Mechanisms of Syneresis in Rennet Curd without Mechanical Treatment

An original method for syneresis evaluation was developed, based on image analysis which enabled direct monitoring of curd shrinkage. Image analysis was employed in combination with the customary tracer method to follow the effects of cutting on curd syneresis. The two methods were then utilized to study curd syneresis in the presence of ethanol, a solvent less polar than water. Curd syneresis was reduced in the presence of diluted ethanol which led to the hypothesis that the mechanism which causes rennet curd to synerese probably relates to changes in protein conformation, essentially due to hydrophobic bonding.     

Synthesis of the Bi2Sr2CaCu2O8+δ superconductor following a polymer matrix route

This paper presents a convenient polymer matrix synthesis route based on the use of metal acetates, which yields Bi2Sr2CaCu2O8+δ (2212 phase) with a Tc higher than 93 K, in air and within a short sintering time (6–12 h) at 855°C. The phase quality of the intermediate and final products has been analysed by powder X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy. Macroscopic superconducting characteristics have been determined by alternating-current magnetic susceptibility and direct-current magnetization. The results obtained on a large number of samples suggest that this preparation route is highly reproducible and leads to black-grained powders with excellent homogeneity within a relatively short processing time. The usefulness of these powders to obtain textured superconducting products has been explored and representative results are presented here. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rheological Properties of Renneted Reconstituted Milk Gels by Piezoelectric Viscoprocess: Effects of Temperature and Calcium Phosphate

Scanning electron microscopy (SEM) and rheology and permeability were used to evaluate effects of clotting temperature and calcium phosphate on reneted reconstituted milk gels, especially microstructure and permeability. Dynamic measurements with a piezoelectric Viscoprocess determined rheological properties of curd after various times of renneting. An increase in clotting temperature accelerated specific rennet action and gel formation; maximum stiffness was reached faster but its value was weaker. Addition of calcium phosphate increased rate of gel formation. Maximum stiffness was reached faster but its value was not changed. Gel microstructure was not very different but its permeability coefficient decreased.     

Polysaccharide Effects on Cross-Flow Microfiltration of Two Red Wines with a Microporous Alumina Membrane

Two red wines, from the same cultivar, differing by polysaccharide content were filtered using cross-flow microfiltration with an alumina membrane, pore size 0.2 μm. Filtration flow rates were different. The wine richest in polysaccharides (A) showed a typical external fouling profile with progressive stabilization, while wine B showed a permanent decrease in flux (external and internal fouling). Retention of polysaccharides negatively correlated with their hydrodynamic volume, overall losses being higher for wine B. Polysaccharides had a possible role in fouling related to their structures and molecular weight distribution in the two wines.     

Application of two forms of silicon and their impact on the postharvest and the content of bioactive compounds in cucumber (<i>Cucumis sativus</i> L.) fruits

The metabolic activity of the fruits continues even after harvest, which results in the loss of bioactive compounds, a decrease in the quality of the fruits, softening and browning, among other negative effects. The use of certain elements such as silicon can improve postharvest quality, since it is involved in the metabolic, physiological and structural activity of plants, moreover can increase the quality of the fruits. In addition, nanotechnology has had a positive impact on crop yield, nutritional value, fruit quality and can improve antioxidant activity. For these reasons, the use of beneficial elements such as silicon in the form of nanoparticles can be a viable option to improve the characteristics of the fruits. In the present study was evaluated the application of potassium silicate (125, 250 and 500 mg L⁻¹) and SiO₂ nanoparticles (125, 250 and 500 mg L⁻¹) during the development of the crop. The results showed that the application of silicon (potassium silicate and silicon nanoparticles) increased the content of total soluble solids (up to 15.6% higher than control), titratable acidity (up to 38.8% higher than control), vitamin C (up to 78.2% higher than control), phenols (up to 22% higher than control), flavonoids (up to 64.6% higher than control), and antioxidant activity in lipophilic compounds (up to 56.2% higher than control). This study suggests that the use of silicon can be a good option to increase the content of bioactive compounds in cucumber fruits when they are applied during the development of the crop.     

Functions of Added Calcium in Acid Milk Coagulation

Changes in calcium solubilization and rennet reaction rate were investigated after the addition of calcium (6.25 mM) to reconstituted milk. Analysis of soluble and ionic calcium showed that distribution of added calcium was very fast (60 and 25 min, respectively), and that micelles of enriched milk remained more mineralized at pH 6.70 to 5.00. In the presence of sufficient amounts of rennet and hydrogen ions, K-CSSeh hydrolysis and aggregation rates were increased by addition of calcium. The calcium supplement led to extended maintenance of the micellar structure and to a more easily drained curd. Thus, it could be used advantageously for lactic acid cheese making.     

Fouling Colloids During Microporous Alumina Membrane Filtration of Wine

Three different wines, from the same cultivar, were microfiltered using a device operating under static conditions and equipped with a section of monochanelled tubular alumina membrane, pore size 0.2 μm. Filtration flux curves resembled permeation profiles observed in dynamic cross-flow conditions using a similar membrane. Minute amounts of colloidal material deposited on the surface and inside the membrane were isolated for the first time and characterized as mixtures of high molecular weight moderately acidic grape proteoglycans (surface deposit) and low molecular weight acidic moieties (inner deposit). These fouling colloids were likely deposited by trace amounts not detectable in crude starting wines.     

Acid Milk Gel Formation as Affected by Total Solids Content

Reconstituted skim milk with varying concentrations of total solids was coagulated using glucono-δ-lactone (GDL). Microscopic, turbidimetric and rheological procedures were used to examine mineral solubilization, buffering capacity, casein dissociation and micellar solvation during gelation. Total solids of the milk affected pH of the onset of gelation attributable to differences in colloïdal calcium phosphate in the casein particles during acidification. Firmness and elasticity of the resulting gel increased with total solids from a more direct contribution of dry matter during the last stage of acid milk gel formation.     

Ultrafiltration of milk on mineral membranes: improved performance*

A new generation of inorganic ultrafiltration (UF) alumina membranes was tested on milk considering membrane porosity, UF pressure. duration of treatment. concentration factor, and ultrafiltrate flux. The experiments were carried out on whole raw milk at a low temperature to simplify the technology and to limit bacteriological growth while considerably reducing the cost of energy. Different modifications were studied to improve the UF process: pre-remitting, enrichment of milk with calcium, and flow regime. The results indicate that it is possible to increase the ultrafiltrate flux considerably while maintaining a high level of retention of nitrogen compounds, thus opening the way to industrial application. A simple cleaning and sanitizing cycle ensures a longer live for this new alumina material and eliminates the technical and economical problems associated with current polymeric membranes.