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Summary Food Structuring
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IntroductionUnstructured foods are rare and not so attractive. Structured foods are more attractive.
Challenges in food structuring are:
- Make new & attractive products
- Make existing product with other ingredients (allergens, caffeine)
- Make healthy products that they are equally or more attractive as unhealthy products
- Feed all the people on earth without destroying the earth (meat analogues)
The three main components are lipids, carbohydrates and proteins. They are not (well) miscible and are
dispersed into each other. Other main ingredient is air.
Food structure consists out of molecular scale (nano), micro and macrostructure. The nanostructures
determines the microstructure, the nano & micro-structure determine the macrostructure.
Food structure is important for:
- Health and quality of life
1. Bioavailability & release of nutrients, supplying micronutrients, nutrition related
diseases/aging population
2. Mashing/juicing increases bioavailbility and cooking decrease the availability.
- Safety and stability
1. Protecting unstable ingredients, keeping low and high water activity separate,
compartmentalization (suppression of mass transfer)
- Taste and sensory
1. Structure determines how and when flavours are released, the state of ingredient
determines properties (crispy, smooth, creamy, etc.)
- Sustainability
1. Optimal use of raw materials and energy, minimal production of waste use of more
sustainable food sources
Effect matrix/natural structure: it is more important to keep the original matrix. Micronutrients are
more efficient in the original plant matrix than in the isolated form. It implies that the natural matrix
improves the recovery, stability and bio-accessibility of micronutrients.
1
,Chapter I: Structures, building blocks and properties
Type of structures:
- Anisotropic structures: properties are direction dependent (e.g. meat, broccoli)
- Isotropic structures: there is no direction dependency, (regular) emulsions (e.g. milk, cheese)
Making food structures can be done in different ways:
- Self assembly happens spontaneously for small structures (e.g. crystals, fibril micelles, <100nm)
- Flow can induce larger structures: it can order the food building blocks (forced assembly, for e.g.
emulsions, product shape, meo-structures >1 µm)
- Solidification is needed to entrap structure (by cooling or evaporation water)
Water is a polar component. It is the only component where the density of the solid phase is lower than
for the liquid phase. There is a fast diffusion of acids and bases and there is exchange of protons. The
hydrogen molecules are located in an angle that makes the molecule polar. It can act as a bigger
molecule by hydrogen bonds. Formation of hydrogen bonds effects:
- High boiling point (compared to its size)
- Large heat of evaporation
- High heat capacity
- High heat conductivity
- Fast diffusion of acids
Lipids are oils and fats, often triglycerides. Lipids are long & linear a-polar chains. They have a high
melting point. Unsaturated fats have lower melting points because of the bend chains. Linear chains
form crystals better and have a higher melting point. The shorter the chain, the lower the melting point.
Also the structure and length of side groups determine the melting temperature
The formation of crystals depends on processing (time-temp profile).
The melting effect is the release of energy with the formation of crystals (melting requires energy),
which gives a cooling effect.
In olive oil clouding can occur. This sedimentation is heavier than oil. The formation is due to the droplet
formation of water. Polyphenol-protein complexes form and the saturated fat crystallizes.
It is prevented by the use of highly purified oil.
Biopolymers are carbohydrates and proteins. Proteins are globular or non-globular. The non-globular
proteins can form colloidal particles and micelles (caseinates), but can also have no clear structure
(gluten). In foods biopolymers are always combined with water. The water content determines sensory
properties. A product with a low water content is solid, with a high water content it is liquid. Exceptions
are gels (high water content, but solid) and cellular products (apples and carrots).
The main ingredients in food are lipids, proteins, carbohydrates, water and air. The solubility of protein,
carbohydrates (and lipids) are influenced by ionic strength, multivalent ions, pH, emulsifiers, etc.
Preparation processes changes the temperature, pH, etc. and deformation.
Other components are humectants and plasticisers which are miscible with water and soften materials
(e.g. glucose, glycerol). Salts is also another component and can have a lot of effects:
- alter molecular interacts in products,
- change properties of biopolymers,
- influences water activity (water availability)
- influences structure formation process
Salts have big effect and humectants & plasticizers have a small effect on formation processes.
2
,It is hard to remove the sugar in gingerbread. Gingerbread is soft because of the big amount of glucose
in it. By reducing the sugar, the product would be less soft. This is fixed by adding other plasticiser
(water, glycerol), sweetener and bulk agent (dietary fibre). By adding water the water activity is higher.
This causes microbial spoilage, so also a preservation agent is added.
Water-biopolymer mixtures have a typical sorption isotherm. In the rubbery state the aw changes very
fast with changing moisture content, it shows instable behaviour. At higher moisture contents, the aw
hardly changes with moisture content.
A rubbery material easy to deform and is tough. A gel is a rubber.
The water activity equals RH in case product is in thermodynamic equilibrium. However, the RH in the
environment is most often not same as the aw of the product. If RH<aw, water will be absorbed
/evaporated out of product (e.g. bread). RH>Aw, products takes up the water (e.g. crisps).
The higher the temperature of air, the more water it can contain.
The water activity is used for describing the products (e.g. describing the crispiness intensity)
Phase transitions are important in food processing. Possible transitions are solid to liquid, liquid to solid,
solid to gas, solid to solid (crystalline to amorphous, amorphous to crystalline)
A product is solid or liquid, depending on the moisture content and temperature.
A system is not always in a thermodynamic equilibrium. A TD equilibrium might not be reached during
processing. Kinetics are influenced by fixing and flow conditions.
phase diagram, state diagram
3
, In the phase diagram the eutectic point is given where the solute and solvent equilibrium curve
intersect. Below the point a eutectic solid is formed (no liquid present). However, water as a very low
molecular weight compared to solute molecule, so it is unusual that a eutectic solid will form. It is more
common that the water continues to freeze at low temperature and the solute does not crystallize
because of kinetic constraints.
Glass is formed if the viscosity is so high that the molecules cannot move and no crystals can be formed.
A glass is hard, crispy, brittle and has extremely slow mass diffusion. The glass state is not in
thermodynamic equilibrium. It is a kinetic effect. A state diagram is a phase diagram that includes kinetic
effects such as the glass transition.
A glass is a solid material without any ordering at molecular scale. The product can be transparent. The
volume fraction of glass systems are generally larger than that of crystalline systems.
Liquid nitrogen is used to produce glassy foods. At higher temperatures a high content of sugar is
needed. A glass transition is the transition from a melt to a frozen, amorphous liquid. This can occur
when cooling is taken place so rapidly that crystallisation cannot take place. Crystallisation is also
suppressed in impure systems. The glass transition temperature depends on type of biopolymer and
chain length. The higher the molecular weight of a product, the higher the glass transition.
If you heat up a glass material, it can form crystals if it is above the Tg.
The glass transition is a second order transition, as the heat capacity changes over a temperature rate.
With first order transitions, such as ice melting involve latent heat effects and are much clearer seen.
The driving force for crystallization is a difference in temperature. A lower temperature leads mostly to a
higher rate of crystallization.
Water is a plasticiser for many biopolymers. Plasticizers reduce intermolecular forces (softens the
structure), they increase plasticity of polymer, increase flexibility and lower the Tg. The water content
and sate of water determines the heat capacity and heat conductivity as well. The heat capacity is the
amount of heat necessary to heat up. The heat conductivity is the speed at which heat is transported.
Most properties are not dependent on the structural changes (heat capacity, density). But diffusion,
heat conductivity and viscosity do differ by structural changes.
The preparation process has influence on the properties of the components and together with
deformation they influence the structure.
If liquid is cooled very fast, it can’t form crystals. A glass transition is characterized by change in slope of
volume, enthalpy and entropy. The lower curve (crystals) is a real transition, whereby the density
decreases when the solid state is formed (only not true for water).
Bread and emulsions are multi-phase products. The properties can be estimated on overall composition
and some properties are dependent on the structure.
4
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