Water activity, controlling

Table 9.3 Saturated salt solutions suitable for water activity control. Values are given for 25°C.

In order to study the effects of water activity on an enzymatic reaction, there is a need for practical methods to adjust the water activity in the reaction mixture. Likewise, it is highly desirable to keep this parameter close to the optimal value during large-scale conversions. A range of water activity control methods have been developed [11], and which one to choose depends on the scale of reaction, the quantities of water to be removed or added, and availability of equipment. 

Water Activity Control Using Saturated Salt Solutions... 

Water Activity Control Using Pairs of Salt Hydrates... 

An alternative method for water activity control is based on the fact that salt hydrates containing different numbers of water molecules are interconverted at fixed water activities [15]. The first salt hydrate used was Na2C03- 10H2O. This is converted to Na2C03-7H20 at a water activity of 0.74 at 24 °C. The salt hydrates act as a buffer of the water activity. As long as both salt hydrates are present, the water activity remains at 0.74. If another water activity is desired, another pair of salt hydrates should be chosen. The salt hydrates can be added directly to the organic reaction mixture. One should be careful that the salt hydrates do not interfere with the enzyme or the enzymatic reaction. 

Table II shows similar relative rates and AFImax measured over saturated aqueous NaCl (water activity 0.76). Limited studies were also made over saturated KI solutions (water activity 0.60). Rate was somewhat greater at the higher water activity, but there was not sufficient difference to warrant further comparative study. The same relative rate order for sugar type occurs over NaCl as over water. Table III reports similar studies over activated silica. The experimental scheme does not permit rigid water activity control at near zero activity, but it is clear that rates are much lower and that the same relative rate order for pentose and hexose sugars is preserved, although the differences are much less pronounced. The triose is relatively less reactive here.

Both are a function of the ionomer water activity (controlled by its hydration state and relative humidity (RH)) and the CD (controlled by the fuel-air segment). ... 

Selection of reaction operating conditions (e.g., temperature and solvent system programming, water activity control, procedure used to form the medium)... 

Colombie, S., Tweedell, R. J., Condoret, J. S., and Marty, A., Water activity control a way to improve the efficiency of continuous lipase esterification, Biotechnol. Bioeng., 60, 362-368, 1998. 

Rossell, C. M., Vaidya, A. M., and Hailing, P. 1., Continuous in situ water activity control for organic phase biocatalysis in a packed bed hollow fiber reactor, Biotechnol. Bioeng., 49, 284-289, 1996. 

Ujang, Z., Al-Sharbati, A. N., and Vaidya, A. M., Organic-phase enzymatic esterification in a hollow fiber membrane reactor with in situ gas-phase water activity control, Biotechnol. Prog., 13, 39 2, 1997. 

Resell, C. M. and Vaidya, A. M., Twin-core packed-bed reactors for organic-phase enzymatic esterification with water activity control, App. Microbiol. Biotechnol., 44, 283-286, 1995. 

Table 8-4. Selected salt pairs found u seful for water activity control in biocatalysis. ...

Many chemists have adopted the direct addition of salt hydrates as a simple method of water activity control. However, it does require a little thought and care to make sure the desired water activity is really produced. In particular, it must be ensured that both solid salt forms really will be present at equilibrium. It is best to estimate a water budget for the system, to ensure that enough of the right salt forms are being added. Table 8-5 shows an example of this, for a system made up of... 

Berberich et al. used salt hydrate pairs to control water activity in [BMIM][PF6]. The results were in good agreement with that obtained for water activity control using saturated salt solutions. The advantage of pre-equilibration is that the contact of the enzyme with the used salt and thus enzyme deactivation can be avoided. On the other hand it is only applicable for initial rate measurements. This disadvantage can be overcome by controlling water activity with salt hydrate pairs. Berberich et al. measured initial rate - water activities for the transesterification reaction of methyl methacrylate with 2-ethylhexanol in either hexane or [BMIM][PF6]. Both reaction systems gave similar profiles [72],... 

Plasmaphan HF/single-pass recycle Esterification of dodecanol and decanoic acid with in situ water activity control [119]... 

Wehtje, E., Kaur, J., Adlercreutz, P., Chand, S., Mattiasson, B. (1997). Water activity control in enz)Tnatic esterification processes. Enzyme and Microbial Techrwlogy, 21, 502—510. 

Han, JJ and Yamane, T (1999) Enhancement of both reaction yield and rate of synthesis of structured triacylglycerol containing eicosapentaenoic acid under vacuum with water activity control. Lipids, 34, 989-995. 

Kim J, Kim BG. 2000. Lipase-catalyzed synthesis of lysophosphatidylcholine using organic cosolvent for in situ water activity control. J Am Oil Chem Soc 77 791-797. 

Water activity control needed for processes involving condensation reactions. 

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