1. WATER AND WATER ACTIVITY
OBJECTIVES
1. To understand what is bound water;
2. To learn what is water activity;
3. Importance of water activity for food chemistry and food technology;
4. Influence of water activity on major processes in foodstuffs.

2. WATER AND WATER ACTIVITY WATER – a matter of life and death1
WATER – a matter of life and death
Essential for the life on Earth:
- as medium for development of organisms
- as solvent
- as regent in chemical and biochemical reactions
- as cooling substance
- as carrier of nutrients and wastes
- as lubricant and plasticizer
- as stabilizer of biopolymer conformation

3. WATER AND WATER ACTIVITY2
Product
WATER, %
Meat pork
- beef
- chicken
~ 74
- fish
Fruits strawberry, cherry, pear, banana
- apple, peach
- orange, grapefruit
Vegetable green pea
- potato, carrot, red beet, broccoli
- cabbage, tomato, lettuce, green bean, spinach
Beverages beer 90
- juices 87
- whiskey, ouzo
- milk
Others cheese 37
- white bread 35
- salami 30
- honey 20
- dry fruits 18
- butter, shortening (margarine) 16
- flour 12
- dry milk 4

4. WATER AND WATER ACTIVITY Structure and physical properties3
Structure and physical properties
Hydrogen bonds formation with
neighbors water molecules (or other subst.)
Н-bond energy – kJ/mol

5. WATER AND WATER ACTIVITY Structure and physical properties4
Structure and physical properties
Extended structure of ice – hexagonal lattice
Ice density at 0оС (0,9168 g/cm-3)
and density of the liquid water
in it - (0,9998 g/cm-3)
The highest density water has at
3,98 оС.

6. WATER AND WATER ACTIVITY Interactions of water with solutes!
WATER AND WATER ACTIVITY 5
Interactions of water with solutes
Macroscopic level (hydration, water-binding capacity)
Molecular level (bound water)
- Bound water is that which does not freeze at some arbitrary low temperature (usually -40°C or lower).
- Bound water is that which is unavailable as a solvent for additional solutes.
- Bound water is that which moves with a macromolecule in experiments involving sedimentation rates, viscosity, or diffusion.
- Bound water is that which exists in the vicinity of solutes and other nonaqueous substances and has properties differing significantly from those of “bulk” water in the same system.

7. WATER AND WATER ACTIVITY Interactions of water with solutes!
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Interactions of water with solutes
Type
Example
Strength of interaction compared to water-water hydrogen bonda
Dipole-ion
Water-free ion
Water-charged group on organic
molecule
Greaterb
Dipole-dipole
Water-protein NH
Water-protein CO
Water-sidechain OH
Approx. equal
Hydrophobic hydration
Water + Rc - R(hydrated)
Much less
a About kJ/mol.
b But much weaker than strength of single covalent bond.
c R is alkyl group.

8. WATER AND WATER ACTIVITY Interactions of water with solutes7
Interactions of water with solutes
1. Interaction with ions and ionic groups
Ion – ion (charged groups);
Ion – dipole;
H-bonds between water molecules
Likely arrangement of water molecules
adjacent to sodium chloride. Only water molecules in plane of paper are shown

9. WATER AND WATER ACTIVITY Interactions of water with solutes8
Interactions of water with solutes
2. Interaction with neutral groups (having H-bonding capabilities
Hydrogen bonding (dotted lines) of
water to two kinds of functional groups
occurring in proteins.
dipole – dipole interactions
Examples of a three-molecule water bridge in papain; 23, 24, and 25 - water molecules.
(From Berendsen, H. J. C. (1975). Specific interactions of water with biopolymers, in Water—A Comprehensive Treatise, vol. 5 (F. Franks, ed.), Plenum Press, New York, pp. 293–349.)

10. WATER AND WATER ACTIVITY Interactions of water with solutes9
Interactions of water with solutes
3. Interaction with nonpolar substances
Schematic depiction of a globular protein undergoing hydrophobic interaction. Open circles are hydrophobic groups, “L-shaped” entities around circles are water molecules oriented in accordance with a hydrophobic
surface, and dots represent water molecules associated with polar groups.
Proposed water orientation at a
hydrophobic surface.
(Adapted from Lewin, S. (1974). Displacement of Water and Its Control of Biochemical Reactions, Academic Press, London.)

11. WATER AND WATER ACTIVITY!
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Water activity (RVP)
aw = Pw / Pºw
where Pw – partial pressure of water vapors above solution or product
Pºw – partial pressure of water vapors above pure water at
the same temperature

12. WATER AND WATER ACTIVITY11
Water activity (RVP)
Product aw
% water
water 1
100
meat
53-75
milk
0.97 85
Fruit juice
0.95 – 0.97 87
Cheese
35-37
Bacon
< 0.85 -
jam
0.82 – 0.94
saturated solution of NaCl
0.75
room air
honey 20
dry fruits 18

13. WATER AND WATER ACTIVITY!
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Water activity (RVP)
Moisture sorption isotherm
(according to Labuza, T. P., Kinetics of lipidoxidation in foods. Crit. Rev. Food Technol. 2, 355 (1971)).
a Food with high moisture content;
b Food with low moisture content

14. Water activity (RVP) – moisture sorption isotherms
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Water activity (RVP) – moisture sorption isotherms
Information derived from MSIs are useful :
1). for concentration and dehydration processes, because the ease or difficulty
of water removal is related to aw;
2) for formulating food mixtures so as to avoid moisture transfer among the
Ingredients;
4) to determine what moisture content will curtail growth of microorganisms of interest;
5) to predict the chemical and physical stability of food as a function of water content.

15. ! Water activity (RVP) – moisture sorption isotherms
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Water activity (RVP) – moisture sorption isotherms
Values of monomolecular layer of water for
different dehydrated products –
bound water (inaccessible microorganisms)
Product
g water/ 100 g product
Dehydrated potatoes
5 - 8
Dehydrated green been
4 - 6
Dehydrated onion 4
Dehydrated biscuits
Dehydrated milk powder 2
Dehydrated beef
Generalized moisture sorption
isotherm for the low-moisture segment of a food
Layers of water on food surface

16. WATER AND WATER ACTIVITY
Water activity (RVP)
Values of water BET monolayer value of a food provides a good first estimate of the
water content providing maximum stability of a dry product
BET equation
𝑎 𝑤 𝑚(1− 𝑎 𝑤 ) = 1 𝑚 𝑖 .𝑐 + 𝐶 −1 𝑚 𝑖 .𝑐 . 𝑎 𝑤
p/p0 of 0.2
BET plot for native potato starch (resorption data, 20°C).
Layers of water on food surface

17. ! Water activity (RVP) – moisture sorption isotherms
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Water activity (RVP) – moisture sorption isotherms
Product aw
water 1
meet
milk
0.97
juice
cheeses
bacon
< 0.85
jams
0.82 – 0.94
saturated solution of NaCl
0.75
room air
honey
dry fruits
microorganism aw
Clostridium botulinum
0.97
Pseudomonas fluorescens
Escherichia coli
0.95
Clostridium perfringens
Salmonella
Vibrio cholerae
Clostridium botulinum A, B
Bacillus cereus
0.93
Listeria monocytogenes
0.92
Bacillus subtilis
0.91
Staphylococcus aureus
0.87
Most of the molds (fungi)
0.70
No development
0.60

18. Relative Vapor Pressure and Growth of Microorganisms in Food
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Relative Vapor Pressure and Growth of Microorganisms in Food
Range of aw
Microorganisms generally inhibited by lowest p/p 0 in this range
Foods generally within this range
1.00–0.95
Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens, some yeasts
Highly perishable (fresh) foods and canned fruits, vegetables, meat, fish and milk; cooked sausages and breads; foods containing up to approximately 40% (w/w) sucrose or 7% sodium chloride
0.95–0.91
Salmonella, Vibrio parahaemolyticus, C. botulinum, Serratia, Lactobacillus, Pediococcus, some molds, yeasts (Rhodotorula, Pichia)
Some cheeses (Cheddar, Swiss, Muenster, Provolone), cured meat (ham), some fruit juice concentrates; foods containing up to 55% (w/w) sucrose or 12% NaCl
0.91–0.87
Many yeasts( Candida, Torulopsis, Hansenula), Micrococcus
Fermented sausage(salami), sponge cakes, dry cheeses, margarine; foods containing up to 65% (w/w) sucrose (saturated) or 15% NaCl
0.87–0.80
Most molds (mycotoxigenic
penicillia), Staphylococcus aureus,
most Saccharomyces (bailii spp.),
Debaryomyces
Most fruit juice concentrates, sweetened condensed milk, chocolate syrup, maple and fruit syrups; flour, rice, pulses containing 15–17% moisture; fruit cake; country-style ham, fondants, high-ratio cakes

19. Relative Vapor Pressure and Growth of Microorganisms in Food
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Relative Vapor Pressure and Growth of Microorganisms in Food
Range of aw
Microorganisms generally inhibited by lowest p/p 0 in this range
Foods generally within this range
0.80–0.75
Most halophilic bacteria, mycotoxigenic asperg illi
Jam, marmalade, marzipan, glacé fruits, some marshmallows
0.75–0.65
Xerophilic molds( Aspergillus
chevalieri, A. candidus, Wallemia
sebi), Saccharomyces bisporus
Rolled oats containing approximately 10% moisture; grained nougats, fudge, marshmallows, jelly, molasses, raw cane sugar, some dried fruits, nuts
0.65–0.60
Osmophilic yeasts( Saccharomyces
rouxii) ,few molds( Aspergillus
echinulatus, Monascus bisporus)
Dried fruits containing 15–20% moisture; some toffees and caramels; honey
0.50
No microbial proliferation
Pasta containing approximately 12% moisture; spices (10% moisture)
0.40
Whole egg powder (5% moisture)
0.30
Cookies,crackers, bread crusts(3–5% moist.)
0.20
Whole milk powder containing 2–3% moisture; dried vegetables app. 5% moisture;
corn flakes containing approximately 5% moisture;country style cookies, crackers

20. ! Water activity (RVP) – moisture sorption isotherms
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Water activity (RVP) – moisture sorption isotherms
Speed of deterioration of foods and speed of some important reactions
depending of water activity
(according to T.P. Labuza, Sorption phenomena in foods: Theoretical and practical aspects. In “Theory, determination and
control of physical properties of food materials”, C. K. RHA ed., p.197, Riedel, Dordreht, 1975)

21. Water activity (RVP) – influence of the physical state
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Water activity (RVP) – influence of the physical state
MSI of sucrose – amorphous and crystalline state
Transition from amorphous to
crystalline – release of solvent and volatiles

22. Water activity (RVP) – influence of the physical state
WATER AND WATER ACTIVITY
19a
Water activity (RVP) – influence of the physical state
In addition to chemical reactions and microbial growth, p/p0 also influences the texture of dry and semidry foods. For example, suitably low RVPs are necessary if crispness of crackers, popped corn, and potato chips is to be retained; if caking of granulated sugar, dry milk, and instant coffee is to be avoided; and if stickiness of hard candy is to be prevented [*]. The
maximum p/p0 that can be tolerated in dry materials without incurring loss of desirable properties ranges from 0.35 to 0.5, depending on the product. Furthermore, suitably high water activities of soft-textured foods are needed to avoid undesirable hardness.
* Labuza, T. P., and R. Contreras-Medellin (1981). Prediction of moisture protection
requirements for foods. Cereal Foods World 26(7):335–344.

23. Molecular mobility and food stability. Glass transition temperature
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Molecular mobility and food stability. Glass transition temperature
State diagram of a binary system. Assumptions: maximal freeze concentration, no solute crystallization, constant pressure, no time dependence. Tmɤ is the melting point curve, TE is the eutectic point, Tms is the solubility curve Tg is the glass transition curve, and Tg’ is the solute-specific glass transition temperature of a maximally freeze concentrated solution. Heavy dashed lines represent conditions of metastable equilibrium.

24. Molecular mobility and food stability. Glass transition temperature
WATER AND WATER ACTIVITY 21
Molecular mobility and food stability. Glass transition temperature
State diagram of a binary system showing stabilities of diffusion-dependent properties of food. It is assumed that the “Tg’ zone” is the appropriate Tg for frozen foods, but this is still a matter of some disagreement.

25. Molecular mobility and food stability. Glass transition temperature
WATER AND WATER ACTIVITY 22
Molecular mobility and food stability. Glass transition temperature
State diag ram of a binary system showing possible paths for freezing (unstable sequence ABCDE; stable sequence ABCDETg’F, drying (unstable sequence AHIJK; stable sequence AHIJLG) and freeze drying (unstable sequence ABCDEG; stable sequence ABCDETg’FG.

26. Combined methods approach to food stability and safety
WATER AND WATER ACTIVITY 23
Combined methods approach
to food stability and safety