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     

The effect of low-temperature blanching on the quality of fresh and frozen/thawed mashed potatoes

The effect of low‐temperature blanching (LTB) prior to cooking on colour, textural, firmness and oscillatory parameters, sensory attributes and overall acceptability of either fresh or frozen/thawed mashed potatoes was studied using response surface methodology (RSM) to establish the optimum temperature and time for blanching in both types of mashed potatoes. A central composite rotatable design was used to study the effects of variation in levels of blanching temperature (57.93–72.07 °C) and time (15.86–44.14 min) on the quality parameters. Stationary points showing maximum thickening had critical temperatures (approximately 67–69 °C) and times (approximately 26–30 min) in the ranges of temperature and time used for each independent variable for both fresh and frozen/thawed mashed potato. Results showed a high correlation between structural reinforcement and overall acceptability under optimum experimental blanching conditions. This demonstrates the potential of this experimental approach in terms of tailoring physical properties to predetermined levels in order to meet consumer preferences in mashed potatoes, and of altering the changes that occur after freezing and thawing.     

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.     

The effect of long-term frozen storage on the quality of frozen and thawed mashed potatoes with added cryoprotectant mixtures

Cryoprotectant mixtures were added to frozen/thawed (F/T) mashed potatoes in the form of amidated low-methoxyl (ALM) pectin and xanthan gum (XG), kappa-carrageenan (κ-C) and XG and sodium caseinate (SC) and XG, and the effect of frozen storage was examined. F/T mashed potatoes without added biopolymers had higher storage modulus G ' after freezing and frozen storage, associated with sponge formation due to amylose retrogradation. Oscillatory measurements indicated weakening of the structure of mashed potatoes without biopolymers and with added κ-C/XG and SC/XG mixtures at the end of storage due to ice recrystallisation, whereas the structure of samples with added ALM/XG mixtures was reinforced by increasing time in storage. Mashed potatoes with added mixtures exhibited water-holding capacity for 1 year. Samples with added κ-C/XG mixtures were more structured, although when both κ-C/XG and SC/XG mixtures were included in mashed potato, very acceptable sensory quality was maintained in usual frozen storage conditions.     

The effect of sample temperature on instrumental and sensory properties of mashed potato products

The effect of sample temperature on texture, firmness and the sensory attributes of fresh and frozen/thawed natural and commercial mashed potatoes was studied. Scanning electron microscopy was used to obtain information on the structure of potato cells and how this is affected by processing. Various parameters were measured and were found to increase at 55 °C, these changes could be related to gelatinisation of the starch remaining in the products. In natural mashed potato, texture profile analysis (TPA) and firmness parameters were lower in the processed than in the fresh samples, indicating a weakening of the structure; this was confirmed by photomicrographs. In commercial mashed potato, TPA parameters measured at 55 and 65 °C and firmness parameters measured at 25 °C were higher in the processed samples, indicating enhanced structural strength; this was confirmed by photomicrographs. Firmness per gram of product was the best instrumental parameter for measuring texture in these products. The results of the different analyses indicated that 55 °C was the preferred temperature for consumption of mashed potatoes.     

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     

Influence of osmotic dehydration and freezing on the volatile profile of kiwi fruit

The effect of osmotic dehydration on the volatile fraction of kiwi fruit was studied, as well as the effect of freezing and frozen storage. Osmotic treatments were carried out in sucrose solutions until the kiwi fruit reached 30°Brix, at atmospheric pressure (OD) and by applying a vacuum pulse (PVOD), by using 45 and 65°Brix sucrose. Volatile compounds of fresh, dehydrated and frozen-stored (at −18 °C for 1 month) samples were obtained by simultaneous distillation-extraction, and analyzed by GC-MS. Osmotic dehydration provoked formation of esters and a decrease in aldehydes and alcohols, depending on the dehydration treatment applied, which is similar to what occurs during kiwi ripening. A severe reduction of all volatile compounds occurred after one month in frozen storage, which smoothes the changes induced by osmotic treatments. Only small differences between dehydrated and non-pretreated frozen/thawed samples could be recognized.     

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     

Preservation of sweet cherry by isochoric (constant volume) freezing

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

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.     

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.     

Comparative Characterization of Dietary Fibre-Enriched Frozen/Thawed Mashed Potatoes

The potential use of commercial fibres (pea fibre, inulin, and their blends), as fibre-enriching agents in frozen/thawed mashed potatoes was reported. Pea fibre and inulin supplementations conferred hardness and softness to the product, respectively. Differences were attributed to the relationship of the fibre with the potato starch matrix. The association of pea fibre at low concentration (<15 g/kg mashed potatoes) and inulin at high concentration (>45 g/kg) is strongly encouraged to fortify the diet without promoting negative effects on textural and rheological properties of frozen/thawed mashed potatoes or colour and overall acceptability of the resulting products.     

Sensory and Texture Properties of Mashed Potato Incorporated with Inulin and Olive Oil Blends

The effect of adding different ratios of inulin and extra virgin olive oil blends, formulated without (MPA) and with cryoprotectants (MPB), on texture properties of fresh mashed potatoes and frozen/thawed mashed potatoes was studied. Inulin and extra virgin olive oil behaved like soft fillers, but inulin was associated with increased fibrousness and extra virgin olive oil with increased creaminess. In the total dataset and frozen mashed potatoes, frozen/thawed mashed potatoes, and MPA subgroups, component 1 was a contrast between mechanical and surface textural attributes, whereas in MPB samples component 1 was determined by geometrical attributes. Addition of inulin at 30 g/kg and extra virgin olive oil at 45 g/kg is recommended.     

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.     

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.     

Freezing/defrosting/frying of sardine fillets. Influence of slow and quick defrosting on protein quality

The effect of sequential freezing/defrosting/frying on protein quality is not well known. With this in mind, fillets of fresh sardine were stored frozen, then thawed, either conventionally at 4 °C in a refrigerator or with the use of a microwave oven, and subsequently deep‐fried. Proximate and amino acid compositions, protein solubility in sodium dodecyl sulphate/β‐mercaptoethanol (SDS/β‐ME), total ? SH group content and amino acid chemical score were determined. The lowest protein concentration was observed in frozen/4 °C‐thawed sardines (CR), whilst the lowest fat content was found in both fresh/fried sardines (F) and 4 °C‐thawed/fried sardines (CF). Every step of each process studied caused a decrease in cyst(e)ine; the most important loss was recorded in CF samples and in frozen sardines fried without defrosting (Fro‐F). The lowest solubility in SDS/β‐ME and the lowest total ? SH group content were observed for Fro‐F samples and microwave‐thawed/fried sardines (MF). On the other hand, the lowest chemical score was found for Fro‐F, CF and MF samples. Although weight loss and proximate composition seemed to change less when defrosting sardine fillets using a microwave oven rather than at 4 °C, the results for SDS/β‐ME solubility and total ? SH group content suggest that a slow defrosting process (refrigerator at 4 °C) is preferable to a much quicker process (microwave oven) for thawing frozen sardine fillets before frying. Copyright © 2003 Society of Chemical Industry     

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     

Differentiation of fresh and frozen/thawed fish, European sea bass (Dicentrarchus labrax), gilthead seabream (Sparus aurata), cod (Gadus morhua) and salmon (Salmo salar), using volatile compounds by SPME/GC/MS

BACKGROUND: A simple method based on solid phase microextraction/gas chromatography/mass spectrometry (SPME/GC/MS) was applied for studying the volatile profiles of whole fish samples of European sea bass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) and fillets of cod (Gadus morhua) and salmon (Salmo salar) during frozen storage in order to be able to differentiate a fresh product from one that has been frozen. Analysis of volatile compounds was performed on these two product types, fresh and after freezing/thawing following storage at ? 20 °C for 30 and 90 days. RESULTS: More than a hundred volatile compounds were found by SPME/GC/MS. Statistical processing by principal component analysis and ascending hierarchical classification was used to classify the samples into categories and verify the possibility of separating fresh samples from those that had been frozen and thawed. The compounds to be used as differentiators were identified. Four compounds were common to all species: dimethyl sulfide, 3‐methylbutanal, ethyl acetate and 2‐methylbutanal. Not only were they found in larger quantities after thawing but they also increased with the duration of storage at ? 20 °C. CONCLUSION: These four compounds can therefore be considered as potential markers of differentiation between a fresh product and one that has been frozen. Copyright © 2012 Society of Chemical Industry     

The Effects of Applying Oscillating Magnetic Fields During the Freezing of Apple and Potato

The use of Oscillating Magnetic Fields (OMF) during freezing has been advocated by some for preserving the intrinsic ‘fresh’ qualities of the subsequently thawed food. This paper describes studies to investigate the effects of a range of different OMF freezing settings using an ABI Cells Alive System (CAS) freezer on characteristics of the freezing process and food-quality parameters. The effects of short-term frozen storage on sample properties were also evaluated. Apple and potato samples were frozen at ??30 or ??45 °C with a range of OMF settings whilst measuring sample temperatures. Samples were then either immediately thawed or stored frozen for 1 month. Effects of OMF on freezing temperature curve texture parameters, sample dimensions, sample weights and sample texture parameters of thawed samples were determined. Overall, results showed no statistically significant continuum of effect in these trials that prevails across all CAS settings and/or temperatures for apple and potato for any parameter measured. However, significant effects were seen at some settings for some parameters for each product, and this may suggest that OMF may possibly be tuneable for specific situations. Where significant changes did occur, these only appeared at isolated settings, temperatures or storage periods. This suggests that OMF may not affect all foods in an equal manner and that any effect depends on an interrelationship of food type, freezing rate, magnetic field frequency and storage conditions. We conclude that there is no major effect of OMF on freezing parameters and there is no overall pattern of OMF on freezing processes.     

Changes in Texture of Green Peas during Freezing and Frozen Storage

The effects of freezing (still-air, air-blast, and Freon immersion freezing) and frozen storage (?5°, ?10° and ?15°C for 0 to 48 wk) on texture of cooked frozen peas were examined. Peas frozen by a Freon-12 immersion method with no appreciable damage to cell structure had firmer and more chewy sensory textural quality compared to those peas frozen by a slower freezing method. Sensory tenderness decreased initially and then increased with storage time to the maximum storage times of 6 wk in ?5°C storage, 16 wk in ?10°C storage, and 48 wk in ?15°C storage. Sensory chewiness increased initially, then remained constant with storage time except in ?15°C storage. Generally, the lower storage temperature resulted in less sensory chewiness. The correlation coefficients (r) between sensory and objective measurements for tenderness and chewiness were 0.76 and 0.88 (P < 0.05), respectively.