Texture and Histological Structure of Carrots Frozen at a Programmed Rate and thawed in an Electrostatic Field

To investigate the most suitable rate of freezing and method for thawing, raw and blanched carrots were frozen with LN₂ (freezing rate: –5°or -2°C/min, final temp: -30°C) using a program freezer (PF), or were frozen using conventional freezers (F: -80°C, -30°and -20°C). Then, they were thawed in five different ways: electrostatic thawing (ET, -3°C, 17 hr); -3°C, 17 hr; 5°C, 17 hr; 20°C, 30 min; 100°C, 3 min. Firmness of thawed carrots and amount of undamaged tissues by LM and TEM observations were greatest to least: PF -5°C/min < PF-2°C/ min <-80°C CF<-30°CF<-20°CF, and ET ≧-3°< 5°< 20°< 100°C, respectively. Results suggest the optimum rate of freezing was -5°C/ min. The frozen disks were defrosted comparatively fast even at -3°by ET. Drip, cell damage and softening of disks were prevented by ET.     

Frozen Carrots Texture and Pectic Compotients as Affected by Low-Temperature-Blanching and Quick Freezing

Carrots preheated for 2 hr at 60°C and then cooked became firmer than raw or cooked carrots. After preheating, the amount of high methoxyl pectin decreased, and low methoxyl pectin increased. Firmness of carrots decreased through freezing then thawing, but preheated carrots retained firmer texture than those blanched in boiling water. Quick-freezing resulted in better texture than slow-freezing. Loss in texture was accompanied by release of pectin. Slow-freezing accelerated release of pectin as compared to quick-freezing. Preheated carrots were slower in release of pectin. The degree of esterification of pectin substances in raw carrots decreased during preheating, freezing and thawing. Cell damage in quick frozen carrots was slight. Optimum preheating occurred with 30 min at 60°C or 5 min at 70°C. Preheating and then quick freezing were effective in improving texture of frozen carrots.     

High-Pressure-Freezing Effects on Textural Quality of Carrots

Raw or 3 min blanched carrots were pressurized for 45 min at ?18°C ～–20°C and then thawed at 20°C. When carrots were frozen at 100Mpa (ice I), firmness decreased and strain increased. Textural values of carrots pressurized at 200MPa (liquid), 340MPa (ice III), 400MPa (ice V) at ca. –20°C were acceptable. When pressure was increased above 500MPa, the strain increased. Release of pectin and histological damage in carrots pressurized at 200, 340 and 400MPa were less than carrots frozen at 100 and 700MPa (ice VI). After pressurization at 200 and 340MPa at —20°C, carrots were stored in a freezer (–30°C). Firmness decreased and strain increased, but textural values were higher and histological structure were more intact than those frozen at –30°C (0.1MPa) then stored. Thus, high-pressure-freezing at 200, 340 and 400MPa appeared to be effective in improving both the texture and histological structure of frozen carrots.     

EFFECT OF BLANCHING AND SULFITE TREATMENT ON THE QUALITY OF FROZEN CAULIFLOWER

The influence of blanching time and post-blanching sulfite treatment on the sensory quality and texture of frozen cauliflower were assessed after storage at -18°C for up to one year. The treated cauliflower florets, sealed in polyethylene bags, were placed in waxed paperboard cartons and frozen in a contact plate freezer at -35°C. Samples blanched for 3 min and dipped in a solution containing 1000ppm of SO₂ for 5 min gave a significantly (p<0.05) superior product even when stored for one year. The residual SO₂ content of 50 ppm found in these stored samples disappeared after a 3 min cooking in boiling water. Cauliflower texture was influenced by blanching time but the textural differences of blanched samples diminished following freezing and storage. After a 3 min cooking, the texture of all thawed samples were comparable to that of fresh cauliflower cooked for 10–12 min.     

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     

Effects of Processing Treatments on Recovery of Capsaicin in Jalapeno Peppers

Capsaicin content in frozen, cooked and canned jalapeno peppers was quantified using GLC analysis and compared to the capsaicin content in raw peper. The frozen peppers were blanched for 3 min and stored at -18°C; canned peppers were blanched for 3 min and processed at 100°C for 50 min; cooked peppers boiled at 100°C for 10 min. Results demonstrated a significant difference (a = 0.05) between each treatment and the raw pepper. Frozen and canned peppers retained approximately one-half of the capsaicinoid compounds that were present in raw pepper.     

Texture and Rehydration of Dehydrated Carrots as Affected by Low Temperature Blanching

Whole carrots were blanched at four temperatures for five time periods, then blanched again for 6 min at 100°C. A control sample was blanched 8 min at 100°C. All samples were then dehydrated. Very slight differences in rehydration ratios between treatments were observed. The 50°C blanch gave a firmness equal to or less than the control for all blanch times. The carrots blanched at 55°C for 15, 30 and 45 min were less firm than the control while the 60 and 90 min blanched samples were firmer than the control. The 60 and 65°C blanched samples had significantly firmer texture than the control when blanch time was > 30 min. Blanching carrots for 45 min at 65°C increased firmness of the rehydrated product by 51% for uncooked and 27% for cooked.     

Impact of frozen storage on polyacetylene content, texture and colour in carrots disks

This study investigated the effect of freezing method (slow or blast freezing) with or without blanching during storage at −20 °C on the levels of three polyacetylenes, falcarinol (FaOH), falcarindiol (FaDOH), falcarindiol-3-acetate (FaDOAc) in carrot disks. The quality of the carrot disks was also assessed using instrumental texture and colour measurements. Blast frozen carrot disks retained higher amounts of polyacetylenes compared to their slow frozen counterparts. Whilst the levels of retention of total polyacetylenes was higher in unblanched than blanched disks prior to freezing there was a sharp decrease in the levels of polyacetylenes in unblanched frozen carrots during the storage period for 60 days at −20 °C. FaDOH was observed to be the most susceptible to degradation during frozen storage of unblanched carrot disks, followed by FaOH and FaDOAc. The changes in the level of polyacetylenes during storage were adequately described by using Weibull model. The texture and colour were also found to decrease during frozen storage compared to fresh carrots.     

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.     

Structural and Textural Quality of Kinu-Tofu Frozen-then-Thawed at High-Pressure

To determine effects of high-pressure thawing on quality of high-pressure frozen tofu, kinu-tofu (soybean curd) was frozen 90 min at ca ?20°C at 100 MPa (ice I), 200 MPa (liquid phase), 340 MPa (ice III), 400, 500 or 600 MPa (ice V), then thawed at the same pressure. Texture and structure of this tofu (D) were compared with high-pressure-frozen tofu thawed at atmospheric pressure (A: 90 min frozen; B: 90 min frozen then 2 days at ?30°C; C: 160 min frozen). When tofu was frozen at 200- 500 MPa, ice crystals were largest to smallest in B > A and C > D; pore size of D was the same as untreated tofu. Results indicated ice crystals never grew when frozen at 200–500 MPa. Growth occurred during reduction of pressure at ca ?20°C, frozen storage or while thawing at atmospheric pressure due to phase transition.     

Histological Changes in High-Pressure-Frozen Carrots

Histological changes in carrots frozen using a computer-programmed high pressure pilot unit for food processing were examined by light microscope. When raw carrots were frozen at 50 MPa, – 15°C; 100 MPa, –15°C; 150 MPa, –25°C; 200 MPa, –28°C, they were extremely damaged due to volume expansion by the formation of ice I. Conversely, carrots pressurized at 100 MPa, – 10°C (between liquid phase and ice I) and 200 MPa at –20°C (liquid phase) were not damaged because they were frozen rapidly during pressure reduction. They were not damaged even after pressurizing-then-immersing in LN₂. Carrots frozen at 240 MPa, –28°C and at 280 MPa, –25°C, were also not damaged, although ice III formed. The structure of carrots frozen at 400 MPa, –20°C (ice V) was comparatively intact. When carrots were preheated at 60°C for 30 min and frozen at 100 MPa, –15°C or at 400 MPa, –20°C, damage was reduced further.     

Chinese Cabbage Midribs and Leaves Physical Changes as Related to Freeze-processing

Differences were investigated in texture, drip and pectic composition of raw or blanched Chinese cabbage midribs and leaves after conventional (F) or program freezing (PF). Softening of tissues and amount of drip were least to greatest: PF-5°C/min < PF-2°C/min < F-35°C < F-20°C, respectively. Freezing-thawing accelerated release of pectin but the freezing rate did not affect pectin release much. Total pectin in raw midribs was less than in raw leaves. The leaves contained more low methoxyl pectin than midribs, but the high methoxyl pectin was almost the same. Cell damage in frozen-thawed midribs and leaves by light-and electron-microscopy appeared extensive.     

Effect of modified starch concentration and freezing and thawing rates on properties of mashed potatoes (cv. Kennebec)

Effects of modified cornstarch concentration, freezing rate and thawing mode on quality properties of mashed potatoes were examined. The product was tested by oscillatory rheometry, instrumental texture profile analysis (ITPA) and cone penetration, colour, dry matter (DM) and sensory analyses. Oscillatory parameters showed that increasing starch concentration resulted in a softer product in which gel strength decreased in direct proportion to concentration in quick‐frozen and microwave‐thawed product. Quick freezing made for firmer mashed potatoes, whereas oscillatory, ITPA and penetration parameters were lower in the samples thawed at 4 °C. Starch concentration and instrumental consistency and firmness were directly proportional to one another. Adding starch at the highest concentration, quick freezing and slow thawing produced a lighter‐coloured mashed potato, although the loss of colour in the processed product was not detected by panellists in the microwave‐thawed samples. DM was higher in the quick‐frozen samples and lower in air‐thawed samples. The factors studied affected different sensory texture parameters but had hardly any effect on attributes perceived during final and residual phases of mastication. Only starch concentration had a significant effect on sensory acceptability, but interactions between factors showed that starch addition followed by quick freezing was judged undesirable by the panellists due to excessive softening. Oscillatory parameters correlated well with sensory texture attributes and proved quite precise. They would therefore appear to be the best test to monitor properties of frozen mashed potatoes. Copyright © 2007 Society of Chemical Industry     

The Effect of Endogenous Pectinases on the Consistency of Tomato–Carrot Purée Mixes

Tomato and carrot were subjected to a split-stream process designed to produce a tomato–carrot suspension with reduced consistency. Raw tomatoes, containing pectinmethylesterase and endo-polygalacturonase, were mixed with thermally pretreated (blanched versus cooked) carrots containing different levels of solubilized pectin. After mixing the vegetables, tomato pectinases were shown to act on both tomato and carrot pectin in case an incubation step at medium temperature level (30 min, 40 °C), to allow enzyme action, was performed. Carrot pectin, when present in a mix of tomato and blanched (5 min, 95 °C) carrot, was solubilized as well as depolymerized, whereas depolymerization of the thermo-solubilized carrot pectin by the tomato pectinases was observed in the tomato–carrot purée containing cooked (30 min, 95 °C) carrots. The final serum pectin properties were however similar for both purée types. Carrot contributed more to the consistency of the purée mix compared with tomato but by stimulating the action of the tomato pectinases at mild temperature (30 min, 40 °C), this contribution was lost which resulted in a consistency reduction of the purée mix. This purée liquefaction was larger for the tomato–carrot purée containing blanched instead of cooked carrots. Based on the results, it is suggested that the liquefying effect is related to solubilization and degradation of pectin that is counteracted by a reduction in particle size. The purée mix containing cooked carrot showed in this respect smaller particle sizes than the mix containing blanched carrot.     

Effect of freezing rate and programmed freezing on rheological parameters and tissue structure of potato (Cv. Monalisa)

 Compression, shear and tension tests were carried out to determine the effect on potato tissues of different freezing rates (0.5, 1.25 and 2 °C/min down to −18 °C), thawing up to +20 °C at the same rates, and one, two, three or four successive freeze/thaw cycles. The effect of freezing rate on the zone of maximum crystallization was also examined, along with different combinations of programmed freezing, and the effect of prior cooling was assessed. Minimum alteration of the rheological behaviour of slowly thawed tissues was achieved by pre-freezing (3 °C for 30 min), slow cooling phases (0.5 °C/min) before and after the phase of maximum ice crystallization and quick freezing (2 °C/min) in the same phase. The shear test was found to be well suited to the study of these effects. Examination of the tissues by SEM revealed differing degrees of mechanical damage to tissue structure, which accounted for the rheological behaviour of the samples. Coefficients of softening per freeze/thaw cycle were determined for the various rheological parameters, the highest value being given by the modulus of rigidity (17.75%). Received: 29 July 1996     

Autoxidation of Bluefin Tuna Myoglobin Associated with Freezing and Thawing

Bluefin tuna oxymyoglobin (oxyMb) solutions were frozen either quickly at ?80°C or slowly at ?2O°C, thawed, and the metmyoglobin (metMb) to total myoglobin (Mb) ratio (metMb%) determined. When quick freezing was applied, autoxidation was clearly pH-dependent, with a minimum (metMb%, 15–40) at around pH 5.9–6.3. When slowly frozen and thawed, on the other hand, tuna Mb generally showed an extremely high metMb%. Freezing and thawing partially insolubilized tuna Mb, depending upon pH. The Mb showed the minimum ratio of insolubilization at pH 6–6.3 again. Insolubilization of tuna Mb was markedly accelerated by NaCl.     

Bioactivity of falcarinol and the influenceof processing and storage on its content in carrots (Daucus carota L)

The concentration‐dependent activity of the polyacetylene falcarinol ((9Z)‐heptadeca‐1,9‐dien‐4,6‐diyn‐3‐ol), isolated from carrots, was investigated in a bioassay with primary mammary epithelial cells in collagen gels and compared with that of β‐carotene, the orange pigment in carrots. Falcarinol showed biphasic activity, having stimulatory effects between 0.01 and 0.05 µg ml^?1 and inhibitory effects between 1 and 10 µg ml^?1, whereas β‐carotene showed no effect in the concentration range 0.001–100 µg ml^?1. The results are discussed in relation to the health‐promoting effects of carrots and related vegetables. Falcarinol was quantified in the carrot cultivars Bolero, Rodelika and Fancy by analytical reverse phase HPLC, subjected to various processing and storage conditions in order to study how long‐term storage, blanching, freezing and boiling influence the content of falcarinol. Long‐term storage of raw carrot cubes (1 cm³) reduced the falcarinol content by almost 35%. A similar reduction was found in steam‐blanched carrot cubes (1 cm³). Long‐term storage at ?24 °C of steam blanched carrot cubes did not reduce the falcarinol content further. A reduction of almost 70% in the falcarinol content was found in carrot pieces boiled in water for 12 min compared with raw carrots. Copyright © 2003 Society of Chemical Industry     

The effect of freezing compared with chilling on selected physico-chemical and sensory properties of sous vide cooked carrots

Sous vide is increasingly used to process convenience foods including ready-meals as it is reputed to give superior quality because of the mild process and the absence of oxygen in the pack. The potential for temperature abuse is greater for sous vide ready-meals than for sous vide catered items as the distribution and retailing chain for the former is usually longer. Freezing sous vide foods is, therefore, a potentially safer alternative to sous vide processed foods followed by chilling. The objective of the current study was to investigate production of sous vide frozen sliced (discs) carrots, with similar quality to sous vide chilled product. Sensory trials were used to determine the acceptable shear texture range (1.0–2.8 kN) for steamed carrots and this was the target texture in the processing trials. A Barriquand Steriflow retort was used for sous vide processing and the optimized conditions using carrot discs were: low temperature blanch (50 °C/30 min; firms product); blanch (90 °C/3 min; inactivates enzymes); sous vide cook (90 °C to core P₉₀¹⁰ values > 4 min and < 6 min); blast freeze (? 35 °C/2 h); and store (? 25 °C). Carrot discs prepared from untreated raw carrots had a firmer texture than those prepared from either blanched or blanched and frozen material. Freezing post-sous vide cooking softened texture in comparison with chilling, but freezing rate (nitrogen; blast; cabinet methods) and frozen storage (9 months) had a minimal effect on product texture and quality. Scanning electron microscopy showed no significant structural differences between sous vide frozen and chilled carrots of similar shear values. Paired comparison sensory testing of sous vide frozen and chilled sliced carrots indicated a significant preference for the latter at the 20-day storage date.Industrial relevance: The trials dealt with the effects of freezing on the quality of sous vide processed carrots. This is of relevance to companies who wish to produce texturally acceptable sous vide frozen or chilled components as part of ready-meals.     

Effect of freezing on theβ-hydroxyacyl-CoA-dehydrogenase (HADH) activity of fish meat

Changes in the β-hydroxyacyl-CoA-dehydrogenase (HADH) activity of squid (Loligo vulgaris), mackerel (Scomber scombrus), tuna (Thunnus alalunga), sea bream (Pagellus centrodontus), sole (Solea solea), hake and small hake (Merluccius merluccius) meat due to freezing treatment at ?10° C, ?18° C, ?35° C, ?80° C or ?196° C were investigated. With the exception of the small hake, the HADH activity of aqueous extracts from meat was significantly higher (p<0.0001) in all frozen/thawed fish species studied than in unfrozen animals because during freezing there was a release of HADH. HADH activity values of frozen/thawed squid, unfrozen mackerel, frozen/thawed and unfrozen sea bream and unfrozen hake were affected by the storage time in crushed ice.     

Impact of pretreatment and freezing conditions on the microstructure of frozen carrots: Quantification and relation to texture loss

The textural quality of carrots subjected to pretreatments affecting the pectin structure in combination with different freezing conditions was studied. Carrot samples frozen under different conditions were extensively studied by light microscopy quantifying the freezing damage based on the analysis of different parameters (number, area, perimeter, and shape factor of tissue particles) associated with carrot tissue damage. The reduced texture loss of rapidly or cryogenically frozen carrots, compared to slowly frozen samples, was associated with the reduction in cell wall damage in the carrot tissue. In case no pretreatment was used, carrot texture was only slightly improved by using high-pressure shift freezing instead of slow freezing. Detailed analysis of the different steps involved showed that severe tissue damage occurred during the completion of the high-pressure freezing process at atmospheric pressure. However, tissue damage, and thus texture loss, of high-pressure frozen carrots could be minimized by applying pretreatments consisting of a thermal treatment at 60 °C and a high-pressure treatment at 300 MPa and 60 °C.