Module 4 - Response lecture

Water - Introduction: different states of water

phase diagram

• solid = ice → different polymorph crystals

• liquid

• vapour = steam

if you want to vaporize liquid water, you need a lot of energy (latent heat)D

Density in function of temperature

• density frozen temperature < density room temperature

• ice is conducting better than water

Water - Introduction: Why do we freeze foods?

• effect on shelf life (~ storage temperature)

Water - Introduction: Pressure-assisted freezing and thawing

advantages over confentional freezing and thawing

• pressure-assisted freezing

  • steps: increase pressure until you reach point B → freeze to -20°C (water stays liquid due to high pressure) → lower pressure → crystallization happens all throughout food instead of only on the outside

• pressure-assisted thawing

  • steps: Start in point D → increase pressure until you reach point C → heat up to liquid situation → decrease pressure until point A → thawing all throughout the food

advantages

• less cell damage

• less leakage

• higher quality

Water - water activity and glass transition: Timeline

<1950: higher water content → more risk of spoilage

1957: ‘water activity’ definition

• = to which degree the water is associated with food components → will impact its availability to act as a solvent and contribute in physical, chemical and microbiological processes

• impact of the solute is depending on type and concentration → affects colligative properties

  • boiling point

  • boiling point elevation

  • freezing point

  • freezing point depression

  • vapour pressure

  • density

  • osmotic pressure

1988: Glass transition

• polymer science → food science

• changes

  • thermodynamic properties

  • molecular mobility

  • dielectric constants + mechanical properties

• → influences food quality and stability

Water - Question: The water activity is a measure for the water availability

True

Water - Question: The water activity is a measure for the water content

False

Water - Question: The water activity increases with increasing temperature

True

Below 0.6 → microbiological stability

Water - Water activity meter (niet zo belangrijk denk ik)

• headspace equilibration

• measurement temperature

Water - Question:

When equivalent weights of either glucose or sucrose are added to a sugar solution, there is a greater change in the colligative properties with the glucose/sucrose solution than the sucrose/glucose solution due to smaller size of sucrose/glucose molecules (higher number of moles for a given weight addition). The interactions between solute and water molecules cause a decrease/increase in the vapor pressure above the solution, which leads to an decrease/increase in the boiling point temperature. The increase/decrease in vapor pressure above a sugar solution means it must be heated to a higher/lower temperature in order for the vapor pressure to be equal to the atmospheric pressure, the condition necessary for boiling to occur.

→ the more sugar present → the higher boiling point elevation

Water - Raoult’s law

• calculate water vapor above food/solution based on molar fraction of water

• ideal mixture

• only for low solute concentrations

• not for complex foods

Comparison glucose vs salt

• molecular weight sucrose > molecular weight salt

• lower molecular weight molecules → larger impact on the collegative properties

ex. salt will dissolve in ice and decrease water freezing point

Water - Time dependent changes in water activity

super saturated sugar sollutions (when you give it time, they will crystallize)

= sugar crystal + solution: complex mixture of sugars (glucose, fructose, sucrose)

→ honey with a high glucose/fructose ratio will crystallize more rapidly

→ paper mentions different strategies to prevent crystallization

→ when the sugar concentration in the remaining liquid decreases, the water activity increases → yeasts can start to grow

Conclusion

• sugar crystallization has an effect

  • texture

  • microbiological stability

  • solubility

Water - Glass transition

Solid: crystalline vs amorphous

Amorphous: glass vs rubber

Phase change vs glass transition

phase change (e.g. melting)	glass transition
solid → liquid well-defined temperature T_melt latent heat	glass → rubber remains amorphous temperature range no latent heat
First order process	Second order process

Water - Question: Define glass transition temperature in maximum 2 lines using your own words

• This is the temperature at which a amorphous food matrix containing water and solutes undergoes a state change from glassy to rubbery. Solutes in glassy state are so concentrated that there mobility is so low and the food is ‘fixed’.

• It is the temperature range from which molecular mobility of amorphous material is reduced and molecules cannot move freely, just rotate and vibrate.

• The glass transition temperature range is the temperature where an amorphous material transitions from a hard, brittle, glassy state to a soft flexible rubber state upon heating.

• Glass transition temperature is the temperature region where an amorphous material changes between a rigid, glassy state to a softer state due to changes in molecular mobility. Water acts as a plasticiser, so increasing water content generally decreases Tg.

Perfect answer:

The glass transition is the temperature range over which an amorphous material changes, upon heating, from a rigid, glassy state to a softer, rubbery state because molecular mobility increases.

Water - Williams-Landel-Ferry (WLF) model

• C2 + (T- Tg)

Relaxation time

• the shorter relaxation time → the faster molecules can move

Water - Question:

(A: emulsifier started to cake, which is a big problem because it meant that the consumer had problems of using it)

when stored at 5°C it remained perfectly free flowing

above 20°C → more mobility → more stickiness → cake was formed

Water - Question: Freezing does not affect the water activity

False

Water - Question: During freezing, all the water is transformed in the ice state

False

Water - Freezing

Water

• cooling → freezing point lower than actual freezing point (= undercooling) → nucleation starts → actual freezing point → further cool ice to deeper temperature

Sugar solution

• sugar depresses the initial freezing point

• freeze-concentration of the sugars → temperature-dependent equilibrium between ice crystals and unfrozen solution

• sufficiently low temperature: freeze-concentrated unfrozen phase → glassy

Water - Freezing graph

ice crystals (pure water) + solutes in unfrozen water

A = 20% sucrose solution (RT)

C = start freeze-concentration

Te = eutectic temperature

• = homogenous mixture that has a melting point lower than those of the constituents

B = initial glass transition T unfrozen fraction

→ moves up as a consequence of freeze-concentration

D = max freeze-concentration → solute-specific

• 1. Thermodynamic vs kinetic

• 2. Importance of storage T

Water - Question:

A small-scale ice cream manufacturer wants to develop yoghurt ice cream. His first trial contains full-fat yoghurt, cream, sucrose and an emulsifier. His taste panel thought the ice cream has just the right hardness, but it is too sweet for a yoghurt ice cream. Just decreasing the sucrose concentration is an obvious solution but made the ice cream too hard. Can you suggest a strategy to improve the taste without affecting the texture? Hint: Think about Raoult’s law

Raoults law

= change in the vapor pressure of the solvent that occurs when a solute is added to the solvent is proportional to the mole fraction of the solute

1. Sugars with a lower sweetening capacity and lower molecular weight

2. Other sugars: glycerol, glucose, sorbitol, maltose, lactose, maltodextrin, polysaccharides

Water - Question:

The microstructure of chocolate can be defined as a dispersion. It contains sugar particles, cocoa particles and/or milk particles in a continuous cocoa butter phase, depending on the type of chocolate. Relate the type of chocolate (dark chocolate, milk chocolate, white chocolate) to the schematic representation of the microstructure (1, 2, 3) and relate the chocolate ingredients (cocoa particle, sugar particle, cocoa butter, milk particle) to their schematic representation (A, B, C, D).

→ see solution in wooclap

Water - White chocolate

Reason of solidifying in Cargill tank: too much amorphous lactose in the chocolate

amorphous lactose → crystalline lactose + H2O → interparticulate bridges → agglomeration → thickening and solidifying above melting point of cocoa butter

Remark

• T-dependent

• Humidity dependent

• at higher T → the transitions can occur at lower moisture content

How does the relaxation time of amorphous sugar in chocolate change when moisture lowers its Tg?

Water - Question

Jelly candy: liquid

• water

• hydrocolloids

• sugars: sucrose + glucose

  • liquid state

  • sucrose is completely solubilized

Caramel candy: amorphous

• water

• milk ingredients: condensed milk and fat

• sugars: sucrose + glucose

• Caramel and fudge contain lipid emulsions and protein aggregates in an amorphous sugar matrix

Cotton candy: glass

• 100% sugar

• glass state

→ when you start eating it becomes stickier

• it attracts water → crosses glass line → becomes stickier

Tablet candy: crystalline

• 90 - 95% sugar (mainly sucrose)

• 2-8% binder syrup

Water - Question:

Tagatose has a lower glycaemic impact than sucrose. Would you consider to use it in the production of a lollypop? Explain.

If the goal was producing a candy that has lower impact in the raise of blood glucose it would be useful. However, lollipops are suppossed to be hard candies that would be hard at normal T, since tagatose has a Tgaround 15°c then at room T the can would already be in a more rubber state so it can become more sticky which is not desirable

→ glass transition appearing around room temperature

How to prevent stickiness and structural collapse in amorphous food products when the storage temperature is close to their glass transition temperature

1. Formulation

2. Proces: cooling rate, temperature

3. Storage conditions

Water - Question

The figure represents the state diagram of a sucrose/water mixture (Van der Sman & Renzetti, 2019). Relate the symbols indicated on the state diagram to the correct technical terminology.

Cookie baking

• Flour → starch + proteins

• Sucrose

• Fat

• Water

• A or B: dough: crystalline sugar dissolving during mixing

• A or B → C: baking

• C → E: cooling to partially crystallie/amorphous product

if he gives an equation you need to be able to explain it