The standard color of glucose syrups when offered for sale is usually described as ‘water white’ or colorless. It is interesting to note that many food manufacturers specify water white and then proceed to add caramel or other colors to the syrup. In many applications, water white can be replaced by less refined (and less costly) syrups. Water white syrups will not remain that way indefinitely as even fully refined syrups contain traces of proteins and amino acids, the reactions of which are described below. Color formation (browning) in glucose syrups may occur as a result of two types of reaction:
- Maillard browning from reactions between reducing sugars and proteins or amino acids.
- Caramelization due to the effect of heat on carbohydrates.
Maillard reactions
The interactions of carbohydrates and proteins are well documented and represent a complex series of chemical reactions, with the brown color formation being the final stage in the series. The reactions are basically those between reducing sugars (aldoses and ketoses) and amino groups in proteins or free amino acids. Thus all glucose syrups and dextrose are potentially reactive as they contain free aldehyde groups whilst those containing fructose with free ketone groups tend to be even more reactive. It follows that the higher the DE of the syrup the more reactive it will be towards proteins/amino acids. This is well documented in the literature (e.g. Kearsley, 1978a). Amino acids are similarly more reactive than larger proteins.
In some food products browning reactions are desirable, e.g. in bread to give a brown crusty loaf and in toffees to give acceptable color and flavor, but generally, they are not desirable. Brown-boiled sweets or condensed milk do not appeal to the majority of people.
There is little that can be done to control these free aldehyde or ketone groupings without changing the essential characteristics of the glucose syrups themselves and control of Maillard browning, therefore, centers around the control of free proteins or amino acids in the finished syrup or addition of chemical retardants to the syrup to slow the reactions to acceptable levels. Maillard browning is accelerated by heat and the brown colors in the syrups tend to develop during evaporation as the finished product is concentrated or during storage where the syrup must be kept warm (45°C-55°C typically) either to prevent crystallization (high DE syrups) or to facilitate handling (reduce viscosity) of the product (low DE syrups).
Dextrose is a particularly potent reactant in this respect being much more reactive than maltose for example. It is reported that a high maltose syrup of the same DE as an acid-converted syrup (with higher dextrose content) browns much less readily than the higher dextrose-containing product. Thus composition, as well as DE, should be considered when selecting a glucose syrup for an application where browning may be a potential problem.
The proteins in the syrups are present as impurities from the manufacturing process. Glucose syrups are mainly manufactured from maize or wheat starch both of which sources typically contain up to 0.3% protein and residual amino acids. These proteins are carried through to the glucose syrup in reduced concentrations, insoluble proteins being removed during the filtration stages of refining. The modern practice of fully refining glucose syrups also reduces protein/amino acid concentrations in the finished product. For example, in addition to removing inorganic impurities, demineralization will also remove charged proteins/amino acids and carbon treatment, in addition to removing colored compounds, also removes many of the precursors to the brown-colored compounds themselves. Syrups produced from other sources, such as potato or tapioca starch, suffer less in this respect owing to lower protein levels in the starting material.
Sulfur dioxide is commonly added to glucose syrups as a chemical retardant for browning reactions where levels up to 450 ppm are often used. It is not added as a preservative to glucose syrups although at high concentrations undoubtedly has some antimicrobial action. The sulfur dioxide forms a complex with the free aldehyde or ketone groups which reduces their further participation in reactions with proteins/amino acids. Fully refined products require lower concentrations of sulfur dioxide to give the same effect. It should be noted that some countries, e.g. Japan do not allow sulfur dioxide in foods and manufacturers producing foods for these markets usually specify very low levels of sulfur dioxide in the raw materials they purchase. Where sulfur dioxide is not added even greater care must be taken during the handling and storage of the syrup.
Lowering of the pH of the product will also reduce Maillard browning as the initial reactions between reducing sugars and amino acids proceed more quickly at a higher pH. This has limited potential in glucose syrups which are typically produced to have a pH between 4.5-5.5.
Whilst it is possible to deliver a colorless glucose syrup to a food manufacturer the syrup producer has no control over how the user handles and stores the syrup and even more importantly with respect to browning, what other food ingredients are mixed with the syrup and what heat treatment this mixture receives. It is also important to remove impurities (e.g. amino acids as seen above) from glucose syrups during the refining stage as they can in some cases catalyze color formation or maybe precursors to colored compounds. Demineralization removes many charged species (minerals/amino acids) and carbon treatment removes any colored compounds already formed in the syrup.
Caramelization
Caramelization of carbohydrate occurs when it is heated excessively and particularly in the presence of an alkali. This forms the basis of a whole industry dedicated to the production of caramel colors for food use, e.g. in cola beverages, beers, and gravy browning. The final colored compounds produced during caramelization are not dissimilar to those produced as a result of the Maillard reactions although proteins are not of course involved directly in caramelization. The reaction develops round-free aldehyde or ketone groups and caramelization is therefore most likely in higher DE glucose syrups. Ketone groups are much more reactive than aldehyde in this respect and fructose-containing syrups will begin to caramelize at about 70°C. This is of course very important with respect to the storage of these products where on the one hand high temperatures are needed to prevent dextrose crystallization and low temperatures are required to prevent browning. Prolonged storage of fructose syrups is not recommended or normally carried out.
From a glucose syrup manufacturing viewpoint, caramelization reactions are less important than Maillard reactions although potentially they will occur if ‘hot spots’ are present in the process.
Whilst browning is essentially a problem associated with aqueous systems, it also occurs with some crystalline products. Crystalline fructose will for example start to brown above about 70°C whilst maltodextrins are stable even in the presence of proteins.
Since hydrogenation eliminates free aldehyde and ketone groups this process can be used to control both types of browning. Thus dextrose is converted to sorbitol, fructose to a mixture of sorbitol and mannitol and the other components of glucose syrups to the corresponding sugar alcohol.