Protein gelation is important to obtain desirable sensory and textural structures in foods. Gelation phenomenon requires a driving force to unfold the native protein structure, followed by an aggregation retaining a certain degree of order in the matrix formed by association between protein strands. Protein gelation has been traditionally achieved by heating, but some physical and chemical processes form protein gels in an analogous way to heat-induction. A physical means, besides heat, is high pressure. Chemical means are acidification, enzymatic cross-linking, and use of salts and urea, causing modifications in protein-protein and protein-medium interactions. The characteristics of each gel are different and dependent upon factors like protein concentration, degree of denaturation caused by pH, temperature, ionic strength and=or pressure.

The textural attributes of 8 different heat-induced protein gel preparations evaluated by torsion failure testing and Instron texture profile analysis (TPA) were compared to sensory ratings by a trained texture profile panel. The gels presented a wide range of textural properties as determined b y the instrumental and sensory parameters. Among the instrumental parameters, true shear strain at failure was the most frequent and significant predictor of sensory notes. Init,ial shear modulus and 50% compression force had the poorest correlations with sensory notes. Comparison of the two instrumental tests produced high correlations between shear stress at failure and TPA hardness; true shear strain at failure and TPA cohesivencDss; and, initial shear modulus and 50% compression force. High correlations were also observed among various panel notes. Canonical correlation analyses showed that sets of linear combinations of pcirameters from each one of the 3 tests (torsion, TPA or sensory) ulere highly correlated to sets from either of the other two. Regression equations relating each of the instrumental tests to sensory notes were developed. Of the torsion failure parameters, the logarithm of true shear strain most commonly appeared in the equations. Of the TPA parameters, cohesiveness and its logarithm were the terms that were most frequent. High R2 values were obtained for regression equations developed for predicting torsion failure parameters based on TPA parameters.

Sols were prepared from cornminuted fish (surimi), beef, pork and turkey muscles. Continuous evaluation of changes in structural rigidity and energy damping during heating of the sols from 3" to 95°C was performed in a nondestructive, temperature-controlled Thermal Scanning Rigidity Monitor. Surimi presented major rigidity transitions at 40". 48" and 65°C; beef at 43". 56" and 69°C; pork at 44", 53" and 69°C; and turkey at 50", 53" and 79'C. All materials exhibited rapid decrease in energy damping (i.e. increase in elasticity) over a short temperature span. Failure testing of gels indicated differences in strength and deformability. SEM micrographs provided an insight into structural features of the gels.

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Evaluacio´n de cambios proteolı´ticos y fisicoquı´micos durante almacenamiento de queso panela de Quere´taro, Me´xico, y su impacto en la textura J.A. Guerra-Martı´nez, J.G. Montejano and S.T. Martı´n-del-Campo* Mexican Panela cheeses manufactured with pasteurized cow's milk at a pilot plant scale were analyzed in order to determine the relationship between physicochemical and textural changes during 15-day storage. Changes in hardness, cohesiveness, springiness and chewiness were measured by texture profile analysis (TPA). Moisture, pH (superior and inferior rind and center) and total protein were evaluated with official methods. Proteolytic indexes (acid soluble nitrogen (ASN), non-protein nitrogen and 70% ethanol soluble nitrogen (EtOH-SN)) were obtained by selective precipitation and quantified by Kjeldahl method. Organic acids (lactic (LA), propionic (PA) and butyric acid (BA)) quantitation was done by high performance liquid chromatography. Analysis of variance (ANOVA) showed significant differences for all the physicochemical parameters evaluated, but only cohesiveness and springiness showed significant differences in texture parameters. By principal component analysis (PCA) two groups were separated, one from day 1 to 7 and one from day 9 to 15. Keywords: Panela cheese; storage; textural changes; physicochemical changes; PCA Quesos panela mexicanos elaborados a escala piloto con leche pasteurizada de vaca fueron analizados para determinar la relacio´n entre los cambios fisicoquı´micos y de textura durante 15 dı´as de almacenamiento. Los cambios en dureza, cohesividad, resortividad y masticabilidad fueron medidos a partir de pruebas de perfil de textura. Los cambios fisicoquı´micos en humedad, pH (corteza superior, inferior y centro) y proteı´na total fueron evaluados con me´todos oficiales. Los ı´ndices proteolı´ticos (nitro´geno soluble en a´cido y en etanol 70% y nitro´geno no proteico) fueron precipitados selectivamente y cuantificados con me´todo Kjeldahl. Los a´cidos la´ctico, propio´nico y butı´rico se cuantificaron por cromatografı´a de lı´quidos. El ana´lisis de varianza arrojo´diferencias significativas en todos los para´metros fisicoquı´micos, pero solo cohesividad y resortividad mostraron diferencias significativas en los para´metros de textura. El ana´lisis de componentes principales diferencio´dos grupos, uno del dı´a 1 al dı´a 7 y otro del dı´a 9 al dı´a 15.