Water activity logic

Question 1: 

Question 1 How does water vapor partition from a liquid into the surrounding gas?

Slide2: 

H2Ol H2Ol H2Ol H2Ol H2Og H2Ol H2Ol H2Og H2Og

Vapor Pressure of Pure Water: 

Vapor Pressure of Pure Water

Vapor Phase above Water: 

Vapor Phase above Water Water will evaporate until the moisture content reaches a temperature dependant equilibrium. Or in an open container, eventually all of the water will evaporate

Question 2: 

Question 2 How does water vapor partition from a solution into the surrounding gas?

Slide6: 

H2Ol H2Ol H2Ol H2Ol H2Og solute H2Og H2Og solute solute solute

Vapor Pressure above a Solution: 

Vapor Pressure above a Solution Mole fraction of water 1 Vapor pressure of water p0 0 IDEAL

Vapor above a Solution: 

Vapor above a Solution A solution will lose moisture to the atmosphere The partial pressure of water above the solution is reduced by the presence of solute Vapor pressure depends on concentration of solute as well as temperature

Saturated Solutions: 

Saturated Solutions Mole fraction of water 1 Vapor pressure of water p0 0 Solubility limit Liquid phase concentration=solubility limit

Question 3: 

Question 3 How does the partitioning of water vapor from a solution depend on solution composition?

Slide11: 

Mole fraction of water 1 Vapor pressure of water p0 0 IDEAL

Vapor Depends on Solution Composition: 

Vapor Depends on Solution Composition Non-ideal properties of solutions mean some have a greater affinity for water than others. The vapor pressure will always be less than above pure water but not necessarily the same over all solutions Different saturated solutions will have different partial pressures of water

Question 4: 

Question 4 What happens when two different solutions are placed in the same container?

Two solutions/One container: 

Two solutions/One container Both solutions will exchange water with the atmosphere The whole system will finally come to equilibrium The equilibrium concentration of both solutions will be such that they are in equilibrium with the atmosphere. The moisture content need not be the same. The partial pressure will be.

Water Activity: 

Water Activity aw=p/po~%ERH Partial pressure of water above the solution normalized to the partial pressure above pure water.

Question 5: 

Question 5 How do these analogies translate to food?

Slide19: 

Undissolved solute

Moisture Sorption Isotherm: 

Moisture Sorption Isotherm aw Moisture content (d.w.b.)

Slide22: 

Moisture Sorption Isotherm aw Moisture content (d.w.b.)

Slide23: 

Moisture Sorption Isotherm

Slide24: 

Temperature Dependency aw Moisture content (d.w.b.) cold hot

Sorption and Desorption: 

Moisture content (d.w.b.) Sorption and Desorption desorption sorption

Moisture Sorption Isotherms: 

Moisture Sorption Isotherms Highly product specific (physical and chemical structure) Highly temperature dependant Show sorption/desorption hysteresis Affect both physical/chemical reactivity of the food and the dynamics of water transport

Zones in Isotherms: 

Zones in Isotherms Zone 3: Bulk water Zone 2: Loosely bound water Zone 1: Tightly bound water.

Reaction Rates and Water Activity: 

Reaction Rates and Water Activity

Rate of Oxidation of Potato Chips: 

Rate of Oxidation of Potato Chips Monolayer moisture

The GAB Model: 

The GAB Model mo monolayer value K multilayer parameter C temperature dependency parameter

Texture Changes: 

Texture Changes Moisture content (d.w.b.) Soft Crispy/crunchy 0.2-0.5

Slide32: 

Powder Changes Moisture content (d.w.b.) Agglomerated Free flowing ~0.4

Slide33: 

Dynamics of Moisture Exchange

Slide34: 

aw Moisture content (d.w.b.) Moisture Sorption Isotherm

Slide35: 

aw Moisture content (d.w.b.) Moisture Sorption Isotherm

Multicomponent Foods: 

Multicomponent Foods Cheese and crackers Baked products and filling Cereal and fruit Yogurt and cereal Ice cream and cone

PowerBar: 

PowerBar INGREDIENTS: High Fructose Corn Syrup With Grape And Pear Juice Concentrate, Maltodextrin, Raisins, Milk Protein Isolate, Whole Oats, Oat Bran, Rice Crisps (Milled Rice, Rice Bran), Brown Rice, Almond Butter, Glycerin, Natural Flavors, Spices MINERALS: Calcium Phosphate, Magnesium Carbonate, Zinc Gluconate, Iron (Ferrous Fumarate), Copper Gluconate, Chromium Aspartat VITAMINS: Vitamin C (Ascorbic Acid), Vitamin E Acetate, Vitamin B3 (Niacin), Pantothenic Acid (Calcium Pantothenate), Vitamin B6 (Pyridoxine Hydrochloride), Vitamin B2 (Riboflavin), Vitamin B1 (Thiamin Hydrochloride), Folic Acid, Biotin, Vitamin B12, ESSENTIAL AMINO ACIDS: Leucine, Valine, Isoleucine. 

Slide38: 

Cheese Sauce Mix; mix well. If cheese sauce appears thin, do not put back in microwave. Cheese sauce will thicken upon standing.

Raisin Bran: 

Raisin Bran Shelf Life 12 Months Kellogg's®

Humectants: 

Humectants e.g.: sucrose, propylene glycol, glycerol Be careful of: Solubility, MW Flavor Crystallization on storage Chemical reactivity Toxicity