FOOD CHEMISTRY: PROTEINS
Diagram
The same twenty units, two different sources
Essential amino acids
Nine for adults. The body cannot make them in enough quantity, so they must come from food.
Non-essential amino acids
The rest of the twenty. The body can make them for itself, so food need not supply them.
Figure 1: Twenty amino acids are commonly found in proteins. Nine are essential for adults, and the rest are non-essential. Drawn in our own style from the amino acid discussion in the FTN Food Chemistry notes, Unit 4.
Introduction
Twenty amino acids are commonly found in proteins, and they are the same twenty across almost all living things. The difference between essential and non-essential is about the body, not about the amino acid: essential amino acids are those the body cannot make in enough quantity and must get from food, nine of them for adults, while non-essential amino acids are those the body can make for itself.
Quick comparison
Table 1: Essential amino acids compared with non-essential amino acids
| Aspect | Essential amino acids | Non-essential amino acids |
|---|---|---|
| Number (adults) | Nine | Eleven of the twenty |
| The nine | Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine | Not listed by name here: the books used here name the nine essential acids, and the rest are the non-essential group |
| Made by the body | No, not in enough quantity | Yes, the body can make them for itself |
| Must come from food | Yes | No |
| Changes with age | Yes: an amino acid that is essential for a growing child may be less so for an adult, and need changes with age and health | The same: the split depends on the body’s need at the time |
| Short in cereals | Lysine is usually short in cereals | Not applicable |
| Short in legumes | Methionine is usually short in legumes | Not applicable |
| Complete protein | A protein that supplies all nine essential amino acids in about the right proportions: most animal proteins, such as egg, milk, and meat, are complete | A protein can be rich in non-essential acids and still be incomplete if it lacks an essential one |
| Loss in processing | Lysine is the amino acid most often lost in Maillard browning, and harsh heat with alkali can turn L amino acids into D forms, lowering value | The same processing damage applies, but the nutritional cost is counted on the essential acids |
| Reference protein | Egg protein is often used as the reference, because its amino acid pattern matches human needs closely and it is highly digestible | Not applicable |
Scientific explanation
Definition
Each amino acid has an amino group, a carboxyl group, a hydrogen atom, and a side chain, all joined to a central carbon. The side chain is what makes each amino acid different. Essential amino acids are those the body cannot make in enough quantity and must get from food: nine of them for adults (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine). Non-essential amino acids are those the body can make for itself.
Working principle
The nutritional value of a protein depends first on its amino acid composition, and second on how digestible it is. A protein that supplies all nine essential amino acids in about the right proportions is a complete protein: most animal proteins, such as egg, milk, and meat, are complete. Most plant proteins are short of one amino acid: cereals are usually short of lysine, legumes are usually short of methionine. Eaten together, cereals and legumes cover each other’s gaps, which is why that pairing appears in diets around the world. Milk proteins, casein and whey, are rich in lysine, which is why milk improves a cereal diet. Gelatin sets a beautiful gel but is a poor protein for the body, because it lacks some essential amino acids.
Apparatus
The Kjeldahl method for protein uses digestion glassware with hot concentrated sulfuric acid and a catalyst, a distillation set, and a burette for titration. Formol titration uses a burette for the standard alkali. No amino acid by amino acid apparatus is named in the books used here, so no further apparatus is listed.
Measurement procedure
Protein quality is read from its amino acid composition and its digestibility, and the amount of protein is measured by 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, which is multiplied by 6.25 to give crude protein. Formol titration estimates amino nitrogen: formaldehyde is added to block the amino groups, and the acid freed is titrated. The books used here give the definition and principle of formol titration only, and no full laboratory protocol with volumes, reagents, and steps, so no protocol is invented here.
Factors affecting readings
Digestibility matters as much as composition: a protein locked inside a tough cell wall, or damaged by harsh heat, is less available to the body’s enzymes, so its value falls even if its amino acid pattern looks good on paper. Harsh heat drives the Maillard reaction, in which lysine reacts with sugars and is lost to the body. Alkali treatment can turn L amino acids into D forms, which lowers nutritional value because the body’s enzymes are built for L amino acids. The Kjeldahl factor also matters: milk uses 6.38, wheat uses 5.70, and soy uses 5.71, while the general factor is 6.25.
Formulas and calculations
Kjeldahl percent protein. 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 for the general factor.
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. This gives the total protein, not the essential amino acid content: a complete protein and an incomplete protein can show the same total protein and still differ in value, because only the essential amino acids decide completeness.
Industrial applications
- Essential amino acids: complete proteins from egg, milk, and meat in the diet, and cereal and legume pairings that cover each other’s gaps, as in diets around the world.
- Essential amino acids: milk proteins improving a cereal diet, because milk is rich in lysine, the acid cereals lack.
- Non-essential amino acids: the body makes them, so foods are not judged on them alone.
- Both: protein quality checks in food manufacture, where composition and digestibility decide the value of a protein ingredient.
Advantages and limitations
- Essential amino acids, advantage: they decide protein quality, and a food that supplies all nine in about the right proportions is a complete protein.
- Essential amino acids, limitation: they must come from food, and the body cannot make up a shortfall, so a diet short of lysine or methionine is short in value even if total protein looks high.
- Non-essential amino acids, advantage: the body can make them, so they need no special place in the diet.
- Non-essential amino acids, limitation: they cannot cover a gap in an essential acid, so a protein rich in them can still be incomplete.
FAQ
What makes an amino acid essential?
The body cannot make it in enough quantity, so it must come from food.
How many essential amino acids are there for adults?
Nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
What is a complete protein?
A protein that supplies all nine essential amino acids in about the right proportions. Most animal proteins, such as egg, milk, and meat, are complete.
Why do cereals and legumes eaten together give better protein than either alone?
Cereals are usually short of lysine and legumes are rich in it, while legumes are short of methionine and cereals supply it: together they cover each other’s gaps.
Can processing damage essential amino acids?
Yes. Harsh heat drives the Maillard reaction, in which lysine reacts with sugars and is lost to the body, and alkali treatment can turn L amino acids into D forms.
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:
