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Rheological methods are continually evolving to encompass novel technologies and measurement methods. This review highlights novel techniques used to analyze the rheological properties of foods over the previous decade. Techniques reviewed include large amplitude oscillatory shear (LAOS) testing and rheological techniques coupled with other measurement methods, such as microscopy and nuclear magnetic resonance (NMR). Novel techniques are briefly overviewed and discussed in terms of advantages and disadvantages, previous use, and suggested future utilization.

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Reduced- and low-fat cheeses are desired based on composition but often fall short on overall quality. One of the major problems with fat reduction in cheese is the development of a firm texture that does not break down during mastication, unlike that observed in full-fat cheeses. The objective of this investigation was to determine how the amount of fat affects the structure of Cheddar cheese from initial formation (2 wk) through 24 wk of aging. Cheeses were made with target fat contents of 3 to 33% (wt/wt) and moisture to protein ratios of 1.5:1. This allowed for comparisons based on relative amounts of fat and protein gel phases. Cheese microstructure was determined by confocal scanning laser microscopy combined with quantitative image analysis. Rheological analysis was used to determine changes in mechanical properties. Increasing fat content caused an increase in size of fat globules and a higher percentage of nonspherical globules. However, no changes in fat globules were observed with aging. Cheese rigidity (storage modulus) increased with fat content at 10°C, but differences attributable to fat were not apparent at 25°C. This was attributable to the storage modulus of fat approaching that of the protein gel; therefore, the amount of fat or gel phase did not have an effect on the cheese storage modulus. The rigidity of cheese decreased with storage and, because changes in the fat phase were not detected, it appeared to be attributable to changes in the gel network. It appeared that the diminished textural quality in low-fat Cheddar cheese is attributed to changes in the breakdown pattern during chewing, as altered by fat disrupting the cheese network.

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This study investigated the effects of aging and fat content on the texture of Cheddar cheese, both mechanical and sensory aspects, over a 9-mo aging period. Cheeses of 6, 16, and 33% fat were tested at 0.5, 3, 6, and 9 mo of aging. Cheeses were evaluated by a trained sensory panel using an established texture lexicon as well as instrumental methods, which were used to probe cheese structure. Sensory analysis showed that low-fat cheeses were differentiated from full-fat cheeses by being more springy and firm and this difference widened as the cheeses aged. In addition, full-fat cheeses broke down more during chewing than the lower fat cheeses and the degree of breakdown increased with aging. Mechanical properties were divided by magnitude of deformation during the test and separated into 3 ranges: the linear viscoelastic region, the nonlinear region, and fracture point. These regions represent a stress/strain response from low to high magnitude, respectively. Strong relationships between sensory terms and rheological properties determined in the linear (maximum compliance) and nonlinear (critical stress and strain and a nonlinear shape factor) regions were revealed. Some correlations were seen with fracture values, but these were not as high as terms related to the nonlinear region of the cheeses. The correlations pointed to strain-weakening behavior being the critical mechanical property. This was associated with higher fat content cheeses breaking down more as strain increased up to fracture. Increased strain weakening associated with an increase in fat content was attributed to fat producing weak points in the protein network, which became initiation sites for fracture within the structure. This suggests that fat replacers need to serve this functional role.

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Whey protein, at one time considered a by-product of the cheese-making process, is now commonly used in foods for its thickening and emulsifying properties. Currently, approximately 30% of these proteinaceous resources remain under-utilized. Previously, an acidified, thermally treated whey protein concentrate (mWPC) was developed to produce a cold-set thickening ingredient. Mass spectroscopy revealed an approximate 2.5-fold decrease in the lactosylation of beta-lactoglobulin in mWPC starting materials compared with commercial whey protein concentrates, manufactured at a higher pH. Potentially, this should increase the number of reactive sites that remain available for carbohydrate attachment. With this study, the formation of glycoprotein complexes was demonstrated between the mWPC ingredient and lactose, naturally occurring in mWPC powders, or between mWPC protein components with dextran (35 to 45 and 100 to 200 kDa) materials at low pH. In fact, additional dry heating of mWPC powders showed a 3-fold increase in the amount of lactosylated beta-lactoglobulin. Evidence of Maillard reactivity was suggested using colorimetry, o-phthaldialdehyde assays, and sodium dodecyl sulfate PAGE followed by glycoprotein staining. Resultant glycoprotein dispersions exhibited altered functionality, in which case steady shear and small amplitude oscillatory rheology parameters were shown to be dependent on the specific reducing sugar present. Furthermore, the emulsion stability of mWPC-dextran fractions was 2 to 3 times greater than either mWPC or commercial WPC dispersions based on creaming index values. The water-holding capacity of all test samples decreased with additional heating steps; however, mWPC-dextran powders still retained nearly 6 times their weight of water. Scanning electron microscopy revealed that mWPC-dextran conjugates formed a porous network that differed significantly from the dense network observed with mWPC samples. This porosity likely affected both the rheological and water-binding properties of mWPC-dextran complexes. Taken together, these results suggest that the functionality of mWPC ingredients can be enhanced by conjugation with carbohydrate materials at low pH, especially with regard to improving the emulsifying attributes.

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A soy protein isolate (SPI) was thermally denatured at a critical concentration of 8% protein for 3 h at 95 degrees C, resulting in a powder that was readily reconstituted at ambient temperature and that demonstrated improved heat stability and cold-set gel functionality when compared to a control SPI. When SPI was heated at 3% protein equivalently, prior to reconstitution to 8% protein, the final viscosity was about 3 orders of magnitude less than the original sample. The viscosity of SPI heated at 3% protein was still nearly 2 orders of magnitude less than the original sample after both samples were reheated at 8% protein. These results suggested that heat denaturation at low protein concentrations limited network formation even after the protein concentration and interaction sites increased, impacting the isolate's cold gelling ability. Gelation was prevented upon treatment of SPI with iodoacetamide, which carbaminomethylated the cysteine residues, establishing the role of disulfide bonds in network formation. The viscosity of the 8% protein dispersion was also reduced by 2 orders of magnitude when treated with 8 M urea, and when combined with 10 mM DTT the gel viscosity was decreased by another order of magnitude. These results suggested that hydrophobic interactions played a primary role in gel strength after disulfide bonds form. The need for a higher concentration of protein during the heating step indicated that the critical disulfide bonds are intermolecular. Ultimately, the functionality produced by these protein-protein interactions produced a powdered soy protein isolate ingredient with consistent cold-set and thermal gelation properties.

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Two sets of cheeses were evaluated to determine factors that affect shred quality. The first set of cheeses was made up of 3 commercial cheeses, Monterey Jack, Mozzarella, and process. The second set of cheeses was made up of 3 Mozzarella cheeses with varying levels of protein and fat at a constant moisture content. A shred distribution of long shreds, short shreds, and fines was obtained by shredding blocks of cheese in a food processor. A probe tack test was used to directly measure adhesion of the cheese to a stainless-steel surface. Surface energy was determined based on the contact angles of standard liquids, and rheological characterization was done by a creep and recovery test. Creep and recovery data were used to calculate the maximum and initial compliance and retardation time. Shredding defects of fines and adhesion to the blade were observed in commercial cheeses. Mozzarella did not adhere to the blade but did produce the most fines. Both Monterey Jack and process cheeses adhered to the blade and produced fines. Furthermore, adherence to the blade was correlated positively with tack energy and negatively with retardation time. Mozzarella cheese, with the highest fat and lowest protein contents, produced the most fines but showed little adherence to the blade, even though tack energy increased with fat content. Surface energy was not correlated with shredding defects in either group of cheese. Rheological properties and tack energy appeared to be the key factors involved in shredding defects.

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A mathematical model was developed to quantitatively analyze the rheological data of rennet casein gelation at different cooling rates. Kinetic parameters were estimated and correlated with the microstructure development of the protein network. The kinetic model identified structure development upon cooling to be first order, and the network forming energies were estimated for four protein concentrations cooled at four rates. A lower energy for network formation was observed for a slower cooling rate and a higher protein concentration. This observation resulted from the availability of more flocs at a slower cooling rate and a higher casein concentration, simplifying floc cross-linking. By analyzing the kinetics during the aging process of casein gels, no difference in the reaction mechanism was observed. This study illustrated that structure formation resulted from the addition of flocs into the protein network: not all flocs were part of the network at a defined gel point. The incubation period following cooling integrated idle flocs into the network, thereby strengthening the gel. By understanding the gelation mechanism during cooling of rennet casein gels, the structure and thus quality of dairy products, such as processed cheese, may be better controlled.

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This study investigated the sensory and rheological properties of young cheeses in order to better understand perceived cheese texture. Mozzarella and Monterey Jacks were tested at 4, 10, 17, and 38 d of age; process cheese was tested at 4 d. Rheological methods were used to determine the linear and nonlinear viscoelastic and fracture properties. A trained sensory panel developed a descriptive language and reference scales to evaluate cheese texture. All methods differentiated the cheeses by variety. Principal component analysis of sensory texture revealed that three principal components explained 96.1% of the total variation in the cheeses. The perception of firmness decreased as the cheeses aged, whereas the perception of springiness increased. Principal component analysis of the rheological parameters (three principal components: 87.9% of the variance) showed that the cheeses' solid-like response (storage modulus and fracture modulus) decreased during aging, while phase angle, maximum compliance, and retardation time increased. Analysis of the instrumental and sensory parameters (three principal components: 82.1% of the variance) revealed groupings of parameters according to cheese rigidity, resiliency, and chewdown texture. Rheological properties were highly associated with rigidity and resiliency, but less so with chewdown texture.

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Pregelatinized starch is employed in many food applications due to the instantaneous nature of thickening and stability imparted by modification. Proteins, however, have been excluded as a viscosifying agent due to requisite thermal treatments required to create structure. Whey protein isolate gels were produced while manipulating heating time, pH, and mineral type/content, producing a variety of gel types/networks. Gels were frozen, freeze-dried, and ground into a powder. Once reconstituted in deionized water, gel powders were evaluated based on solubility studies, rotational viscometry, and electrophoresis. The protein powder exhibiting the largest apparent viscosity, highest degree of hydrolysis, and greatest solubility was selected for pH and temperature stability analyses and small amplitude oscillatory rheology. This processing technique manipulates WPI into a product capable of forming cold-set weak gel structures suitable for thickening over a wide range of temperature and pH food systems.

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Advancing age is increasingly associated with confounding chronic and acute ailments, predisposing elderly individuals to conditions such as malnutrition and swallowing dysfunction. This enhanced susceptibility to malnutrition and dysphagia in this aging demographic lends itself to exacerbating, disabling conditions that may result in increased morbidity and mortality in the event of an aspiration episode. Early identification of substandard nutritional status and subsequent interventiion in the elderly dysphagic population may circumvent the deleterious effects of malnutrition.

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