Kinetic analysis of soy-protein denaturation by a temperature-programmed heat-denaturation technique.

Abstract

The soy-protein denaturation rates of defatted soy flour and isolated soy protein of varying moisture content were analyzed by a temperature-programmed heat-denaturation (TPHD)technique with a hand-press reactor. The degree of protein denaturation of the soy proteins, which is defined as the solubility in 0.1 m phosphate buffer (pH 8.0) containing 1% SDS and 1% 2-ME, could be estimated with the TPHDmodel as a first-order heat-denaturation process. A kinetic compensation effect was observed for the protein denaturation of both soy proteins, the activation energy of the denaturation rate decreasing linearly with a moisture content below 0.4.Cooking screw extruders are being used extensively in the manufacture of texturized soy protein products and other food products.1>2) The extruder is a high-temperature, short-residence-time reactor which generates frictional heat as it compresses and propels material within the barrel and out through the extrusion dies. developed a theoretical model which described the effects of temperature-time history, temperature, shear rate and moisturecontent on the apparent viscosity of defatted soy dough. The temperature-time history function depended on the activation energy involved in the denaturation of soy dough. They assumed that the soy-protein denaturation effects on viscosity could be approximated as a pseudo firstorder reaction. However, little information has been obtained regarding the basic kinetic data for soy-protein denaturation, especially in the case of a low moisture content. As Saio5) and Noguchi6) have reviewed, the polymerization reaction of protein inside an extruder had a complex dependence on the temperature and the moisture content of the protein. However,in order to understand the extrusion cooking process quantitatively, it is necessary to treat the process on the basis of Materials and Methods 1) Defatted soy flour and isolated soy potein. The defatted soy flour (Protein-S) used in this study was from Ajinomoto (Tokyo). This flour contained a minimumof 48% protein, a maximumof 28% carbohydrate, and less than 1.5% fat, and had a nitrogen solubility index (NSI) of 60. Over 85%of the flour particles had a diameter of more than 200-mesh size. The isolated soy protein was from Ajinomoto (Tokyo), and contained a minimumof 89.6% protein. This isolated soy protein had an NSI of 97.5.2) Chemicals. 2-Mercaptoethanol (2-ME) and sodium laurylsulfate (SDS) were specially prepared reagents from Nakarai Chemicals Ltd. The other chemicals used were of reagent grade.3) Samplepreparation. The defatted soy flour or isolated soy protein was hydrated to the desired moisture content by adding distilled, deionized water. The samples were initially mixed in a kitchen mixer (Matsubara Ltd.), andwere further mixed in a kitchen blender (Multi-Blender Mill, Nihon Seiki). The c. 2g samples were each compressed into a button-like mold (20mm diameter and 5mmthickness) with a hand press (S04-15, Riken) in a stainless reactor, as shown in Fig. 1. In this compression process, all the sampl...

Relationships between processing conditions and starch and protein modifications during extrusion‐cooking of pea flour

“…The temperature at which IER began to decrease (approx 120"C), ie the temperature at which denaturation started, agrees with the upper values measured by DSC for thermal denaturation of legumins as a function of water/protein ratio (Arntfield et a1 1985). Similar values have also been reported for experiments carried out on a static temperature reactor (Yoshii et al 1990). However, direct comparison with extrusion behaviour is difficult since the heating kinetics are not the same and shear effects are not taken into account in these experiments.…”