Rheological Properties of Chymosin-Induced Skim Milk Gels as Affected by Milk Storage Time and Temperature

Fundamental dynamic rheological tests were used to measure the rigidity (|G*|) and viscoelasticity (tan δ) of gels made from skim milk samples that were (1) never cooled, (2) stored at 4°C, or 3) stored at 4°C and then rewarmed to 25°C. Milk cooled and held at 4°C prior to gel formation produced gels with lower |G*| and higher tan(δ) than fresh milk gels. Chilled milk samples that were rewarmed to 25°C for 16h prior to gel formation produced gels with rheological properties similar to fresh milk gels. The solubilization of β-casein during cold storage was hypothesized to be the cause of observed differences in gel rheology.     

Effect of various refrigeration temperatures on quality of shell eggs

BACKGROUND: The objective of this study was to evaluate the effects of low storage temperatures on shell egg quality. RESULTS: Approximately 2100 shell eggs were collected and stored at ? 1.1, 0.6, 2.2, 3.9, 5.6 and 7.2 °C for up to 4 weeks. Eighteen eggs at each storage temperature were evaluated after 0, 2, 7, 14, 21 and 28 days of storage. Haugh units (HU), yolk index (YI), albumen pH (pHA), yolk pH (pHY) and angel food cake density (CD) were measured. Shell egg quality tended to be preserved better at below 2.2 °C, as high HU and YI values relative to eggs stored at 7.2 °C were determined on day 28. However, storage at ? 1.1 °C tended to cause the opposite effect, especially highly declined HU values over time. Significantly different HU values of shell eggs were measured after 14 days of storage, with eggs stored at 0.6 and 2.2 °C having the highest HU values, 80.42 and 77.97 respectively. CONCLUSION: A lower temperature limit for shell egg storage could be established between 0.6 and 2.2 °C, as both temperatures showed the highest HU values, 77.88 and 77.60 respectively, after 28 days of storage. Copyright © 2011 Society of Chemical Industry     

Texture and Microstructure of Cooked Whole Egg Yolks and Heat-Formed Gels of Stirred Egg Yolk

Hardness, cohesiveness, and springiness of heated intact egg yolks were determined by an Instron compression test. Hardness increased as temperature was increased from 75° to 90°C and as time of heating was increased from 10 to 30 min. Cohesiveness and springiness increased as temperature was increased from 75° to 80°C and as time was increased from 10 to 30 min. Egg yolks from cooked shell eggs were lower in all textural parameters than gels formed from stirred egg yolk. Low scores for cohesiveness and springiness were indicative of the crumbliness of yolk from cooked shell eggs; crumbliness was attributed to structural characteristics. The microstructure consisted of adjoining polyhedral grains, which showed no evidence of crosslinking. Grains appeared to be equivalent to yolk spheres, ranging in size from 40 to 100 μm. The structure of a stirred yolk gel consisted of a highly crosslinked protein matrix containing spheres (5–35 μm) and granules (l–2 μm).     

Rheological Properties of Heat-induced Gels from Egg Albumen Subjected to Freeze-thaw

A back-extrusion device was used to measure gel strength, elasticity, and viscosity index of heat-induced gels of albumen maintained at 80°C from 5 to 30 min. These rheological properties were the same in albumen refrigerated 24 hr at 3°C as those observed in gels from fresh albumen suspensions. Short term frozen storage (?10°C for 24 hr) significantly reduced each gel parameter compared with fresh (control) albumen gels. Incorporation of sucrose into fresh albumen protected rheological properties of the heat-induced gels from deleterious effects of freezing the albumen suspensions. Addition of 5 or 10% NaCl to albumen reduced or eliminated the ability of albumen to form heat-induced gels.     

EGG QUALITY AS AFFECTED BY STORAGE AND HANDLING METHODS

The quality and shelf-life of fresh eggs were studied under simulated climatic conditions as found in Kuwait. Three groups of eggs (unwashed, washed, and washed and oil treated) were used. The treated eggs were stored at 5°, 25°, 37° and 42°C. Internal and external quality and sensory evaluation of the eggs stored up to 75 days were determined. The quality of the unwashed and washed eggs stored at 37°C and 42°C deteriorated rapidly while the wash and oil treatment extended shelf-life but decreased in quality to C over a period of time. The percent weight loss in the oil treated eggs at all temperatures was about 0.6% while the washed and unwashed lost more depending upon the temperature and storage time. All parameters studied for interior quality showed that oil treated eggs were significantly better than untreated eggs even at 42°C. Sensory evaluation on boiled eggs indicated that changes in flavor and overall quality for untreated eggs stored at high temperatures were obvious while little change in texture was observed. It was concluded that under Kuwaiti conditions eggs should be oil treated directly after collection and stored at or below 25°C so that the quality and shelf-life can be extended significantly.     

Rheological Properties and Microstructure of Acid Milk Gels as Affected by Fat Content and Heat Treatment

Gels were made from recombined milks containing 0, 1.5 or 3.5% fat that were heated at 75, 80 or 90°C for 30 min, followed by acidification with glucono-8-lactone at 30°C. The rheological and microstructural properties of acid gels were investigated using dynamic low-amplitude oscillatory rheology and confocal scanning laser microscopy (CSLM). Heating milks, at temperatures ≥80°C, increased the storage moduli (G′) and decreased the gelation time. Recombined milks containing high fat (3.5%) had higher G′ than gels made from low-fat or skim milk. Milk heat treatment resulted in gels with a cross-linked microstructure. Recombined fat globules appeared to be embedded in the protein matrix.     

Antigenicity of Ovomucoid Remaining in Boiled Shell Eggs

Antigenicity of egg white proteins which remained in heated shell eggs was analyzed quantitatively by using rabbit antibodies specific to egg white proteins and human antibody from patients allergic to egg. The major antigenic component in heated (100°C for 20 min) and coagulated egg white was ovomucoid. A considerable amount of ovomucoid remained immunoreactive even after heating at 100°C for 15 min, though ovomucoid in stored eggs was a little less stable to heating than that of fresh eggs. Even long-time heating (100°C for 45 min) could not completely eliminate the ovomucoid immunoreactivity to human IgE antibody.     

Correlation of the Rheological Behavior of Egg Albumen to Temperature, pH, and NaCl Concentration

Rheological parameters of viscosity, gel strength, and elasticity were determined on heat-set egg albumen gels over various treatment combinations ranging from 65-90°C, pH 6.4-9.6, and NaCl concentrations of 0.0-0.1M added NaCl. Maximum viscosity was measured at a treatment combination of 77.5°C, pH 8.00, and 0.1M NaCl. Elasticity and gel strength were highest in gels with a treatment combination of 85.2°C, pH 9.0, and 0.08M NaCl. Temperature had the greatest effect on all three rheological parameters. Gels heated above 80°C were of unusual character, exhibiting syneresis and shrinkage.     

Modification of rheological properties of whey protein isolates by limited proteolysis

Whey protein isolate (WPI) was subjected to limited tryptic hydrolysis and the effect of the limited hydrolysis on the rheological properties of WPI was examined and compared with those of untreated WPI. At 10% concentration (w/v in 50 mM TES buffer, pH 7.0, containing 50 mM NaCl), both WPI and the enzyme-treated WPI (EWPI) formed heat-induced viscoelastic gels. However, EWPI formed weaker gels (lower storage modulus) than WPI gels. Moreover, a lower gelation point (77 °C) was obtained for EWPI as compared with that of WPI which gelled at 80 °C only after holding 1.4 min. Thermal analysis and aggregation studies indicated that limited proteolysis resulted in changes in the denaturation and aggregation properties. As a consequenece, EWPI formed particulated gels, while WPI formed fine-stranded gels. In keeping with the formation of a particulate gel, Texture Profile Analysis (TPA) of the heat-induced gels (at 80 °C for 30 min) revealed that EWPI gels possessed significantly higher (p < 0.05) cohesiveness, hardness, gumminess, and chewiness but did not fracture at 75% deformation. The results suggest that the domain peptides, especially β-lactoglobulin domains released by the limited proteolysis, were responsible for the altered gelation properties.     

Storage stability of an egg yolk cream formulation: texture and microbiological assessment

BACKGROUND: Egg yolk cream is confectioned with egg yolk, sugar and water. Pasteurized liquid eggs may be used in order to increase product safety, although these samples may differ from the classic ones produced with raw eggs. The objective was to evaluate and compare the stability (physicochemical characteristics, such as texture assessment, pH and water activity, and microbiological assessment) of a classic formulation and an alternative one produced with pasteurized eggs, during storage at 6, 26, 30 and 37 °C. RESULTS: From a microbiological point of view (mesophile and psychrotrophic activity), no differences were observed in either formulations. At 6 and 26 °C, rheological behaviour of both formulations remained approximately constant. At 30 and 37 °C, differences were only detected after 20 days of storage. Texture was better retained in samples prepared with pasteurized eggs, while the classic samples showed an increase in complex viscosity. CONCLUSION: Cream storage did not require refrigeration. In terms of texture and microbial load, results obtained at 6 and 26 °C were identical. The formulations only differed in texture when stored at 30 and 37 °C and for long periods. These conclusions may allow reduction of costs related to refrigerated distribution/storage of either classic or alternative formulations. Copyright © 2008 Society of Chemical Industry     

Study on retrogradation of maize starch–flaxseed gum mixture under various storage temperatures

This study aimed to investigate the effect of flaxseed gum (FG) on the retrogradation of maize starch (MS) gels under different storage temperatures. In this work, MS‐FG gels with 0–0.4% FG were stored under different temperatures (–20, 4 and 20 °C). Under all storage temperatures, the addition of FG inhibited the retrogradation process of MS, suppressed the recrystallisation of starch molecules, reduced the water loss of starch gels and improved the texture of the gels. Among the three storage temperatures, ?20 °C could almost stop the starch retrogradation but leading it results in the hardest gel texture. The retrogradation of MS‐FG gels stored under 4 °C was most serious with the fastest rate of recrystallisation. When stored at 20 °C, the gels had a retrogradation degree that was lower than when they were stored at 4 °C and also had the softest texture.     

Influence of Thermal Treatment on Gelation of Actomyosin from Different Myosystems

We analyzed the influence of temperature and heating time on rheological properties, and types of proteins involved in the gelation process of natural actomyosin from pork, chicken, and hake. At low temperatures (40–50°C) hake gels were stiffer than chicken or pork gels (due to setting); at higher temperatures (60°C), actomyosin from all three formed gels with similar rheological characteristics. Types of protein in the different fractions (analyzed by electrophoresis) were consonant with the rheological behavior of the AM gels for each heat treatment and species.     

Structural and rheological properties of amaranth protein concentrate gels obtained by different processes

The heat-induced gelation of amaranth protein concentrates (APCs) by three processes was studied. The first was the conventional process for isolating protein (standard process-st), the second included an acid washing step prior to protein extraction (acid washing process-aw) and the third included heating (50 °C) during the alkaline extraction step (heat process-ht). The dispersions (12%, w/v) were heated to 55–90 °C and assessed by rheological measurements made under small deformations, whereas the gels obtained by heating at 70, 80 or 90 °C/30 min were subjected to uniaxial compression measurements (TPA and mechanical properties). The rheological parameters associated with the network structure, elasticity modulus (E) and storage modulus (G′), increased with increasing gelation temperature. For the APCst and APCht gels, protein aggregation occurred in two steps, whereas for APCaw, gelation occurred in a single step. The APCht gels showed the highest fracturability, hardness and adhesiveness, followed by the APCst and APCaw gels (p < 0.05). Similar results were obtained for the mechanical properties at fracture. Increasing the heat treatment temperature from 80 to 90 °C resulted in a more structured matrix with greater water-holding capacity as compared to gels obtained at 70 °C, and these properties were influenced by the extraction processes used to obtain the APCs. Heat extraction (APCht) improved the gelation and water-holding properties, whereas the acid treatment had the opposite effect.     

Frozen storage of high-pressure- and heat-induced gels of blue whiting (Micromesistius poutassou) muscle: rheological, chemical and ultrastructure studies

This paper examines the influence of frozen storage over 34 weeks on the rheological properties as well as the chemical and microstructural characteristics of gels made from muscle of blue whiting (Micromesistius poutassou) subjected to different gelling treatments entailing three combinations of pressure, temperature and time: 200 MPa, <10°C, 10 min (lot L), 375 MPa, 38°C, 20 min (lot H) and atmospheric pressure, 37°C, 30 min and then 90°C, 50 min (lot T). Freezing at –40°C caused certain changes in rheological parameters. In heat-induced gels, breaking deformation, elasticity and cohesiveness increased. Of the high-pressure-induced gels, breaking force increased and cohesiveness decreased in the gel formed at lower pressures, while the only change in the gel formed at higher pressure was some loss of elasticity. There was a general fall in water holding capacity (WHC) values. Lightness remained stable. In terms of protein solubility, there was an increase in covalent bonds in lot L. As for the ultrastructure, all gels matrixes were more disorganized as a result of freezing. In the course of frozen storage, the greatest changes in rheological parameters generally took place during the first 8 weeks, and in all the gels there was a decrease in WHC. In the heat-induced gel the changes were less marked over the storage period compared with those in the high-pressure-induced gels, but the heat-induced gel was more brittle and did not maintain maximum folding test scores. Covalent bonds increased and hydrophobic interactions decreased in all lots. The general appearance of the structure of gel T remained more homogeneous, while the pressurized gels exhibited more and larger cavities.     

Heat-induced Gelation of Minced Rainbow Trout (Salmo gairdneri:) Effect of pH, Sodium Chloride and Setting

The structure formation of minced rainbow trout was monitored during heating from 30°C to 85°C by a dynamic rheological method. The maxima of storage modulus (G') were found at 37°C and in the range of 45°C to 60°C. The largest differences in rigidity resulted from seasonal variations. Firmest protein matrix was obtained by using well-fed fish as raw material, by setting the gels at 35°C and using a pH value around 6.1. Sodium chloride in the range 1–2% had little effect on the final rigidity of the minced rainbow trout gels.     

Effect of thermochemical treatment on the structure of inulin and its gelling properties

The aim of this study was to investigate rheological properties of inulin gels and the changes in the inulin structure after heating the high polymerised inulin (DP ≥ 23) solutions at different pH. The 20% inulin solutions were heated at pH 3, 5 and 7 at 60 and 80 °C followed by 21 h storage at 5 and 25 °C. Rheological properties evaluation indicated that the increase in the heating time and temperature significantly (P ≤ 0.05) influenced on the inulin structure. Reducing sugars, cryoscopy and HPLC analysis revealed that chemical structure of inulin is stable in neutral and slightly acidic conditions (pH 5). Gelation of the high polymerised inulin solution after heating at pH 7 and 80 °C could be inhibited after dissolving the inulin crystallites which act like seeding crystals. At lower temperatures in which, not all crystallites were dissolved; it was feasible for the solutions to form firm inulin gels.     

Rheological properties of patatin gels compared with β-lactoglobulin, ovalbumin, and glycinin

BACKGROUND: The thermal unfolding and rheological properties of patatin gels were compared with those of commonly used proteins (β‐lactoglobulin, ovalbumin, glycinin). RESULTS: A significant difference between these proteins was observed in both the denaturation temperature (59 °C for patatin; about 20 °C lower than the other proteins) and the onset temperature of gel formation (50–60 °C, compared to 70–85 °C for the other proteins). At low ionic strength the minimal concentration was only 6% (w/v) for patatin, compared to 8–11% for the other proteins. This effect was attributed to the relatively high exposed hydrophobicity of patatin as determined by hydrophobic interaction chromatography. For gels compared at ‘iso‐strength’, the frequency dependence was found to be close to identical, while small differences were observed in the strain at fracture. CONCLUSIONS: Patatin was found to form gels with comparable small‐deformational rheological properties as typical food proteins. In addition, at concentrations where the elastic modulus was similar for all proteins, the frequency and strain dependence were also comparable. From this it is concluded that patatin is a promising protein to be used in food applications as a gelling agent. Copyright © 2010 Society of Chemical Industry     

Pasteurization of intact shell eggs

The feasibility of pasteurization for destruction ofSalmonella enteritidisin artificially- inoculated intact shell eggs was investigated. Water-bath and hot air ovens were used for pasteurization of intact shell eggs under different conditions using these two heating methods, either individually or in combination. Eggs pasteurized in a 57°C circulating water-bath for 25 min gave reductions inS. enteritidisATCC 13076 of about 3 log cycles whereas heating at 55°C in a hot air oven for 180 min gave a 5 log reduction ofS. enteritidis. A combin?ation of the two methods (water-bath heating at 57°C for 25 min followed by hot-air heating at 55°C for 60 min) produced 7 log reductions inS. enteritidisATCC 13076 in shell eggs. Examination of lysozyme activity and other physical properties of egg white upon heating indicated that the overall functionality of pasteurized shell eggs are acceptable under the heating conditions defined in this study. This process may be used to produce safe, pasteurized shell eggs.sed     

Dynamic rheological behaviour and biochemical properties of rabbit skeletal actomyosin during storage at 0° C

The relationship between the dynamic rheological properties of heat induced gels of actomyosin (natural actomyosin) and the denaturation of actin in actomyosin during storage without ATP at pH 6.0 and 0°C was investigated using biochemical and dynamic rheological measurements. The complex modulus of gels after heating actomyosin containing 0.5 or 1.5 M KCl (pH 6.0) at 80°C increased with increasing storage time. The dynamic rheological behaviour during heat gelation of actomyosin in 1.5 M KCl indicated that the first rheological transition peak in the 50–53°C range induced by the presence of F-actin gradually disappeared with increasing storage time. However, in 0.5 M KCl, this transition peak clearly remained even after 15 days. The time course of denaturation of actin in actomyosin treated with 1.5 M KCl at pH 6.0 showed an increase in the percent denaturation after the storage was started, and about 100% of the actin became denatured after 21 days. In the case of 0.5 M KCl, unlike 1.5 M, the denaturation of actin occurred quickly within the first 5 days and then did not proceed. A sigmoidal relationship was found between the percent denaturation of actin and the KCl concentration added, the greatest change occurring at KCl concentrations between 0.5 and 1.0 M. The data indicated that the change in the property of actin in actomyosin during storage at low temperature exerts a great influence on the viscoelasticity of heat-induced gels of actomyosin.     

Rheological Changes During Slow Acid Induced Gelation of Milk by D-glucono-δ-lactone

Alteration in the rheological behavior of milk during acidification were studied. Structure formation during gelling was monitored by a small strain rheological scanner. Dynamic viscoelastic properties were monitored as a function of time and temperature. Time of onset of gelation, rate of network formation and development of the elastic component during acidification were accelerated at higher temperatures (55°C). Addition of calcium (0–40 mM) facilitated gelation. Preheating milk at 90°C for 5 min prior to acidification enhanced gelation and induced protein-protein network formation. The number or strength of elastically effective bonds between protein molecules were reduced and gels of lower firmness were obtained with preheated milk, compared to gels made from unhealed milk.