Caramelization reaction

Caramelization is a type of non-enzymatic browning reaction of sugars which takes place under the action of dry heat and results in the formation of a characteristic caramel flavour.

Chemistry behind the reaction

Caramelization reactions take place under a variety of conditions and with various catalysts. Caramelization temperature ranges from 110 degree Celsius for fructose, 160 degree Celsius for glucose, galactose, to about 180 degree Celsius for Maltose, etc.

The caramelization reactions are mostly linked with inversion, condensation, isomerisation, dehydration and fragmentation of sugars, which result in formation of unsaturated polymers. A variety of catalysts can be used in these reactions to form caramels which either add flavour or colour to the product.

Caramelization reactions take place at temperatures above 120 degree Celsius and mostly under pH below 5. The sugars first undergo enolization, rearrangement and then a series of reactions like aldolisation, etc. The release of H+ ions during this reaction lowers the pH. There can be a variety of colours obtained as a result of caramelization reaction from yellow to brown based on the time and temperature conditions. The taste of the caramel progresses from slightly caramel-like to bitter.

It is quite difficult to ascertain the mechanism of complex reactions like caramelization, but it can be simplified under the following heads:

  • Initial dehydration of sugars  when exposed to heat and introduction of double bonds into the sugar skeleton
  • The next step leads to formation of a number of aromatic compounds (enediols, diacetyl, furans, etc.) both heterocyclic and homocyclic, which are formed as intermediates of enolization reactions. All of these reactions are associated with the elimination of water molecules.
  • Finally the small sugar molecules undergo condensation reactions to form medium to highly pigmented polymer compounds which also are responsible for characteristic caramel colour.

The below picture depicts the mechanism of caramelization of sugars like sucrose and compares the literature proposed mechanism with the experimentally observed mechanism (obtained through multi-response modelling procedures)


Figure 1 Schematic representation of sucrose caramelization mechanism (a) based on literature search, and (b) proposed reaction pathways based on experimental data and multiresponse modelling procedures.
Source: https://www.researchgate.net/figure/Schematic-representation-of-sucrose-caramelisation-mechanism-a-based-on-literature_fig1_240435055

APPLICATIONS IN FOOD INDUSTRY

Caramelization finds various uses in the food industry. Some of them are as follows:

  • Caramelization reactions are useful in adding desirable light to dark brown colours to food items like breads, juices, dessert wines, hard candies, etc.
  • Caramelization reactions generate innumerable heterocyclic and homocyclic aromatic compounds like diacetyl (one of the most important), hydroxymethyl furfural, hydroxyl acetyl furone, furfural, and other furans, pyrones, carbocyclic compounds, which add to flavour and aroma of foods like soya sauce, butter scotch flavours, ice creams, and also during cooking e.g. caramelized onions, sugar glazing in cakes, etc.

However, it must be clearly understood that the time and temperature conditions during the caramelization reaction, stage till which the sugars are caramelised, are major determinants of the kind of caramels which are produced and their respective applications. Thus, caramelization reactions find their application in the production of frostings, caramel candy or cake fillings to hard candies or as colouring agents.

The table below depicts the variety of uses that caramelization reactions can serve, based on the stage of caramelization and uses:

StageTemperature (°C)DescriptionUse
Evaporation of water100The sugar first melts. Any impurities which are present rise to the surface______
Small Thread102In this stage, no change of colour or flavour takes place.It can be used in frostings
Large Thread104In this stage, no change of colour or flavour takes place.It can be used in preserves
Small Ball110 – 115No colour or flavour change takes place. The sugar is semi-soft on cooling.It is used in candy fillings, meringues, fondants, marshmallows and fudges
Large Ball119 – 122No colour or flavour change takes place. The sugar gets firm on cooling.It is used in soft caramels
Light Crack129No colour or flavour change takes place. The sugar gets firm on cooling.It is used in semi hard candies
Hard Crack165-166No colour or flavour change takes place. The sugar gets hard on cooling.It is used in making butterscotch and hard candies
Extra Hard Crack168There is little change in colour that takes place with no change in flavour, the sugar shatters like glass during cooling;It is used in hard candies.
Light Caramel180The sugar attains a pale brown to golden brown colour.It can be used as flavouring agent.
Medium Caramel180-188Here the colour of the sugar ranges from golden brown to chestnut brown.Mostly used for imparting a rich caramel flavour to cakes and ice creams.
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