Reduction of Cr(VI) by immobilized cells of Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776.

Hexavalent chromium, a carcinogen and mutagen, can be reduced to Cr(III) by Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776. This study examined Cr(VI) reduction by immobilized cells of the two strains in a common solution matrix using various entrapment matrices. Chitosan and PVA-borate beads did not retain integrity and supported low or no reduction of Cr(VI) by the cells. A commercial preparation (Lentikats) was stable but also did not support Cr(VI) reduction. K-carrageenan beads were stable in batch suspensions but gel integrity was lost after only 5 h in a flow-through system in the presence of 100 microM Cr(VI). The best immobilization matrices were agar and agarose, where the initial rates of reduction of Cr(VI) (from 500 microM solution) for D. vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776 were 127 (agar) and 130 (agarose), and 15 (agar) and 12 (agarose) nmol h(-1) mg dry cell wt(-1), respectively. The higher removal of Cr(VI) by D. vulgaris was also seen in 14-mL packed-bed flow-through columns, where, at a flow rate of 2.4 mL h(-1), the percentage removal of Cr(VI) was approximately 95% and 60% for D. vulgaris and Microbacterium sp., respectively (agar-immobilized cells). The Cr(VI) reducing activities of D. vulgaris and Microbacterium sp. were lost after 159 and 140 h, respectively. Examination of the beads for structural integrity within the columns in situ using magnetic resonance imaging after 24 and 100 h of continuous operation against Cr(VI) (with negligible Cr retained within the columns) showed that agar beads were more stable with time. The most appropriate system for development of a continuous bioprocess is thus the use of D. vulgaris NCIMB 8303 immobilized in an agar gel matrix.

Continuous removal of Cr(VI) from aqueous solution catalysed by palladised biomass of Desulfovibrio vulgaris.

Growth-decoupled cells of Desulfovibrio vulgaris NCIMB 8303 can be used to reduce Pd(II) to cell-bound Pd(0) (Bio-Pd(0)), a bioinorganic catalyst capable of reducing hexavalent chromium to less toxic Cr(III), using formate as the electron donor. Magnetic resonance imaging showed that Bio-Pd(0), immobilized in chitosan and agar beads, is distinguishable from the surrounding gel and is evenly dispersed within the immobilization matrix. Agar-immobilized Bio-Pd(0) and 'chemical Pd(0)' were packed into continuous-flow reactors, and challenged with a solution containing 100 microM Cr(VI) (pH 7) at a flow rate of 2.4 ml h(-1). Agar-immobilized chemical Pd(0) columns lost Cr(VI) reducing ability by 160 h, whereas columns containing immobilized Bio-Pd(0) maintained 90% reduction until 680 h, after which reduction efficiency was gradually lost.

MRI phase mapping of temperature distributions induced in food by microwave heating.

A range of temperature-sensitive MRI parameters of water (T2, T1, diffusion coefficient, and chemical shift) were evaluated to map in three dimensions the non-uniform temperature distributions induced by microwave heating in both model and real food systems. Phase mapping was found to be the most robust method, and evaluations of possible experimental errors were based on semi-quantitative studies of homogeneous and heterogeneous systems. The MRI protocol provides complementary phase and magnitude data, which are related to the sample temperature and structural heterogeneity, respectively. Used together, they relate the temperature changes to the differential thermal properties of the various components within a heterogeneous sample. The potential applications of this technique to microwave and other forms of heating is discussed.

Quantitative magnetic resonance imaging of fresh and frozen-thawed trout.

Magnetic resonance imaging (MRI) has been used to visualise the major organs and muscular-skeletal frame-work of fresh rainbow trout (Salmo gairdneri) in two dimensions, and to identify the spatial distribution of lipid- and collagen-rich tissues. Quantitative MRI provides the MR parameters (T1, T2, M0, T1sat, Msat/M0, and the Magnetisation Transfer (MT) rate) for the tissue water; variations in those parameters enable distinction to be made between a freshly killed trout and one which has been frozen-thawed. The effects of freezing method, repeat freeze-thawing, and storage time on the MR parameters are discussed.

Measurement of textural changes of food by MRI relaxometry.

This article summarises the current status of, and future prospects for, the use of magnetic resonance imaging (MRI) to evaluate the texture of a range of foods, and changes therein which accompany pathogen infection, natural ageing, damage, ripening and processing. The basic concept is that the magnetic resonance parameters of water are sufficiently sensitive to the texture of the food matrix, that magnetic resonance images of the spatial distribution of those parameters are effectively maps of the structural status of the foodstuff. This is illustrated in the context of the effects of pathogen infection in cucumbers, internal necrosis of melons, bruising in peaches, ripening of pineapples, and the effect of freeze-thawing on meat and fish.