RESUMO

The sole effect of the microstructure of biodegradable isodimorphic poly(butylene succinate)-ran-poly(ε-caprolactone) random copolyesters on their rheological properties is investigated. To avoid the effect of molecular weight and temperature, two rheological procedures are considered: the activation energy of flow, Ea, and the phase angle versus complex modulus plots. An unexpected variation of both parameters with copolyester composition is observed, with respective maximum and minimum values for the 50/50 composition. This might be due to the peculiar chain configurations of the copolymers that vary as a function of comonomer distribution within the chains. The same chain configuration variations are responsible for the isodimorphic character of the copolymers in the crystalline state. Tack tests, performed to study the viability of the copolyesters as environmentally friendly hot melt adhesives (HMA), reveal a correlation with rheological results. Tackiness parameters, particularly the energy of adhesion obtained from stress-strain curves during debonding experiments, are enhanced as melt elasticity increases. Based on the carried-out analysis, the link microstructure-rheology-tackiness is established, allowing selecting the best performing HMA sample considering the polymer chemistry of the system.

RESUMO

This work studies how sucrose (S) addition modifies the thermal properties of cassava starch (CS). Neat CS and CS-S blends with 4, 6 and 8% sugar contents (CS-S-4%, CS-S-6% and CS-S-8%) were prepared and analyzed by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA), in a wide range of moisture levels (2-20%). In equilibrated samples with moisture contents lower than 10%, twoendothermic steps were observed during first DSC heating scans and two corresponding relaxation maxima in tan δ were detected by DMTA. The first transition, detected at around 45-55°C by both DSC and DMTA, is frequently found in starchy foods, while the second observed at higher temperatures is associated to the glass transition temperature of the blends. At higher moisture contents, only one thermal transition was observed. Samples analyzed immediately after cooling from the melt (i.e., after erasing their thermal history), exhibited a single glass transition temperature, regardless of their moisture content. Addition of sugar promotes water plasticization of CS only at high moisture contents. In the low moisture content range, anti-plasticization was observed for both neat and sugar-added CS samples. Addition of sugar decreases the moisture content needed to achieve the maximum value of the glass transition temperature before plasticization starts. The results of this work may be valuable for the study of texture establishment in low moisture content extruded food products.

RESUMO

Glass transition temperatures and physical aging of amorphous cassava starch and their blends with corn oil were assessed by differential scanning calorimetry (DSC). Two enthalpic relaxation endotherms, well separated in temperature values, were exhibited by neat amorphous cassava starch with 10.6% moisture content, evidencing two amorphous regions within the starch with different degrees of mobility. The phase segregation of these two amorphous regions was favored by added corn oil at low moisture contents during storage. The presence of amylose-lipid complexes in this matrix, may also affect the molecular dynamics of these two amorphous regions at low moisture contents. Increasing moisture content, leads to a homogeneous amorphous phase, with an aging process characterized by a single enthalpic relaxation peak. In all cases, after deleting the thermal history of the samples only one glass transition temperature was detected (during DSC second heating runs) indicating that a single homogeneous amorphous phase was attained after erasing the effects of physical aging. Trends of the enthalpic relaxation parameters were also different at the two moisture contents considered in this work.

Assuntos

RESUMO

The effect of adding 1-8% amylose complexing fatty acids (CFA), such as linoleic and oleic acids, on the glass transition temperature (Tg) of cassava starch (CS) with moisture content varying from 5 to 35% (dry basis) was studied. The main relaxation temperature (Tα), associated with the glass transition temperature of the samples (Tg), was determined by dynamic-mechanical-thermal analysis. The plasticizing behavior of water in the blends was evidenced by a decrease of Tα values with moisture content. The effect of CFA on CS was found to be a function of moisture content. At low moisture (<11%) it caused an anti-plasticization effect, while at higher moisture contents it produced plasticization. The anti-plasticizing effect of CFA on CS was attributed to amylose-lipid complex formation.

Assuntos

RESUMO

Heat induced color change kinetics in a tuna-vegetable mixture was evaluated by measuring color parameter "L" (Hunter-Lab) and 5-hydroxi-methyl-furfural (5-HMF) accumulation. For this purpose small reusable stainless steel TDT cans were used and the kinetic studies performed in a temperature range characteristic of thermal processing of low acid canned foods (110-125°C). The color parameter L was better described by a pseudo zero order while a pseudo first order reaction was found for 5-HMF accumulation, indicating the progression of the Maillard reaction during heating. In both cases, temperature dependence of the rate constants followed the Arrhenius relationship, with Ea= 92.0 and 38.9kJ·mol-1 for lightness L and for 5-HMF accumulation, respectively. The color parameter L was more sensitive to temperature changes than 5-HMF accumulation, and therefore is suggested for evaluation of heat induced color changes in this product.

Se evaluó la cinética del cambio de color inducido por el calor en una mezcla de atún con vegetales mediante la medición del parámetro de color "L" (Hunter-Lab) y la acumulación de 5-hidroxi-metil-furfural (5-HMF). Los estudios cinéticos se llevaron a cabo en un rango de temperatura característico del procesamiento térmico de alimentos de baja acidez (110-125°C), empleando envases TDT reutilizables. El parámetro de color L tuvo un mejor ajuste a una cinética de pseudo orden cero, mientras que la acumulación de 5-HMF se ajustó a una cinética de pseudo orden uno, indicando el desarrollo de la reacción de Maillard durante el calentamiento. En ambos casos, la dependencia de la constante de la velocidad de reacción con la temperatura se ajustó a la ecuación de Arrhenius con Ea= 92,0 y 38,9kJ·mol-1 para la luminosidad L y para la acumulación de 5-HMF, respectivamente. El parámetro de color L fue más sensible a los cambios de temperatura que la acumulación de 5-HMF y por consiguiente se sugiere para la evaluación de los cambios de color inducidos por el calor en este producto.

Foi avaliada a cinética da mudança de cor induzido pelo calor em uma mistura de atum com vegetais mediante a medição do parâmetro de cor "L" (Hunter-Lab) e a acumulação de 5-hidroxi-metil-furfural (5-HMF). Os estudos cinéticos foram levados a efeito em uma faixa de temperatura característica do processamento térmico de alimentos de baixa acidez (110-125°C), empregando envases TDT reutilizáveis. O parâmetro de cor L teve um melhor ajuste a uma cinética de pseudo ordem zero, enquanto que a acumulação de 5-HMF foi ajustada a uma cinética de pseudo ordem um, indicando o desenvolvimento da reação de Maillard durante o aquecimento. Em ambos os casos, a dependência da constante de velocidade de reação com a temperatura foi ajustada à equação de Arrhenius com Ea= 92,0 y 38,9kJ·mol-1 para a luminosidade L e para a acumulação de 5-HMF, respectivamente. O parâmetro de cor L foi mais sensível às mudanças de temperatura que a acumulação de 5-HMF e, por tanto, é sugerido para a avaliação das mudanças de cor induzidas pelo calor em este produto.