Calculating the Weight of Desiccant or Moistener to Adjust a Dry Mixture's Water Activity

Using Excel*

This Microsoft Excel workbook file takes an entered equilibrium water activity value, 'New (final or target) Aw', of a dry multicomponent food or pharmaceutical mixture stored in a hermetically sealed container and computes the water activity decrease or increase from the initial Aw including the addition of a single desiccant or moistener component to the mixture. It is assumed that the ingredients do not interact chemically, that the temperature is constant and that the amount of moisture absorbed by or released to the atmosphere trapped in the container is negligible. The model uses from 2 (minimum) to 15 (maximum) ingredients' moisture sorption isotherm equations, each one of which may or may not be the same as the equation of another ingredient. Input moisture values should be entered on a wet basis and will be converted to a dry basis for calculation and plotting.

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Download the Demonstration Excel File

The demonstration worksheet is available in file formats for Windows Excel 2003 (also runs in Excel 2007 and Excel 2010) and MacOS Excel 2004 (also runs in Excel 2011). If you have a modern web browser you should be able to download a binary Excel file directly by clicking on one of the links below.

For Windows:
Excel binary file (336K) for Windows Excel 2003, 2007 and 2010: Desiccants and Moisteners.xls

For MacOS:
Excel (283K) for MacOS Excel 2004 and 2011: Desiccants and Moisteners Mac.xls

Opening the Excel File

You should be able to open the downloaded workbook file in Microsoft Excel by double-clicking on its icon. Alternatively, you can open the workbook file in Excel by first starting Excel, choosing Open from the File menu and then selecting the Excel file that was downloaded. If the file does not appear on the list of file names in Excel for Windows, set the 'Files of type:' pop-up menu to 'All Files (*.*)'. When you are shown a dialog box asking about enabling macros, click the 'Enable Macros' button. The workbook contains VBA (Visual Basic for Applications) macros that need to run in order to obtain results. In Windows Excel 2003 and MacOS Excel 2004 and 2011 you must therefore click the 'Enable Macros' button when opening the file. In Windows Excel 2007 you must enable the marcos if they have been disabled for security purposes by clicking on the 'Options...' button next to the Security Warning then clicking the 'Enable this content' button and then the 'OK' button. In Windows Excel 2010 you must enable the macros if they have been disabled by clicking the 'Enable Content' button. Note that the macros run approximately ten times slower in MacOS (typically taking about 30 seconds) than in Windows (typically taking about 3 seconds).

Worksheet Comment Notes

The Desiccants and Moisteners Excel file contains two worksheets: Sheet1 and Sheet2. Sheet1 contains instructions, an embedded chart, colored user-input cells and a button labeled 'Update Workbook'. Sheet2 is used only for settings and data for the chart on Sheet1. Neither sheet is protected which means you may change the contents of any cell so be careful where you click and what you enter. Cells in either sheet that have a red triangle marker in their upper right corner have a comment note attached to them. Click here to see the text of all comment notes from both sheets. If a cell contains the name of one or more cells below it, the note explains how those cells are used. In Windows or MacOS Excel the note pops up automatically when you use the mouse to point to a marked cell. To edit or change the size of a note click on a marked cell to select it, then choose Edit Comment from the Insert menu. To see the VBA macro code chose Macro> Visual Basic Editor from the Tools menu then click on the View Code button in the Projects window if the code window is not visible. To return to Excel, choose 'Close and Return to Microsoft Excel' from the Excel menu. Making modifications to formulas on either sheet or to any of the Visual Basic macro code may cause the macros to stop working properly.

How to Run the Workbook

Instructions on how to run the workbook are also found within the file in columns A and B of Sheet1 and in comment notes attached to cells having a red triangle in their upper-right corner. All comment notes are also collected together on this page. Locate the worksheet cell labeled 'Aw (in equations)' (in cell C1, colored light turquoise). If you type a water activity value from 0.00 to 0.99 into that cell and then enter it (by pressing Enter, Tab, an arrow key or by clicking the 'Enter' checkmark button or clicking on any other cell) the cells in column G ('Equations') that contain a moisture sorption isotherm equation formula will show the percent moisture of that ingredient, desiccant or moistener at the entered Aw value. In the 'New (final or target) Aw' cell below it (cell C2, colored light turquoise) you should enter the new (final or target) water activity value that you want the mixture to achieve at equlibrium. The vertical dashed gray New Aw line will move to the entered New Aw location shown on the x-axis of the moisture sorption isotherms chart. Cell E20 labeled 'Delta Water (g)' (colored sea green) will immediately show the total amount of water that needs to be added to (if it is a positive value) or removed from (if it is a negative value) the mixture to allow it to reach equilibrium at the entered New Aw value. In colunm C enter the Initial Aw value of each ingredient in the mixture (up to a maximum of 15 ingredients) into the variously colored cells C4 to C18 and of each of 3 possible desiccant (in cells C22 to C24) and moistener (in cells C25 to C27) components each of which is considered to be included alone (i.e., not in combination with any other desiccant or moistener) in the mixture. If the mixture is in equilibrium at the start, set the Initial Aw cell of each ingredient to the same equilibrium Aw value. The Initial Aw of the desiccant cells C22 to C24 should typically be set to 0.00. The Initial Aw of the moistener cells C25 to C27 should be set to values greater than the value entered in the New Aw cell C2. To remove an ingredient, desiccant or moistener from the model, from both sheets and from the chart enter a negative value in its 'Initial Aw' cell of column C. Enter a name for each ingredient, desiccant or moistener included in the mixture in column D (cels D4 to D18 and D22 to D27) by typing its name in place of the color name found in those colored cells. Enter the initial wet mass in grams (greater than or equal to 0, including the water) for each ingredient in the mixture into the 'Initial Wet Mass (g)' colored cells E4 to E18. Enter the percent moisture on a wet basis (a number from 0 to 100) of each ingredient in the mixture into the '% Moisture (wb)' colored cells F4 to F18. Enter the moisture sorption isotherm equation of each ingredient into the 'Equation' colored cells G4 to G18 as an Excel formula (i.e., beginning with an '=' character, entering one ingredient's equation per cell) and into the light turquoise colored cells G22 to G25 for desiccants and G25 to G27 for moisteners. After the 'UpdateWorkbook' macro is run, other computation cells of Sheet1 and the chart will be updated to reflect any newly entered values and Sheet2 will contain new data for the chart. Column A of Sheet2 contains some cells where you can enter values that affect the appearance of the chart on Sheet1.

To update the model and the chart on Sheet1, you need to run a VBA macro named 'UpdateWorkbook'. Do this by clicking on the 'Update Workbook' button. (The macro can also be run by selecting Macro> Macros... from Excel's Tools menu which brings up the Macro dialog box. Click on 'UpdateWorkbook' to highlight it in the list of defined macros then just click the Run button to run it.) After the macro has run, if the value of Delta Water in cell E20 is negative, the three yellow-orange colored cells E22 to E24 will show the number of grams of each desiccant that needs to be added to the mixture to achieve the New (final or target) Aw value entered in cell C2, and the three bluish colored cells E25 to E27 will show 0.00. If the value of Delta Water in cell E20 is positive, the three bluish colored cells E25 to E27 will show the number of grams of each moistener that needs to be added to the mixture to achieve the New (final or target) Aw value entered in cell C2, and the three yellow-orange colored cells E22 to E24 will show 0.00.


Iglesias, H.A. and Chirife, J. 1982. Handbook of food isotherms: water sorption parameters for food and food components. Academic Press, New York.

Peleg, M. and Normand, M. D. l992. Estimation of the water activity of multicomponent dry mixtures. Trends Food Sci. & Technol., 3:l57-l60.

Peleg, M. l993. Assessment of a semi-empirical four parameter general model for sigmoid moisture sorption isotherms. J. Food Proc. Engng., l6:2l-37.

Wolf, W., Spiess, W.E.L. and Jung, G. 1985. Sorption isotherms and water activity of food materials. Elsevier, New York.

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Content last updated: April 27, 2012