Preservation of spinach by isochoric (constant volume) freezing

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.     

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     

Viability of Listeria monocytogenes and Salmonella Typhimurium after isochoric freezing

Isochoric freezing, different from isobaric (conventional) freezing, allows for storage below freezing temperatures without significant damage from ice formation. While several types of tissues have been successfully stored in sub-zero isochoric conditions, it is unknown how isochoric freezing affects pathogenic microorganisms. Thus, the objective of this study was to investigate the survival of Salmonella Typhimurium and Listeria monocytogenes at below freezing storage (<0°C) in isochoric conditions. Tested conditions included storage at −4, −7, and −15°C for 24 hr and at −15°C for 1, 2, 3, 6, 12, and 24 hr. A comparison of bacterial survival during isobaric freezing was included with every trial. Additionally, bacterial cells were examined for morphological damage using transmission electron and field-emission scanning electron microscopes. Isochoric freezing at −15°C for 24 hr reduced both species of bacteria down to unrecoverable levels and maximum efficacy achieved after the 6 hr timepoint for L. monocytogenes and the 12 hr timepoint for S. Typhimurium. When viewed using electron microscopy, S. Typhimurium cells were noticeably disfigured with regions of cytosol separated from the cell wall. The results of this study demonstrate that isochoric freezing is capable of substantial levels of pathogen reduction. Unlike conventional nonthermal interventions, isochoric freezing does not require additional devices such as elevated pressure machines or pulsed electric fields and can be achieved with simple, inexpensive, rigid closed volume containers such as household freezers or commercial cold storage facilities.     

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.     

Protein changes in shrimp (Metapenaeus ensis) frozen stored at different temperatures and the relation to water-holding capacity

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.     

Cover Caption

September Online Cover : Cryo-scanning electron microscopy images of potato parenchyma tissue microstructures before and after freezing for 4 weeks under different freezing methods, and photographs of fresh and thawed potatoes frozen for different periods of time under isochoric, isobaric, or individually quick frozen conditions, from “Effect of isochoric freezing on quality aspects of minimally processed potatoes” by Cristina Bilbao-Sainz, Yuanheng Zhao, Gary Takeoka, Tina Williams, Delilah Wood, Bor-Sen Chiou, Matthew J. Powell-Palm, Vivian C.H. Wu, Boris Rubinsky, and Tara McHugh. p. 2656.     

Synthesis and degradation of adenosine triphosphate in cod (Gadus morhua) at subzero temperatures

This study has demonstrated that the extraction step is very important when analysing ATP and its degradation products. An important factor is whether the sample is fresh, frozen or thawed when homogenised since thawing of the sample will lead to rapid loss of ATP. During frozen storage it was found that ATP in cod (Gadus morhua) was stable at −40 °C in small samples for at least 12 weeks. At −20 °C it was found that ATP content increases initially and thereafter falls. It was demonstrated that degradation of ATP in small samples occurs faster at 0 °C than at −2 and −5 °C. Furthermore, it was found that in whole cod ATP could be synthesised at a significant rate at −7 °C. © 1999 Society of Chemical Industry     

Impact of extended refrigerated storage and freezing/thawing storage combination on physicochemical and microstructural characteristics of raw whole and skimmed sheep milk

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.     

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     

Capsule-Packed Freezing of Cooked Rice and Glutinous Rice Cakes

Cooked rice and commercial glutinous rice cakes were frozen by a capsule-packed freezing method we developed, then stored. Characteristics of samples frozen by this new method were compared with those of samples frozen in deep freezers (–20°C and –50°C), or chilled in an ordinary (5°C) or Cold Fog refrigerator (0°C). Texturometer measurements, glucoamylase digestion, and X-ray diffraction analysis of specimens thawed to room temperature indicated that the samples prepared by our new method were superior to those frozen in conventional freezers or chilled in refrigerators. Simulation experiments in a program freezer showed that rapid freezing and adequate tempering were characteristic of our capsule-packed method.     

Effect of freezing conditions and storage on ice crystal and drip volume in turbot (Scophthalmus maximus): Evaluation of pressure shift freezing vs. air-blast freezing

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.     

Effect of various thermal pre-treatments on the texture of frozen cherries (Prunus avium L.). Related enzyme activities

The effect produced on the texture of cherries by heating at 50, 60 and 70° C for 3, 6, 9, and 12 mins before freezing followed by 3 and 6 months of frozen storage was studied. Variations in cell-wall enzyme activity were also analysed. To this end, objective measurements were carried out on the firmness of the cherries, by means of mechanical penetration, shear and Kramer Shear Cell tests. Pectinesterase and polygalacturonase activity were also measured. Correlations were established between the mechanical test results and pectinesterase activity in cherries subjected to different treatments. Significant correlations were found between this activity and the final firmness of the frozen fruit. Low-temperature heating (70° C) prior of freezing significantly improves the final texture of the product.     

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

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

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

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

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.     

Changes in optical and mechanical properties during osmodehydrofreezing of kiwi fruit

The influence of osmotic dehydration and freezing–thawing on optical (colour and translucency) and mechanical properties of kiwi slices were analysed. Osmotic treatments were carried out in sucrose solutions up till the soluble solids in kiwi fruit reached 30 °Brix, both at atmospheric pressure (OD) and by applying a vacuum pulse (PVOD). Analyses were carried out on fresh and dehydrated samples before and after frozen storage (at −18 °C for 1 and 30 days). Reflexion spectra (400–700 nm) were measured to obtain the Kubelka–Munk coefficients and CIE-L*a*b* colour co-ordinates. Mechanical properties were analysed through the compression test. A transparency gain was observed in PVOD treated samples and in frozen–thawed samples, which implied a reduction in product clarity and chrome. Colour hue did not change notably, due to either osmotic treatments or freezing. Samples treated with 45 °Brix osmotic solution at atmospheric pressure were the best preserved in mechanical properties after freezing–thawing.     

Synergistic mechanism of steam blanching and freezing conditions on the texture of frozen yellow peaches based on macroscopic and microscopic properties

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.     

Lipoamid-dehydrogenase,citratsynthase und β-hydroxyacyl-coa-dehydrogenase des skelettmuskels

European Food Research and Technology - Samples of bovine muscle (post rigor) were frozen at −30 °C at two different rates (1.27 min/°C and 13.10 min/°C) and thawed at...     

Microstructure of high-pressure vs. atmospheric frozen starch gels

The aim of this study was to find out whether the ice crystal size of a starch gel, a model food system, could be reduced by high-pressure freezing compared with freezing at atmospheric pressure. The size and number of pores in thawed gels was determined by light microscopy and image analysis, and was taken as an indirect measure of ice crystals formed during the different freezing processes studied.The pore size and the total area occupied by the pores were clearly reduced by high-pressure freezing at 150–240 MPa compared with freezing at atmospheric pressure at the same cooling rate. The pore size in the high-pressure (nor in the atmospheric) frozen gels did not increase during a storage time of 3 months at − 24 °C (still air) at atmospheric pressure.Industrial relevanceHigh-pressure processing at subzero temperatures is not yet industrially applied. More evidence on the benefits of high-pressure freezing or thawing on the quality of real food materials as well as development of processing equipment is needed for commercialization of the processes. This study demonstrates that the pore size of frozen and thawed starch gels can be reduced by high-pressure freezing compared with freezing at atmospheric pressure. The reduced pore size was assumed to be a result of smaller ice crystals formed in the high-pressure freezing process. Based on this study, no conclusions can be drawn on the possibility of high-pressure freezing to improve the quality of real foods of a more complex composition and structure.     

Quick-freezing based on a nitrogen reversed Brayton cryocooler prototype: Effects on the physicochemical characteristics of beef longissimus thoracis muscle

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.