Principal component analysis to study the effect of temperature fluctuations during storage of frozen potato

 The effects of temperature fluctuation ranges, number of fluctuations carried out, and packaging during frozen storage on the texture of potato tissue in terms of compression, shear, and tension rheological parameters were assessed through data generated according to a factorial design using principal component analysis (PCA). Five ranges of fluctuation (–24  °C to –18  °C, –18  °C to –12  °C, –12  °C to –6  °C, –24  °C to –12  °C and –18  °C to –6  °C) applied 2, 4, 8, 16, 24, and up to 32 times on unpacked and pre-packed frozen potatoes, were considered. The controls were unpacked and prepacked frozen tissues thawed immediately without undergoing any fluctuation. In addition, several geometrical, technological, and chemical parameters were determined. PCA showed that maximum shear force, F_s was the best rheological parameter for differentiation of the structural damage and softening occurring in the tissue at each treatment, which was closely related to its duration, TT _d . PCA did not permit complete discrimination between the five fluctuation ranges, but it clearly separated samples subjected to –18  °C/–6   °C from those subjected to –24  °C/–18  °C. Frozen samples undergoing up to four fluctuations formed a separate cluster from those undergoing a higher number. Analysis also clearly separated unpacked from pre-packed samples in response to slower freezing rates reached in the latter. Received: 17 December 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.     

The effect of the pre-treatments and the long and short-term frozen storage on the quality of raspberry (cv. Heritage)

Results are presented of the effect of pre-treatments before freezing followed by long and short-term frozen storage (12 months at –18 °C and 24 days suffering temperature fluctuations between –18 °C and –12 °C) on quality parameters of raspberry. Pre-treatments were carried out with calcium, low methoxyl pectin, a combined solution, and results compared with untreated control fruits. Kramer shear, back extrusion, compression, and multiple penetration tests were used to measure rheological behavior. One-hundred mM CaCl₂ reduced the long and short-term frozen storage induced loss of firmness. For long-term storage at –18 °C, a softening of the tissue became evident between 3 and 12 months and at each date test the stored fruits were firmer than those without storage. For short-term storage with fluctuations, the loss of firmness was evident between 0 and 24 days, and at all the testing dates the stored fruits were softer than those without storage. Results evidenced a higher cell damage in the short-term frozen storage. Coefficients of softening per day suffering fluctuation were determined, the highest values being given by Kramer shear energy and back extrusion maximum force (>1%). Short-term frozen storage affected physical and physico-chemical characteristics, increasing the saturation (r) and the anthocyanins and decreasing the ascorbic acid of the raspberries. In both storage conditions, pre-treatments reduced the drip loss, which correlated best with the Kramer shear energy. Panelists detected mainly time effect on the sensory firmness. For long-term, sensory firmness and juiciness gave the highest correlations with back extrusion maximum force, while for the short-term, sensory firmness and drip loss gave the highest correlations with the Kramer shear energy. SEM revealed different degrees of mechanical damage to structure, which accounted for rheological behavior of the fruits.     

Optimization of stepwise blanching of frozen-thawed potato tissues (cv. Monalisa)

 Response surface methodology was used to compare the effect of temperature and time of the first step of blanching on compression, shear, tension and stress-relaxation parameters of frozen-thawed potato tissues. A central, composite rotatable design was used to study the effects of variation in levels of temperature (52.93–67.07  °C) and time (15.86–44.14 min) on rheological parameters. Blanching temperature was the most important factor affecting the mechanical properties tested. The models fitted for the apparent modulus of elasticity in compression, maximum tension force, and relaxed force in the first cycle (F _r1); all had R ²>0.85 (P≤0.01) and were used for doing predictions. Optimum conditions were with in the ranges of temperature (60–65  °C) and time (25–35 min) used for each factor. In the experimental verification of the models at 65  °C during 30 min, the lowest percentage residual between experimental and predicted values was obtained for F _r1 (0.644), which was therefore the most appropiate parameter for detecting the firming effect that the pectinesterase activity produced on frozen potato tissues as a consequence of stepwise blanching under these conditions. Received: 3 February 1999 / Revised version: 12 April 1999     

Freeze- and bake-stable glazing for confectionery products characterized by differential scanning calorimetry

 A freeze- and bake-stable glazing for pastry has been developed based on an optimization procedure in which starches and carrageenans as thickening agents were combined with sugar alcohols and sodium tripolyphosphate. Instrumental gloss measurement on test biscuits showed that glazing consisting of 3% ι-carrageenan and 15% sorbitol (w/w) had maximal gloss. This was confirmed on pastry products. In contrast to glazing based on mannitol, no crystallization problems were encountered for this glazing, for which differential scanning calorimetry showed that some water freezes at –3  °C followed by freezing of supercooled encapsulated water at –20  °C and by a glass transition at –61  °C. During normal frozen storage of preglazed, nonbaked pastry, the glazing is thus in a nonstable rubbery state with a limited resistance to water migration and ice crystal formation, which can be substantially improved by storage at temperatures below –61  °C. Received: 1 March 1999 / Revised version: 19 April 1999     

Effect of freezing rate and programmed freezing on rheological parameters and tissue structure of potato (Cv. Monalisa)

 Compression, shear and tension tests were carried out to determine the effect on potato tissues of different freezing rates (0.5, 1.25 and 2 °C/min down to −18 °C), thawing up to +20 °C at the same rates, and one, two, three or four successive freeze/thaw cycles. The effect of freezing rate on the zone of maximum crystallization was also examined, along with different combinations of programmed freezing, and the effect of prior cooling was assessed. Minimum alteration of the rheological behaviour of slowly thawed tissues was achieved by pre-freezing (3 °C for 30 min), slow cooling phases (0.5 °C/min) before and after the phase of maximum ice crystallization and quick freezing (2 °C/min) in the same phase. The shear test was found to be well suited to the study of these effects. Examination of the tissues by SEM revealed differing degrees of mechanical damage to tissue structure, which accounted for the rheological behaviour of the samples. Coefficients of softening per freeze/thaw cycle were determined for the various rheological parameters, the highest value being given by the modulus of rigidity (17.75%). Received: 29 July 1996     

Use of β-hydroxyacyl-CoA-dehydrogenase (HADH) activity to differentiate frozen from unfrozen fish and shellfish

 Further work on an enzymic method to differentiate frozen from unfrozen fish and shellfish is reported. The method is based on the release of the β-hydroxyacyl-CoA-dehydrogenase (HADH) from mitochondria during freezing. Enzymic activity was evaluated in fresh and frozen thawed samples from sole (Solea solea), sea bream (Pagellus centrodontus), hake (Merluccius merluccius), gilt headed bream (Sparus aurata), sea bass (Dicentrarchus labrax), salmon (Salmo salar), prawn (Penaeus japonicus) and Norwegian lobster (Nephrops norvegicus). Changes in the HADH activity of fresh and frozen thawed samples were compared after freezing at –196  °C for 15 min. Two values were obtained: U (by dividing: HADH activity of samples frozen at –196  °C, then thawed/HADH activity of unfrozen samples) and F (by dividing: HADH activity of samples frozen at –18  °C, thawed, then frozen at –196  °C /HADH activity of samples frozen at –18  °C, then thawed). Statistical analysis showed significant differences (P≤0.05) between both quotients for gilt headed bream, salmon, sea bream, sole and prawn, and an arbitrary limit was set at 2 to differentiate frozen thawed from unfrozen samples. The application of this limit made it possible to discriminate the unfrozen from the frozen thawed state of around 90% of the total samples analysed. Best results were obtained for prawn (100% of samples differentiated). In the present paper, a laboratory routine is proposed based on the comparison of the HADH activity of a sample analysed straight away and that of a sample frozen at –196  °C and then thawed. The reported method is simple and fast. The entire laboratory procedure can be performed in 45 min. Received: 20 July 1998 / Revised version: 2 November 1998     

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

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

Effect of temperature fluctuations during frozen storage on the quality of potato tissue (cv. Monalisa)

 Results are presented on the effect of different ranges of temperature fluctuation (–24 to –18°C, –18 to –12°C, –12 to –6°C, –24 to –12°C and –18 to –6°C) on the compression, shear and tension parameters of packed and unpacked frozen potato tissue. The initial temperature, duration and number (2, 4, 8, 16, 24 and 32) of fluctuations were varied. The highest parameter values occurred in samples subjected to fluctuations between –24°C and –18°C, and the lowest values in the range –18 to –6°C. The mechanical strength of the frozen tissue decreased with an increase in the number of fluctuations and in most cases was lower in the packed samples. Moisture loss was greatest in the –18 to –6°C range for pre-packed samples. Changes in the maximum compression force, as a measure mechanical damage, showed the greatest level of significance. Received: 17 June 1997     

Effect of temperature fluctuations during frozen storage on the quality of potato tissue (cv. Monalisa)

 Results are presented on the effect of different ranges of temperature fluctuation (–24 to –18°C, –18 to –12°C, –12 to –6°C, –24 to –12°C and –18 to –6°C) on the compression, shear and tension parameters of packed and unpacked frozen potato tissue. The initial temperature, duration and number (2, 4, 8, 16, 24 and 32) of fluctuations were varied. The highest parameter values occurred in samples subjected to fluctuations between –24°C and –18°C, and the lowest values in the range –18 to –6°C. The mechanical strength of the frozen tissue decreased with an increase in the number of fluctuations and in most cases was lower in the packed samples. Moisture loss was greatest in the –18 to –6°C range for pre-packed samples. Changes in the maximum compression force, as a measure mechanical damage, showed the greatest level of significance. Received: 17 June 1997     

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.     

Kinetics of thermal softening of potato tissue (cv. Monalisa) by water heating

The temperature dependence of rheological parameters as firmness indicators for potato tissue was determined within the temperature range 50-100 °C using four different objective methods. The rate of thermal softening of potato tissue by water treatment at 50 °C, 90 °C, and 100 °C was consistent with one pseudo first-order kinetic mechanism, while at 70 °C and 80 °C the rate of softening was consistent with two simultaneous pseudo first-order kinetic mechanisms. Kinetic theory was suitable to detect an increase of firmness through heating at 60 °C, mainly between 20 min and 40 min, presumably by pectinesterase activation. This study shows that two substrates S_a and S_b may be involved in providing firmness to potato tissue at 70 °C and 80 °C. For these temperatures, mechanism 1 is more probably due to gelatinization and light cooking, whereas mechanism 2 is more likely to represent the changes of the pectic substances in the cell wall and interlamellar region. At 90 °C and 100 °C the gelatinization process was fast and therefore the simple mechanism fitted presumably reflects the degree of solubilization of the pectic substances. At 50 °C and 60 °C there was practically no gelatinization, so that the simple mechanism fitted presumably represented incipient solubilization of pectic material. In water heating, gelatinization contributes less than the cell wall structure to potato tissue firmness on the basis of either kinetic parameters or microscopic observations. Maximum breaking compression force and modulus of rigidity were the most suitable rheological parameters for studying the softening of potato tissue in water heating.     

Structure of northern snakehead (Channa argus) meat: Effects of freezing method and frozen storage

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.     

Rheological and microstructural changes in gels made from high and low quality sardine mince with added egg white during frozen storage

 This paper examines the influence of freezing temperature (–40°C or –18°C) and frozen storage temperature (–18°C or –12°C) on gels made from two different sardine minces (M1, high functional quality; M2, low functional quality), with the addition of egg white as a gel enhancing ingredient. To characterize the washed mince, proximate analyses and protein functionality were determined. Freezing at either –40°C or –18°C caused no drastic changes in gel structure. Throughout the course of frozen storage of all samples, a decrease in the water holding capacity (WHC) was detected, along with an increase in the amount of protein soluble in 8 M urea. At 90 days the gels frozen at –40°C exhibited numerous ice micro-crystals; however, they did not affect the external appearance of the gel and had hardly any effect on gel strength, shear strength, hardness, cohesiveness or elasticity. On the other hand, at 90 days the gels frozen at –18°C and stored at either temperature exhibited large, macroscopically visible ice crystals. In these samples, gel strength and shear strength increased while hardness decreased. No definite changes attributable to mince quality were detected during frozen storage. Received: 23 June 1997     

Pressure-shift freezing and its influence on texture,colour, microstructure and rehydration behaviour of potato cubes

Temperature changes during pressure-shift freezing (400 MPa) of potato cubes and its effects on the drip loss (weight and conductivity), texture (shear and compression tests), colour (L, a, b values), drying behaviour, rehydration properties (water uptake, texture after rehydration) and visible cell damage after thawing (micrographs) were investigated and compared with conventional freezing (0.1 MPa, -30 °C), subsequent frozen storage (-18 °C) or pressure treatment (400 MPa) at +15 :C. Pressure-shift freezing resulted in increased crystallization rates compared to conventional freezing at -30 °C. Crystallization and cooling to ?8 =C took 2.5 min during and after pressure release versus 17 min at atmospheric pressure. Drip loss was reduced from 12.0 to 10.8g/100g. Water uptake during 10 min of rehydration (93.9g/100g compared to 77.4g/100g and incomplete rehydration) and texture values were improved. Browning after thawing or after fluidized bed drying was reduced (increased a value, lower L value), suggesting partial enzyme inactivation during pressure treatment. Differences in colour and texture to the untreated controls were smaller after pressure-shift freezing than after conventional freezing. Cooling to ?30 °C after pressure-shift freezing did not significantly affect the results, whereas subsequent frozen storage at ?18 °C resulted in quality deterioration, as observed after frozen storage of conventionally frozen samples. The improved preservation of cell structure was demonstrated using scanning electron microscopy.     

Effect of modified starch concentration and freezing and thawing rates on properties of mashed potatoes (cv. Kennebec)

Effects of modified cornstarch concentration, freezing rate and thawing mode on quality properties of mashed potatoes were examined. The product was tested by oscillatory rheometry, instrumental texture profile analysis (ITPA) and cone penetration, colour, dry matter (DM) and sensory analyses. Oscillatory parameters showed that increasing starch concentration resulted in a softer product in which gel strength decreased in direct proportion to concentration in quick‐frozen and microwave‐thawed product. Quick freezing made for firmer mashed potatoes, whereas oscillatory, ITPA and penetration parameters were lower in the samples thawed at 4 °C. Starch concentration and instrumental consistency and firmness were directly proportional to one another. Adding starch at the highest concentration, quick freezing and slow thawing produced a lighter‐coloured mashed potato, although the loss of colour in the processed product was not detected by panellists in the microwave‐thawed samples. DM was higher in the quick‐frozen samples and lower in air‐thawed samples. The factors studied affected different sensory texture parameters but had hardly any effect on attributes perceived during final and residual phases of mastication. Only starch concentration had a significant effect on sensory acceptability, but interactions between factors showed that starch addition followed by quick freezing was judged undesirable by the panellists due to excessive softening. Oscillatory parameters correlated well with sensory texture attributes and proved quite precise. They would therefore appear to be the best test to monitor properties of frozen mashed potatoes. Copyright © 2007 Society of Chemical Industry     

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.     

Impacts of Low and Ultra-Low Temperature Freezing on Retrogradation Properties of Rice Amylopectin During Storage

Amylopectin retrogradation is a serious problem in starch-based ready meals. In the current research, rice amylopectin was frozen by low temperature (−20, −30, and −60°C) and ultra-low temperature (−100°C), and then stored at 4°C for 21 days or at −18°C for up to 5 months to evaluate the retrogradation properties. Amylopectin retrogradation enthalpy of rice was determined by a differential scanning calorimetry. The results showed that low temperature and ultra-low temperature freezing can effectively retard amylopectin retrogradation during the freezing process and during frozen storage (−18°C) for at least 5 months. However, rice amylopectin still retrograded after the freezing process during chill storage at 4°C. The methods of low and ultra-low temperature freezing combined with frozen storage might be potentially very useful for food industry to produce high quality starch-based ready to eat meals.     

Differential lipid damage in various muscle zones of frozen hake (Merluccius merluccius)

 A comparison of the lipid damage produced in different hake zones was carried out during frozen storage at –11 and –18  °C. Three light muscle zones and the dark muscle were considered. Lipid oxidation [conjugated dienes; thiobarbituric acid index (TBA-i); fluorescence formation] and hydrolysis (free fatty acids, FFA) were determined. The most predominant lipid damage in all zones was hydrolysis, at the end of storage reaching values of about 40% (for the light muscle zones) and 12% (for the dark muscle) of the total lipids at –11  °C. Significant (P<0.05) correlation value (r=0.67–0.85) relationships between the frozen storage time and the FFA content were obtained for the four muscle zones at both temperatures. A comparison of the regression lines slopes in the different zones showed that a lower (P<0.05) lipolitic activity was produced in the dark muscle compared to the three light zones at both temperatures. A low lipid oxidation development was produced in the three light muscle parts, so that no significant differences between them could be assessed. However, the dark muscle showed a higher oxidation development (TBA-i and fluorescence formation) as a result of a higher lipid content and the presence of prooxidant constituents. Received: 16 June 1998     

Differential lipid damage in various muscle zones of frozen hake (Merluccius merluccius)

 A comparison of the lipid damage produced in different hake zones was carried out during frozen storage at –11 and –18  °C. Three light muscle zones and the dark muscle were considered. Lipid oxidation [conjugated dienes; thiobarbituric acid index (TBA-i); fluorescence formation] and hydrolysis (free fatty acids, FFA) were determined. The most predominant lipid damage in all zones was hydrolysis, at the end of storage reaching values of about 40% (for the light muscle zones) and 12% (for the dark muscle) of the total lipids at –11  °C. Significant (P<0.05) correlation value (r=0.67–0.85) relationships between the frozen storage time and the FFA content were obtained for the four muscle zones at both temperatures. A comparison of the regression lines slopes in the different zones showed that a lower (P<0.05) lipolitic activity was produced in the dark muscle compared to the three light zones at both temperatures. A low lipid oxidation development was produced in the three light muscle parts, so that no significant differences between them could be assessed. However, the dark muscle showed a higher oxidation development (TBA-i and fluorescence formation) as a result of a higher lipid content and the presence of prooxidant constituents.