question 1 how does water vapor partition from a liquid into the surrounding gas?
TRANSCRIPT
Question 1
How does water vapor partition from a liquid into the surrounding gas?
H2Ol
H2Ol H2Ol
H2Ol
H2Og
H2Ol
H2Ol
H2Og
H2Og
Vapor Pressure of Pure Water
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Temperature/ oC
Vap
or
pre
ssu
re /
atm
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?
H2Ol
H2Ol H2Ol
H2Ol
H2Og
solute
H2Og
H2Og
solutesolute
solute
Vapor Pressure above a Solution
Mole fraction of water
1
Vap
or p
ress
ure
of w
ater
p0
0
IDEAL
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
Mole fraction of water
1
Vap
or p
ress
ure
of w
ater
p0
0
Solubility limitLiquid phase concentration=solubility limit
Question 3
How does the partitioning of water vapor from a solution depend on
solution composition?
Mole fraction of water
1
Vap
or p
ress
ure
of w
ater
p0
0
IDEAL
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 Sorption Isotherm
aw
Moi
stur
e co
nten
t (d.
w.b
.)
Moisture Sorption Isotherm
aw
Moi
stur
e co
nten
t (d.
w.b
.)
Moisture Sorption Isotherm
aw
Moi
stur
e co
nten
t (d.
w.b
.)
Zone 3
Zone 2
Zon
e 1
Temperature Dependency
aw
Moi
stur
e co
nten
t (d.
w.b
.)
cold
hot
Moi
stur
e co
nten
t (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.
Reaction Rates and Water Activity
aw
Moi
stur
e co
nten
t (d.
w.b
.)
log
(RA
TE
)
Lipi
d ox
idat
ion
Mos
t rea
ctio
nsMicrobial growthSORPTION ISOTHERM
Rate of Oxidation of Potato Chips
0.1
1
10
100
0 0.2 0.4 0.6 0.8
aw
Rel
ativ
e ra
te c
onst
ant
Monolayer moisture
0
0.05
0.1
0.15
0 0.5 1
aw
Moi
stur
e co
nten
t (d
wb
)The GAB Model
Mm Cka
ka kaw Ckaw
w w
0
1 1( )( )
mo monolayer valueK multilayer parameterC temperature dependency parameter
Texture ChangesM
oist
ure
cont
ent (
d.w
.b.)
Soft
Crispy/crunchy
0.2-0.5
Powder ChangesM
oist
ure
cont
ent (
d.w
.b.)
Agglomerated
Free flowing
~0.4
aw
Moi
stur
e co
nten
t (d.
w.b
.)Moisture Sorption Isotherm
aw
Moi
stur
e co
nten
t (d.
w.b
.)Moisture Sorption Isotherm
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 AspartatVITAMINS: 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
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?
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
Kellogg's®
Humectants
• e.g.: sucrose, propylene glycol, glycerol
• Be careful of:– Solubility, MW– Flavor– Crystallization on storage– Chemical reactivity– Toxicity