Food and Health

Formulating novel processed foods to meet consumer demands for healthy, low-fat and/or low sugar foods while optimizing the desired flavour and texture characteristics presents a continuing challenge to food technologists. Reformulating a food can change its flavour so the ...

We explore the implications of motional relativity in NMR and show that sample translation can be used to acquire NMR signals without the need for pulsed RF excitation or pulsed magnetic field gradients. Novel single-shot, on-line NMR acquisition protocols for samples being conveyed at high speed are discussed and preliminary results using a low-cost, on-line prototype NMR sensor are presented.

By periodically flipping the longitudinal magnetisation with a chain of 180 degrees pulses it is possible to establish a steady-state of longitudinal polarisation that effectively stores the information of T(1) relaxation. The pulse sequence for achieving this, called steady-state Flipped LOngitudinal Polarisation (FLOP) can be used for the fast acquisition of a two-dimensional T(1)-T(2) relaxation time spectrum in both periodic and a-periodic modes. We have therefore called this new class of sequences periodic or a-periodic FLOP-T(1)-T(2).

A general theory of spatially dependent bacterial growth in heterogeneous systems is developed by combining a structured-cell kinetic model of bacterial metabolism with reaction-diffusion equations describing the transport of nutrients in the growth medium. Rate constants in the theory are determined for Listeria monocytogenes by fitting the viable count batch growth data acquired in homogeneous, shake-flask cultures. The rate constants are then used to calculate the form of bacterial growth when it occurs as localized microcolonies in structured systems. The same theory is used to analyse the effects of transient variations in environmental variables.

A stochastic computer simulation is used to investigate the effects of restricted diffusion in NMR microscopy. It is shown that diffusion contributes to a loss of interfacial resolution through two main mechanisms. The first applies to spatial regions bound by impermeable interfaces and involves diffusive averaging of the frequency differences set up by the applied field gradients. This effect can be made arbitrarily small by increasing the magnitude of the field gradient. The second mechanism involves diffusion through permeable membranes or interfaces defining the sample morphology. This effect can, in principle, be reduced by multiple echo imaging with short pulse spacings. The possibility of imaging diffusive flow through a permeable interface is discussed.

A theoretical analysis is given of the response of particular nuclear magnetic spin systems to resonant pulse sequences of the form 90x-tau-(-thetavarphi-ts-)n- with nthetavarphi = Thetavarphi, where the sequence 90x-tau-Thetavarphi is known to excite particular forbidden transitions in the spin system for definite values of the product Omegatau, Omega being a spin coupling giving rise to splittings in the spectrum of the spin system. The systems investigated are (i) a spin-1 with a finite, first order quadrupole coupling and (ii) an AX scalar-coupled spin-½ pair. In the spin-1 case it is shown, for example, that excitation of double quantum coherence (DQC) by the 90x-tau-90x sequence is made increasingly selective on the value of the quadrupole coupling OmegaQ by using the sequences 90x-tau-(-thetax-4tau-)n- or 90x-tau-(-thetax-2tau-theta-x-2tau-)n/2>-. These sequences excite DQC for OmegaQ values around OmegaQtau = (2q + 1)pi/2, q integer, and increasing n leads to increasing selectivity with respect to OmegaQ. A similar result is obtained for excitation of a state of J order in the AX spin system. Experimental measurements are reported for the spin-1 system using the 2H resonance in an oriented soap/D2O mesophase. The results agree quantitatively with the theory and show the increasing selectivity with the number of pulses as predicted.

... is as completely described as possible if the density operator or matrix is known [9]. For nuclear spin systems it ... describe new pulse sequences, particularly for the usual scalar-coupled spin systems encountered in high-resolution liquid-state NMR spectroscopy. ... ls* + lI* + ~ss*, ...