Sugar

Introduction

What is sugar?

Sugar is a type of carbohydrate. Carbohydrates are molecules produced by plants that contain carbon, hydrogen, and oxygen. Typically, these elements occur in a consistent ratio. Thus, the general formula for a carbohydrate is:

C_m(H₂O)_n

Carbohydrates are also called saccharides. They come in three forms. Simple carbohydrates or sugars are monosaccharides or disaccharides. That means that they are made up of one (mono-) or two (di-) saccharide units. The other types of carbohydrates are starches (complex carbohydrates) and fiber (cellulose, a substance that makes up the plant cell wall).

In colloquial or everyday usage, sugar typically refers to sucrose. Sucrose is the substance that makes up table sugar or white sugar. It is a disaccharide. Sucrose is made up of two monosaccharides bonded together, one unit of fructose and one unit of glucose.

Sugar-producing crops

Sugar typically comes from one of two crop plants: sugarcane (Saccharum officinarum and its hybrids), a tall grass that concentrates sugar in its stem, and sugar beet (Beta vulgaris), a plant in the amaranth family (Amaranthaceae) that concentrates sucrose in its root. Another grass, sorghum (Sorghum bicolor), comes in a sweet variety that is less frequently cultivated to produce sorghum syrup. Because all of these plants are high in sugar, they are also targets for production of biofuels, in which sugar is fermented to produce ethanol. So far, however, only sugarcane has been heavily exploited for this purpose, mostly in Brazil.

Today, many processed foods and artificially sweetened drinks (like sodas) are sweetened using corn syrup or high fructose corn syrup (HFCS) instead of or in addition to cane or beet sugar. Corn syrup is made by breaking down the starch in maize (Zea mays) kernels and consists of glucose and water. HFCS contains glucose, fructose, and water.

Sugar from sugarcane

Sugarcane is a tall tropical grass in the genus Saccharum that is cultivated primarily as a source of sucrose. The juice in the sugarcane stem may be processed to produce refined sugar and/or other edible products. The juice or its derivatives like molasses can also be fermented to produce ethanol, which is used to make alcoholic beverages like rum and also ethanol biofuel.

Cultivation of sugarcane takes place primarily in the tropics, as sugarcane cannot survive winter freezes and needs a relatively long growing season to mature. Unlike grasses grown for grain, sugarcane is propagated asexually from whole stems or from pieces of stem called billets or setts. In regions where the climate allows, sugarcane can be grown as a perennial crop, meaning that it can be harvested and allowed to re-sprout to produce additional crops.

Refined (white) sugar

After harvesting, sugarcane must be processed into raw sugar, a precursor to refined sugar, quickly to prevent the sugary juices in the stalks from spoiling. Thus, at harvest time, sugar milling may go on 24 hours a day, 7 days a week.

Extraction of juice

After sugarcane is harvested and transported to the mill, it is typically shredded, then crushed and washed to extract the sucrose-containing juice. The fibrous plant material left over after sugarcane juice is extracted is called bagasse. Bagasse is often burned at the sugar mill to provide power or heat.

Clarification and filtering

After the juice is extracted, it is clarified (meaning, impurities are removed) and filtered. During clarification, chemicals are added to the diluted sugarcane juice; these chemicals react with impurities in the juice to form solids. The treated juice sits in a settling tank, where the solids are allowed to settle out, accumulating at the bottom of the tank as a substance called mud.

The slurry of mud and some of the sugarcane juice from the bottom of the tank is collected and moved to a rotary filter, which filters out the mud. The solid mud can be recycled as a fertilizer or used for other industrial or agricultural purposes. The juice from the slurry is captured and mixed back in with unclarified sugarcane juice to go through the clarification process again. The rest of the juice, called clarified juice, moves on to the next step in processing. 

Evaporation (concentration)

The clarified juice is next heated so that the some of the water in it evaporates, concentrating the liquid and thus raising its sucrose content. The concentrated liquid is now called syrup. Modern sugar mills use vacuum evaporators to concentrate the juice. Heating the sugary juice under vacuum means that the liquid will boil at a lower temperature than it would in an open container.

Crystallization, centrifugation, and drying

The concentrated syrup is transferred to a vacuum pan. Sugar crystals are added to the heated syrup to spur the sucrose to crystallize, producing a thick liquid called massecuite. The massecuite is moved to centrifugals that spin, separating the solid sugar crystals from the liquid in the massecuite. The liquid, called molasses, is another product of sugar; it is used as food, to make rum, or produce livestock feed. The crystals are dried to make raw sugar. 

Raw sugar produced at the sugar mill may be non-food-grade, meaning that it is not considered suitable for human consumption. The raw sugar is shipped to a refinery before it is processed further.

Refining

Once at the refinery, the raw sugar is melted (mixed with liquid) and passed through a series of steps to refine it: clarification, filtration, and decolorization. Afterwards, the melted sugar is heated again under vacuum to remove the liquid. The sugar is crystallized and then centrifuged again to remove the remaining molasses. If white sugar is being produced, the sugar can then be dried, packaged, and distributed. Modern, commercially produced brown sugar is typically made from refined white sugar with molasses added back. Powdered sugar or confectioner's sugar is made of finely ground sugar mixed with a little cornstarch.

Cane syrup

Cane syrup is produced by extracting the juice from sugarcane, straining it, then heating it so that it that it thickens into syrup. Cane syrup should not be confused with simple syrup (made by heating a mixture of sugar and water) or invert syrup (made by adding an acid to a sugar and water solution to break down the sucrose into fructose and glucose molecules). Cane syrup is a traditional product of the southeastern U.S.

Non-centrifugal sugar

Non-centrifugal sugar is a traditional solid sugarcane product with a brown color. It goes by various regional names, including kokuta in Japan, gur in India, jaggery in Africa and parts of Asia, and panela, piloncillo, or rapadura (among other names) in Latin America. Non-centrifugal sugar is made by heating and evaporating the juice squeezed from the sugarcane stalk, leaving a thick syrup. This syrup is cooled in a mold, yielding a solid sugar product.

The main difference between non-centrifugal and refined sugar is that in refined sugar, molasses is separated from the sugar syrup and the sugar is processed to produce uniform white crystals that are visible to the naked eye. While non-centrifugal sugar contains a large amount of sucrose, it is made up of very small, irregular crystals. Non-centrifugal sugar also contains other natural compounds.

Sweet sorghum & sorghum syrup

Sweet sorghum is a type of domesticated sorghum (Sorghum bicolor subspecies bicolor) that accumulates sucrose in its stem. Sorghum is thought to have been domesticated in tropical Africa. Nevertheless, it can be grown as an annual crop in temperate regions. In other words, while sorghum has to be replanted every year, it can grow to maturity in one growing season in temperate regions. In regions that do not experience freezing, it can be grown as a perennial crop. Thus, sorghum stands in contrast to sugarcane, which can only be grown in tropical to subtropical regions with extended growing seasons and mild winters. Sorghum is grown from seed.

Because of its high sucrose content, ability to be grown in temperate zones, and drought tolerance, sweet sorghum is seen as a potentially attractive plant for producing biofuels. Nevertheless, it is currently not widely grown for this purpose, as maize and sugarcane are the primary biofuel crops.

Sweet sorghum is grown in tropical to subtropical regions for use as human food, livestock fodder, and a source of sugary juice for making sorghum syrup, non-centrifugal sugar, and sorghum beer. Sweet sorghum is traditionally used to make sorghum syrup (sometimes incorrectly called sorghum molasses) in a process similar to that used to make sugarcane syrup in the United States. 

Corn syrup

Corn syrup and high fructose corn syrup (HFCS) are made mostly from kernels of yellow dent corn, which is a type of corn that has dented kernels. Maize was domesticated in southern Mexico, but it is widely grown as a crop in the temperate zone, since it can complete its life cycle in the shorter temperate growing season.

Unlike sugar and sorghum, maize does not accumulate high concentrations of sucrose in its stem. Rather, like other grains, maize accumulates starch in its kernels. Starch molecules are complex, so they must be broken down in order to yield sugar molecules. Sugar made from maize kernel starch is used to produce corn syrup for human consumption. Sugar from maize kernel starch may also be fermented to produce ethanol, which is used to make ethanol biofuel or alcoholic beverages, like bourbon.

Production of corn syrup starts with whole maize kernels. The first step is to isolate the starch from the kernels in a process called wet milling. Wet milling involves soaking the kernels in a large vat of slightly acidic hot water. The soaked kernels are then ground into a slurry, which is spun to separate the germ (oil-containing embryos) from the other components of the kernels. The remainder of the mixture is ground again, then filtered to remove the fiber. Finally, the mixture is spun and thoroughly washed to remove the protein, leaving nearly pure cornstarch.

Starch in maize kernels consists of long chains of glucose molecules that must be broken apart. In order to do this, two enzymes are added to the cornstarch, one after the other. The first breaks the starch into simpler carbohydrates, and the second breaks the simpler carbohydrates into glucose molecules, yielding corn syrup.

High fructose corn syrup (HFCS)

Today, many processed foods and sweetened beverages use high fructose corn syrup rather than cane or beet sugar. HFCS is made by treating corn syrup with a third enzyme that converts glucose molecules into fructose molecules, which are sweeter in flavor. There are several types of HFCS, which are differentiated by the percent fructose sugar they contain. The two most common used in food and drinks are HFCS 42 (42% fructose) and HFCS 55 (55% fructose).

HFCS has advantages over sucrose for food and drink manufacturers. HFCS is cheaper than sucrose. It is liquid, so it is a convenient sweetener for use in beverage production. It is also more stable than sucrose in certain processed foods and has preservative qualities, helping to keep food from spoiling.

Some have criticized the widespread use of HFCS in food and drink products. Some consumers dislike the flavor of HFCS. Health advocates argue that widespread use of HFCS is linked to poor health. While HFCS is sometimes singled out for contributing to conditions like fatty liver disease, diabetes, and cardiovascular disease, there is currently no evidence that HFCS is worse for health than sucrose. Rather, high sugar consumption in processed foods and sweetened drinks regardless of the form or source of sugar has probably contributed to the rise in certain health problems in the U.S. population and worldwide.