CHANGES IN VISCOELASTIC PROPERTIES DURING THE RIPENING OF ARZÚA-ULLOA CHEESE. EFFECT OF PROTEOLYSIS

Changes in viscoelastic properties during the ripening of three different batches of Arzúa-Ulloa cheese, a traditional Spanish variety, were studied. Proteolysis changes were determined in terms of the typical nitrogen fractions, these fractions were significantly correlated with the measured viscoelastic properties. Substantial changes in stress amplitude over the linear viscoelastic range were observed at 20C, as were significant differences in storage modulus (G') and viscous modulus (G") throughout the ripening period. Both moduli decreased markedly over the first two weeks, but exhibited an opposite trend from the third week. Mechanical spectra were recorded at 20, 25, 30 and 35C throughout the ripening period. The viscous component significantly increased compared to the elastic one as function of temperature and ripening time.     

MICROBIOLOGICAL STUDY OF ARZÚA CHEESE (NW SPAIN) THROUGHOUT CHEESEMAKING AND RIPENING

Changes during production and ripening in the microbial flora of 11 batches of Arzúa, a soft cheese made from raw cow's milk, were investigated. The following microbial groups were counted on the surface and interior of the cheese: total viable count (TVC), lactic acid bacteria (LAB), halotolerant flora, enterococci, proteolytic enterococci, staphylococci, Staphylococcus aureus, Enterobacteriaceae, faecal coliforms, molds, yeasts, Listeria spp. and (in milk) Brucella spp. pH and water activity were also determined. TVC and LAB were, generally, more than 9 log (cfu/g). Enterococci counts increased gradually, reaching values in excess of 6 log units. Halotolerant flora and staphylococci remained practically constant throughout ripening, at 6–8 and 5–7 log units, respectively. Maximum Enterobacteriaceae and faecal coliform counts exceeded 7 and 6 log units, respectively. Brucella spp. were not detected in any of the milk samples. Listeria spp. were detected in four batches, and Listeria monocytogenes in two.     

LINEAR VISCOELASTIC BEHAVIOR OF FRANKFURTERS

Normal creep and stress relaxation experiments were performed on seven commercial brands of frankfurters. A nonlinear procedure was used to determine the model best fitting the data for each test. Re assumption of linear viscoelasticity was tested by the equivalence of model parameters. Results showed fankfurters to be linear viscoelastic materials at strain levels up to 3.8%.     

VISCOELASTIC PROPERTIES OF WHITE FRESH CHEESE FILLED WITH SODIUM CASEINATE

The yield, moisture loss and viscoelastic properties of white fresh cheeses containing sodium caseinate were determined and compared to those of a reference cheese without sodium caseinate. Added sodium caseinate resulted in higher cheese yields and lower moisture losses during aging. Creep compliance tests were performed on the cheeses using a parallel plate viscoelastometer. Multiple regression analysis of the experimental data provided first order models that explain the variation of most of the viscoelastic parameters of the white fresh cheeses in terms of sodium caseinate level, moisture content, aging and pH. The instantaneous elastic compliance and the Newtonian viscosity increased with moisture content and pH, respectively. The retarded elastic compliances increased and the first retardation time decreased during aging. The influence of sodium caseinate on most of the viscoelastic parameters was significant.     

VISCOELASTIC BEHAVIOR OF FRIED POTATO CRUST*

A restructured potato model system was fried for 5, 10 or 15 min at 170, 180, or 190C and tested in a dynamic mechanical analyzer. Stress relaxation phenomena was described by a two‐element Maxwell model with a parallel spring. The viscoelastic behavior of the crust resulted from its porous structure of open and closed foam cells embedded in oil. Relaxation of stress in the fried crust was a result of slippage of the dispersed oil through the crust and from rearrangements of the air cells. Elastic modulus of the fried crust increased for higher frying times and temperatures and were the most significant elements in distinguishing between samples.     

FLOW BEHAVIOR of PROCESSED CHEESE MELTS

Flow behavior of processed cheese melts during pipe flow was investigated using a magnetic resonance imaging (MRI) tube viscometer technique. the technique utilized an MR velocity profile and an independent pressure drop measurement to evaluate flow properties. the test fluids were three commercial processed cheese products which were evaluated at a number of volumetric flow rates under melt conditions. MR image analysis was performed using a graphic user interface Matlab program. the three processed cheeses were best characterized as shear thinning power law fluids with flow behavior indices that ranged from 0.28 to 0.40 and consistency indices that ranged from 10 to 79 Pa sⁿ over the shear rate range of 0.1 to 10 s^?1. These flow properties are relevant to the mixing and pumping of these products under melt conditions during manufacture.     

EFFECTS OF TEMPERATURE AND MOISTURE CONTENT ON THE VISCOELASTIC BEHAVIOR OF COWPEAS

The viscoelastic parameters of cowpeas (Vigna unguiculata) were studied at different temperature and moisture content combinations. Cowpeas were tested at temperatures of 25°, 45° and 70°C and moisture contents of 13.9, 19.6 and 22.4% dry basis on an Instron Universal Testing Machine using stress relaxation techniques. The response of cowpeas was characterized by two Maxwell bodies in parallel without the incorporation of a residual spring. The relaxation curves (uniaxial modulus versus time) were used to calculate temperature and moisture shift factors, which subsequently led to the construction of a single master response curve. Cowpeas are both thermo-rheologically and hydro-rheologically simple biological materials.     

A COMPUTERIZED METHOD TO ANALYZE THE CREEP BEHAVIOR OF VISCOELASTIC FOODS

A computer-aided method was developed to simplify the analysis of creep behavior and to determine the elasticity, the Newtonian viscosity and individual viscosities of viscoelastic foods. The method enabled the inclusion of all creep compliance data in the analysis, and reduced the analysis time from hours to minutes. The method was tested in the calculation of rheological properties of wheat flour dough.     

LINEAR AND NONLINEAR VISCOELASTIC BEHAVIOR OF OIL-IN-WATER EMULSIONS STABILIZED WITH POLYSACCHARIDES

The rheological behavior and stability of oil-in-water emulsions stabilized by different thickening agents were analyzed. Food emulsions were prepared with commercial sunflower oil (40% w/w oil-in-water) and stabilized with 1% emulsifier. The tested thickeners were: (1) 1% w/w xanthan gum (XG), (2) 5% w/w potato starch (PS), (3) 5% PS + 0.5% XG, (4) 1% w/w guar gum (GG), and (5) 0.5% XG + 0.5% GG. Mean droplet size and droplet size distribution (DSD) of emulsions were determined by static light scattering. Steady flow (viscosity versus shear rate), transient flow (viscosity versus time) and oscillatory shear tests (linear viscoelasticity) were performed. The addition of thickening agents improved the stability of the emulsions, the effect was less marked in systems containing only GG. DSD was not significantly modified in emulsions containing starch or hydrocolloids. Microscopic observations showed that all the tested emulsions were flocculated due to the presence of hydrocolloids. The observed shear thinning behavior was attributed to the molecular structure of the polysaccharides and to the flocculation/deflocculation process; viscosity data were satisfactorily fitted to the Cross model. Frequency sweeps showed that emulsions with PS or XG have a weak gel structural network (G' > G); those with GG correspond to a polymeric solution where G' and G" curves intersect within the range of tested frequencies. The viscoelastic linear behavior was described according to the Maxwell generalized model. The discrete relaxation spectrum and relaxation times were estimated from the experimental values of G' and G" for emulsions with PS, PS + XG, and XG. Nonlinear viscoelasticity was also studied from stress relaxation curves at different shear strains. The damping function was calculated and the Soskey-Winter parameters were determined. Transient flow viscosities at different shear rates were comparable to the values estimated from stress relaxation measurements.     

REVERSIBLE GELATION OF WHEY PROTEINS: MELTING, THERMODYNAMICS AND VISCOELASTIC BEHAVIOR

Whey protein isolates formed reversible gels following heating at 90°C for 15 min under certain conditions i.e., pH 6.5 to 8.5 with protein concentration of 9.0–10.5%. The melting temperatures of the gels formed at pH 8.0 ranged from 24.5°C to 57.8°C. The maximum enthalpy of formation (ΔH_f) was –858 call mole of crosslinks. A maximum storage modulus (G') of 240 dynes/cm² was obtained following holding for 7 h at 8°C.     

EFFECT OF METHOCEL AS A WATER BINDER ON THE LINEAR VISCOELASTIC PROPERTIES OF MOZZARELLA CHEESE DURING EARLY STAGES OF MATURATION

Changes in the dynamic and transient rheological character of highmoisture, skim milk (HMSM) Mozzarella cheese due to the addition of 0.2% Methocel (methyl cellulose) as a water binder at room (25C) and refrigeration (7C) temperatures during early stages of maturation (1, 5, 7, and 14 days after manufacture) were investigated. The HMSM Mozzarella with 0.2% Methocel was softer (lower dynamic storage and loss modulus, and higher creep and recovery compliance) compared to HMSM Mozzarella without Methocel, due to improved water holding capacity. The age-dependent frequency dispersions of dynamic mechanical spectra (storage and loss modulus) were fitted to a power-law model. A six-element Voigt-Kelvin mechanistic model described the age-dependent retardation spectra (compliances, viscosities, and retardation times) obtained from creep experiments. Strong correlation was obtained between the viscoelastic     

BEHAVIOR OF MOZZARELLA, CHEDDAR AND PROCESSED CHEESE SPREAD IN LUBRICATED AND BONDED UNIAXIAL COMPRESSION

The response of a material under uniaxial compression depends both on the bulk material properties and on the frictional effects at the sample-platen interface. Three commercial cheeses (cheddar, mozzarella and processed cheese spread) were studied in uniaxial compression. Frictional effects at the sample surface were examined under three conditions: (1) lubrication of the sample-platen interface; (2) bonding of the sample to the instrument platens; (3) samples neither bonded nor lubricated. The forces required to deform the cylindrical sample to a given strain level were highest for the bonded cases and lowest under lubricated conditions. The stress in lubricated compression, ρ_L, was calculated from Fh(πR_ρ²h_ρ)     

CONSIDERATIONS OF A GENERAL RHEOLOGICAL MODEL FOR THE MECHANICAL BEHAVIOR OF VISCOELASTIC SOLID FOOD MATERIALS*

A model describing the rheological behavior patterns of solid foods is suggested and discussed. It consists of a parallel array of a generalized Maxwell body in which the Maxwell elements have been modified by incorporation of two fracture elements. The model enables the anticipation of response patterns of real food materials under various force—deformation histories without becoming inconsistent. It also predicts the viscoelastic phenomena of materials after their failure and does not exclude the elastic deformation stage. The effects of experimental conditions, differences between various types of foods, as well as variations and changes in the same food can be expressed or explained in terms of model components and their relative distribution. The application of the model is demonstrated with a small number of representative elements.