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1.
Turbot fillets were frozen either by pressure shift freezing (PSF, 140 MPa, −14°C) or by air-blast freezing (ABF), and then stored at −20°C for 75 days. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF compared with air-blast frozen ones. Ice crystals area in PSF samples was approximately 10 times smaller than that of ABF samples, on average. The PSF process reduced thawing drip compared with air-blast freezing. Conversely to this classical freezing process, the storage time did not adversely influence the thawing drip of PSF samples. In addition, PSF appeared to reduce cooking drip after 45 days of storage at −20°C. Differential scanning calorimetry analysis showed a significant reduction of the total enthalpy of denaturation for the pressure shift frozen samples compared to fresh and conventional frozen samples. Besides, a new melting transition appeared on the thermogram of PSF samples at approximately +40°C.  相似文献   

2.
This study was conducted to investigate the potential of cryogenic freezing with liquid nitrogen in the shelf-life extension of northern snakehead (Channa argus) and clarify the effects of temperature fluctuations after freezing on the quality attributes and tissue microstructure during frozen storage. The fish fillets were frozen by three methods including freezing using an ultra-low-temperature freezer (?80°C) to the core temperature of ?60°C (T1) or ?18°C (T2), or liquid nitrogen (T3) followed by storage at ?20°C for five months. Cryogenic freezing with liquid nitrogen postponed the decrease in pH and protein extractability. Temperature fluctuations after freezing might promote the accretion of ice crystals and resulted in the loss of tissue integrity and disorganization of myofibrils. The microstructural changes contributed greatly to the increased thawing loss and decreased resilience, as indicated by the enlarged extracellular spacing and the flakiness of myofibrils. Cryogenic freezing with liquid nitrogen showed no superiority in maintaining the microstructure of northern snakehead fillets, which was supposedly attributed to the cracking in tissue during freezing and the accretion of ice crystals during frozen storage.  相似文献   

3.
To study the effects of freezing temperature on muscle proteins, shrimp (Metapenaeus ensis) were frozen stored at either −18 or −60 °C up to 90 and 210 days. Shrimp frozen at −18 °C had higher thawing and compression loss and poor myofibril water-holding capacity compared with those frozen at −60 °C. In terms of protein characteristics, shrimp frozen at −18 °C had higher levels of carbonyls and reduced sulphhydryls. Moreover, the shrimp frozen at −18 °C had higher surface hydrophobicity and reduced Ca2+-ATPase activity, indicating increased protein denaturation. Proteomics revealed that seventy-five proteins were classified as differentially abundant proteins (DAPs) following freezing. There were sixty-four DAPs in the F18-CON comparison group (shrimp frozen at −18 °C vs. control) and thirty-two DAPs in the F60-CON comparison group (shrimp frozen at −60 °C vs. control), suggesting that freezing at −18 °C results in more DAPs than freezing at −60 °C. A comparison between F18 and F60 revealed that ribosomal proteins (L44, L7a) and heat shock protein 21 were downregulated in F18. These results increase our understanding of the variable quality associated with shrimp frozen at different temperatures.  相似文献   

4.
The objective of this study was to use a new self-cooling laboratory system for carrying out the pressure shift freezing (PSF) and evaluate the influence of PSF at 150 MPa on the microstructural properties of largemouth bass relative to liquid immersion freezing (LIF) and conventional air freezing (CAF). CAF, LIF and PSF showed average total freezing times of 176 ± 7.4, 65.3 ± 6.8 and 23.2 ± 3.1 min, and the cross sectional area of ice crystals in the muscle were 1002 ± 778, 501 ± 248 and 143 ± 50.6 μm2, respectively, demonstrating a significant reduction in crystal size to be associated with PSF. It was observed that damage caused by the ice crystals during the freezing to the muscle microstructure was irreversible. The thawing and cooking losses of largemouth bass after the freezing were lower for PSF as compared to the other two freezing methods. PSF reduced the damage to myocytes and resulted in lower drip loss due to reduced microstructure disruption due to their small ice crystals, thereby maintaining the muscle tissue to better retain the fluids. Color and texture properties were less affected by PSF.Industrial relevanceFreezing is the most used preservation method for aquatic products. Rapid freezing results in better texture retention while the slow freezing damages the product texture because of the formation of extracellular large ice crystals developed during the freezing process. Thus, the nature of freezing affects the quality of frozen foods. Successful freezing processes aim at employing rapid freezing conditions which result in the formation of small ice crystals. Pressure shift freezing (PSF) is a novel technique with advantages of high degree of super-cooling, short phase transformation time, and results in very small ice crystals. This study makes use of a laboratory self-cooling system to carryout PSF of largemouth bass. This cooling system overcomes the limitation of previous studies on PSF which are expensive, limited to small size and impractical for commercial exploitations. The cooling system employed in this study can be easily adapted to large-scale production of PSF aquatic products. Test results provide a basis for the commercial exploitation of PSF for largemouth bass and such other aquatic foods for driving the quality advantage.  相似文献   

5.
The objective of this study was to investigate the effects of electrostatic freezing on the quality attributes of freeze-thaw agar gel. Agar gels were frozen under static electric field 0–5.8 × 104 V m 1 at − 20 °C. Freezing rate and energy consumption were monitored during the freezing process and microstructures of the formed ice crystals were also analyzed by light microscopy techniques. Agar gel quality changes (syneresis and texture) were evaluated after thawing the frozen samples at + 4 °C. Results showed that the energy used by a DC high voltage generator was negligibly small as compared to the energy consumption by a freezer, and the freezing rate was not significantly influenced by electrostatic freezing (ESF). ESF also reduced the size of ice crystals but did not cause obvious changes in syneresis and texture of the samples.Industrial relevanceThe effects of electrostatic field freezing on the quality attributes of agar gel have been investigated. The results showed that the electrostatic freezing can be used as a potential tool to improve the microstructure of foodstuffs during freezing.  相似文献   

6.
Efforts are currently directed towards improving the quality of vegetables after freezing and thawing. One of the methods under investigation is isochoric freezing. In this study, we evaluated isochoric freezing for preserving the quality of baby-leaf spinach. We compared the properties of thawed spinach frozen to −4°C in an isochoric system with those of fresh spinach, thawed spinach frozen to −4°C in an isobaric system and thawed spinach that were commercially frozen. Spinach leaves frozen under isobaric conditions lost mass and thickness, making them softer and translucent. They also lost much of their nutrient content. In comparison, isochoric freezing maintained cell integrity and turgidity. Thawed leaves remained crunchy with characteristics similar to fresh leaves. Isochoric freezing also preserved nutritional content better than isobaric freezing, although significant nutrient losses still occurred.  相似文献   

7.
Apple texture is one of the critical quality features for the consumer. Texture depends on several factors that are difficult to control and which change with freezing. To better understand the mechanisms involved in the texture degradation of apple tissues following freezing/thawing, our approach was to combine mechanical properties, cellular structure and cell wall composition measurements on fresh and thawed apples (Granny Smith) after three different freezing protocols (at ?20 °C, ?80 °C and ?196 °C). This work highlighted the interest of applying macrovision and image texture analysis to quantify the freezing effects on cellular structure and ice crystal size. Freezing at ?20 °C and after immersion into liquid nitrogen were the protocols affecting the most fruit texture leading to cell membrane breakage resulting in cell wall collapse and tissue breakage, respectively, which accounted for the mechanical behaviour of the samples. All freezing protocols induced vacuole burst showing that the turgor pressure preservation remains critical during the freezing process.  相似文献   

8.
To enhance the freezing rate and thawed product quality of pork tenderloin, an experimental study was conducted using the high voltage electric field and static magnetic field separately during freezing. Pork tenderloin pieces were frozen at −20 °C under several high voltage electric fields (10 kV/m (HVEF1), 30 kV/m (HVEF3), 50 kV/m (HVEF5)) and magnetic fields of 2 mT (MF2), 4 mT (MF4), 6 mT (MF6) and 8 mT (MF8). The effects of different methods on freezing rate, ice crystal size as well as the distribution, and product quality after thawing were investigated. The freezing time of pork tenderloin was reduced by 40.04% and 37.81% respectively, under the optimal electric and magnetic field conditions tested. The thawing loss decreased from 5.7% of conventional freezing to 1.7% of HVEF1 and 2.4% of MF2, respectively. In addition, both high-voltage electric field freezing and magnetic field freezing can better maintain the moisture state in the sample. The results for color and pH confirmed that the thawed product quality using HVEF1 and MF2 was superior to that obtained under other conditions. The myofibrillar protein in the thawed products obtained from HVEF1 and MF2 treatments was also found to be thermally more stable. It is noteworthy that the HVEF1 treated sample has the highest umami signal and the lowest salty signal. Considering the enhanced freezing efficiency and improved quality, application of HVEF1 is recommended as a viable strategy to produce high-quality frozen pork tenderloin.Industrial relevanceThe slow freezing rate of frozen meat products and serious deterioration of product quality are the key problems. Therefore, improving the efficiency of freezing is desirable. This study provides ideas for pork preservation. It caters to the need of industrial production of meat product where better efficiency freezing process is highly desirable, and the findings of this study is beneficial to the meat processing industry.  相似文献   

9.
The effect of freezing treatments on sweet dough was studied. The dough was frozen at ?20 °C, ?30 °C and ?40 °C in air-blast freezer cabinet and by immersion in liquid nitrogen. The yeast viability, gassing power, dough volume and dough network integrity from fresh and thawed sweet doughs were assessed. The results showed that both parameters depend on the freezing rate, which controls ice crystals size and location. Dough volume loss after freezing was attributed to reduced yeast fermentative activity and gluten network alteration in frozen dough. Fermentative activity reduced significantly in frozen dough using liquid nitrogen, causing 70% decrease on yeast population. Gluten integrity seemed to be affected by slow freezing treatment, i.e. ?20 °C and ?30 °C. Gas loss was also evaluated as a decrease of 25% ± 2 in dough volume. A correlation was observed between the freezing rate and osmotic pressure effects which influence strongly the yeast viability.  相似文献   

10.
《Meat science》2008,78(4):616-625
Meat high-hydrostatic pressure treatment causes severe decolouration, preventing its commercialisation due to consumer rejection. Novel procedures involving product freezing plus low-temperature pressure processing are here investigated. Room temperature (20 °C) pressurisation (650 MPa/10 min) and air blast freezing (−30 °C) are compared to air blast freezing plus high pressure at subzero temperature (−35 °C) in terms of drip loss, expressible moisture, shear force, colour, microbial quality and storage stability of fresh and salt-added beef samples (Longissimus dorsi muscle). The latter treatment induced solid water transitions among ice phases. Fresh beef high pressure treatment (650 MPa/20 °C/10 min) increased significantly expressible moisture while it decreased in pressurised (650 MPa/−35 °C/10 min) frozen beef. Salt addition reduced high pressure-induced water loss. Treatments studied did not change fresh or salt-added samples shear force. Frozen beef pressurised at low temperature showed L, a and b values after thawing close to fresh samples. However, these samples in frozen state, presented chromatic parameters similar to unfrozen beef pressurised at room temperature. Apparently, freezing protects meat against pressure colour deterioration, fresh colour being recovered after thawing. High pressure processing (20 °C or −35 °C) was very effective reducing aerobic total (2-log10 cycles) and lactic acid bacteria counts (2.4-log10 cycles), in fresh and salt-added samples. Frozen + pressurised beef stored at −18 °C during 45 days recovered its original colour after thawing, similarly to just-treated samples while their counts remain below detection limits during storage.  相似文献   

11.
《Food chemistry》2003,82(3):447-453
Frozen stored fillets from Norwegian spring spawning herring (Clupea harengus L.) become discoloured. Based on the hypothesis that lipid oxidation in the fillets caused discoloration, we designed an experiment where ascorbic acid was added to the water (2 g l−1) in RSW-tanks on board a fishing vessel and/or by spray (20 g l−1) after filleting but before freezing. Treatment with ascorbic acid on board and on the fillet line led to 8- and 17–24-fold increases, respectively, in the ascorbic acid concentration in the herring fillets. Ascorbic acid treatment on the fillet line, but not the treatment on board, protected the herring fillets against oxidation during freezing, an effect that could be detected until 9 weeks of storage at −30 °C. Between 9 and 14 weeks of storage there was a new burst of lipid oxidation where the treatments had no effect. There was a large variation in colour within each fillet and between fillets in the same group. During freezing, the amount of yellow colour increased substantially and the amount of red colour increased in the posterior end of the fillets, causing a visible change in appearance. Thereafter only minor changes occurred until week 10 but, in week 30, the fillets had become less red, causing a less fresh and more grey appearance. Compared to the within fillet and within group variations, there were only minor effects of ascorbic acid treatment on colour. It is suggested that shelf life of frozen herring fillets should be set at between 9 and 14 weeks at −30 °C. Treatment with ascorbic acid will not alleviate the discoloration of frozen herring fillets.  相似文献   

12.
Salmon fillets were frozen either by pressure shift freezing (PSF, 200 MPa, − 18 °C or 100 MPa, − 10 °C) or by air-blast freezing (ABF, − 30 °C, 1 m/s or 4 m/s) or direct-contact freezing, and then stored at − 20 °C for 6 months. The influence of these treatments on the microstructure of Salmon fillets was studied. The equivalent diameter of the intracellular ice crystals were 14.69 ± 4.11, 5.52 ± 2.11, and 30.65 ± 6.31 μm for the samples subjected respectively to PSF at 100, 200 MPa and ABF (− 30 °C, 4 m/s) after 2 days of storage. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF (200 MPa) compared with PSF (100 MPa), ABF and direct-contact frozen ones. Significant differences were observed between the size of the ice crystals obtained after conventional freezing process and PSF. Large and extracellular ice crystals were observed in fillets frozen by ABF (1 m/s) and direct-contact frozen. Minimal changes in the size of ice crystals were observed during a 3 months storage.

Industrial relevance

This paper compares different freezing methods and subsequent frozen storage with respect to their effect on microstructures of salmon fillets. Pressure shift freezing at 200 MPa was superior to conventional freezing regarding small and regular ice crystal formation. Interestingly, during frozen storage for up to 3 months the high quality product obtained via pressure freezing at 200 MPa could be retained. For longer storage periods lower pressures (100 MPa) seem sufficient to achieve stable ice crystals.  相似文献   

13.
Taking air freezing (AF) as the reference, the effects of four types of multi-compound freezing medium for cryogenic liquid quick-freezing (immersion freezing, IF) on the freezing rate, quality and myofibrillar protein (MP) denaturation of red drum (Sciaenops ocellatus) fillets during frozen storage (−18°C) from days 0 to 90 were studied. Samples were gathered on days 0, 15, 30, 45, 60 and 90 for analysis. The results showed that IF groups (IF-1: 20% ethanol, 30% propylene glycol, 10% sodium chloride aqueous solution; IF-2: 20% ethanol, 20% propylene glycol, 10% glycerol, 10% sodium chloride aqueous solution; IF-3: 20% ethanol, 20% propylene glycol, 10% polyethylene glycol, 10% sodium chloride aqueous solution; IF-4: 20% ethanol, 20% propylene glycol, 5% glycerol, 5% polyethylene glycol, 10% sodium chloride aqueous solution) significantly shortened the time to cross the formation zone of maximum ice crystals while the freezing rate was 6.13 times higher than the AF group after adding 30% propylene glycol as the freezing medium. Furthermore, compared with the AF group, the IF groups significantly reduced losses in the water-holding capacity, the myofibrillar fragmentation index, microstructure damage, texture characteristics, drip loss and water migration (P < 0.05). In addition, the MP of IF groups had higher maximum transition temperatures (Tmax1 and Tmax2), total sulfhydryl content, Ca2+-ATPase activity and relative α-helix content compared with the AF group. In conclusion, IF could significantly increase the freezing rate of red drum fillets, and slow down quality deterioration and denaturation of MP during frozen storage for 90 days (−18°C). In particular, IF-2 (20% ethanol, 20% propylene glycol, 10% glycerol, 10% sodium chloride aqueous solution) was found to be more suitable for the immersion freezing of marine fish among the four multi-compound freezing medium.  相似文献   

14.
The effects of freezing temperature (−80, −40 or −24 °C) and thawing mode (microwave or overnight at 4 °C) on quality parameters of mashed potatoes made from tubers (cv Kennebec) and from potato flakes were examined, as was the effect of long‐term frozen storage on the quality of mashed potatoes. Mashed potatoes were tested for texture profile analysis (TPA) and cone penetration, oscillatory and steady rheometry, colour, dry matter, Brix and sensory analyses. In natural mashed potatoes, TPA hardness and oscillatory parameters showed that processing resulted in a softer product than the fresh control. The parameters were lower in the samples thawed at 4 °C than in those thawed by microwave at all the freezing temperatures used, which may be ascribed to gelatinisation of the starch released from damaged cells. Differences from the freshly prepared product decreased when the samples were frozen at −80 °C and thawed by microwave. No difference was found in sensory acceptability between samples frozen at −80 and −40 °C, which probably reflects the panellists' mixed preferences for air‐thawed versus microwave‐thawed samples. Increasing the time in frozen storage led to a natural mash with a firmer texture, higher L*/b* value and Brix; nonetheless, panellists found the samples at 0, 3 and 12 months of frozen storage equally acceptable. In commercial mash, penetration and oscillatory parameters showed that processing made for a firmer product than the fresh control, probably owing to retrogradation of gelatinised starch. Thawing mode had a significant effect on parameters, which were lower in the samples thawed at 4 °C. The structure and quality of commercial mash was more detrimentally affected by freezing and, therefore, we would not recommend either freezing or frozen storage of this mashed potato in the used conditions. Natural mash made from Kennebec potatoes should be frozen quickly and thawed by microwave in the conditions described to obtain a product more similar to that freshly made. If the samples are frozen by air blasting at −40 °C, the product can withstand frozen storage for one year. Copyright © 2005 Society of Chemical Industry  相似文献   

15.
The paper presents a study of food freezing using hydrofluidisation method, characterised by very high heat transfer coefficients exceeding 2500 W · m−2 K−1, which constitutes a novel and promising technology. A numerical analysis of a system with a stationary group of food products was performed. This study aimed to investigate the geometrical parameters governing the freezing process, i.e., the position of food products above the orifices (from 50 mm to 70 mm), their mutual position, the diameter of orifices (3 mm or 5 mm), and the spacing of the orifices in an array (from 8 mm to 12 mm). In this method, the freezing time was from 3 min for 10 mm spherical food sample in moderate refrigerating medium temperatures up to 12 min for 30 mm spherical sample. Moreover, the freezing times at different liquid temperatures were compared in the range of −20 °C to −5 °C. Reducing the temperature by 5 K may lead to shortening the process by up to 50%. The hydrofluidisation method was assessed versus the immersion freezing for spherical products of different sizes showing the reduction of the process time from about 35% to over 60%.  相似文献   

16.
The effect of freezing at −30 °C, frozen storage at −22 °C for 30 days and thawing at 5 °C on proteolysis during ripening of Port Salut Argentino cheese was studied. Cheeses were sampled at different ripening times (1, 6, 13, 27 and 56 days) and two sampling zones (central and external). Moisture content, salt concentration and RP-HPLC of the nitrogenous fractions (water-insoluble fraction, water-soluble fraction and free amino acids in the sulfosalicylic acid-soluble fraction) were analysed. The freezing process did not affect moisture and salt contents at the beginning of the ripening period nor moisture and salt redistribution during the ripening period studied. However, the freezing process affected proteolysis during ripening of Port Salut Argentino cheeses that had been frozen prior to ripening. There was increased breakdown of αs1-casein and αs1-I-casein, and increased breakdown of peptides of the water-soluble fraction (including αs1-CN (f1-23)) along with an early development of free amino acids.  相似文献   

17.
Freezing plays an important role in food preservation and shelf-life extension. This study assessed the effectiveness of an innovative quick-freezing plant for meat freezing, based on a nitrogen reversed Brayton cryocooler prototype able to reach very cold temperatures (< −100 °C), by evaluating the effects on the physicochemical characteristics of frozen/thawed beef steaks. A comparison with two other standard freezing methods was conducted, and unfrozen beef steaks were used as a reference. The time-temperature profiles were monitored during the freezing and thawing processes, and physicochemical analyses were performed on unfrozen and thawed steaks. The results of the experimental campaign showed that the prototype plant made it possible to achieve a faster freezing rate (50 °C h−1) than standard plants, leading to a significant reduction in the thawing losses of frozen meat. In addition, the steaks processed in the prototype plant had larger hue values, indicating less red colour.Industrial relevanceIn the food cold chain, especially for highly perishable products such as meat, freezing plays a significant role in preservation and shelf-life extension, thus improving safe food consumption for humans. In this study, a prototype of a nitrogen reversed Brayton cryocooler was developed and tested under the realistic conditions expected for quick meat freezing. The results showed a very fast freezing rate of the food product. Our findings demonstrated that reversed Brayton cryocoolers show great promise in the food industry for quick freezing at very low temperatures, with the potential to freeze several kinds of food products.  相似文献   

18.
The impact of freezing (1 month + thawing at 7 or 25 °C) and extended refrigeration (4 days, 7 °C) on physicochemical and microstructural characteristics of raw whole and skimmed sheep milk were assessed. Refrigerated storage resulted in higher sedimentation and creaming (whole milk), possibly due to proteases and agglutinins. Freezing/thawing processes in whole milk increased the particle size and creaming when samples were thawed at 7 °C. Skimmed milk showed an increase in buffering capacity and a reduction in soluble calcium immediately after thawing at 25 °C, suggesting that although the changes in fat are the main alterations caused by slow freezing of sheep milk, minor changes in saline balance can occur. An evaluation of the results showed that frozen and thawed milk in domestic equipment (commonly found in smallholdings) alter the milk microstructure, and it is therefore preferable to use extended refrigeration to accumulate the milk before dairy production.  相似文献   

19.
Due to the perishable nature of fruits and the importance of reducing food waste, an effective preservation technique is required to prolong the shelf life and maintain the physical and nutritional properties of seasonal fruits. In this study, we evaluated isochoric freezing for preserving the quality of sweet cherries. We examined the physical characteristics and nutritional values of thawed cherries frozen to −4 °C or −7 °C in an isochoric system and compared them with those of fresh cherries, thawed cherries that were individually quick frozen and thawed cherries frozen to −4 °C or −7 °C in an isobaric system. We found that isochoric freezing decreased the drip loss and better preserved the color, texture, structure, ascorbic acid, phenolic and antioxidant content of frozen cherries, thereby proving their potential in frozen fruit applications.  相似文献   

20.
Freezing and blanching are essential processing steps in the production of frozen yellow peaches, inevitably leading to texture softening of the fruit. In this study, the synergistic mechanism of stem blanching, freezing conditions (−20°C, −40°C, −80°C, and liquid nitrogen [−173°C]), and sample sizes (cubes, slices, and half peaches) on macroscopic properties of texture, cellular structure, and ice crystal size distribution of frozen yellow peaches were measured. Blanching enhanced the heat and mass transfer rates in the subsequent freezing process. For nonblanched samples, cell membrane integrity was lost at any freezing rate, causing a significant reduction in textural quality. Slow freezing further exacerbated the texture softening, while the ultra-rapid freezing caused structural rupture. For blanched samples, the half peaches softened the most. The water holding capacity and fracture stress were not significantly affected by changes in freezing rate, although the ice crystal size distribution was more susceptible to the freezing rate. Peach cubes that had undergone blanching and rapid freezing (−80°C) experienced 4% less drip loss than nonblanched samples. However, blanching softened yellow peaches more than any freezing conditions. The implementation of uniform and shorter duration blanching, along with rapid freezing, has been proven to be more effective in preserving the texture of frozen yellow peaches. Optimization of the blanching process may be more important than increasing the freezing rate to improve the textural quality of frozen yellow peaches.  相似文献   

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