Temporal leeway: can it help to reduce biomechanical load for older workers performing repetitive light assembly tasks?

Current industrial production systems allow assembly of customised products which include additional elements distinguishing them from a reference model. This customisation can result in significant additional time constraints which compel workers to complete their tasks faster, which may pose problems for older workers. The objective of this laboratory study was to investigate the impact of restrictive or flexible pacing during assembly of customised products among groups of younger and older participants. The data gathered were used to analyse cycle-time, assembly performance, muscular load, and kinematic adaptations. The flexible pacing condition was found to improve production performance, increasing customised assembly cycle-time and reducing biomechanical load, for both young and older participants. However, as the task required fine manual dexterity, older participants were subjected to a higher biomechanical load, even in the flexible pacing scenario. These results should encourage assembly-line designers to allow flexible time constraints as much as possible and to be particularly attentive to the needs of older workers.

Permeation of volatile compounds through starch films.

The aim of this study was to gain insight into the factors that affect the permeation of volatiles through starch films. These films were obtained by casting gelatinized starch/water/glycerol mixtures. The films were dried and conditioned under different conditions (temperature and relative humidity) resulting in films that vary in the degree of starch crystallinity and glycerol and water content. The permeation of two model volatiles (carvone and diacetyl) at 20 degrees C and at 30, 60, or 90% relative humidity (RH) was analyzed gravimetrically. Further, the solubility of the two model compounds (under conditions where the permeation experiments were carried out) was determined. From the obtained permeation and solubility data, the diffusion coefficients of these compounds in the different starch films were calculated. The crystallinity in the starch films increased with increasing water content of the films during preparation. The water content of the resulting films in turn increased with increasing glycerol and when the films were exposed to a higher RH during drying or conditioning. For films with the same composition, the flux for diacetyl was greater than for carvone. The solubilities of diacetyl and carvone were slightly dependent on the properties of the films. It was found that with increasing starch crystallinity the diffusion coefficient for both compounds decreases, which is probably due to the impermeability of starch crystallites. Interestingly, in films with about the same extent of crystallinity, the diffusion can be described with the free volume model, with water and glycerol determining the amount of free volume.

Modulated release of a volatile compound from starch matrixes via enzymatically controlled degradation.

The release of a model volatile (diacetyl) from a system based on a starch matrix, in which the volatile is dispersed, was studied. Kneading was used to obtain a homogeneous mixture (melt) composed of starch, glycerol alpha-amylase, and diacetyl. Samples were then ground to powders. When the starch powders were exposed to 30% relative humidity (RH) at 20 degreesC, no degradation of the starch matrix occurred. The samples only showed an initial burst release of diacetyl (around 10% of the loaded dose), whereas the remaining amount of diacetyl was not released, most likely due to the glassy character of the matrix and the low solubility of diacetyl in the matrix. However, when the samples were incubated at 90% RH, due to the uptake of moisture by the particles full release of the entrapped volatile occurred. The release of diacetyl from the matrix without enzyme followed first-order kinetics and, as expected, the release rate increased with decreasing particle size. Due to absorption of water, the enzyme became active and starch degradation occurred. The initial release of diacetyl from amylase-containing matrixes followed first-order kinetics as well. However, once the matrix was degraded to a certain extent, the particles collapsed, which was associated with concomitant rapid increase in release. The time at which the particle collapse occurred decreased with increasing enzyme concentration in the matrix. In conclusion, it is demonstrated that the release of a volatile from starch matrixes can be modulated both by the amount of coencapsulated matrix-degrading enzyme and by the humidity of the environment.