MECHANICAL PROPERTIES OF UNPRESSED SEMI-HARD CHEESES BY UNIAXIAL COMPRESSION

Different lots of Tilsit cheeses, which are characterized by their open structure caused by pressing under their own weight during manufacture, were subjected to uniaxial compression, a quasi-standard for the evaluation of mechanical properties of hard and semi-hard cheeses. Additionally, video films showing the behaviour of the specimens during compression were analysed. For standard Tilsit with a fat content of 35% in dry matter, the cheese mass proved to be highly anisotropic with respect to stress response and lateral expansion expressed as apparent Poisson number. Specimens cut perpendicular to the flat side of the cheese wheels, which had lentil-shaped eyes with their major axis parallel to the compression plates, showed significantly higher stresses and lower Poisson numbers than specimens sampled parallel to the flat side of the cheese wheels. Therefore, stress values calculated on the basis of actual specimen diameter were different from those calculated by assuming constant specimen's volume. However, for samples with a fat content of approx. 60% in dry matter no differences caused by compression direction were observed.     

PRESENCE OF LISTERIA IN MEXICAN CHEESES

The presence of Listeria was investigated in ripened cheeses (Chihuahua, Manchego) and fresh cheese (Panela) from street vendors and retail stores in Mexico City. Cheeses were tested for Listeria, pH, NaCl, moisture and fat. Listeria selective cold enrichment was used to recover Listeria from positive samples. Fresh cheese had the lowest pH and NaCl contents and the highest moisture Chihuahua and Manchego cheeses made with pasteurized milk were negative for Listeria. Panela cheese samples were the most contaminated. The presence of Listeria was 65% in fresh cheeses: Listeria murrayi 20% , Listeria inoccua 15% , Listeria grayi 15%, and Listeria monocytogenes 15%. L. monocytogenes serotypes 1/2a, 1/2b and 4b were isolated from positive samples.     

OCCURRENCE OF MOLDS IN FRESH CHEESES

The purpose of our work was isolation, enumeration and identification of molds in fresh cheeses. A total of 112 cheese samples were obtained from different retail markets in Santa Cruz de Tenerife. Of the samples, 57.2% were positive for industrially craft produced cheeses. The values for the industrially produced cheeses ranged between 10² and 10⁶ CFU/g, for the craft produced cheeses the corresponding values were 10²-10⁷ CFU/G and 4.4x103. The genera identified most often in both types of cheeses were Geotrichum and Penicillium with 49.4 and 29.1% of isolates, respectively. Species of the Aspergillus genus, the main producer of aflatoxins, was detected in 1.3% of the total samples analyzed. Nonetheless, the presence of such genera as Fusarium, Cephalosporium, Cladosporium, etc. is considered to be undesirable, as they are capable of producing other mycotoxins related to specific pathological processes.     

MECHANICAL BEHAVIOUR OF COOKED MEAT UNDER SINUSOIDAL COMPRESSION

The rheological behaviour of raw meat, and of meat cooked under different conditions, has been studied with sinusoidal compression in a transient state. Meat samples underwent compression perpendicular to the myofibre axis in a cell equipped with lateral walls that made it possible to limit the free strain of samples to only one direction viz. parallel (longitudinal configuration) or perpendicular (transverse configuration) with respect to the myofibres. Both raw and cooked meat exhibit visco-elastic behaviour with a slight viscous component. Cooking meat results in both increased resistance and more elasticity. The variations in these parameters depend on the configuration and compression ratio used. With raw or slightly cooked meat, myofibrillar and connective structures can be analysed selectively in the longitudinal configuration by using compression ratios that are, respectively, nondestructive and destructive. When cooking is carried out at a high temperature, the myofibrillar resistance can be analysed preferentially in the longitudinal configuration, and connective tissue in the transverse configuration.     

奶酪风味物质形成机理的研究

奶酪中的风味物质主要包括脂肪酸、酯、醛、醇、酮和硫化物等，这些物质主要是通过蛋白水解、氨基酸的分解代谢、乳糖代谢、柠檬酸盐和脂类代谢等途径产生。对奶酪风味物质形成机理进行了综述。     

EFFECT OF LUBRICATION ON THE COMPRESSION BEHAVIOUR OF CHEESE AND FRANKFURTERS

Cylindrical samples of cheese and frankfurters were subjected to a 50% uniaxial compression between flat plates and artificial molars, mounted on a Texture Analyser, operating at two speeds, with and without lubrication. At both speeds and with both types of probe the maximum force was less for the lubricated probes than the unlubricated probes. In all cases the maximum force recorded at the higher speed was greater than at the lower speed and greater when plates were used than when teeth were used. Through observations made during the testing and from photographs taken, it was concluded that the frictional effect of the cusps on the molar teeth was greater than the effect of lubrication. Observations made when samples were compressed between molar teeth in the mouths of panellists confirmed the conclusion that compression of foods in the mouth follows the nonlubricated pattern despite the lubrication provided by saliva.     

INCIDENCE OF MYCOTOXIC MOLDS IN DOMESTIC AND IMPORTED CHEESES12

Domestic and imported cheeses were studied to determine the incidence of mycotoxin producing molds. The total incidence of molds in visibly non-moldy cheese was very low. Isolation of molds from plate counts, and directly from samples, showed that the major portion of the flora was made up of Penicillium species; 86% in domestic cheeses and 80% in imported cheeses. Many of the Penicillium isolates were capable of growing at low storage temperatures. Mold counts done at 5°C, and prolonged storage of cheese samples at 5°C indicated a potential for considerable mold growth on cheese during refrigerated storage. While the overall incidence of known mycotoxin producing molds was low, a number of potentialy toxic species were found, including P. cyclopium, P. viridicatum, A. flavus and A. ochraceus. These species accounted for 4.4% of all the isolates from domestic cheeses and 4.0% from imported cheeses. Screening of all mold isolates for production of several known mycotoxins showed that a number of isolates (14.1% of all molds in domestic cheeses and 11.5% in imported cheeses) were capable of producing certain mycotoxins including patulin, penicillic acid, ochratoxin A, citrinin and aflatoxins.     

MOLD GROWTH AND PRESENCE OF AFLATOXIN IN SOME TURKISH CHEESES

Twenty-five random fresh market samples of Van herby cheese and pickled white cheese were examined for molds and aflatoxins. The mean total mold count in Van herby cheese was 2.50 × 10⁵/g; in pickled white cheese it was 4.95 × 10⁴/g. The mycoflora on the cheeses were determined. In all cheeses, over 65% of molds were Penicillium species . Aspergillus made up 0 to 1.6 % and 2.6 % to 4.0 % of the mold on pickled white cheese and Van herby cheese, respectively. Other isolated molds belonged to Mucor, Geotrichum and Trichoderma genera. None of the samples contained aflatoxins and none of the 6 Aspergillus isolates was an aflatoxin producer .     

SOME MICROBIOLOGICAL CHARACTERISTICS OF HERBED CHEESES

Fifty herbed cheese samples, in which herbs (Allium spp.) were used at production, were analyzed for some microbiological quality characteristics. Cheese samples were collected from different retailers in four counties of Eastern Turkey. Presumptive coliform bacteria, generic Escherichia coli, Staphylococcus aureus, lactic acid bacteria, yeast, mold and sulfite‐reducing anaerobic bacteria counts were examined as general microbiologic quality parameters, and the average bacteria counts were 2.1 × 10⁵, 3.1 × 10⁴, 1.1 × 10⁵, 2.2 × 10⁷, 6.8 × 10², 7.0 × 10⁶ and 3.2 × 10¹ cfu/g, respectively. The number of samples, in which the above selected bacteria have been detected, were 32 (64%), 29 (58%), 23 (46%), 50 (100%), 5 (10%), 46 (92%) and 9 (18%), respectively. Salmonella spp., Listeria monocytogenes and E. coli O157:H7 were targeted as foodborne pathogens, and the presence of those pathogens in samples were 10% (in five samples), 8% (in four samples) and 4% (in two samples), respectively. It was found that herbed cheeses containing foodborne pathogens raise an important risk for consumers’ health. Standardization and modernization of herbed cheese production, in which hygienic conditions and quality controls would be applicable, seems a necessity in order to prevent possible public health risks from herbed cheeses.     

THE EFFECT OF TIME OF DRESSING ON THE FLOWERING BEHAVIOUR AND YIELD OF HOPS VARIETY LATE CLUSTER

The Late Cluster variety of hops when introduced to Kashmir (India) from USA gave two flowering flushes in a growing season, thus giving two harvests. By adjusting the date of dressing (root pruning) from December to May 15 the abnormal flowering behaviour of the plant could be altered from two flushes to only one. Plants dressed late in spring, April 15 to May 1, gave the highest cone yield which was expressed in only one harvest. This also produced the highest α-acids content.     

TEXTURE EVALUATION OF FRENCH ACID-TYPE FRESH CHEESES

A range of French acid-type fresh cheeses of various commercial brands was tested by a trained sensory panel which detected significant differences in thickness, smoothness and slipperiness. The cheeses exhibited thixotropic behaviour. The perceived thickness of the samples was highly correlated with the shear stress values obtained for a limited range of shear rates (30–80 _S⁻¹). The area of the hysteresis loop formed during the first shearing cycle of the samples over a wide range of shear rates (4.5–450 _S⁻¹) was negatively correlated to slipperiness. The correlation matrix showed that the sensory attributes were independent of the chemical composition of the products. Principal components analysis indicated the way the three sensory attributes were related to each other and to the rheological parameters.     

BIOGENIC AMINE CONTENT OF SOME TURKISH CHEESES

Biogenic amines in ten samples of Kaşar (aged), five samples of Kaşar (fresh), four samples of Mihaliç, nine samples of Van Otlu (herbed), nine samples of Örgü, nine samples of Urfa and six samples of Civil cheeses were obtained from local supermarkets of Ankara. Tryptamine (TRY), phenylethylamine (PHA), putrescine (PUT), cadaverine (CAD), histamine (HIT), tyramine (TYA), spermine (SPM) and spermidine (SPD) contents of Turkish cheese samples were screened by high performance liquid chromatography (HPLC).
PUT, CAD, HIT, TYA and SPD were the predominant biogenic amines in maturated Kaşar (aged), vacuum-packed Kaşar (fresh), Mihaliç, Otlu (herbed) and Örgü cheeses. PHA, CAD, TYA and SPD were the predominant biogenic amines in Urfa cheese samples.
Civil cheeses were found to contain the highest HIT and TYA content with an average of 94.76 mg/100 g and 138.16 mg/100 g, respectively, much higher than the toxic dose. HIT level of Mihaliç cheeses were also higher than the accepted limit. The concentration of amines in all other cheeses was much lower than the toxic dose limits.     

MECHANICAL PROPERTIES AND SENSORY TEXTURE ASSESSMENT OF CHEESES

The viscoelastic behaviour of hard and semi-hard cheeses was investigated in relation to sensory assessment of texture. The experiments consisted of two separate studies which involved sensory and uniaxial compression tests on eight cheeses in each study. The stress-strain data of the cheeses were approximated well by a power law relationship, showing that the strain and the time dependent variables were separable. The cheeses exhibited similar time dependent behaviour, so the degree of correlation between sensory and instrumental measurements was not influenced significantly by changing the deformation rate in the instrumental tests. Sensory firmness appeared to correlate with instrumental stiffness measurements in one study, but not in the other. This was probably due to the use of different panels and methods of sensory assessment.     

TORSION ANALYSIS OF FRESH AND AGED CHEESES1

Six cheese varieties encompassing a wide compositional and age range were analyzed fresh and after aging to determine correlations between casein proteolysis products, fat content, moisture content, and torsion results. Shear stress, a measure of the strength of the casein matrix, was dependent on the amount of intact α_sl‐casein. Shear strain, a measure of the cohesiveness of a structure, exhibited a negative correlation with fat content and a positive correlation with moisture content. Shear strain was also correlated with the ratio of moisture to protein, an indication of the interaction between casein particles. Torsion tests provide information on the level of degradation of the casein network in cheese, which can be used by manufacturers to alter storage conditions to optimize the quality of their aged cheese., April 10, 2003     

A DEVICE FOR EVALUATING MELT/FLOW CHARACTERISTICS OF CHEESES

A device to objectively measure the melt/flow behavior of cheeses at different temperatures was built consisting of a temperature-controlled heater, an LVDT (linear variable differential transformer), and a personal computer with a data acquisition system. This device, named UW Meltmeter, operates in the lubricated squeeze-flow configuration and can be used either alone at a constant force or in conjunction with a universal testing machine at a constant deformation rate. A marker technique was used to verify the shear-free squeezing flow assumption. The flow of Mozzarella cheeses with 14 and 43% fat content was studied at 40 and 60C under constant force (0.7 and 0.9 N) and constant cross-head speed (0.5 and 2.5 cm/min). Biaxial extensional strain rate and biaxial stress growth coefficient were calculated. The results supported the expected trend of increased meltability at higher fat levels and at higher temperatures.     

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.