Cryo-Scanning Electron Microscopy Investigation of the Texture of Cooked Potatoes
The texture of steam-cooked potatoes was investigated by examining the fracture planes of four different cultivars, using cryo-scanning electron microscopy (cryo-SEM), which yielded a good preservation of the hydrated structures in potato tissue . For all cultivars , fracturing after steam -cooking took place between cells preferentially alongside the cell walls. However, textural difference appeared from the degree of intercellular contact, the cell shape and the appearance of cell surfaces. Cells in the fracture planes of firm potatoes had large intercellular contacts. In this case, most of the cells were flat and cell surfaces showed folds and cracks . For mealy potatoes , it appeared that cells in the fracture planes had little intercellular contacts . The cells were round and turgid and had smooth surfaces . In conclusion, the structure of cell wall and middle lamella , and the starch content , appear to be important parameters to distingui sh firm and mealy cultivars. Key \Vords: Cell, cryo-scanning electron microscopy , potato, cultivar , steam-cooking, texture, fracture plane, pectin, starch, cell wall. Initial paper received March 19, 1992 Manuscript received September 8, 1992 Direct inquiries to J.T. van Marte Telephone number: 31 8370 75060 Fax number: 31 837
Effects of multivalent cations on cell wall-associated Acid phosphatase activity.
Primary cell walls, free from cytoplasmic contamination were prepared from corn (Zea mays L.) roots and potato (Solanum tuberosum) tubers. After EDTA treatment, the bound acid phosphatase activities were measured in the presence of various multivalent cations. Under the conditions of minimized Donnan effect and at pH 4.2, the bound enzyme activity of potato tuber cell walls (PCW) was stimulated by Cu(2+), Mg(2+), Zn(2+), and Mn(2+); unaffected by Ba(2+), Cd(2+), and Pb(2+); and inhibited by Al(3+). The bound acid phosphatase of PCW was stimulated by a low concentration but inhibited by a higher concentration of Hg(2+). On the other hand, in the case of corn root cell walls (CCW), only inhibition of the bound acid phosphatase by Al(3+) and Hg(2+) was observed. Kinetic analyses revealed that PCW acid phosphatase exhibited a negative cooperativity under all employed experimental conditions except in the presence of Mg(2+). In contrast, CCW acid phosphatase showed no cooperative behavior. The presence of Ca(2+) significantly reduced the effects of Hg(2+) or Al(3+), but not Mg(2+), to the bound cell wall acid phosphatases. The salt solubilized (free) acid phosphatases from both PCW and CCW were not affected by the presence of tested cations except for Hg(2+) or Al(3+) which caused a Ca(2+)-insensitive inhibition of the enzymes. The induced stimulation or inhibition of bound acid phosphatases was quantitatively related to cation binding in the cell wall structure.
Characterization of Aqueous Dispersions and Gels Made of Sodium Caseinate and Basil Seed Gum: Phase Behavior, Rheology, and Microstructure
The interactions between sodium caseinate (NaCas) and basil seed gum (BSG) in the presence of calcium chloride (CaCl 2 ) were investigated. The phase behavior of the mixed aqueous dispersions and their gels revealed a homogeneous mixture, obtained at the higher concentrations of both CaCl 2 and BSG. The Herschel-Bulkley model sufficiently fitted the flow behavior of the mixture solution data. Apparent viscosity increased significantly ( p  < 0.05) by increasing the concentration of BSG, where the addition of CaCl 2 had no significant effect on the viscosity of the samples ( p  > 0.05). Furthermore, there was an increase in thixotropy due to the higher concentrations of BSG and CaCl 2 . Based on the frequency sweep test, at the low frequencies, a more gel-like behavior was observed in the case of the higher concentrations of either BSG or CaCl 2 . The rheological and SEM data suggested that the stronger structure of NaCas-BSG gel in the presence of the higher concentrations of CaCl 2 was related to the induction of complex formation between the two biopolymers.