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In food science, water activity (a w) of a food is the ratio of its vapor pressure to the vapor pressure of water at the same temperature, both taken at equilibrium. [1] Pure water has a water activity of one. Put another way, a w is the equilibrium relative humidity (ERH) expressed as a fraction instead of as a percentage.
An increase in a w is usually accompanied by an increase in water content, but in a non-linear fashion. This relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. These curves are determined experimentally and constitute the fingerprint of a food system. [2]
The kinetic data (Figure 1a) shows the change in mass and humidity as a function of time. From the kinetic results, the rate of water uptake and water diffusion coefficients can be determined. The equilibrium mass values at the end of each humidity step were used to calculate the sorption and desorption isotherms (Figure 1b).
in the condensation of the water-vapour of the air on the cold surface of a glass; in the capillarity of hair, wool, cotton, wood shavings, etc.; in the imbibition of water from the air by gelatine; in the deliquescence of common salt; in the absorption of water from the air by concentrated sulphuric acid; in the behaviour of quicklime". [4]
In biochemistry, the molar absorption coefficient of a protein at 280 nm depends almost exclusively on the number of aromatic residues, particularly tryptophan, and can be predicted from the sequence of amino acids. [6] Similarly, the molar absorption coefficient of nucleic acids at 260 nm can be predicted given the nucleotide sequence.
where is the sum over the participating pressures, such as the atmospheric pressure , the hydrostatic pressure and the equivalent pressure due to capillary forces . η {\displaystyle \eta } is the viscosity of the liquid, and ϵ {\displaystyle \epsilon } is the coefficient of slip, which is assumed to be 0 for wetting materials.
The Atwater system, [1] named after Wilbur Olin Atwater, or derivatives of this system are used for the calculation of the available energy of foods.The system was developed largely from the experimental studies of Atwater and his colleagues in the later part of the 19th century and the early years of the 20th at Wesleyan University in Middletown, Connecticut.
where temperature T is in degrees Celsius (°C) and saturation vapor pressure P is in kilopascals (kPa). According to Monteith and Unsworth, "Values of saturation vapour pressure from Tetens' formula are within 1 Pa of exact values up to 35 °C." Murray (1967) provides Tetens' equation for temperatures below 0 °C: [3]