You can measure the aw by moisture sorption isotherm study ..........

U can also use moisture analyser, a machine which gives the moisture content of a food in minutes.....

There is not any closed mathematical function between these two value, so you can not calculate the water activity even if you have measured the water content. The function is the moisture sorption isotherm, which you can use after measuring the water activity as the function of the water content of every different food, regarding the difference of water binding capacity of different food components and structures.

Dear, you need a moisture analyser like this http://www.aqualab.com/products/?utm_source=decagon&utm_medium=redirect&utm_campaign=awproductgroup

Dear, you need an aw meter like this http://www.aqualab.com/products/?utm_source=decagon&utm_medium=redirect&utm_campaign=awproductgroup

Dear, you need an aw meter.

http://www.aqualab.com/products/?utm_source=decagon&utm_medium=redirect&utm_campaign=awproductgroup

Dear Sekar, see:Water Properties in Food, Health, Pharmaceutical and Biological Systems, ISOPOW 10, Chapter 30, Weglarz et al, Molecular Mobility Interpretation of Water Sorption Isotherms of Food Materials by Means of Gravimetric NMR and Witek, et al, The structural and hydration properties of heat-treated rice studied at multiple length scales, Food Chemistry 120 (2010) 1031-1040.

You obviously need help in understanding the principles 1stly a hydrometer is not an aw instrument although the density of a pure solute (e.g. sugar or NaCl) plus water can be correlated with water activity. If you took your food system and exposed it to different humidities over salt solutions you can generate a moisture sorption isotherm Lecture notes and readings for my course (FScN 4342) are available ( http://www.ardilla.umn.edu/Ted_Labuza/FScN4342-08/default.html ) Also my workbook for isotherms can be found at

http://www.ardilla.umn.edu/Ted_Labuza/tpl-books-main.html or at amazon

http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Dstripbooks&field-keywords=Labuza

The key is that the actual aw of a complex system is hard to predict without isotherms or each ingredient

There is a particular relationship between moisture content and water activity of a food materials. I think , from the similar material's curve of water activity and water content you can predict your one,although it will not be exact one .

The concept of water activity was developed because things like susceptibility to mold and bacterial growth are not dependent on moisture content alone. The amount of water which is available for microbial growth depends on the ability of the other components in a material to immobilize water. Therefore the water activity cannot be calculated from the moisture content, it must be measured independently.

Water content and water activity are two different parameters. You can measure water activity by measuring water sorption isotherm. The method involves determining water uptake by samples kept in atmosphere of known relative humidities, which can be managed by using designated salts. We have used it for many ingredients and products. It works well, though should be done properly with all precautions and with a good balance. I have enclosed two publications, which have used such experiments.

Look for these publications,

Sheetal Gupta and Jamuna Prakash. Utilization of micronutrient rich dehydrated green leafy vegetables in formulation of traditional products. Advances in Food Science. 33. (1): 34-43. and

Nagaprabha P. and Jamuna Prakash. Development and acceptability of green gram based instant ‘dosai’ mix. Journal of Food Science and Technology. 46 (5): 418-422.

You can get water activity from sorption nature of the product.

Dear Sekar Ramarajan ,

If you know the composition of your food product, and its water content, you can find an Excel spreadsheet at my Researchgate.net page which may be able to calculate the water activity.

1) If the food is a (multi-component) aqueous solution, the spreadsheet already contains the aW constants by Raoult, Norrish, Ross, Samapundo, Favetto, Pitzer, Labuza, Chirife and Baeza for some 70 different, listed food components.

https://www.researchgate.net/publication/292657199_Water_activity_aW_calculations_21_of_aqueous_solutions_and_of_solids_mixturesxlsx_-_Raoult-Norrish-Ross_Samapunto-Favetto-Pitzer_and_Labuza-Chirife-Baeza

Go to tab page 3 in the spreadsheet; your input then should be: the total mass of water and the total mass of each of the listed components. The spreadsheet next calculates the total aW of the multi-component solution.

Several worked examples explain how to use the spreadsheet.

2) If the food is a solid food product, or a mixture of solid food products, the spreadsheet can only be used if you have available the sorption isotherms of each of the solid food components.

Data Water activity aW calculations (2.1) of aqueous solutions an...