CYSTINE LYASES IN PLANTS: A COMPREHENSIVE REVIEW

Cystine lyases cleave L-cystine through aβ-elimination reaction producing thiocysteine (cysteine persulfide), pyruvate and ammonia. They are responsible for the initial reaction that produces characteristic flavors and aromas in important vegetables of the genus Brassica. In this review, an overview of these plant C-S lyases, comparison with alliin lyases andβ-cystathionases, purification protocols, substrate specificity, function and biochemical characteristics will be discussed. The importance of these enzymes to food science will also be examined.     

MODELING THE BEHAVIOR OF LISTERIA MONOCYTOGENES IN pH-MODIFIED CHICKEN SALAD DURING COLD STORAGE AND TEMPERATURE ABUSE CONDITIONS

Listeria monocytogenes grows at refrigeration temperatures (5C or below) and tolerates various environmental stressors. The Food and Drug Administration specifies a zero tolerance for this pathogen in certain ready‐to‐eat processed foods. Modeling its dynamic behavior to fluctuation in temperature at various pH levels is critical to the safety of food. This study presents linear and nonlinear models to predict the behavior of L. monocytogenes in pH‐modified chicken salad at various cold storage and temperature abuse conditions. A linear model of the kinetics accounting for simple and interactive effects of storage time, temperature and pH was developed. Predictions of the linear model were inconsistent with laboratory observations. The limitations of the linear model were reflected in the poor correlation of model predictions to the observed values (r² = 0.58). A proposed nonlinear model was therefore used to model the observed data. The four model parameters (N(0), C_c(0), k_max and N_res ) were optimized for each of the nine treatments. Correlation coefficient (r²) values ranged from 0.70 (pH 5.2, 7.2C) to 0.99 (pH 4.0, 21.1C), indicating an improved accuracy. Developing a functional and validated microbial predictive model for chicken salad requires further analyses and collection of data at additional pH and temperature values to determine a single set of parameter values that would represent the microbial behavior at the full range of pH and temperatures observed under storage conditions. Future experiments should address the adaptive nature of L. monocytogenes, as the response to environmental stressors affects the survival of the organism in food systems.     

Formation of a Gel from a Heated Emulsion of Alfalfa Leaf Protein and Peanut Oil

A new method was investigated for extracting the proteins from green leaves (alfalfa). An opaque white gel formed when an emulsion of chloroplast-free leaf tissue juice and peanut oil was heated for 10 min at 90°C. The proximate composition of this gel was 40% moisture, 2% protein, 57% fat, 0.2% ash, and <1% carbohydrate. SDS-PAGE analysis of the gel suggested that fraction 1 or ribulose bisphosphate carboxylase is the major protein component present. The apparent viscosity of the gel, 5,257 m Pa.sec, was less than one-tenth of mayonnaise.     

Predicted Static Fatigue Behavior of Specially Coated Optical Glass Fibers

Long-term static fatigue behavior of optical glass fibers is controlled by their strength, fatigue resistance, and zerostress aging behavior. The effectiveness of four special coatings in preventing the long-term static fatigue deterioration of optical glass fibers was evaluated by determining the dynamic fatigue behavior and the effects of zero-stress aging on strength of the four specially coated optical glass fibers in water from 25° to 85°C. The results clearly show that the strength, fatigue resistance, and aging behavior varied significantly between these specially coated fibers. By analysis of these experimental results in terms of fracture mechanics principles, the predicted static fatigue behaviors of the four fibers were compared. Ideally the optimum fiber is one that exhibits a high strength, low strength variability, high fatigue resistance, and high aging resistance. Each of these specially coated fibers had a deficiency in at least one of these properties.     

Structural Aspects of the Lattice Thermal Expansion of Hexagonal Cordierite

The mechanism of lattice thermal expansion in pure hexagonal cordierite prepared by glass devitrification was investigated by time-of-flight neutron powder diffraction at temperatures from 22° to 750°C under vacuum. Full matrix least-squares refinement of the diffraction spectra was carried out in space group P6/mcc using the Rietveld method. The structural data from the refinements were resolved into components parallel and normal to the c axis. The negative c axis expansion results from two effects: (1) increasing distortion of the T₂ tetrahedral (ring tetrahedra) with temperature, primarily due to displacement of the T₂ cations toward the c-axis channels in the structure; (2) decreasing distortion of the T₁/M tetrahedra/ octahedra with increasing temperature, due to octahedral I (M-O₁) bond expansion and an associated decrease in torsional distortion of the T₁ tetrahedra.     

Recrystallization and Phase Transformation in Reaction-Sintered Sic

Reaction-sintered Sic specimens prepared at ∼1500°C and heat-treated at ∼1850°C in the presence of molten silicon were examined by transmission electron microscopy. The β-Sic grains in as-prepared specimens grew to a large size when heat-treated in the presence of molten Si; the number of growth twins in such °-Sic crystals was very small. The boundary areas of α- and β-Sic crystals were microsyntactic, consisting of thin strips of α- and β-Sic after heat-treating. These changes were due mostly to thin lamellar growth of α-Sic into β-Sic grains along the basal plane. There is also a high density of similar growth of transformation twins in β-Sic crystals. The results provide clear evidence of in situ solid-state transformations in sic.     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.