Even before caramelization occurs you may have Maillard reaction products. These arise from the combination of reducing sugars (such as glucose) and amines in proteins (such as the amino groups of lysine). These so called “Maillard compounds” (such as furosine) are yellow or brown in solution depending on their concentration. Pure recrystallized glucose in distilled water solution does not yellow until high temperatures are attained and these are maintained for prolonged periods of time or the water starts evaporating resulting into a syrup. You can continue until true caramel is formed or even until you carbonize the sugar. However, you may be able to get decent translucent (very viscous) glucose candy with slow evaporation at lower temperatures. If you have high pH in the solution, “peeling” may occur and color may develop but for different reasons than Millard browning.

In other words, to answer your question properly it is important that you describe the conditions of your experiment or particular experience, i.e. the purity of the glucose, its concentration, the solvent and its purity, temperature, pH. etc.

You can get an idea of Maillard compound formation by comparing the color that results from heating solutions of different carbohydrates in 2mM Lysine. When the carbohydrate lacks a reducing end (such as non-reducing fructooligosaccharides - FOS) only a slight yellow color develops. This color is due to the presence of tiny amounts of free glucose and traces of nitrogen. Other carbohydrates that have reducing ends generate color depending on the molar concentration of their free reducing termini.

To illustrate this I am attaching a photograph that was produced many years ago for a laboratory class. I have to recognize that while I am asking you to define your experiment I am failing to define mine since I do not have notes on the experiment. All I can surmise is that the experiment was run in pairs. The first two volumetric flasks correspond to non-reducing fructooligosaccharides (I do not remember the concentration); the first volumetric flask (left to right) has these FOS in distilled water, the second in a 2mM solution of Lysine. The next pair is equivalent for a preparation of GOS available almost 20 years ago. I do not remember what the third pair was, nor do I remember the concentration of carbohydrate. This is a very artificial situation; the experiment was designed to illustrate to students the importance of regulating carbohydrate protein interactions in the elaboration of beverages. Nothing else was pursued.

Caramelization occurs later on in solutions or early in solid foods. At that time there are already many Maillard compounds around. The difference is that caramel is a type of pyrolysis that produces several compounds including some that are volatile and generate aromas and flavors that may be complex and attractive. Coffee and chocolate develop their flavor through controlled pyrolysis called roasting. This process is actually akin to a “controlled burning” . Molecularily it is a more complex process; carbohydrates are hydrolyzed, combined and fragmented in many ways during caramelization. If you heated pure solid glucose then you are indeed producing caramel.

Hope this helps.

If you keep heating the syrup you would get caramel! Please read about Caramelization.

Even before caramelization occurs you may have Maillard reaction products. These arise from the combination of reducing sugars (such as glucose) and amines in proteins (such as the amino groups of lysine). These so called “Maillard compounds” (such as furosine) are yellow or brown in solution depending on their concentration. Pure recrystallized glucose in distilled water solution does not yellow until high temperatures are attained and these are maintained for prolonged periods of time or the water starts evaporating resulting into a syrup. You can continue until true caramel is formed or even until you carbonize the sugar. However, you may be able to get decent translucent (very viscous) glucose candy with slow evaporation at lower temperatures. If you have high pH in the solution, “peeling” may occur and color may develop but for different reasons than Millard browning.

In other words, to answer your question properly it is important that you describe the conditions of your experiment or particular experience, i.e. the purity of the glucose, its concentration, the solvent and its purity, temperature, pH. etc.

You can get an idea of Maillard compound formation by comparing the color that results from heating solutions of different carbohydrates in 2mM Lysine. When the carbohydrate lacks a reducing end (such as non-reducing fructooligosaccharides - FOS) only a slight yellow color develops. This color is due to the presence of tiny amounts of free glucose and traces of nitrogen. Other carbohydrates that have reducing ends generate color depending on the molar concentration of their free reducing termini.

To illustrate this I am attaching a photograph that was produced many years ago for a laboratory class. I have to recognize that while I am asking you to define your experiment I am failing to define mine since I do not have notes on the experiment. All I can surmise is that the experiment was run in pairs. The first two volumetric flasks correspond to non-reducing fructooligosaccharides (I do not remember the concentration); the first volumetric flask (left to right) has these FOS in distilled water, the second in a 2mM solution of Lysine. The next pair is equivalent for a preparation of GOS available almost 20 years ago. I do not remember what the third pair was, nor do I remember the concentration of carbohydrate. This is a very artificial situation; the experiment was designed to illustrate to students the importance of regulating carbohydrate protein interactions in the elaboration of beverages. Nothing else was pursued.

Caramelization occurs later on in solutions or early in solid foods. At that time there are already many Maillard compounds around. The difference is that caramel is a type of pyrolysis that produces several compounds including some that are volatile and generate aromas and flavors that may be complex and attractive. Coffee and chocolate develop their flavor through controlled pyrolysis called roasting. This process is actually akin to a “controlled burning” . Molecularily it is a more complex process; carbohydrates are hydrolyzed, combined and fragmented in many ways during caramelization. If you heated pure solid glucose then you are indeed producing caramel.

Hope this helps.

To see what is going on to glucose being heated up (pyrolysis) go to

---John B. Paine III , Yezdi B. Pithawalla , John D. Naworal, Journal of Analytical and Applied Pyrolysis, 82(1), 2008, 42–69

---John B. Paine III , Yezdi B. Pithawalla , John D. Naworal, Journal of Analytical and Applied Pyrolysis, Volume 83 (1), 2008,37–63

---John B. Paine III , Yezdi B. Pithawalla , John D. Naworal , Charles E. Thomas Jr., Journal of Analytical and Applied Pyrolysis, Volume 80(2), 2007, 297–311

---Chong Shen, Igor Ying Zhang, Gang Fu and Xin Xu, Chin. J. Chem. Phys. 24, 2011, 249.

to find out that a variety of products starting C1 up to ---C6

Good luck

Heating glucose truly result in caramelization and millard reaction .However there are differences. Caramelization is an entirely different process from Maillard browning, though the results of the two processes are sometimes similar to the naked eye (and tastebuds). They both are promoted by heating, but the Maillard reaction involves amino acids, whereas caramelization is simply the pyrolysis of certain sugars.

What you observed is one the most commom chemical process.Thats why one never autoclave glucose if added to media.If you look over there is lot of literature on this.

Moreover look how bakery does it...320 deg F,Clear liquid. 338 deg F,Brown liquid,356 deg F Medium Brown.Amazing Science with different taste and colour.

Njoy!

@ Adel Amer Thank you for the articles. But when I looked for what is pyrolysis it means thermal decomposition of organic compounds by excessive temperature in the absence of oxygen. But in my case the temperature may not have gone to that higher and also it was in the presence of oxygen.

The real scenario in my question is that I do phytochemistry studies and currently dealing with sugar rich fractions. Most of my sugar containing fractions are colored and I am sure they are decomposed compounds from sugars. But they are not showed up in TLC. So I am little curious to know what they are really.

By the way many thanks for letting me know of caramelization.

It may due to degradation products of glucose like Hydroxymethylfurfural (HMF)

Selvaraju: Yes, it is correct!

HMF is formed as minor (or trace) product in glucose decomposition under neutral conditions. Under acidic conditions HMF can be a major product.

"The First Molecular Level Monitoring of Carbohydrate Conversion to 5-Hydroxymethylfurfural.."

ChemSusChem, 2012, 5, 783.

http://dx.doi.org/10.1002/cssc.201100670

There are many degradation/dehydratation and oxidation products arising from heating glucose. It depends on the conditions of heating /presence or absence of air oxigen, pH and temperature/. In the case o heating this monosacharide in absence of O2 to 200 oC, levoglucosan /1,6 anhydroglucose/ is formed, also 1-2 and 1-4 anhydroglucose may be present among the thermodegradation products. Later Levoglucosenone , 3-deoxyglucosenone and a number of side products are formed /5-hydroxymethyl 2-furaldehyde, glyolaldehyde, formaldehyde etc./. I am not sure, but the color of pyruvate is not brown- it does not contain chromophoric group absorbing in visible region of light.

Best regards,

R. Solár.

I find that I often made the world's most expensive caramel by recrystallizing carefully differentially protected carbohydrates when I started doing research---and forgetting about the solvent on the hot plate as I try to dissolve/reduce the added solvent. Trityl groups add that special floral note to the caramel!