Maillard and enzymatic browning
Why foods turn brown: the Maillard reaction and caramelisation, enzymatic browning of cut fruit, acrylamide, controlling browning, and a shelf life sum.
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The crust of bread, the flavour of roasted coffee and grilled meat, the colour of chocolate and beer: all owe much to browning reactions. So do some unwanted changes: a sliced apple turning brown, milk powder darkening in storage, or potatoes developing acrylamide when fried. Food scientists distinguish non-enzymatic browning, mainly the Maillard reaction and caramelisation, from enzymatic browning, driven by enzymes in damaged plant tissue. Controlling them is central to food quality.
Browning reactions are standard topics in food chemistry and food processing exams.
The Maillard reaction
Described by the French chemist Louis-Camille Maillard in 1912, it is a complex series of reactions between reducing sugars (glucose, fructose, lactose, maltose) and amino groups of amino acids, peptides and proteins.
- Early stage: the sugar and amino group condense to a glycosylamine, which rearranges to an Amadori product (or Heyns product from fructose). No colour yet, but lysine, an essential amino acid, becomes nutritionally unavailable.
- Intermediate stage: Amadori products break down into reactive dicarbonyls, furfurals and other compounds. Strecker degradation of amino acids produces aldehydes and pyrazines responsible for roasted, nutty, malty and meaty aromas.
- Final stage: polymerisation into brown nitrogen-containing melanoidins.
Factors affecting the Maillard reaction
- Temperature: rate rises steeply with temperature; it is fast during baking, roasting and frying but also proceeds slowly in storage.
- Water activity: fastest at intermediate water activity, about 0.6 to 0.8; slow in very dry and very wet foods.
- pH: faster at neutral to alkaline pH; alkaline conditions are used to darken pretzels.
- Sugar type: pentoses react faster than hexoses; sucrose is not a reducing sugar and must first be hydrolysed.
- Amino acids: lysine is especially reactive.
Effects
- Desirable: colour and flavour of bread crust, roasted coffee and cocoa, grilled and roasted meat, malted barley, fried foods, toffee and condensed milk.
- Undesirable: darkening and off-flavours in milk powder, dried fruits, and UHT milk during storage; loss of available lysine; formation of acrylamide from asparagine and reducing sugars in fried potatoes, crisps, bread, biscuits and coffee above about 120 °C, a probable human carcinogen whose levels are now regulated in some regions.
Browning in feeds and dairy products
The Maillard reaction also matters outside the kitchen. Hay and silage that heat in storage turn brown and smell of tobacco or caramel; much of their protein becomes bound to fibre and indigestible, measured as acid detergent insoluble nitrogen. In dairy products, controlled Maillard browning gives the colour and flavour of dulce de leche, khoa-based sweets and condensed milk, while unwanted browning limits the shelf life of milk powders and whey products.
Caramelisation
Heating sugars alone, above about 120 to 160 °C depending on the sugar, gives caramelisation: dehydration and polymerisation into brown caramel colours and flavours. It needs higher temperatures than the Maillard reaction and involves no amino compounds. Caramel colours are widely used as food colourings.
Enzymatic browning
When fruits and vegetables such as apples, bananas, potatoes, avocados, mushrooms and aubergines are cut, bruised or peeled, the enzyme polyphenol oxidase (PPO) comes into contact with phenolic compounds and oxygen. It oxidises phenols to quinones, which polymerise into brown melanins. Enzymatic browning is welcome in tea (fermented black tea), cocoa, dates and prunes, but unwanted in most fresh-cut produce.
Controlling enzymatic browning
- Blanching to inactivate PPO before freezing or drying.
- Acidulants such as citric acid and antioxidants such as ascorbic acid (vitamin C), which reduce quinones back to phenols.
- Excluding oxygen by immersion in water or syrup, vacuum or modified-atmosphere packaging.
- Sulphites, effective but restricted because some people are sensitive.
- Low-PPO varieties, including gene-silenced non-browning apples and potatoes approved in some countries.
- Chilling, which slows enzyme activity.
Controlling Maillard browning
- Limit reducing sugars: choosing potato varieties and storage temperatures that keep sugars low, blanching potatoes before frying.
- Control temperature and time of cooking.
- Use asparaginase enzymes to reduce acrylamide in some products.
- Store dried products at low water activity and cool temperatures.
Storage temperature and browning of milk powder
Non-enzymatic browning in a whole milk powder has a Q10 of about 3: the rate triples for each 10 °C rise. The powder keeps acceptable colour for 12 months at 25 °C.
- At 35 °C, browning is 3 times faster: shelf life ≈ 12 ÷ 3 = 4 months.
- At 15 °C, browning is 3 times slower: shelf life ≈ 12 × 3 = 36 months.
For a 5 °C rise, the factor is 30.5 ≈ 1.73, so at 30 °C shelf life ≈ 12 ÷ 1.73 ≈ 7 months. This is why milk powder exported to hot countries needs cool storage and why the same product can have very different shelf lives in different climates.
Key points
- Non-enzymatic browning: the Maillard reaction (reducing sugars + amino groups) and caramelisation (sugars alone at high temperature).
- Maillard stages: glycosylamine and Amadori product, dicarbonyls and Strecker degradation (aroma), melanoidins (colour).
- Fastest at intermediate water activity (about 0.6–0.8), higher temperature and neutral to alkaline pH; reduces available lysine; can form acrylamide.
- Enzymatic browning: PPO oxidises phenols to quinones and melanins in cut produce.
- Control enzymatic browning with blanching, acids, ascorbic acid, oxygen exclusion, sulphites and low-PPO varieties.
- Browning rate rises with temperature, often with a Q10 of about 2–5.
What the exams ask
- School-leaving and entrance papers: why cut apples turn brown and examples of browning in cooking.
- Degree and research-fellowship entrances: Maillard reaction mechanism and factors, Strecker degradation, caramelisation, PPO and its control, acrylamide, and nutritional effects.
- Food technology examinations: controlling browning in processing and storage, shelf-life prediction, and regulation of acrylamide.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does lemon juice stop apples browning?
Its acidity lowers PPO activity, and its vitamin C reduces quinones back to colourless phenols before they form brown pigments.
Is browning always bad for nutrition?
Maillard reactions reduce available lysine and can form undesirable compounds such as acrylamide, but they also create flavours and some antioxidant melanoidins. Moderate browning is part of most cooked diets.
Why do potatoes stored in the fridge fry darker?
Cold storage converts starch to sugars (cold-induced sweetening), and the extra reducing sugars brown, and form acrylamide, faster when fried.
Sources
- Damodaran, S. and Parkin, K. L. (eds) Fennema's Food Chemistry, 5th edition. CRC Press, 2017.
- Maillard, L.-C. "Action des acides aminés sur les sucres: formation des mélanoïdines par voie méthodique." Comptes Rendus de l'Académie des Sciences 154, 66–68 (1912).
- Martins, S. I. F. S., Jongen, W. M. F. and van Boekel, M. A. J. S. "A review of Maillard reaction in food and implications to kinetic modelling." Trends in Food Science and Technology 11, 364–373 (2000).
- Mottram, D. S., Wedzicha, B. L. and Dodson, A. T. "Acrylamide is formed in the Maillard reaction." Nature 419, 448–449 (2002).
- Commission Regulation (EU) 2017/2158 establishing mitigation measures and benchmark levels for the reduction of acrylamide in food. European Union, 2017.
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