Food Chemistry Quiz 1

Front 1

How do water activity and moisture content differ?

Back 1

-Moisture content, the basics are simple enough: The amount of moisture in a material is usually expressed as a percentage of the total. This is easy to understand until you start the measurement process.

- Water Activity, on the other hand, is a measure of water that is available to react with or attach itself to another material.

Front 2

What is water activity?

Back 2

Fugacity of water in the sample compared to fugacity of pure water in same conditions

Front 3

Explain how and why the water activity of 50 gsand in 100g water is different from that of 50 g sucrose in 100 g water.

Back 3

The water activity of sand and water compared to sucrose and water is different because sucrose is soluble in water and leads to hydrogen bonding.

Front 4

Define a hydrogen bond. List two substances whose molecules formhydrogen bonds with water.

Back 4

The hydrogen bond is an attractiveinteraction between a hydrogen atomfrom a molecule or a molecular fragmentX–H in which X is more electronegativethan H, and an atom or a group of atomsin the same or a different molecule, inwhich there is evidence of bond formation.

- Two substances include oxygen and nitrogen

Front 5

Compare and contrastcrystalline and amorphous solids

Back 5

-Crystalline Solids: The highly ordered 3-dimensionalstructure of the crystal lattice over longdistances provides optimal :PurityPhysical and Chemical StabilityControl of Particle Morphology

- The Amorphous State Represents a solid state thatlacks the long-range threedimensiona

Front 6

Tg

Back 6

The glass transition temperature. The temperature at which anamorphous glassy materialsoftens (becomes rubbery orviscous) due to the onset oflong-range coordinatedmolecular motion.

Front 7

water-solid interactions

Back 7

Critical to water – solid interactions are :

1. The small molecular sizeof water and its ability to move

2. The strong directional hydrogen bonding that can occur at 4 charge centers, with interaction energies of ~ 20kJmol-1

3. The abundance of water molecules in the atmosphere under most ambient conditions.

Front 8

free volume

Back 8

Free water

Front 9

Describe the differences in an amorphous solidabove and below its Tg. If a small amount of water was added to this amorphoussolid, what would happen to the Tg (and why)? What changes the Tg of amaterial?

Back 9

Above, glassy. Below, rubbery. Below, Increasing the amount of water absorbed into the amorphous solidwill decrease the Tg.

 Increasing moisture content ofan amorphous solid lowers theTg.

Front 10

Scenario: You are working on a fruit filled barproduct that has a crispy crust and a soft filling. You need to make sure water does not migratefrom the filling into the crust. Do youfocus on equilibrating the moisture contents or the water activities ofthe crust and filling (and why)? Whatwill you do to stop the moisture migration (how will you equilibrate themoisture content or water activity to maintain product quality)?

Back 10

Water activity needs to be focused on.

Front 11

Draw a type 2 moisture sorption isotherm (thatis representative of most foods). Labelthe axes appropriately. Describethe ‘types’ of water found in each ‘zone ‘of the isotherm.

Back 11

- Zone I Water present is most strongly sorbed and least mobile. Before the slope up.

-Zone II Water added occupies first-layer sites that are still available. The point of equilbrium.

-Zone III Further water addition causes glass-rubber transition;additional water at this point acts more as bulk water. More slope up.

Front 12

Rank the following types of bonds/interactionsfrom highest to lowest energy. Provide examples of foods where each of thesebonds/interactions is important:a. Peptideb. Van der Waalsc. Hydrogen bondd. Ion dipole bonde. Ionic interactionf. Disulfide

Back 12

Top is lowest energy

- Van der Waals = Melting butter

- Ionic interaction

- Hydrogen Bond = Water molecules

- Ion Dipole

- Disulfide

- Peptide

Front 13

Describe the relative size scale of commonmacronutrients in foods.

Back 13

Top is smallest

- Water

- Sugars, salts, organic acids, O_2

- Lipids

- Proteins

-Hydrocolloids

-Amylopectin!!!!

Front 14

Water has an equal numberof hydrogen donors and hydrogen acceptors. Why is this significant?

Back 14

Can lead up to 4 hydrogen bonds

Front 15

Why is the dielectricconstant a good indicator of the solvent power of water? Briefly describe the processes involved inthe hydration of ionic and nonionic solutes.

Back 15

Dissolving of ions by high di-electric constants of water is given by Coulomb’s law: F = Q1x Q2/ DR2where:Q1, Q2 are magnitude of chargesR= distance between chargesD = dielectric constant of the medium separating chargesF = forces of electrostatic attraction

Front 16

Define the boiling point ofwater using the concept of vapor pressure. How do solutes affect the boiling point of a solution?

Back 16

For water, the vapor pressure reaches the standard sea level atmospheric pressure of 760 mmHg at 100°C.

- The boiling point of the solvent above a solution will be greater than the boiling point of the pure solvent whether the solution contains a non-volatile solute or a volatile solute.

Front 17

How do surface activeagents overcome the large surface tension of water? Proteins lower surface tension differentlythan other surface-active agents, explain their mode of action.

Front 18

What are the five mechanisms by which water interacts with solids.

Back 18

-Adsorption

-Capillary condensation

-Deliquescence

- CRYSTAL HYDRATE FORMATION

- Absorption

Front 19

Why are amorphous solidsmore hygroscopic than crystalline solids?

Back 19

Amorphous solids do not have a crystal lattice structure

Front 20

Define colligative property andthen list the colligative properties.

Back 20

-Colligative properties are properties of solutions that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the type of chemical species present.

Front 21

Describe general methods tomeasure aw.

Back 21

Measure activity of vapor abovesaturated solution in equilibrium with the solid phase.

-water activity meter, dew point analyzer, capacitance prob

Front 22

Sigmoidal or S-shapedsorption isotherms are most common for food components. Describe the three classifications of waterbased on such isotherms.

Back 22

-Type 1, upside down J. Anticaking agents,such as calciumsilicate and silicondioxide

-Type 2, S shape.Multicomponentfood materials,such as starch andraisins

- Type 3, J shape, Crystalline foodmaterials, such assucrose and NaCl

HORIZONTAL IS WATER ACTIVITY, MOISTURE CONTENT IS VERTICAL

Front 23

What is glassy-rubberytransition temperature? As a hydrophilicpolymer (e.g. protein) is being hydrated from bone dry at room temperature,what happens to the rheological property?

Back 23

Amorphous Solids Can Exist in Two States Super-cooled liquid/Rubbery solid – a viscous equilibriumliquid form of the material Glass – a solid non-equilibrium form of the same material

Front 24

Draw a food stability map(reaction rate vs. water activity) for the following reactions: lipidoxidation, Maillard browning, enzyme reactions

Back 24

Front 25

Adsorption

Back 25

First mechanism by which water interacts with solids.A process wherein molecules of water contact and adhere to a hydrophilic solidsurface, interacting with available surface sites via hydrogen bonding, electrostatics, or charge transfer

Front 26

Capillary condensation

Back 26

Second mechanism by which water interacts with solids. It occurs when water vapor present in the ambient air condenses in the region of contact between solid particles to form a liquid bridge connecting the two surfaces, or in pores or other surface irregularities.

Front 27

Deliquescence

Back 27

-Third mechanism by which water interacts with solids. A first order phase transformation wherein certain crystalline solids undergo dissolution when the relative humidity of the surroundings exceeds the critical relative humidity of the solid, known as the deliquescence point RH0.

Front 28

Crystal Hydrate Formation

Back 28

Fourth mechanism by which water interacts with solids.A solid crystalline compound containing water molecules as an integral part ofthe crystal (stoichiometric or non-stoichiometric)

-Many food ingredients are crystalline solids, and a subset of these, including glucose, are capableof forming crystalline hydrates at ambient conditions (Figure 8).

Front 29

Absorption

Back 29

Fifth mechanism by which water interacts with solids. A process in which water vapor permeates into an amorphous solid.A process in which one substancepermeates another;