RESUMO
BACKGROUND: Water activity is measured by equilibrating a test portion with a sealed head space and measuring the test portion temperature and the vapor density of the head space. The water activity is the ratio of head space vapor density to the saturation vapor density at test portion temperature. Headspace vapor density is typically measured using capacitance or chilled mirror sensors, but, when volatiles in significant concentration are present, these measurements may fail. OBJECTIVE: Evaluate the accuracy of a tunable diode laser (TDL) for measuring the headspace vapor density and water activity of pharmaceutical preparations and food in the presence of non-aqueous volatiles. METHODS: A commercial TDL water activity meter was calibrated against standards of known water activity and used to measure water activity of pharmaceutical preparations and food with high concentrations of non-aqueous volatiles. RESULTS: When no volatiles other than water vapor are present, this method is capable of measuring water activity with an accuracy of 0.005 or better. When high concentrations of volatiles such as ethanol, isopropanol, propylene glycol, tetrahydrofuran, or acetonitrile are present the uncertainty of the measurement increases. This is at least partly due to the uncertainty of the standards. CONCLUSION: Based on uncertainties in the water activity estimates of the water-organic mixtures, the uncertainties in water activity measurements with high concentrations of non-aqueous solvents is 0.02 or less up to mass fractions of the organic of 0.97. HIGHLIGHTS: The theoretical background for TDL measurement of water activity is presented showing that the water vapor concentration is proportional to the area of the absorption line which is not affected by the presence of other volatiles.
Assuntos
Lasers Semicondutores , Vapor , Etanol , Preparações FarmacêuticasRESUMO
A number of recent outbreaks related to pathogens in low-moisture foods have created urgency for studies to understand the possible causes and identify potential treatments to improve low-moisture food safety. Thermal processing holds the potential to eliminate pathogens such as Salmonella in low-moisture foods. Water activity (aw ) has been recognized as one of the primary factors influencing the thermal resistance of pathogens in low-moisture foods. But most of the reported studies relate thermal resistance of pathogens to aw of low-moisture foods at room temperature. Water activity is a thermodynamic property that varies significantly with temperature and the direction of variation is dependent on the product component. Accurate methods to determine aw at elevated temperatures are needed in related research activities and industrial operations. Adequate design of commercial thermal treatments to control target pathogens in low-moisture products requires knowledge on how aw values change in different foods at elevated temperatures. This paper presents an overview of the factors influencing the thermal resistance of pathogens in low-moisture foods. This review focuses on understanding the influence of water activity and its variation at thermal processing temperature on thermal resistance of pathogens in different low-moisture matrices. It also discusses the research needs to relate thermal resistance of foodborne pathogens to aw value in those foods at elevated temperatures.
RESUMO
Stem water content, ice fraction, and losses in xylem conductivity were monitored from November 1996 to October 1997 in an even-aged stand of Pinus contorta (lodgepole pine) near Potlatch, Idaho, USA. A time domain reflectometry (TDR) probe was used to continuously monitor stem water contents and ice fractions. Stem sapwood water contents measured with TDR were not different from water contents measured gravimetrically. The liquid water content of stems ranged from 0.70 m3 m-3 to 0.20 m3 m-3 associated with freezing and thawing of the wood tissue. Ice fraction of the stem varied from 0-75% during the winter suggesting liquid water was always present even at ambient temperatures below -20°C. Shoot xylem tensions decreased through the winter to a minimum of ca. -1.4 MPa in February then increased to -0.4 MPa in May. Shoot xylem tensions decreased during the growing season reaching -1.7 MPa by September. Annually, low shoot water potentials were not correlated to decreases in stem hydraulic conductivity. Xylem conductivity decreased due to cavitation through the winter and was 70% of summer values by March. Decreases in xylem conductivity were correlated to low shoot water potentials and cumulative freezing and thawing events within the xylem. Xylem conductivity increased to pre-winter values by May and no reductions in xylem conductivity were observed during the growing season.
