Water, the most abundant constituent of natural foods, is a ubiquitous plasticizer of most natural and fabricated food ingredients and products. Many of the new concepts and developments in modern food science and technology revolve around the role of water, and its manipulation, in food manufacturing, processing, and preservation. This article reviews the effects of water, as a near-universal solvent and plasticizer, on the behavior of polymeric (as well as oligomeric and monomeric) food materials and systems, with emphasis on the impact of water content (in terms of increasing system mobility and eventual water "availability") on food quality, safety, stability, and technological performance. This review describes a new perspective on moisture management, an old and established discipline now evolving to a theoretical basis of fundamental structure-property principles from the field of synthetic polymer science, including the innovative concepts of "water dynamics" and "glass dynamics". These integrated concepts focus on the non-equilibrium nature of all "real world" food products and processes, and stress the importance to successful moisture management of the maintenance of food systems in kinetically metastable, dynamically constrained glassy states rather than equilibrium thermodynamic phases. The understanding derived from this "food polymer science" approach to water relationships in foods has led to new insights and advances beyond the limited applicability of traditional concepts involving water activity. This article is neither a conventional nor comprehensive review of water activity, but rather a critical overview that presents and discusses current, usable information on moisture management theory, research, and practice applicable to food systems covering the broadest ranges of moisture content and processing/storage temperature conditions.

Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety

Advances in experimental and computational methodologies have led to a recent renewed interest in the Hofmeister series and its molecular origins. New results are surveyed and assessed. Insights into the underlying mechanisms have been gained, although deeper molecular understanding still seems to be elusive. The principal reason appears to be that the Hofmeister series emerges from a combination of a general effect of cosolutes (salts, etc.) on solvent structure, and of specific interactions between the cosolutes and the solute (protein or other biopolymer). Hence every system needs to be studied individually in detail, a state of affairs which is likely to continue for some time. A deeper understanding of the Hofmeister series can be an extraordinarily valuable guide to designing experiments, including not only those probing the series per se, but also those designed to elucidate the adsorption, aggregation and stabilization phenomena which underlie so many biological events. The aim of this review is to provide an up-to-date framework to guide such understanding, consolidating recent advances in the many fields on which the Hofmeister series impinges.

The Hofmeister series: salt and solvent effects on interfacial phenomena

Dielectric Relaxation and Solvation Dynamics of Water in Complex Chemical and Biological Systems

Almost all biological macromoleculess proteins
(enzymes) and DNAs are inactive in the absence of
water. Hydration of a protein/enzyme is particularly
important for the stability of the structure and for
the function, especially the recognition at a specific
site. This role of hydration in enzyme catalysis is well
known and has recently been reviewed in a number
of publications. In one of these studies it was
shown that the dehydration of a protein, which
makes it more rigid and increases its denaturation
temperature, is correlated with the loss of its physiological function.

Dynamics of water in biological recognition.

Dynamic properties, derived from dielectric relaxation spectra of glass-formers at variable temperature and pressure, are used to characterize and classify any resolved or unresolved secondary relaxation based on their different behaviors. The dynamic properties of the secondary relaxation used include: (1) the pressure and temperature dependences; (2) the separation between its relaxation time τβ and the primary relaxation time τα at any chosen τα; (3) whether τβ is approximately equal to the independent (primitive) relaxation time τ0 of the coupling model; (4) whether both τβ and τ0 have the same pressure and temperature dependences; (5) whether it is responsible for the “excess wing” of the primary relaxation observed in some glass-formers; (6) how the excess wing changes on aging, blending with another miscible glass-former, or increasing the molecular weight of the glass-former; (7) the change of temperature dependence of its dielectric strength Δeβ and τβ across the glass transition temperature Tg; ...

Classification of secondary relaxation in glass-formers based on dynamic properties.

Total reflection methods and instrumentation for their use are described for measurements of dielectric permittivity and loss at frequencies to 10 GHz or more. Several cell designs are shown, together with analyses of their performance. Procedures are given for correcting effects of wave propagation in the cells and residual reflections in the cells by bilinear analysis with calibrations using dielectrics of known permittivity. Representative results are presented for highly polar liquids, dilute solutions of polar molecules in nonpolar solvents, electrolyte solutions, and ionic glasses with appreciable ohmic conduction.

Time domain reflection methods for dielectric measurements to 10 GHz

The dielectric behavior of living tissues and a number of biological materials was examined by new equipment of the time domain reflectometry method in a wide frequency range of 10/sup 7/-10/sup 10/ Hz. The authors found two peaks of Debye absorption around 100 MHz and 20 GHz for all the materials. The low-frequency absorption is probably due to bound water while the high-frequency absorption to free water. From the observed relaxation times of bound water a hypothesis is ventured on the structure of bound water and its relaxation mechanism.

Dielectric relaxation time and structure of bound water in biological materials

By the use of the time domain reflectometry method dielectric measurements were carried out first on methanol mixtures with ethanol and 1‐propanol, and second, water mixtures with methanol, ethanol and 1‐propanol in the frequency range 10 MHz–20 GHz. The first mixtures show a Debye relaxation and logarithm of the relaxation time is given by a linear function of the mole fraction of methanol. These mixtures have the same chainlike cluster of pure alcohol. The second mixtures show the same trend of the relaxation time in a region 0≤xw 0.83 and that the cluster must be cyclic, consisting of six molecules.

Structures of water and primary alcohol studied by microwave dielectric analyses

Dielectric measurements over a microwave frequency range 10 MHz–15 GHz were carried out by the use of new time domain reflectometry equipment on the mixtures of water with five primary alcohols, viz., methanol, ethanol, and n‐propanol in the concentration range 0≤x≤1 and n‐butanol and amyl alcohol in the range 0≤x≤0.5 at room temperature; x being the mole fraction of water. The systems of water and two alcohols of low molecular weight are characterized by a single relaxation with a distribution parameter of the unity or near to it. The molecular reorientation in the mixtures as well as water and these alcohols is a cooperative process involving a large number of molecules with the hydrogen‐bond linkages (O–H⋅⋅⋅O). Dielectric behavior of the mixtures of water and methyl or ethyl alcohol is due to the structure of a hydrogen‐bonded network being microscopically homogeneous. Microscopic heterogeneity occurs in the mixtures of water and higher alcohols.

Satoru Mashimo

Papers

Time domain reflection methods for dielectric measurements to 10 GHz

Dielectric relaxation time and structure of bound water in biological materials

Structures of water and primary alcohol studied by microwave dielectric analyses

The dielectric relaxation of mixtures of water and primary alcohol

Evaluation of dielectric behavior by time domain spectroscopy. 3. Precision difference methods