CHARACTERIZATION OF GELLAN GELS BY UNIAXIAL COMPRESSION,STRESS RELAXATION AND CREEP 1

The rheological properties of 0.5–2.5% gellan gels were evaluated by uniaxial compression, stress relaxation and creep tests. The gel's strength was in the range of 0.1–1 kg.cm^-2 and their deformability modulus 1–6 kg.cm^-2. The asymptotic modulus determined in relaxation tests at two strains and the asymptotic creep compliance determined under two loads indicated that gellan gels have a yielding structure. The strains sustained in creep were considerably higher than the failure strain in uniaxial compression. Weight loss due to syneresis was on the order of 4–38% depending on the gum concentration, the deformation level in relaxation or the load in the creep tests.     

ROLE OF OIL ON PHYSICAL PROPERTIES OF CORN MASA FLOURS AND SENSORY CHARACTERISTICS OF CORN TORTILLAS

This study was conducted to evaluate the effect of oil on physical properties of corn masa and sensory of tortillas. Oil from commercial and lab-made whole dry corn masa (DCM) was extracted with hexane. Both flours were reconstituted with 2, and 4% w/w of commercial corn oil, to have 50 and 100% oil reconstituted flours. These levels of reconstitution were to determine the impact of oil on dough properties and sensory characteristics of corn tortillas. The sensory analysis of both types of flours showed the same patterns. The absence of oil did not affect dough stickiness or tortilla reliability. But, low content of oil did adversely and significantly (p<0.05) affect the typical corn tortilla flavor. Presence of oil significantly improved tortilla firmness and chewiness.     

LIPID-AUTOXIDATION-LIMITED SHELF-LIFE OF NIXTAMALIZED INSTANT CORN MASA

Consumers often report the presence of off-flavors and off-odors characteristic of lipid oxidation in commercial dry instant corn masa, but the extent of this problem is unknown. The lipid oxidation states of two common brands of nixtamalized instant corn masa produced in Mexico were evaluated during 23-weeks of storage at 15, 25, 35, 45, and 55C. Lipid autoxidation was evaluated by determining peroxide values (PV) and conjugated dienoic acids (CD) of extracted lipids, and sensory analyses of flours and tortillas made from stored flours. Increased storage time significantly increased lipid oxidation (P < 0.001). Similarly, increased storage temperature significantly decreased the time to reach rancid flavor thresholds. The Q₁₀ values for flours and tortillas were 1.58 and 1.75, respectively. The shelf-lives of instant corn masa were 5.8 months when stored at 25C and only 2 months at 45C.     

PARTIAL PURIFICATION AND CHARACTERIZATION OF SWEET CORN GERM LIPOXYGENASE

Lipoxygenase (LPO) extracted from the germ fraction of sweet corn (Zea mays L. var. Jubilee) was purified by acetone powder preparation; extraction with 0.1 M Tris-HCl, pH 8; 40–60% fractionation with ammonium sulfate; and conventional column chromatography on Sephacryl S-300 HR and Fast Protein Liquid Chromatography (FPLC) on a Mono Q column. A 124-fold purification was achieved with 26.3% recovery. Further purification was achieved by FPLC chromatofocusing on a Mono P column and size exclusion chromatography on Superose-12, with a resultant 436-fold purification and 16.8% recovery. The apparent molecular weight and isoelectric point (pI) determined by FPLC on Superose-12 and Mono P columns were 90,500 and 5.06, respectively. Lipoxygenase-catalyzed formation of conjugated dienes was inhibited by both synthetic (BHA, BHT) and natural phenolic antioxidants (quercetin, chlorogenic acid) at a concentration of 0.2 mM with 57.2, 16.3, 61.4 and 32.3% inhibition, respectively. The activation energy for thermal inactivation of sweet corn germ lipoxygenase from the ammonium sulfate preparation at 55–70C was 56.3 kcal/mol.     

SWELLING STRESS AND HYDROSTATIC COMPRESSIBILITY OF GROUND CORN AND ITS CONSTITUENTS

Swelling stresses and hydrostatic compressibilities of ground whole corn, endosperm and germ were determined by using the theory of swelling stresses in gels. The ground corn and its constituents were considered as gels. The magnitude of swelling stresses in the ground corn materials was in order of several thousand pounds per square inch at low equilibrium moisture contents, when subjected to increased vapor pressure. The hydrostatic compressibility for ground whole corn, endosperm and germ increased with an increase of moisture content.     

THE ANALYSIS OF STRESS RELAXATION DATA OF SOME VISCOELASTIC FOODS USING A TEXTURE ANALYZER*

Uniaxial compression test for dough and several commercial products like jello, mozzarella cheese, cheddar cheese, tofu and sausage (cooked and uncooked) was performed using a texture analyzer (TA). Percent stress relaxation (%SR ), k1 (initial rate of relaxation), k2 (extent of relaxation) and relaxation time (RT) were calculated and compared for different products. The TA software was used to convert the raw SR data into a linear form. Constants k1 and k2 were determined from the intercept and slope of the linear data. Higher values of %SR and k2 (90 and 9, respectively) indicated higher elasticity for jello, whereas wheat flour dough samples showed the lowest values (20–30) for %SR and 1 to 2 for k2. The RT and k1 values were not good indicators for differentiating different products based on their viscoelastic behavior. Measurement of RT was limited by the maximum time for which the data were collected, whereas k1, because of its mathematical form, needed careful interpretation. In this study, %SR was found to be a good measure to interpret viscoelasticity of different food samples.     

FLOW CURVES, STRESS RELAXATION AND CREEP MEASUREMENTS OF STARCH GELS

Flow curves, stress relaxation, and creep compliance were measured for various types of corn starch gel that underwent different degrees of modification. The four starches were native corn starch, medium hydroxypropylated distarch phosphate, highly hydroxypropylated distarch phosphate, and distarch phosphate. All the starches exhibited pseudo-yield stress behavior at low shear rates and shear-thinning at higher shear rates. Stress relaxation data at different strains indicated strain-softening phenomenon for all starches. Higher degrees of cross-linking decreased the half relaxation time due to the addition of rigid filler into the matrix. Increasing the concentration increased the relaxation modulus for all starches. Creep compliance and recovery measurements were conducted over the range of 3.2 Pa-320 Pa. In the linear viscoelastic region, the compliance data were not dependent on the magnitude of stress applied. As concentration increases, higher stress magnitude leads to nonlinear behavior. The creep compliance data were fitted into the Ninomiya and Ferry relation from dynamic viscoelastic data and the Maxwell-Voigt model. Ninomiya and Ferry overpredicted the relaxation data while the Maxwell-Voigt model fitted the data reasonably well.     

STRESS RELAXATION AND TENSILE STRENGTH TESTING OF A PROCESSED FISH PRODUCT

Minced silvercarp was prepared as cooked loaves containing only various added salts. The loaves were then subjected to stress-relaxation and tensile strength tests. The various treatments gave different compression forces, relaxation T_50% values and tensile strengths. The compression force measurements were closely correlated with tensile strength values. Both compression force and tensile strength might be related to the binding quality (cohesiveness) of the loaves. The relaxation T_50% values were not closely correlated with the compression forces nor the tensile strengths, and might be related to the product's viscoelastic properties. Different percent deformation did not have a significant effect on the relative order of the compression force values, but did affect the relaxation T_50% values, with larger differences between samples at 25% deformation.     

STRESS RELAXATION BEHAVIOUR OF FROZEN SUCROSE SOLUTIONS

The stress relaxation behaviour of two frozen sucrose solutions (7% and 19%) during indentation in the temperature range of -20C to -40C were investigated. The stress relaxation is similar to that of pure polycrystalline ice, which is controlled by steady-state creep. The steady state creep rate exponent , m, of 7% and 19% sucrose solutions lies between 2.3 and 3.6. The steady state creep rate constant , B, of 19% sucrose solution is greater than that of 7% sucrose solution. It is suggested that the steady-state creep rate exponent m depends on contributions from the proportions of favourably oriented grains, unfavourably oriented grains and grain boundaries to creep and that these components depend on the value of internal stress which is related to the hardness of samples at the different testing temperatures. The steady-state creep rate constant B depends on the mobility of dislocations in sucrose solutions which, in turn, depends on the temperature and the concentration of sucrose.     

STRESS RELAXATION BEHAVIOR OF MICROWAVE-VACUUM-DRIED ALGINATE GELS

Swelling of alginate polymer matrix in water involves a build up of network pressure due to an elastic extension of the polymeric matrix. When this network pressure undergoes relaxation by means of dehydration, shrinkage may take place. Three different types of wet alginate gels were prepared and dried using microwave‐vacuum‐drying technique. Dried alginate gel solids had a porous structure. To understand the stress relaxation behavior of alginate gel‐based porous solid structures, uniaxial compressive relaxation studies were performed at selected strain rates, preloads and relaxation times Experimental relaxation curves were normalized and fitted to an empirical relationship, and relaxation behavior was explained. Stress relaxation data were also fitted to another empirical model. All three types of gels had similar elastic components. At lower strain rate, all samples had more resistance to elastic deformation. Stress relaxation information of the dried gel was related to its microstructure. Type 2 gel had more stiffness than type 1 and type 3 gels. The mechanism involved in stress relaxation was entanglement coupling of larger polymer chains in covalently cross‐linked alginate gels.     

DYNAMIC VISCOELASTICITY AND STRESS RELAXATION IN STARCH PASTES

The dynamic viscoelasticity and stress relaxation of ten types of starch pastes were measured. The dynamic moduli increased with increasing frequency, and were different for each starch paste, changes in tan δ for cassava and potato starch pastes being about 0.5–0.6. The stress relaxation behaviour of all the samples tested could be represented by a four-element mechanical model. The relationship between spinnability and stress relaxation was determined, the spinning distance increasing with increased relaxation time (τ₂). No satisfactory linear relationship between spinning distance and relaxation time (τ₂) was apparent with the 10 starch paste samples.     

STRESS RELAXATION OF STORED STIRRED CHEDDAR CURD

An introspective empirical method for reducing stress relaxation data of certain foods was proposed by Peleg (1979). In his method relaxation curves are normalized with respect to initial force. In the method described herein his resulting equation is rearranged as a straight line but with the normalized force and time variables written as reciprocals 1/f(t) = (1/ab)(1/t) + (1/a), a plot of 1/f(t) versus (1/t) will yield a more severe test of the experimental data since the time variable "t" no longer appears on both sides of the equation as it does in the former case. This empirical method has been applied to relaxation data of stirred curd cheddar cheese manufactured under government specifications and stored at −2 to 0°C. With this method there are apparently at least two relaxation periods for cheese, a primary period which occurs for about 2 min and a secondary period which commences thereafter. When the separate periods are compared to one another values of the "a" and "b" constants are completely different and show that the secondary relaxation rate is substantially slower than the first. This method provides a more sensitive tool for comparing aging effects in stored cheese.     

EVALUATION OF THE RHEOLOGICAL PROPERTIES OF FRESH CORN MASA USING SQUEEZING FLOW VISCOMETRY: BIAXIAL EXTENSIONAL VISCOSITY1

Alkaline corn was cooked for 20, 55 or 80 min, and the nixtamal was ground into masas with medium or coarse particle size distributions. Water was added to each corn masas in order to obtain the same adhesiveness. Rheological properties of these masas were evaluated with the squeezing flow viscometry technique with lubrication at crosshead speeds of 0.05, 0.2 and 0.5 cm/min, respectively. From the data collected, stress-strain and biaxial elongational viscosityradial extension rate curves were obtained. All the fresh corn masas gave the same stress and elongational viscosity at any given specific strain and radial extension rate, respectively, regardless of the cooking time and degree of grinding. The elongational viscosity could be useful to predict fresh corn masa behavior during molding and cutting steps in the tortilla-making process.     

玉米皮膳食纤维的双酶法制备及其性质研究

采用蛋白酶和淀粉酶结合水解制备玉米皮膳食纤维，研究确立了玉米皮膳食纤维的双酶法制备工艺，进一步探讨了食品体系对玉米皮膳食纤维持水力的影响。研究表明，双酶法制备的膳食纤维中蛋白质、淀粉、灰分残留少，得率高，持水力优于传统酸碱法制备的产品；同时常规食品体系中的氯化钠浓度、蔗糖浓度以及食品体系的pH对玉米皮膳食纤维的持水力影响不大，使其在食品加工中能充分发挥生理活性。