Strategies for raw sheep milk storage in smallholdings: Effect of freezing or long-term refrigerated storage on microbial growth

We assessed the effects of freezing and refrigeration over long periods on the microbiological quality of sheep milk. The raw milk was frozen in 1-L plastic bags or 5-L milk buckets and, after 1 mo, thawed at 7 or 25°C. We evaluated these samples immediately after thawing (d 0) and after 1 d of storage at 7°C. Furthermore, we stored fresh raw milk at 7°C for 10 d in the same packages and in a bulk milk cooler at 4°C (adding 10% of fresh raw milk daily). The total bacterial, total psychrotolerant, and proteolytic psychrotolerant counts were evaluated before and after thawing (for previously frozen milk) and daily (for refrigerated milk). The frozen-thawed milks showed no significant increase in bacterial counts immediately after thawing for all samples (<0.7 log cfu/mL), but only the samples packaged in 1-L bags and thawed at 7°C remained microbiologically adequate after 1 d of storage. Findings of the refrigerated samples were modeled using a modified Gompertz equation, obtaining a lag phase of around 0.5 (5-L bucket), 2.6 (1-L bag), and 7.0 (bulk milk cooler) d for total bacterial and total psychrotolerant counts. Maximum growth rates (µ_max) were 1.0 and 1.0 (5-L bucket), 1.2 and 1.3 (1-L bag) and 3.0 and 1.5 (bulk milk cooler) ln(cfu/mL) per day for total bacteria and total psychrotolerant counts, respectively. Compared with total bacteria and total psychrotolerant bacteria, psychrotolerant proteolytic bacteria grew slowly, reaching unacceptable counts only after 9 to 10 d of storage. The studied methods are interesting alternatives for preserving raw sheep milk on smallholdings.     

Prevalence of Yersinia enterocolitica in milk and dairy products and the effects of storage temperatures on survival and virulence gene expression

Yersinia enterocolitica was isolated from raw milk and dairy products from 10% of examined samples. The highest isolation rate was 22%, from raw milk, followed by 12%, 4% and 2% from fermented milk (Rayeb), pasteurised milk and ripened salted cheese, respectively. The virulence-associated genes ail and yst were detected in 30% and 10% of the isolates, respectively, while these genes were present simultaneously in 10% of the isolates. All the isolates showed susceptibility to gentamicin, ciprofloxacin and chloramphenicol, while only two of the isolates exhibited multidrug resistance. Storage of inoculated pasteurised milk at refrigeration (4 °C), freezing (−20 °C) and room (25 °C) temperatures revealed significant differences in Y. enterocolitica counts and relative expression of the two virulence genes. The isolation of potentially pathogenic Y. enterocolitica isolates from retail dairy products indicates risk to consumers; screening of prevalence, pathogenicity potential and antibiotic resistance is essential to implement control measures.     

Effect of freezing on Fossomatic cell counting in ewe milk

Using the Fossomatic method, a total of 10,072 analytical somatic cell count (SCC) observations were carried out on 4760 aliquots taken from 70 individual ewe milk samples with the objective of studying whether freezing showed significant differences of SCC in comparison with refrigeration, according to different analytical conditions. These conditions were four preservation procedures (without preservation, potassium dichromate, azidiol, and bronopol), two storage temperatures (refrigeration and freezing), five milk ages within storage (24 h postcollection in refrigeration, and 24 h, 15, 30, and 60 d postcollection in freezing), two thawing types (rapid and slow), and two analytical temperatures (40 and 60 degrees C). Preservation, storage, and analytical temperature, type of thawing and milk age within storage, and most of the interactions showed a significant effect on the SCC variation. On average, the SCC was lower after freezing than in refrigeration. This effect depended specifically on type of preservation and analytical temperature of milk. The SCC of milk unpreserved or preserved with bronopol or potassium dichromate, and analyzed at 40 degrees C, was not affected by freezing; however, use of azidiol as a preservative before freezing, and heating the milk to 60 degrees C following thawing resulted in significantly decreased SCC. Milk age had little quantitative influence on SCC of thawed milk. The type of thawing (rapid and slow) did not significantly influence SCC of milk analyzed at 40 degrees C. As a result, when using properly handled samples, the Fossomatic method could be used to enumerate SCC in samples frozen over the 60 d.     

Short communication: Effect of refrigerated storage on the pH and bacterial content of pasteurized human donor milk

Once pasteurized donor milk is thawed for its administration to a preterm or sick neonate, and until it is administered, it is kept refrigerated at 4 to 6°C for 24 h. After this time, unconsumed milk is discarded. This time has not been extended, primarily because of the concern of bacterial contamination. The aim of this study was to determine the changes in pH and bacterial count when pasteurized donor milk was kept under refrigeration for a prolonged period (14 d). In this prospective study, 30 samples of pasteurized donor milk from 18 donors were analyzed. Milk was handled following the regular operating protocols established in the neonatal unit and was kept refrigerated after thawing. pH measurements and bacteriology (on blood agar and MacConkey agar plates) were performed on each sample at time 0 (immediately after thawing) and then every day for 14 d. Changes in pH of samples over time were evaluated with linear mixed-effects regression models. A slow but gradual increase in milk pH was observed starting from the first day [mean (±SD) pH of 7.30 (±0.18) at time 0 and 7.69 (±0.2) on d 14]. No bacterial growth was observed in any of the samples throughout the complete trial except in one sample, in which Bacillus flexus was isolated. In conclusion, pasteurized human donor milk maintains its microbiological quality when properly handled and refrigerated (4–6°C). The slight and continuous increase in milk pH after the first day could be due to changes in the solubility of calcium and phosphate during refrigerated storage.     

Conventional and ohmic heating pasteurization of fresh and thawed sheep milk: Energy consumption and assessment of bacterial microbiota during refrigerated storage

This study aimed to assess the energy consumption of ohmic heating (OH) and conventional heating (CH) for pasteurization of fresh and thawed sheep milk and their impact on bacterial microbiota throughout refrigerated shelf-life (4 °C ± 1, 15 days). OH pasteurization using 8.33 and 5.83 V/cm electric field strength spent 72–73% less energy than CH pasteurization (515 KJ). The cultivation-dependent approach showed that at least 4.2 log cycle reductions were achieved in sheep milk submitted to CH and OH pasteurization, regardless sheep milk was fresh or thawed. Data from amplicon sequencing indicated that Staphylococcus was the most prevalent genus in raw samples at day 1 (F1_D1: 58.35%; T1_D1: 69.50%), while Pseudomonas became the most abundant after 15 days under cold storage (F1_D15: 50.15% and T1_D15: 54.50%). The relative abundance of all bacterial genera assessed remained similar on pasteurized samples by CH and OH throughout refrigerated storage.Industrial relevanceOhmic heating (OH) presents as a critical advantage rapid and uniform heating of fluid food material. No studies assessed the use of OH for pasteurization of sheep milk and to evaluate the impact of this technology on sheep milk bacterial microbiota during refrigerated storage. The findings of this study prove the feasibility of sheep milk pasteurization using OH_{8.33 V/cm}. OH also ensured the bacteriological stability of sheep milk during 2 weeks of storage under refrigeration conditions prior to dairy products manufacturing. This approach comprises a cheaper and easier way to store milk when compared to frozen storage, with potential benefits to small farmers and dairy industries.     

Evolution of quality parameters of high pressure processing (HPP) pretreated albacore (Thunnus alalunga) during long-term frozen storage

The aim of this work was to study the application of high pressure processing (HPP) before freezing for maintaining as much as possible the fresh characteristics of albacore steaks after long-term storage. HPP treatments were applied at 200 MPa for 0–6 min. Then, samples were immediately frozen (−20 °C) and stored (−20 °C) for up to 12 months. Once thawed (4 °C; 24 h), weight losses, color, texture, lipid oxidation (TBARS) and salt-soluble protein content were analyzed.After 12 months of frozen storage, 200 MPa for 6 min minimized thawing loss inherent to freezing and frozen storage and decreased TBARS (53.9%) with respect to the control. However, it resulted in changes in color (higher L*, b* and ΔE values) and texture (higher adhesiveness and springiness) and decreased the salt-soluble protein content with respect to non-pretreated samples. Nevertheless, after cooking, there were no differences in color and texture between HPP pretreated fish and the controls.     

Effect of various storage conditions on the stability of quinolones in raw milk

Research on the storage stability of antibiotic residues in milk is important for method development or validation, milk quality control and risk assessment during screening, confirmation, qualitative or quantitative analysis. This study was conducted using UPLC-MS/MS to determine the stability of six quinolones – ciprofloxacin (CIP), danofloxacin (DAN), enrofloxacin (ENR), sarafloxacin (SAR), difloxacin (DIF) and flumequine (FLU) – in raw milk stored under various conditions to investigate if quinolones degrade during storage of milk, and finally to determine optimal storage conditions for analysis and scientific risk assessment of quinolone residues in raw milk. The storage conditions included different temperatures and durations (4°C for 4, 8, 24 and 48 h; –20°C for 1, 7 and 30 days; –80°C for 1, 7 and 30 days), thawing temperatures (25, 40 and 60°C), freeze–thaw cycles (1–5), and the addition of different preservatives (sodium thiocyanate, sodium azide, potassium dichromate, bronopol and methanal). Most quinolones exhibited high stability at 4°C for up to 24 h, but began to degrade after 48 h. In addition, no degradation of quinolones was seen when milk samples were stored at –20°C for up to 7 days; however, 30 days of storage at –20°C resulted in a small amount of degradation (about 30%). Similar results were seen when samples were stored at –80°C. Moreover, no losses were observed when frozen milk samples were thawed at 25, 40 or 60°C. All the quinolones of interest, except sarafloxacin, were stable when milk samples were thawed at 40°C once and three times, but unstable after five freeze–thaw cycles. Preservatives affected the stability of quinolones, but the effects differed depending on the preservative and quinolone. The results of this study indicate optimum storage protocols for milk samples, so that residue levels reflect those at the time of initial sample analysis, and should improve surveillance programmes for quinolones in raw milk.     

Combined effects of high-pressure treatment and storage temperature on the physicochemical properties of caprine milk

The effects of high-pressure (HP) treatment (200–500 MPa for 25 min at 25 °C) combined with storage temperature (25 and 4 °C) on the physicochemical properties of raw caprine milk were studied. Storage of HP-treated and untreated milk samples at 25 °C considerably affected the changes in the conformation of milk proteins, which were reflected by changes in the protein sedimentation rate, gradual decreases in the soluble calcium and phosphorus contents, a slight decrease in pH, an insignificant decrease (P > 0.05) in viscosity, and a decrease in the casein hydration level of milk at the end of the storage time. In contrast, the HP-treated and untreated milk samples stored at 4 °C demonstrated different characteristics than the samples stored at 25 °C. These results could be due to calcium and phosphate association with caseins, which screen charges and reduce the repulsion of micelles during the storage time.     

Proteolysis and storage stability of UHT milk produced in Turkey

The effect of raw milk quality (total and psychrotrophic bacterial and somatic cell counts, proteinase and plasmin activity) and UHT temperature (145 or 150 °C for 4 s) on proteolysis in UHT milk processed by a direct (steam-injection) system was investigated during storage at 25 °C for 180 d. High proteinase activity was measured in low-quality raw milk, which had high somatic cell count, bacterial count and plasmin activity. The levels of 12% trichloroacetic acid–soluble and pH 4.6-soluble nitrogen in all milk samples increased during storage, and samples produced from low-quality milk at the lower UHT temperature (145 °C) showed the highest values. Bitterness in UHT milk processed from low-quality milk at 145 °C increased during storage; gelation occurred in that milk after 150 d. The RP-HPLC profiles of pH 4.6-soluble fraction of the UHT milk samples produced at 150 °C showed quite small number of peaks after 180 d of storage. Sterilization at 150 °C extended the shelf-life of the UHT milk by reducing proteolysis, gelation and bitterness.     

Fate of Listeria monocytogenes during freezing,thawing and home storage of frankfurters

Little information is available regarding the fate of Listeria monocytogenes during freezing, thawing and home storage of frankfurters even though recent surveys show that consumers regularly store unopened packages in home freezers. This study examined the effects of antimicrobials, refrigerated storage, freezing, thawing method, and post-thawing storage (7 °C) on L. monocytogenes on frankfurters. Inoculated (2.1 log CFU/cm²) frankfurters formulated without (control) or with antimicrobials (1.5% potassium lactate plus 0.1% sodium diacetate) were vacuum-packaged, stored at 4 °C for 6 or 30 d and then frozen (−15 °C) for 10, 30, or 50 d. Packages were thawed under refrigeration (7 °C, 24 h), on a countertop (23 ± 2 °C, 8 h), or in a microwave oven (2450 MHz, 1100 watts, 220 s followed by 120 s holding), and then stored aerobically (7 °C) for 14 d. Bacterial populations were enumerated on PALCAM agar and tryptic soy agar plus 0.6% yeast extract. Antimicrobials completely inhibited (p < 0.05) growth of L. monocytogenes at 4 °C for 30 d under vacuum-packaged conditions, and during post-thawing aerobic storage at 7 °C for 14 d. Different intervals between inoculation and freezing (6 or 30 d) resulted in different pathogen levels on control frankfurters (2.1 or 3.9 log CFU/cm², respectively), while freezing reduced counts by <1.0 log CFU/cm². Thawing treatments had little effect on L. monocytogenes populations (<0.5 log CFU/cm²), and post-thawing fate of L. monocytogenes was not influenced by freezing or by thawing method. Pathogen counts on control samples increased by 1.5 log CFU/cm² at d-7 of aerobic storage, and reached 5.6 log CFU/cm² at d-14. As indicated by these results, consumers should freeze frankfurters immediately after purchase, and discard frankfurters formulated without antimicrobials within 3 d of thawing and/or opening.     

Freezing and Thawing Conditions Affect the Gel Stability of Different Varieties of Rice Flour

The freeze‐thaw stabilities of three different rice flour gels (amylose rice flour with 28% amylose, Jasmine rice flour with 18% amylose and waxy rice flour with 5% amylose) were studied by first freezing at –18 °C for 22 h and subsequent thawing in a water bath at 30 °C, 60 °C and 90 °C, or by boiling in a microwave oven. The freeze‐thaw stability was determined for five cycles. Starch gels thawed at higher temperature exhibited a lower syneresis value (percent of water separation) than those thawed at lower temperature. Amylose rice flour gels gave the highest syneresis values (especially at the first cycle). The Jasmine rice flour gels gave a higher syneresis value than the waxy rice flour gel. Except for freezing by storage at –18 °C and thawing at 30 °C, there was no separation of water at any cycle when waxy rice flour gel was thawed at any temperature, irrespectively of the freezing methods used. Cryogenic Quick Freezing (CQF) followed by storage at –18 °C and then thawing (by boiling or by incubation at any other temperatures) gave lower syneresis values than all comparable samples frozen by storage at –18 °C. The order of syneresis values for the three types of rice flour was waxy rice flour < Jasmine rice flour < amylose rice flour. The syneresis values and the appearance of starch gels, which had gone through the freeze‐ thaw process, suggested that the order of freeze‐thaw stability of gels for the three types of rice flour was waxy > Jasmine > amylose rice flour.     

Effects of freezing and thawing methods and storage time on thermal properties of freshwater prawns (Macrobrachium rosenbergii)

Thermal stability of proteins in frozen and thawed freshwater prawns was measured by differential scanning calorimetry. The onset and peak melting temperatures corresponding to myosin denaturation, as well as total enthalpy of denaturation of prawn muscle, decreased after freezing–thawing treatments. There were no significant differences in the thermal properties of prawns with changes in the rate of freezing, ie blast (fast) vs still (slow) freezing. However, the thermal properties were influenced by the rate of thawing. Rapid thawing using a combination of microwave and tap water resulted in a lower (P⩽0·05) thermal stability of prawn proteins compared to slow (refrigeration) or moderately fast (tap water) thawing methods. Keeping prawn shells intact or not intact during freezing–thawing did not alter the thermal properties of the prawn proteins. © 1997 SCI.     

Influence of storage and preservation on fossomatic cell count and composition of goat milk

This study was designed to evaluate the effects of different test conditions on the somatic cell count (SCC) and composition of goat milk. To this end, 3600 tests were performed on 1800 aliquots taken from 40 goat milk samples using a combined instrument set-up based on flow cytometry for SCC and Fourier transform infrared analysis for fat, total protein, lactose, total solids, and freezing point determinations. The conditions tested were storage temperature (refrigeration and freezing), use of a preservative [no preservative (NP), azidiol (AZ), and bronopol (BR)], and age of the milk samples at each storage temperature (24 h to 42 d at refrigeration temperature and 21 to 105 d at freezing temperature). Significant effects on logSCC variation were shown by the storage temperature, the preservation treatment, the interaction of storage temperature × preservation treatment, and milk age within the interaction of storage temperature × preservative. Highest counts were recorded in the BR-preserved milk samples (logSCC = 5.877), and lowest counts were recorded in milk samples preserved using AZ (logSCC = 5.803). The use of frozen/thawed samples led to a significantly decreased logSCC for the treatments AZ and NP; the logSCC was not modified when BR-preserved frozen/thawed samples were analyzed. During storage, variations in the SCC observed for BR-preserved samples stored at refrigeration temperature for up to 25 d and at freezing temperature for all times tested were always <10%. The preservation treatment was the main factor affecting the milk composition variables examined. Highest values of most variables were obtained in the BR-preserved samples, and the lowest values were obtained in the AZ-preserved samples. The freezing point was lower in the preserved samples than in the NP samples. The levels of milk constituents recorded in the BR-preserved samples were independent of both the storage temperature and age of milk sample. Our findings indicate that the freezing point of goat milk must be interpreted according to the preservative used.     

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     

Changes in texture,cellular structure and cell wall composition in apple tissue as a result of freezing

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.     

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.     

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.     

Fourier transform infrared spectroscopy analysis of physicochemical changes in UHT milk during accelerated storage

The feasibility of using Fourier transform infrared spectroscopy (FTIR) to detect heat induced conformational rearrangements of proteins, protein–protein and protein–lipid interactions was studied with accelerated shelf-life protocols. Ultra-high temperature treated whole (WM) and skim milk (SM) were stored at 20, 30, 40 and 50 °C for 28 days. The changes leading to increased sedimentation in SM and WM at higher temperatures (≥40 °C) were observed during first 14 days of the storage period. Milk samples stored at 40 and 50 °C showed marked changes in the bands corresponding to conformations of milk lipids and formation of intermolecular β sheet of proteins, indicating protein–lipid interactions and aggregation. Dried sediment contained fat confirming protein–lipid participation in the sedimentation. FTIR was also able to detect changes that led to increased sedimentation in SM at temperatures lower than 40 °C, but only after 28 days.     

Kinetics of softening of potato tissue by temperature fluctuations in frozen storage

 Pre-packed and unpacked potato tissues were frozen, subjected to temperature fluctuations and then thawed. Temperature fluctuations ranged from –24  °C to –18  °C and from –18  °C to –6  °C. The number of fluctuacions ranged from 0 (that is to say, only freezing and thawing processes) to 32 at each above fluctuation range, simulating practical frozen storage conditions. Compression, shear and tension tests were carried out to measure the extent of structural damage caused to the potato tissue. Plots of log (rheological parameters and moisture content) versus number of temperature fluctuations showed two distinct regions; the first was a rectilinear plot with a steep negative slope up to four fluctuations. The second was also a rectilinear plot with a shallow negative slope beyond four fluctuations. For higher number of fluctuations, most of rheological parameters reached a value almost constant. These two-stage softening rate curves are consistent with the biphasic model and qualitatively similar to those for thermal softening of the vegetables. This study shows that two substrates S_a and S_b may be involved in providing firmness to potato tissue in freezing and frozen storage conditions. By analogy with earlier works, the term "frozen storage firmness" can be proposed to describe the amount of firmness that is resistant to degradation by freezing with temperature fluctuations during frozen storage and final thawing of the product. Received: 30 April 1999     

Freezing, thawing and cooking effects on quality profile assessment of green beans (cv. Win)

Results are presented of the effect of freezing followed by thawing (air and water immersion, both at environmental temperature) and cooking (traditional boiling in a covered pot) on quality profile (in terms of objective texture, colour, chlorophylls and pheophytins and sensory attributes) and structure of green beans (cv. Win). Freezing was carried out at three different rates by forced convection with liquid nitrogen vapour. Kramer shear cell (KSC) and Warner–Bratzler (WB) tests were used for objective assessment of the texture. The highest parameter values occurred in beans frozen at the highest rate and air-thawed at the slowest rate. Also, minimum alteration of the rheological behaviour of cooked beans was achieved by freezing at the highest rate. The best parameter for assessing the texture of frozen green beans after thawing and cooking was the Warner–Bratzler slope (S _WB). Coefficients of softening estimated for S _WB in the thawed beans showed that the texture of the beans frozen at −24 °C was almost four and almost five times softer than that of the beans frozen at −70 °C, for air and water thawing respectively. Frozen and thawed green beans were darker than fresh control, whereas freezing prior to cooking produced lighter-coloured beans than direct cooking. The freezing rate affected colour parameters differently depending on the process that followed. When beans were thawed, increasing the freezing rate produced lighter-coloured beans, whereas when beans were cooked, increasing the rate produced darker-coloured beans. No difference was found in sensory assessments between cooked samples frozen at −24 °C, −35 °C and −70 °C, which probably reflects the panellists' mixed preferences for quickly and slowly frozen samples. Scanning electron microscopy (SEM) revealed different degrees of mechanical damage to tissue structure, which accounted for the rheological behaviour of the beans.