Week 2 Chemistry
Solvent properties
If you look at the water molecule, it demonstrates the partially negative and positive regions of the water molecules, so whilst the overall charge is neutral, there are regions within the molecule that are more positive and negative, making it a di-polar molecule. This is what gives it unique solvent properties, allowing water molecules to stack closely together (as you can see on the diagram showing hydrogen bonding) and easily break down and bond or reform with other molecules.
Other polar molecules such as sugars, starches, gums and proteins, utilize water’s powerful solvent properties that can be very useful in food and cooking
Hydrogen bonds are weak bonds
Water is a liquid at room temperature. Individually, hydrogen bonds are considered to be weak bonds as they are constantly forming and reforming. However, collectively they are very strong bonds due to their polarity. This is why a lot of energy is needed to bring water to a boiling point to break these molecular bonds, which are released as steam in cooking..
Hydrogen bonding
Water is bound together via covalent bonding, which is a strong intramolecular bond; however, water molecules interact with each other via intermolecular hydrogen bonding, which is a weak bond.
Reactions and subtractions: Hydrolysis and condensation reactions
Many of the common chemical reactions occurring in food and drink that will be explored throughout this subject involve water. These reactions include hydrophobic interactions with lipids, dipole-dipole interactions with amino acids, carboxyl groups ionisation, pH changes, oxidation-reduction and carbon dioxide release, and hydrolysis/condensation.
The following are examples of the primary forms of interactions:
Hydrophobic reactions
Dipole-ion – amino acids
Dipole–dipole–carboxyl group ionisations
PH changes – enzymatic activity
Oxidation and reduction + carbon dioxide release
Hydrolysis and condensation reactions are happening all the time in the atmosphere, as well as in growing, cooking and digesting food and drink. Two examples are:
Digestion of food: Enzymatic hydrolysis of proteins into peptides and amino acids
Cooking process: Condensation of a reducing sugar or polysaccharide with protein or peptide, by linking the reducing end carbonyl groups in the former to the amino groups in the latter, in the Maillard reaction.
Condensation reactions
A condensation reaction produces a water molecule from a chemical reaction, as a result of the 'leftovers' from molecules being 'condensed' by becoming smaller, changing shape/configuration, or two molecules joining together. For example, two monosaccharides are joined by a glycosidic bond, and a water molecule is released. This will be further demonstrated when we discuss carbohydrate chemistry and the breaking of glycosidic bonds.
You may hear a condensation reaction is referred to as a 'dehydration' reaction ('de' meaning undo, and 'hydration' referring to water). However, these terms cannot be interchangeably used, even though dehydration does refer to a chemical reaction that results in a water molecule being released; the difference is outlined below:
Condensation reaction involves the removal of -H and -OH from different molecules.
Dehydration reaction involves the removal of -H and -OH from the same molecule.
In a condensation reaction, hydrogen is removed from an oxygen or a nitrogen, while in a dehydration reaction, hydrogen is removed from a carbon.
Hydrolysis reaction
A hydrolysis reaction, on the other hand, uses a water molecule in a chemical reaction. Reactions are the opposite of condensation reactions, in that they can break a chemical bond by using a water molecule.
The term 'hydro' refers to water and 'lysis' means the disruption. So, hydrolysis is the addition of a water molecule in a chemical reaction.
An example is the production of lactose-free milk, which involves hydrolysis of lactose, most frequently using the enzyme lactase to produce a mixture of glucose and galactose. In this process, the enzyme uses water to break the glycosidic bond.
Ionic interactions
Salt is an electrolyte. When salt comes into contact with water, it dissociates as it is ionic. Ionic interactions with water are a quite common occurrence that happens with naturally occurring 'salts' cellular processes in the body such as blood and fluid balance, and plants, food, and drink. The below example happens when adding salt to boiling water when cooking pasta.
We have a solution of water and table salt (also known as sodium chloride or NaCl). This molecule is held together by ionic bonds with their positive and negative charges attracting them to 'stick together', much like metal to a magnet. When salt is added to water, the hydrogen bonds within the water rearrange, and the sodium and chloride ions separate.
Sodium
Sodium is a positively charged atom, so the negative (oxygen, red) side of the water molecules move to surround and therefore neutralise the positive sodium atom.
Chloride
Chloride is a negatively charged atom, therefore the positive regions of the water molecules (hydrogen, white) do the same, and move to surround and neutralise the negative charge.
Water in food and drink
Where is it?
Water is present in many foods and drinks, even those that do not appear to be moist or have visible water. Many foods contain water as the highest amount of this nutrient, this can range from 0-95%. Foods such as fruits and meat have a water content of 70-95%, whereas foods containing a high proportion of lipids (fats) such as butter and can range in water content between 10-20%. Dry foods such as dehydrated grains may have a water content of 0%.
Bound
Water molecules are very tightly bound within the chemical structure of another molecule and cannot freely move.
Like any chemical reaction, bound water requires energy to remove the water from its bound state and break free the bonds that hold it within other molecules.
Foods such as meat and bread have a high proportion of bound water stored within their constituent molecules: carbohydrates, lipids (fats) and proteins.
For example, in meat, water is bound within the protein matrix (a concept we will explore further) and requires energy from heat to break the chemical bonds that secure water within the protein matrix.
Bound water can be very helpful in food preservation and cooking, as bound water is resistant to freezing and drying.
Free
There is little to no restriction of the movement of water molecules.
Free water is easily removed and can be readily available to act as a solvent for salts, acids or sugars.
It is stored within the cells of animal and plant tissues.
For example, free water can be observed when cutting into a piece of fruit. Water stored within in the cells’ vacuoles freely moves out of the cell walls when the cells are cut open, as can be observed when an apple is cut open with a knife.
Immobilised
In immobilised form, movement of water molecules is restricted
Nutrients in food and water
In the following weeks, we will be featuring the chemical composition, functional properties and key chemical reactions in food. Using the Australian Guide to Healthy Eating (AGHE) food groups chart as our base, we will take a closer look at a key chemical reaction happening in one food from each food group. Below is an overview of this application, starting with the AGHE food groups, then we define further into how these relate to the macro and micronutrient groups, then the chemical base unit groups: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur (CHNOPS). Lastly, we investigate the key chemical reactions occurring in food and drink.
WATER:
water is a polar molecule, with 2 distinctly charged areas of opposing charge. This is why the molecule creates a tetrahedral shape. the positive and negative charges repell eachother.
Bound by intramolecular bonds
water in food can be bound or free
H2O Water activity is measured by (aw) 0.00 (lowest) to 1.00 (highest). Food spoilage is inhibited when water activity is below aw 0.85. By lowering the amount of water activity in foods, microbial growth can be lowered. This is most evident in preserved foods like jams and salami. Sugar and salt are added in large amounts to bind the free water, allowing for the diffusion of water out of the surrounding areas and cells within the food via osmotic pressure, which reduces available water, inhibiting microbial growth, and preserving the food.
Osmosis
Osmosis is the net movement of water to a solute. This is specifically happening through a semipermeable membrane, such as those found within the cell walls of plant and animal foods. Water is a powerful solvent, dissolving solvents such as salt and sugar. Where there is an uneven gradient of a solute, osmotic pressure is created. Water will diffuse out through the cell wall membrane to achieve equilibrium so that the concentration of solute to solvent is equal on both sides of the membrane
Functional properties of water
Colligative properties of water
The functional properties of water are described in chemistry terms as colligative properties. This term describes the way water behaves in a solution, depending on the number of molecules present in a defined volume of water, rather than the chemical structure of the molecule and its own properties. Examples of colligative properties are:
the lowering of vapour pressure in cooking at elevation, thus lowering the boiling point of water
osmotic pressure caused by the presence of small molecules (solute) that dissolve in water
phase transitions of water from solid, liquid to gas
Solvent: a substance usually a liquid in which another substance is dissolved Example is water is the solvent in a saline solution.
Solute: a Solid, Liquid, or gas dissolved in another substance. For example, salt is the solute in a saline solution.
Solubility: degree or ability of substance to blend uniformly with another. For example, salt has a high solubility in water.
Saturation: when a liquid can no longer dissolve a solute (all molecules are 'used up'), and a precipitate (solid) is formed.
True Solution: has no gravitational pull, so all particles are completely dissolved, and no precipitate (solid) forms.
Dispersion: mixtures that contain particles that are dispersed within another substance.
Solution:
Definition:
Particles that come from a uniform (homogenous) mixture of two or more substances that are evenly distributed
Examples:
Saline (salt in water)
Alcohol (alcohol in water)
Sugar (solid in water)
Carbonated water (gas in water)
Properties:
Particles are small once dissolved and cannot be seen with the naked eye
Can't be separated by filtering, wont precipitate out
Suspension
Definition:
Heterogenous mixture of undissolved particles in a medium (water)
Examples:
Oil in water
Tea
Properties:
Non-uniform composition
Particles can be seen with naked eye
Over time, gravity causes visible layers to appear
Suspended when shaken, separates upon standing
Colloid
Definition: An intermediate state between a suspension and solution
Examples:
Milk (water and fat)
Foam (gas in liquid) such as whipped cream or whipped egg white
Emulsion (water in oil solution) such as mayonnaise, milk and butter
Solid emulsions like curd and custard
Properties:
Particles are small and suspended over time into layers and will not separate