lec 4 water Flashcards

Question

The positive and negative ends of dipole creates what?

Answer 1

Forms a hydration sphere about one layer deep around the ion. The water in this hydration sphere is typically called “bound water”.

Answer 2

Free water can make up over 90% of water in a food, as in orange juice. It is available for microbial growth and chemical reactions and can be easily removed from the product by squeezing or pressing.

Answer 3

It refers to water molecules associated directly with food macromolecules. They are: -exchangeable, not covalently or H-bonded, -give structure to macromolecules, -do not support microbial growth or chemical reactions, -do not contribute to product deterioration. This water type is present even in dry foods, like powders.

Answer 4

Trapped water makes up about 5% of a food’s total water, -is associated with cellular structures, -cannot be easily pressed out, -can be removed by drying. It does not support microbial growth but provides a "moist" texture, as seen in dried fruits or vegetables

Answer 5

Bacterial growth stops below a water activity (aw) of 0.9.

Answer 6

Below 0.8, yeasts and bacteria stop growing; below 0.7, molds, yeasts, and bacteria stop growing.

Answer 7

Enzymatic reactions slow or stop at aw = 0.5, and chemical reactions slow or stop at aw = 0.3.

Answer 8

Salts and sugars strongly interact with water, while proteins do not interact as strongly.

Answer 9

Water can exist in gaseous, liquid, and solid states, unique among substances. Both temperature and pressure can change its state, useful in processes like evaporative drying and freeze drying

Answer 10

Freezing affects foods microbially (some survival), physically (ice formation), and chemically (cryo-concentration). Storage prevents microbial growth but can lead to recrystallization and slow chemical reactions, while thawing allows microbial growth, drip loss, and chemical decompartmentalization.

Answer 11

Water undergoes a liquid to solid state transition, forming ice

Answer 12

For most compounds, as the temperature of the liquid increases, the density decreases.

Answer 13

In liquid water, each molecule hydrogen bonds with approximately 3.4 other water molecules, while in ice, each molecule is hydrogen-bonded to 4 other molecules.

Answer 14

Ice floats because it crystallizes into an open structure that is 91% as dense as liquid water, making it less dense overall.

Answer 15

Ice formation can disrupt tissue structure.

Answer 16

When ice melts, only 15% of hydrogen bonds are broken. This explains why water’s latent heat of fusion (333 kJ/kg) is much lower than its latent heat of vaporization (2230 kJ/kg). It also shows that liquid water is still very structured.

Answer 17

Colligative properties are properties of solutions that depend on the concentration of solute molecules or ions, but not on the identity of the solute.

Answer 18

Examples include vapor pressure lowering (Raoult’s Law), boiling point elevation, freezing point depression, and osmotic pressure.

Answer 19

Freezing point depression depends on solute concentration and molecular weight; freezing causes solute molecules to concentrate in the unfrozen water, producing an unfrozen phase.

Answer 20

Metastable, supercooled liquid state achieved by rapid cooling, which increases viscosity and prevents crystallization.

Answer 21

they are metastable and show no significant change in physical properties despite their solid-like viscosity.

Answer 22

Glass behavior is challenging to study because extremely high viscosities require millions of years to observe any significant physical changes.

Answer 23

Examples include dried pasta, spray-dried powders (e.g., protein supplements, starches, proteins, instant coffee), hard sugar candies, sugar in chocolate, and boiled sweets.

Answer 24

Pure sucrose cannot form a glass because it crystallizes instead, a process known as "doctoring." This matters in confections because controlling crystallization affects the texture of candies.

Answer 25

Molecular mobility provides a kinetic approach to food stability, while water activity is a thermodynamic approach.

Answer 26

Low molecular mobility in glasses is a predictor of food stability because it slows down deteriorative reactions, with some exceptions like free-radical oxidation that doesn’t rely on diffusion.

Answer 27

increasing concentration or lowering water content, especially in high molecular weight materials, or by lowering temperature.

Answer 28

Tg marks the point where a material transitions into a glass state. Higher molecular weight materials have higher Tg values, while low molecular weight carbohydrates have lower Tg values.

Answer 29

The two main types of emulsions are water-in-oil and oil-in-water.

Answer 30

Butter, margarine, and lard are water-in-oil emulsions, while salad dressing and mayonnaise are oil-in-water emulsions.

Answer 31

it is an emulsifier and is necessary because it helps stabilize the emulsion by allowing the hydrophobic phase to remain in the hydrophilic phase, preventing spontaneous separation.

Answer 32

The main components needed are water, oil, an amphiphilic molecule (emulsifier), energy, and time.

Answer 33

Emulsion destabilization occurs because the hydrophobic phase is insoluble in the hydrophilic phase and without an emulsifier, separation happens due to density differences and droplet size variations.

Answer 34

Smaller droplets have higher internal pressure which increases the solubility of the dispersed phase, causing large droplets to grow at the expense of smaller droplets.

Answer 35

Proteins stabilize emulsions reducing interfacial tension where their hydrophobic sections interact with oil phase, and hydrophilic sections interact with the water.

Answer 36

The Hydrophilic Lipophilic Balance (HLB) is a measure of the balance between the hydrophilic and lipophilic portions of an emulsifier. It is calculated as HLB=20xMh/M Mhis the molar mass of the hydrophilic portion, and M is the total molar mass.

Answer 37

Provides electrostatic repulsion (DLVO theory) or stearic repulsion (Gibbs Marangoni effect). This repulsion prevents particles from clumping together. It helps stabilize emulsions, keeping them smooth and consistent.