Enrico Maltini

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.

Evaluation of viscosities of amorphous phases in partially frozen systems by WLF kinetics and glass transition temperatures

Abstract The viscosities of concentrated solutions of glucose, fructose, sorbitol and xylitiol were measured by a falling sphere method. The measured viscosities were compared with those predicted by WLF kinetics from glass transition temperatures. Discrepancies between measured and predicted viscosities were found to be within an order of magnitude of 0.3 on log ϵ. Experimental data on sucrose solutions obtained from literature sources were also compared with WLF predicted values. The viscosities of the amorphous phases in frozen or partially frozen sucrose-, glucose-, fructose-, citric acid-, dimethyl sulfoxide-, polyvinylpyrrolidone- and glycerol-water systems were estimated by WLF kinetics and freezing temperatures. At equal sub-freezing temperature, the viscosities increased with the increasing of molecular weight. In addition, viscosity changes during devitrification and recrystallization of 60–70% sucrose solutions, which do not freeze upon cooling, were evaluated.

Polyphenoloxidase and peroxidase activity in partially frozen systems with different physical properties

Abstract Enzyme catalysed reactions can occur at significant rates at low temperatures, thus exerting a major control on the quality of frozen foods. The temperature-concentration effect in partially frozen systems can make enzyme reactions controlled by diffusion and thus rate limited by viscosity according to the Stokes–Einstein relation. Polyphenoloxidase (PPO) activity in sucrose, glycerol and fructose water media and (POD) activity in fructose and glycerol water media, were assayed spectrophotometrically from 20°C to −30°C. The media have at equal sub-freezing temperature marked differences in viscosities and others physical properties. For both enzymes, the activity data showed similar dependence on temperature and, above freezing, on the concentration of the media. A relation with the viscosity of the media at equal sub-freezing temperature was not evident. POD activity showed a “break” between −10°C and −20°C, suggesting a reversible conformational change. The temperature dependence of PPO rate constants were well described by both the Williams–Landel–Ferry ( r 2 =0.998) and the Arrhenius ( r 2=0.985) models. A hypothesis for the non conformity of the measured kinetics with the theoretical model is that in the temperature range considered the reactions are still activation controlled or in a transition zone, and should become diffusion limited only at lower temperatures and higher viscosities.

Functional properties of reduced moisture fruits as ingredients in food systems

Abstract Fruit pieces of some species have been processed to be used as ingredients in various food systems, either by partial dehydration alone or by osmosis and partial dehydration, to achieve different levels of water activity and solids contents in the final products. The relationships existing among processing, phase composition after processing and functional properties of the products have been assessed within a useful range of water content and water activity. The functional properties are expressed as diagrams relating the phase composition (soluble solids, insoluble solids and water) to the range of consistencies obtainable at various water activities. These diagrams provide a useful tool for preparing fruit ingredients suitable for specific food systems.

Water activity and viscosity—relations with glass transition temperatures in model food systems

Abstract Both thermodynamic and kinetic parameters are important in food processing and stability. Thermodynamic parameters are represented by water activity and generally applied to high and medium moisture content materials, where molecules can freely diffuse, without restrictions. In contrast, kinetic properties mainly apply to intermediate and low moisture content foods, where diffusional mobility of reactants is more or less restricted and where water acts as a plasticizer of amorphous compounds. Kinetic properties can be represented by viscosity and related to the glass transition temperature ( T g ) by the Williams-Landel-Ferry equation. Thus, in a food system passing from high to intermediate and low moisture content and/or from high to low temperature, the prevailing control mechanism will gradually change from thermodynamic to kinetic. The relationship between the water activity, temperature and viscosity is presented for some model solutions.

Physical changes induced by the Maillard reaction in a glucose–glycine solution

Abstract Some physical changes associated with the development of Maillard reaction in a glucose–glycine aqueous solution were detected by means of thermal and rheological analysis as well as by more conventional determinations. The increase in water content, water activity ( A w ) and enthalpy of ice melting ( Δ Hm ) during the reaction reflects both the formation of new water molecules and the consumption of low molecular weight species. The concomitant arrangement of more complex molecular structures was indicated by changes in the glass transition temperature of the maximally freeze-concentrated solution ( T g′) and of the freeze-dried solution ( T g (dry) ) as well as in the estimated unfrozen water (UFW). After fractionation of the samples by solid phase extraction, the non-polar fractions exhibited much higher T g (dry) than the non-fractionated samples. The T gs (dry) of samples at increasing heating time were treated with the Gordon–Taylor equation and results were discussed. The expected increase in viscosity due to polymerisation was compensated by the formation of water, but it could be detected when viscosities were measured on samples equilibrated at a same A w

Growth of Bacillus cereus on Solid Media as Affected by Agar, Sodium Chloride, and Potassium Sorbate

The effect of two independent variables: microstructure, as modified by the agar content (1.0, 4.0, 7.0%), and water activity (a(w)), as modified by the NaCl content (0.5, 2.5, 4.5%), in the absence or in the presence of potassium sorbate (0.0; 2,000 ppm) on Bacillus cereus growth on solid media was studied. The time to visible growth (TVG) and the radial growth rate (RGR) of colonies were evaluated. TVG was not affected by microstructure and K-sorbate, although when a(w) was reduced, TVG tended to increase. RGR depended on linear effects of microstructure and a(w) variables and their interaction. When K-sorbate was added to cultural media, RGR was reduced significantly. However, in the presence of K-sorbate, RGR was found to change only when a(w) vas varied.

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.