THE TEACHING OF FOOD RHEOLOGY

A brief account of the rheology course for undergraduates at the Procter Department of Food and Leather Science of Leeds University is given. Both the practical and theoretical approaches are considered. The former deals with present methods of rheological assessment, the latter should provide a sound foundation for future rheological research.     

食品流变学的研究进展

简要介绍了食品流变学的起源、发展、分类、数学模型，测量仪器和方法的研究进展情况。     

AN ADAPTATION OF THE CASSON EQUATION FOR THE RHEOLOGY OF CHOCOLATE

The generalized Casson equation with exponent m was considered. The plastic viscosities and yield values corresponding to different ranges of shear rate D were calculated for different values of exponent m between 0.5 and 1. All the plastic viscosities tend to about the same value at high shear rates, while at low shear rates all the yield values tend to the same value which is supposed to be the "true" yield value. For the exponent m = 0.6, the yield value is least dependent on the shear rate range and the yield value calculated for D > 5 sec⁻¹ is generally much closer to the "true" yield value than the Casson value (m = 0.5). Therefore, choice of the exponent m = 0.6 is proposed.     

THE USE OF RHEOLOGY TO CHARACTERIZE FLOW BEHAVIOR OF LIQUORICE SOLUTIONS

The rheological behavior of liquorice solutions at different concentrations (ranging between 30 and 60 Brix) and temperatures (in the range between 25 and 80C) has been investigated performing both dynamic tests in the linear viscoelastic region and steady shear tests, in 0.1–100 s^?1 shear rate range, by using a stress controlled rheometer (DSR 200, Rheometric Scientific, USA) and parallel plate geometry (φ = 40 mm). Small amplitude oscillations has shown the presence of both moduli also at the lowest tested concentration, with G′ greater than G″, except for the 30 Brix sample. It has been found that experimental viscosity data show a good agreement with a power law model, thus allowing the estimation of both the friction factor and the pressure drops in a typical pipeline used for liquorice solution conveying. It has been observed that the assumption of Newtonian behavior can lead to the erroneous design of the apparatuses that have to process liquorice extracts.     

LIMITATIONS OF RHEOLOGY IN FOOD TEXTURE MEASUREMENTS1

Rheological measurements constitute an important part of food texture measurements but they d o not cover all the factors that make up the “texture” of a food. Mastication is a process in which lumps of food are ground into a fine state, mixed with saliva and converted into a liquid slurry at approximately body temperature ready. for swallowing. The processes of size reduction, wetting with saliva, melting, release of moisture or fat are not rheological processes. The perception of moistness, size, shape and roughness of food particles are important factors in texture sensations that are not rheological and yet these oral sensations are an important aspect of the acceptability of a food. This leads to the conclusion that food texture measurement lies partly with-in and partly outside the field of conventional rheology.     

THE RHEOLOGY OF GELS

The following aspects of the rheological behavior of polysaccharide and protein gels are discussed with particular emphasis on recent investigations: (1) Viscoelasticity of gels; (2) Validity of rubber elasticity theory; (3) Rupture strength of gels; and (4) Single-point measurements of gel strength.
It is concluded that in contrast to most food materials gels show linear viscoelastic behavior up to strains of the order of 0.1. Results obtained from creep and stress relaxation experiments would suggest that noncovalent crosslinks in gels move or break when the gel is stressed. Activation energies associated with crosslink breakage have been estimated from the temperature dependence of viscoelastic parameters. For gelatin and polysaccharide gels energies ranging from 5–65 Kcals/mole have been reported.
The stiff and extended nature of polysaccharide chains make it unlikely that polysaccharide gels obey rubber elasticity theory, though it is possible that this theory holds for gelatin gels and also for ovalbumin gels in 6 M urea.
It is emphasized that the rupture strength of a gel is not necessarily related to its elastic modulus and therefore 'single point' measurements of 'gel strength' based on rupture tests will not always rank a series of gels in the same order as tests which involve small deformations without rupture. One of the reasons for this is that the elastic modulus and rupture strength depend in different ways on the primary molecular weight of the polymer from which the gel is formed.     

THE PURIFICATION OF PROTEASE FROM COWSLIP (PRIMULA VERIS) AND ITS USE IN FOOD PROCESSING

Proteases perform very important functions and are used in different objectives as in vitro and in vivo. In recent years, proteases have been used for clinical, pharmaceutical and industrial applications. Most of these proteases are obtained from animal sources. News of diseases passing over from animals to human (severe acute respiratory syndrome [SARS], mad cow) has created suspicion of contamination in animal food and its product. Therefore, a new vegetal protease source for use in the food industry was studied. Ammonium sulfate fractionation and CM‐sephadex ion exchange chromatography were used in the purification of the enzyme. The purified enzyme has an optimum pH of 7.0 and its optimum temperature was 70C. The V_max and K_m values determined by Lineweaver–Burk graphics were 0.44 mg/mL.min and 40 mM, respectively. The purification degree was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE ). The native molecular weight of the enzyme was 46 kDa using a gel filtration chromatograph. SDS‐PAGE results show that the enzyme has two subunits, which have protease activity at 32 and 14 kD. It was investigated whether the purified and characterized protease enzyme would cause to congeal milk or could produce digestion of chicken and cow meat. The results showed that the protease enzyme can be used for industrial production.     

新型食品添加剂在鲜湿方便食品改良中的应用

阐述当前鲜湿方便面、鲜湿方便米粉的应用现状,重点分析鲜湿方便面、鲜湿方便米粉中的食品添加剂及应用情况,提出采用新型食品添加剂时改良鲜湿方便面、鲜湿方便米粉品质的构想.     

谷氨酰胺转胺酶在食品加工中的应用

介绍了谷氨酰胺转胺酶的一些功能特性，并对其在食品中的应用进行了综述。     

乳酸链球菌素的特性及其在食品中的应用

乳酸链球菌素是由乳酸链球菌产生的一种高效、无毒、安全、无副作用的天然食品防腐剂.介绍乳酸链球菌素的特性、效用及在食品加工中的应用.表明乳酸链球菌素在食品加工中是理想的食品防腐剂.     

THE USE OF FORMALDEHYDE IN RESTEEPING

Small-scale malting trials showed that resteeping in a solution of formaldehyde was more uniformly effective than resteeping in water in reducing rootlet yield and the overall malting loss. The hot water extract, the fermentability of the wort, and the formol nitrogen were the same in worts from malts prepared by resteeping in water or formaldehyde solutions. However, cold water extract was reduced, as was the total soluble nitrogen, by the presence of formaldehyde. The anthocyanogen level in the wort was reduced by about 73% when formaldehyde was used in the resteep liquor. The residue of formaldehyde in the unboiled wort was about I p.p.m. It is considered that, in view of the improved resteeping performance and the expected increase in beer stability resulting from the reduction in anthocyanogens and wort nitrogen, the use of formaldehyde in resteeping is worth evaluating on a larger scale. In addition the use of formaldehyde in the resteep liquor prevented fouling by micro-organisms and resulted in a cleaner malt.     

甜菊及甜菊糖研究进展

甜菊糖甜度是蔗糖的200-350倍，是一种可替代蔗糖非常理想的甜味剂。本文扼要介绍了甜菊糖的生理功能和在食品工业上的应用，并简要介绍了甜菊糖加工成果糖和低聚果糖的基本工艺。菊糖作为一种绿色、安全的天然食品配料，其应用前景不可估量。