YIELD STRESS OF FOOD DISPERSIONS WITH THE VANE METHOD AT CONTROLLED SHEAR RATE AND SHEAR STRESS

The vane method was used to measure yield stresses of 15 commercial food dispersions under controlled shear stress (Cσ) and controlled shear rate (C     ) operating conditions. Magnitudes of yield stress (σ_σ) at Cσ were higher than those of yield stress (σ     ) at C     on tomato products and baby foods. There were good linear correlations ( R ²= 0.96, 0.86) between γ     and σ^σ for food dispersions with undisturbed structure (UDS) and with broken down structure (BDS). Data obtained under Cσ were also used to calculate shear moduli of foods.     

MEASUREMENT OF FLOW PROPERTIES OF FOOD SUSPENSIONS WITH A MIXER

Abstract. Flow properties of selected food suspensions (tomato puree, apple sauce, and apple pulp) were studied with a mixer. Power law constants for the suspensions were determined by employing an approximate method based on power correlations for mixers in the laminar flow regime. The studied suspensions cannot be characterized with narrow gap viscometers because of the size and segregation of the particles (formation of a dilute phase next to the rotor).     

LIQUID FOODSTUFFS EXHIBITING YIELD STRESS AND SHEAR-DEGRADABILITY

Two rheological properties of liquid foodstuffs often encountered in practice are the yield stress and the shear-degradability (irreversible time-dependence). Both these properties were determined for meat and yeast extracts using two rotational viscometers. The material constants were obtained and fitted with Bingham Plastic and Herschel-Bulkley models for the meat extracts. The shear-degradability exhibited by the yeast extract was quantified in terms of the energy required to destroy the material structure.     

DETERMINATION OF THE YIELD STRESS OF HYDROCOLLOID DISPERSIONS

The yield stresses of dispersions of guar gum (0.5–2.0% by weight) and gelatinized cornstarch (2.5–5.0% by weight) were measured using several different established experimental methods. The yield stresses in these polysaccharide dispersions result from intermolecular hydrogen bonding and molecular entanglements, thus the higher yield stress and hysteresis effect in guar gum dispersion was interpreted in terms of the greater hydrogen-bonding density and molecular entanglements caused by the more extended chain conformation. The results confirm that there is no one absolute value for the yield stress of the hydrocolloid dispersions and that the viscoelastic characteristics of the test materials influence the shear rate and time dependence of yield stress. Therefore, experimental determination of yield stress must be made relevant to the practical application.     

NEW METHOD FOR THE DETERMINATION OF YIELD STRESS1

A method is proposed for determining the yield stress of liquid and semi-liquid foods. The described technique makes use of a modified Fisher Autotensiomat apparatus. A yield stress range of from 20 dynes/cm² to about 900 dynes/cm² was covered by products such as condensed milk and tomato paste, respectively. The data were compared with the results obtained with a Ferranti-Shirley cone and plate viscometer, a Chainomatic Beckers balance and a Thomas-Stormer viscosimeter. With the exception of the data from the latter instrument, the results were in good agreement.     

COMPARISON BETWEEN PROJECTED MECHANICAL EQUILIBRIUM CONDITIONS OF SELECTED FOOD MATERIALS IN STRESS RELAXATION AND CREEP1

Asymptotic modulus versus strain relationships in stress relaxation tests of potato flesh, cheddar cheese and 1.5% agar gels were compared to similar relationships derived from creep experiments. The asymptotic moduli (or compliance reciprocals) were on the same order of magnitude and their dependency on the strain of a similar character. These suggest that despite the history sensitive response of food materials, their mechanical equilibrium conditions are governed by the same structural features.     

RHEOLOGICAL BEHAVIOUR OF THREE BIOPOLYMER/WATER SYSTEMS, WITH EMPHASIS ON YIELD STRESS AND VISCOELASTIC PROPERTIES

Flow and viscoelastic properties of three thickened systems are presented: aqueous solutions of locust bean gum, of xanthan and a dispersion of a crosslinked waxy corn starch. Creep measurements allow investigations at very low shear rates. The existence of a yield stress is discussed. Flow was observed at low shear rates, but the starch paste exhibited pseudo-yield stress behaviour. Dynamic measurements were performed and the relationships between flow curves and mechanical spectra are discussed.     

ENZYMATIC SYNTHESIS OF STRUCTURED LIPIDS: TRANSESTERIFICATION OF TRIOLEIN AND CAPRYLIC ACID

Structured lipids were successfully synthesized by lipase-catalyzed trans-esterification (acidolysis) of caprylic acid and triolein in nonaqueous medium. Twelve commercially available lipases (10%, w/w substrates) were screened for their ability to form structured lipid by incubating 100 mg triolein and 65.3 mg caprylic acid in 3 ml hexane at 55C for 24 h. The products were analyzed by reverse-phase high performance liquid chromatography (HPLC) with evaporative light scattering detection. Monocapryloolein was the major component of the products (57.4 mol %) and IM60 lipase from Rhizomucor miehei was the best biocatalyst. Dicapryloolein and triolein contents were 35.4% and 5.3%, respectively. Temperature, mole ratio, time course, incubation media, added water, enzyme load, and substrate concentration were also investigated in this study. The results suggest that it is possible to synthesize structured lipids with lipase as biocatalyst.     

EFFECT OF CONCENTRATION ON THE RHEOLOGY AND SERUM SEPARATION OF TOMATO SUSPENSIONS

The °Brix value of the tomato concentrate, from which tomato suspensions were prepared, was shown to have a large effect on the resulting apparent viscosity and storage modulus. The apparent viscosity of a tomato suspension prepared from a 30 °Brix tomato concentrate was only 35% of that of a suspension prepared from a 4.9 °Brix juice, both standardized on the same water unextractable solids (WUS) (0.65%) and total tomato solids (TS) level. After homogenizing the difference in apparent viscosity was only about 10–15%. A similar trend was found for the dynamic moduli. The decrease in rheological parameters of the nonhomogenized suspensions is probably partly related to the decrease in diameter of the tomato particles due to the concentration process. Moreover, microscopic fracture of the cellulosic, microfibrillar network of the cell wall plays a part. Serum separation of diluted nonhomogenized tomato suspensions was higher if made from a concentrate that was concentrated to a higher °Brix value. This phenomenon is discussed in terms of variations in uniaxial compression of the network in the diluted tomato suspensions caused by gravitational force.     

保健食品的现状及其开发前景

介绍了保健食品目前的概况,并对今后开发的前景进行了总结.     

抗菌食品包装技术研究与应用进展

抗菌包装作为一种新型活性包装技术,将会对食品安全性的提高和货架期的延长产生重大的积极影响;对不同的抗菌食品包装系统的特性进行描述,其中包括抗菌食品包装的原理、未来的应用、检测方法、立法现状以及未来的发展趋势.     

马齿苋的营养及其食品的开发

主要介绍了马龄苋的分布、生长习性、营养价值、药用价值和医疗保健作用。将马齿苋加工成保健食品，以满足消费者营养和保健的多种要求。     

REGULATORY AND TOXICOLOGICAL ASPECTS OF FOOD IRRADIATION

A major difficulty with food irradiation from a regulatory point of view is that it is related to many food issues, such as safety, labeling and consumer acceptance. Food that is intentionally irradiated is judged to be adulterated under current law unless there is a regulation for such use. As a first step in the rulemaking process, the Bureau of Foods has developed criteria useful for evaluating the safety of irradiated food by considering present knowledge in radiation chemistry and toxicity testing. These criteria will be used for evaluating existing toxicology studies and for developing policy on food irradiation.     

YIELD AND SENSORY EVALUATION OF POULTRY MARINATED PRE AND POSTRIGOR

The effect of time of cut-up and needle injection of phosphate marinades into poultry was evaluated. Poultry were collected after the chiller, cut and needle injected immediately with phosphate containing marinade, or needle injected after 5 h. Whole birds were aged 5 h before cut-up and needle injection as a control. The percentage weight gain and the percentage net weight increase was less for poultry which was cut or needle injected without aging, but varied depending on marinade composition. Cook yield was not affected by time of cut-up or needle injection of marinade. No differences in tenderness and juiciness between treatments were detected by untrained or trained panelists (p<0.05).