Rheological,surface and colorbvietric properties of egg albumen gel affected by pH

Abstract

Studies were conducted to evaluate the effects of albumen at naturally existing pH ranges on the functional and gel characteristics. Albumen was obtained from freshly laid shell eggs. The pH of albumen was altered using either IN NaOH or IN HC1 to obtain a pH of 7.5, 8.0, 8.5, 9.0, 9.5 and 10.0. The altered albumen was tested for its foaming capacity (FC), foam stability (FS), and bake volume (BV). Albumen gels were prepared and tested for firmness (FM), strain at failure (NF), and stress at failure (SF) as well as their color attributes (Hunter L, a, and b). In general, FC and BV of the albumen decreased (P<0.05) as the pH increased. The BV of the albumen was affected by FS than FC for all the adjusted pH ranges. A denser structure was observed for albumen gels as the pH increased. The FM, NF and SF of albumen gels increased, and Hunter L and a values decreased with the increasing adjusted pH. The FM, NF, SF and Hunter L value of albumen gel could be used as quality indicators in evaluating the changes of texture and color with pH.     

Characteristics of stirred low-fat yoghurt as affected by high pressure

The effects of high pressure on the physicochemical, chemical, microbiological and sensory characteristics of stirred low-fat yoghurt were studied. Laboratory-made yoghurts were treated at high pressure (100–400 MPa) for 15 min at 20°C. No significant changes in pH and total organic acids were observed after pressuring the yoghurt. Pressures over 200 MPa prevented post-acidification of the yoghurt during chilled storage. Pressurized yoghurts exhibited higher viscosity and amino acid contents than did the untreated controls, and the differences were maintained after chilled storage. High-pressure treatments at 300 and 400 MPa reduced the number of viable cells of lactobacilli to below the legal minimum permitted in many countries. Significant differences in sensory characteristics between untreated and pressurized yoghurts (200–300 MPa) were detected after chilled storage.     

Structural properties of stirred yoghurt as influenced by whey proteins

The effect of whey protein addition on structural properties of stirred yoghurt systems at different protein and fat content was studied using laser diffraction spectroscopy, rheology and confocal laser scanning microscopy (CLSM). The composition of heated milk systems affected micro- and macroscopic properties of yoghurt gels. Particle size increased as a function of increasing whey protein content and decreased as a function of increased fat level. Firmness (elastic modulus) and apparent viscosity of manufactured yoghurt samples increased as a function of increased interparticle interactions, mainly caused by self-aggregation of whey proteins or aggregated whey protein-coated fat globules, respectively. The resistance towards shear-induced disruption of yoghurt gels increased with an increasing proportion of casein protein in the protein mixture, whereas products with high whey protein level revealed lower resistance behaviour towards shear-forces. CLSM images illustrated that the presence of large whey protein aggregates and lower number of fat globules lead to the formation of an interrupted and coarse gel microstructure characterised by large interstitial spaces. The higher the casein fraction and/or the fat level, the less interspaced voids in the network were observed. However, it is evident that the addition of whey proteins reinforces firmness properties of low-fat yoghurts comparable to characteristics of full-fat yoghurt.     

Rheological and sensory properties of stirred yoghurt with inulin‐type fructans

This study concerns physicochemical and sensory analysis of stirred yoghurts, in which skim milk powder was partly replaced by different inulins. The composition of basic solutions contained 12% skim milk powder and 4% inulin, while reference yoghurts contained 16% SMP or whole milk powder. Rheological and sensory analysis showed that one‐quarter of milk powder could be substituted by long‐chain inulin in low‐fat stirred yoghurt. Short‐chain inulin and oligofructose can potentially be applied in the production of drinking yoghurt. Sensory evaluation showed that milk fat cannot be successfully substituted by inulin.     

Physicochemical and pasting properties of maize as affected by storage temperature

Maize grains are used as raw material in various food products. In countries where the production is seasonal, the grains must be stored throughout the year in order to provide sufficient maize supply for the food industries and consumers. During storage, the environmental temperature is considered as one of the most critical variables that affects grain quality. This study aimed to evaluate the effects of various storage temperatures (5, 15, 25 and 35 °C) on the proximate composition, pH, fat acidity, percentage of grains infected by molds, grain color, protein solubility and pasting properties of maize stored for 12 months. Grains stored at 35 °C during the 12 months period showed the greatest decrease in grain yellowness, pH, protein solubility and breakdown viscosity. An increase in disulfide bonds within the protein structure and interaction between starch and non-starch components seems to be responsible for the changes in protein solubility and pasting properties determined in maize during the storage period. Fat acidity and the percentage of grains infected by visible molds were concluded to be very dependent of moisture content. The result of this study demonstrated that the minimum temperature of 5 °C was able to maintain the quality of maize stored for up 12 months.     

Rheological and viscoelastic properties of colloidal solutions based on gelatins and chitosan as affected by pH

The objective of this study was to evaluate the influence of pH on rheological and viscoelastic properties of solutions based on blends of type A (GeA) or type B (GeB) gelatin and chitosan (CH). Solutions of GeA, GeB, CH, GeA:CH, and GeB:CH were prepared in several pH (3.5–6.0) and analyzed for determination of zeta-potential. Rheological analyses (stationary and dynamic essays) were carried out with blends allowing to study the effect of pH on shear stress, apparent viscosity, loss (G”) and storage (G’) moduli, and angle phase (Tanδ). Zeta potential of all biopolymers decreased linearly as a function of pH. CH presented higher values, and GeB, the lowest one, being the only having negative values at pH > 5. Overall, the pH influenced the rheological and viscoelastic properties of the colloidal solutions: shear stress and apparent viscosity increased as a function of pH. Other assays were carried out at 3% and 5% strain, for GeA:CH and GeB:CH, respectively. In the sol domain, G’ and G” (1 Hz) increased linearly for GeA:CH. But for GeB:CH, they increased in two linear different regions: one function between pH 3.5 and 5.0 and another one between 5.0 and 6.0, being a more important effect was visible in this last domain probably due to the negative net charge of gelatin, above it pI. An effect in two domains was also visible for Tanδ, explained in the same manner as previously. The GeB:CH blends behaved like diluted solutions, and transition temperatures increased as a function of pH.     

Rheological monitoring of structure evolution and development in stirred yoghurt

The objective of the present work was to study the rheological changes of yoghurt at different stages in the manufacturing process and to try to gain some measure of understanding of the morphological changes that occur. Rheological measurements were made on yoghurt samples for different post-sampling periods, allowing the study of the influence of time in the structure and flow properties of the material to be performed. This was accomplished using both steady and oscillatory rheometry. Up and down shear rate sweeps, constant and oscillatory shear measurements were carried out. The study of the effect of temperature allowed the observation of two distinct regions of temperature dependency, in all samples, the transition point being approximately 25°C. The structural and rheological changes that occurred during post-incubation were observed during the analysis of several yoghurt samples, indicating that there is a partial structure recovery upon cessation of flow.     

Graininess and roughness of stirred yoghurt as influenced by processing

The aim of this work was to study how heat treatment, the type of starter culture, incubation temperature, and storage time can affect the physicochemical characteristics of stirred yoghurt. A four-factor experimental design was used for data analysis. Yoghurt milk was heated at 95 °C for 5 min or 130 °C for 80 s. Yoghurts were produced with three different starter cultures that had been incubated at 37, 42 or 45 °C and stored at 4 °C for 15 days. Visual roughness, number of grains, perimeter of grains, storage modulus, and yield stress all decreased when heating temperature was increased, when an exopolysaccharide-producing starter culture was used, or when incubation temperature was decreased. Storage time did not affect any of the physicochemical properties of yoghurt, except for the pH.     

The sol–gel transition temperature of skim milk concentrated by microfiltration as affected by pH and protein content

This study aimed at determining sol–gel transition temperatures of microfiltered skim milk retentates for different protein levels (6, 8 and 10% (w/w)) and a wide pH range (native pH to 4.6) by means of small‐amplitude oscillatory shear rheology. For a pH of 5.4 to 5.0, the sol–gel transition temperatures decreased significantly with increasing protein content, which did not differ for pH 4.8 and 4.6. The sol–gel transition temperatures of retentates with 10% (w/w) protein at pH 5.4 and 4.6 were 58.4 °C and 10.9 °C, respectively.     

Textural and pasting properties of potatoes (Solanum tuberosum L.) as affected by storage temperature

Fresh tubers from five potato (Solanum tuberosum L.) cultivars were stored at different temperatures (4, 8, 12, 16 and 20 °C) and 80–90% relative humidity for 18 weeks after harvest to examine the effect of storage temperature on their textural and pasting properties. Texture profile analysis was performed on raw and cooked potatoes using an Instron universal testing machine to measure textural parameters such as fracturability, hardness, cohesiveness, adhesiveness and springiness. Both raw and cooked potato tubers showed a considerable reduction in all textural parameters upon storage, irrespective of the storage temperature employed. Raw potatoes showed a decrease in fracturability and hardness with increasing storage temperature, whereas their cooked counterparts showed the opposite trend. The extent of change in the textural properties of both raw and cooked potatoes also varied among the different cultivars. Fresh and stored tubers from all cultivars were freeze‐dried, ground into flours and analysed for amylose content and pasting properties. The amylose content of flours prepared from potatoes stored at 4 and 8 °C was observed to be considerably lower than that of flours prepared from potatoes stored at 16 and 20 °C. Pasting characteristics such as peak viscosity, setback and final viscosity increased with increasing storage temperature, while the reverse was observed for pasting temperature, when studied using a rapid visco analyser. Breakdown in viscosity of the flour pastes from all cultivars was considerably reduced during storage, irrespective of the storage temperature employed. Copyright © 2006 Society of Chemical Industry     

Survival,growth, and inactivation of acid-stressed Shigella flexneri as affected by pH and temperature

A study was done to determine the survival, growth, and inactivation characteristics of unadapted, acid-adapted, and acid-shocked Shigella flexneri 2a cells as affected by pH and temperature. The pathogen was grown at 37 degrees C for 18 h in tryptic soy broth containing no glucose (TSBNG) (unadapted cells) and TSBNG supplemented with 1% glucose (TSBG) (acid-adapted cells). Cells grown in TSBNG were acid-shocked by adjusting 18-h cultures to pH 4.5+/-0.05 with lactic acid. All three cell types were separately inoculated into tryptic soy broth (6.6-7.0 log(10) cfu/ml) containing 0.25% glucose (TSB) acidified to pH 3.5-5.5 with lactic acid and incubated at 4, 12, 21, 30, and 48 degrees C for up to 144 h. Overall, inactivation of S. flexneri cells at low pH was enhanced with an increase in incubation temperature. All three types of cells survived for 144 h at 4 degrees C in TSB acidified to pH 3.5, compared to < 24 h at 30 degrees C and 2 h at 48 degrees C. The population of all three cell types increased significantly (alpha = 0.05) within 24 h when cells were incubated at 12, 21, or 30 degrees C in TSB at pH 5.0, 5.5, or 7.3. Prior exposure of the S. flexneri to an acidic environment (acid-adapted or acid-shocked cells) resulted in increased resistance to extreme acid and temperature conditions. Acid-adapted cells decreased by approximately 2.5 log(10) cfu/ml when incubated at 4 degrees C for 144 h, compared to a 6-log(10) reduction in control (unadapted) cells. When cells were exposed to low pH (3.5-4.5) and high temperature (48 degrees C), significantly higher (alpha = 0.05) populations were recovered on tryptic soy agar (TSA) than on TSA supplemented with 4% NaCl (TSAS), indicating that a portion of S. flexneri cells were injured. Results show that the ability of S. flexneri to survive and grow at a given pH is influenced by previous exposure to acidic environments and by incubation temperature.     

Modelling Bacillus cereus adhesion on stainless steel surface as affected by temperature,pH and time

The adhesion of Bacillus cereus to stainless steel was modelled as a function of pH (4.0–8.0), time (2–24 h) and temperature (4.0–36.0 °C) using response surface methodology. Based on the initial inoculum (3 or 6 log cfu mL⁻¹), two equations describing B. cereus adhesion to stainless steel were obtained. The results indicated that B. cereus was able to reach up to 5.5 cfu cm⁻² and 6.4 cfu cm⁻² when the initial inocula were 3 log cfu mL⁻¹ and 6 log cfu mL⁻¹, respectively. The significance of the factors varied with the model; i.e., inoculum of 3 or 6 log cfu mL⁻¹. Bias and accuracy factors showed that the models are adequate to predict B. cereus adhesion to stainless steel surface under conditions assessed and to assess the adhesion of B. cereus under a range of conditions to which this microorganism can be exposed during either milk processing or cleaning procedures.     

Predicting microbial growth: growth responses of salmonellae in a laboratory medium as affected by pH, sodium chloride and storage temperature

The growth responses of salmonellae (mixed inoculum of Salmonella thompson, S. stanley and S. infantis) as affected by NaCl concentration, pH level and storage temperature were studied in laboratory medium. Growth curves were obtained at 5 concentrations of NaCl (0.5-4.5%, w/v), 5 pH levels (5.6-6.8) and 5 storage temperatures (10-30 degrees C). Sigmoid curves (Gompertz form) were fitted to the data and the curve parameters used to produce a polynomial model from which predicted growth curves could be generated for any combination of NaCl, pH and storage temperature within the limits studied. From those growth curves values for growth rate, generation time, lag time and other values such as time to a 1000-fold increase in numbers were derived. Such a model offers a cost-effective approach to understanding the microbial growth response in foods, and forms a data-base against which other controlling factors could be evaluated. Some problems of fitting curves to microbial growth data and of modelling such data are discussed.     

Rheological properties of model dairy emulsions as affected by high pressure homogenization

The aim of this work was to evaluate the effect of different homogenization pressures on the rheological properties of model dairy emulsions. Solutions of sodium caseinate and whey protein concentrate at 2% + 1% and 2% + 2% (w/v) were emulsified with milk fat at the final contents of 5%, 8% and 15% (w/v). Raw and pasteurized emulsions were subjected to conventional (15/3 MPa) and high pressure (97/3 MPa and 147/3 MPa) homogenization. Differences in viscosity were observed according to fat content, protein ratio, heat treatment and pressure homogenization conditions. Gel networks formed in 8% and 15% fat content emulsions after pasteurization and high pressure homogenization. Gel strength was mostly affected by protein and fat contents.Dynamic high pressure may be used to produce emulsion gels with modulated structures which could be exploited low fat formulations.Industrial relevanceTexture influences people's acceptance of food and may be more important than the flavor in some products. Gelation of emulsions was observed after pasteurization and high pressure homogenization treatments, depending on fat and protein contents. Dynamic high pressure may be an alternative tool to improve viscoelastic properties and mouth feel of emulsions by providing a weak gel structure in the system. These characteristics can be exploited for low fat and low additive formulations.     

发酵剂的筛选及其对搅拌型酸奶理化特性的影响

通过调查单菌株发酵特性、球菌/杆菌比例的选择、菌株复配后的性能,得到了4组优化的发酵剂配方。在不添加任何增稠剂的条件下,其发酵的酸奶在28d货架期内质构、后酸化程度表现优异且稳定。此外,为了预测酸奶货架期后酸化程度的强弱,比较了15组28d储存期实验与加速实验中所得的pH值及其变化趋势。结果表明,加速试验中趋于定值时的pH值可以作为判断后酸化程度强弱的先行指标。     

Rheological properties of instant fried noodle dough as affected by some ingredients

An experiment was set up to investigate the influence of ingredients such as moisture, gum and starch on the rheological properties of instant fried noodle dough. The noodle dough was formulated using a mixture of wheat flour (100%), 0–0.3% guar gum, 0–7.5% starch and 30–42% moisture. The rheological properties of noodle dough were influenced by the ingredients and their interactions. Increasing moisture content decreased Young's modulus (E) of dough samples. Addition of gum to dough at a lower moisture content of 30% increased E, decreased the energy at break (EB) and increased the storage modulus (G′) values. Interactions of starch and gum influenced changes in the rheological parameters at higher moisture contents. Increasing starch content in noodle dough with a lower gum concentration increased EB to a point beyond which further increase in starch concentration decreased EB. Gum and starch improved the binding and mechanical network in the dough. Insufficient water in the dough apparently reduced cohesion in the dough whereas excess water reduced the functionality of gum and starch. Copyright © 2005 Society of Chemical Industry     

搅拌型酸奶生产过程中粘度变化的研究

对搅拌型酸奶生产过程中粘度变化进行了研究。通过采集不同酸奶生产厂的相关数据，就生产中搅拌、冷却、输送泵和管路、灌装、冷藏各个环节的综合分析，找出了影响酸奶粘度的主要工艺过程，并给出了合理的工艺参数。通过改善生产工艺提高酸奶的粘度和品质，指导生产实践。