DIRECTION REVERSALS IN THE MECHANICAL SIGNATURE OF CELLULAR SNACKS: A MEASURE OF BRITTLENESS?

The “mechanical signatures” of brittle cellular foods are known to be irregular and irreproducible. Nevertheless, their degree of jaggedness is remarkably similar and can be a measure of such foods' brittleness and perceived crunchiness/crispiness. An irregular curve's “jaggedness” is manifested by frequent direction reversals. Hence, their number can serve as a jaggedness's index. Counting the number of direction reversals in a digitized force‐displacement curve can be done with a simple algorithm, implemented in any general‐purpose software. The number of direction reversals is strongly correlated with the signature's apparent fractal dimension. It is also a reproducible enough parameter to monitor the loss of brittleness in snacks as a result of moisture sorption. This measure of jaggedness, however, is inapplicable to smooth force‐displacement curves because in such records, the small random force fluctuations produced by the digitizing process can create a false impression of “jaggedness.” Although a correction for this artifact can be introduced into the program, its utility is doubtful.     

Controllability and duration of stress alter central nervous system depressant-induced sleep time in rats.

Rats were exposed to either 80 escapable shocks or yoked inescapable shocks and then injected with several hypnotic doses of sodium pentobarbital, midazolam, or ethanol; their sleep-time duration was compared with that of naive controls. Inescapable shock exposure resulted in a significant increase in ethanol-induced sleep time compared with the escapable shock and naive control groups. Both escape and yoked groups showed an increase in barbiturate-induced sleep time compared with controls, although no difference was observed for midazolam. Acute stress (20 5-sec inescapable shocks) did not alter the depressant-induced sleep time for any of the drugs tested. These results illustrate the importance of psychological aspects of stress and its influence on the potency of certain depressants. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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.     

DETERMINATION OF PRACTICALLY SIGNIFICANT DIFFERENCES IN THE SENSORILY PERCEIVED CONSISTENCY OF SEMILIQUID FOODS1

The consistency of commercial salad dressing, chocolate pudding and mustard samples before and after controlled disruption was assessed by squeezing flow viscometry and evaluated sensorily by an untrained panel. The instrumental parameters were the apparent stress at 1.5 and 1.0 mm height and the apparent residual stress after 60 and 120 s relaxation (at 1.0 mm height). The panelists were asked to identify the sample(s) with the stronger consistency in a triangular test. The percent correct identifications was plotted against the difference in the magnitude of the instrumental parameters and their ratios. The scatter of these plots suggests that the effective stimulus for consistency perception was primarily the overall resistance to deformation or flow and to a much lesser extent to more subtle rheological characteristics like the ‘degree of solidity'. In the range of consistencies examined, the detection thresholds were on the order of 0.5‐0.7 kPa and 0.4‐0.6 kPa at 1.0 and 1.5 mm height, respectively. They corresponded to stress ratios on the order of 0.75‐0.85. Results of this kind will enable setting objective standards for the consistency of commercial semi liquid foods and tolerance margins to deviations from them.     

EFFECT OF SUCROSE ON THE MECHANICAL AND ACOUSTIC PROPERTIES OF FREEZE‐DRIED AGAR,K‐CARRAGEENAN AND GELLAN GELS1

Agar, K‐carrageenan and gellan gels (2%) with and without infused sucrose (5, 10 and 12.5%) were freeze dried and the mechanical and acoustic signatures of the resulting solid sponges recorded. For comparison, agar gels with 5 and 10% starch were also prepared and likewise tested. The presence of the sugar, or starch, in the dried gels increased their density in a manner that could not be predicted from the corresponding stoichiometric relations between the gum and additive indicating that the sugar, or starch, were not inert fillers. In general, the presence of the sucrose in the dried gel solid matrix increased the brittleness of the sponges. The effect could be quantified in terms of the increase in the mechanical signature's apparent fractal dimension (Richardson's and Kolmogorov's). The more brittle dried gels also had a “richer” acoustic signature whose apparent fractal dimension was determined with the “blanket” algorithm. Despite the irregular and jagged appearance of all the dry gels stress‐strain relationships, they could be described by the same kind of a three parameter empirical model originally derived for soft baked products and polymeric foams. Although the added sugar, or starch, stiffened the dehydrated gels the effect could not be quantified unambiguously because of the differences in the density and in the overall shape of the stress‐strain curve.     

ASSESSMENT OF THE EXTENT OF CONSISTENCY LOSS IN SEMILIQUID FOODS BY COMPRESSION AND SHEAR1

The consistency of commercial mayonnaise, mustard and tomato paste samples was assessed by squeezing flow before and after compression or stirring, by hand and with a domestic blender which caused intensive shearing. The specimens before and after these treatments were squeezed between Teflon® coated parallel plates mounted on an Instron UTM model 5542 to a final height of 0.8mm and had been let to relax before the crosshead was withdrawn. Their consistency was assessed in terms of their apparent stress at three heights, and after relaxation for 1 and 2 min at 0.8mm height. Compression of the specimen prior to its testing (1–3 times) has very little effect on the specimen's consistency which supports the view that there was little friction between the Teflon® coated plates and the samples. Shearing caused measurable loss of consistency in the mustard and tomato paste which was not fully restored even after three hours. The sheared mayonnaise restored much of its consistency after about an hour as judged by the apparent stress at a given height, and almost fully recovered it if judged by the apparent residual stress after relaxation.     

Induced co-flow and laminar-to-turbulent transition with synthetic jets

A detailed numerical solution of the fluid flow patterns engendered by a synthetic jet has been carried out. The synthetic jet is caused by a reciprocating piston assembly which is attached to a large, stationary cavity. It was found that a significant momentum efflux is produced by the synthetic jet assembly. Also, fluid entrainment by the synthetic jet causes a coincident flow around the exterior of the cavity (self-induced co-flow). Numerical solutions allow the investigation of the effect of reciprocation stroke length and piston speed on the resulting flow patterns and momentum flows. For all investigated cases, the contribution made by the co-flow to the momentum flowrate is found to be small. In order to account for the simultaneous existence of both laminar and turbulent regions, two numerical approaches were taken. One approach used the Shear Stress Transport (SST) turbulence model while the other used a newly devised transitional turbulence model. Of particular interest was a comparison between the predicted locations of the laminar-to-turbulent transition based on the two independent models. The excellent agreement between the two models reinforces the use of the SST model throughout the domain.