80 × 10⁸ g mol^?1 for potato and normal maize starches respectively. Gyration radius (R?_G) varied from 157 nm for ginger starch to 209 nm for normal maize starch. Gelatinisation enthalpy (ΔH) ranged between 9.8 and 20.7 J g^?1 for wheat and Florido starches respectively. Gelatinisation peak temperature (T_g) varied from 58.1 °C for wheat starch to 87.3 °C for ginger starch. Native starch granule mean diameter ranged between 5.1 and 44.5 µm for Esculenta and potato starches respectively. Cassava and potato starches had the highest swelling power and dispersed volume fraction at all treatment temperatures, while ginger starch had the lowest. Cocoyam starch had the highest and ginger starch the lowest solubility at 85 and 95 °C. Cassava starch was the most stable under cold storage conditions. Roots and tubers such as ginger and cassava produced in the Ivory Coast are new sources of starches with very interesting properties. Thus these starches could be isolated on an industrial level in order to market them. Copyright © 2007 Society of Chemical Industry     

Patulin Adsorption Kinetics on Activated Carbon, Activation Energy and Heat of Adsorption

The kinetics of adsorption of patulin on activated carbon were studied at different initial patulin concentrations (100–400 ppb) for the temperature range 20–80°C. Apparent adsorption rate constants (k_aapp) were changed from 1.07 × 10^?3 to 1.86 × 10^?3 g^?1 min^?1 while the temperature increased from 20 to 80°C. For equilibrium adsorption curves; the Langmuir model was attempted and model parameters (K and Q°) were obtained for different temperatures. Energy of activation and heat of adsorption were determined in a batch adsorption system (E_a= 2.02 kcal/mol and ΔH = 2.24 kcal/mol). The adsorption occurred endothermically and by physical mechanisms.     

Thermal Properties of Sweet Potato with its Moisture Content and Temperature

Thermal properties of sweet potato were experimentally determined and modeled as a function of temperature and moisture content. The purpose is to develop empirical correlations that could predict thermal properties during sweet potato processing. Thermal conductivity from the study was 0.49 ± 0.038 Wm^?1K^?1 (mean ± s.d.), thermal diffusivity was 1.2?×?10^?7 ± 9.05?×?10^?9 m²s^?1, specific heat was 3660 ± 477.4 Jkg^?1K^?1, and density was 1212 ± 73.5 kgm^?3. All properties were determined within temperature range of 20 to 60°C and moisture content range of 0.45 to 0.75 w.b. Prediction models for the thermal properties of sweet potato were developed as a function of product temperature and moisture content with experimental data from this study. Mechanistic models were also developed for thermophysical properties of sweet potato using major food components of the product. Developed models were all presented and compared.     

Evolution de la chaleur specifique apparente des fromages fondus entre 40 et 100°C. Influence de leur composition

The technique of differential scanning calorimetry was used to determine the apparent specific heat of mixtures for processed cheese manufacture.In the ranges of: dry matter content, 31·6–57·5% (kg kg^?1); fat content, 13·5–40·5% (kg kg^?1) and temperature, 40–100°C; the following relationship was obtained:

$\text{c}{}_{\mathrm{p}}\text{= 4,101+0,0012θ ? (1,673+0,00027θ)x}{}_{\mathrm{f}}\text{?(2,716?0,0011θ)x}{}_{\mathrm{s}}$

where c_p = apparent specific heat (kJ kg^?1 °C^?1), θ = temperature (°C), x_f = mass fraction of fat (kg kg^?1) and x_s = mass fraction of non-fat solids (kg kg^?1).After comparison of these experimental results and extrapolated values with those from the literature, it is concluded that this relationship is appropriate to describe the specific heat of processed cheeses.     

Winged bean lipoxygenase—Part 2: Physicochemical properties

Winged bean lipoxygenase (linoleate: oxygen oxidoreductase EC 1.13.11.12) isoenzymes FI and FII were isolated and purified according to the method of Truong et al. (1980).FI and FII were both highly specific for linoleic acid. They exhibited optimal activity at pH 6·0 and 5·8, respectively at 30°C. An activation energy of 4·5 kcal mol^?1 was calculated for this lipoxygenase within the temperature range of 30–50°C.At 0·075% Tween 20, FI and FII had K_m values for linoleic acid of 0·44 and 0·37 × 10^?3M, respectively, compared to 0·4 × 10^?3M for the crude enzyme. Maximal activity was obtained at 1·6 × 10^?3M. Higher levels of Tween 20 inhibited the lipoxygenase activity.Both isoenzymes had identical average molecular weight of 80 000 daltons by gel filtration and SDS gel electrophoresis.FI and FII isoenzymes were strongly inhibited by Hg⁺⁺, Mn⁺⁺, Mg⁺⁺ and Fe⁺⁺⁺ and activated by Zn⁺⁺, Co⁺⁺ and Fe⁺⁺. A difference in the degree of inhibition or activation was observed between FI and FII response. Ca⁺⁺ inhibited both FI and FII but the former was more sensitive to Ca⁺⁺. KCN also inhibited the two isoenzymes.Among the antioxidants tested, butylated hydroxytoluene and butylated hydroxyanisole most effectively inhibited both FI and FII at only 10^?6M. Sulphydryl reagents such as iodoacetamide and dithiothreitol have little effect on the lipoxygenase isoenzyme activity.The lipoxygenase isoenzymes were more stable at neutral pH. The enzyme in the crude extract and especially in situ was more stable to heat treatment.     

The fate of Escherichia coli O157 : H7 in Myzithra,Anthotyros, and Manouri whey cheeses during storage at 2 and 12°C

Myzithra, Anthotyros and Manouri whey cheeses were inoculated the day after production withEscherichia coli O157 : H7 at concentrations of approx. 1·8×10⁶cfu g⁻¹, and stored at 2 and 12°C for 30 and 20 days, respectively. The pH of the whey cheeses decreased from an initial value of approx. 6·20 to 5·83 or 5·60 (Myzithra) 5·75 or 5·20 (Anthotyros) and 5·80 or 5·30 (Manouri) by the end of the corresponding storage periods at 2 and 12°C, respectively. Escherichia coli O157 : H7 populations in the whey cheeses at the end of the 12°C storage period, had grown with an increase of approx. 1·3 log₁₀cfu g⁻¹. E. coli O157 : H7 populations in whey cheeses at the end of the 2°C storage period did not grow and decreased, with an approx. 2·5 log₁₀cfu g⁻¹reduction. Results showed that E. coli O157 : H7 can grow at 12°C and survive at 2°C storage in Myzithra, Anthotyros and Manouri whey cheeses, and therefore post-manufacturing contamination with this pathogen must be avoided by employing hygienic control programmes such as HACCP.     

Modelling of air drying of fresh and blanched sweet potato slices

Effects of blanching, drying temperatures (50–80 °C) and thickness (5, 10 and 15 mm) on drying characteristics of sweet potato slices were investigated. Lewis, Henderson and Pabis, Modified Page and Page models were tested with the drying patterns. Page and Modified Page models best described the drying curves. Moisture ratio vs. drying time profiles of the models showed high correlation coefficient (R² = 0.9864–0.9967), and low root mean squared error (RMSE = 0.0018–0.0130) and chi‐squared (χ² = 3.446 × 10^–6–1.03 × 10^–2). Drying of sweet potato was predominantly in the falling rate period. The temperature dependence of the diffusion coefficient (D_eff) was described by Arrhenius relationship. D_eff increased with increasing thickness and air temperature. D_eff of fresh and blanched sweet potato slices varied between 6.36 × 10^–11–1.78 × 10^–9 and 1.25 × 10^–10–9.75 × 10^–9 m² s^–1, respectively. Activation energy for moisture diffusion of the slices ranged between 11.1 and 30.4 kJ mol^–1.     

ESTIMATION OF MOISTURE LOSS FROM THE COOLING DATA OF POTATOES

A procedure was developed to predict moisture loss from cooling data of potato packed in gunny bags and stacked on wooden platforms in commercial cold stores. To predict the moisture loss, mass transfer coefficients k_cand k_mwere estimated during the storage period, which were found to decrease with time. The calculated time average k_cand k_mvalues were 1.83 × 10^?4 m/s and 2.31 × 10^?10 kg/s·m²·Pa during the transient cooling period and 1.59 × 10^?4 m/s and 2.27 × 10^?10 kg/s·m²·Pa for the rest of the storage period, respectively. The estimated moisture losses were 4.8, 4.74 and 4.78%, at the center of three different stacks, for a storage period of 8 months. The corresponding experimentally measured weight losses at the center of the same stacks were 5.2, 5.1 and 5.26% with a variation of 11, 7.5 and 10.2%, respectively. Therefore, the procedure adopted in this study may be used to assess the moisture loss from potatoes under the different storage conditions. The effect of relative humidity (RH) and potato temperature on moisture loss was also predicted using the developed procedure. Decrease in RH of the storage air increased the moisture loss. The potatoes stored below 85% RH incurred more than 7% water loss. Therefore, 88–90% RH in the cold store may be used to limit the maximum moisture loss within the permissible limit of 5% even after 8 months of storage. It was also found that increasing the potato temperature exponentially increased the rate of moisture loss.     

Thermal conductivity of tomato paste

Experimental measurements of the thermal conductivity and thermal diffusivity of tomato paste at concentrations of 27–44° Brix are reported. The thermal conductivity was measured in a guarded hot-plate apparatus while the diffusivity was estimated by a simplified transient method. The thermal conductivity (λ) values fell in the region of 0·460–0·660 W m^?1 K^?1, decreasing with increasing solids concentration and increasing as the temperature was raised from 30 to 50°C. The temperature effect was less pronounced at higher solids concentration. The thermal diffusivity of tomato paste at 35° Brix and 20°C was estimated as 1·42 × 10^?7 m² s^?1, which is in good agreement with data from the steady-state method.     

Nonenzymatic Browning in Pear Juice Concentrate at Elevated Temperatures

The effect of temperature and soluble solids (°Brix) on nonenzymatic browning in pear juice concentrate was determined by following absorbance at 420 nm (A₄₂₀) over the temperature range of 50–80°C. Browning could be modeled as a zero order rate process with rates of 22.2 × 10⁻⁴ (45.2 °Brix), 36.9 × 10⁻⁴ (55.4 °Brix), 53.5 × 10⁻⁴ (65.1 °Brix) and 107 × 10⁻⁴ (72.5 °Brix) A₄₂₀· min⁻¹ at 80°C. Temperature dependence was described by the Arrhenius relationship with an average activation energy of 21.9 kcal · mole⁻¹. Formol titration indicated a 20% loss of amino acids during heating 4.4 hr at 80°C and no loss of carbohydrates was observed after any heating period.