Flavour molecules, the effect of cooking on flavour, the difference between aldehydes and ketones, oxidation of aldehydes, and the volatility and solubility of flavour and aroma compounds.

What this key area is asking

The SQA wants you to describe flavour molecules, explain how cooking can change flavour, distinguish aldehydes from ketones by their structure, describe the oxidation of aldehydes, and link the volatility and solubility of molecules to how we perceive flavour and aroma. The aldehyde-versus-ketone test is a near-certain exam item, so the structural reason and the test results must be precise.

Flavour and aroma molecules

The taste and smell of food come from small organic molecules. We detect flavour on the tongue and aroma through volatile molecules reaching the nose. Cooking changes flavour because heat can break down, oxidise or release flavour molecules, and can also create new ones (for example, the browning reactions that give roasted food its taste).

Aldehydes and ketones

Oxidation of aldehydes

The reason is structural: the aldehyde's carbonyl carbon carries a hydrogen that can be replaced by an $-OH$ to form the acid, whereas the ketone's carbonyl carbon has no such hydrogen.

Worked example: moles of a flavour aldehyde

Volatility and solubility

A molecule's behaviour depends on its size and the forces between molecules:

• Volatility: smaller molecules and those with weaker intermolecular forces vaporise more easily, so they are detected as aroma.
• Solubility: molecules that can hydrogen bond dissolve in water; non-polar molecules dissolve in oil. This is why some flavours wash out in water while others stay in the fat.

Examples in context

The chemistry of cooking explains why a steak smells so good. Heating drives the volatile aldehydes and other small carbonyl molecules off the surface so they reach the nose, while browning reactions between sugars and amino acids generate hundreds of new aroma compounds. Vanilla flavour comes from vanillin, an aldehyde that gives a positive Fehling's test, which is why food chemists can confirm its presence chemically. The contrast between water-soluble and fat-soluble flavours is why a cook adds fat to a sauce: many flavour molecules are non-polar and dissolve in the oil, carrying taste through the dish, whereas water-soluble flavours such as those in a stock are extracted into the cooking water.

Try this

Q1. State the test that distinguishes an aldehyde from a ketone. [2 marks]

• Cue. A mild oxidising agent (Fehling's blue to brick-red, Tollens' silver mirror, or acidified dichromate orange to green) reacts with the aldehyde but not the ketone.

Q2. Why must an aroma molecule be volatile? [1 mark]

• Cue. It must vaporise easily so that it can travel to and be detected by the nose.

Q3. Calculate the number of moles in $5.80 \text{ g}$ of the aldehyde ethanal ($GFM = 44.0 \text{ g}$). [1 mark]

• Cue. $n = 5.80 / 44.0 = 0.132 \text{ mol}$.