A NEW EMPIRICAL MODEL TO SIMULATE TRANSIENT SHEAR STRESS GROWTH IN SEMI-SOLID FOODS1

A family of models has been developed which has the ability to simulate transient shear stress growth in foods. The models can be written as follows: Three relaxation terms (seven parameters) are necessary to stimulate transient shear stress growth for stick butter, tub margarine and peanut butter at shear rates of 10 s^?1 and 100 s^?1. At small times the model tends to: which simulates the stress growth part of the curve. At long times the models tend to the power-law. The series of relaxation terms allows for the introduction of several time scales in the relaxation behavior of each food.     

A SINUSOIDAL COMPRESSION SYSTEM TO STUDY RHEOLOGICAL PROPERTIES OF FOODS IN THE TRANSIENT STATE

A testing device specially designed to study rheological properties of meats and their relation to tenderness is described. Samples are submitted to a sinusodal compression during 1 to 2 cycles with recovery delay of 0 to 10 s at frequencies between 0.05 and 25 Hz so that deformation rates thus attained overlapped those of chewing. The compression can be regulated between 10 and 90%, and adjusted to initial sample height from 3 to 30 mm. Sample strength is measured by a force quartz transducer connected to a high speed data acquisition and storage system, versus the deforming tool displacement whose specific law is given.     

VISCOELASTIC AND THERMAL CHARACTERISTICS OF VEGETABLE PUREE-BASED BABY FOODS

Viscoelastic properties of three commercial vegetable puree‐based baby foods (pea, corn and wax bean) were evaluated in the temperature range of 20 to 80C using dynamic oscillatory viscometry, and the thermal properties were evaluated using a standard differential scanning calorimeter (DSC) (?10 to 100C). Test samples behaved like viscoelastic fluids with consistently higher magnitudes of storage modulus (G′) as compared to loss modulus (G″) in an oscillatory frequency (ω) range between 0.628 and 62.800 rad/s. Both G′ and G″ versusωdata were well fitted by a power‐type relationship at all temperatures employed in the study. However, the derived power‐law parameters of G′ and G″ showed no consistent trends with temperature. Both pea and wax bean purees exhibited similar viscoelastic patterns with an increase in both G′ and G″ at 50C, somewhat coinciding with the initiation of starch gelatinization. However, the corn puree exhibited a stronger elastic characteristic with starch gelatinization occurring around 80C. The rheological measurements were well supported by the calorimetric data from the DSC.     

A COMPUTERIZED METHOD TO ANALYZE THE CREEP BEHAVIOR OF VISCOELASTIC FOODS

A computer-aided method was developed to simplify the analysis of creep behavior and to determine the elasticity, the Newtonian viscosity and individual viscosities of viscoelastic foods. The method enabled the inclusion of all creep compliance data in the analysis, and reduced the analysis time from hours to minutes. The method was tested in the calculation of rheological properties of wheat flour dough.     

ORAL PERCEPTION OF VISCOSITY IN FLUID FOODS AND MODEL SYSTEMS

The perceived in-mouth thickness ( T ) of a range of fluid foods and model systems was assessed by a trained sensory panel, using a ratio scaling technique, and correlated with objective measurements of viscosity (η_N). For Newtonian samples a simple linear correlation (r²= 0.98) is observed between log T and log η with exponent η= 0.22. The linear relationship between log T and log η for Newtonian materials was used to calculate the equivalent Newtonian viscosity (η_N) from subjective panel scores for the non-Newtonian samples studied. The shear rate at which observed viscosity is equal to η_N decreases with η_N, as reported previously by Shama and Sherman, but also decreases with increasing shear-rate dependence of viscosity. This implies oral perception of viscosity over a range of shear rates. Comparison of flow curves for samples assigned similar thickness scores, but showing very different shear thinning behaviour, suggests that the perceived thickness of extremely shear-thinning materials is dominated by their high viscosity at low rates of shear. As a simple practical index, viscosity at 10 s⁻¹ shows a better correlation (r²= 0.95) with the perceived thickness of the samples studied than values calculated by the methods suggested by Wood, or by Shama and Sherman (r²= 0.90 in both cases). The close agreement between panel scores for perceived thickness and perceived stickiness previously observed for concentrated solutions of random coil polysaccharides does not apply for several of the food systems studied, or for very concentrated solutions of xanthan.     

TRANSIENT FLOW OF MAYONNAISE DESCRIBED BY A NONLINEAR VISCOELASTICITY MODEL

Dynamic and stress relaxation tests were carried out to verify the applicability of the Wagner model for describing transient flow of mayonnaise. Tests were done on two commercial brands at temperatures ranging between 10 and 40C. It was necessary to modify the model by introducing the Soskey-Winter damping function to describe the nonlinear relaxation modulus of mayonnaise. This modified Wagner model fits transient flow better at low shear rates, because the damping function estimated from stress relaxation tests does take into account the irreversible breakdown of mayonnaise.     

MAXIMUM CHANGE IN PHYSICAL AND QUALITY PARAMETERS OF FLUID FOODS DURING CONTINUOUS FLOW HEATING. APPLICATION TO LIQUID WHOLE EGG

A fluid in continuous flow heating approaches maximum thermal treatment as the viscosity profile becomes plug flow. This effect is examined for physical and quality changes in tubular heating of fluid foods. Arrhenius mathematical forms of constituent change are incorporated into the thermal profile models for the general case. Application to soluble protein loss (SPL) and viscosity change in liquid whole egg (LWE) are used as examples. Time and temperature schedules are suggested for pasteurization of LWE at elevated temperatures and shorter times, relative to standard schedules, based on the plug flow velocity profile assumption.     

AN ATTEMPT AT ESTIMATING THE SIZE DISTRIBUTION OF OIL GLOBULES IN EMULSION TYPE COMMERCIAL FOODS BY MEANS OF A MODIFIED CENTRIFUGAL PHOTOSEDIMENTOMETER

The size distribution of oil globules in emulsion type foods (mayonnaise, cow's milk, and ice cream) was estimated with the homogeneous (simple dilution) technique in the modified disk-centrifugal photosedimentometer, for which data were available for calculating the cumulative size distribution. It was confirmed that, in many of the tested samples, a large number of oil globules are in the sub-micron range. These very small globules have an important effect on the rheological properties of emulsion type foods.     

SUITABILITY OF SINGLE-COMPRESSION AND TPA TESTS TO DETERMINE ADHESIVENESS IN SOLID AND SEMI-SOLID FOODS

Single compression tests were applied to carrageenan plus locust bean gum model gels (1:1) without sucrose and with 40% added sucrose, and two commercial samples of quince jelly containing high levels of sucrose (55 and 72 °Brix). The samples were glued to the main platform of the texturometer. The tests were performed up to two different levels of compression. Negative area (A_adh), the time that the samples remained adhering to the probe (t_adh), and maximum negative force (F_max) during the decompression stroke were determined. All the parameter values for the commercial samples were greater than those for the model systems. A_adh and F_max were good indices of adhesiveness. The highest discriminative capacity in detecting sucrose levels of these two parameters was obtained at a low degree of compression; this could be because the samples had not suffered damage to their internal structure. Since the samples had high instantaneous recoverable springiness, the application of automated TPA proved inappropriate for obtaining a measurement of adhesiveness.     

MICROBIAL ENUMERATION IN READY-TO-EAT FOODS AND THEIR RELATIONSHIP TO GOOD MANUFACTURING PRACTICE

As a contribution to Good Manufacturing Practice (GMP) programs, we have analyzed products produced and offered by ready-to-eat food stores. Two hundred and seventy four samples of a wide variety of foods from 19 different shops were analyzed. Aerobic counts, total coliforms and yeast and molds were enumerated in each sample. Food stores were evaluated using a GMP check-list. They were grouped in three classes: Class III, nonsatisfactory GMP; Class II, partially satisfactory GMP; and Class I, satisfactory GMP. From the results of microorganism counts in the Class I food shops the following maximum counts are proposed for cooked ready-to-eat foods: aerobic colony count: 10⁵ cfu g⁻¹. It is coincident with satisfactory microbiological quality of many category 3 foods drawn up by Public Health Laboratory Service (PHLS).     

BITE FORCE AND SAMPLE DEFORMATION DURING HARDNESS ASSESSMENT OF VISCOELASTIC MODELS OF FOODS

The sensory mechanisms underlying oral perception of hardness of silicone elastomers was studied using three approaches: (1) compression between parallel plates up to 10% (2) Bite forces of 10 subjects for hardness comparisons. (3) Deformations were evaluated during controlled mechanical indentations corresponding to the maximum force previously recorded. Over the mechanical stress range studied, the bite forces remained constant for soft samples and then increased for hard ones, while the resulting deformation followed an opposite variation with a decrease for soft samples and a plateau for hard ones. When considering the hardness comparisons between samples of a given series, the harder sample was detected in 10 pairs out of 17. This detection was obtained under almost constant bite force which shows that deformation should be the sensory clue for hardness perception.     

TEMPERATURE DISTRUBITIONS AND LIQUID-SIDE HEAT TRANSFER COEFFICIENTS IN MODEL LIQUID FOODS IN CANS UNDERGOING FLAME STERILIZATION HEATING

Heat transfer studies were performed with clay suspensions and sugar solutions in 303 × 406 cans heated singly in a flame process simulator. Experimental results show that the can wall temperature does not reach excessive temperatures and that the temperature distribution throughout the can is uniform. The slowest heating point was on the axis near the end of the can but this temperature lagged the center temperature by less than 3°C even with adverse heating conditions (i.e., no head space). The liquid-side heat transfer coefficient in 60% sucrose solution at 30 rpm was measured to be 466 ± 34 W/m²K based on the cylindrical area of the can. Rotational speed had only a small effect on heating rate in the range of 10 to 120 rpm.     

MODIFICATIONS AND APPLICATIONS TO FOODS OF THE GENERAL FOODS SENSORY TEXTURE PROFILE TECHNIQUE

The primary attributes of the General Foods sensory texture profile technique are standardization, reproducibility, and ease of correlation with instrumental measurements. Since the original publication in 1963, the method has undergone modifications and refinements such as: addition of an initial stage for evaluation of surface properties, evaluation of certain mechanical characteristics of elasticity and cohesiveness on specific products, and development of texture profile techniques and terminology for the evaluation of semi-solids. Application of this expanded texture profile method is demonstrated using foods such as vanilla cookies, frankfurters, and whipped toppings.     

高效快速检测食物中猪成分的PCR和RT—PCR方法

依据猪线粒体DNA中的细胞色素b基因序列设计一对特异性引物,用于检测食物中的猪源成分DNA扩增片段(130 bp)可以从猪的基因组中检出而对其它物种(牛、羊、鸡等)样品的检测为阴性,且在各种组织中稳定存在,亦可应用于实时荧光PCR从加工的熟食和混合食物中灵敏地栓出痕量猪DNA.     

AFLATOXIGENIC ASPERGILLI IN FOODS AND FEEDS IN UGANDA

Studies conducted during the sixties and the seventies on food crops in Uganda showed that the populace was exposed to consumption of aflatoxin-contaminated foods. These studies also linked the highest incidence of liver cancer in the world to the presence of high levels of aflatoxins in the food and beverages. After a lapse of a decade, it was of interest to investigate the occurrence of aflatoxins and aflatoxigenic fungi in staple Ugandan food crops and poultry feeds derived from these foodstuffs. A simple, rapid and reproducible procedure was used. The procedure consisted of growing or culturing feed grains on a selective medium, Aspergillus flavus/parasiticus agar (AFPA) followed by screening for aflatoxin producing fungi on a coconut agar medium (CAM) under UV light with a subsequent confirmatory screening method for aflatoxin production by the fungi in pure culture. Fifty-four samples consisting of corn and peanuts, soybean and poultry feed were analyzed for content of aflatoxigenic. A. flavus/parasiticus and 25 of the samples were also screened for aflatoxins B₁ and G₁, zearalenone, sterigmatocystin, ochratoxin A, citrinin, vomitoxin, and diacetoxyscirpenol (DAS). Aflatoxigenic A. flavus/parasiticus was detected from the majority of corn (77%), peanuts (36% human food and 83.3% animal feed) and poultry feed (66.6%). but not from soybean samples. Two samples out of 25 contained detectable levels of aJatosin B, (20 ppb). For the jirst time other mycotoxins, zearalenone (3 samples) and vomitoxin (2 samples) were detected in corn from Uganda.     

HEAT TRANSFER IN THIN-FILM WIPED-SURFACE EVAPORATION of MODEL LIQUID FOODS

Experimental product-side heat transfer coefficients were studied in a laboratory scale thin-film wiped-surface (TFWS) evaporator under different rotational speeds, liquid flow rates and temperatures of the heating medium. Experiments were conducted with water, and 10, 30, and 45% sugar solutions in order to simulate the conditions during evaporation of fruit juices. Dimensionless correlations for heat transfer coefficient were established. It was shown that the “equivalent diameter” of the liquid bow wave formed in the front of wiper blade could be used as a characteristic dimension. Magnitudes of unstable and stable modes of evaporator performance were determined and different regimes related to the liquid flow were revealed.     

A SIMPLIFIED ANALYTICAL MODEL FOR FREEZING TIME CALCULATION IN FOODS

A simplified analytical model for the freezing time prediction of simple-shaped foodstuffs was developed. It was assumed in the model that the solution to the unsteady state, unidirectional heat conduction equation with constant thermophysical properties was valid during cooling and freezing. the latent heat effects during freezing were incorporated into an effective diffusivity term. the predictions of the model were compared to the available experimental data on freezing of infinite slabs, infinite cyliners and spheres, and to the experimental data obtained in this research for the freezing of apples. the level of agreement between the predictions and the experimental data was quite satisfactory.