Water activity levels

Choose a salt standard with a water activity level that is slightly below the water activity of sample to be measured. Make sure that salt standard is at ambient temperature before loading it into the instrument. 

Dew-point instrument manufacturers also provide certified unsaturated salt solutions at various water activity levels in individual prepackaged vials. 

Unstable water-soluble vitamins, such as ascorbic acid, are sensitive to drying. The loss of ascorbic acid is dependent on the presence and type of heavy metals, such as copper and iron, light, water activity level in the product, dissolved oxygen, and the temperature of drying. The losses of ascorbic acid during drying have been between 10% and 50%. 

Fig. 2 presents the oxygen permeability coefficient for PLA 98% L-lactide (98L-PLA) as a function of water activity. A significant increase of the oxygen permeability coefficient as temperature increases can be observed for 98L-PLA. 98L-PLA shows an increase of the oxygen permeability coefficients from 3.5 X 10 kg m/(m s Pa) at 5°C to 11 X 10 kg m/(m s Pa) at 40°C when moisture is not present. At a higher water activity level (A = 0.9), the value of the oxygen permeability coefficient increases only to 8.5 X 10 kg m/(m2 s Pa) at 40°C. This reduction of the oxygen permeability coefficient as water activity increases at a constant temperature is observed in Fig. 2 for the three temperatures tested. The reduction is more pronounced at 40°C. 

Other factors also impact the type of crystals formed upon cooling of hot soap. Water activity or moisture content contribute to the final crystal state as a result of the different phases containing different levels of hydration. Any additive that changes the water activity changes the crystallization pathway. For example, the addition of salt reduces the water activity of the mixture and pushes the equiUbrium state toward the lower moisture crystal stmcture. Additionally, the replacement of sodium with other counter cations influences the crystallization. For example, the replacement of sodium with potassium drives toward the formation of 5-phase. 

The inhibitory activity of sorbates is attributed to the undissociated acid molecule. The activity, therefore, depends on the pH of the substrate. The upper limit for activity is approximately pH 6.5 in moist appHcations the degree of activity increases as the pH decreases. The upper pH limit can be increased in low water activity systems. The following indicates the effect of pH on the dissociation of sorbic acid, ie, percentage of undissociated sorbic acid at various pH levels (76,77). 

The guarded hot-plate method can be modified to perform dry and wet heat transfer testing (sweating skin model). Some plates contain simulated sweat glands and use a pumping mechanism to deUver water to the plate surface. Thermal comfort properties that can be deterrnined from this test are do, permeabihty index (/ ), and comfort limits. PermeabiUty index indicates moisture—heat permeabiUty through the fabric on a scale of 0 (completely impermeable) to 1 (completely permeable). This parameter indicates the effect of skin moisture on heat loss. Comfort limits are the predicted metaboHc activity levels that may be sustained while maintaining body thermal comfort in the test environment. 

In this process the addition of water vapor to the sweep stream can be controlled so that the water activity of the gas phase equals that of the beverage. When this occurs, there is no transport of water across the membrane. The water content of both the beverage feed and the sweep stream is kept constant. These conditions must be maintained for optimum alcohol reduction. The pervaporation system controls the feed, membrane, airstream moisture level, and ethanol recovery functions. An operational system has been developed (13). 

Weigh out 2 mg of sulfo-SMCC and add it to the above solution. Mix gently to dissolve. To aid in measuring the exact quantity of crosslinker, a concentrated stock solution may be made in water (or DMSO) and an aliquot equal to 2 mg transferred to the reaction solution. If a stock solution is made, it should be dissolved rapidly and used immediately to prevent extensive hydrolysis of the active ester. As a general guideline of addition for a particular protein activation, the use of a 40- to 80-fold molar excess of crosslinker over the amount of protein present usually results in good activation levels. 

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