Gas production and transport in artificial sludge depots.

This paper presents a study to determine the impact of gas production in dredging sludge on the storage capacity of artificial sludge depots. Gas is produced as a result of the decomposition of organic material present in dredging spoil. This process, in which methane and carbon dioxide are formed, may lead to expansion of sludge layers, partly or even completely counterbalancing consolidation. The study shows that, even with a very conservative estimation of the rate of gas production, accumulation of gas occurs as convective and diffusive transport proceed very slowly. Nucleation of gas bubbles occurs already at a limited oversaturation of pore water. During their growth, bubbles push aside the surrounding grain matrix. Resulting stresses may initiate cracks around bubbles. If these cracks join, they may form channels stretching out to the depot surface and along which gas may escape. However, channels are only stable to a limited depth below which bubble accumulation may continue. The gas content at which sufficient cracks and channels are formed to balance the rate of gas production with the rate of outflow strongly depends on the constitutive properties of the dredging sludge considered. In sludge with a high shear strength (> 10 kPa), stable channels are created already at low deformations. However, a large expansion may occur in sludge with a low strength. The present study shows that accumulation of gas may continue until a bulk density less than that of water is attained. This is equivalent to a gas fraction of about 25-37%, depending on the initial water content of the sludge. Only then can gas escape as a result of instabilities in the sediment matrix. This should be well taken into account during the design and management of artificial depots.

Measurement of trench depth by infrared interferometry.

Measurement and control of high-aspect-ratio structures such as dynamic random-access memory trenches is an important step in the manufacture of modern memory devices. We present a novel technique based on infrared interferometry that has been implemented in manufacturing and is capable of measuring sub- 0.25-mum -wide and 10-mum -deep trenches nondestructively and with an accuracy of better than 0.1mum .

Yeast meiotic mutants proficient for the induction of ectopic recombination.

A screen was designed to identify Saccharomyces cerevisiae mutants that were defective in meiosis yet proficient for meiotic ectopic recombination in the return-to-growth protocol. Seven mutants alleles were isolated; two are important for chromosome synapsis (RED1, MEK1) and five function independently of recombination (SPO14, GSG1, SPOT8/MUM2, 3, 4). Similar to the spoT8-1 mutant, mum2 deletion strains do not undergo premeiotic DNA synthesis, arrest prior to the first meiotic division and fail to sporulate. Surprisingly, although DNA replication does not occur, mum2 mutants are induced for high levels of ectopic recombination. gsg1 diploids are reduced in their ability to complete premeiotic DNA synthesis and the meiotic divisions, and a small percentage of cells produce spores. mum3 mutants sporulate poorly and the spores produced are inviable. Finally, mum4-1 mutants produce inviable spores. The meiotic/sporulation defects of gsg1, mum2, and mum3 are not relieved by spo11 or spo13 mutations, indicating that the mutant defects are not dependent on the initiation of recombination or completion of both meiotic divisions. In contrast, the spore inviability of the mum4-1 mutant is rescued by the spo13 mutation. The mum4-1 spo13 mutant undergoes a single, predominantly equational division, suggesting that MUM4 functions at or prior to the first meiotic division. Although recombination is variably affected in the gsg1 and mum mutants, we hypothesize that these mutants define genes important for aspects of meiosis not directly related to recombination.

Degradation and debittering of a tryptic digest from beta-casein by aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2.

The mode of action of purified aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2 on a complex peptide mixture of a tryptic digest from bovine beta-casein was analyzed. The oligopeptides produced in the tryptic digest before and after aminopeptidase N treatment were identified by analysis of the N- and C-terminal amino acid sequences and amino acid compositions of the isolated peptides and by on-line liquid chromatography-mass spectrometry. Incubation of purified peptides with aminopeptidase N resulted in complete hydrolysis of many peptides, while others were only partially hydrolyzed or not hydrolyzed. The tryptic digest of beta-casein exhibits a strong bitter taste, which corresponds to the strong hydrophobicity of several peptides in the tryptic digest of beta-casein. The degradation of the "bitter" tryptic digest by aminopeptidase N resulted in a decrease of hydrophobic peptides and a drastic decrease of bitterness of the reaction mixture.