I cannot find any information on the straightforward reaction that takes place in a heated solution of starch and sugar. I would like to know more about this reaction; preferably via references to textbooks or peer-reviewed papers, please, although it seems to be an obscure topic.
Presumably this reaction is not a hydrolysis of the starch - since that generally requires more aggressive conditions (see the process for creating modified starch) - but it must be something more than a mere gelatinisation, because gelatinisation is rapid and takes place below 100 C, whereas the reaction between starch and sugar is slow and requires a higher temperature.
A demonstration of the reaction: Prepare a solution of sugar (sucrose), boil it to 'soft ball' and then slowly pour it into a cold slurry of cornflour (a.k.a. cornstarch outside the UK) and water, to which a dash of cream of tartar has been added. Bring the liquid back to the boil and simmer very gently for several hours, stirring..
Result: With the sugar, the mixture starts out white and cloudy but, after some time, turns clear and golden. Without the sugar (but still with the cream of tartar) the mixture thickens, and remains white and opaque. The latter reaction is clearly just gelatinisation but, equally obviously, the former reaction with sugar must be something else. So what is happening?
I have consulted a couple of text books but the descriptions of the chemistry are vague and confused. On the one hand, the books state that starches have a gelatinisation temperature which can vary, but which is always less than 100 C. (The chemical process is described and it would seem clear that this process takes only a few minutes to achieve the thickening of a starch slurry as it is heated). But on the other hand, the books also say that the starch/sugar reaction (which they also call gelatinisation) is slow and can take several hours. Those two points would appear to be contradictory - the starch/sugar reaction is not (I assert) gelatinisation.
A further point is that there seems to be a distinction between 'thickening' and 'gel forming' ability of starch. It seems to be stated that starches that thicken well, will tend to form poor gels. However, I have not been able to find a reference that adequately confirms that, or explains the chemistry of those two related processes.
The effect of moderate heat and weak acid on sucrose is to hydrolyse or 'invert' it by splitting the sucrose (a di-saccharide) into dextrose and fructose (mono-saccharide); reducing sugars as they are known. This gives rise to my speculation that it is the reducing sugar that is reacting with the starch.
So, in summary...
1. What is this reaction between starch and sugar?
2. Is a mono-saccharide (i.e. a reducing sugar) necessary for the reaction with the starch?
3. The text books say that starches that thicken well, will tend to form poor gels. Why is this so?
Starch, a complex carbohydrate, has powerful thickening properties. When starch is combined with water or another liquid and heated, individual starch granules absorb the liquid and swell. This process, known as Gelatinization , is what causes the liquid to thicken. Gelatinization occurs at different temperatures for different types of starch. As a general rule of thumb, root-based starches (potato and arrowroot, for instance) thicken at lower temperatures but break down more quickly, whereas cereal-based starches (corn and wheat, for example) thicken at higher temperatures but break down more slowly. High levels of sugar or acid can inhibit gelatinization, while the presence of salt can promote it. Starch-sugar interactions may play a role in delaying starch gelatinization. Interaction of sugar with starch chains in the amorphous regions of the starch granule stabilizes those regions, thus increasing the energy required for gelatinization.
Hi. Thanks for taking the trouble to reply, but Im not sure that your answer gets to the specific point of my question. In the example I gave, the starch solution appears to gelatinise immediately then, after an hour or more, a separate reaction occurs that turns it from a thick opaque white "gelatinous" mass to a clear golden product. That reaction requires the sugar to be present, so it is not (I am surmising) a simple gelatinisation; it must be a specific reaction between the starch and the sugar. One of my questions is whether the reaction requires a reducing sugar?
Im hoping someone can give me a reference to a text-book or paper that describes the chemistry that is going on here - it is a well-known reaction, so someone must have described it in the literature, surely? However I have checked a number of textbooks, including a large volume dedicated to starch chemistry, but the reaction is not described.
to prepare a true solution of starch in water we have to dissolve solid starch in few cold water and this suspension will be poured into a boiling water solution. sucrose however dissolve easily in cold water and much more in hot water. together these sugars depending on their concentrations could give viscous solutions: hich concnentrations give more viscosity . Heating this solution at 80 degree Celcius and around 120 do not give the same result mainly the gel formation. high temperatures increases dissolution of starch granules which absorb more water and undergo degradation.
heating sugars until water evaporation could lead to non enzymatic browning and caramelisation
I need to clarify my question:
In one experiment, I heated a slurry of cornstarch and dextrose in a waterbath at 100 C. I repeated the test without the dextrose. WITH the dextrose, the slurry thickens almost immediately, to a white paste then, after 2-3 hours it turns a clear golden colour. WITHOUT the dextrose, the slurry thickens almost immediately, to a white paste, but the second reaction does not take place. My question is: what is the reaction that is taking place between the sugar and the starch?
Clearly it is not (I assert) gelatinization, because that is the initial thickening. It is not (I assert) "non enzymatic browning and caramelisation" because the temperature is not high enough, and no (or very little) water is lost. So what is going on? I Im suggesting that it is due to the reducing property of the sugar but what, precisely, is the reaction?
If there is just a very little of water that is lost. So, may be you have to though to the role of the dextropse as an emulsifiant
I must admit that as a starch scientist I have never heard of the phenomenon that you describe. From your initial question and your answer to J.M. Koli I guess that cream of tartary is not essential to induce the effect and that it occurs both with flour and with starch. In my opinion it might be due to the thermal degradation of glucose in water according to a paper of Woo et al. (attached). These authors describe a pH decrease, the formation of colour, hydroxymethylfurfural and organic acids, when heating glucose and maltose solutions for several hours at 110-150 degrees C at moderate concentration. The acids formed may eventually induce starch hydrolysis, which would explain the system's clarity.
To test this hypothesis I recommend to monitor the reaction at various times (including t = 0) for pH, colour, and viscosity (to keep it simple). More advanced tests could include monitoring the formation of HMF and organic acids. Another option is to repeat the experiment in a near to neutral buffer where this phenomenon should not occur.
In principle, there is no interaction between starch and sugar in a sucrose solution. Heating starch in the solution postpones starch gelatinization since water absorption by starch granules happens at the later starge of viscosity profile. Based on this, depending on the sucrose concentration in the solution, the swelling of starch granules partly happens, due to less water absorption, therefore the rupture happens with delay.
Thank you all for your answers. Peter A. M. Steeneken : you make an interesting suggestion, which I shall follow up; thank you. Salomeh Ahmadi-Abhari : I appreciate that gelatinisation is delayed in the presence of sugar, but my point was what causes the colour change? This work was something I did some time ago, and I think I need to repeat it, paying very careful attention to the reaction conditions before reporting further.
Colour change may be caused due to caramelization of sucrose during heating.
But, Krishan Kumar, I said that "In one experiment, I heated a slurry of cornstarch and dextrose in a waterbath at 100 C ". Surely that is too low a temperature for caramelisation? However, as I also said, this experiment was some time ago, and I must repeat it to be absolutely sure of my facts. (i.e. was it that particular experiment I used the water bath for?) One thing is certain though, the temperature was not high enough for most caramelisation reactions, which need about 160 C. Only fructose caramelises at a lower temperature (around 110 C) and I was using dextrose (glucose). I think some sort of thermal degradation may be the key, as Peter A. M. Steeneken suggested. I will try this out as soon as I can.
David,
While I would agree with previous comments suggesting the thermal degradation of sucrose (beginning of caramelisation). The degradation products (probably among those some hydroxymethylfurfural) may just have better solubility parameters for starch than the pure saccharose.
In which proportion starch/sugar did you notice this phenomenon?
Have you tried to dissolve the clear mixture?
If so what did you notice?
Hi, in a experiment with corn flour, Maillard reaction happened between protein and sugar at 100 degree is possible; and cream of tartar exhibit acid condition, may boost the reaction
Thank you, Xiaohong Lanm but there is no protein in cornflour (also known as cornstarch) - it is pure starch.
I know that cream of tartar is potassium bitartarate KC4H5O6. I also know that corn flour should contain 10-12% protein and should not be named corn starch, because in the process of preparing corn starch, other intefering nutrirnts are usually removed to obtain the starch. Since wothout the sugar, the mixture remained white and opaque, althogh it contained cream of tartar. The difference is the two mixtures is the presence of sugar.
Just to be clear , I was using corn starch. In the UK corn starch is colloquially referred to as corn flour. As it says at https://en.wikibooks.org/wiki/Cookbook:Cornstarch ... Cornstarch is a starch made from corn. It is not to be confused with cornmeal or corn masa. Both cornstarch and finely-ground cornmeal are sometimes called cornflour; this is thus a bad term to use. Cornstarch is commonly used to thicken sauces.
If the corn starch is relatively pure and free of adulteration, the occurence of protein sugar reaction (Maillard reaction) should not be reponsible for your observation. Starch is a homopolymer of glucose. It contains a straight chain portion (amylose) and a branched chain portion (amylopectin). Amylose gives a blue black coloration hen it reats ith iodine, hile amylopectin gives a puurple coloration with iodine. In nature, starch molecules are enclosed in granules. The granules may be small medium or large. Large granules gelatinize faster than small granules. This means that different starches ill have different gelatinization temperatures and times. When starch is heated in a moist environment, the granules imbibe water and swell until the ganules burst and the starch is said to gelatinize.On cooling, gelatinized starch tries to re-organize itself by forming new hydrogen bonds, trying to hold on to the water which it imbibed during swelling. This process is called retrogradation. This process leads to textural changes in the starch. Cream of tartar is acidic and can infleunce acid modification of starches . Acid modified starches are usually thinner than their native counterpartes. This could account for the clearer gel with a slight change in colour. I hope you find this contribution helpful.
All the best!
Philippa
Important, apart from the temperature, there is the type of starch. It may be native, it may be a modified starch. Starch, not equal to starch. It will be different in the case of potato starch and different in the case of e.g. corn starch. Another factor is the relationship between the amounts of starch, sucrose and water. Although at low temperatures I would not expect any chemical reactions, but increasing it may cause phase trasitions. Thermal decomposition will take place at high temperatures, mainly after water has evaporated.
To your question as to why the color change. It occurs to me that it may simply be caramelizing of the dextrose. Boiling dextrose water yields a golden brown syrup. Repeating the experiment with just dextrose might explain the color change and might also prove that indeed no reaction is taking place. Perhaps both chemicals have retained their original properties and are now in solution?
There is no reaction or interaction happening between starch and mono/disaccharides. There is however an indirect interaction. Sugars are immediately soluble in water; therefore there remains insufficient water for starch to get compl gelatinized. It is called immature gelatinization.
the discoloration of starch in the presence of sugar has nothing to do with Miaillard reaction since the protein content of starch is not high enough to take part in this reaction at sugar presence.
this discoloration is very much depending on the water content of your starch suspension. For a complete starch gelatinization, in the presence of no or low sugar ratio, you need min. 60% water. If the suspension is deprived of water, starch is physically hydrolyzed and the outcome is discoloration. This is enhanced by increasing the ratio of sugar.
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