FOOD CHEMISTRY: PROTEINS
Diagram
From amino acids to a working protein
Amino acids
Single units, each with an amino group, a carboxyl group, and a side chain.
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Peptide bonds join them
Two amino acids make a dipeptide, a few make an oligopeptide, many make a polypeptide.
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Peptide
A short chain of amino acids, such as a hormone or a flavour peptide.
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Protein
One or more polypeptide chains folded into a working shape, often with hundreds of amino acids.
Figure 1: Amino acids join into peptides, and peptides grow into proteins that fold into a working shape. Drawn in our own style from the protein structure discussion in the FTN Food Chemistry notes, Unit 4.
Introduction
A protein and a peptide are both chains of amino acids joined by peptide bonds, and the difference is size and shape. A peptide is a short chain: two amino acids make a dipeptide, a few make an oligopeptide, and many make a polypeptide. A protein is one or more polypeptide chains folded into a working shape, often with hundreds of amino acids. The shape is the function: an enzyme works because its folded shape holds a pocket that fits its target exactly.
Quick comparison
Table 1: Protein compared with peptide
| Aspect | Protein | Peptide |
|---|---|---|
| Size | Large: one or more polypeptide chains, often hundreds of amino acids | Short: a few amino acids up to a short chain |
| Structure | Four levels: primary sequence, secondary helix and sheet, tertiary fold, quaternary joining of chains | A short chain, with no folded working shape of its own |
| Shape | Folded into a working shape, held by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulphide bonds | A short chain, too small to fold into a working shape |
| Function in food | Sets, gels, foams, and emulsifies: egg sets, yoghurt thickens, mayonnaise holds | Flavour and signals: peptides formed during fermentation and ripening give flavour |
| Examples | Egg white protein, milk casein and whey, meat protein | Insulin, a hormone that controls blood sugar; flavour peptides in ripened cheese and soy sauce |
| Denaturation | Yes: heat, acid, or force undoes the fold, and the protein loses its function | Not in the same way: a short chain has no folded shape to lose |
| Test | Biuret test turns violet when peptide bonds are present; Kjeldahl method measures nitrogen | Ninhydrin test turns purple with free amino acids; peptides are found where proteins are cut by proteolysis |
| Nutrition | Valued for amino acid composition and digestibility; a complete protein supplies all nine essential amino acids | A short chain that the body can use, and a step in digestion, when enzymes cut proteins into peptides and amino acids |
Scientific explanation
Definition
Amino acids join by peptide bonds: the carboxyl group of one joins the amino group of the next, with a water molecule lost. Two amino acids make a dipeptide, a few make an oligopeptide, and many make a polypeptide. A protein is one or more polypeptide chains folded into a working shape, often with hundreds of amino acids. A peptide is a short chain of amino acids, shorter than a protein, and it does more than wait to become a protein.
Working principle
The structure of a protein is described in four levels. Primary structure is the sequence of amino acids along the chain. Secondary structure is the local folding into shapes such as the alpha helix and the beta sheet, held by hydrogen bonds. Tertiary structure is the overall three dimensional folding of one chain. Quaternary structure is the joining of several chains into one working unit, as in haemoglobin, which holds four chains. The shape is the function: change the shape and the function goes with it. Peptides work as short chains: some are hormones, some carry minerals or act as signals between cells, and in foods the peptides formed during fermentation and ripening give flavour.
Apparatus
The Kjeldahl method uses digestion glassware with hot concentrated sulfuric acid and a catalyst, a distillation set, and a burette for titration. The biuret and ninhydrin tests use test tubes and a colour reading.
Measurement procedure
Protein is tested in two ways. Qualitative tests show whether protein or amino acids are present: the biuret test turns violet when peptide bonds are present, and the ninhydrin test turns purple with free amino acids. Quantitative methods measure how much is present. The Kjeldahl method measures nitrogen and multiplies by 6.25, and it is the reference method, though it counts any nitrogen compound, not only protein. In the Kjeldahl method, the sample is digested with acid so its nitrogen becomes ammonium, the ammonium is distilled into a standard acid, and the acid used is titrated to find the nitrogen.
Factors affecting readings
Any non-protein nitrogen counts as protein in the Kjeldahl method: urea, ammonia, nitrates, and free amino acids all add to the titre, which is why the result is called crude protein. A missing or wrong blank is the commonest lab error, because the reagents themselves carry a little nitrogen. The factor is not universal: milk protein uses 6.38, wheat protein uses 5.70, and soy uses 5.71. Using 6.25 for wheat overstates its protein by about 10 percent.
Formulas and calculations
Kjeldahl percent nitrogen. Percent N = (V_sample minus V_blank) x N_acid x 1.4007 / W, where volumes are in millilitres and W is the sample weight in grams. Percent protein = percent N x 6.25, because most proteins are about 16 percent nitrogen, so 100 / 16 = 6.25.
Worked example: a 1.000 g sample of flour is titrated with 0.1 N acid. Sample titre 20.0 mL, blank titre 0.2 mL. Percent N = (20.0 minus 0.2) x 0.1 x 1.4007 / 1.000 = 2.77 percent. Percent protein = 2.77 x 6.25 = 17.3 percent.
Worked example for milk: a milk powder with 5.0 percent nitrogen gives 5.0 x 6.25 = 31.25 percent protein with the general factor, and 5.0 x 6.38 = 31.9 percent with the milk factor. The milk factor is the one that goes on the report.
Industrial applications
- Protein: egg sets and foams, milk casein clumps to make yoghurt and cheese, meat firms on cooking, and egg yolk proteins stabilise mayonnaise.
- Protein: texturised plant protein made into a fibrous, meat-like texture by heat and pressure, as in meat analogues.
- Peptide: flavour in ripened cheese and soy sauce, where proteolysis releases peptides and free amino acids.
- Peptide: hormones such as insulin, which controls blood sugar, and signals between cells.
- Both: proteolysis, the cutting of proteins by enzymes or microbes, softens a food as in ripening cheese, or spoils it as in meat going off.
Advantages and limitations
- Protein, advantage: a working shape that gives functional properties, hydration, gelation, emulsification, and foaming, plus a full set of amino acids in complete proteins such as egg, milk, and meat.
- Protein, limitation: denaturation undoes the fold and loses the function, and harsh heat drives the Maillard reaction, in which lysine reacts with sugars and is lost to the body.
- Peptide, advantage: small and active, so peptides can act as hormones and carry flavour, and they form naturally during fermentation and ripening.
- Peptide, limitation: a short chain has no folded working shape, so it cannot gel, foam, or emulsify the way a protein can, and some food peptides studied for effects such as lowering blood pressure belong to research, not to claims on a food label.
FAQ
What is the difference between a protein and a peptide?
A peptide is a short chain of amino acids. A protein is one or more long polypeptide chains folded into a working shape, often with hundreds of amino acids.
What holds a protein in its shape?
Hydrogen bonds, ionic bonds, hydrophobic interactions, and disulphide bonds between sulphur atoms.
What is a peptide bond?
The bond joining the carboxyl group of one amino acid to the amino group of the next, with water lost.
How is protein measured in the laboratory?
By the Kjeldahl method: nitrogen is measured by digestion, distillation, and titration, and percent nitrogen is multiplied by 6.25, or by the food’s own factor.
Why is Kjeldahl protein called crude protein?
It measures all nitrogen, including non-protein nitrogen such as urea and nitrates, and converts it with a factor, so it estimates protein rather than measuring protein itself.
References and further reading
- FTN subject: Food Chemistry, Unit 4: Proteins.
- Zeece, Michael. Introduction to the Chemistry of Food. Academic Press, 2020.
- Coultate, Tom. Food: The Chemistry of Its Components. 7th ed., Royal Society of Chemistry, 2023.
- Damodaran, Srinivasan, and Kirk L. Parkin. Fennema’s Food Chemistry. 5th ed., CRC Press, 2017.
- Wang, Dongfeng, Hong Lin, Jianqian Kan et al. Food Chemistry. De Gruyter, 2015.
- Google Books pages read for this article:
