IMPERFECT SQUEEZING FLOW VISCOSIMETRY OF NEWTONIAN LIQUIDS – THEORETICAL AND PRACTICAL CONSIDERATIONS

Imperfect squeezing flow viscosimetry is an experimental procedure based on pressing a liquid specimen in a shallow container by a wide plate. Unlike in proper squeezing flow viscosimetry the specimen does not have a perfect cylindrical shape. Consequently, the flow regime is affected by factors other than squeezing and hence the name. Newtonian fluids, silicone oils of known viscosity and two honeys, were employed to evaluate the roles of the hydrostatic pressure and entry effects on the method's performance. Because of these, the shape of the initial part of the flow curve differs significantly from the ideal and it appears that a fully developed squeezing flow is only reached after a penetration of several millimeters. The magnitude of the viscosity calculated through nonlinear regression is extremely sensitive to the mathematical model used. Thus, added terms, to account for secondary effects, which improve the fit almost to perfection, can also result in a substantial error – or underestimate of the viscosity. Theoretical analysis shows that with a sensor of a size of Petri dish (plate and container diameters on the order of 120 and 140 mm respectively, and a specimen height on the order of 5 mm) the method is only suitable for fluids whose viscosity is in excess of about 100 Pa.s. Nevertheless, the method was still sensitive enough to monitor differences in the viscosity of commercial honeys which was merely on the order of 5–10 Pa.s at ambient temperature.     

IMPERFECT SQUEEZING FLOW VISCOSIMETRY WITH A WIDE PLATE AND A SHALLOW CONTAINER 1

“Imperfect squeezing flow” is defined as the outward and upward flow of a liquid in a shallow container compressed at a constant displacement rate with a wide plate (liquid depth to plate diameter on the order of 1:10–1:20). The viscosity of Newtonian silicon oils could be determined with an accuracy of about 5% using such an array, with a plate and containers having a diameter on the order of 10 and 14 cm, respectively, and liquid depth on the order of 5–6 mm. The same procedure, however, was not very effective for determining the rheological constants of a commercial mustard and especially ketchup. In the latter case this was probably the result of self-lubrication by a low viscosity liquid film, which distorted the force-time relationship. The effect was even more prominent when samples of the same ketchups were tested in a coaxial viscosimeter. Because of this problem the procedure as described can only be used as a crude empirical method to distinguish between liquid foods having very different rheological properties.     

RHEOLOGICAL CHARACTERIZATION OF RICOTTA CHEESES BY IMPERFECT SQUEEZING FLOW VISCOMETRY 1

Samples of three commercial ricotta cheeses made from whole milk, part skim milk and fat free, were compressed in a 140 mm diameter shallow Teflon® container with a wide Teflon® plate, of 64 or 100 mm in diameter, to induce imperfect lubricated squeezing flow. The log force versus log specimen height relationship had a linear region from which the region of a dominant squeezing flow regime could be clearly identified. Its slope was on the order of ?1.0 to ?0.8 in the whole milk ricotta, ?1.6 to ?0.9 in the part skim milk and ?1.7 to ?1.2 in the fat free cheese. The apparent stresses at two selected heights (1 and 2 mm) and at two given times after relaxation (60 and 120 s) were sensitive measures of consistency and could be used to distinguish between the ricottas of the three types. The coefficient of variation of these measures was 10% or less, while the differences between the ricottas or the different brands were 20 to 100%. The apparent stresses, before and after relaxation, had a modest dependence on the upper plate diameter, but the latter had no effect on their sensitivity as measures of consistency. Doubling the compression rate, from 0.1 to 0.2 mm. s^?1, had a relatively small effect on the magnitude of the apparent stresses, probably a consequence of the high yield stress.     

FLOW PARAMETERS OF SELECTED COMMERCIAL SEMI-LIQUID FOOD PRODUCTS2

The rheological characteristics of samples of commercial apple sauce, baby food (banana and peach), mustard and ketchup were evaluated by two successive shearing cycles in a coaxial viscosimeter. It was found that generally, both the Herschel-Bulkley and the modified Casson equations were equally good mathematical representatives of the experimental flow curves, especially after the sample has already been sheared once. The original Casson equation was generally a lesser appropriate model especially for the first flow curve. Time and shear effects on the flow curve features, and on the magnitude of the yield stress, were also studied. The results showed that although all the effects were roughly on the same order of magnitude, they varied considerably between the different types of products and within the same products of different manufacturers.     

FLOW BEHAVIOUR OF TOMATO KETCHUPS

The flow behaviour of 13 tomato ketchups was characterized using a coaxial cylinder viscosimeter. Power law values for the flow behaviour index n averaged 0.35 ± 0.016 which indicated the pseudoplastic nature of ketchups. The consistency coefficient K values for ketchups depended more on temperature, pectin and pulp contents than on total solids. Activation energy (Ea) values for flow of ketchups varied from 1.99 to 4.88 kcal/mole and were significantly related with pectin contents. The Casson's yield values for ketchups showed a significant correlation with pectin contents. Michaels and Bolger model parameters showed that much greater forces were involved in the breaking of network structure and aggregates in ketchups than needed for flow. Multiple regressions of K and apparent viscosity on pectin and pulp contents of ketchups were highly significant.     

FLOW PROPERTIES OF TOMATO CONCENTRATES

The flow properties of over seventy concentrates made from four cultivars of tomatoes were determined with a concentric cylinder viscometer. The apparent viscosity (100 s⁻¹, 25°C) of the concentrates of each variety was proportional to the 2.5 power of the concentration (% total solids). The apparent viscosity of the Nova cultivar obeyed the Arrhenius temperature relationship; the activation energy for the concentrates was 2.3 ± 0.3 kcal/mole. The simple power law and the Casson model described satisfactorily the flow data of the concentrates but the power law gave higher correlation coefficients. The Herschel-Bulkley model with the Casson yield stress was also satisfactory but the correlation coefficients were lower than for the simple power law. The consistency index of the power law model also showed a power dependence on the concentration. The natural logarithm of the yield stress and the concentration could be correlated by a quadratic equation.     

RHEOLOGICAL CHARACTERIZATION OF BUTTER USING LUBRICATED SQUEEZING FLOW

Rheological characteristics of butters made from anhydrous milk fat (AMF) and supercritically fractionated high melting triglyceride (HMT) fraction were compared. An Instron Universal Testing Machine was used to compress the samples between two lubricated plates at constant deformation rate (0.5 mm/min). Stress growth coefficients, computed from the ratios of stresses and biaxial strain rates, were plotted as function of time. HMT butter had higher stress growth coefficients than AMF butter, indicating its greater degree of firmness. Stress growth coefficients were negatively correlated to temperature due to lower crystallinity of fat at higher temperatures and increased on storage due to formation of weak van der Waal forces of attraction between crystalline fat chains.     

EFFICIENCY OF ANACARDIC ACID AS PRESERVATIVE IN TOMATO PRODUCTS

Anacardic acid, 6[8'(Z), 11'(Z),14'-pentadecatrienyl] salicylic acid is the main active principle of Anacardium occidentale ( Anacardiceae ), the cashew. Cashew nut shell liquid (CNSL) contains approximately 70% anacardic acids, 18% cardol, 5% cardanol, and some other phenols and less polar substances. Because of the presence of anacardic acids, the CNSL exhibits potent antimicrobial activity. In view of this and the potent antimicrobial activity of anacardic acid observed in our previous study, the present study was planned to evaluate the anacardic acid isolated by us in our previous study as a food preservative. The effect of anacardic acid at 0.014% (w/w) was studied in tomato paste and tomato ketchup inoculated with 2  ×  10⁴ cfu/g of Staphylococcus aureus, Bacillus subtilis and Escherichia coli and stored at room temperature. Good activity was observed, as no viable growth after 28 days in both the products. Results showed that anacardic acid was active against both gram-positive and gram-negative bacteria. Results of the study indicated that anacardic acid can act as a potential preservative in tomato products and can be considered as an alternative natural preservative over the synthetic preservatives.

PRACTICAL APPLICATIONS

When safeties of synthetic preservatives are questioned, natural substances of plant origin may appeal to the public. The anacardic acid isolated form cashew nut shell is present in cashew nut, which is being consumed by a number of people as a food material. So it is relatively safe to use as an alternate to the existing synthetic preservatives.     

APPLICABILITY OF FLOW MODELS WITH YIELD FOR TOMATO CONCENTRATES

Shear rate-shear stress data on about fifty tomato concentrates were employed to study the applicability of three flow models: Herschel-Bulkley (H-B), Mizrahi-Berk (M-B), and Vocadlo. The magnitudes of the three parameters of each model were determined by means of nonlinear regression analysis. The H-B and M-B models described very well the flow data. The logarithm of the apparent yield stress values predicted by each model and total solids of the concentrates were related by a quadratic equation. The magnitudes of the flow behavior index of each model did not change significantly over the total solids range studied: 6–36%. The consistency index of the Vocadlo and H-B model, and the concentration of the tomato concentrates were related by a power relationship. Over the temperature range 15–55°C, the flow behavior index of each model did not change significantly. The yield stress decreased with increase in temperature reaching a nearly constant value at 55°C. The consistency index of each model and temperature could be correlated by the Arrhenius equation.     

EVALUATING RHEOLOGICAL PROPERTIES OF MOZZARELLA CHEESE BY THE SQUEEZING FLOW METHOD

We adapted the squeezing flow technique to make compression and relaxation tests on Mozzarella cheese at temperatures ranging from 30 to 60C during one month of refrigerated storage. The deformability modulus of the cheese decreased with aging and temperature but increased with deformation rate. The average decrease in the deformability modulus was 0.4 kPa/day and 0.5 kPa/C. Relaxation data also indicated a continuous increase in viscous character of Mozzarella with aging and temperature. Apparent relaxation time was less than 10 s. Relaxation parameters showed a strong temperature dependency in the range of 10-60C. Lubricated squeezing flow data showed that the resistance of Mozzarella to flow decreased with aging and temperature indicating an increased meltability.     

FLOW BEHAVIOR of TOMATO SAUCE WITH OR WITHOUT PARTICULATES IN TUBE FLOW1

Viscosity and density are two properties of fluids that are affected by temperature changes. the flow behavior of tomato sauce (7° and 14° Brix) in a tube viscometer was studied at varying concentrations of added particulates (2 and 5 wt%) at temperatures in the range of 70 to 95°C. the results indicate that the Power Law model described the flow behavior of the non-Newtonian fluids better than the Casson model. the effects of temperature, concentration of carrier fluids and particulates on the flow behavior indices and viscosity was studied. the effect of temperature on the flow behavior index (n) and consistency index (K) were modeled as exponential relationships. the values of n and K for 7° Brix sauce ranged from 0.30 to 0.86 and 0.04 to 0.34, respectively. For the 14° Brix sauce these values were 0.27 to 0.53 and 0.20 to 0.60 respectively. the values of n decreased and K increased with increase in concentration of solids and particulates. the relative changes in K and n were lower for higher concentrations of solids and particulates. It was found that temperature had a greater effect on K than on n. The effects of temperature on viscosity was modeled as an Arrhenius type equation from which the activation energies were calculated. the combined effects of temperature, concentration of solids in the sauce and added particulates on apparent viscosity can be predicted by a nonlinear model.     

IN-LINE CONSISTENCY MONITORING OF TOMATO BASED PRODUCTS USING VIBRATIONAL PROCESS VISCOMETRY

A vibrational process viscometer was evaluated as an in-line consistency monitoring technique for tomato based products. This sensor was installed in both pilot and industry scale production processes. Accuracy of the sensor and its ability to distinguish between varying concentrations of non-Newtonian fluids was evaluated with flowrate and temperature closely controlled. The sensor was shown to have the least inherent variability when compared to off-line techniques. However it was found to be flowrate dependent for the tomato based products. Good correlations between the sensor and off-line techniques were developed for the pilot scale trials. Plant trials revealed a number of problems in developing such correlations under industrial process conditions, however the instrument proved capable of tracking out of specification sauce as determined by the off-line Brookfield viscometer. It was concluded that use of the sensor would result in increased measurement precision over the off-line instruments coupled with the benefits of in-line monitoring.     

SENSORY THRESHOLDS OF CONSISTENCY OF SEMILIQUID FOODS: EVALUATION BY SQUEEZING FLOW VISCOMETRY*

Identification of subtle textural differences or changes in foods in isolation has been a difficult task because they are usually accompanied by differences or changes in other sensory properties. Textural modification of semi liquid foods can easily be produced by mechanical means without affecting their taste and appearance. The changes can be then quantified using squeezing flow viscometry without introducing new uncontrolled modifications. Comparison of the measurements with sensory evaluations of the same samples can establish the threshold of sensory detection, at least in principle. To test this hypothesis, the consistency of practically intact mayonnaise, tomato paste, yogurt and strawberry jam of two commercial brands each were evaluated by squeezing flow viscometry using an Instron UTM. Other samples of the same brands were stirred mildly to modify their consistency and then compared with the undisturbed samples. The textural differences were expressed in terms of the apparent stress at three compressed specimen heights, and the residual stress after 60 and 120 s at the final height. Relatively large differences, as judged by these mechanical measures were also detected by an untrained sensory panel, but the ‘correct identifications’ never reached a 100%. In the case of the two different brands of yogurt, the panel failed to establish the existence of differences, which were clearly detected by the instrument. This suggests that not every difference, detectable instrumentally, has a corresponding sensory response. Although the number of results in this preliminary work was too small to establish the exact threshold for sensory detection, the methodology seems to be appropriate for such a purpose. This is because of the high reproducibility of the squeezing flow parameters and their sensitivity to even subtle difference in the tested specimens consistency.     

RESEARCH NOTE EFFECT OF SELECTED ADDITIVES ON THE FLOW PARAMETERS OF 1:1 MIXTURES OF CARRAGEENAN-GUAR AND CMC-LOCUST BEAN GUM

Flow parameters of 1:1 mixtures of carrageenan-guar and CMC-locust bean gum in the presence of common food ingredients, namely, sugar, salt and proteins, were determined by using a coaxial viscometer. The effect of additives on yield stress, and power law model constants varied depending on the type of ingredient and its concentration, and the nature of the gum. Although, it is interesting to see the flow behavior of gum blends together with the additives in concentrations that are common in food products, it is necessary to study an enormous number of combinations in order to analyze a multicomponent system quantitatively.