Structure/processing relation of vacuum infused strawberry tissue frozen under different conditions

Strawberry halves were infused with pectinmethylesterase (PME) and calcium and subsequently frozen by slow, rapid, cryogenic or high-pressure shift freezing (HPSF). A microscopic procedure including detailed image analysis was used to study the effect of vacuum infusion (VI) and/or freezing on the strawberry tissue structure. Hereto, parameters characterizing the size, the shape and/or smoothness of tissue particles were compared for the different samples. It was found that VI seemed to stabilize the cell walls and the cell–cell contact and did not affect the integrity of the strawberry tissue. Structural damage of untreated strawberry tissue due to freezing was large for all freezing methods applied. Rapid and cryogenic freezing conditions were most favorable. VI was effective to minimize the structural damage of rapidly, cryogenically and high-pressure shift frozen strawberries. Infused strawberries frozen by HPSF showed the most smooth and largest particles from all the frozen samples, indicating the importance of both infusion and a high and homogenous degree of undercooling to obtain maximal structure retention of frozen strawberries.     

Effect of freezing/thawing conditions and long‐term frozen storage on the quality of mashed potatoes

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     

High-pressure shift freezing: recrystallization during storage

High-pressure shift freezing has been proposed as a method to produce frozen food with smaller ice crystal size and, consequently, with reduced tissular damage and higher overall quality. The fate of this initially improved crystal size distribution, decisive for the long-term value of this procedure, is unclear. The recrystallization behaviour of partially frozen aqueous solutions, as food models, is here compared with that of similar classically frozen samples. A microscopic observation cell has been specially designed for this purpose. The temporal evolution of high-pressure shift frozen ice crystals has been fitted to different mechanism models and is found to be similar within experimental error to that of classically frozen samples. However, differences in the shape evolution of crystals have been detected, which can be ascribed to small differences in the initial distribution. The implications of these observations for the long-term storage of frozen food are discussed.     

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.     

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.     

冻结和冻藏对冷冻面团流变学以及汤包皮质构特性的影响

研究了冻结温度、冻藏温度、冻藏时间对冷冻面团流变学特性和汤包皮质构特性的影响。实验结果表明,冻结改变了面团的流变性质,降低了面团的弹性;并且-18℃下冻结对汤包皮TPA参数的影响比-30℃的影响大。-30℃的冻藏温度对面团的粘弹性影响较大,而不同冻藏温度对汤包皮的TPA参数影响不大;冻藏时间对面团粘弹性、面团拉伸特性以及汤包皮的TPA参数影响大。     

Effects of freezing and frozen storage on the microstructure and texture of ewe's milk cheese

Semi-hard ewe's milk cheeses, frozen immediately after manufacture either slowly at –35 °C or rapidly at –80 °C and stored at –20 °C for 4 months were studied for microstructural and textural characteristics during subsequent ripening. Two control groups were used to establish the effect of freezing: the fresh unfrozen cheese and cheese thawed immediately after freezing. Freezing proper did not result in any marked changes in the textural parameters of the cheeses, but considerable changes were found in slowly frozen cheeses after 4 months of frozen storage. Shear strength values were lower in all frozen and stored cheeses, particularly in cheese samples frozen slowly compared to those in the unfrozen control batch. This parameter and firmness values were significantly lower in both slowly and rapidly frozen cheeses at the completion of ripening. Ripening tended to offset differences in elasticity, noticeable in the cheeses during the first 30 days of ripening. Light microscopy and electron microscopy revealed small cracks and ruptures in the cheeses which could not be observed by the naked eye. More extensive damage to the cheese microstructure was found in slowly frozen cheese samples stored frozen for 4 months.     

Influence of frozen storage and packaging on oxidative stability and texture of bread produced by different processes

The influence of packaging barrier properties and frozen storage on phenolic and phytosterol content, oxidative stability and crumb texture of frozen dough, part-baked and fully baked frozen bread was investigated in comparison to conventionally produced bread. Samples were stored either in blue coloured high density polyethylene (PE-HD) or transparent polyester-polyethylene-ethylene-vinyl alcohol copolymer (PET-PE/EVAL/PE) pouches for 22 days at −18 °C. Packaging materials were different in oxygen permeability: 3.67 cm³m⁻²day⁻¹ bar⁻¹ for PET-PE/EVAL/PE and 2080 cm³m⁻²day⁻¹ bar⁻¹ for PE-HD material, which did not significantly change during storage. Total phenolic content and oxidative stability of bread samples decreased during storage depending on the process. Frozen dough bread had the lowest phenolics decrease and the highest oxidative stability. Total phenolic content and oxidative stability of frozen breads during 8 days were similar to conventional bread. The phenolics reduction was higher for samples stored in PET-PE/EVAL/PE laminate than in PE-HD packaging. Total sterol content did not significantly change during bread storage in investigated packaging and did not contribute to the oxidation. Bread firmness was affected only by the process and not by the storage time and packaging material.     

Quality changes of pork during frozen storage: comparison of immersion solution freezing and air blast freezing

This study evaluated the effects of freezing method and storage time on the quality changes of pork longissimus thoracis (LT). The results showed that, compared to air blast freezing (AF), better microstructure was observed in pork under immersion solution freezing (ISF) mainly due to higher freezing rate. ISF group had significantly lower thawing loss at 1, 31 and 91 days and higher shear force at 1 day than AF (P < 0.05). A significant reduction in TBARS value at 61 and 91 days was observed in ISF samples (P < 0.05). No significant differences were observed between the two treatments in the colour, cooking loss and sulfhydryl groups (P > 0.05). All quality indicators were significantly affected by storage time (P < 0.05). It is concluded that ISF could maintain better microstructure, improve water-holding capacity and inhibit lipid oxidation during pork LT frozen storage.     

Effects of Kudoa spores,endogenous protease activity and frozen storage on cooked texture of minced Pacific hake (Merluccius productus)

The effects of Kudoa infection of Pacific hake (Merluccius productus) on endogenous protease activity and on cooked mince texture were investigated. Texture was significantly (p < 0.05) negatively correlated with spore counts as well as protease activity. Soft texture (maximum force <150 g) was observed in fish with 10⁴–10⁶Kudoa thyrsites spores g⁻¹ mince, compared to 10⁵–10⁸K. paniformis spores g⁻¹ mince, suggesting that especially for fish having lower infection levels, K. thyrsites may have a greater impact than K. paniformis on Pacific hake quality. Pre-incubation for 15 min at 52 °C prior to cooking resulted in softer texture in some samples due to endogenous proteolytic action. This pre-incubation effect was not consistently observed in fish held 6 months or longer at −25 °C or after freeze-thaw cycling, which may be explained by an opposing toughening effect attributed to protein denaturation and aggregation during prolonged or abusive frozen storage.     

Effect of high-pressure versus conventional thawing on color, drip loss and texture of Atlantic salmon frozen by different methods

Atlantic salmon (Salmo salar) samples were frozen by conventional air freezing, plate freezing and liquid nitrogen (LN) freezing, and subjected to different thawing treatments: water immersion thawing (WIT) (4°C and 20°C) and high-pressure thawing (HPT) at 100, 150 and 200 MPa with water (containing 2 g oil/100 g) as pressure medium at 20°C. Temperature and phase change behavior of fish samples were monitored during freezing and thawing. The phase change point of frozen salmon was lowered to −14°C, −19°C and −25°C for the HPT processes at 100, 150 and 200 MPa, respectively. These phase change temperatures were lower than for pure ice at the same pressures possibly due to the presence of solutes in salmon. The HPT times were 22.6±1.4, 18.1±1.4 and 17.0±1.3 min at 100, 150 and 200 MPa, respectively, as compared with 26.6±2.1 and 94.3±3.4 min for the WIT process at 20°C and 4°C, respectively. Employing pressures above 150 MPa caused noticeable color changes in salmon during the HPT process and the product texture was significantly modified during HPT at 200 MPa. Different freezing rates prior to thawing resulted in differences in drip loss in salmon samples, but they did not induce specific color and texture changes. A significant (P<0.05) reduction of drip loss by the HPT process was observed only for the LN frozen samples in which mechanical cracking occurred and much of the drip appeared after WIT process. Drip loss formed during pressure thawing seems to be a complicated process, for which further studies are needed.     

Effect of frozen storage conditions (temperature and length of storage) on microbiological and sensory quality of rustic crossbred beef at different states of ageing

The effect of frozen storage conditions on meat from 36 Morucha × Charolais crossbred yearlings was studied. Slices of M. Longissimus thoracis were randomly assigned to groups arising from the combination of experimental factors. These factors were: ageing extent (3 and 10 days), length of frozen storage (0, 30, 75 and 90 days) and temperature (−20 and −80 °C). Regarding microbiological counts, although values were acceptable in all cases, longer storage time and longer previous ageing extent provided higher phychrotrophic bacteria counts. As frozen storage period increased, colorimetric parameters L^∗, a^∗ and C^∗ decreased, but H^∗ increased. Regarding Warner–Braztler shear force and tenderness values, an interaction (p < 0.05) between frozen storage and post-mortem ageing resulted from larger differences between frozen storage periods at shorter ageing periods than those at longer ageing periods. Frozen storage for 90 days resulted in a reduction in water holding capacity, without differences in juiciness. No effect of freezing temperature was observed in any of the parameters studied.     

Quality changes during the frozen storage of sea bass (Dicentrarchus labrax) muscle after pressure shift freezing and pressure assisted thawing

Quality of frozen sea bass muscle stored (1, 3 and 5 months) at two levels of temperature (−15 and −25 °C) after a pressure shift freezing process (200 MPa) — PSF — and/or a pressure assisted thawing process (200 MPa) — PAT — was evaluated in comparison with samples frozen and thawed using conventional methods (air-blast AF and AT, respectively). Frozen storage of high-pressure treated samples did not significantly affect initial quality of frozen muscle. Thus, parameters related to protein denaturation and extractability, water holding capacity and color presented similar values than those obtained for not stored samples. In addition, the improvement of the microstructure achieved by PSF application remains unchanged during frozen storage. On the other hand, conventional treated samples experienced significant changes during frozen storage, such as protein denaturation, and water holding capacity and color modifications. Storage temperatures did not have influence in the quality of PSF and PAT samples, but it showed some effects in AF muscle.Industrial relevance: This work demonstrates the potential application and benefits of high pressure (HP) in the freezing and thawing of fish meat in comparison to conventional methods, due to an improvement on the cellular integrity of the tissue. Although some negative effects are produced during processing with HP, no additional modifications occur during the frozen storage. The studied methodologies seemed to be very suitable for fish freezing and thawing, especially for products which will be frozen stored and/or cooked.     

Effects of sodium bicarbonate containing traces of citric acid in combination with sodium chloride on yield and some properties of white shrimp (Penaeus vannamei) frozen by shelf freezing, air-blast and cryogenic freezing

Effects of sodium bicarbonate with traces of citric acid in combination with sodium chloride on yield, freezing time, freezing rate, freezing loss and cutting force of white shrimp frozen by shelf, air-blast and cryogenic freezing with/without precooking were investigated. Shelf freezing was done at −40 °C ± 2 °C while air-blast freezing was carried out at −35 °C ± 2 °C, and cryogenic freezing was done at −35 °C, −40 °C and −60 °C. The freezing loss in the non-treated samples was 8.25, 4.6-5.84 and 1.92-3.48 g/100 g fresh shrimp for peeled samples frozen without precooking and increased to 21.85, 17.54-26.97, 17.92-20.31 g/100 g fresh shrimp in the precooked samples frozen by shelf, air-blast and cryogenic freezing, respectively. The treatment of sodium bicarbonate containing traces of citric acid at 4 g/100 ml with sodium chloride at 3 g/100 ml lead to the increase of yield thus reduced the freezing loss by about 6.83-10.28 and 6.41-12.4 g/100 g fresh shrimp for the frozen-thawed samples frozen as uncooked and cooked products, respectively. The toughening of shrimp was observed while sodium bicarbonate containing traces of citric acid treatment with sodium chloride could reduce the texture change occurred during the freezing.     

不同保鲜剂结合冰温对玫瑰香葡萄贮藏品质和生理生化的影响

以玫瑰香葡萄为试材,研究了不同保鲜剂结合冰温（-0.5～-0.2℃）对玫瑰香葡萄采后贮藏品质及生理的影响。结果显示:保鲜剂结合冰温能有效减少葡萄贮藏期间的失重、脱粒、腐烂,延迟可溶性固形物、可滴定酸、VC含量的下降,较好保持葡萄的外观和风味;并能使葡萄果肉的硬度、弹性、凝聚性、咀嚼性维持在较高的水平;另外,能很好的抑制丙二醛的积累、PPO活性,保持较高的POD、SOD活性,延缓衰老,增长贮藏时间。其中以处理Ⅲ即8gCT2保鲜剂+1gCT5保鲜剂处理对玫瑰香葡萄保鲜效果最好。      

Gelation process in two different squid (Dosidicus gigas) surimis throughout frozen storage as affected by several cryoprotectants: Thermal, mechanical and dynamic rheological properties

Differential scanning calorimetry (DSC) was used to follow the thermal denaturation of two differently processed surimis from giant squid (Dosidicus gigas), A (isoelectric precipitation) and B (acid washing). Both surimis were added with several cryoprotectants (sucrose, sorbitol and trehalose), and kept in frozen storage at −15 °C for 6 months. DSC profiles and transition enthalpies showed that protein quality was better in B than in A surimis whether fresh or frozen. It was also demonstrated that the sucrose-plus-sorbitol mixture and trehalose were the most efficient for preserving B and A type formulations respectively after frozen storage. Mechanical (puncture), small amplitude oscillatory (SAOS) and dynamic-mechanical thermoanalyses were performed on both gels from A and B series surimis throughout frozen storage. Gels from surimi A (GA) were considerably brittler than gels from surimi B (GB) and presented significantly lower strain amplitudes. Several correlations among different experimental data have been developed.     

Influence of the in vivo addition of alpha-tocopheryl acetate with three lipid sources on the lipid oxidation and fatty acid composition of Beluga sturgeon, Huso huso, during frozen storage

This study was conducted to investigate the effect of dietary alpha-tocopheryl acetate (vitamin E) and oil sources on fish flesh quality characteristics of Huso huso during frozen storage. Practical-type diets containing 0 or 250 mg vitamin E kg⁻¹ with three lipid sources, fish oil (FO), soybean oil (SO) and canola oil (CO), were fed to H. huso for 120 days. Fillet samples were analysed fresh or after storage at −18 ± 1 °C for 12 months. Replacement of FO by SO and CO in diets for H. huso significantly altered the fatty acid (FA) profile, which also influenced the FA composition during frozen storage. Dietary vitamin E had a significant effect on muscle vitamin E content and lipid oxidation during storage (P > 0.05). Oxidation was reduced for fish fed vitamin E and results showed that dietary vitamin E supplementation can slow down the level of lipid oxidation in H. huso muscles during frozen storage.