Southern Matters

Southern Matters

Sugar Cane - Essays - Sugars

In the end, our cane mills are about producing syrup. Sugars are the principal ingredients by weight of syrup and confer protection against spoilage, provide sweetness and body, and contribute to color if caramelized. Therefore, sugars merit some mention on this site. First, a general description of carbohydrates is given. Second, relevant carbohydrates are described. Third, the conversion of selected carbohydrates is outlined. Finally, why this matters to syrup makers is addressed.

Sugars are ubiquitous, diverse, and abundant in plants, where they serve a myriad of functions. The simplest of sugars are strings of 3 to 7 carbon atoms. Carbon forms 4 bonds, and the two bonds remaining (after the formation of bonds with neighboring carbons) are used to bond with a hydrogen atom on one "side" and a hydroxyl (OH) moiety on the other. Simple sugars, therefore, are characterized as having the ratio of C:H:O = 1:2:1; in short, sugars are carbohydrates. In the following narrative, I have placed in bold the main carbohydrates that syrup makers are concerned with. Glucose (= dextrose), fructose (= levulose, fruit sugar), and sucrose (= table sugar, cane sugar, beet sugar) are the main sugars in cane juice, and starch is the starting point for making corn syrup (which is blended with cane syrup by some syrup makers).

Glucose and fructose have six carbon atoms. Because each of the carbon atoms along the string can have the H on the "left" or on the "right" if a flat projection of the string is made, many different sugars have six carbons, and, of course, the same goes for sugars of other sizes.

Simple sugars can linked together to form larger molecules, such as sucrose. Note that sucrose is made from one glucose molecule (the six-membered ring on the left) joined to a fructose molecule. This figure also makes it easy to visualize, as discussed above, how so many different sugars can result simply by "flipping" the H and the OH. Lactose (milk sugar) and maltose (discussed later) are other examples of sugars that are formed from 2 units of 6-carbon sugars. You have probably noticed that the trivial name of many sugars ends in -ose.

Finally, very large carbohydrates can be made by linking up to tens of thousands of simple sugars to form long chains. Cellulose-cotton fiber is almost pure cellulose-is an extremely abundant resource and is used to make a variety of products such as paper and plastics. Cellulose is a chain of glucose molecules joined from the # 1 carbon on one to the # 4 carbon on the next. Starch is another large carbohydrate made exclusively from glucose. Starch consists of two kinds of chains, though. One kind of chain (amylose) is linear (again, the # 1 and # 4 are linked to form the chain). The other kind of chain (amylopectin) has a 1­>4 backbone, but also has branches; amylopectin is very similar to glycogen, which is the main readily available food reserve stored in animals ranging from oysters to humans.

Two important points arise. The physical and biological properties of apparently very similar carbohydrates are vastly different. Take cellulose and amylose from the preceding paragraph. Both are simply chains of glucose units connected between the #1 and # 4 carbons. However, the orientation of the connecting bond is different. Cellulose is very strong with a tensile strength exceeding steel; starch has no strength. Starch forms a gel in hot water, as when we make gravy; cellulose is unphased by water, as we observe each time we wash cotton clothes. Starch from wheat, rice, and corn are the foundation of a good diet; we cannot digest cellulose.

Sucrose and starch can be broken down using simple chemical means. Sucrose is broken down into glucose and fructose by heating with acid. Starch is also broken down into fragments by acid treatment; depending on the duration, the strength of the acid and the temperature, breakdown can be complete, all the way to individual glucose units. Less complete breakdown of starch yields, among other products of various lengths, maltose (a fragment of two glucose units). Some types of corn syrup are prepared by partial or complete acid hydrolysis of corn starch. There are many different corn syrup formulations. I chromatographed one source of corn syrup used by many syrup makers in South Georgia, and it was pure glucose. One the other hand, I chromatographed a sample of syrup blend produced in South Georgia from a different brand of corn syrup and it could not have been pure glucose.

Carbohydrates can be converted from one to another using enzymes (proteins that specifically speed certain reactions.) A commonly used enzyme is an invertase obtained from yeast. This enzyme catalyzes the breakdown of sucrose into glucose and fructose. (Glucose and fructose are thus invert sugars because a physical property, rotation of plane-polarized light, has been inverted [changed from plus to minus] by the conversion of sucrose to the two 6-carbon sugars). If the treatment is too short or if the conditions (e.g., amount of invertase) are not optimum, all the sucrose may not be broken down. A second commonly used enzyme is one of the amylases, which partially break down starch. Amylases are used to manufacture some corn syrups and to prevent gelling of sorghum syrup. Finally, another enzyme, an isomerase, converts a portion of glucose to fructose. Use of this enzyme permits the production of High-Fructose Corn Syrup (after the starch has been converted to glucose). Corn syrup is, of course, a cheap sweetner and various formulations find their way into many foods such as breakfast cereals, canned fruit, ketchup, soft drinks . . . . As you have noticed, the trivial names of enzymes end in -ase.

What does all this mean to a syrup maker?

  • Sugaring. Of the three sugars in cane juice, sucrose is the least soluble (i.e., syrup will "hold" less sucrose before it forms crystals). Although some sugaring is acceptable, it has two drawbacks (esthetics to some consumers and potential spoilage of the remainder of the syrup). Fortunately for the syrup maker, some sucrose is chemically degraded to glucose and fructose by heating because the juice is slightly acidic (pH ~ 5.2) [see section above]. (An early extension bulletin indicated that the longer heating time in kettles is an advantage because the syrup contains less sucrose, if all else is equal.) There are three remedies for sugaring under the control of the syrup maker. First, he or she can choose a variety of cane that is less likely to form sugar in syrup. This is a very effective strategy to produce excellent excellent syrup. Second, he or she can add corn syrup to the sugar-cane syrup, effectively diluting the sucrose. Many syrup makers opt for this choice. Whether to blend or not to blend is a matter of personal preference, of course, and some prefer the milder taste of the blend anyhow. Third, and finally, the sucrose content of the syrup can be diminished by the use of invertase (see Walton CF, EK Ventre 1935 How to prevent sugaring of sugarcane sirup. USDA Circular). Walton and Ventre suggested taking the juice to semi-syrup (20 º Baumé) before treatment. As I understand it, invertase is considered a "processing aid," not an ingredient, and therefore does not need to be listed on the label.
    Fructose is the most soluble of the three sugars and is often chosen in food manufacture for that reason (e.g., to prevent "sandiness" in ice cream).
    Interestingly, honey bees are faced with the same problem as syrup makers. Nectar, like cane juice, is mostly sucrose, glucose, and fructose. Honey bees inject invertase into the nectar as it is converted to honey. Thus, honey is mainly glucose and fructose. Even so, some nectars, like that produced by the mustard family, contain so much glucose that the honey crystallizes quickly. On the other hand, tupelo nectar has so much fructose that honey produced from it never crystallizes.
  • Spoilage. Of course, the first line of defense against spoilage is sterilization. A second potential line is the addition of a "preservative" such as benzoate. Again, this is a matter of personal preference, but note that we accept benzoate in products such as soft drinks. However, the sugar composition may also play a role. A primary historical means of preventing the growth of spoilage organisms is to deny the organisms adequate water. This goal is accomplished directly by drying the product such as fruit slices or by "drawing" the water out with usually salt. The relevant comparative physical-chemical parameter to measure concentration in this context is osmolality (osm; not osmolarity), which in this case is the ratio of sugar molecules to water molecules. What all this means theoretically is that for a syrup of given sugar content (weight of sugar per volume of syrup), a sucrose syrup would be the easiest for the spoilage organism to thrive in.

    Another property of a solution that depends on osmolality is elevation of boiling point. For a typical cane syrup, this value is about 12-13 º F above the boiling point of water, or about 225 º F along the coastal areas. As indicated in the discussion above, this value will depend on which sugar (and other components plus deviations from ideality) in the syrup predominates. (If sucrose predominates, a lower boiling point would be expected, and if the 6-C sugars predominate, a higher boiling point would be expected.) As a general rule, thermometers, which provide a continuous reading, are used in evaporators whereas hydrometers are used in kettles, as discussed below.

  • Thickness. The viscosity, or resistance to flow, comes into play in two different ways. First, some syrup makers rely on viscosity (i.e., flaking off a dipper) to determine when the syrup is finished. Second, some people have a preference for thicker or thinner syrups. The least viscose sugar is fructose, with glucose being slightly more viscose. Sucrose is, by far, the most viscose (the relative viscosity at room temperature is almost twice as high for sucrose). Ironically, a syrup made of sucrose will be thicker than a syrup made of fructose (or glucose) even when the latter contains more sugar by weight! These facts, along with the sharp temperature dependence of viscosity, explain the difficulty of judging when the syrup is done by using "flaking." On the other hand, the densities of solutions made from different sugars at the same concentration (weight per volume) vary by less than 1 %, attesting to the utility of a hydrometer. Density, like viscosity, is affected by temperature, however. For example, a desirable density for finished cane syrup is 38.5 to 39 º Baumé at 70º F. This same syrup would test at 34.5 º Baumé at 210 ºF. (Thanks to McCalip and Walton for these figures from an article in USDA Bulletin 1370, 1925).

  • Sweetness. Fructose is the sweetest.