Lecture 3: Water Activity

104.5°

Covalent bond that has a positive and negative end • O has larger nucleus and more protons than H, so shared electron pairs orbit more around oxygen • Oxygen end has slightly negative charge • Hydrogen has slightly positive charge

Negative oxygen from next molecule somewhat attracts the positive hydrogen end of first water molecule due to polarity. Explains a lot of characteristics of water • Surface tension - water molecules forming H-bonds, pulling molecules on outside edge towards them • High solubility - proteins, starches, and other sugars have a lot of OH groups, making it easy to form H-bonds with water. This helps the substance to stay distributed inside water • Why water has a relatively high BP - Have to break all the H-bonds before water can escape and turn to gas This is why water can dissolve so many substances in it Much weaker than covalent bond - eg: two people holding hands vs. an arm attached to body; less energy needed to break this bond

Proteins, starches, and other sugars - Anything with a lot of OH groups will readily form hydrogen bonds with water

B/c of H-bonding - when try to pack H2O into solid form, H wants to repel H and O wants to repel O (due to slight polarity); O and H try to form H-bonds. • When reaches equilibrium forms unique pyramid shape distance between oxygen molecules is smaller compared to liquid form • However, distance between water molecules is larger than in liquid; D=M/V (mass stays the same, but volume is larger, so is less dense) Closer O distance but is less dense

Ice: 109° Liquid: 104.5°

• Atmospheric pressure - Higher you go up, less pressure • Effect on BP - water molecules need to escape from liquid surface; if have less air pressing on it, easier for water to escape → Lower boiling point • Principle behind the pressure cooker - once water reaches boiling point, it cannot go any higher, just turns to gas; if want to cook >100°C you have to add pressure to increases bp of water - Can also add a lid - water vapor can’t escape, so builds up pressure, increasing BP to 108C • Impurities - (Salt, sugar considered impurities) when added to water begins forming H-bonds; so now you need more energy to boil the water to break the H-bonds first → BP increases with impurities in it

A lot of flavor, color and texture compounds dissolve more readily in water than steam

• Readily separates from foods that are sliced • Held inside cells or tiny tunnels • Most susceptible to bacterial growth

• Cannot be easily separated in foods • Is part of molecular structure - water slides between other molecules, forms H-bonds with water limiting water movement. Locked unless apply heat, acid, etc to break bond • Cannot react with other substances as free water • Freezes only at very low temp • Does not evaporate

(eg: coffee) Forms a stronger chemical bond (sometimes covalent), tightly locking water; Almost impossible to separate water from molecule

• Biological pollutants • Human & animal feces • Daily waste of organic compounds • Chemical contaminants • Acids – carbonic acids, sulfuric acid… • Toxic compounds • Metal ions - calcium & magnesium • Physical contaminants - garbage, diapers, cans, plastics

The driving force that determines how matter flows (high energy → low energy until all domains reach equilibrium/same amount of energy) • Chemical potential is movement from high energy to low energy; so if one domain has higher energy is has higher chemical potential • Water activity is the same as chemical potential, but just specific to water

• Is the same as chemical potential, just specific to water * It is what determines how water moves from one domain to another

Aw = Pwater above food/Ppure water • This is for in a closed space • Range of Aw is always from 0-1 (1 is pure water)

Cheese and crackers in a room example - water activity, NOT water content, determines how water moves around • Cracker gradually absorbs moisture from the air; stops at equilibrium point and won’t absorb any more (15% water) • Cheese loses moisture to air (not to the cracker); keeps losing water until it reaches a plateau (25%) • Air, cheese, and cracker all have water activity; by time each reaches equilibrium the water activity of cracker = water activity of cheese = water activity of air

Water activity is not determined by water content, but the two are correlated - the more water content generally the higher water activity; this is because it will have more free water

Mold (starts at aw 0.60, but most start to grow above 0.8) → yeast ( aw 0.88)→ bacteria (aw 0.91) in terms of first things to grow

Enzymatic activity happens from 0-1, but increases with higher activity

Non-enzymatic browning (Maillard reaction) - peaks around medium range of water activity; don’t see in very dry or very wet situation

Lipid oxidation - smiley-face shape in water activity; increases at low or high water activity, decreases in mid-range

• Reformulate to same aw while maintaining initial texture - use humectants • Slow down internal diffusion - eg: keep in fridge so transfer rate slows down • Edible barrier between domains - eg: coat of chocolate on surface • Use external barrier • Drying foods, using salts and or sugar