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.     

Influence of Pretreatment Conditions on the Texture and Cell Wall Components of Carrots During Thermal Processing

ABSTRACT: Diced carrots ( Daucus carota var. Nerac) were subjected to different pretreatment conditions. The pretreated carrots were subsequently thermally processed in an oil bath (100°C) and in a static retort (equivalent processes [F_oΔ 6 min] at 115°C, 120°C, and 125°C). Changes in texture were analyzed as well as changes in the degree of methylation (DM) of pectin. From all the pretreatment conditions tested, high-pressure pretreated carrots (400 MPa, 60°C for 15 min) exhibited the highest resistance to texture loss. The textural properties were significantly improved when calcium infusion was combined with low-temperature blanching condition (60°C for 40 min). A significant reduction in the DM of carrot pectin was observed for all pretreatment conditions that resulted in a reduced texture loss after thermal processing. A strong negative correlation (r ≥−0.90) exists between the changes in the degree of methylation of carrot pectin and the observed changes in texture.     

Effects of High-Pressure Pretreatment and Calcium Soaking on the Texture Degradation Kinetics of Carrots during Thermal Processing

ABSTRACT: Carrots ( Daucus carota ) pretreated under different high-pressure conditions were thermally processed at temperatures in the range of 90°C to 110°C. Texture degradation (hardness) was monitored objectively using a texture analyzer. For a given thermal treatment, the rate constant (k-value) decreased with increasing pretreatment pressure. A high-pressure pretreatment (200 to 500 MPa) at 60°C for 15 min resulted in a more pronounced texture improvement compared with the same pretreatment at 20°C and 40°C, respectively. Calcium impregnation conferred more beneficial effects when applied immediately after the high-pressure pretreat-ment. The observed changes in texture characteristics were associated with the degree of methylation of carrot pectins, which is dependent on pectinmethylesterase (PME) activity.     

Effect of high-pressure induced ice I/ice III-transition on the texture and microstructure of fresh and pretreated carrots and strawberries

The effect of pressure treatments at −25 °C between 150 and 300 MPa, indicated as high-pressure induced crystallization (HPIC) processes if formation of ice III occurs during pressurization, on the texture and structure of frozen strawberries and carrots were studied. The formation of ice III, which has been proven to inactivate the microbial load of a frozen food, occurred when pressure was increased to 250 MPa or higher. Volume changes related to the formation of ice III affected the cell wall integrity of infused frozen strawberries and caused a 42–46% reduction of the fruit’s hardness. These textural and structural changes were not affected by the pressure holding time (30 s versus 10 min), and thus by partial thawing during the pressure holding time, and were absent in frozen fruits treated at pressures lower than 250 MPa. The structure and texture of frozen carrots were respectively not and only slightly altered during high-pressure–low-temperature (HP–LT) treatments at all pressure levels studied. However, if carrots were blanched (30 min at 60 °C, 2 min at 90 °C and a combination of both) prior to freezing, structural damages during pretreatment and freezing made the tissue, in terms of both structural and textural quality, unsuitable for a post-freezing HP–LT treatment. These observations should be taken in mind when analyzing the possibilities of HPIC processes as a tool for post-freezing microbial reduction when applied to tissue based systems.     

Textural and Ultrastructural Changes in Carrot Tissue as Affected by Blanching and Freezing

ABSTRACT: The impact of blanching and freezing conditions on firmness retention and ultrastructural changes in the cell wall and middle lamella of carrot tissues were studied. Freezing caused extensive degradation of cell wall pectins as evident from the rapid loss in tissue firmness. High-temperature short-time blanching (100°C, 0.58 min.; 90°C, 2.12 min.) retained firmer texture than low temperature long time blanching (80°C, 11.64 min.; 70°C, 71.1 min.). Freezing at rapid rates of -4.5°C/min and -2.4°C/min showed less softening than slow rates of -0.19°C/min and -0.05°C/min. Softening was further enhanced in blanched-frozen carrots. Severe structural damage due to growing ice crystals and substantial loss of pectic material were seen at slower freezing rates.     

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.     

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.     

Effect of temperature, pressure and calcium soaking pre-treatments and pressure shift freezing on the texture and texture evolution of frozen green bell peppers (Capsicum annuum)

The firmness of green bell pepper (Capsicum annuum) was studied under different processing conditions. Thermal texture degradation kinetics of pepper tissue between 75 and 95 °C could be accurately described by a fractional conversion model. The firmness of pre-processed pepper increased when the samples were submitted to several heat, pressure, and combinations of heat/pressure and calcium soaking pre-treatments. Pre-heating at 55 °C during 60 min and mild heat/high-pressure treatments (200 MPa at 25 °C, 15 min) yielded the best results, which were further improved when combined with calcium soaking. These pre-treatments significantly slowed down thermal texture degradation of pepper at 90 °C, a typical temperature used for pepper blanching prior to freezing. The above-mentioned pre-treated samples showed a significant reduction in firmness when frozen by regular freezing at 0.1 MPa. The same samples showed no changes in firmness when frozen by high-pressure shift freezing at 200 MPa. When freezing was carried out by high-pressure shift and after frozen storage (−18 °C) for 2.5 months, pressure pre-treated pepper showed a better retention of texture than thermal pre-treated pepper.     

Minimizing texture loss of frozen strawberries: effect of infusion with pectinmethylesterase and calcium combined with different freezing conditions and effect of subsequent storage/thawing conditions

Vacuum infusion (VI), freezing, frozen storage and thawing conditions were optimized in order to minimize the texture loss of frozen strawberries. Slow freezing caused severe loss in textural quality of the strawberries. This quality loss could not be prevented by the application of VI prior to slow freezing, or by the application of rapid, cryogenic or high-pressure shift freezing conditions on non-infused fruits. A remarkable texture improvement was noticed when infusion of pectinmethylesterase (PME) and calcium was combined with rapid or cryogenic freezing. The highly beneficial effect of PME/Ca-infusion followed by HPSF on the hardness retention of frozen strawberries was ascribed to the combined effect of the infused PME (53% reduction in degree of esterification (DE) of the strawberry pectin) and the high degree of supercooling during HPSF. During frozen storage, textural quality of PME/Ca-infused high-pressure frozen strawberries was maintained at temperatures below −8 °C, whereas the texture of PME/Ca-infused strawberries frozen under cryogenic freezing conditions was only preserved at temperatures below −18 °C. Thawing at room temperature seemed to be an appropriate method to thaw strawberries. Fast thawing by high-pressure induced thawing (HPIT) did not prevent textural quality loss of frozenstrawberries.     

Programmed Freezing Affects Texture, Pectic Composition and Electron Microscopic Structures of Carrots

Raw and blanched carrots (3 min, boiling water) were frozen at ?2°C, ?3°C, ?4°C or ?5°C/min (final ?20°C or ?50°C) then thawed at 20°C or 100°C. Firmness of thawed raw carrots was: ?5°C > ?4°C > ?3°C > ?2°C/min. Effect of freezing rate on blanched carrots was less than that on raw carrots, but firmness of thawed carrots was not affected by final temperature of freezing. When raw carrots were thawed at 20°C, high methoxyl pectin decreased. Pectin decrease in blanched carrots caused by freezing was greater than that in frozen raw carrots. Effects of slow-freezing, programmed-freezing (slow + quick + slow) and quick-freezing showed quick freezing (—5°C/min) best for texture. As freezing rate decreased, drip increased. A wide difference among experimental samples in fine structure was revealed by cryo-scanning electron microscopy.     

HISTOLOGICAL AND PHYSICAL CHANGES IN CARROTS AS AFFECTED BY BLANCHING, COOKING, FREEZING, FREEZE DRYING AND COMPRESSION

SUMMARY— The effect of processing variables on the cell structure and physical characteristics of carrots were determined. The phloem portion of fresh carrots was subjected to one of the following treatments: blanching; cooking for 10 min; freezing at 0°F, −30°F or −320°F; freeze drying, compressing after freeze drying at approximately 1500 psi. Carrots at each treatment were tested for: (1) texture by means of the Ailo-Kramer Shear Press; (2) water holding capacity by centrifuging at 500, 1000, 1500, 2000 and 2500 rpm; (3) histological changes by microscope observation of the tissue structure. Results indicate that among all treatments, freezing temperature is the most critical factor affecting the cell structure of the carrots. Freezing at 0°F or −30°F results in considerable disruption of the cellular structure, whereas it was minimal at −320°F. Carrots frozen at −320°F showed firmer texture as well as higher water holding capacity than the rest. Significant correlation coefficient was established between the shear press values and percent weight loss measured by centrifugation. This suggests that the latter may be used as an objective test for measuring textural changes in processed carrots and perhaps other foods.     

Changes in Temperature, Texture, and Structure of Konnyaku (Konjac Glucomannan Gel) During High-pressure-freezing

To determine the effects of high-pressure-freezing, changes in temperature, texture, and structure of konnyaku (a gel with high water content) were measured during freezing for 60 min at 0.1 to 700 MPa and -20 °C. During freezing at 0.1, 100, 500, 600, and 700 MPa, exothermic peaks were detected (konnyaku froze). However, at 200 to 400 MPa, exothermic peak was not detected and temperature rose when pressure was released at -20 °C; the supercooled konnyaku froze by pressure-shift-freezing. The coarse gel network observed in unfrozen konnyaku was compressed by freezing due to formation of ice crystals. The rupture stress increased and strain decreased in all frozen konnyaku. High-pressure-freezing was ineffective in improving the texture of frozen-then-thawed konnyaku.     

Changes in Temperature and Structure of Agar Gel as Affected by Sucrose During High-pressure Freezing

ABSTRACT: To determine the effects of high-pressure freezing, agar gel with 0, 5, 10, or 20% sucrose were frozen at 0.1 to about 686 MPa and -20 °C. Exothermic peaks were detected at 0.1, 100, 500 to about 686 MPa (freezing). However, at approximately 200 to 400 MPa, gel did not freeze but froze during pressure release. Thus, structure of gel frozen at approximately 200 to 400 MPa was better than other samples due to quick freezing. The phase transition from high-pressure-ices to ice I at -20 °C might have promoted the growth of ice crystals. With the addition of sucrose, the initial freezing temperature decreased and structural quality improved. Keywords: high pressure, agar gel, freezing, texture, ice crystals     

Changes in texture,microstructure and nutritional quality of carrot slices during blanching and freezing

Thermal processing of vegetables has pronounced effects on the cell structure, often lowering the final textural properties of the product. In order to investigate the effect of thermal processing on carrot, slices were subjected to different blanching and freezing treatments before frozen storage. Microwave-, steam- or water-blanched material was frozen and then stored at −24 °C. Steam-blanched carrots were subjected to blast freezing or cryogenic freezing at different temperatures before frozen storage. The influence of these process conditions on the texture (maximum load and slope), microstructure, dry matter, sugars, carotene and drip loss was investigated. Microwave blanching differed from the other blanching methods by resulting in a heterogenic cell structure. The content of dry matter, carotene and sucrose was higher following microwave blanching. Blast freezing resulted in low maximum load which seemed to be caused by major tissue damage. Concerning cryogenic freezing, lowering the temperature from −30 °C to −70 °C resulted in better preservation of the native microstructure together with an increase in maximum load, which was most pronounced after one month of storage. No significant effect was observed when lowering the temperature from −30 °C to −70 °C for any of the other measured parameters. © 1999 Society of Chemical Industry     

Effect of Novel Processing Techniques on Texture Softening and β-Carotene Content of Thermally Processed Carrots

Effect of novel processing methods was evaluated on product texture and β-carotene content of carrots following acidification to reduce pH from 6.0 to 4.4. Thermal treatments under Conventional (CH-T) and Ohmic heating (OH-T) conditions at 87, 92, and 97 °C, individually and/or in combination with high-pressure processing (HP-T; 400–600 MPa/40–60 °C), were given up to 90 min. A fractional conversion model was used to compute texture softening rate constant, k, and activation energy, E _a. Acid-infused carrot samples had lower k values than the control, implying a better texture retention in acidified products. In order to explore this further, acid-infused and control samples were subjected to selected processing methods for 0, 7, and 25 min representing minimal, optimum, and over-processing conditions, respectively. Texture value, pectin depolymerization by β-elimination, demethoxylation, cell microstructure modification, and β-carotene content were evaluated. Results showed that acid-infused samples retained significantly (p?≤?0.05) better texture than the untreated ones. Pectin depolymerization by β-elimination was greater (p?≤?0.05) in control samples than acid-infused samples. In contrast, pectin depolymerization by demethoxylation showed no such differences (p?>?0.05) with acid-infused samples. This indicates that pectin degradation was more dominated by β-elimination than demethoxylation, and these results concurred with the cell microstructure observations of processed carrots. Thermal and HP-T processing after acid infusion reduced the β-carotene content of carrots more than in control. However, mild heat treatment of carrots at 97 °C under CH-T and OH-T enhanced the β-carotene levels to higher than in raw control carrot samples.     

Non-enzymatic Depolymerization of Carrot Pectin: Toward a Better Understanding of Carrot Texture During Thermal Processing

ABSTRACT Pretreated carrot discs were thermally processed (90 °C to 110 °C) in closed containers and the resulting textural characteristics were analyzed. The pretreatment conditions used include conventional high‐temperature blanching (90 °C, 4 min), low‐temperature blanching (LTB = 60 °C, 40 min), LTB combined with 0.5% calcium chloride soaking, LTB combined with 2% sodium chloride soaking, high pressure pretreatment (HP = 400 MPa, 60 °C, 15 min), HP combined with 0.5% calcium chloride soaking, and control (non‐pretreated sample). Alcohol insoluble residues (AIR) from the pretreated carrot discs were characterized in terms of degree of methoxylation (DM). The AIR samples were further subjected to fractionation into water‐soluble pectin (WSP), chelator‐soluble pectin (CSP), and sodium carbonate‐soluble pectin (NSP). Heat depolymerization patterns and β‐elimination kinetics were investigated on the different pectin fractions. Thermal texture degradation was strongly influenced by the pretreatment condition used and the processing temperature during subsequent thermal treatment. Pretreatment conditions that showed a significant reduction in DM exhibited decreased WSP content, reduced β‐elimination, and consequently superior textural characteristics. β‐elimination was markedly pronounced in the highly methoxylated WSP fractions. CSP and NSP fractions were insensitive to β‐elimination. A strong correlation (r> 0.95) between thermal texture loss of carrots and β‐elimination kinetics exists. Overall, the benefits of controlled pectinmethylesterase activity in carrot processing were pointed out.     

ULTRASTRUCTURAL AND TEXTURAL CHANGES IN PROCESSED CARROT TISSUE1

Processed carrot tissues were studied using a transmission electron microscope (TEM) and an Instron Universal Testing Instrument. Carrot slices were evaluated raw, after thermal processing and after freezing and thawing. Cell walls of carrot tissues, frozen and thawed, were intact and dense. The middle lamella lost pectic materials. Overall, frozen and thawed cells were not so closely pressed as those of raw tissues. In thermally processed carrots the cell wall and the middle lamella were less densely stained compared to raw or frozen-thawed carrots. Thermally processed carrots exhibited significantly smaller modulus of deformability and stiffness and toughness values than raw carrots. There were no significant differences between the modulus of deformability and stiffness values for frozen-thawed and thermally processed carrots. Toughness values were, however, significantly lower for the thermally processed in comparison with the frozen-thawed carrots. Instrumental texture measurements indicated that different areas of carrot tissue behave differently to thermal processing.     

High-Pressure and Freezing Pretreatment Effects on Drying, Rehydration, Texture and Color of Green Beans, Carrots and Potatoes

High-pressure pretreated and frozen green beans, carrot dice or potato cubes were fluidized bed dried and compared to untreated, pressure-treated or water-blanched dried samples. Drying rates varied with pre-treatments. Freezing resulted in highest drying rates. Pressure-treated and water-blanched samples retained highly acceptable colors. Freezing or hot-water blanching or high-pressure pretreatment, followed by freezing, gave good rehydration. High-pressure treatment resulted in incomplete rehydration but combined with freezing, water uptake was between 2.1 and 4.8 mL/g. Retention of cell wall structures of frozen samples during drying was presumed responsible for more efficient mass transfer. Texture measurements revealed significant effects of pretreatments. Pressure-treated samples had texture nearest that of the raw material. No major differences in color were observed.     

Comparison of air freezing,liquid immersion freezing and pressure shift freezing on freezing time and quality of snakehead (Channa Argus) fillets

The objective of this study was to investigate the freezing time and quality differences in Snakehead fillets frozen by pressure shift freezing (PSF), conventional air freezing (AF) and liquid immersion freezing (LIF) at −20 °C, −40 °C and − 60 °C, respectively. The results showed that liquid immersion freezing at −60 °C maintained the quality best, with a freezing time of 3.62 min and the cross sectional area of 209.11 um². Air freezing at −20 °C had the longest freezing time (184.58 min) and the largest cross sectional area (4470.79 um²), and lowest hardness and springiness of the fillets. Pressure shift freezing did not demonstrate the well established advantages of maintaining better product quality found in similar technique with some other foods. The samples of pressure shift freezing also had higher thawing loss and free water ratio after thawing. Therefore, the liquid immersion freezing at lower temperatures was demonstrated to better maintain the quality of frozen products and held significant potential for commercial application.Industrial relevanceFreezing is a widely used method for extending the shelf life of aquatic products, but some freezing methods, especially the slower ones, often lead to the decrease in the quality and commercial value of frozen products during storage. This paper explored the comparison of industrially used freezing techniques (air freezing and liquid immersion freezing) with the novel pressure shift freezing technique. Liquid immersion freezing at −60 °C was found to be the preferred freezing method for Snakehead fillets, which maintained better frozen product quality, with a simple freezing process and low cost.     

The Effect of Brine Ingredients on Carrot Texture during Thermal Processing in Relation to Pectin Depolymerization due to the β-Elimination Reaction

ABSTRACT:  Thermal texture degradation of carrots was studied at a temperature of 100 °C in aqueous solutions containing sodium chloride, citric acid, ascorbic acid, and ethylenediaminetetraacetic acid (EDTA) at different concentrations. To enhance the texture of the final product, the carrot samples were pretreated at 65 °C for 30 min in an aqueous calcium chloride solution (5 g/L). For all case studies considered, the pH of the solutions was adjusted to pH = 6.0. In parallel, both the changes in degree of esterification (DE) and the progress of the β-elimination reaction of carrot pectin under the same conditions were investigated. The kinetic parameters for texture degradation (rate constant k_t and final texture value [TP_∞/TP₀]) were estimated using a fractional conversion model. The results indicate that both the rate constant for texture degradation ( k_t ) and the rate constant for the β-elimination reaction ( k_b ) increased with increasing additive concentration, while the final texture values (TP_∞/TP₀) and DE decreased with increasing additive concentration in all systems studied. A high correlation was observed between the relative rate constant for texture degradation and the relative rate constant for the β-elimination reaction on the one hand, and the relative final texture value and the relative rate constant for the β-elimination reaction on the other hand, suggesting that the influence of the solutes on texture degradation can be explained by their influence on the β-elimination reaction.