Amphiphilic polysaccharides. Evidence for a competition between intra and intermolecular associations in dilute system

Amphiphilic carboxymethylpullulans differing in their degree of octyl-bearing groups were investigated in dilute solution. Viscometry, flow field-flow fractionation with on-line coupling multi-angle laser light scattering (F4/MALLS) were used to examine conformations and aggregation states in dilute solutions. Polymer/polymer interactions tendency increases with grafting degree. At lower grafting degree, the main species in solution are composed of isolated chains, the conformation of which is close to the precursor. For greater grafting degree, strongly compact structure has been evidenced due to strong intramolecular hydrophobic associations. The transition between dilute and semi-dilute cannot be described by a classical overlapping of chains. The critical concentration (C_cr) appears to be largely function of the content of C₈ groups and presents a minimum for about 20% of octyl groups: at this critical content aggregation trend is found maximum.     

Significance of pre-incubation temperature and inoculum concentration on subsequent growth of Listeria monocytogenes at 14 degrees C

The influence of the bacterial concentration of an inoculum (10(1) or 10(3) cfu ml-1) of two strains of Listeria monocytogenes (Scott A: serotype 4b and V7: serotype 1) and one strain of L. innocua (Lin 11), and the time and temperature at which the inoculum was stored (cold storage: 4 degrees C for 4 weeks, or without cold storage: -20 degrees C before immediate transfer), and the temperature at which cells were pre-incubated (30 degrees C and 14 degrees C) on subsequent growth in Richard's broth at 14 degrees C was investigated. Richard's broth at a pH 5.9 was used to simulate potential growth in soft cheese (camembert type) and an incubation temperature of 14 degrees C was used to simulate storage-temperature ripening of cheese. Enumeration of the number of viable cells was by plate count method, except where viable cell numbers were less that 10(3) cfu ml-1, when the MPN (Most Probable Number) technique was used. With cold storage and an inoculum of 10(3) cfu ml-1 (high bacterial concentration) the pre-incubation temperatures (30 degrees C and 14 degrees C) did not significantly influence the subsequent growth curve: there was no significant lag (less that 21 h) and cell numbers peaked in about 8.5 d. However, with cold storage and an inoculum of 10(1) cfu ml-1 (low bacterial concentration) and a pre-incubation temperature of 30 degrees C a significant shift in the growth curve was observed over that pre-incubated at f14 degrees C, with the appearance of a lag of about 7.7 d. At a pre-incubation temperature of 14 degrees C with the low inoculum concentration, there was a measurable lag of about 1 d. Without cold storage and a pre-incubation temperature of 30 degrees C, there was a lag time of 2.3 d. Storage conditions, pre-incubation temperature and inoculum concentration therefore appear to influence the subsequent growth curve. Importantly, however, the growth curves for cultures from inocula, pre-incubated at either 30 degrees C or 14 degrees C, appeared to involve two distinct values of the exponential growth rate (k): the initial portion of the growth curve described by a low value of k and the subsequent portion by a consistently and significantly greater value. The appearance of two distinct growth phases was reproduced in further data determined for all the studied strains of the microorganism. Further study to explain these unexpected and reproducible findings is being conducted.     

Nested Quantum Search and NP-Hard Problems

A quantum algorithm is known that solves an unstructured search problem in a number of iterations of order , where d is the dimension of the search space, whereas any classical algorithm necessarily scales as O(d). It is shown here that an improved quantum search algorithm can be devised that exploits the structure of a tree search problem by nesting this standard search algorithm. The number of iterations required to find the solution of an average instance of a constraint satisfaction problem scales as , with a constant α < 1 depending on the nesting depth and the problem considered. When applying a single nesting level to a problem with constraints of size 2 such as the graph coloring problem, this constant α is estimated to be around 0.62 for average instances of maximum difficulty. This corresponds to a square-root speedup over a classical nested search algorithm, of which our presented algorithm is the quantum counterpart. Received: August 17, 1998; revised version: December 1, 1999     

Cloning a qutrit

Several classes of state-dependent quantum cloners for three-level systems are investigated. These cloners optimally duplicate some of the four maximally-conjugate bases with an equal fidelity, thereby extending the phasecovariant qubit cloner to qutrits. Three distinct classes of qutrit cloners can be distinguished, depending on whether two, three, or four maximally-conjugate bases are cloned as well (the latter case simply corresponds to the universal qutrit cloner). These results apply to symmetric as well as asymmetric cloners, so that the balance between the fidelity of the two clones can also be analysed.     

Analysis of arabic gum: Study of degradation and water desorption processes

Arabic gum is very much used in the agroalimentary industry, mainly as an emulsifier. It is a natural origin polysaccharide and therefore subject to more or less important variabilities of its properties. This work deals with two similar polysaccharides exuded by Acacia senegal of different history and an Acacia seyal sample. To characterize these samples, wide angle X-ray scattering (WAXS) and thermogravimetric analysis in dynamic and isothermal mode are used.The initial structure checked by X-ray diffraction allows to affirm that samples are equivalent in their crystalline structure. The activation energy used to characterize the main degradation process is the same for A. senegal samples, but higher for the A. seyal gum sample. Differences are furthermore observed for the kinetics of water desorption. We conclude that the samples from the same type of gum (A. senegal) are very similar and that A. seyal exudates can be detected by this technique. The water desorption of the two exudates is different.     

Hydrogels Based on Pullulan Crosslinked with sodium trimetaphosphate (STMP): Rheological study

Summary Hydrogels from pullulan crosslinked with sodium trimetaphosphate (STMP) under alkaline condition have been investigated by rheological measurements. The influence of reagents concentration (pullulan, STMP, NaOH) on kinetics of reaction and rheological properties is reported. Increasing concentrations of any reagents lead to an increase of the crosslinking kinetics. Increasing the polymer concentration leads to a stronger gel whereas an increase of crosslinking reagent (STMP) evidences a critical concentration above which the rheological properties of the gel do not change anymore. This may be explained by the specificity of the system for which the polymer concentration is high and negative charges appear.     

Quantitative risk assessment of human salmonellosis from the consumption of a turkey product in collective catering establishments

The quantitative risk assessment (QRA) approach recommended by the Codex Alimentarius Commission was used to assess the risk of human salmonellosis from the consumption of 'cordon bleu', a specific turkey product, in collective catering establishments (CCEs) of a French department. The complete process was modeled and simulated, from the initial storage in the CCE freezer to the consumption, using a Monte Carlo simulation software. Data concerning the prevalence of contaminated 'cordon bleu', the level of contamination of Salmonella, the cooking and storage process were collected from 21 CCEs and 8 retailers of 'cordon bleu' in the selected department. Thermal inactivation kinetics for Salmonella were established to estimate the effect of heat treatment on the concentration in the product and to calculate the dose that could be ingested by the consumer. The Beta-Poisson dose-response model of Rose and Gerba [Water Science and Technology 24 (1991) 29] with the specific parameters for Salmonella was used to estimate the probability of infection related to the ingestion of a particular dose and a factor was applied to estimate the probability of illness from ingestion. The individual risk of salmonellosis, the risk of outbreak and the number of cases were calculated using Monte Carlo simulation method. The risk of salmonellosis was close to zero when the 'cordons bleus' were cooked in the oven. Therefore, the risk was calculated for the fryer cooking since the insufficient cooking time observed was, sometimes, at the origin of low temperatures (37-89 degrees C). The influence of both the initial concentration of Salmonella in the product and the heat storage before consumption on the final risk was studied. For a high initial concentration of Salmonella in the product, when the 'cordons bleus' are fryer cooked, the average risk of salmonellosis was equal to 3.95 x 10(-3) without storage before consumption and 2.8 x 10(-4) if the product is consumed after storage. This paper presents the results of the QRA and discusses risk management options to minimize the risk of salmonellosis.     

A Protocol for Packet Network Intercommunication

A protocol that supports the sharing of resources that exist in different packet switching networks is presented. The protocol provides for variation in individual network packet sizes, transmission failures, sequencing, flow control, end-to-end error checking, and the creation and destruction of logical process-to-process connections. Some implementation issues are considered, and problems such as internetwork routing, accounting, and timeouts are exposed.     

Entanglement May Enhance Channel Capacity in Arbitrary Dimensions

We consider explicitly two examples of d-dimensional quantum channels with correlated noise and show that, in agreement with previous results on Pauli qubit channels, there are situations where maximally entangled input states achieve higher values of the output mutual information than product states. We obtain a strong dependence of this effect on the nature of the noise correlations as well as on the parity of the space dimension, and conjecture that when entanglement gives an advantage in terms of mutual information, maximally entangled states achieve the channel capacity.