EFFECT OF SAMPLE DIMENSIONS AND DEFORMATION RATE ON THE TORSIONAL FAILURE OF DUMBBELL SHAPED GELS

Torsional failure measurements were made on gels with dumbbell and capstan geometries. As a comparison, cylindrical specimens were tested in uniaxial compression. Both brittle and elastic gels, made from carrageenan and locust bean gelling agents by selecting appropriate ratios of the two polymers, were investigated. Using the dumbbell geometry, the stress at failure was found to be independent of the sample dimensions while the failure strain decreased with increase in the aspect ratio. In addition, neither the stress nor the strain at failure depended on the strain rate. The failure parameters from the two torsion geometries and the compression test were found to agree provided comparison was made using appropriate sample dimensions.     

INFLUENCE OF UNIAXIAL COMPRESSION RATE ON RHEOLOGICAL PARAMETERS AND SENSORY TEXTURE PREDICTION OF COOKED POTATOES

The effect of uniaxial compression rate (20–1000 mm/min) on the parameters: Stress (σ_ftrue), strain (ε_fHencky) and work to fracture (W_f), modulus of deformability (E_d), maximum slope before fracture (E_max) and work during 75% compression (W_total) was investigated for ten potato varieties. Multivariate data analysis was used to study the correlation between and within the sensory and nonsensory measurements by Principal Component Analysis (PCA) which showed σ_ftrue, E_max, W_f, and W_total to explain the same type of information in the data, and ε_fHencky versus E_d another type of information in the data. The deformation rate had a large effect on ε_fHencky. Nine sensory texture attributes covering the mechanical, geometrical and moistness attributes were evaluated. Relationships between uniaxial compression data at various deformation rates and the sensory texture attributes were studied by Partial Least Squares Regression (PLSR). A minor effect of deformation rate on the correlation with the sensory texture properties was obtained. Mechanical properties were predicted to a higher extent than the geometrical attributes and moistness. The prediction of the mechanical, geometrical and moistness attributes increased largely by using uniaxial compression supplemented by chemical measures such as dry matter and pectin methylesterase, but here no relevant effect of deformation rate was obtained.     

FREEZING OF RAW BEEF: INFLUENCE OF AGING, FREEZING RATE AND COOKING METHOD ON QUALITY AND YIELD

The combined effects of aging, freezing rate, frozen storage and cooking method on beef quality and yield were studied in a multifactorial experiment, using 1.5 cm slices of LD muscles from young steers. Aging for 2 wk resulted in significantly more tender, but slightly less juicy frozen meat than aging for 4 days. Prolonged frozen storage gave tougher meat and a larger tenderness difference between aging times. Freezing rates of 13, 2.0 and 0.04 cm/h differed but little in their effect on sensory quality, but slightly lower yield and redness value were noted for the lowest rate. Pan frying directly from the frozen state resulted in slightly higher juiciness and cooking yield, with larger differences in yield between freezing rates, than cooking after previous thawing. Significant interactions were noted for cooking method × freezing rate and for aging time × frozen storage time     

EFFECTS OF SAMPLE DIMENSIONS ON THE SNAPPING FORCE OF CRISP FOODS *

Abstract. According to well-known formulae for the mechanics of flexure stresses, the force required to snap a test specimen of uniform rectangular cross-section is inversely proportional to its length and directly proportional to the width and the square of the thickness. For a specimen of circular cross-section, the snapping force is directly proportional to the cube of its radius and inversely proportional to its length. In order to test the validity of the theoretical models for foodstuffs, three snappy foods (cookie, beet, carrot) were fashioned into bars of rectangular or circular cross-section in lengths varying from 2.0 to 5.8 cm. The rectangular bars ranged from 0.4 to 1.0 cm in thickness and 1.5 to 3.0 cm in width. The cylindrical bars ranged from 0.253 to 0.578 cm in diameter. The bars were snapped in two by mounting them in a horizontal plane on two fixed vertical supports and applying force to fracture by means of a descending beam centered between the supports in a snapping rig mounted in an Instron Universal Testing machine. Experimental data for the commodities tested were in satisfactory agreement with the theoretical model.     

EFFECT OF SAMPLE THICKNESS ON THE DEFORMATION OF CHEWING GUM

Thickness-time data from plastometer tests of gums of different thicknesses using a range of compressive loads and specimen sizes yield consistent indices of compliance suggesting that thicker samples are softer than thinner ones. On staling of these gums by exposure to low humidity, the compliance of each gum decreases in a regular manner with exposure time. At comparable staling times, the thinner samples are found to be harder than the thicker ones by the plastometer criteria.
Limited panel evaluation shows that the trend for each gum thickness to become harder during staling is perceived by the chewers, consistent with the physical data and with prior experience. In contrast, the panelists do not readily differentiate the hardnesses among samples of different thicknesses chewed at comparable staling times unless the difference in thickness is large; in such cases the thinner samples are judged to be softer in chew than the thicker, contrary to the plastometer indications. This suggests that cues other than compliance are invoked by chewers when sample dimensions differ widely but this issue will require more extensive and detailed study for resolution.     

THE INFLUENCE OF THE FIRMNESS OF RAW TOMATOES ON THE PHYSICAL, CHEMICAL AND SENSORY CHARACTERISTICS OF CANNED PRODUCTS

The effects of firmness of raw tomatoes on selected quality characteristics of canned products was the primary objective of this study. Firm fruits, as sorted with Holme's Vibratory Sorter, were found to be significantly (P≤ .05) higher in drained weight, Agtron E-5 color readings, total acidity, sodium hexametaphosphate-soluble, and sodium hydroxide-soluble pectic substances, and lower in pH, soluble solids, soluble solids/acid ratio, and water soluble pectin than soft and unsorted fruits. In determining the sensory quality, firm fruits were superior in flavor, texture, and overall quality but inferior in color compared to soft and unsorted fruits. This investigation revealed that sorting fresh tomatoes with a vibratory sorter and canning only the firm fruits had a positive impact on improving the quality of canned products.     

UNIAXIAL COMPRESSION AND STRESS RELAXATION TESTS ON ALGINATE GELS

The rheological behavior of alginate gels was analyzed using uniaxial compression and stress relaxation tests. The engineering compressive stress (σ_E)-deformation (ɛ_E curves were found to be concave upward and independent of the crosshead speed (V_t for V_t≥ 120 mm/min. By considering all the stress (σ_E)-time (t) data for t greater than 10 times the average loading period, the viscoelastic response of these gels was described by means of a 3-element Maxwell body characterized by a relaxation time of 300 s. Such gels exhibited an apparent linear viscoelastic solid behavior for ɛ_E≤ 8% and a nonlinear one for larger deformations. However, for ɛ_E>32% their solid viscoelastic behavior tended to a liquid one. Empirical models allowed the viscoelastic properties, as well permanent deformation, of these gels to be quantitatively described as functions of the deformation applied and/or loading rate.     

INFLUENCE OF SURFACE FRICTION ON THE STRESS RELAXATION OF GOUDA CHEESE1

The stress relaxation behaviour of Gouda cheese cylinders with a diameter/length ratio of 0.7 was studied at compression rates of 5 and 100 cm min^-1. Tests were made in three ways: (1) normal test procedure i. e., the samples were not lubricated, (2) with the samples' upper and lower ends lubricated with mineral oil, (3) with emery paper inserted between the samples and the compression plates so as to exaggerate the friction effects. Surface friction affected stress relaxation behaviour using the normal and emery paper test procedures at compressions up to about 20%. At higher concentrations cracks are initiated and propagated in all three test procedures. The values of the rheological parameters derived from relaxation data obtained using compressions not exceeding 20% were lower for lubricated samples. This is attributed to the elimination of that component of the initially applied force which was required to overcome surface friction and which does not decay with time, in contrast to the component actually involved in the initial compression which exhibits stress relaxation.     

BEHAVIOR OF DIFFERENT FOODS IN THE STANDARD SHEAR COMPRESSION CELL OF THE SHEAR PRESS AND THE EFFECT OF SAMPLE WEIGHT ON PEAK AREA AND MAXIMUM FORCE

Abstract Twenty-four foods (encompassing fruits, vegetables, meat, bread, cheese and rice) were tested in the Allo Kramer Shear Press at various weight levels ranging from those needed to cover the bottom of the cell to those required for a complete fill. The relationship between maximum force values and sample weight was found to be different for different foods. Products could be grouped into three general categories: those exhibiting a constant force to weight ratio (e.g. white bread, sponge cake), those exhibiting a continuously decreasing force to weight ratio (e.g. raw apples, cooked white beans), and those exhibiting a constant force, independent of sample weight, beyond a certain cell fill level (e.g. canned beets, canned and frozen peas). Fitting the observed behavior into mathematical models allows to infer that extrusion, in addition to the previously recognized shear and compression, plays an important role in the behavior of foods in the standard cell of the Shear Press, and that most foods are subject to various combinations of these forces. The contribution of extrusion was confirmed by actual extrusion tests. Peak areas showed an exponential relationship to sample weight following the equation A = l0^aW^b. Some evidence was obtained that exponents a and b are related to the amount and the general type of resistance offered by the food.     

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.     

EFFECTS OF SAMPLE THICKNESS ON BITE FORCE FOR RAW CARROTS AND FISH GELS

The effects of food thickness on human bite force were investigated using a multiple‐point sheet sensor and electromyography. The total force and contact area were directly measured in real time using the sensor, and the average stress detected between the teeth and a food sample (active stress) was derived from the ratio of the force to the contact area. Raw carrots and fish gels with different thicknesses were bitten between the upper and lower incisors or molars by healthy women. For hard and brittle carrots, the peak force, contact area and active stress at the fracture point were greater in thicker samples when biting with incisors. For soft and tough gels, the peak force and contact area were increased as thickness increased; however, the active stress was similar in incisor bites. These values did not differ significantly with thickness in the molar bites of both samples. Although the force and contact area were greater for molar bites than incisor ones, the active stresses at fracture point were independent of tooth position. The sample thicknesses influenced the bite force of humans and the stress applied to the sample, and the effects differed with sample texture.     

BEHAVIOR OF MOZZARELLA, CHEDDAR AND PROCESSED CHEESE SPREAD IN LUBRICATED AND BONDED UNIAXIAL COMPRESSION

The response of a material under uniaxial compression depends both on the bulk material properties and on the frictional effects at the sample-platen interface. Three commercial cheeses (cheddar, mozzarella and processed cheese spread) were studied in uniaxial compression. Frictional effects at the sample surface were examined under three conditions: (1) lubrication of the sample-platen interface; (2) bonding of the sample to the instrument platens; (3) samples neither bonded nor lubricated. The forces required to deform the cylindrical sample to a given strain level were highest for the bonded cases and lowest under lubricated conditions. The stress in lubricated compression, ρ_L, was calculated from Fh(πR_ρ²h_ρ)     

小麦和甘薯抗性淀粉得率的工艺研究

采用酸变性和沸水浴的方法，对小麦和甘薯淀粉进行处理，以抗性淀粉得率作为评价指标，研究了在不同的淀粉乳浓度、盐酸用量、酸解时间、沸水浴时间、不同冷藏温度和时间条件下，小麦和甘薯淀粉糊化后形成抗性淀粉得率的变化情况，得到在不同条件下抗性淀粉得率受影响的趋势。