Elena Venir

Water activity and the preservation of plant foods

The general constitution of plant foods, namely of vegetables and fruits, may be described as a watery solution of low molecular weight species, mainly sugars, salts and organic acids and of high molecular weight hydrocolloids, contained in a water insoluble cellular matrix of macromolecules, mostly carbohydrates, including insoluble pectic substances, hemicelluloses and proteins. All of these constituents, to different extents, interact with water and have the ability to lower its vapour pressure; with small molecules this is mainly through polar binding, and with large biopolymers, through surface interactions and capillary effects. For a long time, scientists have realised that the relative vapour pressure, i.e. the water activity, could be much more important to the quality and stability of food than the total amount of water present. Over the past few decades, the glass transition temperature, as well as its dependence on moisture content has also gained interest, as it could become the main determinant of the dynamic of changes in many non-equilibrium, lowmoisture systems. Processed fruits and vegetables are often lowmoisture, sugar-rich foods, characterised by colour, flavour and structural properties. The relationships between both aw and Tg and some important chemical, physical and structural changes which can occur during processing and preservation of fruits and vegetables and their derivatives, are presented. # 2003 Elsevier Science Ltd. All rights reserved.

Effect of high-pressure homogenization on droplet size distribution and rheological properties of ice cream mixes.

The effect of different homogenization pressures (15/3 MPa and 97/3 MPa) on fat globule size and distribution as well as on structure-property relationships of ice cream mixes was investigated. Dynamic light scattering, steady shear, and dynamic rheological analyses were performed on mixes with different fat contents (5 and 8%) and different aging times (4 and 20 h). The homogenization of ice cream mixes determined a change from bimodal to monomodal particle size distributions and a reduction in the mean particle diameter. Mean fat globule diameters were reduced at higher pressure, but the homogenization effect on size reduction was less marked with the highest fat content. The rheological behavior of mixes was influenced by both the dispersed and the continuous phases. Higher fat contents caused greater viscosity and dynamic moduli. The lower homogenization pressure (15/3 MPa) mainly affected the dispersed phase and resulted in a more pronounced viscosity reduction in the higher fat content mixes. High-pressure homogenization (97/3 MPa) greatly enhanced the viscoelastic properties and the apparent viscosity. Rheological results indicated that unhomogenized and 15/3 MPa homogenized mixes behaved as weak gels. The 97/3 MPa treatment led to stronger gels, perhaps as the overall result of a network rearrangement or interpenetrating network formation, and the fat globules were found to behave as interactive fillers. High-pressure homogenization determined the apparent viscosity of 5% fat to be comparable to that of 8% fat unhomogenized mix.

Involvement of alanine racemase in germination of Bacillus cereus spores lacking an intact exosporium

The l-alanine mediated germination of food isolated Bacillus cereus DSA 1 spores, which lacked an intact exosporium, increased in the presence of d-cycloserine (DCS), which is an alanine racemase (Alr) inhibitor, reflecting the activity of the Alr enzyme, capable of converting l-alanine to the germination inhibitor d-alanine. Proteomic analysis of the alkaline extracts of the spore proteins, which include exosporium and coat proteins, confirmed that Alr was present in the B. cereus DSA 1 spores and matched to that encoded by B. cereus ATCC 14579, whose spore germination was strongly affected by the block of conversion of l- to d-alanine. Unlike ATCC 14579 spores, l-alanine germination of B. cereus DSA 1 spores was not affected by the preincubation with DCS, suggesting a lack of restriction in the reactant accessibility.

Structure and Water Relations of Melanoidins Investigated by Thermal, Rheological, and Microscopic Analysis

Water soluble melanoidins (WSM) from a glucose-glycine model system were obtained and separated into high (HMW) and low (LMW) molecular weight fractions. Adsorption isotherm and GAB and BET fitting parameters of WSM were determined. Physical and structural properties were investigated through thermal and rheological analysis. The LMW fraction exhibited thermal properties similar to that of oligosaccharides and the HMW fraction was compatible with a complex and large molecular size biopolymer structure, while both exhibited gel like properties. Intrinsic viscosity values were consistent with spherical hydrodynamic volumes.