Thermal and Calcium Pretreatment Affects Texture, Pectinesterase and Pectic Substances of Frozen Sweet Cherries

Freezing causes loss of turgidity and firmness in sweet cherries. Thermal pretreatment at 50°C for 10 min followed by immersion in 100 mM CaCl₂ and thermal pretreatment at 70°C/2 min with or without immersion in 100 mM CaCl₂ prevented freezing-induced loss of firmness. Thermal pretreatments increased the pectin fraction soluble in EDTA, reduced the degree of pectin esterification, and increased both the concentration of divalent cations in the cell wall and the pectinesterase activity bounded to the cell wall. Immersion in CaCl₂ increased the concentration of Ca²⁺ cations in the cell wall and enhanced the effect of thermal pretreatments on pectinesterase activity.     

INFLUENCE OF FRICTION, SAMPLE DIMENSIONS AND DEFORMATION RATE ON THE UNIAXIAL COMPRESSION OF RAW POTATO FLESH

The influence of surface friction and lubrication on the compression behavior, at compression rates of 5–100mm min^−l, of cylindrical samples of potato flesh have been examined with an Instron Universal Testing Machine at room temperature. The samples had length/diameter ratios of 0.2–0.8. Parameters derived from compression to failure were: failure stress; failure strain; and apparent modulus of elasticity. Stress relaxation with time was studied using samples with length/diameter ratios of 0.4–0.8 following compressions of 10 and 30%. The length/diameter ratio, rate of compression, surface friction and lubrication influenced the parameters derived from both the compression and the stress relaxation tests. Relaxation times increased following sample lubrication. The effect due to surface lubrication was smaller than in previous compression and relaxation tests on Gouda cheese. This was attributed to release of fluid from the damaged cellular tissue of the potato flesh which reduced the effectiveness of the lubricant film.     

THE EFFECT OF TEMPERATURE, GEOMETRY, GAP AND SURFACE FRICTION ON OSCILLATORY RHEOLOGICAL PROPERTIES OF MASHED POTATOES

Oscillatory data obtained for mashed potatoes made from fresh tubers from measuring geometries of different designs and sizes were analyzed and compared at the sample temperatures of 25 and 55C. The measuring geometries used were smooth parallel plates (PP40 and PP20, 40‐ and 20‐mm‐diameter plates, respectively); truncated cone and plate (CP 4/40, 40‐mm‐diameter cone having an angle of 4°); FL25 four‐bladed vane tool (diameter: 25 mm, height: 55 mm); and serrated parallel plate (SPP40, 40‐mm‐diameter plate). Three different gaps between plates were used (2, 1.5 and 1 mm) to simulate the range of aggregate spacing in fresh mashed potatoes. The measured oscillatory rheological properties were significantly affected not only by the sample temperature but also by the test geometry used and the friction of and gap between its shearing surfaces. Dynamic parameter values were higher at 55 than at 25C, possibly indicating the onset and propagation of gelatinization of the potato starch grains. Unexpectedly, test geometries with higher friction capabilities, such as the serrated plate and vane tool, presented lower viscoelastic properties compared with results of other test geometries with lower friction capabilities. This result provides evidence of the influence of other factors on the measured oscillatory rheological properties of the mashed potato, such as measurement accuracy of the rheometer at different very small strains, homogeneity of the temperature for the different samples, existence of a complex dependency on the possibly complex stress condition that is imposed on the material upon loading, or even samples being prone to drying. The effect of the variation of the gap between parallel plates on oscillatory parameters depended on the shearing wall friction. The study highlights the difficulty in reconciling rheological results from different sources and the need for standardized rheological test methods so that rheological interlaboratory results can be critically analyzed and compared.     

Comparison of several blanching methods on the texture and ascorbic acid content of frozen potatoes

The effect of stepwise blanching and a mixed method, in which the first step of the stepwise blanching was replaced by a short microwave treatment, have been compared with the conventional process for blanching potatoes for freezing with respect to physical and sensory properties, and ascorbic acid content. After freezing and reheating the mixed method produced a similar texture to that obtained with stepwise blanching, and both were significantly better than conventional blanching. The mixed method shortened the process time and improved retention of ascorbic acid.     

1H and 23Na NMR Relaxation Studies of the NaCl/β-Lactoglobulin System Equilibrated at Various Water Activities

NMR relaxation times (T₁) of ¹H and ²³Na were determined at 8, 14, 60, MHz and at 14, 52 MHz, respectively, in the NaCl/β-lactoglobulin system equilibrated at water activities ranging from 0.11 to 0.97. A two-state model was used for both nuclei which allowed the calculation of proportions of bound and free species. It was shown that bound water and Na⁺ increased at medium water activities in the presence of increasing NaCl. The discrepancy with adsorption isotherms data was explained by (1) limitations of the mass balance equation used to calculate interacting salt by gravimetry and (2) the fact that NMR probed the totality of ions in the vicinity of the protein, whereas adsorption isotherms were indicative of sodium choride tightly bound to the protein. It was also shown that the protein and Na⁺ shared available water molecules at medium and low water activity.