Question 1
How does water vapor partition from
a liquid into the surrounding gas?
H2O g
H2 Og
H2Og
H2Ol
H2Ol
H2Ol
H2Ol
H2Ol
H2 Ol
Vapor Pressure of Pure Water
Vapor pressure /atm
1
0.8
0.6
0.4
0.2
0
0
20
40
60
Temperature/ oC
80
100
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
How does water vapor partition from
a solution into the surrounding gas?
H2O g
H2 Og
H2Og
H2Ol solute
H2Ol
solute
solute H O
2 l
H2 Ol
solute
Vapor pressure of water
Vapor Pressure above a Solution
p0
1
0
Mole fraction of water
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
Liquid phase
concentration
=solubility
limit
Vapor pressure of water
Saturated Solutions
Solubility limit
p0
1
0
Mole fraction of water
Question 3
How does the partitioning of water
vapor from a solution depend on
solution composition?
Vapor pressure of water
p0
1
0
Mole fraction of water
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
What happens when two different
solutions are placed in the same
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
aw=p/po~%ERH
Partial pressure of water above the
solution normalized to the partial
pressure above pure water.
Question 5
How do these analogies translate to
food?
The typical water activity of some foodstuffs
Type of product
Water Activity (aw)
Fresh meat and fish
.99
Bread
.95
Aged cheddar
.85
Jams and jellies
.8
Plum pudding
.8
Dried fruit
.6
Cookies
.3
Milk powder
.2
Instant coffee
.2
Undissolved solute
Moisture content (d.w.b.)
Moisture Sorption Isotherm
aw
Moisture content (d.w.b.)
Moisture Sorption Isotherm
aw
Zone 1
Moisture content (d.w.b.)
Moisture Sorption Isotherm
Zone 2
Zone 3
aw
Moisture content (d.w.b.)
Temperature Dependency
cold
hot
aw
Moisture content (d.w.b.)
Sorption and Desorption
desorption
sorption
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
• Zone 3: Bulk water
• Zone 2: Loosely bound water
• Zone 1: Tightly bound water.
log (RATE)
Moisture content (d.w.b.)
Reaction Rates and Water
Activity
Microbial growth
aw
Rate of Oxidation of Potato
Chips
Relative rate constant
100
10
1
Monolayer moisture
0.1
0
0.2
0.4
aw
0.6
0.8
The GAB Model
Moisture content
(dwb)
m0Ckaw
M
(1  kaw )(1  kaw  Ckaw )
mo monolayer value
K multilayer parameter
C temperature dependency parameter
0.15
0.1
0.05
0
0
0.5
aw
1
Moisture content (d.w.b.)
Texture Changes
Crispy/crunchy
Soft
0.2-0.5
Moisture content (d.w.b.)
Powder Changes
Free flowing
Agglomerated
~0.4
Dynamics of Moisture Exchange
Moisture content (d.w.b.)
Moisture Sorption Isotherm
aw
Moisture content (d.w.b.)
Moisture Sorption Isotherm
aw
Multicomponent Foods
•
•
•
•
•
Cheese and crackers
Baked products and filling
Cereal and fruit
Yogurt and cereal
Ice cream and cone
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.
Macaroni into 2-cup
microwavable cereal bowl. Add
2/3 cup water.
Macaroni and water,
uncovered, on HIGH
3-1/2 to 4 minutes or
until Macaroni is
tender. DO NOT
DRAIN. Some water
remaining in bowl is
desirable and
necessary to make
cheese sauce.
CAUTION: Bowl will be
Very Hot.
Cheese Sauce Mix; mix well. If
cheese sauce appears thin, do not put
back in microwave. Cheese sauce
will thicken upon standing.
Raisin Bran
Kellogg's®
Whole wheat, raisins, wheat bran, sugar,
high fructose corn syrup, salt, malt
flavoring, niacinamide, reduced iron, zinc
oxide, pyridoxine hydrochloride (vitamin
B6), riboflavin (vitamin B2), thiamin
hydrochloride (vitamin B1), vitamin A
palmitate, folic acid, vitamin B12 and
vitamin D.
Shelf Life 12 Months
Did you know results from a recent in-home taste test with
raisin bran users - like you - showed that our flakes are
crispier than Kellogg's® Raisin Bran's and stay crispier longer
in milk?
Humectants
• e.g.: sucrose, propylene glycol, glycerol
• Be careful of:
–
–
–
–
–
Solubility, MW
Flavor
Crystallization on storage
Chemical reactivity
Toxicity