Effect of processing on the texture and structure of raspberry (cv. Heritage) and blackberry (cv. Thornfree)

This paper reports separate studies of the effect of pre-treatments (CaCl₂, low methoxyl pectin (LMP), and combined solutions) and the effect of freezing method (at four different rates) and thawing mode (at two different rates) on objective parameters, structure and sensory characteristics of fresh raspberries and blackberries. After that, the effect of a complete freezing process combining the best pre-treatments with the best freezing/thawing conditions found for each fruit was investigated. Kramer Shear Cell (KSC), back extrusion, compression and multiple penetration tests were used to measure fruit texture objectively. For calcium and LMP pre-treatments, which were applied separately, texture parameters were significantly higher in samples treated at the highest concentrations (100 mM of CaCl₂ for both fruits and 0.3 and 3% of LMP for raspberry and blackberry, respectively) compared to fresh controls. Blackberry structure was more susceptible than raspberry structure to the effect of pre-treatments. For the combined pre-treatments, the highest texture parameters were found in the samples treated with CaCl₂ (100 mM) and LMP (0.1%) in the case of raspberries and CaCl₂ (100 mM) and LMP (3%) in the case of blackberries. Combined pre-treatment did not increase firmness with respect to that of samples treated only with calcium, which indicates that CaCl₂ preserved the raspberry structure more efficiently during processing. Fruits frozen by forced convection with liquid nitrogen vapour at –40 °C were significantly firmer. Raspberries should be thawed at 5 °C, whereas blackberries may be thawed at room temperature. Sensory analysis showed that the blackberry structure was more resistant to freezing. In both fruits, over the complete process parameter values were again highest in the samples treated with 100 mM CaCl₂, applied either separately or in combination with LMP. In raspberry, panellists detected no significant differences between sensory texture parameters of the different samples, and in blackberry, panellists found no significant differences between any of the sensory characteristics. Multiple penetration maximum force (F _MP) was the parameter that best expressed product firmness for both fresh and frozen raspberries, whereas compression slope (S _C) best reflected changes in blackberries. SEM mainly corroborated results from objective texture parameters.     

EFFECT OF PRESSING ON MOISTURE LOSS AND SENSORY PROPERTIES OF BEEF

Paired halves of top round (Semimembranosus and Adductor muscles) sub-primals were removed from each of ten, USDA Choice, approximately 272 kg steers, freezer wrapped, frozen, and stored at -24° C for approximately 1 month. Following freeze-tempering, each primal was divided longitudinally with one half being pressed at 344 Newtons/cm² to a specified shape in a Bettcher Model 70 Press, while the other half served as nonpressed control. Moisture loss and sensory properties were determined for 2.54 cm thick steaks. No significant differences were found in juiciness, flavor or Kramer shear values between pressed and non-pressed top round steaks, however, sensory tenderness scores were lower (P < 0.05) for steaks that were pressed than for those that were not pressed. Holding steaks at 2° C for 0, 1, 2, or 3 days did not significantly alter Kramer shear or sensory panel evaluations. Cooking loss, drip loss and total loss were not affected by pressing. Drip loss was lower (P < 0.05) and cooking loss was higher (P < 0.05) for steaks held one day at 2° C than for steaks held 2 or 3 days at 2° C. When drip loss and cooking loss were combined (total loss) there was no difference among the four storage times. Drip loss following portion cutting could result in a problem in fresh merchandising of freeze-tempered cuts.     

Kinetic study of the effect of the osmotic dehydration pre-treatment to the shelf life of frozen cucumber

Quality of frozen cucumber can significantly be improved by means of osmotic dehydration as a pre-freezing treatment. The objective of this work was to study the effect of osmotic pre-treatment with alternative osmotic solutes – oligofructose and a high DE maltodextrin – on the quality and functional properties of frozen cucumber tissue. Colour, texture and sensory characteristics of pre-treated and conventionally frozen samples were comparatively measured at 3 months time, during storage at four different temperatures, − 5, − 8, − 12, − 15 °C. Colour change was kinetically studied, and its temperature dependence was modelled by the Arrhenius equation. Dehydrofrozen samples exhibited significantly improved stability, with the rates of colour change being reduced up to 36.7% for osmotically pre-treated cucumbers, compared to the untreated samples. The results indicate that osmodehydrofrozen compared to conventionally frozen sliced cucumbers show improved firmness for prolonged storage period. Sensory evaluation also showed good organoleptic quality in osmodehydrofrozen cucumber slices.

Industrial relevance

The poor quality of frozen cucumber can significantly be improved by means of the cryoprotection accomplished by a pre-freezing osmotic dehydration step. Osmotic dehydration at mild temperatures can preserve product texture, and enhance flavour and other sensory properties (colour). In this paper, the effect of osmotic pre-treatment on the quality and shelf life of frozen cucumber tissue was investigated. The aim was to develop a method of processing cucumber that maintains palatable product while optimizing quality at the beginning and during frozen storage.     

Optimization of low-temperature blanching for retention of potato firmness: Effect of previous storage time on compression properties

Low-temperature blanching (LTB) of potatoes (cv. Kennebec), both without further processing and prior to cooking or freezing + cooking, significantly increased firmness retention as measured from compression parameters. The increase in firmness with respect to that of unblanched potatoes diminished in the order: blanched at 60 °C for 60 min and cooked > blanched at 60 °C for 60 min frozen and cooked > blanched at 60 °C for 60 min. Potato tubers were kept in refrigerated storage, and firmness, PME activity and dry matter (DM) content were periodically sampled over a period of 80 days. In the early stages of storage, PME activity lost 40% of its original value after 60 min at 60 °C, indicating that the contribution of starch breakdown products to the firmness of cooked and frozen cooked potatoes predominated over the effect of enzyme activity. With increasing time in storage, PME activity measured in the fresh tissue increased by 95% of its original value after 35 days; this resulted in changes in the pectic polymers which made for a firmer texture and different PME behaviour versus LTB temperature and time. A central composite rotatable design was used to study the effects of variation in levels of temperature (52.93–67.07 °C) and time (31.72–88.28 min) on compression parameters and PME activity. Stationary points showing maximum mechanical resistance had critical temperatures and times in the ranges of temperature (58–60 °C) and time (66–75 min) used for each independent variable. Results show a high correlation between PME activity and tissue firmness, suggesting that the contribution of the changes in the composition of the cell wall to the firmness of frozen cooked potatoes increased with increasing time in storage and reached a maximum in the intermediate stages of storage (35 days). Engineering stress (_u) proved to be the most appropriate compression parameter for detecting the firming effect that the PME activity produced on the frozen-cooked potato tissues as a consequence of LTB under these conditions.     

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.     

Effect of freezing rate and frozen storage on the texture and sensory analysis of two pineapple fruit cultivars

The effects of freezing rate (in a cold room at –18°C and in an air-blast freezer at –50°C) and frozen storage (at –18°C for 0–12 months) on texture and the sensory quality of pineapple fruit slices of Smooth Cayenne and Red Spanish cultivars were examined. The texture of pineapple fruit slices was evaluated by sensory and instrumental measurements. Sensory texture was highly (P0.05) and positively correlated with overall fruit acceptability and appearance and negatively correlated with flavour in the two cultivars studied. Sensory analysis showed no significant differences (P0.05) in the appearance, colour, odour, texture, flavour and overall acceptability between the two varieties studied when frozen at different rates, as compared to the fresh product and after 1 year of frozen storage at –18°C. The two cultivars are suitable for freezing.     

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

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

Blanching effects on chemistry, quality and structure of green beans (cv. Moncayo)

Green beans (cv. Moncayo) were blanched at 65, 70, 75, 80, 85, 90 and 97 °C for 2.5, 5, 10, 20 and 40 min. Pectinesterase (PE) activity was highest in cell-wall-bound extracts of beans blanched at 70 °C/10 min. The lowest water-soluble pectin fraction, the highest EDTA-soluble pectin fraction and the lowest degree of esterification of the EDTA-soluble fraction were all recorded for the same temperature/time combination; these effects can therefore be attributed to PE activity. Chemical changes did not affect initial firmness of the beans, which was practically constant after blanching at 65, 70, 75 and 80 °C. Simple first-order models were adequate to establish softening kinetics for beans blanched at 85, 90 and 97 °C. In this temperature range, Kramer maximum force was the mechanical parameter that best characterised bean softening by blanching. For all temperatures, short-time blanching increased the coloration and total chlorophyll content of the samples with respect to fresh control, thus precluding the use of simple models. In the treated beans, the ascorbic acid content was consistently lower than in the control and decreased continuously with increasing time. Microphotographs showed no appreciable differences in morphology between fresh and blanched beans at 65, 70 and 75 °C, which would explain the similarity of mechanical behaviour in these samples. Blanching at 85, 90 and 97 °C caused loosening and swelling of the cell walls owing to breakdown of the pectic material, which again helps to explain the observed loss of firmness.     

Freshness Assessment of Thawed and Chilled Cod Fillets Packed in Modified Atmosphere Using Near-infrared Spectroscopy

Near-infrared reflectance (NIR) spectra was recorded of 105 samples of cod mince prepared from chill stored thawed cod fillets of varying quality in modified atmosphere packaging (MAP). Traditional chemical, physical, microbiological and sensory quality methods developed for assessing fresh fish products were determined on the same cod fillets. The purpose was to evaluate the potential of NIR spectroscopy for estimating (i) frozen storage temperature, (ii) frozen storage period and (iii) chill storage period of thawed-chilled MAP Barents Sea cod fillets. Furthermore, the potential for measuring of selected quality attributes as drip loss, water holding capacity and content of dimethylamine by NIR was evaluated. The results of the investigation were presented using multivariate modelling methods such as partial least-squares regression (PLSR) and discriminant partial least-squares regression (DPLSR). Systematic differences in the NIR measurements on minced cod fillets were primarily due to the chill storage duration (days at 2°C) on thawed-chilled MAP fillets. PLSR models based on wavelengths selected by a new Jack-knife method resulted in a correlation coefficient of 0.90 between measured and predicted duration of chill storage period (days at 2°C). The root-mean-square error of cross-validation (RMSECV) was 3.4 d at 2°C. NIR measurements provided promising results for evaluation of freshness for thawed-chilled MAP cod fillets completing the traditionally quality methods. However, it is necessary to study the effect of e.g. sample preparation, season, fishing ground and cod size together with more sophisticated pre-treatments of NIR spectra before the NIR method can be integrated as a method for evaluation of thawed-chilled MAP cod fillets.     

Drip loss,peroxidase and sensory changes in kiwi fruit slices during frozen storage

The biochemical, sensory and drip loss changes that occur during processing and prolonged frozen storage of kiwi fruit slices (cvs Abbott and Hayward) were studied. Fruit slices were frozen at ?40°C, packed in polyethylene bags and stored at ?18°C for 11 months. Maturity characteristics (pH, acidity, dry matter, soluble solids) were determined on raw fruit. Objective (proteins, peroxidase (EC 1.11.1.7) activity, drip loss) and subjective (sensory tests) analyses were carried out during processing and storage, and indicated a good quality of the frozen kiwi fruit slices after 11 months of storage. There were significant differences (P < 0.05) between the studied varieties with respect to drip loss during frozen storage and colour after 11 months. Best results were obtained with cv Hayward. This variety showed less drip loss after thawing and after 11 months storage presented the same green colour as after freezing, while cv Abbott became yellowish-green.     

Osmotic dehydrofreezing of strawberries: Polyphenolic content, volatile profile and consumer acceptance

We evaluated dehydrofreezing in strawberries in terms of retention of healthy compounds (i.e. polyphenolics) and sensory qualities for direct consumption in substitution of fresh fruit. Different osmodehydration (OD at 30 °C and 5 °C), vacuum osmodehydration (VOD at 30 °C) and immersion chilling freezing (ICF) processes were applied in sucrose syrup. Samples were analyzed for dry matter, soluble solids, pH, titratable acidity, mass transfers, polyphenolic content by HPLC-DAD/MSD, volatile profile by SPME-GC/MSD and consumer acceptance. The results of sensory evaluation, in particular, confirmed the cryoprotective effects of osmotic processes of fruits with respect to untreated frozen control samples. The OD samples at 5 °C presented a water loss to solid gain ratio comparable to OD at 30 °C and VOD samples. Moreover, while osmo-dehydrofreezing at relatively high temperatures caused a slight depletion of phenolic compounds, the samples osmodehydrated at 5 °C showed higher polyphenolic retention. Data on aromatic compounds showed that some compounds increased (e.g. ethanol and acetaldehyde), while others did not appear to be affected by the pre-treatments.     

微冻对南美白对虾质构的影响

以南美白对虾为对象,研究了其在-3℃微冻条件下质构参数(硬度、弹性、剪切力,咀嚼性、黏附性、胶黏性)的变化,同时考察了TVBN值、汁液流失率和感官特性的变化。结果表明:随着贮藏时间的延长,TVBN和汁液流失率均呈上升趋势;在贮藏至第14天以前,硬度、弹性、剪切力、咀嚼性呈上升趋势,之后逐渐下降,而黏附性一直呈现上升趋势,胶黏性呈下降趋势;结合TVBN值、汁液流失率和感官特性测定结果,南美白对虾质构测定结果能够较客观的表现其品质。     

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     

Microbial, physical-chemical and sensory spoilage during the refrigerated storage of cooked pork loin processed by the sous vide method

The aim was to study spoilage during the refrigerated storage of cooked pork loin processed by the sous vide method. Samples were packaged under vacuum into polyamide-polypropylene pouches, cooked at an oven temperature/time of 70 °C/12 h, chilled at 3 °C and stored at 2 °C for 0, 5 or 10 weeks. Microbial (psychrotrophs, lactic acid bacteria, Enterobacteriaceae, moulds and yeasts), physical–chemical (pH, water activity, TBARS, acidity, L^*a^*b^* colour, texture profile analysis and shear force) and sensory (appearance, odour, flavour, texture and acceptance) parameters were determined. The results showed that sensory spoilage preceded microbiological spoilage of sous vide pork loin. Counts bellow 1 log cfu/g of psychrotrophs, anaerobic psychrotrophs, Enterobacteriaceae and lactic acid bacteria were detected in any control week, while moderate counts (2–3 log cfu/g) of moulds and yeasts were found. Minor changes in water activity, lipid oxidation, CIELab colour, hardness, cohesiveness or gumminess were associated with spoilage of pork loin, only decreases of lactic acid, springiness and shear force were observed. The pork loin was unacceptable after 10 weeks. This loss of acceptance was mainly due to the deterioration of meaty flavour and odour, although the loss of appearance, juiciness and firmness also contributed. Moderate warmed-over and rancidity were detected. The sensory analysis was the most effective method for determining the shelf life of the sous vide pork-based dishes.     

The Effect of Freezing Conditions on the Quality of Freeze-Chilled Reconstituted Mashed Potato

Freeze-chilling involves freezing and frozen storage of food products followed by thawing and chilled retail display. The objective of this trial was to examine the effect of different freezing conditions (−30 °C, −60 °C, −90 °C) on selected quality parameters of freeze-chilled reconstituted potato flakes. Lowering the freezing temperature reduced the time required for freezing, gave a softer product and led to a reduction in drip loss. Freeze-chilling resulted in a firmer product and a higher drip loss compared to chilling alone. There was no difference in vitamin C content or in taste panel acceptability between the chilled and the freeze-chilled products.     

Freezing Characteristics and Storage Stability of Broccoli (Brassica oleracea L. var. botrytis L.) Under Osmodehydrofreezing and Ultrasound-Assisted Osmodehydrofreezing Treatments

Freezing characteristics (freezing time, latent heat of fusion of ice, and freezable water content) and quality parameters (drip loss, color, firmness, and l-ascorbic acid content) of broccoli (osmodehydrofrozen and ultrasound-assisted osmodehydrofrozen) during frozen storage were investigated. Freezing time, latent heat of fusion of ice, and freezable water content of osmodehydrated samples decreased significantly compared to samples which were not under osmotic dehydration. The changes of drip loss, color, firmness, and l-ascorbic acid content of osmodehydrofrozen and ultrasound-assisted osmodehydrofrozen broccoli during frozen storage were inhibited markedly compared to samples which were not submitted to osmotic dehydration before freezing. Compared to osmotic dehydration, the ultrasound-assisted osmotic dehydration shortened the needed dehydration time and better preserved the firmness and l-ascorbic acid content after osmotic dehydration pretreatment. In addition, the ultrasound-assisted osmotic dehydration minimized the drip loss and loss of l-ascorbic acid content and better maintained the color and firmness when stored at ?25 °C for 6 months. These findings indicate that it is promising to apply ultrasound-assisted osmodehydrofreezing in freezing and frozen storage of food.     

Combination of warm water and hydrogen peroxide to reduce the numbers of Salmonella Typhimurium and Listeria innocua on field salad (Valerianella locusta)

The optimal conditions of decontamination of field salad by treatment with warm water containing hydrogen peroxide were determined with the aid of a central composite design. It was the aim to improve the hygienic status while keeping the sensory quality at the highest level possible. Two series of experiments were performed to study the effects of the following parameters: temperature (45–50 °C), H₂O₂ (1–4%) and treatment time (60–210 s). In series 1 the effect on sensory properties and reduction of total aerobic counts was investigated. To determine sensory quality, treated samples stored at 4 °C for 7 days were subjected to evaluation by 15 panelists. Overall visual quality (OVQ), color, texture and odor correlated well with product acceptability (R²=0.934, 0.946, 0.945 and 0.956, respectively), and OVQ was the most sensitive criterion. An acceptability of 75% in OVQ was chosen as the limit and an acceptance area was defined to deduce permitted conditions for decontamination. Within that area a reduction of total counts by 2.64–3.22 and 1.9–3.48 log₁₀ cfu/g was achieved after treatment and storage at 4 °C for 7 days, respectively. In series 2 field salad was inoculated with Listeria innocua and Salmonella Typhimurium and it was observed that within the area of acceptability a reduction of the numbers of the challenge organisms by 1.27–3.26 and 0.79–3.05 log₁₀ cfu/g, respectively, was achieved immediately after treatment, and by 1.64–3.09 (L. innocua) and 1.1–2.63 log₁₀ cfu/g (S. Typhimurium) after storage. The initial decontaminating effect on L. innocua and S. Typhimurium was sustained or even increased after storage, when the salad was treated below 48.9 °C for more than 68 s. Three independent experiments confirmed the reliability of the models of the count reduction of L. innocua and S. Typhimurium.     

Influence of the modified atmosphere packaging on shelf life and quality of Navalinda sweet cherry

The benefits of different wrap-films for Navalinda sweet cherry, packaged in small-format (consumer packages), were analysed. A storage and distribution process entailing eight days at 4 °C plus four days at 20 °C was simulated. The cherries were packed in punnets and film-wrapped with one macroperforated film and two microperforated films of 0.30 (MAP.30) and 0.55 (MAP.55) µmol cm/cm² atm day of CO₂ permeability at 0 °C. Packing in MAP at 0 °C did not alter the subsequent respiratory intensity of the fruits at 20 °C; however, the respiratory intensity at 20 °C did increase when the storage temperature was raised to 4 °C. The concentration of CO₂ (%) inside the packets at 20 °C was proportional to the permeability of the films: MAP.30 (21%) and MAP.55 (15%). The microperforated films preserved fruit acidity and firmness while slowing down the darkening of colour, loss of quality and decay. Shelf life was most effectively prolonged with MAP.30 film. However, in view of the levels of hypoxia reached at 20 °C, its use is recommended only in the distribution and marketing processes with failsafe cold chains. The recommendations of use of the films, for the different markets, are expressed in light of the obtained results.     

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.