Report of Group 1Moisture Sorption Isotherm, water activity and Food preservationGroup members:Pham Huynh Khanh NamNguyen Khoi Le Tran Quoc ThangContents:Properties of WaterMoisture sorption IsothermHysteresisWater activity and Food preservation

1. Properties of waterHeat capacity of waterWater as high boiling point because of Hydrogen bonds between water moleculesDipole and hydrogen bondsOxygen has high electronegativity so it will pull electrons towards it and become more negatively charged, while Hydrogen is positively charged, also the angles formed by oxygen and hydrogen is 104.5 Water can be a solventMany substances can dissolve in water, these are hydrophilic substance, this is because water has dipole effect, substances that cannot mix well with water are hydrophobic.

2. Solvation and dispersing action of water

Positively charged Ions such as Na interact with slight negatively charged Oxygen atoms in water molecule, water is the solvent and NaCl is the solute, the process of mixing a hydrophilic substance and water is called solvationMixing can occur without solvation and this is called dispersion. Eg of dispersion:Water interact with amphiphilic molecule and the mixture contains micellesEg of micelles:Vitamin A, DMilk3. MicelleWater interact with the hydrophilic heads while the hydrophobic tail is hidden away from water in the center of micelle, also micelle can be in the form of bilayer

4. Moisture sorption isothermsIt shows the relationship between water activity and moisture at a givenTemperature and represent moisture content at equilibrium for each water activity, it allows for predictions in changes of moisture content and its potentialeffect on water activityIf the temperature is altered, then the relationships can not be compared equivalentlyWater activity is very temperature dependent. Temperature changes water activity due to changes in water binding, dissociation of water, solubility of solutes in water, or the state of the matrix.The temperature dependence of water activity varies between substances. Some substances have increased water activity with increasing temperature, while others show a decrease with increasing temperature. Most high moisture foods have negligible change with temperature. One can therefore not predict even the direction of the change of water activity with temperature, since it depends on how temperature affects the factors that control water activity in the substance

5. HysteresisTraditionally based on capillary condensation theoryThe pore is allowed to be adsorbed by vapor (water) molecule, till complete filling of the pore volume and then the vapor (water) is taken out from the pore. The difference between adsorption and desorption is hysteresisEffects of temperature on Hysteresis:Hysteresis size (loop difference), decreases as temperature increases because increased temperature will increase the ability of molecules (vapor water) to diffuse in and out of the pores, therefore will help the system move toward the equilibriumEffects of water removed and Hysteresis:When too much water is desorbed, the water is taken out from the Bound water and hence the food structure changes

6. Summary of water activity, controling water activity and growth of micro organismWater activity is a ratio of the vapor pressure of the water in the substrate (p) to that of pure water at the same temperature (p0)

A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. The water activity increases with temperature.Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mold. The amount of available moisture can be reduced to a point which will inhibit the growth of the organisms.Quality and food security depends on pH and water activity (aw) in food environment. Foods with water activity can be detroyed. Water activity, pH, temperature, and other parameters, have a direct impact on the growth of micro-organisms. Free water that is available to molds, yeasts, and bacteria is responsible for their growth and even toxin production. Water activity (aw) has its most useful application in predicting the growth of bacteria, yeasts and moulds. It is necessary to control either its acidity level (pH) or the level of water activity (aw) or a suitable combination of the two. This can effectively increase the product's stability and make it possible to predict its shelf life under known ambient storage conditions.

References:http://www.fda.gov/iceci/inspections/inspectionguides/inspectiontechnicalguides/ucm072916.htmhttp://www.utm.md/meridian/2012/MI_4_2012/8.%20Art.%20Sandulachi%20E.%20Water.pdfhttp://watereducation.utah.gov/waterscience/Properties/default.asphttp://quizlet.com/27833620/introduction-to-food-science-the-nature-of-matter-lect-3-flash-cards/http://www.science.uwaterloo.ca/~cchieh/cact/applychem/hydration.htmlhttp://jun.bol.ucla.edu/illam/FOA7paper.pdf