J.F. Vélez-Ruiz

Rheological properties of concentrated milk as a function of concentration, temperature and storage time

The flow properties of concentrated milk were evaluated between concentrations of 12.6 and 48.6% solids content, at three temperatures and through 4 weeks of storage. Three rheological models, Newton, power law and Herschel-Bulkley, were applied to fit the flow behavior of milk concentrates depending on the concentration level. The resulting flow behavior indices and consistency coefficients were correlated to concentration by a single term exponential equations, with only two concentrates, 42.4 and 48.6% solids contents exhibiting yield stress. The effect of the temperatures studied on the flow behavior index was minimal, though noticeable on the consistency coefficient, and at the three selected temperatures (5, 15 and 25 °C) the flow behavior index decreased with storage time while the consistency coefficient increased. The energy of activation for flow (evaluated with an Arrhenius type equation) increased with concentration and storage time and ranged from 2.42 to 11.8 kcal gmol−1. A nonlinear model accounting for temperature and concentration effects on the consistency coefficient was identified. The constants of the aforementioned nonlinear model were different for each week of storage. During the 4 weeks of storage the consistency coefficient showed no significant differences, in contrast to the flow behavior index.

EFFECTS OF CONCENTRATION AND TEMPERATURE ON THE RHEOLOGY OF CONCENTRATED MILK

The flow properties of milk concentrates covering a solid content range of 12.7% to 40.4% made from fresh whole milk were determined with a double concentric cylinder geometry in a rotational rheometer. Shear stress or apparent viscosity corresponding to a 500 to 1000 s–1 shear rate range were recorded; and from these shear rate-shear stress data the flow nature of milk concentrates was evaluated. The rheological behavior of milk concentrates was represented by parameters corresponding to Newton, Power Law, or Herschel-Bulkley equations, depending on the solid content level. The flow behavior index (n) and the consistency coefficient (K) were exponentially correlated to milk concentration. The consistency coefficient followed the Arrhenius temperature relationship and the activation energy for the concentrates ranged from 4.3 to 6.1 kcal/gmole. These two rheological parameters (n and K) significantly changed with concentration and temperature. The consistency coefficient was correlated to concentration and absolute temperature by a simple mathematical relationship, which is useful for equipment and process design.

Physicochemical and Rheological Properties of a Dairy Dessert, Enriched with Chickpea Flour

Dairy desserts are complex mixtures and matrices including main components such as milk, sugar, starch, hydrocolloids, colorants and flavors, with a proteinaceous structure; they are widely consumed and present a semisolid consistency. In this work, the physicochemical and rheological properties of a dairy dessert with the addition of chickpea flour (raw and cooked, at four concentrations) were studied to determine the effect of the flour. The results indicated that luminosity (L*: 62.75–83.29), pH (6.35–7.11) and acidity (1.56–3.56) changed with the type of flour. The flow properties of the custards exhibited a non-Newtonian behavior that was well fitted by three flow models. The studied custard systems were stored for twelve days at 4 °C. The physicochemical and flow properties of the custards changed notably as a function of flour addition and storage time. From all samples, only four were analyzed with oscillatory tests, showing their mechanical spectra with elastic behavior. The dessert texture was also measured, founding that those formulated with Blanco Noroeste chickpea flour exhibited the highest values of hardness (0.356–0.391 N) through the twelve days. It can be concluded that those custard systems with the highest content of flour presented a very good response as a potential new dairy product.

Estudio del Efecto Combinado del Secado y Freido en las Propiedades de Tortillas Tostadas

A study of the combination of air drying and immersion frying, and their effect on decreasing fat content and other properties of toasted tortillas was done. Drying with hot air (35, 48 and 68 °C) at 2, 4 and 6 m/s was utilized to dry the tortillas, in which the final moisture content was in a range of 6-12%. Frying was conducted with initial moisture of 10% in the tortillas, at 140, 160 and 180 °C and process times of 30, 60, 90 and 120 s to get the toasted tortillas. Faster moisture loss during drying determined lower evaporation rates through the frying and vice versa. There was a significant effect of the drying conditions on the fried product. The toasted product had fat content lower than commercial products and similar quality attributes. Optimization of the process was carried out using a computer program and the Box-Behnken approach. Model results were compared with experimental data.

Rheological And Physicochemical Properties Of Some Custard Model Systems.

The custards are semisolid dairy desserts of wide consumption in some countries, including Mexico. This product is composed of milk, starch, hydrocolloids, sugar, colorants and aromas. It is a dairy product with limited information about its characteristics. The objective of this research was to study the properties of some custard model systems, following a factorial design, with 4 variables: starch, gum, sweetener and flavor, in two groups. Formulated systems were analyzed during four weeks of storage at 5°C. The obtained results indicated that acidity, color, pH, moisture and density, did not change through the storage. Acidity and pH showed some changes during the storage, but there was not significant difference (p>0.05). Net color difference exhibited a small change, with values 0-5. Adhesive and compressive forces, and serum separation showed significative effect of storage time, with a decreasing trend, as consequence of structural modifications. Syneresis augmented significantly, from 0-16% a the beginning, up to reach 30-60% at the fourth week. All the formulated systems behaved as non-Newtonian fluids and exhibited thixotropy in which the starch influenced the rheological response. Flow properties varied importantly as a function of the storage time and composition; most of the systems exhibited yield stress and were fitted by the Herschel-Bulkley equation. The flow behavior index did not change significantly with the storage, in contrary to the consistency coefficient and yield stress, in which a decreasing trend was observed. From the viscoelastic response, a weak gel was identified. Sensorial assessment did not show differences between custard systems with sucrose and fructose.

Propiedades Fisicoquímicas y de Flujo de un Yogur Asentado Enriquecido con Microcápsulas que Contienen Ácidos Grasos Omega 3

espanolSe han estudiado y analizado las propiedades fisicoquimicas y de flujo de un yogur enriquecido con acidos grasos poli-insaturados omega-3 en microcapsulas a dos concentraciones y almacenado durante 3 semanas en refrigeracion. Los sistemas preparados fueron analizados semanalmente. Con respecto al factor de concentracion, la sineresis disminuyo y los valores de grados Brix aumentaron. Con respecto al tiempo de almacenamiento, los valores de pH disminuyeron, la humedad y la densidad permanecieron estables, el color mostro tendencia hacia el amarillo, perdiendo escasa luminosidad. Las propiedades de flujo mostraron un comportamiento no-Newtoniano de tipo seudoplastico. Los datos fueron ajustados a los modelos de Herschel-Bulkley y Ley de Potencia, permitiendo establecer la correspondiente relacion experimental de los tres parametros de flujo con las variables de estudio. El yogur favorece su funcionalidad y ciertas caracteristicas fisicoquimicas con la adicion de omega-3, ademas de su consistencia, viscosidad y color. EnglishPhysicochemical and flow properties of a yogurt enriched with polyunsaturated fatty acids omega-3 in microcapsules, at two concentrations and during 3 weeks of cold storage, were studied and analyzed. The prepared systems were analyzed every week. Syneresis phenomenon decreased while °Brix increased as a function of microcapsules concentration. With respect to storage, pH values decreased, moisture and density were stable, color showed tendency to yellow with low loss of lightness. Flow properties showed a non-Newtonian behavior of pseudoplastic type. Data were fitted to the Herschel-Bulkley and Power Law models that allow establishing the corresponding relation of the three experimental flow parameters with respect to the studied variables. The yogurt favors its functionality by the addition of omega-3 and improves some of its physicochemical characteristics, besides its consistency, viscosity and color.

Effect of sucrose substitution by low-calorie sweeteners on the characteristics of condensed milk

The inclusion of low-calorie sweeteners, fructose/xylitol and Splenda® (a commercial blend of dextrose, maltodextrine and sucralose) in the formulation of sweetened condensed milk was studied. Carrageenan or xanthan gum at two levels (0.05 and 0.10% w/w) were added to provide adequate consistency. Condensed milks were stored during 4 weeks at room temperature. Color and density remained without variations. Flow and texture parameters changed as a function of the composition and storage time, and behaved as non-Newtonian fluids well fitted by Power law model. This kind of sucrose-free products can be a choice for patients with diabetes or people interested in reducing the calorie ingest from these new items.

Flow and Physical Properties of Concentrated Juices from Prinkly Pear (Opuntia ficus-indica and Opuntia streptacantha)

Opuntia is the fruit from a cactus and a long-domesticated plant important in agricultural economies throughout arid and semiarid parts of the world. This fruit has a short shelf life, it is marketed mainly as a fresh fruit and its industrial products are scarce. The objective of this work was to evaluate the flow and physical properties of concentrated juices from two varieties: O. ficus-indica (greenish) and O. streptacantha (reddish).

Evaluación de Coeficientes de Fricción en el Transporte de Fluidos No-Newtonianos

El objetivo del presente trabajo fue medir las caidas de presion para obtener los coeficientes de friccion de fluidos no-newtonianos. Se empleo un sistema de transporte con varios accesorios en los que se hicieron pasar los fluidos de prueba, midiendo el flujo volumetrico y las perdidas de presion. Los coeficientes de friccion que se obtuvieron para el agua como fluido de referencia fueron entre 0.42 y 25.7, y para las soluciones de sal de sodio de carboximetil celulosa (NaCMC) entre 0.43 y 38.1. Los coeficientes de friccion obtenidos aumentaron con la concentracion, lo que permitio obtener una relacion potencial entre dicho coeficiente y la concentracion de sal para la mayoria de los accesorios y tuberias en que fueron empleados. Las diferencias encontradas al comparar los valores determinados experimentalmente con otros informados en la literatura, se atribuyen al comportamiento reologico del fluido, al circuito de flujo empleado, y al llamado ?efecto de influencia mutua? de los accesorios.