Optimisation of freezing process with pressure steaming of potato tissues (cv Monalisa)

Response surface methodology (RSM) was used for determining optimum conditions of the freezing process on pectinesterase (PE) activity, rheological parameters and textural properties in potato tissues. In the process of production of frozen potatoes, the second step of the stepwise blanching prior to freezing was considered as a fixed factor and performed at 97 °C for 2 min as well as the freezing rate in the freezing step itself, which was carried out at −2 °C min⁻¹. The effects of variation in levels of temperature (57.93–72.07 °C) and time (15.86–44.14 min) in the first blanching step on the PE activity were studied using a central composite rotatable design. A Box–Behnken factorial design was used to investigate the effects of simultaneous variation of temperature (60–70 °C) and time (20–40 min) in the first blanching step and steaming temperature (112–122 °C) and time (1–3 min) on rheological parameters and textural properties. Blanching temperature was the independent variable that most influenced either enzymatic activity or rheological parameters. Stationary points showing maximum PE activity had critical temperature and time values of 64.22 °C and 29.37 min before freezing and 64.39 °C and 28.02 min after freezing and steaming of the tissues, and these values were very close to those obtained for some creep compliance parameters. Results show a high correlation between increases in PE activity and tissue firmness below optimum experimental freezing conditions, proving the role of the enzyme as one of the main contributors to the firmness which determines the textural quality of frozen potato tissues. © 1999 Society of Chemical Industry     

Effect of rice ageing and freeze‐thaw cycle on textural properties of cooked rice (Oryza sativa L.) cv. Khao Dawk Mali 105

This study investigated the effect of rice grain ageing (0.7–12 months) and freeze‐thaw 1–5 cycles on the textural properties of cooked rice. The cooked rice from aged rice grains was freezing and thawing up to five cycles. Ageing of the rice grain increased the hardness and decreased the stickiness of the cooked rice. Repeated freeze‐thaw cycles caused an increase in hardness and a decrease in stickiness of aged cooked rice. Scanning electron micrographs showed a rough surface on the cooked rice after repeated freeze‐thaw cycles, especially for cooked rice from rice aged for 12 months. Differential scanning calorimetry and X‐ray diffraction showed increased starch retrogradation with increased freeze‐thaw cycles. The gelatinisation temperature and gelatinisation enthalpy increased when rice was aged for longer periods. Thus, ageing of rice and the number of freeze‐thaw cycles influence the textural properties of cooked rice.     

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     

萝卜组织孔隙中气体含量对超声辅助浸渍冷冻冻结效果的影响

为了研究组织内部孔隙中气体含量对超声辅助浸渍冷冻冻结效果的影响,加深对空气对超声衰减所造成的影响的了解。本研究以萝卜组织为食品材料模型,采用真空脱气方式对萝卜组织中气体含量进行调节,得到气体含量分别为9.91%、6.46%、4.01%及0的4种萝卜组织,分别在28 kHz、0.33 W/cm2的超声条件下进行超声辅助浸渍冷冻处理,并与常规浸渍冷冻比较。结果表明：与常规浸渍冷冻相比,4种不同气体含量的萝卜组织经过超声辅助浸渍冷冻处理后冻结速率分别提高了20.00%、27.00%、29.00%和33.00%,相变时间分别缩短了16.00%、24.00%、27.00%和31.00%。随着组织中气体含量的降低,超声辅助浸渍冷冻冻结速率得到显著提高（p<0.05）,样品冷冻解冻后的硬度、微观结构及钙离子含量均得到显著的改善（p<0.05）。研究发现组织内部孔隙中气体含量是降低超声辅助浸渍冷冻冻结效果的一大影响因素,脱气处理可以有效改善含气物料超声辅助浸渍冷冻效果。     

不同加工方式对马铃薯原粉特性的影响

以新鲜马铃薯为原料,制备得到马铃薯熟粉、冷冻干燥马铃薯原粉、未经烫漂的热风干燥马铃薯原粉及经烫漂的热风干燥马铃薯原粉,研究不同类别的马铃薯原粉在外观品质、理化特性、功能特性及复水特性等方面的性质。结果表明,不同加工方式下马铃薯原粉的基本理化性质及外观品质均存在显著差异,其中经烫漂的热风干燥马铃薯原粉色泽(L*值为80.78)最佳,为淡黄色粉末,具有高淀粉、高纤维及低脂肪的特点,有利于用作减肥及降血脂健康食品的原料;功能特性结果表明,马铃薯熟粉具有较高的持水能力(3.03±0.01 g/g)、乳化性(21.58%±1.37%)及乳化稳定性(15.98%±0.73%);冷冻干燥马铃薯原粉具有较高的持油能力(3.42±0.08 g/g)和溶解性(16.49%±1.07%);经烫漂的热风干燥马铃薯原粉具有较高的乳化稳定性(16.67%±0.76%)、复水性(复水比为6.43)和复水稳定性(95.76%±0.85%)。同时,结合考察不同加工方式下马铃薯原粉的加工效率与能耗,结果发现,经烫漂的热风干燥原粉具有较高的加工品质且加工能耗低,可作为熟粉的替代品。本研究可为马铃薯不同产品特点选择合适的加工方式,并为马铃薯原粉的研制及代替熟粉的可行性奠定基础。     

Effects of different pre-freezing blanching procedures on the physicochemical properties of Brassica rapa leaves (Turnip Greens, Grelos)

For optimal freeze storage, green vegetables should first be blanched. The present study compared four different procedures for the blanching of grelos (leaves of Brassica rapa L.): steaming for 2 min, immersion in boiling water for 2 min, immersion in boiling water containing 1% citric acid for 1 min, and immersion in boiling water containing 5% citric acid for 1 min. After blanching, the grelos were stored for up to 120 days at ?18 °C, with sampling at two‐weekly intervals for analysis of physicochemical properties (ash weight, vitamin C content, pH, acid value, moisture content and CIE_L*a*b* colour variables). In almost all respects steam blanching gave the best results: notably, vitamin C losses were markedly lower, while moisture content and colour remained closer to those of the fresh product.     

Effect of Freezing Rate and Microwave Thawing on Texture and Microstructural Properties of Potato (Solanum tuberosum)

Food freezing is a preservation process that works by lowering temperature while simultaneously decreasing water activity. It is accepted that although freezing preserves foods, it generally has a negative effect on textural quality. This research investigated the texture response of potatoes (Solanum tuberosum) as a function of time to freeze (defined as the time for the center temperature to reach ?20 °C) and thawing process. Potatoes slices (6 mm) were blanched then frozen in an ethanol/carbon dioxide bath, a pilot scale high velocity air freezer (HVAF) and a still air freezer to achieve various times to freeze. Slices were stabilized at –20 °C and thawed by 2 methods; room temperature air and microwave. Afterwards, samples were allowed to come to room temperature prior to texture profile analysis (TPA). Results indicate a maximum texture loss of the potato was reached at a time to freeze of approximately 8 min (corresponding to the HVAF). The texture difference between room temperature and microwave thawing methods was not shown to be significant (P = 0.05). SEM images showed the cellular structure of the potato in a HVAF to be similar to that of the still air freezer, validating that the matrix was maximally damaged in both conditions. This work created a continuous quality loss model for the potato as a function of time to freeze and showed no textural benefit to high velocity over still air freezing.     

Evaluation of the amino acid content and sensory value of flageolet bean seeds (Phaseolus vulgaris L.) as affected by preprocessing methods before freezing

The aim of the work was to evaluate the content of amino acids in flageolet bean seeds, prepared for consumption from frozen products obtained using different methods. One of the frozen products was obtained using the traditional method (blanching–freezing–frozen storage) and then cooked. The other was obtained using the modified technology (cooking–freezing–frozen storage), followed by defrosting and heating in a microwave oven. The culinary and technological processing applied brought about a pronounced decrease in tyrosine content (35–47% in 16 g N). The content of the remaining amino acids decreased or increased in relation to the raw material within the extremes of ?15% to +17%. Sulphur amino acid was the limiting amino acid. Product obtained using traditional method had the higher quality protein (EAA index 144–166) of seeds prepared for consumption. In sensory test products, blanched before freezing were evaluated a bit higher than those cooked before freezing (ready‐to‐eat).     

微波熟化结合超低温冷冻预处理降低煎炸薯条油脂吸附的研究（网络首发、推荐阅读）

对比分析不同熟化方式(漂烫熟化、微波熟化)和不同冷冻方式(常规冷冻、超低温冷冻)的结合预处理对煎炸薯条油脂吸附的影响。发现与传统的漂烫熟化结合常规冷冻预处理相比,将漂烫熟化改为微波熟化,煎炸薯条的总油含量下降15.86%;将常规冷冻改为超低温冷冻,总油含量下降6.09%;而微波熟化结合超低温冷冻预处理使总油含量降低20.46%。激光共聚焦显微镜(CLSM)、差示扫描量热仪(DSC)和扫描电镜(SEM)结果证实:微波结合超低温冷冻预处理使煎炸薯条外壳层产生了较多VII型淀粉脂质复合物,外壳表面结构致密光滑、附着的油脂较少,内部结构破损较小。结合感官评价结果,表明微波熟化结合超低温冷冻预处理可以在改善薯条品质的同时降低油脂吸附。     

Freezing of tilapia fillets in an air blast freezer

This paper explores the numerical simulation of freezing tilapia fillets of five different geometries, which are the slab, elliptical, disc, spherical and cylindrical shapes, in an air blast freezer. The air velocities and temperatures inside the freezer are varied in the simulation to determine the freezing times and energy consumptions for freezing the tilapia fillets of different shapes. The results show that the cylindrical-shaped fillet has the longest freezing time and also required the most energy for freezing. The slab-shaped fillet has the shortest freezing time, but the spherical-shaped fillet requires the least energy to freeze. The simulation is also applied to a case study to compare the processing rate, energy consumption and the cost of freezing the fillets of different shapes. The freezing cost for a tray of spherical-shaped fillet is the lowest among all.     

Effects of multiple freeze–thaw cycles on meat quality,nutrients, water distribution and microstructure in bovine rumen smooth muscle

This study investigated the effect of 5 freeze–thaw cycles (freezing at −18°C for 12 h and then thawing at 4°C for approximately 12 h) on the meat quality, proximate composition, water distribution and microstructure of bovine rumen smooth muscle (BSM). As the number of freeze–thaw cycles increased, BSM pH, shear force, water content and protein content decreased by 3.06%, 35.50%, 14.49% and 21.11%, respectively, whereas BSM thawing loss, cooking loss, pressing loss, total aerobic count (TAC), ash content and fat content increased by 108.12%, 47.75%, 78.33%, 90.99%, 105% and 35.20%, respectively. The freeze–thaw cycles resulted in greater protein and lipid oxidation, as evidenced by a 36.46% reduction in the sulfhydryl content and a 209.06% and 338.46% increase in the carbonyl and malondialdehyde contents, respectively. Ice crystal formation disrupted the structural integrity of the muscle tissue. Low-field nuclear magnetic resonance results showed that the freeze–thaw cycles prolonged the relaxation times (T_2b, T₂₁ and T₂₂), indicating that immobile water shifted to free water, and consequently, free water mobility increased. After 3 freeze–thaw cycles, the decline in shear force slowed, the increase in thawing loss became accelerated, and the TAC approached the domain value (6 log colony-forming units/g). Therefore, the number of freeze–thaw cycles of smooth muscle during transport, storage and distribution should be controlled to 3 or fewer. The current results provide a theoretical basis and data support for the further utilisation and culinary processing of smooth muscle.     

Effect of different technological and culinary treatments on iron retention,nutritional density and recommended dietary intake in fourteen vegetable species

The present work determined iron content in fourteen species of vegetable prepared for consumption. The experimental material consisted of raw and boiled raw vegetables and two types of frozen product: one traditionally produced (blanching before freezing); the other obtained using the modified method (boiling before freezing), having the characteristics of a ready‐to‐eat convenience product. Nutrient density (ND) and recommended dietary intake (RDI) were established on the basis of iron content. The highest iron content was found in pea seeds (2.03 mg per 100 g fresh weight) and the lowest in root vegetables (0.38–0.60 mg). Iron retention was similar in all pea, broad bean, New Zealand spinach, kale, white cauliflower, celery and red beet products. In the remaining products, the significantly lowest retention was found in the traditionally frozen product (Sample C) and the highest in the ‘convenience’ frozen product (Sample D). The above‐mentioned order also applied to ND and RDI values. Only in the ND of parsnip calculated for women did any value fall below 100%.     

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

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

不同冻结方式对牡蛎品质的影响

本文以冻结曲线、冻结速率、持水性、蛋白质变性程度、色差和质构为指标,研究不同冻结方式(浸渍冻结、鼓风冻结和静置冻结)对牡蛎品质的影响。结果表明:浸渍冻结组冻结速率为6.67 cm/h,分别是鼓风冻结组的5倍,静置冻结组的23倍,且其样品解冻汁液流失率和蒸煮损失最低,分别为6.75%±1.12%、42.08%±3.72%,蛋白变性程度最小,其中盐溶性蛋白含量为(48.15±5.53) mg/g,Ca2+-ATPase酶活为(0.202±0.020) μmol Pi/(mg·h)。浸渍冻结处理的牡蛎的弹性为(3.96±0.25) mm,与鼓风冻结组和静置冻结组比较差异显著(p<0.05)。在不同冻结方式下的色差值(L*、a*、b*)、总色差(ΔE)、硬度和咀嚼性差异性不显著(p>0.05)。浸渍冻结处理牡蛎的所有品质指标都与鲜样差异不显著(p>0.05),但鼓风冻结和静置冻结在盐溶性蛋白含量、Ca2+-ATPase活性和弹性指标上都与鲜样差异性显著(p<0.05)。综上,浸渍冻结对牡蛎品质的保持比鼓风冻结和静置冻结更具优势,且其处理的样品品质更接近鲜样。     

Blanching, Freezing and Frozen Storage Influence Texture of White Asparagus

We studied effects of three methods of blanching in conjunction with freezing, on texture of white asparagus as defined by three measures: maximum shear force, cutting energy, and total fiber content. We also assessed shelf life of asparagus kept in frozen storage at -22°C. Methods of blanching were total immersion in hot water, progressive immersion in hot water and steam. An increase in total fiber content was found throughout frozen storage. This increase correlated with lignification of vascular bundles in the basal segment of spears, even during frozen storage. This was reflected in an increase in maximum shear force and cutting energy required. The shelf life of frozen asparagus was 12 mo using total fiber content as a criterion.     

Thermal Properties of Ration Components as Affected by Moisture Content and Water Activity During Freezing

ABSTRACT: Beef roast with vegetables is an example of a meal, ready‐to‐eat (MRE) ration entrée. It is a mixture of meat, potato, mushroom, and carrot with a gravy sauce. The thermal properties of each component were characterized in terms of freezing point, latent heat, freezable and unfreezable water contents, and enthalpy during freezing using differential scanning calorimetry. Freezing and thawing curves and the effect of freezing and thawing cycles on thermal properties were also evaluated. The freezing points of beef, potato, mushroom, and sauce were all in the range of −5.1 to −5.6 °C, but moisture content, water activity, latent heat, freezable and unfreezable water contents, and enthalpy varied among these components. Freezing temperature greatly affected the unfrozen water fraction. The unfreezable water content (unfrozen water fraction at −50 °C) of ration components was in the range of 8.2% to 9.7%. The freezing and thawing curves of vegetables with sauce differed from those of beef but took similar time to freeze or thaw. Freezing and thawing cycles did not greatly affect the thermal properties of each component. Freezing point and latent heat were reduced by decreasing moisture content and water activity of each component. Water activity was proportionally linear to freezing point at a_w > 0.88, and moisture content was proportionally linear to freezable water content in all ration components. Water was not available for freezing when moisture content was reduced to 28.8% or less. This study indicates that moisture content and water activity are critical factors affecting thermal behavior of ration components during freezing.     

Effect of Processing on Slowly Digestible Starch and Resistant Starch in Potato

The effect of a number of laboratory‐scale pretreatments on the proportions of rapidly digested (RDS), slowly digested (SDS) and resistant starch (RS) in raw and cooked potato has been examined using an in vitro digestion procedure. Potatoes of the variety Frisia were prepared in three states: raw, cooked, and cooked followed by a cold treatment (4°C, two days). Each preparation was then subjected in triplicate to freeze‐drying, coarsely mincing, pasting, freezing, dry‐milling after freeze‐drying, in 22 different combinations, before digesting. In raw potato, very little RDS and SDS (<5% total starch (TS)) were present, and the mechanical treatments of the potato did not affect the amounts of RDS and SDS. Cooking resulted in an almost complete conversion to RDS (>95% TS) in freshly‐cooked potato, but after post‐cooking cold treatment much of the RDS transformed to SDS, which reached a maximum of about 45% TS. SDS formation was independent of the degree of tissue disruption after cooking, and was generally associated with formation of RS, however, freezing after cooking allowed SDS formation without prolonged cold treatment and with very little associated RS (SDS 35% and RS 4% of TS). Freeze‐drying caused an increase in RS in most treatments of the cooked potatoes. The observed effects provided guidance for sample handling in potato research, but also suggested several approaches to the enrichment of SDS and/or RS, with a concurrent reduction in RDS, that could be used to improve the nutritional profile of potato products by decreasing RDS (lowered glycaemic impact), and increasing SDS (more sustained energy availability) and RS (prebiotic benefits).     

Freezing-induced changes in goat cheese: Effect of freezing rate,storage and freeze–thaw cycles

This study compared the effect of various freezing set-ups [freezing in the walk-in freezer (WF), blast freezing and liquid nitrogen freezing (LNF)] on the textural properties of goat cheeses. The influences of freeze–thaw cycles (1, 3 and 5) and storage time (1, 7 and 14 days) at 4°C on the hardness and visual appearance were investigated. The results showed that freezing damaged the cheese structure, particularly for WF. Frozen samples were not suitable for 4°C storage. Freeze–thaw cycles further damaged the texture of both cheese logs and crumbles, resulting in increased dryness. The development of the dry structure was faster for the LNF-treated sample.     

Physicochemical and enzymatic changes of cod muscle proteins subjected to different freeze–thaw cycles

The effects of freeze–thaw cycles on the physicochemical and enzymatic changes of cod muscle proteins were investigated. The activities of α‐glucosidase and β‐N‐acetyl‐glucosaminidase increased as the the number of freeze–thaw cycles increased. A loss of Ca²⁺‐ATPase and Mg²⁺‐Ca²⁺‐ATPase activities was observed, while Mg²⁺‐EGTA‐ATPase activity increased, especially after one cycle of freezing–thawing, with a concomitant decrease in Ca²⁺ sensitivity. The surface sulphhydryl group content decreased with increasing freeze–thaw cycles, while no changes in total sulphhydryl group content were found. The surface hydrophobicity of actomyosin did not change significantly with the number of freeze–thaw cycles. The loss of protein solubility increased with increasing freeze–thaw cycles. However, no cross‐linked proteins induced by formaldehyde were found. The results revealed that freeze–thaw cycles directly affected the physicochemical and enzymatic properties of cod muscle proteins. © 2000 Society of Chemical Industry     

PRESSURE-ASSISTED FREEZING AND THAWING: PRINCIPLES AND POTENTIAL APPLICATIONS

The phase diagram of water as a function of temperature and pressure delimits distinct crystalline ice forms with different specific volumes, melting temperatures, and latent heats of fusion. The melting temperature of ice I decreases to ?22°C when pressure increases to 207.5 MPa. It is possible to freeze a biological or food sample under pressure (obtaining ice I, III, V, VI, or VII), to enhance ice nucleation by fast pressure release, to keep a sample at subzero temperatures without ice crystal formation, to generate pressure through freezing, to reach the glassy state of water by fast cooling under pressure, or to thaw a frozen sample under pressure below 0°C. Fast pressure release from ?10 or ?20°C and 100 or 200 MPa (with a prior cooling step under pressure), called “pressure-shift freezing,” induces significant supercooling (as detected by fast data acquisition) and enhances uniform ice nucleation throughout the sample. When freezing is then completed at atmospheric pressure, different microscopy techniques reveal numerous small ice crystals with no specific orientation or marked size gradient. Crystals are smaller in pressure-shift frozen gels than in similarly frozen oil-in-water emulsions. In the latter, increasing solute concentrations in the aqueous phase tends to reduce ice crystal size. Modeling is proposed for pressure-shift freezing, although the supercooling and nucleation steps are not taken into account. Both freezing under various pressure levels and pressure-shift freezing are reported for gels (mainly heat-induced protein gels), emulsions, and plant and animal tissues. In spite of some discrepancies, gel or tissue structure and texture are generally better maintained after thawing, as compared to control samples frozen by air blast or immersion in a cooling medium at 0.1 MPa. Less liquid exudation is also observed. However, some protein denaturation is detected (unfolding of myofibrillar proteins, toughening of meat or seafood), especially when the initial cooling step is carried out at a high pressure level for a long time. Pressure application at subzero temperature is found to inactivate only some enzymes, but causes a significant degree of microbial inactivation for several species of micro-organisms. Freezing gels or vegetables under pressure with the formation of ice III, V, or VI appears to maintain tissue structure and texture, but the mechanisms for these effects are not fully understood. Pressure-assisted thawing markedly enhances the rate of thawing, mainly due to a greater ΔT between the subzero thawing temperature and that of the heating medium. Specific packaging and equipment requirements for pressure-assisted freezing and thawing are discussed. Suggestions are made for further studies on high pressure–subzero temperature treatments, such as the influence of sample size and composition; the effects on cell membranes; the reduced need for blanching before freezing; the viability of pressure-shift frozen cells, embryos, or organs; the mechanisms of protein denaturation; and texture-promoting effects, especially in ice creams.