Properties of Water in Foods: in Relation to Quality and Stability
It is the most commonly used criterion for safety and quality. Water activity predicts safety and stability with respect to microbial growth , chemical and biochemical reaction rates, and physical properties.
Properties of water in foods in relation to quality and stability - Ghent University Library
Figure 1 shows stability in terms of microbial growth limits and rates of degradative reactions as a function of water activity. Microorganisms have a limiting water activity level below which they will not grow. Water may be present, even at high content levels, in a product, but if its energy level is sufficiently low, the microorganisms cannot remove the water to support their growth. Consequently, the microbes cannot grow. It actually measures water activity with 0. While temperature, pH, and several other factors can influence whether and how fast microorganisms will grow, water activity is often the most important factor.
Water activity may be combined with other preservative factors hurdles , such as temperature, pH, redox potential, etc. The water activity level that limits the growth of the vast majority of pathogenic bacteria is 0. The lower limit for all microorganisms is 0. Water activity influences not only microbial spoilage but also chemical and enzymatic reactivity. Water may influence chemical reactivity in different ways. It may act as a solvent, a reactant, or change the mobility of the reactants by affecting the viscosity of the system. Water activity influences nonenzymatic browning, lipid oxidization , degradation of vitamins and other nutrients, enzymatic reactions, protein denaturation, starch gelatinization, and starch retrogradation.
Typically, as the water activity level is lowered, the rate of chemical degradative reactions decreases. Besides predicting the rates of various chemical and enzymatic reactions, water activity affects the textural properties of foods. Foods with high water activities have a texture that is described as moist, juicy, tender, and chewy. When the water activity of these products is lowered, undesirable textural attributes, such as hardness, dryness, staleness, and toughness, are observed. Low water activity products normally have texture attributes described as crisp and crunchy, while these products at higher water activity levels change to soggy texture.
Critical water activities determine where products become unacceptable from a sensory standpoint. Water activity is an important factor affecting the stability of powders and dehydrated products during storage. Controlling water activity in a powder product maintains proper product structure, texture, stability, density, and rehydration properties.
Knowledge of the water activity of powders as a function of moisture content and temperature is essential during processing, handling, packaging, and storage to prevent the deleterious phenomenon of caking, clumping, collapse, and stickiness. Caking is water activity, time, and temperature dependent and is related to the collapse phenomena of the powder under gravitational force. Because water activity is a measure of the energy status of the water, differences in water activity between components is the driving force for moisture migration as the system comes to an equilibrium.
Thus, water activity is an important parameter in controlling water migration of multicomponent products. Some foods contain components at different water activity levels, such as filled snacks or cereals with dried fruits.
Determining Moisture Content
By definition, water activity dictates that moisture will migrate from a region of high water activity to a region of lower water activity, but the rate of migration depends on many factors. Undesirable textural changes can result from moisture migration in multicomponent foods. For example, moisture migrating from the higher water activity dried fruit into the lower water activity cereal causes the fruit to become hard and dry while the cereal becomes soggy.
Differences in water activity between components or between a component and environmental humidity are a driving force for moisture migration.
Factors Influencing the Freeze‐Thaw Stability of Emulsion‐Based Foods
Knowledge of whether water will absorb or desorb from a particular component is essential to prevent degradation, especially if the substance is moisture sensitive. The answer depends on the water activities of the two components. If the water activities of the two components are the same, then no moisture will exchange between the two components. Irving Langmuir observed a strong repulsive force between hydrophilic surfaces.
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To dehydrate hydrophilic surfaces—to remove the strongly held layers of water of hydration—requires doing substantial work against these forces, called hydration forces. These forces are very large but decrease rapidly over a nanometer or less.
Water has an unusually high surface tension of Because water has strong cohesive and adhesive forces, it exhibits capillary action. Water is an excellent solvent due to its high dielectric constant.
sovinu.com/289.php If a substance has properties that do not allow it to overcome these strong intermolecular forces, the molecules are precipitated out from the water. Contrary to the common misconception, water and hydrophobic substances do not "repel", and the hydration of a hydrophobic surface is energetically, but not entropically, favorable.
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When an ionic or polar compound enters water, it is surrounded by water molecules hydration. The partially negative dipole ends of the water are attracted to positively charged components of the solute, and vice versa for the positive dipole ends. In general, ionic and polar substances such as acids , alcohols , and salts are relatively soluble in water, and non-polar substances such as fats and oils are not. Non-polar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with non-polar molecules.
The ions are then easily transported away from their crystalline lattice into solution. An example of a nonionic solute is table sugar. The water dipoles make hydrogen bonds with the polar regions of the sugar molecule OH groups and allow it to be carried away into solution.
Predicting safety and stability
The quantum tunneling dynamics in water was reported as early as At that time it was known that there are motions which destroy and regenerate the weak hydrogen bond by internal rotations of the substituent water monomers. Unlike previously reported tunneling motions in water, this involved the concerted breaking of two hydrogen bonds. Water is relatively transparent to visible light , near ultraviolet light, and far-red light, but it absorbs most ultraviolet light , infrared light , and microwaves.
Most photoreceptors and photosynthetic pigments utilize the portion of the light spectrum that is transmitted well through water. Microwave ovens take advantage of water's opacity to microwave radiation to heat the water inside of foods. Water's light blue colour is caused by weak absorption in the red part of the visible spectrum. A single water molecule can participate in a maximum of four hydrogen bonds because it can accept two bonds using the lone pairs on oxygen and donate two hydrogen atoms. Other molecules like hydrogen fluoride , ammonia and methanol can also form hydrogen bonds.
However, they do not show anomalous thermodynamic , kinetic or structural properties like those observed in water because none of them can form four hydrogen bonds: either they cannot donate or accept hydrogen atoms, or there are steric effects in bulky residues. This repeated, constantly reorganizing unit defines a three-dimensional network extending throughout the liquid. This view is based upon neutron scattering studies and computer simulations, and it makes sense in the light of the unambiguously tetrahedral arrangement of water molecules in ice structures.
However, there is an alternative theory for the structure of water. In , a controversial paper from Stockholm University suggested that water molecules in liquid form typically bind not to four but to only two others; thus forming chains and rings. The term "string theory of water" which is not to be confused with the string theory of physics was coined. These observations were based upon X-ray absorption spectroscopy that probed the local environment of individual oxygen atoms.
The repulsive effects of the two lone pairs on the oxygen atom cause water to have a bent , not linear , molecular structure,  allowing it to be polar. The hydrogen-oxygen-hydrogen angle is The valence bond theory explanation is that the oxygen atom's lone pairs are physically larger and therefore take up more space than the oxygen atom's bonds to the hydrogen atoms. At standard conditions, water is a polar liquid that slightly dissociates disproportionately into a hydronium ion and hydroxide ion.
Action of water on rock over long periods of time typically leads to weathering and water erosion , physical processes that convert solid rocks and minerals into soil and sediment, but under some conditions chemical reactions with water occur as well, resulting in metasomatism or mineral hydration , a type of chemical alteration of a rock which produces clay minerals.
It also occurs when Portland cement hardens. Water ice can form clathrate compounds , known as clathrate hydrates , with a variety of small molecules that can be embedded in its spacious crystal lattice. Rain is generally mildly acidic, with a pH between 5. Several isotopes of both hydrogen and oxygen exist, giving rise to several known isotopologues of water. Vienna Standard Mean Ocean Water is the current international standard for water isotopes. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium.
Only ppm include deuterium 2 H or D , a hydrogen isotope with one neutron, and fewer than 20 parts per quintillion include tritium 3 H or T , which has two neutrons.
Oxygen also has three stable isotopes, with 16 O present in Deuterium oxide, D 2 O , is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. Tritium is radioactive , decaying with a half-life of days; THO exists in nature only in minute quantities, being produced primarily via cosmic ray-induced nuclear reactions in the atmosphere. The most notable physical differences between H 2 O and D 2 O , other than the simple difference in specific mass, involve properties that are affected by hydrogen bonding, such as freezing and boiling, and other kinetic effects.
This is because the nucleus of deuterium is twice as heavy as protium, and this causes noticeable differences in bonding energies. The difference in boiling points allows the isotopologues to be separated. Consumption of pure isolated D 2 O may affect biochemical processes — ingestion of large amounts impairs kidney and central nervous system function. Small quantities can be consumed without any ill-effects; humans are generally unaware of taste differences,  but sometimes report a burning sensation  or sweet flavor.