Food chemistry

Most food is made of carbohydrates, proteins and fats, and cooking changes all three.

Browning. When bread is toasted or meat is seared, sugars react with the amino acids in proteins (the Maillard reaction), giving brown colours and roasted, savoury smells. Heating sugar on its own until it melts and darkens is a different process, caramelisation.

Fats and oils are esters of glycerol and fatty acids. Unsaturated fats contain C=C double bonds, decolourise bromine water and tend to be liquid at room temperature. Vegetable oils can be hardened into spreads by reacting them with hydrogen at about 60 °C over a nickel catalyst. This adds hydrogen across some double bonds and raises the melting point.

Emulsions are tiny droplets of one liquid spread through another. Milk and mayonnaise are oil-in-water emulsions; butter is water-in-oil. Emulsifier molecules have a water-loving head and an oil-loving tail, so they sit on the droplet surface and stop droplets joining up.

Additives carry E numbers in the UK and EU. They include preservatives, which slow the growth of microbes; antioxidants, which stop fats going rancid; colours; and emulsifiers such as lecithin (E322).

Maillard reaction and caramelisation

The Maillard reaction begins with nucleophilic addition. The lone pair on an amino group (from a free amino acid or the side chain of lysine in a protein) attacks the carbonyl carbon of a reducing sugar such as glucose. Loss of water gives an imine, the N-glycosylamine, and a cascade of rearrangements, dehydrations and fragmentations follows. The products include brown nitrogen-containing polymers (melanoidins) and small volatile flavour molecules. Sucrose does not react directly, because neither of its anomeric carbons is free until it is hydrolysed.

The rate climbs steeply with temperature: browning is quick at 140 to 165 °C and negligible in boiling water (hence grey boiled meat). Alkali speeds it up by deprotonating the amine so it is more nucleophilic: hence the dark crust on pretzels. Caramelisation needs no amino group; it is the thermal breakdown of sugar itself, starting at around 105 °C for fructose and 170 °C for sucrose.

Fats going off

Hydrolytic rancidity breaks ester bonds, helped by lipase enzymes or by heat and water, releasing free fatty acids; the butanoic acid freed from butter smells of sick. Oxidative rancidity is a radical chain reaction with oxygen at the C=C bonds, so polyunsaturated oils are most at risk. Antioxidants such as tocopherols (vitamin E) break the chain by donating a hydrogen atom and forming a stable radical.

Preservation

Microbes need water, warmth and a suitable pH. Salting and sugaring tie up water. Benzoic acid (E210) works best below its pK_a of 4.2, since only the undissociated acid crosses cell membranes before releasing H^+ inside the microbe; that is why it suits soft drinks and pickles. Nitrite in cured meat stops Clostridium botulinum growing, and the nitrogen monoxide it forms binds to myoglobin, giving ham its pink colour.

Maillard mechanism

1. The open-chain aldose condenses with a primary amine to an imine, which cyclises to the N-glycosylamine. 2. Amadori rearrangement: acid-catalysed tautomerisation through an enaminol converts it into a 1-amino-1-deoxy-2-ketose. Ketoses give the analogous Heyns products. 3. The Amadori product enolises and loses water or the amine, releasing reactive 1,2-dicarbonyls such as 3-deoxyglucosone, methylglyoxal and butane-2,3-dione (the buttery note in many baked goods). 4. Strecker degradation: a 1,2-dicarbonyl reacts with an \alpha-amino acid, which is decarboxylated and deaminated to an aldehyde with one carbon fewer. Leucine gives 3-methylbutanal (malty) and methionine gives methional (cooked potato). The leftover \alpha-aminoketones condense in pairs and oxidise to alkylpyrazines, the nutty, roasted notes of coffee and bread crust.

Asparagine takes a related path to acrylamide (prop-2-enamide): its Schiff base decarboxylates and eliminates, mainly above 120 °C in dry, starchy food. Asparaginase-treated dough, lower frying temperatures and potatoes low in reducing sugars all reduce it.

Lipid autoxidation

In linoleic acid the CH_2 between the two double bonds is bis-allylic; removing an H atom there gives a radical delocalised over five carbons, so this C-H bond is unusually weak.

The double bonds shift into conjugation, so conjugated dienes absorbing near 234 nm are a simple marker of early oxidation. Hydroperoxides then cleave to aldehydes such as hexanal, the grassy, cardboard smell of stale oil. Phenolic antioxidants (ArOH) give an H atom to LOO^\bullet faster than LH does, and the resulting aryloxyl radical is too stabilised to continue the chain.

Emulsions

An emulsion is thermodynamically unstable because the oil-water interface carries free energy \gamma \Delta A. Emulsifiers lower \gamma and add a barrier to coalescence. On the HLB scale, small surfactants of roughly 3 to 6 favour water-in-oil and 8 to 18 oil-in-water emulsions; the continuous phase is usually the one the emulsifier dissolves in better (Bancroft's rule). Milk proteins such as casein stabilise droplets by steric and electrostatic repulsion. Creaming follows Stokes' law, v = 2r^2\Delta\rho g/9\eta; homogenisation cuts milk fat droplets to below a micrometre, and because v \propto r^2 the cream no longer rises.

Flavour molecules

Receptors are chiral: (R)-carvone smells of spearmint, (S)-carvone of caraway. Odour thresholds span many orders of magnitude, so a trace compound such as 2-acetyl-1-pyrroline can dominate the aroma of bread crust and basmati rice.